Chapter 7 Alkaloids of Picralima Nitida

-CHAPTER

7-

ALKALOIDS OF PICRALIMA NITIDA J. E. SAXTON The University, Leeds, England

I. Occurrence ........................................................

119

11. Akuammigine ............................................ 111. Akuammicine ......................................................

....... Akuammidine (Rhazine) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Pseudoakuammigine.. . ................ Akuammine .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ....................... Picraline. ............. ....................................... Akuammiline ...................................................... .................................... Akuammenine . . . . . . . . . References ........................................................

123

IV. Pseudoakuammicine.. . . . . . V. VI. VII. VIII. IX. X.

131 134

145 147

155 155 155

I. Occurrence The alkaloids discussed in the succeeding paragraphs occur in the seeds of Picralima nitida Th. and H. Durand (syn. P. klaineana Pierre), which is widely but sparsely distributed throughout tropical Africa. The seeds of this plant are used by the West African natives as a specific for malaria, and as an antipyretic (1, 2). The first chemical examination of Picralima was conducted by Clinquart, who obtained a crystalline alkaloid, mp 242"-243", and a second, amorphous base (3, 4). Various color reactions of these specimens were described, but beyond this no attempts were made t o characterize them completely. The alkaloids were shown to occur chiefly in the seeds, but they are also present in the leaves and bark (3, 4). A thorough investigation by Henry and Sharp ( l ) ,and later by Henry ( 5 ) ,of the constituents of Picralimaseeds,resulted in the isolation of eight alkaloids, of which the principal one, akuammine, appears to be identical with Clinquart's crystalline base. I n common with many other tropical and subtropical plant extracts used locally for their alleged antimalarial properties, the reputation of 119

120

J . E. SAXTON

the Picralima drug has not survived careful clinical and pharmacological testing. As a result of the demonstration that Picralima is inactive in avian, and therefore presumably in human, malaria (6), the chemical investigations were temporarily abandoned. More recent pharmacological inquiries have shown, however, that some of the alkaloids have pronounced activity (7-1 3). I n particular, akuammine augments the hypertensive effects of adrenaline, although, when administered individually without adrenaline, it exerts a hypotensive effect (7-1 1). Akuammine also shows a significant local anesthetic action, almost equal to that of cocaine (8), while a minor alkaloid, akuammidine, is approximately three times as effective (12). On the other hand, akuammigine appears to be almost devoid of physiological activity (14). I n recent years the chemical investigations on the Picralima bases have been resumed, and the structures of several of them have been elucidated. A ninth alkaloid, picraline, has been isolated from the seeds by fractional crystallization and chromatographic separation of the total alkaloid fraction (15, 15a). The occurrence of the typical Picralima alkaloids has occasionally been observed in other genera. Thus, akuammine (vincamajoridine) has been isolated from the leaves and branches of Vinca major L. (16 , 17, 18), and from the roots of Yinca rosea L. (syn. Lochnera rosea Reichb. and Catharanthus roseus G. Don.) (19). Rhazine, one of the constituent bases of the leaves and roots of Rhaxya stricta Decaisne (20), has recently been shown (21) to be identical with akuammidine. A further source of akuammidine has been found in the leaves of Vinca diflormis Pourr. (22). Finally, akuammicine N,-methochloride has recently been identified as one of the quaternary constituents of the root and stem bark of Hunteria eburnea Pichon (22a).

11. Akuammigine Akuammigine is obtained, as its hydrochloride, as the least soluble fraction during the extraction of the crude Picralima alkaloids with dilute hydrochloric acid. The free base, C Z ~ H Z ~ N HzO, Z O ~ . forms -42" colorless, square plates from aqueous ethanol, mp 113", (EtOH), pK,, 6.58. Attempted dehydration of the solvate leads to decomposition. The molecule contains one methoxyl group and one C-methyl group; Kuhn-Roth determination gives a value of 120% for one such group, which was initially interpreted as indicating the presence of two C-methyl groups ( 2 3 ) . However, a similar discrepancy occurs with a closely related alkaloid, tetrahydroalstonine, which is known to contain only one C-methyl group.

