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The Saccharinic Acids
THE SACCHARINIC ACIDS
BY JOHN C . SOWDEN Department of Chemistry. Washington University. Saint Louis. Missouri
I . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ........... I1. The Individual Acids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1. DL-Lactic Acid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2. D L - ( ~ ,4-Dihydroxybutyric Acid) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . a . Preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .............................. 3. ~~-[2,4-Dihydroxy2-(hydroxymethy1)butyric Acid] . . . . . . . . . . . . . . . . . . . . .............................. .............................. 4 The Five-Carbon Metasaccharinic Acids . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
.
....................................................
b . Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . c Configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5. “or”-D-Glucosaccharinic Acid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . a Preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . b . Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . c . Configuration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . d . Miscellaneous Reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 . “cy”-D-Isos&ccharinic Acid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . a Preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . b . Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . c . Configuration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . d . Miscellaneous Reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . e . “0 ”-D-Isosaccharinic Acid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7. The D-Galactometasaccharinic Acids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . a . Preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . b . Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (1) “a”-D-GalactometasaccharinicAcid . . . . . . . . . . . . . . . . . . . . . . . . . . . . (2) “j3”-s-Galactometasaccharinic Acid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . c . Configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 The D-Glucometasaccharinic Acids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . a . Preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . b . Structure ................................................. c . Configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . d . Degradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I11 . Mechanism of Formation of Saccharinic Acids . . . . . . . . . . . . . . . . . . . . . . . . . . . 1. The Fragment-recombination Mechanism of Kiliani and Windaus . . . . . 2 . The Isomerication Mechanism of Nef .................................
. . .
.
35
36 38 38 38 38 39
40 40
40 41 41 41 42 43
43 44
46 47
48 48 49 51 52 52 53 54 55 55 56 59 59
60
61
61
62 62 62 63
36
JOHN C. SOWDEN
3. The Ionic Mechanism of Isbell.. ...................................... 4. Saccharinic Acids from Substituted Sugars.. .......................... 5. Fragment Recombination and Saccharinic Acid Formation. . . . . . . . . . . . . 6. Saccharinic Acid Formation by Various Bases.. ....................... IV. Table of Properties of Saccharinic Acid Derivatives.. ....................
66 69
72 75 76
I. INTRODUCTION In a paper presented before the French Academy of Sciences in 1838,
Eugene Peligot reported that an acid “trds hmgique” was among the products of the action of barium hydroxide or calcium hydroxide on glucose.’ This observation, that acidic materials may result from the treatment of reducing sugars with aqueous alkalis, marked the beginning of investigations that were to uncover one of the most intriguing, and a t the same time one of the most perplexing, reaction sequences in carbohydrate chemistry. Confusion entered the sugar-alkali reaction picture with the report by Mulder2 in 1840 that, apparently, aqueous acid or aqueous alkali act upon glucose in the same manner, to form an acidic product (“glucinic acid”). A natural consequence of this early work was the conclusion that glucose might be an ester whose hydrolysis by acids or alkalis led to acid and alcohol moieties. However, a clear distinction between the products obtainable from the reducing sugars through the action of acids and alkalis, respectively, was eventually achieved. The hexoses were found to afford, through strenuous treatment with acids, a mixture of levulinic and formic acids: whereas with alkalis the principal products are lactic acid plus a series of six-carbon, deoxyaldonic acids (saccharinic acids) isomeric with the starting sugars. In addition, minor amounts of racemic 3-deoxytetronic acid are formed in the hexose-alkali reaction. COZH
COzH
COzH
CHOH
CHa
CHOH
I CHOH I
CHzOH Saccharinic acid
I CHOH I CHZOH Isosaccharinic acid
I I
CHOH CHzOH Metasaccharinic acid
(1) E. Peligot, Compt. rend., 7, 106 (1838). In this same paper it was also recognized that the crystalline sugar obtainable from grapes, honey, starch hydrolyzates, and diabetic urine is one and the same substance, and the name glucose was proposed for it. (2) G . J. Mulder, J . prakt. Chem., 21, 203 (1840). (3) A. F. von Grote and B. Tollens, Ber., 7, 1375 (1874).
THE SACCHARINIC ACIDS
37
Three structurally isomeric forms have been established for the six-carbon saccharinic acids. In the order of their discovery, these are the saccharinic or 2-C-methylpentonic acids, the isosaccharinic or 3-deoxy-2-C(hydroxymethy1)-pentonic acids, and the metasaccharinic or 3-deoxyhexonic acids. Although none of these six-carbon, deoxyaldonic acids has been crystallized, six are known in the form of crystalline lactones (saccharins). All the possible metasaccharinic acids of less than six-carbon content have been obtained, in the form of crystalline derivatives, by the sugaralkali reaction. Only one example of a branched-chain deoxyaldonic acid (the racemic, five-carbon isosaccharinic acid) of other than six-carbon content has been so obtained. The formation of saccharinic acids containing more than six carbon atoms remains to be explored. Isomeric with the saccharinic acids that arise from the sugar-alkali reaction are the w-deoxy- and 2-deoxy-aldonic acids. Both of these latter types may be obtained by oxidation of the corresponding deoxy sugars, and the 2-deoxyaldonic acids also result from the action of lead oxide on the 1,2dideoxy-1 ,2-dihalogeno-aldoses.* In addition, Glattfeld and coworkers6 have synthesized, mainly from non-sugar starting materials, all the racemic deoxytetronic acids possible. However, the present article deals only with those acids which have been isolated from the sugar-alkali reaction. Mention should be made of the origin of the terms saccharin and saccharinic acid. Peligote isolated the first crystalline lactone (“a”-D-glucosaccharin) of a deoxyaldonic acid produced by the hexose-alkali reaction. A slightly erroneous analysis of this new substance led him to believe that it had the same carbon and hydrogen content as has ordinary cane sugar. In addition, the lactone yielded an initially neutral aqueous solution, and Peligot, after concluding that his substance was simply an isomer of sucrose (saccharose), named it saccharin. Although Scheibler’ soon thereafter recognized the lactonic character of Peligot’s saccharin, he retained the name and expanded it to saccharinic acid for the corresponding free acid. Subsequent workers have perpetuated this nomenclature, and the terms saccharinic acid and saccharin are now used extensively in the generic sense. (4) 8. N. Danilov and A. M. Gakhokidze, Z h w . Obshchei Khim., 6, 704 (1936); Chem. Abstracts, SO, 6333 (1936). (5) J. W. E. Glattfeld and G. E. Miller, J . Am. Chem. SOC.,43, 2314 (1920); J. W. E. Glattfeld and F. V. Sander, ibid., 43, 2675 (1921); J. W. E. Glattfeld and L. P. Sherman, ibid., 47, 1742 (1925); J. W. E. Glattfeld and Sybil Woodruff, ibid., 49,2309 (1927); J. W. E. Glattfeld, Gladys Leavell, G. E. Spieth and D . Hutton, ibid., 63,3164 (1931); J. W. E. Glattfeld and J. W. Chittum, ibid., 66, 3663 (1933); J. W. E. Glattfeld and J. M. Schneider, ibid., 60,415 (1938). (6) E. Peligot, Compt. rend., 89, 918 (1879). (7) C. Scheibler, Ber., 13, 2212 (1880).
38
JOHN C. SOWDEN
11. THEINDIVIDUAL ACIDS 1. DL-Lactic Acid COzH
I I
CHOH CHI 3-Deoxy-~~-glyceronic acid
m-Lactic acid is the metasaccharinic acid related to the triose sugars.
It has been obtained as a product of the action of alkali not only on glyc-
erose (glycera1dehyde)a but also on hexosesg and pentoses,*O whence it arises via cleavage and isomerization. The racemic form of the acid is always obtained from the sugar-alkali reaction since the nonasymmetric enediol related to glycerose is an intermediate in its formation (see Section 111). As a consequence of the biochemical importance of lactic acid, its chemistry has been thoroughly studied and is adequately documented elsewhere. The production of lactic acid by the action of alkalis on the sugars has been reviewed by Montgomery." 2. DL- ( 2 ,,&Dihydroxybutyric Acid) COzH
1 I CHa I
CHOH
CHzOH
DL- (3-Deoxytetronic
acid)
Only the racemic form of this acid is obtained from the sugar-alkali reaction. As in the formation of lactic acid, a non-asymmetric enediol is an intermediate in its production (see Section HI), and hence the racemate is the sole representative of the four-carbon metasaccharinic acid class. a. Preparation.-The conversion of the threoses and erythroses to their related metasaccharinic acid by the action of alkali has apparently not been explored because of the relative inaccessibility of these tetroses. The acid is, however, formed as one of the products of the action of hot, concentrated sodium hydroxide on the pentoses or hexoses.12 Its isolation from these (8) J. U. Nef, Ann., 336, 247 (1904). (9) F. Hoppe-Seyler, Ber., 4,346 (1871). (10) T. Araki, Hoppe-Seyler's 2. physiol. Chem., 19,422 (1894). (11) R. Montgomery, Sugar Research Foundation, N . Y., Sci. Rept. Ser., No. 11 (1949). (12) J. U. Nef, Ann., 376, 1 (1910).
39
THE SACCHARINE ACIDS
sources by Nef was accomplished through a complex and arduous sequence of operations which afforded, at the same time, other saccharinic acids produced in the reactions. Nef's method for resolving these complicated mixtures into their component saccharinic acids is here described in general terms only, and the reader is referred to the original work for details.12Following the isomerization with sodium hydroxide, the reaction mixtures were treated with a very slight excess of hydrochloric acid, concentrated, and the residues heated to effect lactonixation. The saccharinic acids and their lactones were then isolated from the sodium chloride by extraction with organic solvents. The acid-lactone mixture was next acetylated, and the products were separated (by extraction with ether) from colored, gummy byproducts. Following deacetylation, the refined acid-lactone mixtures were repeatedly extracted from their aqueous solution with ether and ethyl acetate, to provide fractions of different degrees of solubility in these organic solvents. Finally, fractional recrystallization of the quinine or brucine salts was employed for separating the individual saccharinic acids. Application of the techniques of chromatography and ion exchange, not known to Nef, should greatly simplify the isolation of individual saccharinic acids from such mixtures. b. Structure.12-Nef observed that oxidation of the four-carbon metasaccharhie acid with nitric acid yields DL-malic acid. Moreover, DL- (3,4-dihydroxybutyric acid), prepared by condensation of l-chloro-l-deoxyglyceritol (glycerol a-chlorohydrin, 3-chloro-1,2-propanediol) with potassium cyanide, followed by hydrolysis, provides a phenylhydrazide (m. p., 99") different from that (m. p., 130-131') obtained from the saccharinic acid. Accordingly, the product from the sugar-alkali reaction is ~~-@-deoxytet, acid)]. ronic acid) [ D L - ( ~4-dihydroxybutyric CH aOH-CH 2-CHOH-CO
zH
DL- (2,4-Dihydroxybutyric
acid)
HN03
A
HO &-CH n-CHOH-CO
2H
DL-Malic acid
(phenylhydrazide, m. p., 130-131")
(m. p. and mixed m. p., 128') THNOs
ClCH2-CHOH-CHzOH 1-Chloro-1-deoxyglyceritol
KCN HzO
___f
HOIC--CH~-CHOH-CH~OH D L - ( 4-Dihydroxybutyric ~, acid) (phenylhydrazide, m. p., 99")
Additional evidence regarding the structure of ~~-(S-deoxytetronic acid) was obtained (by Nef) by resolution of the acid through the brucine salt.
40
JOHN C. SOWDEN
One of the two resulting enantiomorphs gave, on oxidation with nitric acid, the enantiomorph of naturally occurring (- )-malic acid. 3.
D L - [ ~ 4-Dihydroxy-2-(hydroxymethyl)butyric , Acid]
COaH
CH~OH
~~-[3-Deoxy-2-C-(hydroxymethyl) tetronic acid]
This recently discovered, racemic acid of the five-carbon series is only the third example of an isosaccharinic acid to be identified as a product of the sugar-alkali reaction. The other examples are the a- and p-D-isosaccharinic acids of the six-carbon series (see pages 48 and 52). a. Preparation.-The crystalline lactone of D L - [ 4-dihydroxy-2-(hydroxy~, methyl) butyric acid] may be isolated after treatment of xyl~biose'~ (4-0P-D-xylopyranosyl-D-xylose) or the related trisaccharide, xylotrio~e,'~ (both obtained by partial hydrolysis of xylan) with lime-water. b. Structure.-The structure of this isosaccharinic acid is established by its synthesis from 1 ,4-butynediol dia~etate.'~ CHaOH CH-0 COCHI
CHa
CHa
C H ~ O CO C H ~
CHaOCOCHs
I
1,4-Butynediol diacetate
I
I
/OH C
I
CHzOH DL- [ Z , 4-Dihydroxy-P(hydroxymethyl) butyric acid]
The branched-chain structure of the acid is further confirmed through its reduction, with hydriodic acid and red phosphorus, to 2-methylbutyric acid.14 Oxidation of one mole of the isosaccharinic lactone with periodate produces approximately one molecular equivalent of f~rrnaldehyde.'~ Whistler and CorbettI4record barely detectable, positive optical rotations (13) G. 0. Aspinall, Mary E. Carter and M. Los, Chemistry & Industry, 1553 (1955). (14) R. L. Whistler and W. M. Corbett, J . Am. Chem. Soc., 78, 1003 (1956).
41
THE SACCHARINIC ACIDS
for the lactone and calcium salt of the acid. These optical rotations, if real, are probably due to unremoved D-xylose, since the accepted mechanism for the production of this five-carbon isosaccharinic acid in the sugar-alkali reaction would predict a racemic product (see Section 111). 4. The Five-Carbon Metasaccharinic Acids COzH II HCOH
I CHz I HCOH I
CHzOH
3-DeoxyD-erythropent onic acid
COzH II
HOCH
I CHa I HCOH I
CHzOH
3-DeoxyD-threopentonic acid
COzH II
HOCH
I CHz I HOCH I
CHzOH
3-DeoxyL-erythropentonic acid
COIH II
HCOH
I I HOCH I
CHI
CHzOH
3-DeoxyL-threopentonic acid
These four acids comprise all the possible five-carbon metasaccharinic acids; and all were obtained by Nef, in the form of crystalline derivatives, from the pentose-alkali reaction. In reading Nef’s description of these substances and their preparation, it must be borne in mind that the available, naturally occurring D-xylose was a t that time called I-xylose. Moreover, Rosanoff’s convention16 for assigning configurational prefixes was then relatively new and was not utilized by Nef. Accordingly, Nef’s I-xylose and I-arabinose, and d-erythro-, I-threo-, I-ergthro-, and d-threo-1,3,4-trihydroxyvaleric acids are respectively, by modern nomenclature, D-xylose and L-arabinose, and D-erythro-, D-threo-, L-erythro-, and L-threo-1 , 3 ,4-trihydroxyvaleric acids (3-deoxypentonic acids). a. Preparation.-Nef applied his conditions of isomerization (with hot, 8 N sodium hydroxide) to L-arabinose and D-xylose, and was able to obtain from the reaction mixtures, through his complicated system of fractionations (see page 39), all four possible 3-deoxypentonic acids. L-Arabinose yielded 3-deoxy-~-erythro-and-L-threo-pentonic acids, and D-xylose provided the corresponding enantiomorphs. The reader is referred to the original work12for details of the separations involved. b. Structure.12-The structures of the four 3-deoxypentonic acids were established through study of their oxidation with nitric acid to the related 2,4-dihydroxyglutaric acids. The interpretation of the results of these oxidation experiments is intimately related to the prior proof of the structure (15) M. A. Rosanoff, J . Am. Chem. Soc., 28,114 (1906).
42
JOHN C. SOWDEN
of “a”-D-isosaccharinic acid (see page 49). Thus, both 3-deoxy-~-erythropentonic acid (from D-xylose) and 3-deoxy-~-erythro-pentonic acid (from L-arabinose) yielded meso-2,4-dihydroxyglutaricacid on oxidation. The last acid, accompanied by an optically active 2,4-dihydroxyglutaric acid, had been obtained previously by Kiliani and M a t t h e P through oxidation, followed by decarboxylation, of “a”-D-iSosacCharinic acid. Moreover, 3-deoxy-~-threo-pentonic acid (from L-arabinose) gave, on oxidation, the enantiomorph of the optically active 2,4-dihydroxyglutaric acid obtained by Kiliani and Matthes from “a”-D-isosaccharinic acid. COzH I
1
C OzH
COsH
\OH
HNOa
- - coz I
CHz
I
HOCH
+
CH2
I
HCOH
COzH
“~”-D-Isosaccharinic acid
I
CHz
-
I HCOH I CH2 I HOCH I
CHz
CHzOH
3-Deoxy-~-threopentonic acid
I I
HCOH
I
COzH
HNO3
HCOH
1
COiH ~-threo-2,4-Dihydroxy- meso-2,4-Dihydroxyglutaric acid glutaric acid
COzH
I
HNO3
HCOH
I
I
HCoH
I
HCOH
CHzOH
COzH
I
I
I HCOH I , CHz I HOCH I
COzH
~-threo-2,4-Dihydroxyglutaric acid
CHz OH 3-Deoxya-erythropentonic acid
COzH
I
HOCH
I
CHz
I I
HOCH CHzOH 3-Deoxy-~-erythropentonic acid
Finally, a comparisonof the properties of the phenylhydrazides of 3-deoxyD-threo-pentonic acid (from D-xylose) and 3-deoxy-~-threo-pentonic acid (from L-arabinose) showed these two acids to be enantiomorphs. c. Configuration.-Much of the evidence quoted above as proof of the structure of the 3-deoxypentonic acids is also applicable to the establishing of their respective configurations. Nef’s theory of the mechanism of formn(16) H. Kiliani and 0. Matthes, Bey., 40,1238 (1907).
43
THE SACCHARINIC ACIDS
tion of the saccharinic acids led to the conclusion that, in the transformation of a pentose or higher sugar into its related metasaccharinic acid, the configuration would not be disturbed beyond C3. The currently accepted modification of Nef’s theory does not alter this conclusion, and the retention of configuration a t C4 in the formation of metasaccharinic acid has been confirmed experimentally in the hexose series (see page 61). Thus, for example, the two 3-deoxypentonic acids obtained from D-xylose have the D-threo and D-erythro configurations, respectively, and the acid possessing the latter configuration provides meso-2,4-dihydroxyglutaric acid on oxidation. It should be noted that this assignment of configuration t o the 3-deoxypentonic acids, taken in conjunction with the evidence cited in the preceding Section, also confirms the D configuration for the penultimate, secondary carbon atom of L‘Q”-D-isosaccharinicacid. The directions of the optical rotations of the lactones (presumably gamma lactones) and phenylhydrazides of the four 3-deoxypentonic acids are in agreement with those predicted, on the basis of the assigned configurations, by the lactonell and phenylhydrazide18 rules. 5. “~”-D-GZucosaccharin~c Acid COzH
I I HCOH I HCOH I
CHZ-COH
CHZOH
2-C-Methyl-~-ribo(?)-pentonic acid a. Preparation.-The lactone of this saccharinic acid is prepared most conveniently by the action of calcium hydroxide on D-glucose, D-fructose, or invert sugar. Peligotlg noted that D-fructose yields the lactone more readily than does D-glucose, and this result was confirmed by Scheibler.7 For preparative purposes, Kiliani preferred invert sugar, and an abstract of his directions20 based on this starting material follows, A cold solution of 1 kg. of inverted sucrose in 9 liters of water is treated with 100 g. of calcium hydroxide and allowed to stand in a stoppered flask with frequent shaking. Fourteen days later, an additional 400 g. of calcium hydroxide is added. After ~~
~
~
~
-
(17) C. S. Hudson, J. Am. Chem. SOC.,32,338 (1910). (18) P . A. Levene, J . Biol. Chem., 23, 145 (1915); P.A. Levene and G. M. Meyer, ibid., 31,623 (1917) ; C. S. Hudson, J. Am. Chem. SOC.,39,462 (1917). (19) E. Peligot, Compt. rend., 90, 1141 (1880). (20) H. Kiliani, Ber., 16, 2953 (1882).
44
JOHN C. SOWDEN
one to two months, with occasional shaking, the solution reduces Fehling reagent only slightly. The mixture is filtered, the filtrate is saturated with carbon dioxide, and the dissolved calcium ions are then precipitated by the addition of an exactly equivalent amount of oxalic acid. After filtration, the solution is concentrated to a thin sirup and allowed to crystallize in the cold. When the crystallization is complete (several days), the mother liquors, from which no appreciable further amount of the lactone can be obtained, are drained from the crystals, and the latter are recrystallized from water. The yield is approximately 100 g. of pure “oc”-D-glucosaccharinic lact one.
Scheibler’ describes a similar preparation of the lactone, except that the long period of standing a t room temperature is replaced by several hours at 100”. However, KilianiZ0states that the yield obtained by this rapid method is unsatisfactory. b. Structure.-In the belief that “a”-D-glucosaccharhic acid possessed a straight, carbon chain, Scheibler? reduced its lactone with hydriodic acid and red phosphorus in an attempt to obtain n-hexanoic acid. He obtained instead, however, a neutral oil of b. p. 203-204’ which he assumed to be the lactone of a hydroxyhexanoic acid. The presence of a methyl group and of a branched, carbon chain in “a))-D-glucosaccharinicacid was established by Kiliani.2°sI1 Oxidation of “a”-D-glucosaccharinic lactone with silver oxidez2gave a mixture of acids
o=c-
I
COzH
I
I
I/CHaI
CHaCOiH
+ CHaOH
I
C02H
I
/CHa C
I\oH
HNOa ____,
A&O
I
CHOH
CHOH
CHO
CHOH
I
CHz OH “a”-D-GlucostLccharinic lactone
I I
COnH Saccharonic acid
1
AgzO but no
(21) H. Kiliani, Ber., 16, 701 (1882). (22) For a more recent study of the oxidizing action of silver oxide on the sugars, see K. G. A. Bwch, J. W. Clark, L. B. Genung, E. F. Schroeder and W. L. Evans, J . Org. Chern., 1, 1 (1936-37).
45
THE BACCHARINIC ACIDS
including formic, glycolic, and acetic, the last indicating the presence of a methyl group in the original lactone. Oxidation of ‘‘a”-D-glucosaccharinic lactone with nitric acid provided in high yield a crystalline monolactone (saccharon, C6HsO6) of a dibasic acid (saccharonic acid, CaHloO?).Oxidation of this dibasic acid with silver oxide yielded acetic acid but no glycolic acid, indicating that, in the original oxidation with nitric acid (“a”-D-ghcosaccharinic lactone + saccharon), a hydroxymethyl group had been oxidized to a carboxyl group. Thus, it was shown that “Cu”-D-glucosaccharinic acid contains the groups -CHI, -CHzOH, and -C02H, and so must possess a branched, carbon chain. The disposition of the functional groups in “a”-D-glucosacCharinic acid was also established by Kiliani.23Reduction of saccharon with hydriodic acid and red phosphorus gave the known:* crystalline ~~-(2-methylglutaric
o=c
CHI
I
CHa-CH--CH*-CH-PO
I
0
I
C
I CHa-CHz-CH2-CH-COzH DL-(2-Methylvaleric acid)
C H O l
I
CHzOH I < 01 1 9 -D-Glucosaccharinic lactone
C I
HI P
~~-(Z-MethyIgIutaric acid)
+
CHI
CHOH
I I
CHOH COzH
I
HOz C-CH=CH-CH-C OIH ~~-(4-Methylglutaconic acid)
Saccharonic acid ~
(23) H. Kiliani, Ann., 216, 361 (1883). (24) J. Wislicenus and L. Limpach, Ann., 192,128 (1878).
46
JOHN C. SOWDEN
acid). A byproduct of the reduction was a crystalline, unsaturated, dibasic acid which later was recognized as ~~-(4-methylglutaconic acid) .26 Thus, the position of attachment of the methyl group in L‘a”-D-glucosaccharinic acid was restricted to one of the penultimate carbon atoms of a pentonic acid carbon chain. A repetition of Scheibler’s reduction’ of “~”-D-glUCOsaccharinic lactone then showed that the neutral product obtained was similar in its properties to known26~~-(2-methylvalero-l ,4-lactone). Accordingly, L‘a”-D-ghcosaccharinicacid must be a 2-C-methylpentonic acid. Confirmation of this structure for “a”-D-glucosaccharinic acid was obtained by Liebermann and Scheibler27when they demonstrated that the known2* ~~-(2-methylvaleric acid) [DL-(methylpropylacetic acid)] was also formed, in low yield, in the reduction of “a”-D-glucosacchariniclactone to the 2-methylvalero-1,4-lactone. Moreover, under more strenuous conditions of reduction with hydriodic acid and red phosphorus (in a sealed tube a t 200°), the ~~-(2-methylvaleric acid) is the principal product. c. Con$guration.-The coilfiguration of “a”-D-glucosaccharinic acid is not known with certainty, although the D-rib0 arrangement is indicated by the evidence so far accumulated. In his theory of the mechanism of saccharinic acid formation. Nefl2,29 assumed that this acid is produced from D-glucose by changes involving only the first three carbon atoms of the sugar and, hence, that the D-erythro configuration is retained in the two lowest asymmetric carbon atoms of the saccharinic acid. This latter contention has not been changed by subsequent modifications of the Nef mechanism (see Section 111) which are currently used for explaining saccharinic acid formation. Supporting evidence for the D classification of “a”-D-glucosaccharinic acid is found in the optical rotation of its lactone. “a”-D-Glucosaccharinic lactone, in view of its marked stability in water or aqueous acids, is almost certainly a gamma lactone. Thus, as pointed out by VotoEek,3° its positive optical rotation indicates the D configuration for C4 on the basis of the lactone rule.” If the two lowest asymmetric carbon atoms possess the D-erythro configuration, ‘(a”-D-glucosaccharinic acid must be either 2-C-methyl-~arabino-pentonic acid or 2-C-methyl-~-ribo-pentonicacid. The initial choice between these two possibilities was made by Nef,12who chose the D-arabino configuration because of similarities between certain alkaloid salts of the saccharinic acid and the corresponding salts of D-arabinonic acid. However, subsequent developments make the D-rib0 configuration appear the more (25) M. Conrad and M. Gutzheit, Ann., 222,249 (1884); see Ref. 60. (26) R. Fittig and L.Gottstein, Ann., 216, 26 (1883). (27) C . Liebermann and C. Scheibler, BeT., 16, 1821 (1883). (28) A. Saytzeff, Ann., 193,349 (1878). (29) J. U.Nef, Ann., 367, 301 (1907);403, 204 (1914). (30) E. VotoEek, Collection Czechoslov. Chem. Communs., 2 , 158 (1930).