7. ALKALOIDS

OB

Picralima nitida

121

The UV-spectrum of akuammigine is almost identical with that of the indole alkaloids containing a heterocyclic ring E, and corynantheine (2, 23, 24). The presence of the /3-alkoxyacrylic ester group is confirmed by the absorption bands at 1706,1684,and 1629 cm-l in the IR-spectrum. The appearance of a second ester band must be owing to intramolecular hydrogen bonding, since it is not observed in the spectrum of a chloroform solution of akuammigine. These data indicate a close relationship with ajmalicine, which is supported by chemical evidence. Thus, selenium dehydrogenation gives alstyrine ; lithium aluminum hydride reduction gives a primary alcohol, akuammigol (I),which possesses a typical indole UV-spectrum, with a deep minimum a t 250 mp. The formulation of this product as an allylic alcohol, stereoisomeric with tetrahydroalstonol, is further shown by its

anomalous behavior with acids and the comparatively facile hydrogenolysis of the primary alcohol function. Since akuammigol does not form crystalline salts with acids, it may be characterized as its methiodide and 0-acetate. From this evidence, akuammigine is formulated as a stereoisomer of ajmalicine and tetrahydroalstonine (11)(2). I n analogy with these two alkaloids, the ester group of akuammigine is not readily saponified, nor is the double bond easily hydrogenated; in contrast to tetrahydroalstonine, however, dinitrophenylhydrazine has no effect on the molecule (2). The IR-spectrum of akuammigine in the 2800 cm-1 region was initially interpreted as indicating that it belongs to the pseudo or epiallo series, which possess /3 hydrogen at C-3 ( 2 5 , 2 6 ); however, the correct deduction to be drawn from the IR-spectrum is that in its preferred conformation akuammigine possesses equatorial hydrogen a t C-3 (27). Since akuammigine is dehydrogenated with palladium and maleic acid much more readily than 3-isoajmalicine, which was also assigned to the pseudo or epiallo series, it was tentatively suggested that akuammigine belongs to the pseudo series (28). The product of this dehydrogenation, isolated as its perchlorate, is identical with alstonine perchlorate ; hence, akuammigine is 3-isotetrahydroalstonine.

J . E. SAXTON

122

Recent work on the stereochemistry of the heteroyohimbine alkaloids has necessitated a revision of these conclusions. Thus, the conversion of tetrahydroalstonine (11) into 19-corynantheidone (111, with u hydrogen at C-20), and isomerization of the latter with sodium methoxide t o the known 18,19-dihydro-19-corynantheone (111,with ,fl hydrogen a t C-20), demonstrates that in tetrahydroalstonine, and therefore in akuammigine, the D/E ring junction must be cis (29). The hydrogenation of tetradehydroakuammigine with Raney nickel in alkaline solution furnishes

IV Akuemmigine

I11

tetrahydroalstonine ; since, in general, the cis D/E tetradehydro derivatives of the yohimbine and heteroyohimbine alkaloids afford predominantly the a110 stereoisomers in this reaction, tetrahydroalstonine must belong to the all0 series, and 3-isotetrahydroalstonine (akuammigine, IV) must belong to the epiallo series. The large spin-spin coupling constant observed in the one-proton octet owing to the C-19 proton in the NMRspectrum of tetrahydroalstonine indicates a trans diaxial orientation of

Akuammigine

the C-19 and C-20 hydrogen atoms (29). Therefore, these two hydrogen atoms must also be oriented trans in akuammigine, which can consequently be completely represented by the conformation V.

7 . ALKALOIDS OF Picralima nitida

123

111. Akuammicine

Akuammicine, C20H22N202, crystallizes from aqueous ethanol as colorless plates, mp 182', pK, 7 . 4 5 ,and ischaracterized by itsremarktibly high rotatory power, [a]:" -745' (5,23). The molecule contains C-methyl and methoxyl groups, and exhibits a UV-spectrum of approximately indoline type, readily distinguishable from the spectra of its congeners (23, 24). The IR-spectrum discloses the presence of an imino group and a 1,2-disubstituted benzene nucleus (747cm-l), and it has a pronounced band at 1658 cm-l, initially ascribed to an amide function (23, 30). The Otto reaction of akuammicine gives a deep blue color, reminiscent of benzylidenestrychnine, which also has a high rotatory power. Akuammicine was therefore regarded as having a similar chromophore, namely, Ph-NH-CO-C=C-

I

I

adjacent to an asymmetric center (30). However, vigorous reduction of the alkaloid with lithium aluminum hydride (it is resistant to this reagent under the usual conditions) gives a crystalline base, ClgH22N2, which behaves in all respects as an indoline derivative (31). The accompanying change of constitution is equivalent to the replacement of -COOMe by a methyl group, and this is supported by an increase in the C-methyl content. The presence of the ester group has been confirmed by saponification, although the intractable amino acid could not be characterized. Cautious acid hydrolysis is accompanied by decarboxylation, the product being an easily oxidized base, ClgH20N2. These data are explained by the assumption that the chromophore in akuammicine is I I Ph-NH--C=C-COOMe

which, as a vinylog of phenylurethane, should readily suffer decarboxylation after hydrolysis. It is significant that the analogous compound, Ph-NH-CMe=CH .COOMe, also exhibits pronounced absorption a t 1658 cm-I. Combining these results with biogenetic considerations, Aghoramurthy and Robinson arrived at the constitution VI for akuammicine. On the basis of this formulation, the aromatic base akuammicyrine, obtained in addition to skatole and 3-ethylindole on selenium dehydrogenation of akuammicine, was assigned the indololepidine structure VII. Synthesis of VII was achieved by the Fischer indole reaction using cyclohexanone 4-methylquinolinyl-7-hydrazone, followed by dehydrogenation of the product. The identity of the synthetic base (VII)

124

J. E. SAXTON

with akuammicyrine was not established, but the absorption spectra are SO closely similar that they must contain the same basic ring system (31). Some further reactions of akuammicine consistent with the structure V I have been reported by Smith and Wrdbel(32). Thus, the isolation of acetaldehyde from the ozonolysis products of akuammicine formally establishes the presence of the double bond in an ethylidene grouping. The acid hydrolysis of akuammicine was earlier stated (31) to give a sensitive indolenine base, C18&&2, mp 147O-148O. I n evacuated sealed tubes, however, akuammicine reacts with hydrochloric acid a t 115' to give a stable, isomeric, indolenine base, mp 80"-84", in very high yield (32). The constitution of Robinson's indolenine base has not been elucidated, but Smith's base is almost certainly VIII, since this formulation accounts extremely satisfactorily for its behavior. Reduction of VIII with lithium aluminum hydride gives an indoline base, which is presumably the expected indoline analog of VIII. Reduction with methanolic potassium borohydride, however, gives an indole base, base A, C18H22N2 (X),which must be formed by reverse Mannich fission of the C-3 t o C-7 bond in VIII in the proton-donating solvent used (see

VI Akuammicine

VII

.

CH * Me VIII

CH Me

IX

1

X

7.

ALKALOIDS OF

Picralima nitida

125

arrows in VIII), followed by reduction of the immonium ion (IX). I n accordance with this suggestion the borohydride reduction of VIIT in acid solution, in which Nb is protonated, affords the corresponding

if*

x1

XI1

C-Fluorocurarine chloride

O d V '

XI11 Dihydrodeoxyisostrychnine

XIV

1

1. [OI 2. 0.4N HClllOOoll hr

I COOH XVI

XVII 19,20-Dihydroakuarnmicine

XVIII

Strychene

126

J . E. SAXTON

indoline base. Further, the analogous degradation of 19,2O-dihydroakuammicine to the 19,20-dihydro derivative of X proves that the isolated double bond is not involved. Zinc dust distillation of the indole base A (X) gives comparatively high yields of 3-ethyl-2-methylindole and 3-ethylpyridine. Thus, it is possible to account for all the carbon atoms of X, and the structure of the latter is confirmed, with the exception of the point of attachment of the asterisked carbon atom (C-16)to the piperidine ring (32). Robinson’s structure for akuammicine (VI) has recently received ample confirmation by several independent routes. The first of these involved the acid hydrolysis of C-fluorocurarine chloride (XI), which gave a deformyl compound identified as XII. The same quaternary chloride was also obtained by hydrolysis and decarboxylation of akuammicine N,-methochloride with dilute hydrochloric acid (33). I n later experiments (33, 34) the permanganate oxidation of dihydrodeoxyisostrychnine (XIII) in neutral solution, followed by treatment with sulfur dioxide in methanol and then with hot, dilute methanolic hydrochloric acid, was shown to give strychene (XVIII), which is the hydrolysis and decarboxylation product of 19,20-dihydroakuammicine(XVII). This oxidation proceeds via the pyridone (XIV) and 19,2O-dihydronorfluorocurarine (XV). A second oxidation product of the pyridone (XIV), obtained in very small (3.5%) yield, is 19,20-dihydroakuammiciiie (XVII); this is presumably formed from the intermediate acid (XVI; not isolated) by esterification and hydrolysis of the amide function during the acid treatment. Thus, strychnine can be degraded in a few stages directly to 19,20-dihydroakuammicine (33, 34). The route adopted by Edwards and Smith (35) involved the conversion of strychnine and akuammicine into the common transformation product XXI. Reduction of akuammicine (VIa)with zinc and methanolic sulfuric acid gave 2,16-dihydroakuammicine (XIX), which, in contrast to akuammicine, exhibited a typical indoline UV-spectrum and normal ester carbonyl absorption in its IR-spectrum. Hydrogenation of XIX gave 2,16,19,20-tetrahydroakuammicine (XX), which, on equilibration with sodium methoxide and magnesium methoxide, was isomerized to isotetrahydroakuammicine (XXI), in which the ester grouping has the preferred equatorial configuration. The convergent route from strychnine proceeded via the Wieland-Gumlich aldehyde (XXII), which was converted first into its oxime and then into the related nitrile acetate (XXIII) by dehydration and concomitant acetylation with acetic anhydride in pyridine. Alkaline hydrolysis of XXIII, followed by esterification, gave the methyl ester (XXIV), the stereochemistry of which was established by its reduction with lithium aluminum hydride