THE SACCHARINIC ACIDS
47
probable. The phenylhydrazide of LLa”-D-ghcosaccharinicacid is strongly dextrorotatory ([a]: +50.3°),12 and the phenylhydrazide rule’s would thus assign the D configuration to the tertiary carbon atom. Obviously, such assignment involves the assumption that replacement of hydrogen by methyl on the a-carbon atom does not invalidate the phenylhydrazide rule. Evidence in support of the presence of a cis arrangement of the hydroxyl groups on C2 and C3 of the ‘La”-D-ghcosacchariniclactone ring is provided , of the lactone for three by its behavior upon a c e t o n a t i ~ n .32~ ~Treatment hours at room temperature with a 2% solution of sulfuric acid in acetone gives a crystalline monoisopropylidene compound [m. p., 62-63’, [a]% -38.4’ (in chloroform)] in 83 % yield. This derivative, after titration with sodium hydroxide to open the lactone ring, consumes one molecular equivalent of sodium metaperiodate, with the production of formaldehyde, and is, lactone. therefore, 2,3-0-isopropylidene-“a”-~-glucosaccharinic The epimer of the known “a”-D-glucosaccharinic acid has not been detected among the products of the D-hexose-alkali reaction, although its concurrent formation is to be expected. d. Miscellaneous Reactions.-Two obviously attractive reactions of ‘(a”D-ghcosaccharinic acid would be its reduction to the corresponding 2-Cmethylaldopentose and its degradation to a 1-deoxy-2-ketopentose. The reduction of “a”-D-glucosaccharinic lactone with sodium amalgam was investigated by S ~ h e i b l e rwho , ~ ~ reported briefly that hydrogen is absorbed by the lactone under these conditions. Subsequently, F i ~ c h e noted r~~ that, if the reduction is carried out at or near neutrality, the product is a reducing sugar. The reduction with sodium amalgam was repeated by VotoEek?O but his attempts to prepare a crystalline hydrazone of the amorphous product with phenylhydrazine, p-bromophenylhydrazine, or 1methyl-1-phenylhydraainewere unsuccessful. However, application of the cyanohydrin synthesis to the sirupy sugar provided what was presumably a mixture of 3-C-methylaldohexoses, from which a crystalline phenylosazone and a crystalline p-bromophenylosazone were obtained. The successful degradation of ‘La”-D-glucosaccharinicacid to a l-deoxypentulose has apparently not been recorded. Experiments in the author’s laboratory have indicated that the oxidation of calcium (‘a”-D-glucoSaccharinate with hydrogen peroxide and ferric acetate (the Ruff degradation) proceeds normally to yield a reducing product. However, no crystalline derivative of the expected deoxypentulose has been obtained as yet. (31) Dorothy J. Kuenne, Ph.D. Dissertation, Washington University, St. Louis, (1953). (32) L. M. Utkin and G. 0. Grabilina, Doklady Akad. Nauk S. 8. S. R., 93, 301 (1953); Chem. Abstracts, 48, 12676 (1954). (33) C. Scheibler, Bes., 18,3010 (1883). (34) E. Fischer, Ber., 22, 2204 (1889).
48
JOHN C. SOWDEN
6. “a”-D-IsoSaccharinic Acid COzH
c I‘oa CHz
I I
HCOH CH2OH 3-Deoxy-2-C-(hydroxymethyl)-(~-erythro or D-threo)-pentonic acid
Shortly after the discovery of Peligot’s “a”-D-glucosaccharin, Dubrunf a u P reported that the calcium salt of a monobasic acid resulted from the action of lime-water on maltose. CuisinieP named the acid isosaccharinic acid, after he had prepared from it a crystalline lactone (CeHloOs) isomeric with Peligot’s (‘a”-D-glucosaccharin.The name was expanded t o (‘a’’-D-isosaccharinic acid after Nef12 obtained evidence of the concurrent formation of its epimer, “p”-D-isosaccharinic acid, in the hexose-alkali reaction. a. Preparatim.--“a”-D-Isosaccharinic lactone is obtained in a 15 to 20 % weight-yield by the action of lime-water on malt0se,~6lactose,8eor cellobiose?’ Somewhat lower yields (4to 12 % by weight) can be obtained from partially degraded cellulose plus lime-water.a8 Relatively little “a”-D-isosaccharinic acid is formed in the reaction of D-glucose or D-galactose with hot 8 N sodium hydroxide.’z The efficacy of the (1+4)-linkeddisaccharides in producing the isosaccharinic acid is discussed in Section 111. Lactose is the most convenient source of “a”-D-kosaccharinic acid and Kiliani’s directionsa@ based on this disaccharide follow. Lactose (1 kg.) in 9 liters of water is treated with 200 g. of calcium oxide (slaked and cooled), and the resulting mixture is maintained in a stoppered flask a t room temperature, with frequent shaking, for 3 days. The solution is then heated in a boiling-water bath for 10 hours, filtered, and evaporated to a volume of 3 liters. The highly insoluble calcium “UJ’-D-isosaccharinate(199g.) crystallizes; i t is accompanied by a small amount (14g.) of calcium carbonate. The salt is separated by filtration and
(35) A. P. Dubrunfaut, Monit. sci. Docteur Quesneville, [3]12, 520 (1882). (36) L. Cuisinier, Monit. en’. Docteur Quesneville, [3]12,521 (1882);Bull. S O C . chim. (France), [2]38, 512 (1882). (37) S. V. Hintikka, Ann. Acad. 81%.Fennicae, Ser. A , ZZ, N o . 9 (1922);Chem. Abstracts, 17, 3486 (1923). (38) 0.von Faber and B. Tollens, Ber., 32, 2589 (1899);J. J. Murumow, J. Sack and B. Tollens, ibid., 34,1427 (1901);C. G.Schwalbe and E. Becker, J . prakt. Chem., [2]100, 19 (1920).J. Palm&, Finska Kemietsamfundets Medd., 38, 108 (1929); Chem. Abstracts, 24, 1625 (1930). (39) 11. Kiliani, Ber., 42, 3903 (1909).
49
THE SACCHARINIC ACIDS
is then heated with a solution of an equivalent amount of oxalic acid. Filtration of the calcium oxalate, followed by concentration of the filtrate to a sirup, affords the readily crystallizable “cr”-D-iaosaccharinic lactone.
b. Structure.-Soon after the discovery of “a”-D-isosacchar~icacid, Kiliani applied to i t 4 0 the same methods he had used previously to establish the structure of “a”-D-glucosaCcharinic acid (see page 44). The reduction of L1a”-D-isosacchariniclactone (CeHloOs) with hydriodic acid and red phosphorus at atmospheric pressure yielded, as had the similar reduction of “a”-D-glucosaccharinic lactone, a 2-methylvalero-1 4-lactone. When the reduction was carried out at higher temperatures in a sealed tube, the prodacid), which also had been obtained similarly uct was ~~-(2-methylvaleric from “a”-D-glucosaccharinic lactone. However, “a”-D-isoSaccharinic lactone, on oxidation with silver oxide, yielded (in contrast to the behavior of “a11-D-glucosaccharinic lactone) no acetic acid. Thus, a methyl group is not present in “a”-D-isosaccharinic acid, and, in view of the formation of the CHs
I
CHa-CH-CHz-CH-C=O
I
HI
AgzO
[no CHsC02Hl
CeHia06 P ‘1 a J 9 -~-Isoaaecharinic
I
2-Methylvalero-l,4-lactone
+
CHa
lactone
CH~-CH~-CH~-~H-CO~H DL- (2-Methylvaleric acid)
2-methylvalero-l , 4-lactone on reduction, it must possess one of two alternate structures (I or 11). COzH I
I/ I
CHzOH
C
COzH I CHzOH
C
‘OH
I
I
CHa
CHOH
CHOH
CHOH
CHzOH I 3-Deoxy-2-C-(hydroxymethy1)pentonic acid
CHzOH
I
I
(40)H. Kiliani, Ber., 18,631 (1885).
I I
I1 2-Deoxy-2-C-(hydroxymethy1)pentonic acid
50
JOHN C. SOWDEN
In agreement with these postulated alternate structures (I or II), it was observed that oxidation of “cr”-D-isosaccharinic lactone with nitric acid yields a tribasic acid, COHsOs Furthermore, the tribasic acid readily loses a molecule of carbon dioxide when warmed to loo”, a behavior consistent with the presence of two carboxyl groups on a single carbon atom. The product initially isolated by Kiliani41from this decarboxylation was an optically inactive dihydroxyglutaric acid. This latter acid was found to differ in properties from a Zf3-dihydroxyglutaric acid obtained by the successive bromination and hydrolysis of glutaconic acid. Accordingly, Kiliani concluded that the dibasic acid (obtained by oxidation, followed by decarboxyl-
.
COzH
COZH
COSH
COaH
CHOH
CHOH
CH
CHOH
I
CHzOH “a”-o-Isosaccharinic acid
I COzH
I
COzH Dihydroxy- Glutaconic glutaric acid acid
(m. p., 106” +)
I
C02H 2,3-Dihydroxyglutaric acid (m. p., 155-156”)
ation, of “Cr”-D-isosaccharinic lactone) must be a 2,4-dihydroxyglutaric acid and, hence, that “a”-D-isosaccharinic acid is a 3-deoxy-2-C-(hydroxymethy1)pentonic acid (I). The above reasoning, based on the dihydroxyglutaric acids, is fallacious, as was recognized subsequently by Kiliani and Herold,42since in no event could the same properties be expected for the dihydroxyglutaric acids obtained, respectively, from “Cr”-D-isosacchariniclactone and from glutaconic acid. Glutaconic acid, on bromination followed by hydrolysis, would yield a mixture of the two possible racemates of 2,3-dihydroxyglutaric acid. In contrast, structure I1 for “cY”-D-isosaccharinic acid would provide, on oxidation and decarboxylation, a single, enantiomorphous 2 , S-dihydroxyglutaric acid. Finally, structure I for “a”-D-isosacchari~cacid would lead to a mixture of diastereoisomeric 2,4-dihydroxyglutaric acids, one of which would be asymmetric and the other meso. The correctness of structure I for “a”-D-isosaccharhic acid was even’ tually confirmed by Kiliani and Matthe@ when they isolated from the oxidation and decarboxylation, not only the previously obtained meso-dihydroxyglutaric acid, but also the accompanying, optically active isomer (41) H. Kiliani, Ber., 18,2514 (1885). (42) H. Kiliani and F. Herold, Ber., 38, 2671 (1905).
1
[:"
51
THE SACCHARINIC ACIDS
COzI[
O
H
O
H
CHa OH CHzOH
,CHIOH ,,CHZOH HCOH
C
I'
COzH
HOCH
C
CHz
CHOH
CHOH
\
- COZ
CHz
I CHO H I
CHZOH
I (Enantiomorph)
CHz
I
CHOH
I
COzH
CH~
1 I CHOH CHOH I I
COzH
CHzOH
I
CHOH
I
CO,H
2,4I1 2,3Dihydroxyglutaric (Enantiomorph) Dihydroxyglutaric acids (one meso, one acid enantiomorph) (enantiomorph)
of this acid. Meanwhile, had also established the presence of a hydroxyl group on the tertiary carbon atom of “a”-D-isosaccharinic acid, by degrading the latter with hydrogen peroxide and ferric acetate to a deoxypentose (CbH1004). Thus, the structure of “a))-D-iSosaccharinic acid is established beyond question as that of a 3-deoxy-2-C-(hydroxymethyl)pentonic acid. c. Configuration.-The D configuration may be assigned to C4, the penultimate secondary carbon atom of “a))-D-isosaccharink acid, from several considerations. The currently accepted mechanism for the formation of this acid from the (1 -+4)-linked disaccharides (see Section 111) involves no change in configuration at C4 (C5 of the original D-glucose moiety of the disaccharide). Moreover, the configuration of this carbon atom has been experimentally related to that of C4 of the 3-deoxy-~-pentonicacids (see page 42). Finally, the positive optical rotation of “a’)-D-isosaccharinic lactone, presumably a gamma lactone, assigns the D configuration for C4 on the basis of the lactone rule.’’ The configuration of C2, the tertiary carbon atom, of “a))-D-isosaccharinic acid has not been established. Unfortunately, application of qualitative rules of configuration based on optical rotation affords disagreeing conclusions in this instance. The positive optical rotation of the phenylhydrazide would indicate the D configuration for C2 on the basis of the phenylhydraBide rule.’s On the other hand, the reported negative optical rotation of the acid amide44would assign the L configuration to this carbon at,om on the (43) 0.Ruff, Ber., 36, 2360 (1902). (44) R. A. Weerman, Rec. trav. chim., 37, 16 (1917).
52
JOHN C. SOWDEN
basis of the amide The amide is, however, reported to be unstable, and assignment of configuration on the basis of the available data for this compound may be unreliable. The anilide, in contrast to the amide, shows a positive optical rotation. d. Miscellaneous Reactions.-The reduction of “a”-D-isosaccharink lactone with sodium amalgam gives a sirupy product from which a crystalline p-nitrophenylhydrazone of the branched-chain sugar, 3-deoxy-2-C-(hydroxymethy1)-(D-erythroor D-threo)-aldopentose, can be ~btained.~” Acetylation of the sirupy sugar yields a mixture of the crystalline, anomeric triacetates; the tertiary hydroxyl group is presumably inert toward acetylation, as is the similar tertiary hydroxyl group of methyl hamameloside.47 The Ruff d e g r a d a t i ~ nof~ ~L‘a”-D-isosaccharinicacid to a deoxypentose has been mentioned above. It is interesting that the soluble, lead salt of the acid was used for the degradation instead of the more usual calcium salt which, in this instance, is only very slightly soluble in water. Although Ruff was able to obtain the crystalline benzylphenylhydrazone of the pentose, the yield was so low that cleavage of this hydrazone to the pure sugar could not be studied. The reaction invites repetition and improvement in view of the rare nature of the product, a 3-deoxy-2-pentulose (3-deoxy-~-glyceropentulose). e. “P”-D-Isosaccharin~cAcid.-During the recrystallization (from ethanol) of the brucine salt of 2,4-dihydroxybutryic acid, obtained from the hexose - alkali reaction, Nef4*always observed the presence of a small amount of a less-soluble brucine salt. He concluded that this latter product was a mixture of the brucine salts of “a”-D-isosaccharhic acid and its epimer. After fractional recrystallization of the mixed salts (22 g.) to remove brucine “a”-D-isosaccharinate, he isolated a minor amount (0.6 g.) of a sirupy lactone that still, however, contained about 10% of “cY”-Disosaccharinic lactone. The principal constituent of the lactone mixture yielded brucine, quinine, and calcium salts, as well as a phenylhydrazide (no optical rotation for which was given), all of which were quite different in properties from the corresponding derivatives of “a”-D-isosacch&rinic acid. The various derivatives of the sirupy lactone were, however, similar to those of the corresponding derivatives of 3-deoxy-~-erythro-pentonic acid (from D-xylose plus alkali). Accordingly, Nef concluded that he must have in hand a 3-deoxy-2-C-(hydroxymethyl)pentonic acid, the epimer of ( I 1) a -D-isosaccharinic acid. (45) C. S. Hudson, J . Am. Chem. Soc., 40, 813 (1918). (46) P. Schorigin and N. N. Makarowa-Semljanskaja, Ber., 66,387 (1933). (47) 0. T . Schmidt, Ann., 476,250 (1929); see F. Shafizadeh, Advances i n Carbohydrate Chem., 11, 270 (1956). (48) Ref. 12, pp. 56-58 and pp. 64-65.
53
THE SACCHARINIC ACIDS
In support of his contention that “/3”-D-isosaccharinic acid is present in the hexose-alkali reaction mixture, Nef also cited certain observations of Kiliani arid Ei~enlohr,4~* 6o who oxidized (with nitric acid) the residue obtained, after substantial removal of LLa”-D-isosaccharinic acid and the metasaccharinic acids, from the lactose-alkali reaction mixture. Among the products identified was the tribasic acid, (H02C)zC(OH)-CH2-CHOHC0211, previously obtained by a similar oxidation of “a”-D-isosaccharinic acid (see page 50). Nef concluded that the tribasic acid must in this instance have arisen from “/3”-D-isosaccharinic acid. This conclusion ignores, however, the experimental demonstration by Kiliani and Eisenlohr60 that the residue subjected to oxidation had still contained a small proportion of ‘(a”-D-isosaccharinicacid, isolable as the slightly soluble calcium salt. The best evidence for the formation of L‘/3J1-D-isosaccharinic acid in the sugar-alkali reaction is the recent observation61that treatment of lactose, maltose, or 4-O-methyl-~-glucosewith lime-water at room temperature provides initially a mixture of saccharinic acids consisting almost exclusively of “a”-D-isosaccharinic acid plus an acid with the properties of Nef’s “/3’1-D-isosaccharinicacid [brucine salt, m. p. 185 to 210’ (dec.), [a]: -20 to -22O; lactone, [a]%+6 to +8.5O]. An experimental proof that this substance possesses the isosaccharinic acid structure would provide the necessary evidence that it is, indeed, the epimer of “a”-D-isosaccharinic acid.
7. The D-Galactometasaccharinic Acids COZH
I
HCOH
I
COzH
I I
HOCH
CHz
CHz
HOCH
HOCH
I
I I
HCOH CHzOH 3-Deoxy-~-xy~o-hexonic acid (“d-D-galactometasaccharinic acid)
I
I I
HCOH CHeOH 3-Deoxy-~-lyxo-hexonicacid (“8”-D-galactometasaccharinic acid)
3-Deoxy-~-x&1-hexonicacid (“a”-D-galactometasaccharinic acid) was first detected as a product of the prolonged action of lime-water on lactose (49) H. Kiliani, Ber., 41, 2650 (1908). (50) H. Kiliani and F. Eisenlohr, Ber., 42, 2603 (1909). (51) W.M.Corbett and J. Kenner, J . Chem. Sac., 2245 (1953); 1789 (1954); J. Kenner and G. N. Richards, ibid., 1810 (1955).
54
JOHN C. SOWDEN
at room temperature. After having removed the very slightly soluble calcium L‘a’’-D-isosaccharinatefrom one of these reaction mixtures, IGliani52 noted the slow deposition of a second calcium salt. This latter material could be recrystallized from hot water; it yielded, after removal of the calcium, a crystalline lactone with the familiar formula, CsH,,05 , of a sixcarbon saccharin. It was recognized later that the initial action of liinewater on lactose yields ‘L~ll-D-isosaccharinic acid and D-galactose, with ensuing conversion of the hexose to the epimeric D-galactometasaccharinic acids. 3-Deoxy-~-lyxo-hexonic acid (“p”-D-galactometasaccharinic acid) was discovered by Kiliani and Sandas3as a minor product of the D-galactose-alkali reaction. Kiliani believed that this product was a new type of branchedchain saccharinic acid, and referred to it throughout subsequent publications as L‘parasaccharinic”acid. The evidence, provided both by his own work and that of Nef, that Kiliani’s “parasaccharinic” acid contained, in fact, the epimer of “rY1’-D-galaetometasaccharinicacid, is outlined on page 56. a. Preparation.-The epimeric D-galactometasaccharinic acids are produced concurrently, in yields of 15 to 20 %, by the action either of hot, concentrated sodium hydroxide12 or of lime-water a t room temperature 011 D-galactose. The xylo epimer apparently predominates in the mixture; it can be readily isolated in pure form through its slightly soluble calcium salt. The lyxo epimer is, however, extremely difficult to purify by recrystallization because of its tendency to form mixed salts with those of the xylo epimer. Kiliani and Sandals directions63 for the preparation of “cr1’-D-ga1aCtOmetasaccharinic acid follow. A solution of one part of D-galactose in ten parts of water is treated with half a part of freshly prepared calcium hydroxide. The mixture is maintained a t room temperature in a stoppered flask for 4 weeks, with initial frequent shaking. The resulting voluminous precipitate is removed by filtration and the filtrate is heated t o boiling, while being maintained a t constant volume, for 3 hours. The new precipitate (of basic calcium salts) is then removed and the filtrate is saturated with carbon dioxide. The solution is again heated, filtered, and concentrated t o about twice the weight of the original D-galactose. After seeding with calcium “a”-D-galactometasaccharinate, if seeding crystals are available, the crystallization of this salt is completed by storing in the cold for about 10 days. The yield is about 14% of the weight of sugar used initially.
From the mother liquors of preparations similar t o the above, Kiliani and coworkerss3,s 4 , 5s isolated a crystalline barium salt of the mixed, (52) (53) (54) (55)
H. Kiliani, Ber., 16, 2625 (1883). H. Kiliani and H. Sanda, Ber., 26, 1649 (1893). H. Kiliani and P. Loeffler, Ber., 37, 1196 (1904). H. Kiliani and H. Naegell, Ber., 36, 3528 (1902).
55
THE SACCHARINIC ACIDS
epimoric D-galactometasaccharinic acids. Their preparations of the lyxo epimer (“parasaccharin”) were obtained from this mixed salt by conversion to the mixed lactones and removal, through crystallization, of the xylo epimer. In some instanceslb4~ 66 they briefly record the crystallization of the lyxo epimer (“@”-D-galactometasaccharin). Nef b7 also isolated the crystalline ‘(@” epimer from the D-galactose-sodium hydroxide reaction. His directions include “protracted” fractional recrystallization of crude brucine salts, followed by successive fractional recrystallizations of the strychnine and barium salts. It appears certain that chromatographic and ion-exchange methods, not known to Kiliani and Nef, could be used to advantage in future preparations of “@”-D-galactometasaccharinicacid. b. Structure.-(1) “a”-D-GalactometaSacchar~n~c Acid.-Kilianib8 observed that reduction of “a”-D-galactometasaccharink? acid with hydriodic acid and red phosphorus, under reflux at atmospheric pressure, yielded n-hexanoic 1,li-lactone. Further reduction, at higher temperature in a sealed tube, gave a low yield of n-hexanoic acid. Thus a straight-chain structure, with a hydroxyl group gamma to the carboxyl group, was established for the
o=c-
AI H T I AI H T I
n-Hexanoic 1,4-1actone
COeH
COzH
CHz CHZ I
CHOH
n-Hexanoic acid
“a’l-D-Galactometasaccharinic acid
I
1 I
I
IHNOa
C OaH
COzH
I
I
cHoH
I
CHz CHOH
- I
I I
(56) H. Kiliani, Ber., 44, 109 (1911). (57) Ref. 12, pp. 62-66 and 76-77. (58) H. Kiliani, Ber., 18, 642 (1885).
CHz CHI
I I
CHOH
CHz
COzH
COzH
Trihydroxyadipic acid
Adipic acid
56
JOHN C. SOWDEN
metasaccharinic acid. Oxidation of the latter with nitric 58 led to a crystalline trihydroxyadipic acid which, on reduction with hydriodic acid and red phosphorus, was converted to the known, crystalline adipic acid. These latter observations established a non-terminal position for the deoxy function in “a”-D-galactometasaccharinic acid. A Ruff degradation, with hydrogen peroxide and ferric acetate, of the calciumss or barium60 salts of “a”-D-gdactometasaccharinic acid provided a crystalline deoxypentose (C5HI004, Limetasaccharopentose”)which failed t o give an osazone on treatment with phenylhydrazine. Oxidation of the deoxypentose with bromine yielded a trihydroxyvaleric acid which, upon lactonization and then reduction with hydriodic acid and red phosphorus,60 gave a n-valero-1 ,$-lactone. The silver salt of the corresponding acid was found to be crystallographically identical with the known silver 4-hydroxyn-valerate. COzH
I
CHOH
1
CHz
I I CHOH I CHOH
CHzOH I t (z
-D-
H2 0
2
-F e w
Galactometasaccharinic acid
CHO
I CHz I
CHOH
I I
o=c-
COzH
I
--Brz -+
CHOH CHzOH
2-Deoxypentose
I I CHOH I
CHOH
CHzOH
Trihydroxyvaleric acid
I I CH2 I CHOI CH2
CH2
HI
CHa
n-Valero-l,4lactone
Considered together, the above observations provided evidence for the presence of hydroxyl groups on C2, C4, C5, and C6 of “a”-D-galactometasaccharinic acid, and, hence, for its formulation as a 3-deoxyhexonic acid. (2) “p”-D-GuZactometusucchur~n~c Acid.-Kiliani and Sanda53 reduced their “parasaccharinic acid” in the usual manner with hydriodic acid and red phosphorus. The product was a hexanoic lactone whose boiling point (217.5’) was precisely intermediate between that (220’) of the n-hexanoic 1,4-1actoneobtained by a similar reduction of “a”-D-galactornetasaccharinic acid and that (215”) reported6I for 2-ethylbutyro-l , 4-lactone. Kiliani, however, chose the latter structure for his lactone, since the corresponding acid, acid) and unlike ejther the enantiomorlike ~~-(2-ethyl-4-hydroxybutyric (59) H. Kiliani, Ber., 18, 1555 (1885). (60) H. Kiliani and P. Loeffler, Ber., 38, 2667 (1905). (61) M. B. Chanlaroff, Ann., 226,340 (1884).
57
THE SACCEIARINIC ACIDS
phous62or racemic 4-hydroxy-n-hexanoic gave a readily crystalline barium salt. On the basis of this identification, “parasaccharinic acid’’ was assigned one of the three structures I, 11, or 111. CHa
CHzOH
CH2 OH
CHOH
CHOH
CH2
I
I
I
I
CHOH
I
CHzOH I
I
CHOH
I
CHZOH I1
I
CH2 OH I11 (“Parasaccharinic acid”)
Structure I was quickly ruled out by the observation that the saccharinic acid contains no methyl group, since it gives no acetic acid on oAdation with silver oxide. Moreover, a Ruff degradationK4of the “parasaccharinic acid” gave a crystalline “parasaccharopentose” (CsHlo04) and, hence, it was concluded that structure 111 shows the correct disposition of the functional groups. As a further observation in support of the branchedchain structure, Kiliani and LoeffleP4 reported that oxidation of the saccharinic acid by nitric acid yields a tribasic acid (presumably a hydroxycitric acid) accompanied by the crystalline monolactone (CeHaOs) of a dibasic acid. Evidence that “parasaccharinic acid” probably contains “p”-D-galactometasaccharinic acid (or its unremoved “a” epimer) was soon forthcoming from Kiliani’s own laboratory. Crystallographic comparison of “parasaccharopentose” with “metasaccharopentose,” of their respective crystalline oximes, and of the phenylhydrazides of their derived deoxypentonic acids, showed the two sugars to be identical.6STo bring this observation into conformity with his proposed branched-chain structure for “parasaccharinic acid,” Kiliani suggested that the latter gives the expected 2-deoxy-3pentulose in the Ruff degradation but that this deoxypentulose structure is unstable and rearranges spontaneously to the 2-deoxypentose, “metasaccharopentose” (IV + V). (62) H. Kiliani and S. Kleemann, Ber., 17,1296 (1884). (63) R. Fittig and E. Hjelt, Ann., 208, 67 (1881). (64) H. Kiliani and P. Loeffler, Ber., 37, 3612 (1904). (65) H. Kiliani and A. Sautermeister, Bey., 40, 4294 (1907); H. Kiliani, ibid., 41, 120 (1908).
58
JOHN C. SOWDEN
CHO
CH~OH’
CHOH
I
CHzOH “Parasaccharinic acid”
I CHz I c=o I CHOH I
CH2OH.
I I CHOH I CHOH I CHz
+
CHzOH
v IV Further serious doubt was cast on the branched-chain structure by NefG6 when he observed that “cr”-D-galactometasaccharink acid can be readily isomerized into a product that very closely resembles Kiliani’s “parasaccharinic acid.” On heating the pure “a” epimer in a sealed tube at 200°, with or without pyridine, it was partly converted to “P”-D-galactometasaccharinic acid, whose brucine salt showed properties in excellent agreement with th‘ose of Kiliani’s “brucine parasaccharinate.” The isomerization product was further characterized, through its strychnine salt and phenylhydrazide, as Nef’s “/3”-D-galactometasaccharinic acid, obtainable directly by the action of alkali on D-galactose. At this stage, the possible presence of a branched-chain saccharinic acid in Kiliani’s preparation was supported only by (a) the properties of the barium salt of the hydroxyhexanoic acid obtained from it on reduction and (b) the reported oxidation of “parasaccharinic acid” with nitric acid t o a tribasic acid. The latter evidence was retracted by Kiliani in his final report on the matter,66when he stated that the previous identification of “hydroxycitric acid” was in error and that this “tribasic acid” is, in fact, (-)-tartaric acid. In addition, he now observed that oxidation of “parasaccharinic acid” with nitric acid, followed by reduction with hydriodic acid and red phosphorus, gives a low yield of adipic acid. The hypothesis of the existence of the branched-chain “parasaccharinic acid” now depended solely on the identity of the reduction product, hydroxyhexanoic acid. To support his previous contention that the barium salt of this acid is, indeed, barium 2-ethyl-4-hydroxybutyrate, Kiliani66 also prepared the calcium salt and found that it, too, closely resembled the corresponding salt of 2-ethyl-4-hydroxybutyric acid. As emphasized by Kiliani, neither the reductions with hydriodic acid and red phosphorus nor the oxidations with nitric acid proceed in good yield to single products. Accordingly, Kiliani remained firm in his conviction that his preparation, although it was apparently a mixture, nevertheless contained the branched(66) Ref. 12, pp. 78-82.
59
THE SACCHARINIC ACIDS
chain “parasaccharinic acid.” The accumulated evidence points overwhelmingly to the presence therein of “/3”-D-galactometasaccharinicacid. Whether or not Kiliani’s “parasaccharinic acid” is also formed in the hexose-alkali reaction is a matter requiring further study. c. Configuration.-The accepted theory of the mechanism of formation of metasaccharinic acids predicts, as mentioned previously, that no change in the configuration of the starting sugar will occur a t carbon atoms below C3. Accordingly, the galactometasaccharinic acids should have the D-threo configuration a t C4 and C5. Thus, “metasaccharopentose,” obtainable from either of the galactometasaccharinic acids by the Ruff degradation, should be 2-deoxy-~-threo-pentose.This sugar, prepared by the glycal rneth0d,~7 shows properties (m. p., and m. p. of the benzylphenylhydrazone) in close agreement with those of “metasaccharopentose.” The product from the glycal synthesis showed a final optical rotation which was slightly negative -2’ (in water)], whereas “metasaccharopentose” was reported64* e6 to be optically inactive. Hence, no decision concerning the D or L classification of the latter is available from these data. The galactometasaccharins are, however, gamma lactones, and both show negative optical rotations, thus permitting assignment of the L configuration to C4 of both on the basis of the lactone rule.’? Thus, “metasaccharopentose” must have the D-threo configuration, and the epimeric galactometasaccharinic acids are the 3-deoxy-~-xylo-and -D-lyxo-hexonic acids. Finally, on the basis of the phenylhydrazide rule,’* the “a” epimer is the 3-deoxy-~-xylo-hexonicacid. It is interesting that Nef assigned the correct configurations to the D-galactometasaccharinic acids, as well as t o the D-glucometasaccharinic acids, on the basis of analogies between the optical rotations of D-tartaric acid, the 2,4-dihydroxyglutaric acids (obtained by oxidation of the fivecarbon metasaccharinic acids), and the 2,3,5-trihydroxyadipic acids (obtained by oxidation of the six-carbon metasaccharinic acids).
8. The D-Glucometasaccharinic Acids COzH
I HCOH I
COzH
I I
HOCH
CH2
CHz
HCOH
HCOH
HCOH
HCOH
I
I
I
CHzOH 3-Deoxy-~-ribo-hexonicacid (“a”-~-glucometasaccharinicacid)
I I
CHzOII 3-~)eoxy-~-arabino-hexonic acid (“B”-n-glucometasaccharinic acid)
(67) P. A. Levene and T. Mori, J . Biol. Chem., 83, 813 (1929).
60
JOHN C. SOWDEN
The epimeric D-glucometasaccharinic acids were first isolated by NefL2 from the interaction of D-glucose and hot, concentrated sodium hydroxide. D-Glucose is isomerized and smoothly degraded under these conditions to a mixture of saccharinic acids, in a yield of over 80 %. a. Preparation.-From the isomerization of 100 g. of D-glucose with hot 8 N sodium hydroxide, Nef reported as products, after careful fractionation, 40 to 45 g. of m-lactic acid, 10 to 15 g. of ~~-(2-hydroxybutyro-l,4lactone), 20 g. of the epimeric D-glucometasaccharinic lactones, and 2 g. of the epimeric D-isosaccharinic lactones. The six-carbon lactones, consisting almost entirely of the D-glueometasaccharhie lactones, were separated with relative ease from the products of lower molecular weight. Accordingly, D-glucose is an attractive source for these metasaccharins. The “/3” epimer (3-deoxy-~-arabino-hexonic acid) is readily isolable in pure form through its calcium salt, which is sparingly soluble in cold water. The “a” epimer (3-deoxy-~-ribo-hexonicacid), however, is relatively difficult to separate from the mixture. Here again, it is probable that chromatographic or ion-exchange methods may serve to good advantage. A recently developed methodE8for preparing the epimeric D-glucometasaccharinic acids is based on the action of lime-water on the seaweed polysaccharide, l a m i n a r i ~ The ~ . ~ ~mixed D-glucometasaccharinic acids are obtainable from this source in practically pure condition, as their calcium salts, after separation from unchanged polysaccharide. The directions for their preparation from “insoluble” laminarin follow. “Insoluble” laminarin (50 g.) is treated with an oxygen-free suspension of calcium hydroxide (509.) in 1 liter of water. After 8 days a t room temperature, the suspension is filtered, and calcium is precipitated by t h e addition of the equivalent amount of oxalic acid. Concentration of the filtrate to a volume of 500 ml. causes precipitation of polysaccharide (21.4 g.). After filtration, and concentration t o a sirup, extraction with ethanol (3 X 100 ml.) leaves further polysaccharide (7.7 g.). Evaporation of the ethanol extract affords a mixture of the sirupy D-glucometasaccharinie lactones (13.8 g.). After their conversion t o t h e calcium salts, and crystallization from water (finally with the gradual addition of ethanol), there is obtained calcium “j3”-D-glucometasaccharinate (5.8 g.), calcium “a”-D-ghcometasaccharinate (0.6 g.), and a residue of the mixed salts (3.7 g., principally “01” epimer). Partial, acid hydrolysis of the recovered polysaccharide, followed by retreatment with lime-water, yields an additional amount (11 g.) of the mixed calcium salts.
Similar treatment of “soluble” laminarin with lime-water, but at 100” for 3 hours, gives approximately the same yield of the calcium D-glucometasaccharinates. (68) W.M.Corbett and J. Kenner, J . Chem. SOC.,1431 (1955). (69) V. C. Barry, Sci. Proc. Roy. Dublin SOC., 21, 615 (1938);22, 59 (1939).
61
THE SACCBARINIC ACIDS
b. Structure.-Nef12 established the structural similarity of the D-glucometasaccharinic acids to the D-galactometasaccharinic acids by oxidizing the former to the two corresponding 2,3,5-trihydroxyadipic acids and converting these individually, by dehydration, to the lactone of 3-hydroxymuconic acid. The latter was also obtained, in a precisely similar fashion, from “a”-D-galactometasaccharinic acid. Thus, since Kiliani had previously established the structure of the galactometasaccharinic acid, Nef concluded that the D-glucometasaccharinic acids are also 3-deoxyhexonic acids. C OzH
I CHOH I CHZ
I I HCOH
COzH
I II
CHOH
CH
I
HNOa
A
HCOH
I
O=C-
I
CHzOH o-Glucometasaccharinic acids
CHz
I HCOH I HCOH I
AcaO
TiF
CO,H 2,3,5-Trihydroxyadipic acids
CH
I II
COCH
I
CO2H 3-Hydroxymuconic lactone
c. Configuration.-On the assumption that the D-erythro configuration had been retained at C4 and C5 in the conversion of D-glucose to the D-glucometasaccharinic acids, Nef assigned the D-rib0 and D-arabino configurations to the latter. Moreover, on the basis of analogies between their optical rotations and those of D-tartaric acid and the five-carbon metasaccharinic acids, he concluded that the “a” epimer is 3-deoxy-~-ribo-hexonicacid and acid. the ((P” epimer is 3-deoxy-~-arabino-hexonic The correctness of Nef’s reasoning has been fully borne out by subsequent observations. Ruff degradation of the D-glucometasaccharinic acids, gives the known 2-deoxy-~-erythro-pentose either mixedloor indi~idually,~~ (“2-deoxy-D-ribose”) . The formulation of the “P” epimer as 3-deoxy-~arabino-hexonic acid is, in view of the negative optical rotation of its phenylhydrazide, in accord with the configurational prediction for C2 by the phenylhydraeide rule.18Finally, the identity of “P”-D-glucometasaccharinic lactone with 3-deoxy-~-arabino-hexonolactone (prepared from authentic by hydrolysis and subsequent oxidamethyl 3-deoxy-~-arabino-hexoside’~ tion) has been e~tablished.~’ (70) J. C. Sowden, J . Am. Chem. Soc., 76, 3541 (1954). (71) G. N. Richards, J . Chem. Soe., 3638 (1954). (73) H. R. Bolliger and D. A. Prins, Helu. Chim. Acta, 29, 1061 (1946).
62
JOHN C. SOWDEN
d. Degradation.-In view of the great biochemical interest in 2-deoxy-~ribose and the many attempts to develop a satisfactory synthesis for this sugar,73it is surprising that the degradation of the D-glucometasaccharinic acids has been investigated only recently.70* 71 The preparation (from D-g1UCOSe by Nef’s method) of the mixed metasaccharinic acids in a state of sufficient purity for the Ruff degradation is readily achieved. The degradation of the calcium metasaccharinates proceeds normally, and the resulting deoxypentose may be isolated as its “anilide” without difficulty. In laboratory scale preparations,70200 g. of D-glucose yields approximately 20 g. of 2-deoxy-N-phenyl-~-ribosylamine. The free, crystalline 2-deoxy-Dribose is obtained from the “anilide” in almost quantitative yield by cleavage with benzaldehyde.
111. MECHANISM OF FORMATION OF SACCHARINIC ACIDS 1. The Fragment-recombination Mechanism of Kiliani and Windaus
Although Kiliani supplied a preponderant amount of the experimental data concerning the preparation and proofs of structure of the saccharinic acids, he theorized but little on the mechanism of their formation. In a footnote74to one of his early articles, he pointed out that glycerose had been reported16 to be one of the products of the action of alkali on D-glUCOSe, and he suggested that glycerose might afford D-glucosaccharinic acid through condensation with the lactic acid also present in the isomerization mixture. HCHO HOz C
\ /
+
CH3
KO2 C
HCOH
+
CHO
I I
CHOH CHzOH
+
CH3
\ / COH I CHOH I CHOH I
CHpOH
n-Glucosaccharinic acid
/
COzH
CHOH
I I CHOH I
--t
7 O Z H COH
I I CHOH I
CHz
CHI
CHpOH
CHzOH
D-Isosacc harinic acid
This idea was expanded by Windaus,7s who suggested that not only (73) See W. G. Overend and M. Stacey, Advances in Carbohydrate Chem., 8 , 45 (1953). (74) Ref. 02, p. 1302. (75) M. Nencki and N. Sieber, J . prakt. Chem., [a] 26, 1 (1882). (76) A. Windaus, Chem. Ztg., 29, 564 (1905).
63
THE SACCHARINIC ACIDS
D-glucosaccharinic acid but also D-isosaccharinic acid and Kiliani’s parasaccharinic acid might be formed by recondensation of appropriate aldehydic fragmentation products with a lower-carbon metasaccharinic acid. He proposed that the unbranched metasaccharinic acids, in contrast, are formed by direct dismutation of the isomeric sugars. CHzOH
I
CHO
CHO
+
/
COzH
CHOH
I CH2 I
CHZOH
CHzOH
I
~
CHOH COzH
\ / COH I CH2 I
CHzOH
I CHOH I CHOH I CHOH I CHOH I
COiH
I I . direct. , CH2 dismutation 1
CHzOH
Kiliani’s Parasaccharinic acid
CHOH
CHOH
I I
CHOH CHzOH Metasaccharinic acid
Recently, C14-labeling experiments, discussed in Section 111, 5 have confirmed that fragment recombination is not involved to any significant extent in the conversion of a sugar to the related metasaccharinic acids. Also confirmed by the C14-labelingdata is the fact that fragment recombination is an important feature of the formation of the branched D-glucosaccharinic acid from an unsubstituted D-hexose. However, the specific fragments suggested by Kiliani, and the direct condensation to the final product, D-glUcosaccharinic acid, now seem improbable. 2. The Isomerixation Mechanism of Nef
Nef’s theory of the mechanism of formation of the saccharinic acids is outlined, in its original form, in a paper published in 1907” and, in its final form, in his comprehensive article of 1910.l2The theory proposes that the reaction takes place in two major steps: (a) the isomerization of the sugar, with loss of water, to an a-dicarbonyl compound, and (b) a benzilic acid type of rearrangement of the latter, with hydration, to the saccharinic acid. The second step involves chain rearrangement in the production of the saccharinic, isosaccharinic, and Kiliani’s parasaccharinic acids, but not in the production of the metasaccharinic acids. (77) J. U. Nef, Ann., 367, 214 (1907).
64
JOHN C. SOWDEN
HCO
HCO
HCO
:0
I CHONa + I
7"""
-HC,
CHOH
I
I Aldose
HCO
I
-'&=O
I
CH2 I
Ho"
I11
I1
-
IV
COzH
I I CH2 I '
CHOH
Metasaccharinic acid
According to the theory, the initial reaction is the formation of an alkoxide (I) between the base and the sugar hydroxyl group vicinal to the .carbony1 group. A molecule of base is then eliminated, to give the free, methylenic intermediate (11). The latter isomerizes to the epoxy compound (111) and thence to the a-dicarbonyl intermediate (IV). Finally, a benzilic acid type of rearrangement, with hydration and dismutation, gives the saccharinic acid. CH2OH
CH20H
'i=" CHONa-
c=o c,
I
I CHOH
I
I CHONa I
c=o I
I 3-Ketose
--+
&OH
I
I 2-Ketose CH2OH
CH2OH
I1
CHzOH I
:7
+
c=o I
CHzOH (C=O I
c=o I
1
I I/ o HC I
HC,
d
- 7'"
I1
l c=o I I
I
CH2
CH2
I11
C+ I 0 HC'
CO2H
I
Isosaccharinic acid
IV
COsH
c=o I IV
I11
Saccharinic acid
The successive isomerization of an aldose to a 2-ketose and then to a 3-ketose was explained by assuming the intermediate formation of 1,2- and 2,3-enediols.?sThus, a single aldose, under the influence of alkali, could produce all three types of saccharinic acid. CHO
I I
CHOH CHOH
I
Aldose
CHOH
CHzOH
CHzOH
CHZOH
COH
C=O
COH
CHOH
II I CHOH I
1,a-Enediol
I I
CHOH
I
2-Ketose
.--)
I II
COH
I
2,3-Enediol
I
7 c=o I
3-Ketose
(78) The possibility of the presence of enediolic forms in alkaline solutions of the sugars had been discussed previously by E. Fischer, Ber., 28, 1145 (1895) and by A. Wohl and C. Neuberg, ibid., 33,3095 (1900).
85
THE SACCBARlNfC ACIDS
To account for the formation of saccharinic acids of carbon content lower than that of the original sugar, it was proposed that the enediols are subject to cleavage at the double bond, to produce lower-carbon sugars which could then also undergo the saccharinic acid rearrangement. Thus, according t o Nef, a molecule of a hexose 3,4-enediol, after cleavage to two molecules of glycerose, could provide two molecules of lactic acid (3-deoxyglyceronic acid). It is now considered more probable79that cleavage of the sugar chain CHzOH
I I
CHOH COH
II
COH
I CHOH I
CHO -+
I
2 CHOH
I
CHzOH
CHzOH
Hexose 3,4-enediol
Glycerose
COzH -+
I I
2 CHOH
CHa Lactic acid
under the influence of alkali takes place by a reverse aldolization (V VI) a t the carbon-carbon single bond situated a,@ to the double bond of the enediol. This mechanism involves a 1,2-enediol, instead of a 3,4-enediol, in the cleavage of a hexose t o two triose fragments. An equally plausible mechanism (VII + VI), that utilizes the 2-ketose as the immediate precursor of the triose fragments,sOwould predict the more rapid cleavage of ketoses than of aldoses. .--)
+GHOH COH
HCVH I CHOH I
CHzOH
V
CHzOH C-OH I
II
CHzOH
JG
CHOH CHO
I
CHOH
I
CHzOH VI
HCOH I CHOH
I
CHzOH VII
I n the original formulation of his theory,77Nef chose the hydroxyl group
p to the carbonyl group as the site of alkoxide formation with the base,
in the initial step of the saccharinic acid rearrangement. This was later amended12 to the formulation shown above, in order to accommodate the (79) 0. Schmidt, Chern. Revs., 17, 137 (1935). (80) H. S. Isbell, private communication.
66
JOHN C. SOWDEN
observation that, in the presence of air or other oxidants, the action of alkali on the sugars leads to aldonic acids instead of to saccharinic acids. Nef’s mechanism for the aldonic acid formation is shown in VIII + IX. HCO
I CHOH I CHOH I
HCO +
I CHONa I
HCO +
CHOH
I
,1 C, I
HCO &‘&=O
CHOH
I
I
COzH
I
CHOH
-1
I
VIII Aldose
CHOH CHOH
I
IX
Aldonic acid
The general statement of the isomerization mechanism, as given in the opening paragraph of this Section, is accepted at the present time as a mechanism of saccharinic acid formation. However, Nef’s concept of the mode of isomerization of the original sugar to the intermediate a-dicarbonyl compound has undergone radical revision.
3. The Ionic Mechanism of Isbell The final phase of the Nef mechanism, which involves a benzilic acid type of rearrangement of a-dicarbonyl intermediates to the saccharinic acids, is at present accepted as a feature of saccharinic acid formation. Nef’s concept of the conversion of reducing sugars to the a-dicarbonyl structures required revision, however, when it became evident that the formation, in this step, of the proposed methylenic intermediates is highly improbable. A departure from the methylenic intermediates was suggested in 1926 by Evans and Benoy,s’ who proposed that the a-dicarbonyl intermediates of the Nef mechanism might arise by successive dehydration and rehydration from the enediols. It is now recognized, however, that formaCHOH
II C-OH I CHOH I
1,a-Enediol
- HzO A
11;’
HC
C
I I
CHOH
+ &O
- HzO
CHI
I I c=o I
c=o
a-Dicarbonyl intermediate
tion of the unsaturated oxide structures (pictured as resulting from the initial (81) W. L. Evans and Marjorie P. Benoy, cited in W. L. Evans, Rachel H. Edgar and G. P. Hoff, J . Am. Chem. Soc., 48,2665 (1926).
67
THE SACCHARINIC ACIDS
dehydration of the enediols) is also improbable. An acceptable course for the initial isomerization, based on consecutive electron-displacement reactions and in accord with the principal experimental facts of saccharinic acid formation, was eventually developed in 1944 by Isbell.@ As a prolog t o the Isbell ionic mechanism, Shaffer and Friedemann had concluded,a3after studying the kinetics of sugar activation by alkali, that saccharinic acids result from spontaneous rearrangement of the unstable, sugar anions that are formed in alkaline solution. They also pointed out that the sugars may behave in such solutions not only as monobasic but also as dibasic or polybasic acids, thus giving rise to unstable mono-, di-, or poly-valent anions as precursors of the saccharinic acids. An experimental demonstration that the Nef mechanism for the initial conversion of a sugar by alkali to the a-dicarbonyl structure is not acceptable was provided by a study of the action of alkali on 2-hydroxy-3-methoxy-3-phenylpropiophenone.Nicolet observeda4that the products in this case are 2,3-diphenyllactic acid and methanol. This conversion, which is completely analogous to the formation of a saccharinic acid from a reducing sugar, demonstrated clearly that carbon-oxygen cleavage occurs at the p-carbon atom, rather than a t the a-carbon atom, with respect to the carbonyl group. The Isbell ionic mechanism for the formation of the various types of saccharinic acid, as well a8 for Nicolet’s conversion of 2-hydroxy-3-methoxy3-phenylpropiophenone to 2,3-diphenyllactic acid, involves the following successive steps: (1) the formation and ionization of a n enediol; (2) the ,&elimination of a hydroxyl or an alkoxyl group; (3) rearrangement to a n a-dicarbonyl intermediate; and (4) a benzilic acid type of rearrangement to the saccharinic acid. H-C-Q@
H-C=O
I
H-C=O
.-
C-OIH 11-CI-OH +OH. I ------ CHOH
I
I
H-b
CHOH
I I
COiH
p,--
I
-
I
I-I-C-H CHOH
I
I I
CHOH II@OH@
- 1
CH2 CHOH
I I
CHOH
CHOH
CHOH
CHOH
CHzOH
CHzOH
CHzOH
CIIzOH
I
I
hletasnccharinic ncid (82) H. S. Isbell, J . Research Null. Bur. Standards, 32,45 (1944). (83) P. A . Shaffer and T. E. Friedemann, J . Biol. Chem., 86,345 (1930). (84) B. H. Nicolet, J . Am. Chem. Soc., 63,4458 (1931).