7.

ALKALOIDS OF

Picralima nitida

127

COOMe XIX 2.1 6-Dihydroakuammicine

VIa Akuammicine

COOMe

xx

2,16,19,20-Tetrahydroakuammicine

dOOMe XXI Is0tetrahydroakuammicine (methyl 2~,16a,20a-curan-l’I-oate)

XXIII

IV“ I

H

1’-H COOMe

XXIV

XXII Wieland-Gumlich aldehyde

I

CHzOH

xxv

I

XXVI

bH2 Me XXVII Anhydro-Z,16-dihydroakuammicinol

128

J . E. SAXTON

to the Wieland-Gumlich diol, and by the configurational stability of its carbomethoxy grouping. Finally, hydrogenolysis of the allylic hydroxyl group in XXIV and reduction of the double bond gave a base, methyl 2/3,16a,20c(-curan-17-oate (XXI), which was identical with isotetrahydroakuammicine (35).

XXVIII

XXIX

Geissoschizoline

A further reference compound in the strychnine-akuammicine series is the alcohol XXVI, which was prepared by Janot and his collaborators (36, 37) from 2,16-dihydroakuammicine (XIX) by saponification, followed by lithium aluminium hydride reduction of the 2,16-dihydroakuammicinic acid (XXV)so obtained. The alcohol XXVI was identical with one prepared earlier (38) from the Wieland-Gumlich aldehyde (XXII) by sodium borohydride reduction to the related diol and subsequent removal of the allylic hydroxyl group by zinc-acetic acid reduction of the corresponding allylic bromide. The lithium aluminum hydride reduction of 2,16-dihydroakuammicine (XIX) and the acid XXV gives rise to different diols; hence, it is clear that epimerization of C- 16 accompanies saponification of the ester XIX. The stereochemistry of the acid XXV follows from that of the alcohol XXVI, which was already known ; consequently, the stereochemistry of 2,16-dihydroakuammicine (XIX)is confirmed (37).I n accordance with this deduction the lithium aluminum hydride reduction of XIX, followed by catalytic hydrogenation of the product, affords geissoschizoline (XXIX) (37,37a). The lithium aluminum hydride reduction of akuammicine itself furnishes an oxygen-free, secondary indoline base, C ~ ~ H Z ~mp NZ 196", , which contains one exocyclic methylene group and one ethylidene group (36, 37). In contrast to an earlier report (31))this base contains only one C-methyl group, and is regarded as the pentacyclic base XXVII, i.e., anhydro-2,16-dihydroakuammicinol. This proposal is supported by the hydrogenation of XXVII to the tetrahydro derivative XXVIII, which can also be prepared by an independent route from geissoschizoline (XXIX) (37, 37a). There remains for discussion the very interesting decomposition

7.

ALKALOIDS OF

Picralima nitida

129

which occurs when akuammicine is heated in methanol solution (39). At 80" the product is a betaine, C1gHzoNzOz (XXX), which contains indole and pyridinium ion chromophores, since its UV-spectrum is almost identical with the summation spectrum of 2,3-dimethylindole

VIa 6OOMe

Me

bOOMe XXXI

/

CHMe

/

V ' N

COOMe

Me

XXXII

XXXIII

xxx

XXXIV

xxxv and 3-picoline ethobromide. The formation of XXX from akuammicine is explained by the degradation of a C-16-protonated species to XXXI (compare VIII --f IX), followed by proton removal a t C-14 to give an intermediate enamine (XXXII) that can break down, by an essentially irreversible retro-Michael reaction, to the ester (XXXIII). The subsequent stages to XXX are then unexceptional. This interpretation of the decomposition of akuammicine is consistent with the observations

130

J. E. SAXTON

that 2,16-dihydroakuammicine and akuammicine methiodide do not break down under comparable conditions; the former is simply epimerized a t c-16, and the latter remains unaffected. At 140" in methanol, a much more extensive decomposition of akuammicine occurs, and the products isolated are 3-ethylpyridine and 2-hydroxycarbazole (XXXV). The formation of 3-ethylpyridine is presumably the result of a normal Hofmann degradation of the pyridinium ester corresponding to XXXIII ; the other product should consequently have been methyl 3-vinyl-2-indolylacetate (XXXIV), but this was not obtained. It was accordingly suggested that the 2hydroxycarbazole obtained was formed from XXXIV by intramolecular nucleophilic attack of the vinylogous enamine methylene group on the carbomethoxy group (arrows in XXXIV), followed by elimination of methanol and aromatization (39).

IV. Pseudoakuammicine Pseudoakuammicine, C Z ~ H ~ Z N Z mp O ~187", , pK, 7.47, was first isolated from Picralima seeds by Henry ( 5 ) )who reported its rotation as [N]E" -83". Robinson and Thomas (30) noted that its I R - and UVspectra are very similar to those of akuammicine, but they also pointed out that pseudoakuammicine does not exhibit an unusually high negative rotation, which was also believed to be associated with the presence of the chromophore I I PhNH-C=C-COOMe

attached to an asymmetric center. I n more recent investigations, Edwards and Smith (40) have carried out further extractions of Picralima seeds, and have found pseudoakuammicine to be optically inactive; its 2,16- and 19,2O-dihydro derivatives are also optically inactive. Pseudoakuammicine and its two dihydro derivatives have IR-spectra identical with those of the corresponding derivatives of akuammicine ; hence, pseudoakuammicine is ( i )-akuammicine. This conclusion was confirmed by resolution of pseudoakuammicine by fractional crystallization of the ( + )-tartrate. The more soluble salt gave akuammicine, and the less soluble salt gave ( + )-akuammicine, mp 181"-182.5", [N]?' + 720" (MeOH). On being mixed with an equimolecular proportion of akuammicine, ( + )-akuammicine gave pseudoakuammicine.

7.

ALKALOIDS OF

PicraEima nitida

131

Pseudoakuammicine is the first racemic base to be discovered in the strychnine-yohimbine series of alkaloids, and the question of its origin naturally arises. The only stage in the extraction of Picralima seeds during which racemization of akuammicine might have occurred invoived prolonged percolation with hot methanol ; however, as already discussed, akuammicine is not racemized under these conditions but suffers a more extensive decomposition. I n any event, such a racemization would necessarily involve fission of the 3,7 and 15,16 bonds, followed by a nonspecific resynthesis, which is considered to be a very unlikely possibility. It was therefore suggested that, in the plant, pseudoakuammicine is produced by a nonspecific biosynthesis; this would accord with its formation from a tryptophan-phenylalanine precursor, but not from an optically pure prephenic acid derivative (40).