68
JOHN C. SOWDEN
__-__
H~c<- OH
@s,C-QQ I
-p. -i:::
HZC3
-
COzH
C-O!H III n,---
C=O
1
y=O CHOH
H@OH~
CHOH
CHOH
CHOH
CHOH
CHOH
CHOH
CHzOH
CHzOH
CHzOH
CHzOH
CHOH
I
I
I
I I
I
Saccharinic acid
2,3-Enediol
CHzOH
CHzOH
C-43"
c=o
I
H-C:-OH
I
.-----
YHoH CHzOH
2,3-Enediol
I I
1
I
-
COzH
HCH
I I
CHOH
CHOH
CHOH
CHzOH
CHzOH
CHzOH
I
I
Isosaccharinic acid
2-Hydroxy-3methoxy-3phenylpropiophenone
It is interesting L a t Isbell's extension of L e mechanism to the hexose 3'4-enediol series would explain the formation of Kiliani's parasaccharinic acid from this source. Indirect evidence is, however, available that the hexose 3,4-enediols are, at most, minor constituents of the hexose-alkali reaction mixtures. A 3,4-enediol (or the 3-hexulose with which it is in equilibrium) should give, by chain cleavage, formaldehyde and a pentose
69
THE SACCHARINIC ACIDS
Kiliani’s Parasaccharinic acid
1 ,a-enediol. The latter would be expected to afford, among other products, five-carbon metasaccharinic acids. Despite his painstaking analyses of the hexose-alkali reaction products, Nef was never able to obtain evidence for the presence therein of five-carbon structures. Recently, however, chromatographic evidence has been obtained86 for the production of S-deoxypentonic acids, in low yield, from D-glucose plus alkali. Thus, if hexose 3,4-enediol is the necessary precursor of Kiliani’s parasaccharinic acid, the latter must be formed to only a minor extent. 4. Saccharinic Acids from Substituted Sugars
It has long been known that D-glucose substituted a t C4, as in the disaccharides lactose, cellobiose, and maltose (see page 48), is particularly suitable for the preparation of ‘La’’-D-isosaccharinicacid through treatment with lime-water. Since this acid is only a minor product of the action of alkali on unsubstituted D-glucose, the presence of a substituent a t C4 must preferentially direct the alkaline degradation to the isosaccharinic structure. The role of substitution in determining the course of saccharinic acid formation has been critically examined recently by Kenner and coworkers. They conclude that an 0-glycosyl or 0-alkyl anion is more readily extruded from the sugar enediol anion of the Isbell mechanism than is a hydroxyl ion.86 I n addition, substitution a t certain positions in the sugar molecule may inhibit competing side-reactions. For example, a substituent a t C4 of the hexose molecule inhibits cleavage (by reverse aldolization) into two three-carbon fragments and the resultant formation of lactic acid:? a result that had been demonstrated earlier by the experiments of Evans and his A combination of the two above effects, then, preferentially (85) J. W. Green, J . Am. Chem. SOC.,78, 1894 (1956). (86) J. Kenner and G . N. Richards, J . Chem. SOC.,278 (1954). (87) J. Kenner and G. N. Richards, J . Chem. SOC., 1784 (1954). (87a) W. L. Evans and Marjorie P. Benoy, J . Am. Chem. SOC.,62, 294 (1930); W. L. Evans and R. C. Hockett, ibid., 63,4384 (1931).
70
JOHN C. SOWDEN
channels the reaction of the substituted sugars, to aff ord specific, saccharinic acid structures. On this basis, isosaccharinic acids are to be expected as the principal products from 4-0-substituted hexoses. This expectation has been experimentally confirmed in studies of the action of lime-water on 61 lact~ lo s e ,~ cellobiulose,68 ' maltu61 c e l l ~ b i o s e 68 ,~~~ 4-O-methyl-~-fructose~~~ and lose,61cellotetraose,684-0-methyl-~-glucose,~~ 4, G-O-benzylidene-~-glucose.~~ The 2 3-enediol precursor for the formation of isosaccharinic acid (by the Isbell scheme) is produced from the above ketoses by a single enolization, whereas the aldoses must proceed through reversible isomerizations, by way of the 1 ,2-enediols and ketoses, t o arrive a t the 2 3-enediol structure. Accordingly, the 4-0-substituted ketoses in the preceding list are all degraded somewhat more rapidly with lime-water than are the related aldoses. I n the case of 3-0-substituted hexoses, the 1,2-enediol anion is the reactive species because of the ready elimination of the 0-alkyl or 0-glycosyl residue. Hence, the metasaccharinic acid structures are the principal products. Experimental confirmation of this concept is seen in the action of lime-water on 3-O-methyl-~-glucose,~~ 3-O-methyl-~-fructose,~~ 3-0-(/3-~glucosy1)-D-glucose (laminaribiose),89 3-O-(a-~-glucosyl)-~-fructose (turanose),89 G-O-benzyl-3-O-methyl-~-glucose,~~ 4 6-0-benzylidene-3-0-methylD - ~ ~ u c oand s ~ ,2,3: ~ ~5 G-di-O-isopropylidene-D-mannose.88 In contrast to the behavior of 4-0-substituted hexoses, the 3-0-substituted aldoses are degraded with lime-water a t approximately the same rate as are the related ketoses. This is to be expected since, in this case, a single enolization of either (aldose or ketose) produces the 1,2-enediol precursor for metasaccharinic acid formation. With a 1-0-substituted 2-hexulose, metasaccharinic acid formation is blocked by the stability of the monosubstituted 1,2-enediol anion (I) t o alkali. However, the alternative 1-0-substituted 2 3-enediol anion (11) CHOR
II c-00 I CHOH I I
CHzOR
I II
C-OH C-00
I
I1
can proceed, through elimination of the OR anion at C l , to the saccharinic acid structure. Experimentally, 1-0-methyl-D-fructose provides a higher (88) W. M. Corbett, J. Kenner arid G . N. Richards, J . Chem. SOC.,1709 (1955). (89) W. M. Corbett and J. Kenner, J . Chem. Sac., 3274 (1954). (90) J. Kenner and G . N . Richards, J . Chem. Sac., 3277 (1954).
THE SACCHARINIC ACIDS
71
yield of ‘‘af’-D-g1ucosacchariiiicacid, when treated with lime-water, than does ~ - f r u c t o s e . ~ ~ As would be expected from the above considerations, substitution a t C6 of a hexose has, a t most, only a minor effect on the course of saccharink acid formation. Thus, 6-O-(a-~-galactosyl)-~-glucose (melibiose) plus lime-water gives lactic acid and a mixture of the corresponding metasaccharinic, isosaccharinic, and saccharinic acids.S7a, 91 The qualitative experiments with melibiose and with 6-O-methyl-~-glucose,based mainly on paper chromatography, suggest that the metasaccharinic acids may be the principal products from hexoses substituted at C6. I n their initial publications on the saccharinic acids, Kenner and his associates utilized the Isbell ionic mechanism, as given in the preceding Section, to interpret their results. S u b s e q ~ e n t l yhowever, ,~~ they have suggested that the enediol di-ion is the reactive species in saccharinic acid formation; this conclusion was reached when it was observed that related 3-0-substituted aldo- and keto-hexoses are degraded by lime-water a t approximately equal rates. This observation does not, however, necessitate any modification of the original Isbell scheme. As pointed out above, the comnion 1’2-enediol is reached from either the aldose or the ketose by a one-step process, and ionization at C1 of the enediol can then initiate the metasaccharinic acid rearrangement. More recently, Whistler and Corbettg2 have cited the moderate stability of 2-O-(~-xylopyranosyl)-L-arabinose t o alkali as further evidence for the necessity of di-ion formation in the saccharinic acid rearrangement. The failure of this substance t o form saccharinic acids under mildly alkaline conditions is adequately explained, however, by the following considerations. The elimination of the hydroxyl CHOe
CHO
COR
COR
II I
HOCH
I
I11
+
I I1 CH I IV
group from C3 of the mono-ion (111) is undoubtedly slow, as is the formation of saccharinic acids from unsubstituted aldoses a t room temperature with lime-water. Moreover, subsequent steps in the Isbell mechanism leading to saccharinic acids are effectively blocked a t structure IV owing to the inability of the glycosyl residue (R) a t C2 t o ionize. At higher tempera(91) W. M. Corbett and J. Kenner, J . Chem. SOC.,3281 (1954); J. Kenner and G. N. Richards, i b i d . , 2916 (1956). (92) R. L. Whistler and W. M. Corbett, J . Am. Chem. Soc., 77,3822 (1955).
72
JOHN C. SOWDEN
tures, the 2-0-substituted pentose is degraded by lime-water to acidic products, presumably because of chain cleavage. The preceding explanation of the failure of a 2-0-substituted aldose to form saccharinic acids (on treatment with alkali) finds substantiation in the results of a study of the action of lime-water, at room temperature, The presence, in this latter reaction mixon 2,3-di-O-methyl-~-glucose.8~~ ture, of an a,p-unsaturated aldehyde(IV, R = CH3) was established by its further, facile conversion to 5-(hydroxymethyl) -2-furaldehyde upon acidification. The fact that an enediol di-ion is not essential for the initiation of the saccharinic acid rearrangement is further amply demonstrated by the observations of Corbett, Kenner and Richards88 on 2 , 3 :5,6-di-O-isopropylidene-D-mannose. This compound is converted to a mixture of the 5 , 6 - 0 -
0-0
HC-Q’
( C H ~ ) ~ C+CH < 1~ . III V
-
COzH
CHO (CH)C-0-C
I
I1
CH
VI
I
1
7
CHOH 1
CH2
I
VII
isopropylidene-D-glucometasaccharinicacids (VII) on treatment with limewater at 100’. The authorss8depict the reaction as occurring through the intermediate VI. Obviously, the formation of an enediol di-ion prior to the anionic elimination at C3 is here ruled out, as it is in the case of 2,3-di-0methyl-D-glucose, and the reactive mono-ion (V) of the normal Isbell mechanism is involved. Thus, although enediol di-ions may well exist in alkaline solutions of reducing sugars, and may participate in rearrangements of the latter to saccharinic acids,83 the evidence so far advanced does not seem to necessitate any embellishment of the original mechanism advanced by Isbell. 5 . Fragment Recombination and Saccharinic Acid Formation
Aldol condensations involving glycerose (glyceraldehyde) and “glycerulose” (dihydroxyacetone) are known to provide ketohexoses of both straightchain and branched-chain structure.93Since these three-carbon sugars are known to be also formed through chain cleavage of hexoses by alkali,7sit is clear that the hexose-alkali reaction mixtures must contain products derived from fragment recombination. It is noteworthy, for example, that DL-sorbose,which is one of the major products of the action of dilute alkali (92a) J. Kenner and G . N . Richards, J . Chem. Sac., 2921 (1956). (93) E. Fischer and J. Tafel, Ber., 20, 1088, 2566 (1887); H . 0. L. Fischer and E . Baer, Helv. Chim. Acta, 19, 519 (1936); L. M. Utkin, Doklady Akad. Nauk S . S. S. R . , 67, 301 (1949); Chem. Abstracts, 44, 3910 (1950).
73
THE SACCHARINIC ACIDS
on ~~-glycerose,9~ has also been obtained by the action of alkali on D-fructose and by treatment of D-glucose with a strong-base resin.96 Tracer experimentsgehave revealed that fragment recombination plays only a minor role in the conversion of D-galactose to the (La”-D-galactometasaccharinic acid by lime-water at room temperature. Thus, the metasaccharinic acid prepared from ~-galactose-l-C’~ was found to contain approximately 95% of the original radioactivity* in C1, in accord with the prediction from the Isbell mechanism. The remaining 5 % of the radioactivity was presumably distributed elsewhere in the molecule as a result of intermediary fragmentation of the D-galactose followed by recombination to hexose. *CO$
I
CHO I
CHZ
HOCH I
I
HCOH Ruff
.degradntion
I
CHzOH 2-Deoxy-D-xylose”
(cu. 5% of original
radioactivity)
o-plicnylene-
diamine; KMn04
’
HFoH CIlzOH
HCOH
‘I
I
CHZ 1 HOCH 1
-
“a”-D-Galactometasaccharinic acid (from D-galactose-I-&4)
heat
N
\;/ I
NH
COzH
Benzimidazole of original radioactivity)
(ca. 95%
In contrast to the above formation of metasaccharinic acid, fragment recombination appears to be a predominant feature in the formation of the branched-chain “a”-D-glucosaccharinic acid from D-mannose-l-CI4 plus g6 In this case, the radioactivity originally present in C1 of lime-~ater.~’’ the hexose was found to have become distributed almost entirely between the methyl carbon atom and the tertiary carbon atom of the saccharinic acid, with the latter atom more heavily labeled than the former. In contrast to these observations, the Isbell mechanism, in the absence of compli(94) E. Schmitz, Ber., 46,2327 (1913). (95) M. L. Wolfrom and J. N. Schumacher, J. Am. Chem. SOC.,77, 3318 (1955); M. Grace Blair and J. C. Sowden, ibid., 77, 3323 (1955). (96) J. C. Sowden and Dorothy J. Kuenne, J. A m . Chem. SOC., 16,2788 (1953).
74
JOHN C. SOWDEN
cations engendered by fragment recombination, predicts the appearance of the radioactivity entirely at the methyl carbon atom. 1
COzH
3 , 2 CH,COH I I 4
HCOH
I
-
coz
1
CH,CO,H
3,
o-phenylenedinmine
NnIO,
HFoH CHzOH
6 “u”-D.Glueoaaccharinic acid (from D-mannose-1-C”)
4+5
HCOzH
6
HCHO
Q-0 oxidation,
N
\&/
3
NH
I
N
NH \z / CH
CH,
2-Rlethylhenzimidazole (ca. 96% of the original radioaitivity)
Benzimidazole of the original radioactivity) (ca. 577,
o-phcnylenediaminc
y/
NH
N
2
I
CO,H
Benaimidazole-’2-cnrboxylic acid (ca. 59%,of the original radioactivity)
Benzimidazole (ca. 2% of the original radioactivity)
The above experimental result has been explained by Kenner and Richards*’ as attributable to fragmentation of the hexose to the trioses, D-glycerose (from C4,C5, and CS) and “glycerulose” (from C1, C2, and C3), followed by aldolization of these same two fragments. This would provide ketohexose labeled equally a t C1 and C3. The latter would then yield, by way of the Isbell mechanism, L‘a”-D-ghcosaccharinicacid labeled equally a t the methyl carbon atom and the tertiary carbon atom. This explanation is attractive by reason of its simplicity, but must be discarded since, although it explains the experimentally observed positions of labeling, it fails to explain the relative extent of labeling in the two principal radioactive atoms of the saccharinic acid. If the acid were formed solely by this route, the methyl and tertiary carbon atoms would each contain 50% of the original radioactivity. Moreover, the formation of uny of the acid without prior fragmentation and recombination would be reflected in a greater extent of labeling at the methyl carbon atom than at the tertiary carbon atom. No combination of the route proposed by Kenner and Richards with the normal Isbell mechanism can explain the appearing of a greater extent of labeling at the tertiary carbon atom than a t the methyl carbon atom. Experimentally, the tertiary carbon atom was found, by the methods depicted above, to contain 57 % of the original radioactivity, whereas the methyl carbon atom contained 39 %. The atoms C l , C4 C 5 ,
+
THE SACCHARINIC ACIDS
75
and CCj of the ‘ L a ~ ’ - ~ - g ~ ~ ~ ~ ~acid a ~ ~contained h a r i n i2,c 2, and 0 %, respectively, of the original radioactivity. From the above considerations, the conclusion may be drawn that, although fragment recombination is implicated in the formation of “ c ~ ” - D glucosaccharinic acid from unsubstituted hexoses, the nature of the fragments involved in the recombination step is not yet known with certainty.
6 . Saccharinic Acid Formation by Various Bases Differences in the details of isomerization of the reducing sugars by different bases were recognized by Lobry de Bruyn and Alberda van Ekenstein.97That these differences extend also into the saccharinic acid rearrangement was apparent from the early work of Kiliani and Nef. Thus, whereas D-glucose plus lime-water at room temperature gives “a”-D-glucosaccharinic acid as the principal six-carbon product (Kiliani) , the same sugar with hot, concentrated sodium hydroxide is reported t o give no trace of this branched-chain saccharinic acid but, instead, the metasaccharinic acids plus a lesser amount of “a”-D-isosaccharinic acid (Nef). Whether all such differences in the saccharinic acid rearrangement are due entirely t o differences in the pH of the reaction mixtures, or whether there is also involved a specific, cationic effect, is not yet known. Corbett and KennerE9 suggest that the difference in behavior of glucose in the above two instances may be due to differences in the ionization behavior of the enediols a t the different pH values. Thus, they propose that, a t low pH, the 2,3enediol di-ion (I) is formed and that a preferential elimination of the hyCH~[OH
I c-08 It c-08 I HCOH I I
CHzOe
I II c-08 I c-08
HCI-OH
I
I1
droxyl group at C l then leads to “a”-D-gIucosaccharinic acid. At higher pH, ionization may also occur a t C l of the 2,3-enediol. Elimination of the hydroxyl group from the resulting 2,3-enediol tri-ion (11) is then restricted to C4, and “a”-D-isosaccharinic acid results. Interesting differences in saccharinic acid formation by different bases (97) C. A. Lobry de Bruyn and W. Alberda van Ekenstein, Rec. trav. chim., 14, 203 (1895); 16,92 (1896); 16,257, 262, 282 (1897); 18, 147 (1899); 19. 1 (1900). See also, A . Kuzin, Ber., 68, 619, 1494 (1935); 69, 1041 (1936). J. C. Sowden and R . Schaffer, J. A m . Chem. Soc., 74, 499 (1952).
76
JOHN C. SOWDEN
are revealed by tracer experiments. Gibbs98 observed that ~-glucose-l-C'~ plus 3 N potassium hydroxide at 50" provide lactic acid labeled equally (and only) in C l and C3. A similar result was obtained in this laboratoryg9 when ~-glycerose-3-C'~ was isomerized with 1.68 N sodium hydroxide at 25". The radioactivity in the lactic acid thus produced was found to be distributed between C1 and C3, with a negligible amount at C2. With saturated lime-water a t 25", however, D-gly~erose-3-C~~ formed lactic acid labeled at all its carbon atoms, in the percentages shown in the formulas. 44%
COzH
I
CHOH
56%
I
CH,
NaOH
CHO
I HCOH I
-
CI~H,OH
COzH
CdOH),
I CHOH I CHI
37% 13% 50%
The following explanation of this result is offered. The reverse isomerization of glycerose with "glycerulose," in the presence of either base, is fast as compared to the rate of saccharinic acid formation. Accordingly, the formation of lactic acid occurs essentially through triose enediol labeled equally at C1 and C3. The a-dicarbonyl intermediate of the Isbell mechanism is in this case pyruvaldehyde, which is subject to migration either of the methyl group (saccharinic mechanism) or of the hydrogen atom (metasaccharinic mechanism) in the rearrangement step. The latter migration is almost exclusive in the presence of sodium hydroxide, and produces lactic acid labeled only at C1 and C3. With lime-water, migration of the hydrogen atom preponderates, but some migration of the methyl group also occurs, t o produce lactic acid labeled at C2 and C3. Any combination of the two mechanisms results in the location of 50 % of the radioactivity in the methyl group. Thus, if methyl-group migration occurs to the extent of 26 %, and hydrogen-atom migration to the extent of the remaining 74 %, the lactic acid obtained will be labeled precisely as is observed experimentally. The possibility of the existence of specific, cationic effects, as opposed to pH effects, in the saccharinic acid rearrangement requires further study. IV. TABLE OF PROPERTIES OF SACCHARINIC ACIDDERIVATIVES The melting points and optical rotations of saccharinic acids and their derivatives are recorded in Table I with the corresponding literature references. (98) M. Gibbs, J . Am. Chem. SOC.,72, 3964 (1950). (99) Eva K. Pohlen, M.A. Thesis, Washington University, St. Louis (1954).
77
THE SACCHARINIC ACIDS
TABLEI Properties of Saccharinic Acid Derivatives Compound
felling point, “C.
Four-carbon Metasaccharinic Acid ~ ~ - ( 3 - D e o x y t e t r o nacid), ic [~~-(2,4dihydroxybutyric acid)] anilide brucine salt phen ylhydrazide Fiue-carbon Zsosaccharinic Acid ~ ~ - [ 3 - D e o x y - 2 -(hydroxymethyl) Ctet,ronic acid], ( ~ ~ - [ 2 , 4 - d i h y d r o xy-2-(hydroxymethyl) butyric acid]) brucine salt lactone Five-carbon Metasaccharinic Acids 3-Deoxy-~-erythro-pentonic acid phenylhydrazide quinine salt 3-Deoxy-~-threo-pentonicacid brucine salt lactone phenylhydrazide quinine salt 3-Deoxy-~-erythro-pentonic acid lactone phenylhydrazide sodium salt 3-Deoxy-~-threo-pentonicacid brucine salt lactone phenylhydrazide quinine salt sodium salt Six-carbon Saccharinic Acid 2-C-Methyl-~-ribo(?)-pentonic acid (“d’-D-glucosaccharinic acid) anhydrobenzimidazole anilide brucine salt lactone phen ylhydrazide quinine salt6
115-116 188 130-131
192-195 95-96
[&”, degrees
-27
References
85 12 12
-27.6
14 13
150 172
- 104
+9.4
12 12
145-150 amorph. 110-112 160-1 62
-34 $42.5 -25 - 119
12 12 12 12
amorph. 150
-45 t o -55 -8.9 -20
12 12 12
160 amorph. 110 172
- 19 -36 t o -40 +26 - 103 +24
12 12 12 12 12
240-241 193-195 152 160-161 167-169 141-142
+58 1-55(95%EtOH
-26 +93.5 +50.3 - 103
96 85 12
7, 12, 19 12, 100 12, 64,101
JOHN C. SOWDEN
TABLE I (continued) COrn~OUfld
Six-carbon Isosaccharinic Acid 3-Deoxy-2-C-(hydroxymethyl) (n-erythro or D-threo)-pentonic acid (“a”-D-isosaccharinic acid) amide anilide brucine salt lactone phenylhydrazide quinine salt strontium saltc Six-carbon Metasaccharinic Acids 3-Deoxy-~-xylo-hexonicacid ( ‘W’-Dgalactometasaccharinic acid) anilide barium salt brucine salt hydrazide lactone phenylhydrazide quinine salt strychnine saltd 3-Deoxy-n-lyzo-hexonic acid ( “ f i ” - ~ galactometasaccharinic acid of Nef; parasaccharinic acid of Kiliani) barium salt brucine salt lactone phenylhydrazide quinine salt strychnine salt 3-Deoxy-~-ribo-hexonicacid ( “ a ” - ~ glucometasaccharinic acid) anilide brucine salt calcium salt lactone phenylhydraeide quinine salt strychnine salt
Welling p o d , “C.
86-89
[aIDa,degrees
References
44
-28 t o -7, 13 days +13 -26 62 +19.6 - 118 -5.8
32, 85, 102 12 12, 36 12 54, 101 32, 50
136, 140 122-1 23 141-142, 144 113-115, 145 134-135, 144 185-195
$57.3 $27.4 -13.2 18 -48.4, -45.3 +34.4 -90 -8.4
32 12 12, 50 103 12, 52 12, 50, 56 12, 54, 101 12
13G137, 137 55-60 85-90 134-135, 142 125-130
-1.3 -27, -25.6 -63 -1.9 -106, -104 -23.5
12 12, 50 12 12 12, 54, 101 12
169-171 164 95-96 120-122 191-192
108-109
89-90 145-150 104 lO(r103 135-140 145-147
+
+
k40 (95%EtOH - 23 -5 +25.3 0 - 101 -19.5
85 12 68 12 12 12 12
79
THE SACCHARINIC ACIDS
TABLEI (continued) Compound
Six-carbon Metasaccharinic Acids 3-Deoxy-~-arabino-hexonic acid (“8”-D-glucometasaccharinic acid) anilide brucine salt calcium salt lactone phenylhy draside quinine salt strychnine salt
Welling point, “C.
[alou, degrees
123-124 ca. 130
-63 (95%EtOH) -33 -23.3 +8.2 -30.7 -113.6 -30.8
92 124-1 26 150-155 180-190
Rejerences
85 12 12 12 12 12 12
The rotation solvent is water unless otherwise noted. copper,20 potassium,7 1 0 and rubidiumIo4salts have also been reported for “a”-D-glucosaccharinic acid. The crystalline calcium saltsbe b2 of “or”-D-isosaccharinic acid (soluble t o t h e extent of 1.19 g. in 100 g. of hot water) was the first known salt of this acid. Crystalline calcium,s* copper,62 lead,62 and strontium64 salts have also been reported for “o”-D-galactometasaccharinic acid.
* Crystalline ammonium,’
(100) (101) (102) (103) (104)
E. Fischer and F. Passmore, Ber., 22, 2728 (1889). H . Kiliani, P. Loeffler and 0. Matthes, Ber., 40, 2999 (1907). B. Sorokin, J. prakt. Chem., [2] 37, 318 (1888). R . A. Weerman, Rec. trau. chim., 37, 52 (1917). E . Rimbach and E . Heiten, Ann., 369, 317 (1908).
BY JOHN C . SOWDEN Department of Chemistry. Washington University. Saint Louis. Missouri
I . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ........... I1. The Individual Acids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1. DL-Lactic Acid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2. D L - ( ~ ,4-Dihydroxybutyric Acid) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . a . Preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .............................. 3. ~~-[2,4-Dihydroxy2-(hydroxymethy1)butyric Acid] . . . . . . . . . . . . . . . . . . . . .............................. .............................. 4 The Five-Carbon Metasaccharinic Acids . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
.
....................................................
b . Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . c Configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5. “or”-D-Glucosaccharinic Acid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . a Preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . b . Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . c . Configuration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . d . Miscellaneous Reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 . “cy”-D-Isos&ccharinic Acid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . a Preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . b . Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . c . Configuration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . d . Miscellaneous Reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . e . “0 ”-D-Isosaccharinic Acid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7. The D-Galactometasaccharinic Acids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . a . Preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . b . Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (1) “a”-D-GalactometasaccharinicAcid . . . . . . . . . . . . . . . . . . . . . . . . . . . . (2) “j3”-s-Galactometasaccharinic Acid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . c . Configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 The D-Glucometasaccharinic Acids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . a . Preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . b . Structure ................................................. c . Configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . d . Degradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I11 . Mechanism of Formation of Saccharinic Acids . . . . . . . . . . . . . . . . . . . . . . . . . . . 1. The Fragment-recombination Mechanism of Kiliani and Windaus . . . . . 2 . The Isomerication Mechanism of Nef .................................
. . .
.
35
36 38 38 38 38 39
40 40
40 41 41 41 42 43
43 44
46 47
48 48 49 51 52 52 53 54 55 55 56 59 59
60
61
61
62 62 62 63
36
JOHN C. SOWDEN
3. The Ionic Mechanism of Isbell.. ...................................... 4. Saccharinic Acids from Substituted Sugars.. .......................... 5. Fragment Recombination and Saccharinic Acid Formation. . . . . . . . . . . . . 6. Saccharinic Acid Formation by Various Bases.. ....................... IV. Table of Properties of Saccharinic Acid Derivatives.. ....................
66 69
72 75 76
I. INTRODUCTION In a paper presented before the French Academy of Sciences in 1838,
Eugene Peligot reported that an acid “trds hmgique” was among the products of the action of barium hydroxide or calcium hydroxide on glucose.’ This observation, that acidic materials may result from the treatment of reducing sugars with aqueous alkalis, marked the beginning of investigations that were to uncover one of the most intriguing, and a t the same time one of the most perplexing, reaction sequences in carbohydrate chemistry. Confusion entered the sugar-alkali reaction picture with the report by Mulder2 in 1840 that, apparently, aqueous acid or aqueous alkali act upon glucose in the same manner, to form an acidic product (“glucinic acid”). A natural consequence of this early work was the conclusion that glucose might be an ester whose hydrolysis by acids or alkalis led to acid and alcohol moieties. However, a clear distinction between the products obtainable from the reducing sugars through the action of acids and alkalis, respectively, was eventually achieved. The hexoses were found to afford, through strenuous treatment with acids, a mixture of levulinic and formic acids: whereas with alkalis the principal products are lactic acid plus a series of six-carbon, deoxyaldonic acids (saccharinic acids) isomeric with the starting sugars. In addition, minor amounts of racemic 3-deoxytetronic acid are formed in the hexose-alkali reaction. COZH
COzH
COzH
CHOH
CHa
CHOH
I CHOH I
CHzOH Saccharinic acid
I CHOH I CHZOH Isosaccharinic acid
I I
CHOH CHzOH Metasaccharinic acid
(1) E. Peligot, Compt. rend., 7, 106 (1838). In this same paper it was also recognized that the crystalline sugar obtainable from grapes, honey, starch hydrolyzates, and diabetic urine is one and the same substance, and the name glucose was proposed for it. (2) G . J. Mulder, J . prakt. Chem., 21, 203 (1840). (3) A. F. von Grote and B. Tollens, Ber., 7, 1375 (1874).
THE SACCHARINIC ACIDS
37
Three structurally isomeric forms have been established for the six-carbon saccharinic acids. In the order of their discovery, these are the saccharinic or 2-C-methylpentonic acids, the isosaccharinic or 3-deoxy-2-C(hydroxymethy1)-pentonic acids, and the metasaccharinic or 3-deoxyhexonic acids. Although none of these six-carbon, deoxyaldonic acids has been crystallized, six are known in the form of crystalline lactones (saccharins). All the possible metasaccharinic acids of less than six-carbon content have been obtained, in the form of crystalline derivatives, by the sugaralkali reaction. Only one example of a branched-chain deoxyaldonic acid (the racemic, five-carbon isosaccharinic acid) of other than six-carbon content has been so obtained. The formation of saccharinic acids containing more than six carbon atoms remains to be explored. Isomeric with the saccharinic acids that arise from the sugar-alkali reaction are the w-deoxy- and 2-deoxy-aldonic acids. Both of these latter types may be obtained by oxidation of the corresponding deoxy sugars, and the 2-deoxyaldonic acids also result from the action of lead oxide on the 1,2dideoxy-1 ,2-dihalogeno-aldoses.* In addition, Glattfeld and coworkers6 have synthesized, mainly from non-sugar starting materials, all the racemic deoxytetronic acids possible. However, the present article deals only with those acids which have been isolated from the sugar-alkali reaction. Mention should be made of the origin of the terms saccharin and saccharinic acid. Peligote isolated the first crystalline lactone (“a”-D-glucosaccharin) of a deoxyaldonic acid produced by the hexose-alkali reaction. A slightly erroneous analysis of this new substance led him to believe that it had the same carbon and hydrogen content as has ordinary cane sugar. In addition, the lactone yielded an initially neutral aqueous solution, and Peligot, after concluding that his substance was simply an isomer of sucrose (saccharose), named it saccharin. Although Scheibler’ soon thereafter recognized the lactonic character of Peligot’s saccharin, he retained the name and expanded it to saccharinic acid for the corresponding free acid. Subsequent workers have perpetuated this nomenclature, and the terms saccharinic acid and saccharin are now used extensively in the generic sense. (4) 8. N. Danilov and A. M. Gakhokidze, Z h w . Obshchei Khim., 6, 704 (1936); Chem. Abstracts, SO, 6333 (1936). (5) J. W. E. Glattfeld and G. E. Miller, J . Am. Chem. SOC.,43, 2314 (1920); J. W. E. Glattfeld and F. V. Sander, ibid., 43, 2675 (1921); J. W. E. Glattfeld and L. P. Sherman, ibid., 47, 1742 (1925); J. W. E. Glattfeld and Sybil Woodruff, ibid., 49,2309 (1927); J. W. E. Glattfeld, Gladys Leavell, G. E. Spieth and D . Hutton, ibid., 63,3164 (1931); J. W. E. Glattfeld and J. W. Chittum, ibid., 66, 3663 (1933); J. W. E. Glattfeld and J. M. Schneider, ibid., 60,415 (1938). (6) E. Peligot, Compt. rend., 89, 918 (1879). (7) C. Scheibler, Ber., 13, 2212 (1880).
38
JOHN C. SOWDEN
11. THEINDIVIDUAL ACIDS 1. DL-Lactic Acid COzH
I I
CHOH CHI 3-Deoxy-~~-glyceronic acid
m-Lactic acid is the metasaccharinic acid related to the triose sugars.
It has been obtained as a product of the action of alkali not only on glyc-
erose (glycera1dehyde)a but also on hexosesg and pentoses,*O whence it arises via cleavage and isomerization. The racemic form of the acid is always obtained from the sugar-alkali reaction since the nonasymmetric enediol related to glycerose is an intermediate in its formation (see Section 111). As a consequence of the biochemical importance of lactic acid, its chemistry has been thoroughly studied and is adequately documented elsewhere. The production of lactic acid by the action of alkalis on the sugars has been reviewed by Montgomery." 2. DL- ( 2 ,,&Dihydroxybutyric Acid) COzH
1 I CHa I
CHOH
CHzOH
DL- (3-Deoxytetronic
acid)
Only the racemic form of this acid is obtained from the sugar-alkali reaction. As in the formation of lactic acid, a non-asymmetric enediol is an intermediate in its production (see Section HI), and hence the racemate is the sole representative of the four-carbon metasaccharinic acid class. a. Preparation.-The conversion of the threoses and erythroses to their related metasaccharinic acid by the action of alkali has apparently not been explored because of the relative inaccessibility of these tetroses. The acid is, however, formed as one of the products of the action of hot, concentrated sodium hydroxide on the pentoses or hexoses.12 Its isolation from these (8) J. U. Nef, Ann., 336, 247 (1904). (9) F. Hoppe-Seyler, Ber., 4,346 (1871). (10) T. Araki, Hoppe-Seyler's 2. physiol. Chem., 19,422 (1894). (11) R. Montgomery, Sugar Research Foundation, N . Y., Sci. Rept. Ser., No. 11 (1949). (12) J. U. Nef, Ann., 376, 1 (1910).
39
THE SACCHARINE ACIDS
sources by Nef was accomplished through a complex and arduous sequence of operations which afforded, at the same time, other saccharinic acids produced in the reactions. Nef's method for resolving these complicated mixtures into their component saccharinic acids is here described in general terms only, and the reader is referred to the original work for details.12Following the isomerization with sodium hydroxide, the reaction mixtures were treated with a very slight excess of hydrochloric acid, concentrated, and the residues heated to effect lactonixation. The saccharinic acids and their lactones were then isolated from the sodium chloride by extraction with organic solvents. The acid-lactone mixture was next acetylated, and the products were separated (by extraction with ether) from colored, gummy byproducts. Following deacetylation, the refined acid-lactone mixtures were repeatedly extracted from their aqueous solution with ether and ethyl acetate, to provide fractions of different degrees of solubility in these organic solvents. Finally, fractional recrystallization of the quinine or brucine salts was employed for separating the individual saccharinic acids. Application of the techniques of chromatography and ion exchange, not known to Nef, should greatly simplify the isolation of individual saccharinic acids from such mixtures. b. Structure.12-Nef observed that oxidation of the four-carbon metasaccharhie acid with nitric acid yields DL-malic acid. Moreover, DL- (3,4-dihydroxybutyric acid), prepared by condensation of l-chloro-l-deoxyglyceritol (glycerol a-chlorohydrin, 3-chloro-1,2-propanediol) with potassium cyanide, followed by hydrolysis, provides a phenylhydrazide (m. p., 99") different from that (m. p., 130-131') obtained from the saccharinic acid. Accordingly, the product from the sugar-alkali reaction is ~~-@-deoxytet, acid)]. ronic acid) [ D L - ( ~4-dihydroxybutyric CH aOH-CH 2-CHOH-CO
zH
DL- (2,4-Dihydroxybutyric
acid)
HN03
A
HO &-CH n-CHOH-CO
2H
DL-Malic acid
(phenylhydrazide, m. p., 130-131")
(m. p. and mixed m. p., 128') THNOs
ClCH2-CHOH-CHzOH 1-Chloro-1-deoxyglyceritol
KCN HzO
___f
HOIC--CH~-CHOH-CH~OH D L - ( 4-Dihydroxybutyric ~, acid) (phenylhydrazide, m. p., 99")
Additional evidence regarding the structure of ~~-(S-deoxytetronic acid) was obtained (by Nef) by resolution of the acid through the brucine salt.
40
JOHN C. SOWDEN
One of the two resulting enantiomorphs gave, on oxidation with nitric acid, the enantiomorph of naturally occurring (- )-malic acid. 3.
D L - [ ~ 4-Dihydroxy-2-(hydroxymethyl)butyric , Acid]
COaH
CH~OH
~~-[3-Deoxy-2-C-(hydroxymethyl) tetronic acid]
This recently discovered, racemic acid of the five-carbon series is only the third example of an isosaccharinic acid to be identified as a product of the sugar-alkali reaction. The other examples are the a- and p-D-isosaccharinic acids of the six-carbon series (see pages 48 and 52). a. Preparation.-The crystalline lactone of D L - [ 4-dihydroxy-2-(hydroxy~, methyl) butyric acid] may be isolated after treatment of xyl~biose'~ (4-0P-D-xylopyranosyl-D-xylose) or the related trisaccharide, xylotrio~e,'~ (both obtained by partial hydrolysis of xylan) with lime-water. b. Structure.-The structure of this isosaccharinic acid is established by its synthesis from 1 ,4-butynediol dia~etate.'~ CHaOH CH-0 COCHI
CHa
CHa
C H ~ O CO C H ~
CHaOCOCHs
I
1,4-Butynediol diacetate
I
I
/OH C
I
CHzOH DL- [ Z , 4-Dihydroxy-P(hydroxymethyl) butyric acid]
The branched-chain structure of the acid is further confirmed through its reduction, with hydriodic acid and red phosphorus, to 2-methylbutyric acid.14 Oxidation of one mole of the isosaccharinic lactone with periodate produces approximately one molecular equivalent of f~rrnaldehyde.'~ Whistler and CorbettI4record barely detectable, positive optical rotations (13) G. 0. Aspinall, Mary E. Carter and M. Los, Chemistry & Industry, 1553 (1955). (14) R. L. Whistler and W. M. Corbett, J . Am. Chem. Soc., 78, 1003 (1956).
41
THE SACCHARINIC ACIDS
for the lactone and calcium salt of the acid. These optical rotations, if real, are probably due to unremoved D-xylose, since the accepted mechanism for the production of this five-carbon isosaccharinic acid in the sugar-alkali reaction would predict a racemic product (see Section 111). 4. The Five-Carbon Metasaccharinic Acids COzH II HCOH
I CHz I HCOH I
CHzOH
3-DeoxyD-erythropent onic acid
COzH II
HOCH
I CHa I HCOH I
CHzOH
3-DeoxyD-threopentonic acid
COzH II
HOCH
I CHz I HOCH I
CHzOH
3-DeoxyL-erythropentonic acid
COIH II
HCOH
I I HOCH I
CHI
CHzOH
3-DeoxyL-threopentonic acid
These four acids comprise all the possible five-carbon metasaccharinic acids; and all were obtained by Nef, in the form of crystalline derivatives, from the pentose-alkali reaction. In reading Nef’s description of these substances and their preparation, it must be borne in mind that the available, naturally occurring D-xylose was a t that time called I-xylose. Moreover, Rosanoff’s convention16 for assigning configurational prefixes was then relatively new and was not utilized by Nef. Accordingly, Nef’s I-xylose and I-arabinose, and d-erythro-, I-threo-, I-ergthro-, and d-threo-1,3,4-trihydroxyvaleric acids are respectively, by modern nomenclature, D-xylose and L-arabinose, and D-erythro-, D-threo-, L-erythro-, and L-threo-1 , 3 ,4-trihydroxyvaleric acids (3-deoxypentonic acids). a. Preparation.-Nef applied his conditions of isomerization (with hot, 8 N sodium hydroxide) to L-arabinose and D-xylose, and was able to obtain from the reaction mixtures, through his complicated system of fractionations (see page 39), all four possible 3-deoxypentonic acids. L-Arabinose yielded 3-deoxy-~-erythro-and-L-threo-pentonic acids, and D-xylose provided the corresponding enantiomorphs. The reader is referred to the original work12for details of the separations involved. b. Structure.12-The structures of the four 3-deoxypentonic acids were established through study of their oxidation with nitric acid to the related 2,4-dihydroxyglutaric acids. The interpretation of the results of these oxidation experiments is intimately related to the prior proof of the structure (15) M. A. Rosanoff, J . Am. Chem. Soc., 28,114 (1906).
42
JOHN C. SOWDEN
of “a”-D-isosaccharinic acid (see page 49). Thus, both 3-deoxy-~-erythropentonic acid (from D-xylose) and 3-deoxy-~-erythro-pentonic acid (from L-arabinose) yielded meso-2,4-dihydroxyglutaricacid on oxidation. The last acid, accompanied by an optically active 2,4-dihydroxyglutaric acid, had been obtained previously by Kiliani and M a t t h e P through oxidation, followed by decarboxylation, of “a”-D-iSosacCharinic acid. Moreover, 3-deoxy-~-threo-pentonic acid (from L-arabinose) gave, on oxidation, the enantiomorph of the optically active 2,4-dihydroxyglutaric acid obtained by Kiliani and Matthes from “a”-D-isosaccharinic acid. COzH I
1
C OzH
COsH
\OH
HNOa
- - coz I
CHz
I
HOCH
+
CH2
I
HCOH
COzH
“~”-D-Isosaccharinic acid
I
CHz
-
I HCOH I CH2 I HOCH I
CHz
CHzOH
3-Deoxy-~-threopentonic acid
I I
HCOH
I
COzH
HNO3
HCOH
1
COiH ~-threo-2,4-Dihydroxy- meso-2,4-Dihydroxyglutaric acid glutaric acid
COzH
I
HNO3
HCOH
I
I
HCoH
I
HCOH
CHzOH
COzH
I
I
I HCOH I , CHz I HOCH I
COzH
~-threo-2,4-Dihydroxyglutaric acid
CHz OH 3-Deoxya-erythropentonic acid
COzH
I
HOCH
I
CHz
I I
HOCH CHzOH 3-Deoxy-~-erythropentonic acid
Finally, a comparisonof the properties of the phenylhydrazides of 3-deoxyD-threo-pentonic acid (from D-xylose) and 3-deoxy-~-threo-pentonic acid (from L-arabinose) showed these two acids to be enantiomorphs. c. Configuration.-Much of the evidence quoted above as proof of the structure of the 3-deoxypentonic acids is also applicable to the establishing of their respective configurations. Nef’s theory of the mechanism of formn(16) H. Kiliani and 0. Matthes, Bey., 40,1238 (1907).
43
THE SACCHARINIC ACIDS
tion of the saccharinic acids led to the conclusion that, in the transformation of a pentose or higher sugar into its related metasaccharinic acid, the configuration would not be disturbed beyond C3. The currently accepted modification of Nef’s theory does not alter this conclusion, and the retention of configuration a t C4 in the formation of metasaccharinic acid has been confirmed experimentally in the hexose series (see page 61). Thus, for example, the two 3-deoxypentonic acids obtained from D-xylose have the D-threo and D-erythro configurations, respectively, and the acid possessing the latter configuration provides meso-2,4-dihydroxyglutaric acid on oxidation. It should be noted that this assignment of configuration t o the 3-deoxypentonic acids, taken in conjunction with the evidence cited in the preceding Section, also confirms the D configuration for the penultimate, secondary carbon atom of L‘Q”-D-isosaccharinicacid. The directions of the optical rotations of the lactones (presumably gamma lactones) and phenylhydrazides of the four 3-deoxypentonic acids are in agreement with those predicted, on the basis of the assigned configurations, by the lactonell and phenylhydrazide18 rules. 5. “~”-D-GZucosaccharin~c Acid COzH
I I HCOH I HCOH I
CHZ-COH
CHZOH
2-C-Methyl-~-ribo(?)-pentonic acid a. Preparation.-The lactone of this saccharinic acid is prepared most conveniently by the action of calcium hydroxide on D-glucose, D-fructose, or invert sugar. Peligotlg noted that D-fructose yields the lactone more readily than does D-glucose, and this result was confirmed by Scheibler.7 For preparative purposes, Kiliani preferred invert sugar, and an abstract of his directions20 based on this starting material follows, A cold solution of 1 kg. of inverted sucrose in 9 liters of water is treated with 100 g. of calcium hydroxide and allowed to stand in a stoppered flask with frequent shaking. Fourteen days later, an additional 400 g. of calcium hydroxide is added. After ~~
~
~
~
-
(17) C. S. Hudson, J. Am. Chem. SOC.,32,338 (1910). (18) P . A. Levene, J . Biol. Chem., 23, 145 (1915); P.A. Levene and G. M. Meyer, ibid., 31,623 (1917) ; C. S. Hudson, J. Am. Chem. SOC.,39,462 (1917). (19) E. Peligot, Compt. rend., 90, 1141 (1880). (20) H. Kiliani, Ber., 16, 2953 (1882).
44
JOHN C. SOWDEN
one to two months, with occasional shaking, the solution reduces Fehling reagent only slightly. The mixture is filtered, the filtrate is saturated with carbon dioxide, and the dissolved calcium ions are then precipitated by the addition of an exactly equivalent amount of oxalic acid. After filtration, the solution is concentrated to a thin sirup and allowed to crystallize in the cold. When the crystallization is complete (several days), the mother liquors, from which no appreciable further amount of the lactone can be obtained, are drained from the crystals, and the latter are recrystallized from water. The yield is approximately 100 g. of pure “oc”-D-glucosaccharinic lact one.
Scheibler’ describes a similar preparation of the lactone, except that the long period of standing a t room temperature is replaced by several hours at 100”. However, KilianiZ0states that the yield obtained by this rapid method is unsatisfactory. b. Structure.-In the belief that “a”-D-glucosaccharhic acid possessed a straight, carbon chain, Scheibler? reduced its lactone with hydriodic acid and red phosphorus in an attempt to obtain n-hexanoic acid. He obtained instead, however, a neutral oil of b. p. 203-204’ which he assumed to be the lactone of a hydroxyhexanoic acid. The presence of a methyl group and of a branched, carbon chain in “a))-D-glucosaccharinicacid was established by Kiliani.2°sI1 Oxidation of “a”-D-glucosaccharinic lactone with silver oxidez2gave a mixture of acids
o=c-
I
COzH
I
I
I/CHaI
CHaCOiH
+ CHaOH
I
C02H
I
/CHa C
I\oH
HNOa ____,
A&O
I
CHOH
CHOH
CHO
CHOH
I
CHz OH “a”-D-GlucostLccharinic lactone
I I
COnH Saccharonic acid
1
AgzO but no
(21) H. Kiliani, Ber., 16, 701 (1882). (22) For a more recent study of the oxidizing action of silver oxide on the sugars, see K. G. A. Bwch, J. W. Clark, L. B. Genung, E. F. Schroeder and W. L. Evans, J . Org. Chern., 1, 1 (1936-37).
45
THE BACCHARINIC ACIDS
including formic, glycolic, and acetic, the last indicating the presence of a methyl group in the original lactone. Oxidation of ‘‘a”-D-glucosaccharinic lactone with nitric acid provided in high yield a crystalline monolactone (saccharon, C6HsO6) of a dibasic acid (saccharonic acid, CaHloO?).Oxidation of this dibasic acid with silver oxide yielded acetic acid but no glycolic acid, indicating that, in the original oxidation with nitric acid (“a”-D-ghcosaccharinic lactone + saccharon), a hydroxymethyl group had been oxidized to a carboxyl group. Thus, it was shown that “Cu”-D-glucosaccharinic acid contains the groups -CHI, -CHzOH, and -C02H, and so must possess a branched, carbon chain. The disposition of the functional groups in “a”-D-glucosacCharinic acid was also established by Kiliani.23Reduction of saccharon with hydriodic acid and red phosphorus gave the known:* crystalline ~~-(2-methylglutaric
o=c
CHI
I
CHa-CH--CH*-CH-PO
I
0
I
C
I CHa-CHz-CH2-CH-COzH DL-(2-Methylvaleric acid)
C H O l
I
CHzOH I < 01 1 9 -D-Glucosaccharinic lactone
C I
HI P
~~-(Z-MethyIgIutaric acid)
+
CHI
CHOH
I I
CHOH COzH
I
HOz C-CH=CH-CH-C OIH ~~-(4-Methylglutaconic acid)
Saccharonic acid ~
(23) H. Kiliani, Ann., 216, 361 (1883). (24) J. Wislicenus and L. Limpach, Ann., 192,128 (1878).
46
JOHN C. SOWDEN
acid). A byproduct of the reduction was a crystalline, unsaturated, dibasic acid which later was recognized as ~~-(4-methylglutaconic acid) .26 Thus, the position of attachment of the methyl group in L‘a”-D-glucosaccharinic acid was restricted to one of the penultimate carbon atoms of a pentonic acid carbon chain. A repetition of Scheibler’s reduction’ of “~”-D-glUCOsaccharinic lactone then showed that the neutral product obtained was similar in its properties to known26~~-(2-methylvalero-l ,4-lactone). Accordingly, L‘a”-D-ghcosaccharinicacid must be a 2-C-methylpentonic acid. Confirmation of this structure for “a”-D-glucosaccharinic acid was obtained by Liebermann and Scheibler27when they demonstrated that the known2* ~~-(2-methylvaleric acid) [DL-(methylpropylacetic acid)] was also formed, in low yield, in the reduction of “a”-D-glucosacchariniclactone to the 2-methylvalero-1,4-lactone. Moreover, under more strenuous conditions of reduction with hydriodic acid and red phosphorus (in a sealed tube a t 200°), the ~~-(2-methylvaleric acid) is the principal product. c. Con$guration.-The coilfiguration of “a”-D-glucosaccharinic acid is not known with certainty, although the D-rib0 arrangement is indicated by the evidence so far accumulated. In his theory of the mechanism of saccharinic acid formation. Nefl2,29 assumed that this acid is produced from D-glucose by changes involving only the first three carbon atoms of the sugar and, hence, that the D-erythro configuration is retained in the two lowest asymmetric carbon atoms of the saccharinic acid. This latter contention has not been changed by subsequent modifications of the Nef mechanism (see Section 111) which are currently used for explaining saccharinic acid formation. Supporting evidence for the D classification of “a”-D-glucosaccharinic acid is found in the optical rotation of its lactone. “a”-D-Glucosaccharinic lactone, in view of its marked stability in water or aqueous acids, is almost certainly a gamma lactone. Thus, as pointed out by VotoEek,3° its positive optical rotation indicates the D configuration for C4 on the basis of the lactone rule.” If the two lowest asymmetric carbon atoms possess the D-erythro configuration, ‘(a”-D-glucosaccharinic acid must be either 2-C-methyl-~arabino-pentonic acid or 2-C-methyl-~-ribo-pentonicacid. The initial choice between these two possibilities was made by Nef,12who chose the D-arabino configuration because of similarities between certain alkaloid salts of the saccharinic acid and the corresponding salts of D-arabinonic acid. However, subsequent developments make the D-rib0 configuration appear the more (25) M. Conrad and M. Gutzheit, Ann., 222,249 (1884); see Ref. 60. (26) R. Fittig and L.Gottstein, Ann., 216, 26 (1883). (27) C . Liebermann and C. Scheibler, BeT., 16, 1821 (1883). (28) A. Saytzeff, Ann., 193,349 (1878). (29) J. U.Nef, Ann., 367, 301 (1907);403, 204 (1914). (30) E. VotoEek, Collection Czechoslov. Chem. Communs., 2 , 158 (1930).