V. Akuammidine (Rhazine) Akuammidine, C21H24N203, mp 234", [c(]gO+ 24" (MeOH), is a monoacidic base which contains one methoxyl group and one C-methyl group, but no methylimino groups. The molecule also contains two active hydrogen atoms and gives rise to monoacetyl and monobenzoyl derivatives ( 5 , 41, 42). Its UV-spectrum is characteristic of the true indole alkaloids (14,41,42) ; hydrogenation gives a dihydroakuammidine by saturation of a double bond, but this is not conjugated with the indole nucleus, since the UV-spectrum of the product is also typically indolic (42). The IR-spectrum of akuammidine discloses the presence of an imino group, a strongly hydrogen-bonded hydroxyl group, and an ester group. The hydroxyl group accounts for the formation of monoacyl derivatives, while the ester group must be a carbomethoxy group, since hydrolysis of akuammidine gives akuammidinic acid, C20H,,Nz03, and reduction with lithium aluminum hydride gives akuammidinol, C20H24N202, the related primary alcohol. The position of the alcohol function in akuammidine relative to the carbomethoxy group is established by its reaction with potassium t-butoxide in benzene at SO", which results in the loss of formaldehyde by retroaldol cleavage. Thus, akuammidine contains a ,f?-hydroxypropionic ester residue, analogous to that in echitamine and dihydropseudoakuammigine. The product, dehydroxymethylakuammidine, on reduction with lithium aluminum hydride, furnishes a primary alcohol, dehydroxymethylakuammidinol, which is identical with normacusine-B (10-deoxysarpagine; XXXVI). Hence, akuammidine must have the structure and absolute configuration shown in XXXVII, and the only

132

J.

E. SAXTON MeOOC

XXXVI

XXXVII

XXXVIII Vincemejine

XLII Vincamedine

XXXIX

XLIII

I

1 HOC-Hz

OHQ

XL Vowhalotine

/

/

XLV

XLIV /KB€?.

XLVI

7.

ALKALOIDS OF

XLV

T

133

Picrulima nitida

XLVI Polyneuridine LiAlH.

KOBU~

MeOOC __t

LiAlHd

I

Me

XXXVIIa Akuammidine

XLI

features of the molecule which remain to be established are the stereochemistry a t C-16 and the configuration at C-19 (42). The configuration of (3-16 can be deduced by correlation of akuammidine and its derivatives with vincamajine and congeneric alkaloids of Vincu species (43).Vincamajine has the structure XXXVIII, in which the configuration of C-16 is rigidly defined, owing to its presence in an ajmaline-like ring system. Oxidation of vincamajine with chromium trioxide in pyridine gives an indole aldehyde (XXXIX), which on reduction furnishes the hydroxy ester (XL), identical with the alkaloid voachalotine. Comparison of the molecular rotation differences between akuammidine (XXXVIIa) or its 0-acetyl derivative and akuammidinol (XLI) on the one hand with the molecular rotation differences between voachalotine or its 0-acetyl derivative and the related diol (AT,-methyl derivative of XLI) on the other, suggests that the configurations of C-16 in akuammidine and voachalotine, and therefore in vincamajine, are not the same. Hence, voachalotine must be epimeric a t C-16 with N,-methylakuammidine ; unfortunately, this conclusion could not be directly verified, owing to the failure to realize the N,-methyIation of akuammidine. Consequently, the converse route involving N,-demethylation of the vincamajine-voachalotine series was adopted. Vincamedine (XLII), which is the 0-acetyl derivative of vincamajine, on oxidation with chromium trioxide in pyridine, affords a mixture of products from which a small yield of the indolenine (XLIII) can be

134

J. E . SAXTON

isolated. I n alkaline solution the acetate grouping in XLIII is readily hydrolyzed with concomitant fission of the C-7 to (2-17 bond, but the expected aldehyde ester (XLIV) cannot be isolated, since it is readily deformylated to the ester (XLV), which is identical with dehydroxymethylakuammidine. However, reduction of the indolenine derivative (XLIII) with potassium borohydride gives the desired hydroxy ester (XLVI), which is the Aspidosperma alkaloid polyneuridine. Polyneuridine and akuammidine should be epimeric a t C- 16 ;this is confirmed by lithium aluminum hydride reduction of the two bases, which gives the same diol, akuammidinol (XLI) (43, 44). The configuration of C-16 in polyneuridine (XLVI) follows from its interrelation with vincamedine; hence, akuammidine has the configuration a t C-16 shown in XXXVIIa (43). These conclusions concerning the structure and stereochemistry of akuammidine have been verified by the X-ray analysis of akuammidine methiodide (21). This investigation has also established the configuration of the C-19 to C-20 double bond, which is as shown in XXXVIIa. The fragmentation pattern observed in the mass spectrum of akuammidine has also been discussed in relation to the structure XXXVIIa (44,45).