THE SACCHARINIC ACIDS
47
probable. The phenylhydrazide of LLa”-D-ghcosaccharinicacid is strongly dextrorotatory ([a]: +50.3°),12 and the phenylhydrazide rule’s would thus assign the D configuration to the tertiary carbon atom. Obviously, such assignment involves the assumption that replacement of hydrogen by methyl on the a-carbon atom does not invalidate the phenylhydrazide rule. Evidence in support of the presence of a cis arrangement of the hydroxyl groups on C2 and C3 of the ‘La”-D-ghcosacchariniclactone ring is provided , of the lactone for three by its behavior upon a c e t o n a t i ~ n .32~ ~Treatment hours at room temperature with a 2% solution of sulfuric acid in acetone gives a crystalline monoisopropylidene compound [m. p., 62-63’, [a]% -38.4’ (in chloroform)] in 83 % yield. This derivative, after titration with sodium hydroxide to open the lactone ring, consumes one molecular equivalent of sodium metaperiodate, with the production of formaldehyde, and is, lactone. therefore, 2,3-0-isopropylidene-“a”-~-glucosaccharinic The epimer of the known “a”-D-glucosaccharinic acid has not been detected among the products of the D-hexose-alkali reaction, although its concurrent formation is to be expected. d. Miscellaneous Reactions.-Two obviously attractive reactions of ‘(a”D-ghcosaccharinic acid would be its reduction to the corresponding 2-Cmethylaldopentose and its degradation to a 1-deoxy-2-ketopentose. The reduction of “a”-D-glucosaccharinic lactone with sodium amalgam was investigated by S ~ h e i b l e rwho , ~ ~ reported briefly that hydrogen is absorbed by the lactone under these conditions. Subsequently, F i ~ c h e noted r~~ that, if the reduction is carried out at or near neutrality, the product is a reducing sugar. The reduction with sodium amalgam was repeated by VotoEek?O but his attempts to prepare a crystalline hydrazone of the amorphous product with phenylhydrazine, p-bromophenylhydrazine, or 1methyl-1-phenylhydraainewere unsuccessful. However, application of the cyanohydrin synthesis to the sirupy sugar provided what was presumably a mixture of 3-C-methylaldohexoses, from which a crystalline phenylosazone and a crystalline p-bromophenylosazone were obtained. The successful degradation of ‘La”-D-glucosaccharinicacid to a l-deoxypentulose has apparently not been recorded. Experiments in the author’s laboratory have indicated that the oxidation of calcium (‘a”-D-glucoSaccharinate with hydrogen peroxide and ferric acetate (the Ruff degradation) proceeds normally to yield a reducing product. However, no crystalline derivative of the expected deoxypentulose has been obtained as yet. (31) Dorothy J. Kuenne, Ph.D. Dissertation, Washington University, St. Louis, (1953). (32) L. M. Utkin and G. 0. Grabilina, Doklady Akad. Nauk S. 8. S. R., 93, 301 (1953); Chem. Abstracts, 48, 12676 (1954). (33) C. Scheibler, Bes., 18,3010 (1883). (34) E. Fischer, Ber., 22, 2204 (1889).
48
JOHN C. SOWDEN
6. “a”-D-IsoSaccharinic Acid COzH
c I‘oa CHz
I I
HCOH CH2OH 3-Deoxy-2-C-(hydroxymethyl)-(~-erythro or D-threo)-pentonic acid
Shortly after the discovery of Peligot’s “a”-D-glucosaccharin, Dubrunf a u P reported that the calcium salt of a monobasic acid resulted from the action of lime-water on maltose. CuisinieP named the acid isosaccharinic acid, after he had prepared from it a crystalline lactone (CeHloOs) isomeric with Peligot’s (‘a”-D-glucosaccharin.The name was expanded t o (‘a’’-D-isosaccharinic acid after Nef12 obtained evidence of the concurrent formation of its epimer, “p”-D-isosaccharinic acid, in the hexose-alkali reaction. a. Preparatim.--“a”-D-Isosaccharinic lactone is obtained in a 15 to 20 % weight-yield by the action of lime-water on malt0se,~6lactose,8eor cellobiose?’ Somewhat lower yields (4to 12 % by weight) can be obtained from partially degraded cellulose plus lime-water.a8 Relatively little “a”-D-isosaccharinic acid is formed in the reaction of D-glucose or D-galactose with hot 8 N sodium hydroxide.’z The efficacy of the (1+4)-linkeddisaccharides in producing the isosaccharinic acid is discussed in Section 111. Lactose is the most convenient source of “a”-D-kosaccharinic acid and Kiliani’s directionsa@ based on this disaccharide follow. Lactose (1 kg.) in 9 liters of water is treated with 200 g. of calcium oxide (slaked and cooled), and the resulting mixture is maintained in a stoppered flask a t room temperature, with frequent shaking, for 3 days. The solution is then heated in a boiling-water bath for 10 hours, filtered, and evaporated to a volume of 3 liters. The highly insoluble calcium “UJ’-D-isosaccharinate(199g.) crystallizes; i t is accompanied by a small amount (14g.) of calcium carbonate. The salt is separated by filtration and
(35) A. P. Dubrunfaut, Monit. sci. Docteur Quesneville, [3]12, 520 (1882). (36) L. Cuisinier, Monit. en’. Docteur Quesneville, [3]12,521 (1882);Bull. S O C . chim. (France), [2]38, 512 (1882). (37) S. V. Hintikka, Ann. Acad. 81%.Fennicae, Ser. A , ZZ, N o . 9 (1922);Chem. Abstracts, 17, 3486 (1923). (38) 0.von Faber and B. Tollens, Ber., 32, 2589 (1899);J. J. Murumow, J. Sack and B. Tollens, ibid., 34,1427 (1901);C. G.Schwalbe and E. Becker, J . prakt. Chem., [2]100, 19 (1920).J. Palm&, Finska Kemietsamfundets Medd., 38, 108 (1929); Chem. Abstracts, 24, 1625 (1930). (39) 11. Kiliani, Ber., 42, 3903 (1909).
49
THE SACCHARINIC ACIDS
is then heated with a solution of an equivalent amount of oxalic acid. Filtration of the calcium oxalate, followed by concentration of the filtrate to a sirup, affords the readily crystallizable “cr”-D-iaosaccharinic lactone.
b. Structure.-Soon after the discovery of “a”-D-isosacchar~icacid, Kiliani applied to i t 4 0 the same methods he had used previously to establish the structure of “a”-D-glucosaCcharinic acid (see page 44). The reduction of L1a”-D-isosacchariniclactone (CeHloOs) with hydriodic acid and red phosphorus at atmospheric pressure yielded, as had the similar reduction of “a”-D-glucosaccharinic lactone, a 2-methylvalero-1 4-lactone. When the reduction was carried out at higher temperatures in a sealed tube, the prodacid), which also had been obtained similarly uct was ~~-(2-methylvaleric from “a”-D-glucosaccharinic lactone. However, “a”-D-isoSaccharinic lactone, on oxidation with silver oxide, yielded (in contrast to the behavior of “a11-D-glucosaccharinic lactone) no acetic acid. Thus, a methyl group is not present in “a”-D-isosaccharinic acid, and, in view of the formation of the CHs
I
CHa-CH-CHz-CH-C=O
I
HI
AgzO
[no CHsC02Hl
CeHia06 P ‘1 a J 9 -~-Isoaaecharinic
I
2-Methylvalero-l,4-lactone
+
CHa
lactone
CH~-CH~-CH~-~H-CO~H DL- (2-Methylvaleric acid)
2-methylvalero-l , 4-lactone on reduction, it must possess one of two alternate structures (I or 11). COzH I
I/ I
CHzOH
C
COzH I CHzOH
C
‘OH
I
I
CHa
CHOH
CHOH
CHOH
CHzOH I 3-Deoxy-2-C-(hydroxymethy1)pentonic acid
CHzOH
I
I
(40)H. Kiliani, Ber., 18,631 (1885).
I I
I1 2-Deoxy-2-C-(hydroxymethy1)pentonic acid
50
JOHN C. SOWDEN
In agreement with these postulated alternate structures (I or II), it was observed that oxidation of “cr”-D-isosaccharinic lactone with nitric acid yields a tribasic acid, COHsOs Furthermore, the tribasic acid readily loses a molecule of carbon dioxide when warmed to loo”, a behavior consistent with the presence of two carboxyl groups on a single carbon atom. The product initially isolated by Kiliani41from this decarboxylation was an optically inactive dihydroxyglutaric acid. This latter acid was found to differ in properties from a Zf3-dihydroxyglutaric acid obtained by the successive bromination and hydrolysis of glutaconic acid. Accordingly, Kiliani concluded that the dibasic acid (obtained by oxidation, followed by decarboxyl-
.
COzH
COZH
COSH
COaH
CHOH
CHOH
CH
CHOH
I
CHzOH “a”-o-Isosaccharinic acid
I COzH
I
COzH Dihydroxy- Glutaconic glutaric acid acid
(m. p., 106” +)
I
C02H 2,3-Dihydroxyglutaric acid (m. p., 155-156”)
ation, of “Cr”-D-isosaccharinic lactone) must be a 2,4-dihydroxyglutaric acid and, hence, that “a”-D-isosaccharinic acid is a 3-deoxy-2-C-(hydroxymethy1)pentonic acid (I). The above reasoning, based on the dihydroxyglutaric acids, is fallacious, as was recognized subsequently by Kiliani and Herold,42since in no event could the same properties be expected for the dihydroxyglutaric acids obtained, respectively, from “Cr”-D-isosacchariniclactone and from glutaconic acid. Glutaconic acid, on bromination followed by hydrolysis, would yield a mixture of the two possible racemates of 2,3-dihydroxyglutaric acid. In contrast, structure I1 for “cY”-D-isosaccharinic acid would provide, on oxidation and decarboxylation, a single, enantiomorphous 2 , S-dihydroxyglutaric acid. Finally, structure I for “a”-D-isosacchari~cacid would lead to a mixture of diastereoisomeric 2,4-dihydroxyglutaric acids, one of which would be asymmetric and the other meso. The correctness of structure I for “a”-D-isosaccharhic acid was even’ tually confirmed by Kiliani and Matthe@ when they isolated from the oxidation and decarboxylation, not only the previously obtained meso-dihydroxyglutaric acid, but also the accompanying, optically active isomer (41) H. Kiliani, Ber., 18,2514 (1885). (42) H. Kiliani and F. Herold, Ber., 38, 2671 (1905).
1
[:"
51
THE SACCHARINIC ACIDS
COzI[
O
H
O
H
CHa OH CHzOH
,CHIOH ,,CHZOH HCOH
C
I'
COzH
HOCH
C
CHz
CHOH
CHOH
\
- COZ
CHz
I CHO H I
CHZOH
I (Enantiomorph)
CHz
I
CHOH
I
COzH
CH~
1 I CHOH CHOH I I
COzH
CHzOH
I
CHOH
I
CO,H
2,4I1 2,3Dihydroxyglutaric (Enantiomorph) Dihydroxyglutaric acids (one meso, one acid enantiomorph) (enantiomorph)
of this acid. Meanwhile, had also established the presence of a hydroxyl group on the tertiary carbon atom of “a”-D-isosaccharinic acid, by degrading the latter with hydrogen peroxide and ferric acetate to a deoxypentose (CbH1004). Thus, the structure of “a))-D-iSosaccharinic acid is established beyond question as that of a 3-deoxy-2-C-(hydroxymethyl)pentonic acid. c. Configuration.-The D configuration may be assigned to C4, the penultimate secondary carbon atom of “a))-D-isosaccharink acid, from several considerations. The currently accepted mechanism for the formation of this acid from the (1 -+4)-linked disaccharides (see Section 111) involves no change in configuration at C4 (C5 of the original D-glucose moiety of the disaccharide). Moreover, the configuration of this carbon atom has been experimentally related to that of C4 of the 3-deoxy-~-pentonicacids (see page 42). Finally, the positive optical rotation of “a’)-D-isosaccharinic lactone, presumably a gamma lactone, assigns the D configuration for C4 on the basis of the lactone rule.’’ The configuration of C2, the tertiary carbon atom, of “a))-D-isosaccharinic acid has not been established. Unfortunately, application of qualitative rules of configuration based on optical rotation affords disagreeing conclusions in this instance. The positive optical rotation of the phenylhydrazide would indicate the D configuration for C2 on the basis of the phenylhydraBide rule.’s On the other hand, the reported negative optical rotation of the acid amide44would assign the L configuration to this carbon at,om on the (43) 0.Ruff, Ber., 36, 2360 (1902). (44) R. A. Weerman, Rec. trav. chim., 37, 16 (1917).
52
JOHN C. SOWDEN
basis of the amide The amide is, however, reported to be unstable, and assignment of configuration on the basis of the available data for this compound may be unreliable. The anilide, in contrast to the amide, shows a positive optical rotation. d. Miscellaneous Reactions.-The reduction of “a”-D-isosaccharink lactone with sodium amalgam gives a sirupy product from which a crystalline p-nitrophenylhydrazone of the branched-chain sugar, 3-deoxy-2-C-(hydroxymethy1)-(D-erythroor D-threo)-aldopentose, can be ~btained.~” Acetylation of the sirupy sugar yields a mixture of the crystalline, anomeric triacetates; the tertiary hydroxyl group is presumably inert toward acetylation, as is the similar tertiary hydroxyl group of methyl hamameloside.47 The Ruff d e g r a d a t i ~ nof~ ~L‘a”-D-isosaccharinicacid to a deoxypentose has been mentioned above. It is interesting that the soluble, lead salt of the acid was used for the degradation instead of the more usual calcium salt which, in this instance, is only very slightly soluble in water. Although Ruff was able to obtain the crystalline benzylphenylhydrazone of the pentose, the yield was so low that cleavage of this hydrazone to the pure sugar could not be studied. The reaction invites repetition and improvement in view of the rare nature of the product, a 3-deoxy-2-pentulose (3-deoxy-~-glyceropentulose). e. “P”-D-Isosaccharin~cAcid.-During the recrystallization (from ethanol) of the brucine salt of 2,4-dihydroxybutryic acid, obtained from the hexose - alkali reaction, Nef4*always observed the presence of a small amount of a less-soluble brucine salt. He concluded that this latter product was a mixture of the brucine salts of “a”-D-isosaccharhic acid and its epimer. After fractional recrystallization of the mixed salts (22 g.) to remove brucine “a”-D-isosaccharinate, he isolated a minor amount (0.6 g.) of a sirupy lactone that still, however, contained about 10% of “cY”-Disosaccharinic lactone. The principal constituent of the lactone mixture yielded brucine, quinine, and calcium salts, as well as a phenylhydrazide (no optical rotation for which was given), all of which were quite different in properties from the corresponding derivatives of “a”-D-isosacch&rinic acid. The various derivatives of the sirupy lactone were, however, similar to those of the corresponding derivatives of 3-deoxy-~-erythro-pentonic acid (from D-xylose plus alkali). Accordingly, Nef concluded that he must have in hand a 3-deoxy-2-C-(hydroxymethyl)pentonic acid, the epimer of ( I 1) a -D-isosaccharinic acid. (45) C. S. Hudson, J . Am. Chem. Soc., 40, 813 (1918). (46) P. Schorigin and N. N. Makarowa-Semljanskaja, Ber., 66,387 (1933). (47) 0. T . Schmidt, Ann., 476,250 (1929); see F. Shafizadeh, Advances i n Carbohydrate Chem., 11, 270 (1956). (48) Ref. 12, pp. 56-58 and pp. 64-65.
53
THE SACCHARINIC ACIDS
In support of his contention that “/3”-D-isosaccharinic acid is present in the hexose-alkali reaction mixture, Nef also cited certain observations of Kiliani arid Ei~enlohr,4~* 6o who oxidized (with nitric acid) the residue obtained, after substantial removal of LLa”-D-isosaccharinic acid and the metasaccharinic acids, from the lactose-alkali reaction mixture. Among the products identified was the tribasic acid, (H02C)zC(OH)-CH2-CHOHC0211, previously obtained by a similar oxidation of “a”-D-isosaccharinic acid (see page 50). Nef concluded that the tribasic acid must in this instance have arisen from “/3”-D-isosaccharinic acid. This conclusion ignores, however, the experimental demonstration by Kiliani and Eisenlohr60 that the residue subjected to oxidation had still contained a small proportion of ‘(a”-D-isosaccharinicacid, isolable as the slightly soluble calcium salt. The best evidence for the formation of L‘/3J1-D-isosaccharinic acid in the sugar-alkali reaction is the recent observation61that treatment of lactose, maltose, or 4-O-methyl-~-glucosewith lime-water at room temperature provides initially a mixture of saccharinic acids consisting almost exclusively of “a”-D-isosaccharinic acid plus an acid with the properties of Nef’s “/3’1-D-isosaccharinicacid [brucine salt, m. p. 185 to 210’ (dec.), [a]: -20 to -22O; lactone, [a]%+6 to +8.5O]. An experimental proof that this substance possesses the isosaccharinic acid structure would provide the necessary evidence that it is, indeed, the epimer of “a”-D-isosaccharinic acid.
7. The D-Galactometasaccharinic Acids COZH
I
HCOH
I
COzH
I I
HOCH
CHz
CHz
HOCH
HOCH
I
I I
HCOH CHzOH 3-Deoxy-~-xy~o-hexonic acid (“d-D-galactometasaccharinic acid)
I
I I
HCOH CHeOH 3-Deoxy-~-lyxo-hexonicacid (“8”-D-galactometasaccharinic acid)
3-Deoxy-~-x&1-hexonicacid (“a”-D-galactometasaccharinic acid) was first detected as a product of the prolonged action of lime-water on lactose (49) H. Kiliani, Ber., 41, 2650 (1908). (50) H. Kiliani and F. Eisenlohr, Ber., 42, 2603 (1909). (51) W.M.Corbett and J. Kenner, J . Chem. Sac., 2245 (1953); 1789 (1954); J. Kenner and G. N. Richards, ibid., 1810 (1955).
54
JOHN C. SOWDEN
at room temperature. After having removed the very slightly soluble calcium L‘a’’-D-isosaccharinatefrom one of these reaction mixtures, IGliani52 noted the slow deposition of a second calcium salt. This latter material could be recrystallized from hot water; it yielded, after removal of the calcium, a crystalline lactone with the familiar formula, CsH,,05 , of a sixcarbon saccharin. It was recognized later that the initial action of liinewater on lactose yields ‘L~ll-D-isosaccharinic acid and D-galactose, with ensuing conversion of the hexose to the epimeric D-galactometasaccharinic acids. 3-Deoxy-~-lyxo-hexonic acid (“p”-D-galactometasaccharinic acid) was discovered by Kiliani and Sandas3as a minor product of the D-galactose-alkali reaction. Kiliani believed that this product was a new type of branchedchain saccharinic acid, and referred to it throughout subsequent publications as L‘parasaccharinic”acid. The evidence, provided both by his own work and that of Nef, that Kiliani’s “parasaccharinic” acid contained, in fact, the epimer of “rY1’-D-galaetometasaccharinicacid, is outlined on page 56. a. Preparation.-The epimeric D-galactometasaccharinic acids are produced concurrently, in yields of 15 to 20 %, by the action either of hot, concentrated sodium hydroxide12 or of lime-water a t room temperature 011 D-galactose. The xylo epimer apparently predominates in the mixture; it can be readily isolated in pure form through its slightly soluble calcium salt. The lyxo epimer is, however, extremely difficult to purify by recrystallization because of its tendency to form mixed salts with those of the xylo epimer. Kiliani and Sandals directions63 for the preparation of “cr1’-D-ga1aCtOmetasaccharinic acid follow. A solution of one part of D-galactose in ten parts of water is treated with half a part of freshly prepared calcium hydroxide. The mixture is maintained a t room temperature in a stoppered flask for 4 weeks, with initial frequent shaking. The resulting voluminous precipitate is removed by filtration and the filtrate is heated t o boiling, while being maintained a t constant volume, for 3 hours. The new precipitate (of basic calcium salts) is then removed and the filtrate is saturated with carbon dioxide. The solution is again heated, filtered, and concentrated t o about twice the weight of the original D-galactose. After seeding with calcium “a”-D-galactometasaccharinate, if seeding crystals are available, the crystallization of this salt is completed by storing in the cold for about 10 days. The yield is about 14% of the weight of sugar used initially.
From the mother liquors of preparations similar t o the above, Kiliani and coworkerss3,s 4 , 5s isolated a crystalline barium salt of the mixed, (52) (53) (54) (55)
H. Kiliani, Ber., 16, 2625 (1883). H. Kiliani and H. Sanda, Ber., 26, 1649 (1893). H. Kiliani and P. Loeffler, Ber., 37, 1196 (1904). H. Kiliani and H. Naegell, Ber., 36, 3528 (1902).
55
THE SACCHARINIC ACIDS
epimoric D-galactometasaccharinic acids. Their preparations of the lyxo epimer (“parasaccharin”) were obtained from this mixed salt by conversion to the mixed lactones and removal, through crystallization, of the xylo epimer. In some instanceslb4~ 66 they briefly record the crystallization of the lyxo epimer (“@”-D-galactometasaccharin). Nef b7 also isolated the crystalline ‘(@” epimer from the D-galactose-sodium hydroxide reaction. His directions include “protracted” fractional recrystallization of crude brucine salts, followed by successive fractional recrystallizations of the strychnine and barium salts. It appears certain that chromatographic and ion-exchange methods, not known to Kiliani and Nef, could be used to advantage in future preparations of “@”-D-galactometasaccharinicacid. b. Structure.-(1) “a”-D-GalactometaSacchar~n~c Acid.-Kilianib8 observed that reduction of “a”-D-galactometasaccharink? acid with hydriodic acid and red phosphorus, under reflux at atmospheric pressure, yielded n-hexanoic 1,li-lactone. Further reduction, at higher temperature in a sealed tube, gave a low yield of n-hexanoic acid. Thus a straight-chain structure, with a hydroxyl group gamma to the carboxyl group, was established for the
o=c-
AI H T I AI H T I
n-Hexanoic 1,4-1actone
COeH
COzH
CHz CHZ I
CHOH
n-Hexanoic acid
“a’l-D-Galactometasaccharinic acid
I
1 I
I
IHNOa
C OaH
COzH
I
I
cHoH
I
CHz CHOH
- I
I I
(56) H. Kiliani, Ber., 44, 109 (1911). (57) Ref. 12, pp. 62-66 and 76-77. (58) H. Kiliani, Ber., 18, 642 (1885).
CHz CHI
I I
CHOH
CHz
COzH
COzH
Trihydroxyadipic acid
Adipic acid
56
JOHN C. SOWDEN
metasaccharinic acid. Oxidation of the latter with nitric 58 led to a crystalline trihydroxyadipic acid which, on reduction with hydriodic acid and red phosphorus, was converted to the known, crystalline adipic acid. These latter observations established a non-terminal position for the deoxy function in “a”-D-galactometasaccharinic acid. A Ruff degradation, with hydrogen peroxide and ferric acetate, of the calciumss or barium60 salts of “a”-D-gdactometasaccharinic acid provided a crystalline deoxypentose (C5HI004, Limetasaccharopentose”)which failed t o give an osazone on treatment with phenylhydrazine. Oxidation of the deoxypentose with bromine yielded a trihydroxyvaleric acid which, upon lactonization and then reduction with hydriodic acid and red phosphorus,60 gave a n-valero-1 ,$-lactone. The silver salt of the corresponding acid was found to be crystallographically identical with the known silver 4-hydroxyn-valerate. COzH
I
CHOH
1
CHz
I I CHOH I CHOH
CHzOH I t (z
-D-
H2 0
2
-F e w
Galactometasaccharinic acid
CHO
I CHz I
CHOH
I I
o=c-
COzH
I
--Brz -+
CHOH CHzOH
2-Deoxypentose
I I CHOH I
CHOH
CHzOH
Trihydroxyvaleric acid
I I CH2 I CHOI CH2
CH2
HI
CHa
n-Valero-l,4lactone
Considered together, the above observations provided evidence for the presence of hydroxyl groups on C2, C4, C5, and C6 of “a”-D-galactometasaccharinic acid, and, hence, for its formulation as a 3-deoxyhexonic acid. (2) “p”-D-GuZactometusucchur~n~c Acid.-Kiliani and Sanda53 reduced their “parasaccharinic acid” in the usual manner with hydriodic acid and red phosphorus. The product was a hexanoic lactone whose boiling point (217.5’) was precisely intermediate between that (220’) of the n-hexanoic 1,4-1actoneobtained by a similar reduction of “a”-D-galactornetasaccharinic acid and that (215”) reported6I for 2-ethylbutyro-l , 4-lactone. Kiliani, however, chose the latter structure for his lactone, since the corresponding acid, acid) and unlike ejther the enantiomorlike ~~-(2-ethyl-4-hydroxybutyric (59) H. Kiliani, Ber., 18, 1555 (1885). (60) H. Kiliani and P. Loeffler, Ber., 38, 2667 (1905). (61) M. B. Chanlaroff, Ann., 226,340 (1884).
57
THE SACCEIARINIC ACIDS
phous62or racemic 4-hydroxy-n-hexanoic gave a readily crystalline barium salt. On the basis of this identification, “parasaccharinic acid’’ was assigned one of the three structures I, 11, or 111. CHa
CHzOH
CH2 OH
CHOH
CHOH
CH2
I
I
I
I
CHOH
I
CHzOH I
I
CHOH
I
CHZOH I1
I
CH2 OH I11 (“Parasaccharinic acid”)
Structure I was quickly ruled out by the observation that the saccharinic acid contains no methyl group, since it gives no acetic acid on oAdation with silver oxide. Moreover, a Ruff degradationK4of the “parasaccharinic acid” gave a crystalline “parasaccharopentose” (CsHlo04) and, hence, it was concluded that structure 111 shows the correct disposition of the functional groups. As a further observation in support of the branchedchain structure, Kiliani and LoeffleP4 reported that oxidation of the saccharinic acid by nitric acid yields a tribasic acid (presumably a hydroxycitric acid) accompanied by the crystalline monolactone (CeHaOs) of a dibasic acid. Evidence that “parasaccharinic acid” probably contains “p”-D-galactometasaccharinic acid (or its unremoved “a” epimer) was soon forthcoming from Kiliani’s own laboratory. Crystallographic comparison of “parasaccharopentose” with “metasaccharopentose,” of their respective crystalline oximes, and of the phenylhydrazides of their derived deoxypentonic acids, showed the two sugars to be identical.6STo bring this observation into conformity with his proposed branched-chain structure for “parasaccharinic acid,” Kiliani suggested that the latter gives the expected 2-deoxy-3pentulose in the Ruff degradation but that this deoxypentulose structure is unstable and rearranges spontaneously to the 2-deoxypentose, “metasaccharopentose” (IV + V). (62) H. Kiliani and S. Kleemann, Ber., 17,1296 (1884). (63) R. Fittig and E. Hjelt, Ann., 208, 67 (1881). (64) H. Kiliani and P. Loeffler, Ber., 37, 3612 (1904). (65) H. Kiliani and A. Sautermeister, Bey., 40, 4294 (1907); H. Kiliani, ibid., 41, 120 (1908).
58
JOHN C. SOWDEN
CHO
CH~OH’
CHOH
I
CHzOH “Parasaccharinic acid”
I CHz I c=o I CHOH I
CH2OH.
I I CHOH I CHOH I CHz
+
CHzOH
v IV Further serious doubt was cast on the branched-chain structure by NefG6 when he observed that “cr”-D-galactometasaccharink acid can be readily isomerized into a product that very closely resembles Kiliani’s “parasaccharinic acid.” On heating the pure “a” epimer in a sealed tube at 200°, with or without pyridine, it was partly converted to “P”-D-galactometasaccharinic acid, whose brucine salt showed properties in excellent agreement with th‘ose of Kiliani’s “brucine parasaccharinate.” The isomerization product was further characterized, through its strychnine salt and phenylhydrazide, as Nef’s “/3”-D-galactometasaccharinic acid, obtainable directly by the action of alkali on D-galactose. At this stage, the possible presence of a branched-chain saccharinic acid in Kiliani’s preparation was supported only by (a) the properties of the barium salt of the hydroxyhexanoic acid obtained from it on reduction and (b) the reported oxidation of “parasaccharinic acid” with nitric acid t o a tribasic acid. The latter evidence was retracted by Kiliani in his final report on the matter,66when he stated that the previous identification of “hydroxycitric acid” was in error and that this “tribasic acid” is, in fact, (-)-tartaric acid. In addition, he now observed that oxidation of “parasaccharinic acid” with nitric acid, followed by reduction with hydriodic acid and red phosphorus, gives a low yield of adipic acid. The hypothesis of the existence of the branched-chain “parasaccharinic acid” now depended solely on the identity of the reduction product, hydroxyhexanoic acid. To support his previous contention that the barium salt of this acid is, indeed, barium 2-ethyl-4-hydroxybutyrate, Kiliani66 also prepared the calcium salt and found that it, too, closely resembled the corresponding salt of 2-ethyl-4-hydroxybutyric acid. As emphasized by Kiliani, neither the reductions with hydriodic acid and red phosphorus nor the oxidations with nitric acid proceed in good yield to single products. Accordingly, Kiliani remained firm in his conviction that his preparation, although it was apparently a mixture, nevertheless contained the branched(66) Ref. 12, pp. 78-82.
59
THE SACCHARINIC ACIDS
chain “parasaccharinic acid.” The accumulated evidence points overwhelmingly to the presence therein of “/3”-D-galactometasaccharinicacid. Whether or not Kiliani’s “parasaccharinic acid” is also formed in the hexose-alkali reaction is a matter requiring further study. c. Configuration.-The accepted theory of the mechanism of formation of metasaccharinic acids predicts, as mentioned previously, that no change in the configuration of the starting sugar will occur a t carbon atoms below C3. Accordingly, the galactometasaccharinic acids should have the D-threo configuration a t C4 and C5. Thus, “metasaccharopentose,” obtainable from either of the galactometasaccharinic acids by the Ruff degradation, should be 2-deoxy-~-threo-pentose.This sugar, prepared by the glycal rneth0d,~7 shows properties (m. p., and m. p. of the benzylphenylhydrazone) in close agreement with those of “metasaccharopentose.” The product from the glycal synthesis showed a final optical rotation which was slightly negative -2’ (in water)], whereas “metasaccharopentose” was reported64* e6 to be optically inactive. Hence, no decision concerning the D or L classification of the latter is available from these data. The galactometasaccharins are, however, gamma lactones, and both show negative optical rotations, thus permitting assignment of the L configuration to C4 of both on the basis of the lactone rule.’? Thus, “metasaccharopentose” must have the D-threo configuration, and the epimeric galactometasaccharinic acids are the 3-deoxy-~-xylo-and -D-lyxo-hexonic acids. Finally, on the basis of the phenylhydrazide rule,’* the “a” epimer is the 3-deoxy-~-xylo-hexonicacid. It is interesting that Nef assigned the correct configurations to the D-galactometasaccharinic acids, as well as t o the D-glucometasaccharinic acids, on the basis of analogies between the optical rotations of D-tartaric acid, the 2,4-dihydroxyglutaric acids (obtained by oxidation of the fivecarbon metasaccharinic acids), and the 2,3,5-trihydroxyadipic acids (obtained by oxidation of the six-carbon metasaccharinic acids).
8. The D-Glucometasaccharinic Acids COzH
I HCOH I
COzH
I I
HOCH
CH2
CHz
HCOH
HCOH
HCOH
HCOH
I
I
I
CHzOH 3-Deoxy-~-ribo-hexonicacid (“a”-~-glucometasaccharinicacid)
I I
CHzOII 3-~)eoxy-~-arabino-hexonic acid (“B”-n-glucometasaccharinic acid)
(67) P. A. Levene and T. Mori, J . Biol. Chem., 83, 813 (1929).
60
JOHN C. SOWDEN
The epimeric D-glucometasaccharinic acids were first isolated by NefL2 from the interaction of D-glucose and hot, concentrated sodium hydroxide. D-Glucose is isomerized and smoothly degraded under these conditions to a mixture of saccharinic acids, in a yield of over 80 %. a. Preparation.-From the isomerization of 100 g. of D-glucose with hot 8 N sodium hydroxide, Nef reported as products, after careful fractionation, 40 to 45 g. of m-lactic acid, 10 to 15 g. of ~~-(2-hydroxybutyro-l,4lactone), 20 g. of the epimeric D-glucometasaccharinic lactones, and 2 g. of the epimeric D-isosaccharinic lactones. The six-carbon lactones, consisting almost entirely of the D-glueometasaccharhie lactones, were separated with relative ease from the products of lower molecular weight. Accordingly, D-glucose is an attractive source for these metasaccharins. The “/3” epimer (3-deoxy-~-arabino-hexonic acid) is readily isolable in pure form through its calcium salt, which is sparingly soluble in cold water. The “a” epimer (3-deoxy-~-ribo-hexonicacid), however, is relatively difficult to separate from the mixture. Here again, it is probable that chromatographic or ion-exchange methods may serve to good advantage. A recently developed methodE8for preparing the epimeric D-glucometasaccharinic acids is based on the action of lime-water on the seaweed polysaccharide, l a m i n a r i ~ The ~ . ~ ~mixed D-glucometasaccharinic acids are obtainable from this source in practically pure condition, as their calcium salts, after separation from unchanged polysaccharide. The directions for their preparation from “insoluble” laminarin follow. “Insoluble” laminarin (50 g.) is treated with an oxygen-free suspension of calcium hydroxide (509.) in 1 liter of water. After 8 days a t room temperature, the suspension is filtered, and calcium is precipitated by t h e addition of the equivalent amount of oxalic acid. Concentration of the filtrate to a volume of 500 ml. causes precipitation of polysaccharide (21.4 g.). After filtration, and concentration t o a sirup, extraction with ethanol (3 X 100 ml.) leaves further polysaccharide (7.7 g.). Evaporation of the ethanol extract affords a mixture of the sirupy D-glucometasaccharinie lactones (13.8 g.). After their conversion t o t h e calcium salts, and crystallization from water (finally with the gradual addition of ethanol), there is obtained calcium “j3”-D-glucometasaccharinate (5.8 g.), calcium “a”-D-ghcometasaccharinate (0.6 g.), and a residue of the mixed salts (3.7 g., principally “01” epimer). Partial, acid hydrolysis of the recovered polysaccharide, followed by retreatment with lime-water, yields an additional amount (11 g.) of the mixed calcium salts.
Similar treatment of “soluble” laminarin with lime-water, but at 100” for 3 hours, gives approximately the same yield of the calcium D-glucometasaccharinates. (68) W.M.Corbett and J. Kenner, J . Chem. SOC.,1431 (1955). (69) V. C. Barry, Sci. Proc. Roy. Dublin SOC., 21, 615 (1938);22, 59 (1939).
61
THE SACCBARINIC ACIDS
b. Structure.-Nef12 established the structural similarity of the D-glucometasaccharinic acids to the D-galactometasaccharinic acids by oxidizing the former to the two corresponding 2,3,5-trihydroxyadipic acids and converting these individually, by dehydration, to the lactone of 3-hydroxymuconic acid. The latter was also obtained, in a precisely similar fashion, from “a”-D-galactometasaccharinic acid. Thus, since Kiliani had previously established the structure of the galactometasaccharinic acid, Nef concluded that the D-glucometasaccharinic acids are also 3-deoxyhexonic acids. C OzH
I CHOH I CHZ
I I HCOH
COzH
I II
CHOH
CH
I
HNOa
A
HCOH
I
O=C-
I
CHzOH o-Glucometasaccharinic acids
CHz
I HCOH I HCOH I
AcaO
TiF
CO,H 2,3,5-Trihydroxyadipic acids
CH
I II
COCH
I
CO2H 3-Hydroxymuconic lactone
c. Configuration.-On the assumption that the D-erythro configuration had been retained at C4 and C5 in the conversion of D-glucose to the D-glucometasaccharinic acids, Nef assigned the D-rib0 and D-arabino configurations to the latter. Moreover, on the basis of analogies between their optical rotations and those of D-tartaric acid and the five-carbon metasaccharinic acids, he concluded that the “a” epimer is 3-deoxy-~-ribo-hexonicacid and acid. the ((P” epimer is 3-deoxy-~-arabino-hexonic The correctness of Nef’s reasoning has been fully borne out by subsequent observations. Ruff degradation of the D-glucometasaccharinic acids, gives the known 2-deoxy-~-erythro-pentose either mixedloor indi~idually,~~ (“2-deoxy-D-ribose”) . The formulation of the “P” epimer as 3-deoxy-~arabino-hexonic acid is, in view of the negative optical rotation of its phenylhydrazide, in accord with the configurational prediction for C2 by the phenylhydraeide rule.18Finally, the identity of “P”-D-glucometasaccharinic lactone with 3-deoxy-~-arabino-hexonolactone (prepared from authentic by hydrolysis and subsequent oxidamethyl 3-deoxy-~-arabino-hexoside’~ tion) has been e~tablished.~’ (70) J. C. Sowden, J . Am. Chem. Soc., 76, 3541 (1954). (71) G. N. Richards, J . Chem. Soe., 3638 (1954). (73) H. R. Bolliger and D. A. Prins, Helu. Chim. Acta, 29, 1061 (1946).
62
JOHN C. SOWDEN
d. Degradation.-In view of the great biochemical interest in 2-deoxy-~ribose and the many attempts to develop a satisfactory synthesis for this sugar,73it is surprising that the degradation of the D-glucometasaccharinic acids has been investigated only recently.70* 71 The preparation (from D-g1UCOSe by Nef’s method) of the mixed metasaccharinic acids in a state of sufficient purity for the Ruff degradation is readily achieved. The degradation of the calcium metasaccharinates proceeds normally, and the resulting deoxypentose may be isolated as its “anilide” without difficulty. In laboratory scale preparations,70200 g. of D-glucose yields approximately 20 g. of 2-deoxy-N-phenyl-~-ribosylamine. The free, crystalline 2-deoxy-Dribose is obtained from the “anilide” in almost quantitative yield by cleavage with benzaldehyde.
111. MECHANISM OF FORMATION OF SACCHARINIC ACIDS 1. The Fragment-recombination Mechanism of Kiliani and Windaus
Although Kiliani supplied a preponderant amount of the experimental data concerning the preparation and proofs of structure of the saccharinic acids, he theorized but little on the mechanism of their formation. In a footnote74to one of his early articles, he pointed out that glycerose had been reported16 to be one of the products of the action of alkali on D-glUCOSe, and he suggested that glycerose might afford D-glucosaccharinic acid through condensation with the lactic acid also present in the isomerization mixture. HCHO HOz C
\ /
+
CH3
KO2 C
HCOH
+
CHO
I I
CHOH CHzOH
+
CH3
\ / COH I CHOH I CHOH I
CHpOH
n-Glucosaccharinic acid
/
COzH
CHOH
I I CHOH I
--t
7 O Z H COH
I I CHOH I
CHz
CHI
CHpOH
CHzOH
D-Isosacc harinic acid
This idea was expanded by Windaus,7s who suggested that not only (73) See W. G. Overend and M. Stacey, Advances in Carbohydrate Chem., 8 , 45 (1953). (74) Ref. 02, p. 1302. (75) M. Nencki and N. Sieber, J . prakt. Chem., [a] 26, 1 (1882). (76) A. Windaus, Chem. Ztg., 29, 564 (1905).
63
THE SACCHARINIC ACIDS
D-glucosaccharinic acid but also D-isosaccharinic acid and Kiliani’s parasaccharinic acid might be formed by recondensation of appropriate aldehydic fragmentation products with a lower-carbon metasaccharinic acid. He proposed that the unbranched metasaccharinic acids, in contrast, are formed by direct dismutation of the isomeric sugars. CHzOH
I
CHO
CHO
+
/
COzH
CHOH
I CH2 I
CHZOH
CHzOH
I
~
CHOH COzH
\ / COH I CH2 I
CHzOH
I CHOH I CHOH I CHOH I CHOH I
COiH
I I . direct. , CH2 dismutation 1
CHzOH
Kiliani’s Parasaccharinic acid
CHOH
CHOH
I I
CHOH CHzOH Metasaccharinic acid
Recently, C14-labeling experiments, discussed in Section 111, 5 have confirmed that fragment recombination is not involved to any significant extent in the conversion of a sugar to the related metasaccharinic acids. Also confirmed by the C14-labelingdata is the fact that fragment recombination is an important feature of the formation of the branched D-glucosaccharinic acid from an unsubstituted D-hexose. However, the specific fragments suggested by Kiliani, and the direct condensation to the final product, D-glUcosaccharinic acid, now seem improbable. 2. The Isomerixation Mechanism of Nef
Nef’s theory of the mechanism of formation of the saccharinic acids is outlined, in its original form, in a paper published in 1907” and, in its final form, in his comprehensive article of 1910.l2The theory proposes that the reaction takes place in two major steps: (a) the isomerization of the sugar, with loss of water, to an a-dicarbonyl compound, and (b) a benzilic acid type of rearrangement of the latter, with hydration, to the saccharinic acid. The second step involves chain rearrangement in the production of the saccharinic, isosaccharinic, and Kiliani’s parasaccharinic acids, but not in the production of the metasaccharinic acids. (77) J. U. Nef, Ann., 367, 214 (1907).
64
JOHN C. SOWDEN
HCO
HCO
HCO
:0
I CHONa + I
7"""
-HC,
CHOH
I
I Aldose
HCO
I
-'&=O
I
CH2 I
Ho"
I11
I1
-
IV
COzH
I I CH2 I '
CHOH
Metasaccharinic acid
According to the theory, the initial reaction is the formation of an alkoxide (I) between the base and the sugar hydroxyl group vicinal to the .carbony1 group. A molecule of base is then eliminated, to give the free, methylenic intermediate (11). The latter isomerizes to the epoxy compound (111) and thence to the a-dicarbonyl intermediate (IV). Finally, a benzilic acid type of rearrangement, with hydration and dismutation, gives the saccharinic acid. CH2OH
CH20H
'i=" CHONa-
c=o c,
I
I CHOH
I
I CHONa I
c=o I
I 3-Ketose
--+
&OH
I
I 2-Ketose CH2OH
CH2OH
I1
CHzOH I
:7
+
c=o I
CHzOH (C=O I
c=o I
1
I I/ o HC I
HC,
d
- 7'"
I1
l c=o I I
I
CH2
CH2
I11
C+ I 0 HC'
CO2H
I
Isosaccharinic acid
IV
COsH
c=o I IV
I11
Saccharinic acid
The successive isomerization of an aldose to a 2-ketose and then to a 3-ketose was explained by assuming the intermediate formation of 1,2- and 2,3-enediols.?sThus, a single aldose, under the influence of alkali, could produce all three types of saccharinic acid. CHO
I I
CHOH CHOH
I
Aldose
CHOH
CHzOH
CHzOH
CHZOH
COH
C=O
COH
CHOH
II I CHOH I
1,a-Enediol
I I
CHOH
I
2-Ketose
.--)
I II
COH
I
2,3-Enediol
I
7 c=o I
3-Ketose
(78) The possibility of the presence of enediolic forms in alkaline solutions of the sugars had been discussed previously by E. Fischer, Ber., 28, 1145 (1895) and by A. Wohl and C. Neuberg, ibid., 33,3095 (1900).
85
THE SACCBARlNfC ACIDS
To account for the formation of saccharinic acids of carbon content lower than that of the original sugar, it was proposed that the enediols are subject to cleavage at the double bond, to produce lower-carbon sugars which could then also undergo the saccharinic acid rearrangement. Thus, according t o Nef, a molecule of a hexose 3,4-enediol, after cleavage to two molecules of glycerose, could provide two molecules of lactic acid (3-deoxyglyceronic acid). It is now considered more probable79that cleavage of the sugar chain CHzOH
I I
CHOH COH
II
COH
I CHOH I
CHO -+
I
2 CHOH
I
CHzOH
CHzOH
Hexose 3,4-enediol
Glycerose
COzH -+
I I
2 CHOH
CHa Lactic acid
under the influence of alkali takes place by a reverse aldolization (V VI) a t the carbon-carbon single bond situated a,@ to the double bond of the enediol. This mechanism involves a 1,2-enediol, instead of a 3,4-enediol, in the cleavage of a hexose t o two triose fragments. An equally plausible mechanism (VII + VI), that utilizes the 2-ketose as the immediate precursor of the triose fragments,sOwould predict the more rapid cleavage of ketoses than of aldoses. .--)
+GHOH COH
HCVH I CHOH I
CHzOH
V
CHzOH C-OH I
II
CHzOH
JG
CHOH CHO
I
CHOH
I
CHzOH VI
HCOH I CHOH
I
CHzOH VII
I n the original formulation of his theory,77Nef chose the hydroxyl group
p to the carbonyl group as the site of alkoxide formation with the base,
in the initial step of the saccharinic acid rearrangement. This was later amended12 to the formulation shown above, in order to accommodate the (79) 0. Schmidt, Chern. Revs., 17, 137 (1935). (80) H. S. Isbell, private communication.
66
JOHN C. SOWDEN
observation that, in the presence of air or other oxidants, the action of alkali on the sugars leads to aldonic acids instead of to saccharinic acids. Nef’s mechanism for the aldonic acid formation is shown in VIII + IX. HCO
I CHOH I CHOH I
HCO +
I CHONa I
HCO +
CHOH
I
,1 C, I
HCO &‘&=O
CHOH
I
I
COzH
I
CHOH
-1
I
VIII Aldose
CHOH CHOH
I
IX
Aldonic acid
The general statement of the isomerization mechanism, as given in the opening paragraph of this Section, is accepted at the present time as a mechanism of saccharinic acid formation. However, Nef’s concept of the mode of isomerization of the original sugar to the intermediate a-dicarbonyl compound has undergone radical revision.
3. The Ionic Mechanism of Isbell The final phase of the Nef mechanism, which involves a benzilic acid type of rearrangement of a-dicarbonyl intermediates to the saccharinic acids, is at present accepted as a feature of saccharinic acid formation. Nef’s concept of the conversion of reducing sugars to the a-dicarbonyl structures required revision, however, when it became evident that the formation, in this step, of the proposed methylenic intermediates is highly improbable. A departure from the methylenic intermediates was suggested in 1926 by Evans and Benoy,s’ who proposed that the a-dicarbonyl intermediates of the Nef mechanism might arise by successive dehydration and rehydration from the enediols. It is now recognized, however, that formaCHOH
II C-OH I CHOH I
1,a-Enediol
- HzO A
11;’
HC
C
I I
CHOH
+ &O
- HzO
CHI
I I c=o I
c=o
a-Dicarbonyl intermediate
tion of the unsaturated oxide structures (pictured as resulting from the initial (81) W. L. Evans and Marjorie P. Benoy, cited in W. L. Evans, Rachel H. Edgar and G. P. Hoff, J . Am. Chem. Soc., 48,2665 (1926).
67
THE SACCHARINIC ACIDS
dehydration of the enediols) is also improbable. An acceptable course for the initial isomerization, based on consecutive electron-displacement reactions and in accord with the principal experimental facts of saccharinic acid formation, was eventually developed in 1944 by Isbell.@ As a prolog t o the Isbell ionic mechanism, Shaffer and Friedemann had concluded,a3after studying the kinetics of sugar activation by alkali, that saccharinic acids result from spontaneous rearrangement of the unstable, sugar anions that are formed in alkaline solution. They also pointed out that the sugars may behave in such solutions not only as monobasic but also as dibasic or polybasic acids, thus giving rise to unstable mono-, di-, or poly-valent anions as precursors of the saccharinic acids. An experimental demonstration that the Nef mechanism for the initial conversion of a sugar by alkali to the a-dicarbonyl structure is not acceptable was provided by a study of the action of alkali on 2-hydroxy-3-methoxy-3-phenylpropiophenone.Nicolet observeda4that the products in this case are 2,3-diphenyllactic acid and methanol. This conversion, which is completely analogous to the formation of a saccharinic acid from a reducing sugar, demonstrated clearly that carbon-oxygen cleavage occurs at the p-carbon atom, rather than a t the a-carbon atom, with respect to the carbonyl group. The Isbell ionic mechanism for the formation of the various types of saccharinic acid, as well a8 for Nicolet’s conversion of 2-hydroxy-3-methoxy3-phenylpropiophenone to 2,3-diphenyllactic acid, involves the following successive steps: (1) the formation and ionization of a n enediol; (2) the ,&elimination of a hydroxyl or an alkoxyl group; (3) rearrangement to a n a-dicarbonyl intermediate; and (4) a benzilic acid type of rearrangement to the saccharinic acid. H-C-Q@
H-C=O
I
H-C=O
.-
C-OIH 11-CI-OH +OH. I ------ CHOH
I
I
H-b
CHOH
I I
COiH
p,--
I
-
I
I-I-C-H CHOH
I
I I
CHOH II@OH@
- 1
CH2 CHOH
I I
CHOH
CHOH
CHOH
CHOH
CHzOH
CHzOH
CHzOH
CIIzOH
I
I
hletasnccharinic ncid (82) H. S. Isbell, J . Research Null. Bur. Standards, 32,45 (1944). (83) P. A . Shaffer and T. E. Friedemann, J . Biol. Chem., 86,345 (1930). (84) B. H. Nicolet, J . Am. Chem. Soc., 63,4458 (1931).