VI. Pseudoakuammigine Pseudoakuammigine, CzzHzsNzO3, crystallizes from ethanol as colorless, square plates, mp 165", [.]go -35" (EtOH), pK, 7.35, and contains methoxyl, methylimino, and C-methyl groups (5, 23, 46). Its UV-spectrum indicates that the molecule is based on an indoline nucleus (23, 46, 47); the IR-spectrum has bands corresponding to a 1,2-disubstituted benzene derivative (754 cm-') and a carbonyl group (1736 cm-l), but there is no absorption corresponding to imino or hydroxyl groups. I n many respects, the behavior of pseudoakuammigine is anomalous, on the assumption that it is an indoline derivative. Thus, its basic strength is closer t o that of strychnine (pK, 7.6) than to that of strychnidine (pK, 8.29). Surprisingly, it does not give all the color reactions exhibited by ajmaline or strychnidine. Nitric acid gives only a brownish-yellow color, instead of the customary deep red; the ferric chloride color is feeble, and appears only on warming; and the base does not couple with diazotized sulfanilic acid, except in dilute, buffered solution, and then very slowly. On the other hand, nitrozation occurs readily, to give a normal green p-nitroso derivative, which can be reduced to a p-aminopseudoakuammigine; this gives the deep red color with nitric acid characteristic of strychnidine (46).

7.

ALKALOIDS OF

Picralima nitida

135

The carbonyl function of pseudoakuammigine is contained in a methyl ester group, since reduction with lithium aluminum hydride gives a primary alcohol, pseudoakuammigol, which behaves in all respects as a typical indoline derivative, except that its ferric chloride reaction is still sluggish. It is noteworthy that the basicity of pseudoakuammigol (pK, 8.22) is almost the same as that of strychnidine. The ester group of the alkaloid is probably attached to quaternary carbon, since it can be recovered almost quantitatively after being refluxed for 7 hours with 10% ethanolic potassium hydroxide (46). I n the earlier work of Robinson and Thomas (46), it was found that neither pseudoakuammigine nor pseudoakuammigol could be smoothly hydrogenated, and the presence of a double bond could therefore not be rigorously established. A similar situation was encountered with the closely related alkaloid akuammine ; however, some (inconclusive) evidence for hydrogenation of this base was obtained, so it was assumed that the molecule contained a double bond. Owing to the close similarity of the IR-spectra of akuammine and pseudoakuammigine (except for the hydroxyl band in the spectrum of akuammine), the presence of a double bond in pseudoakuammigine was also assumed, and it was therefore tentatively formulated as a deoxyakuammine, i.e., XLVII, R = H (46). As expected from this formulation, pseudoakuarnmigine also yields 3-ethylpyridine on distillation with zinc dust (31). The unexpected deactivation of the aromatic position para t o N,, as testified by the anomalous color reactions and the reduced basicity of pseudoakuammigine, were explained by assuming that the proximity of the ester group to the indoline nitrogen atom resulted in deactivation of the potentially basic center across space by the carbonyl group (46). COOMe

Me Me00

XLVIII

XLVII R

=

H ; Pseudoakuammigine R = OH;Akuammine

The recent experimental work of Janot and Smith, and their respective collaborators, has demonstrated the inadequacy of structure XLVII (R = H) to explain the behavior of pseudoakuammigine; consequently, on the basis of their results, the French workers have proposed the constitution XLVIII (R = H ) (48). The reason for the apparently

136

J. E. SAXTON

anomalous color reactions of pseudoakuammigine becomes evident from a comparison of its UV-spectrum in neutral and strongly acid ( 5 N ) solution; in neutral solution, the spectrum is that of a n indoline base, but in acid solution the spectrum is that of the 3H-indolium ion. This behavior, together with the weaker basicity of pseudoakuammigine than is normal for an indoline base, are best explained by the participation of N, in a carbinolamine ether function. Since pseudoakuammigine can be recovered on basification of its solution in 5 N acid, the carbinolamine ether oxygen must be contained in a readily reformed ring, which is therefore likely to be five- or six-membered (48,49). In accordance with the structure XLVIII (R = H ) , reduction of pseudoakuammigine by zinc amalgam and sulfuric acid, or by Raney nickel in boiling dioxan, results in the addition of two hydrogen atoms, with liberation of an acetylatable hydroxyl group, which is strongly hydrogen bonded, presumably to the carbomethoxy group. Unexpectedly, however, the product, a dihydropseudoakuammigine, exhibits UV-spectra in neutral and in acid solution which are characteristic of the geminal dianiino system present in eserine-andfolicanthine. Accordingly, dihydropseudoakuammigine is formulated as LI, and is presumably formed from the pseudoakuammigine ammonium ion (XLIX) by hydrogenolysis of the C-3 to Nb bond, followed by closure of the fivemembered ring in the intermediate (L). Reduction of L I with lithium aluminum hydride gives a dihydropseudoakuammiginol, C Z I H Z ~ N Zby O~, conversion of the carbomethoxy group into a primary alcohol function. The product does not have the spectrographic properties of a base of the eserine type, but instead shows a small bathochromic shift in acid solution similar to, but less pronounced than, that shown by pseudoakuammigine.