68
JOHN C. SOWDEN
__-__
H~c<- OH
@s,C-QQ I
-p. -i:::
HZC3
-
COzH
C-O!H III n,---
C=O
1
y=O CHOH
H@OH~
CHOH
CHOH
CHOH
CHOH
CHOH
CHOH
CHzOH
CHzOH
CHzOH
CHzOH
CHOH
I
I
I
I I
I
Saccharinic acid
2,3-Enediol
CHzOH
CHzOH
C-43"
c=o
I
H-C:-OH
I
.-----
YHoH CHzOH
2,3-Enediol
I I
1
I
-
COzH
HCH
I I
CHOH
CHOH
CHOH
CHzOH
CHzOH
CHzOH
I
I
Isosaccharinic acid
2-Hydroxy-3methoxy-3phenylpropiophenone
It is interesting L a t Isbell's extension of L e mechanism to the hexose 3'4-enediol series would explain the formation of Kiliani's parasaccharinic acid from this source. Indirect evidence is, however, available that the hexose 3,4-enediols are, at most, minor constituents of the hexose-alkali reaction mixtures. A 3,4-enediol (or the 3-hexulose with which it is in equilibrium) should give, by chain cleavage, formaldehyde and a pentose
69
THE SACCHARINIC ACIDS
Kiliani’s Parasaccharinic acid
1 ,a-enediol. The latter would be expected to afford, among other products, five-carbon metasaccharinic acids. Despite his painstaking analyses of the hexose-alkali reaction products, Nef was never able to obtain evidence for the presence therein of five-carbon structures. Recently, however, chromatographic evidence has been obtained86 for the production of S-deoxypentonic acids, in low yield, from D-glucose plus alkali. Thus, if hexose 3,4-enediol is the necessary precursor of Kiliani’s parasaccharinic acid, the latter must be formed to only a minor extent. 4. Saccharinic Acids from Substituted Sugars
It has long been known that D-glucose substituted a t C4, as in the disaccharides lactose, cellobiose, and maltose (see page 48), is particularly suitable for the preparation of ‘La’’-D-isosaccharinicacid through treatment with lime-water. Since this acid is only a minor product of the action of alkali on unsubstituted D-glucose, the presence of a substituent a t C4 must preferentially direct the alkaline degradation to the isosaccharinic structure. The role of substitution in determining the course of saccharinic acid formation has been critically examined recently by Kenner and coworkers. They conclude that an 0-glycosyl or 0-alkyl anion is more readily extruded from the sugar enediol anion of the Isbell mechanism than is a hydroxyl ion.86 I n addition, substitution a t certain positions in the sugar molecule may inhibit competing side-reactions. For example, a substituent a t C4 of the hexose molecule inhibits cleavage (by reverse aldolization) into two three-carbon fragments and the resultant formation of lactic acid:? a result that had been demonstrated earlier by the experiments of Evans and his A combination of the two above effects, then, preferentially (85) J. W. Green, J . Am. Chem. SOC.,78, 1894 (1956). (86) J. Kenner and G . N. Richards, J . Chem. SOC.,278 (1954). (87) J. Kenner and G. N. Richards, J . Chem. SOC., 1784 (1954). (87a) W. L. Evans and Marjorie P. Benoy, J . Am. Chem. SOC.,62, 294 (1930); W. L. Evans and R. C. Hockett, ibid., 63,4384 (1931).
70
JOHN C. SOWDEN
channels the reaction of the substituted sugars, to aff ord specific, saccharinic acid structures. On this basis, isosaccharinic acids are to be expected as the principal products from 4-0-substituted hexoses. This expectation has been experimentally confirmed in studies of the action of lime-water on 61 lact~ lo s e ,~ cellobiulose,68 ' maltu61 c e l l ~ b i o s e 68 ,~~~ 4-O-methyl-~-fructose~~~ and lose,61cellotetraose,684-0-methyl-~-glucose,~~ 4, G-O-benzylidene-~-glucose.~~ The 2 3-enediol precursor for the formation of isosaccharinic acid (by the Isbell scheme) is produced from the above ketoses by a single enolization, whereas the aldoses must proceed through reversible isomerizations, by way of the 1 ,2-enediols and ketoses, t o arrive a t the 2 3-enediol structure. Accordingly, the 4-0-substituted ketoses in the preceding list are all degraded somewhat more rapidly with lime-water than are the related aldoses. I n the case of 3-0-substituted hexoses, the 1,2-enediol anion is the reactive species because of the ready elimination of the 0-alkyl or 0-glycosyl residue. Hence, the metasaccharinic acid structures are the principal products. Experimental confirmation of this concept is seen in the action of lime-water on 3-O-methyl-~-glucose,~~ 3-O-methyl-~-fructose,~~ 3-0-(/3-~glucosy1)-D-glucose (laminaribiose),89 3-O-(a-~-glucosyl)-~-fructose (turanose),89 G-O-benzyl-3-O-methyl-~-glucose,~~ 4 6-0-benzylidene-3-0-methylD - ~ ~ u c oand s ~ ,2,3: ~ ~5 G-di-O-isopropylidene-D-mannose.88 In contrast to the behavior of 4-0-substituted hexoses, the 3-0-substituted aldoses are degraded with lime-water a t approximately the same rate as are the related ketoses. This is to be expected since, in this case, a single enolization of either (aldose or ketose) produces the 1,2-enediol precursor for metasaccharinic acid formation. With a 1-0-substituted 2-hexulose, metasaccharinic acid formation is blocked by the stability of the monosubstituted 1,2-enediol anion (I) t o alkali. However, the alternative 1-0-substituted 2 3-enediol anion (11) CHOR
II c-00 I CHOH I I
CHzOR
I II
C-OH C-00
I
I1
can proceed, through elimination of the OR anion at C l , to the saccharinic acid structure. Experimentally, 1-0-methyl-D-fructose provides a higher (88) W. M. Corbett, J. Kenner arid G . N. Richards, J . Chem. SOC.,1709 (1955). (89) W. M. Corbett and J. Kenner, J . Chem. Sac., 3274 (1954). (90) J. Kenner and G . N . Richards, J . Chem. Sac., 3277 (1954).
THE SACCHARINIC ACIDS
71
yield of ‘‘af’-D-g1ucosacchariiiicacid, when treated with lime-water, than does ~ - f r u c t o s e . ~ ~ As would be expected from the above considerations, substitution a t C6 of a hexose has, a t most, only a minor effect on the course of saccharink acid formation. Thus, 6-O-(a-~-galactosyl)-~-glucose (melibiose) plus lime-water gives lactic acid and a mixture of the corresponding metasaccharinic, isosaccharinic, and saccharinic acids.S7a, 91 The qualitative experiments with melibiose and with 6-O-methyl-~-glucose,based mainly on paper chromatography, suggest that the metasaccharinic acids may be the principal products from hexoses substituted at C6. I n their initial publications on the saccharinic acids, Kenner and his associates utilized the Isbell ionic mechanism, as given in the preceding Section, to interpret their results. S u b s e q ~ e n t l yhowever, ,~~ they have suggested that the enediol di-ion is the reactive species in saccharinic acid formation; this conclusion was reached when it was observed that related 3-0-substituted aldo- and keto-hexoses are degraded by lime-water a t approximately equal rates. This observation does not, however, necessitate any modification of the original Isbell scheme. As pointed out above, the comnion 1’2-enediol is reached from either the aldose or the ketose by a one-step process, and ionization at C1 of the enediol can then initiate the metasaccharinic acid rearrangement. More recently, Whistler and Corbettg2 have cited the moderate stability of 2-O-(~-xylopyranosyl)-L-arabinose t o alkali as further evidence for the necessity of di-ion formation in the saccharinic acid rearrangement. The failure of this substance t o form saccharinic acids under mildly alkaline conditions is adequately explained, however, by the following considerations. The elimination of the hydroxyl CHOe
CHO
COR
COR
II I
HOCH
I
I11
+
I I1 CH I IV
group from C3 of the mono-ion (111) is undoubtedly slow, as is the formation of saccharinic acids from unsubstituted aldoses a t room temperature with lime-water. Moreover, subsequent steps in the Isbell mechanism leading to saccharinic acids are effectively blocked a t structure IV owing to the inability of the glycosyl residue (R) a t C2 t o ionize. At higher tempera(91) W. M. Corbett and J. Kenner, J . Chem. SOC.,3281 (1954); J. Kenner and G. N. Richards, i b i d . , 2916 (1956). (92) R. L. Whistler and W. M. Corbett, J . Am. Chem. Soc., 77,3822 (1955).
72
JOHN C. SOWDEN
tures, the 2-0-substituted pentose is degraded by lime-water to acidic products, presumably because of chain cleavage. The preceding explanation of the failure of a 2-0-substituted aldose to form saccharinic acids (on treatment with alkali) finds substantiation in the results of a study of the action of lime-water, at room temperature, The presence, in this latter reaction mixon 2,3-di-O-methyl-~-glucose.8~~ ture, of an a,p-unsaturated aldehyde(IV, R = CH3) was established by its further, facile conversion to 5-(hydroxymethyl) -2-furaldehyde upon acidification. The fact that an enediol di-ion is not essential for the initiation of the saccharinic acid rearrangement is further amply demonstrated by the observations of Corbett, Kenner and Richards88 on 2 , 3 :5,6-di-O-isopropylidene-D-mannose. This compound is converted to a mixture of the 5 , 6 - 0 -
0-0
HC-Q’
( C H ~ ) ~ C+CH < 1~ . III V
-
COzH
CHO (CH)C-0-C
I
I1
CH
VI
I
1
7
CHOH 1
CH2
I
VII
isopropylidene-D-glucometasaccharinicacids (VII) on treatment with limewater at 100’. The authorss8depict the reaction as occurring through the intermediate VI. Obviously, the formation of an enediol di-ion prior to the anionic elimination at C3 is here ruled out, as it is in the case of 2,3-di-0methyl-D-glucose, and the reactive mono-ion (V) of the normal Isbell mechanism is involved. Thus, although enediol di-ions may well exist in alkaline solutions of reducing sugars, and may participate in rearrangements of the latter to saccharinic acids,83 the evidence so far advanced does not seem to necessitate any embellishment of the original mechanism advanced by Isbell. 5 . Fragment Recombination and Saccharinic Acid Formation
Aldol condensations involving glycerose (glyceraldehyde) and “glycerulose” (dihydroxyacetone) are known to provide ketohexoses of both straightchain and branched-chain structure.93Since these three-carbon sugars are known to be also formed through chain cleavage of hexoses by alkali,7sit is clear that the hexose-alkali reaction mixtures must contain products derived from fragment recombination. It is noteworthy, for example, that DL-sorbose,which is one of the major products of the action of dilute alkali (92a) J. Kenner and G . N . Richards, J . Chem. Sac., 2921 (1956). (93) E. Fischer and J. Tafel, Ber., 20, 1088, 2566 (1887); H . 0. L. Fischer and E . Baer, Helv. Chim. Acta, 19, 519 (1936); L. M. Utkin, Doklady Akad. Nauk S . S. S. R . , 67, 301 (1949); Chem. Abstracts, 44, 3910 (1950).
73
THE SACCHARINIC ACIDS
on ~~-glycerose,9~ has also been obtained by the action of alkali on D-fructose and by treatment of D-glucose with a strong-base resin.96 Tracer experimentsgehave revealed that fragment recombination plays only a minor role in the conversion of D-galactose to the (La”-D-galactometasaccharinic acid by lime-water at room temperature. Thus, the metasaccharinic acid prepared from ~-galactose-l-C’~ was found to contain approximately 95% of the original radioactivity* in C1, in accord with the prediction from the Isbell mechanism. The remaining 5 % of the radioactivity was presumably distributed elsewhere in the molecule as a result of intermediary fragmentation of the D-galactose followed by recombination to hexose. *CO$
I
CHO I
CHZ
HOCH I
I
HCOH Ruff
.degradntion
I
CHzOH 2-Deoxy-D-xylose”
(cu. 5% of original
radioactivity)
o-plicnylene-
diamine; KMn04
’
HFoH CIlzOH
HCOH
‘I
I
CHZ 1 HOCH 1
-
“a”-D-Galactometasaccharinic acid (from D-galactose-I-&4)
heat
N
\;/ I
NH
COzH
Benzimidazole of original radioactivity)
(ca. 95%
In contrast to the above formation of metasaccharinic acid, fragment recombination appears to be a predominant feature in the formation of the branched-chain “a”-D-glucosaccharinic acid from D-mannose-l-CI4 plus g6 In this case, the radioactivity originally present in C1 of lime-~ater.~’’ the hexose was found to have become distributed almost entirely between the methyl carbon atom and the tertiary carbon atom of the saccharinic acid, with the latter atom more heavily labeled than the former. In contrast to these observations, the Isbell mechanism, in the absence of compli(94) E. Schmitz, Ber., 46,2327 (1913). (95) M. L. Wolfrom and J. N. Schumacher, J. Am. Chem. SOC.,77, 3318 (1955); M. Grace Blair and J. C. Sowden, ibid., 77, 3323 (1955). (96) J. C. Sowden and Dorothy J. Kuenne, J. A m . Chem. SOC., 16,2788 (1953).
74
JOHN C. SOWDEN
cations engendered by fragment recombination, predicts the appearance of the radioactivity entirely at the methyl carbon atom. 1
COzH
3 , 2 CH,COH I I 4
HCOH
I
-
coz
1
CH,CO,H
3,
o-phenylenedinmine
NnIO,
HFoH CHzOH
6 “u”-D.Glueoaaccharinic acid (from D-mannose-1-C”)
4+5
HCOzH
6
HCHO
Q-0 oxidation,
N
\&/
3
NH
I
N
NH \z / CH
CH,
2-Rlethylhenzimidazole (ca. 96% of the original radioaitivity)
Benzimidazole of the original radioactivity) (ca. 577,
o-phcnylenediaminc
y/
NH
N
2
I
CO,H
Benaimidazole-’2-cnrboxylic acid (ca. 59%,of the original radioactivity)
Benzimidazole (ca. 2% of the original radioactivity)
The above experimental result has been explained by Kenner and Richards*’ as attributable to fragmentation of the hexose to the trioses, D-glycerose (from C4,C5, and CS) and “glycerulose” (from C1, C2, and C3), followed by aldolization of these same two fragments. This would provide ketohexose labeled equally a t C1 and C3. The latter would then yield, by way of the Isbell mechanism, L‘a”-D-ghcosaccharinicacid labeled equally a t the methyl carbon atom and the tertiary carbon atom. This explanation is attractive by reason of its simplicity, but must be discarded since, although it explains the experimentally observed positions of labeling, it fails to explain the relative extent of labeling in the two principal radioactive atoms of the saccharinic acid. If the acid were formed solely by this route, the methyl and tertiary carbon atoms would each contain 50% of the original radioactivity. Moreover, the formation of uny of the acid without prior fragmentation and recombination would be reflected in a greater extent of labeling at the methyl carbon atom than at the tertiary carbon atom. No combination of the route proposed by Kenner and Richards with the normal Isbell mechanism can explain the appearing of a greater extent of labeling at the tertiary carbon atom than a t the methyl carbon atom. Experimentally, the tertiary carbon atom was found, by the methods depicted above, to contain 57 % of the original radioactivity, whereas the methyl carbon atom contained 39 %. The atoms C l , C4 C 5 ,
+
THE SACCHARINIC ACIDS
75
and CCj of the ‘ L a ~ ’ - ~ - g ~ ~ ~ ~ ~acid a ~ ~contained h a r i n i2,c 2, and 0 %, respectively, of the original radioactivity. From the above considerations, the conclusion may be drawn that, although fragment recombination is implicated in the formation of “ c ~ ” - D glucosaccharinic acid from unsubstituted hexoses, the nature of the fragments involved in the recombination step is not yet known with certainty.
6 . Saccharinic Acid Formation by Various Bases Differences in the details of isomerization of the reducing sugars by different bases were recognized by Lobry de Bruyn and Alberda van Ekenstein.97That these differences extend also into the saccharinic acid rearrangement was apparent from the early work of Kiliani and Nef. Thus, whereas D-glucose plus lime-water at room temperature gives “a”-D-glucosaccharinic acid as the principal six-carbon product (Kiliani) , the same sugar with hot, concentrated sodium hydroxide is reported t o give no trace of this branched-chain saccharinic acid but, instead, the metasaccharinic acids plus a lesser amount of “a”-D-isosaccharinic acid (Nef). Whether all such differences in the saccharinic acid rearrangement are due entirely t o differences in the pH of the reaction mixtures, or whether there is also involved a specific, cationic effect, is not yet known. Corbett and KennerE9 suggest that the difference in behavior of glucose in the above two instances may be due to differences in the ionization behavior of the enediols a t the different pH values. Thus, they propose that, a t low pH, the 2,3enediol di-ion (I) is formed and that a preferential elimination of the hyCH~[OH
I c-08 It c-08 I HCOH I I
CHzOe
I II c-08 I c-08
HCI-OH
I
I1
droxyl group at C l then leads to “a”-D-gIucosaccharinic acid. At higher pH, ionization may also occur a t C l of the 2,3-enediol. Elimination of the hydroxyl group from the resulting 2,3-enediol tri-ion (11) is then restricted to C4, and “a”-D-isosaccharinic acid results. Interesting differences in saccharinic acid formation by different bases (97) C. A. Lobry de Bruyn and W. Alberda van Ekenstein, Rec. trav. chim., 14, 203 (1895); 16,92 (1896); 16,257, 262, 282 (1897); 18, 147 (1899); 19. 1 (1900). See also, A . Kuzin, Ber., 68, 619, 1494 (1935); 69, 1041 (1936). J. C. Sowden and R . Schaffer, J. A m . Chem. Soc., 74, 499 (1952).
76
JOHN C. SOWDEN
are revealed by tracer experiments. Gibbs98 observed that ~-glucose-l-C'~ plus 3 N potassium hydroxide at 50" provide lactic acid labeled equally (and only) in C l and C3. A similar result was obtained in this laboratoryg9 when ~-glycerose-3-C'~ was isomerized with 1.68 N sodium hydroxide at 25". The radioactivity in the lactic acid thus produced was found to be distributed between C1 and C3, with a negligible amount at C2. With saturated lime-water a t 25", however, D-gly~erose-3-C~~ formed lactic acid labeled at all its carbon atoms, in the percentages shown in the formulas. 44%
COzH
I
CHOH
56%
I
CH,
NaOH
CHO
I HCOH I
-
CI~H,OH
COzH
CdOH),
I CHOH I CHI
37% 13% 50%
The following explanation of this result is offered. The reverse isomerization of glycerose with "glycerulose," in the presence of either base, is fast as compared to the rate of saccharinic acid formation. Accordingly, the formation of lactic acid occurs essentially through triose enediol labeled equally at C1 and C3. The a-dicarbonyl intermediate of the Isbell mechanism is in this case pyruvaldehyde, which is subject to migration either of the methyl group (saccharinic mechanism) or of the hydrogen atom (metasaccharinic mechanism) in the rearrangement step. The latter migration is almost exclusive in the presence of sodium hydroxide, and produces lactic acid labeled only at C1 and C3. With lime-water, migration of the hydrogen atom preponderates, but some migration of the methyl group also occurs, t o produce lactic acid labeled at C2 and C3. Any combination of the two mechanisms results in the location of 50 % of the radioactivity in the methyl group. Thus, if methyl-group migration occurs to the extent of 26 %, and hydrogen-atom migration to the extent of the remaining 74 %, the lactic acid obtained will be labeled precisely as is observed experimentally. The possibility of the existence of specific, cationic effects, as opposed to pH effects, in the saccharinic acid rearrangement requires further study. IV. TABLE OF PROPERTIES OF SACCHARINIC ACIDDERIVATIVES The melting points and optical rotations of saccharinic acids and their derivatives are recorded in Table I with the corresponding literature references. (98) M. Gibbs, J . Am. Chem. SOC.,72, 3964 (1950). (99) Eva K. Pohlen, M.A. Thesis, Washington University, St. Louis (1954).
77
THE SACCHARINIC ACIDS
TABLEI Properties of Saccharinic Acid Derivatives Compound
felling point, “C.
Four-carbon Metasaccharinic Acid ~ ~ - ( 3 - D e o x y t e t r o nacid), ic [~~-(2,4dihydroxybutyric acid)] anilide brucine salt phen ylhydrazide Fiue-carbon Zsosaccharinic Acid ~ ~ - [ 3 - D e o x y - 2 -(hydroxymethyl) Ctet,ronic acid], ( ~ ~ - [ 2 , 4 - d i h y d r o xy-2-(hydroxymethyl) butyric acid]) brucine salt lactone Five-carbon Metasaccharinic Acids 3-Deoxy-~-erythro-pentonic acid phenylhydrazide quinine salt 3-Deoxy-~-threo-pentonicacid brucine salt lactone phenylhydrazide quinine salt 3-Deoxy-~-erythro-pentonic acid lactone phenylhydrazide sodium salt 3-Deoxy-~-threo-pentonicacid brucine salt lactone phenylhydrazide quinine salt sodium salt Six-carbon Saccharinic Acid 2-C-Methyl-~-ribo(?)-pentonic acid (“d’-D-glucosaccharinic acid) anhydrobenzimidazole anilide brucine salt lactone phen ylhydrazide quinine salt6
115-116 188 130-131
192-195 95-96
[&”, degrees
-27
References
85 12 12
-27.6
14 13
150 172
- 104
+9.4
12 12
145-150 amorph. 110-112 160-1 62
-34 $42.5 -25 - 119
12 12 12 12
amorph. 150
-45 t o -55 -8.9 -20
12 12 12
160 amorph. 110 172
- 19 -36 t o -40 +26 - 103 +24
12 12 12 12 12
240-241 193-195 152 160-161 167-169 141-142
+58 1-55(95%EtOH
-26 +93.5 +50.3 - 103
96 85 12
7, 12, 19 12, 100 12, 64,101
JOHN C. SOWDEN
TABLE I (continued) COrn~OUfld
Six-carbon Isosaccharinic Acid 3-Deoxy-2-C-(hydroxymethyl) (n-erythro or D-threo)-pentonic acid (“a”-D-isosaccharinic acid) amide anilide brucine salt lactone phenylhydrazide quinine salt strontium saltc Six-carbon Metasaccharinic Acids 3-Deoxy-~-xylo-hexonicacid ( ‘W’-Dgalactometasaccharinic acid) anilide barium salt brucine salt hydrazide lactone phenylhydrazide quinine salt strychnine saltd 3-Deoxy-n-lyzo-hexonic acid ( “ f i ” - ~ galactometasaccharinic acid of Nef; parasaccharinic acid of Kiliani) barium salt brucine salt lactone phenylhydrazide quinine salt strychnine salt 3-Deoxy-~-ribo-hexonicacid ( “ a ” - ~ glucometasaccharinic acid) anilide brucine salt calcium salt lactone phenylhydraeide quinine salt strychnine salt
Welling p o d , “C.
86-89
[aIDa,degrees
References
44
-28 t o -7, 13 days +13 -26 62 +19.6 - 118 -5.8
32, 85, 102 12 12, 36 12 54, 101 32, 50
136, 140 122-1 23 141-142, 144 113-115, 145 134-135, 144 185-195
$57.3 $27.4 -13.2 18 -48.4, -45.3 +34.4 -90 -8.4
32 12 12, 50 103 12, 52 12, 50, 56 12, 54, 101 12
13G137, 137 55-60 85-90 134-135, 142 125-130
-1.3 -27, -25.6 -63 -1.9 -106, -104 -23.5
12 12, 50 12 12 12, 54, 101 12
169-171 164 95-96 120-122 191-192
108-109
89-90 145-150 104 lO(r103 135-140 145-147
+
+
k40 (95%EtOH - 23 -5 +25.3 0 - 101 -19.5
85 12 68 12 12 12 12
79
THE SACCHARINIC ACIDS
TABLEI (continued) Compound
Six-carbon Metasaccharinic Acids 3-Deoxy-~-arabino-hexonic acid (“8”-D-glucometasaccharinic acid) anilide brucine salt calcium salt lactone phenylhy draside quinine salt strychnine salt
Welling point, “C.
[alou, degrees
123-124 ca. 130
-63 (95%EtOH) -33 -23.3 +8.2 -30.7 -113.6 -30.8
92 124-1 26 150-155 180-190
Rejerences
85 12 12 12 12 12 12
The rotation solvent is water unless otherwise noted. copper,20 potassium,7 1 0 and rubidiumIo4salts have also been reported for “a”-D-glucosaccharinic acid. The crystalline calcium saltsbe b2 of “or”-D-isosaccharinic acid (soluble t o t h e extent of 1.19 g. in 100 g. of hot water) was the first known salt of this acid. Crystalline calcium,s* copper,62 lead,62 and strontium64 salts have also been reported for “o”-D-galactometasaccharinic acid.
* Crystalline ammonium,’
(100) (101) (102) (103) (104)
E. Fischer and F. Passmore, Ber., 22, 2728 (1889). H . Kiliani, P. Loeffler and 0. Matthes, Ber., 40, 2999 (1907). B. Sorokin, J. prakt. Chem., [2] 37, 318 (1888). R . A. Weerman, Rec. trau. chim., 37, 52 (1917). E . Rimbach and E . Heiten, Ann., 369, 317 (1908).