XLIX

;i.-$ ._

CHMe

L

7.

ALKALOIDS OF

Picralima nitida

137

The postulated presence in dihydropseudoakuammigine (LI) of a /3-hydroxy-a,a-disubstituted propionic ester grouping, analogous to that in echitamine, finds confirmation in its reaction with potassium tbutoxide in benzene, which results in retroaldol loss of formaldehyde, with formation of dehydroxymethyldihydropseudoakuammigine(LII). The parent alkaloid and dihydropseudoakuammiginol, which do not contain the P-hydroxypropionic ester grouping, do not eliminate formaldehyde under comparable conditions. Reduction of dehydroxymethyldihydropseudoakuammigine with lithium aluminum hydride gives the

aT&

0 7 % Me

Me

H

CHMc

LII

q LIV

LIII

‘CHM~

Et

related alcohol, which can be converted into the corresponding aldehyde by a modified Oppenauer oxidation, and thence by Wolff-Kishner reduction into dehydroxymethyldihydropseudoakuammiginane(LIII). All the bases in this series, from dihydropseudoakuammigine (LI) to the final product (LIII), exhibit UV-spectra consistent with their formulation as bases of the eserine type. The quantitative hydrogenation of LIII, and the results of Kuhn-Roth oxidation, confirm the presence in these transformation products, and therefore in pseudoakuammigine itself, of an ethylidene grouping (48). The preceding degradations and transformations of pseudoakuammigine are in consonance with, but do not prove, the structures proposed; it is therefore noteworthy that the mass spectrum of LIV, the hydrogenation product of LIII, is in complete accord with this geininal diamino structure (48). I n an independent investigation Smith and his collaborators have provided confirmation of the presence in pseudoakuammigine of some of the features contained in the structure XLVIII (R = H), and have also

138

J. E. SAXTON

contributed a fascinating study of the rearrangement of pseudoakuammigine under the influence of strong acid (49, 50, 51). The presence of an ethylidene grouping in pseudoakuammigine is confirmed by the production of acetaldehyde on ozonolysis, and by the NMR-spectrum; the N,-methyl group is also identifiable in the NMR-spectra of pseudoalcuammigine and all its derivatives (49). The presence of an N,-methyl tetrahydro-P-carboline system in pseudoakuammigine is proved by the mass spectrum of LIVa, the lithium aluminum hydride reduction product of Nb-methylpseudoakuammigine dihydromethine, since the second most abundant ion in the spectrum corresponds to the fragment LV (50).

Me

LV

LVI

The lithium aluminum hydride reduction of pseudoakuammigine furnishes an indoline base containing two hydroxyl groups; the carbinolamine ether function and the ester group have therefore been reduced, and the product, pseudoakuammigol (LVI; R = H), earlier formulated as C21H~GN202,is in fact C21H28N202. The two hydroxyl groups in this molecule are situated on alternate carbon atoms since pseudoakuammigol is unaffected by periodic acid but gives a crystalline isopropylidene derivative. Neopseudoakuammigol, the base in which the ester function in pseudoakuammigine is replaced by a primary alcohol group, can be obtained by reduction of the alkaloid with sodium and ethanol (49). The structure LVI (R = H) for pseudoakuammigol receives convincing confirmation from the fragmentation patterns observed in the mass spectra of pseudoakuammigol and its deuterated derivatives LVIa and LVIb ; the structure (XLVIII ; R = H ) for pseudoakuammigine itself can therefore be regarded as firmly established. Since these mass spectra,

7.

ALKALOIDS OF

Picralima nitida

139

when compared with the mass spectra of the alkaloid picraline and its lithium aluminum hydride (or deuteride) reduction products, also provide valuable evidence relating to the structures of the picraline series of bases, they will be discussed in detail later (see Section V I I I ) (50, 51, 51a, 5 3 , 54). HOHzC , ,CX20H

LVIa; X = D , Y = H LVIh; X = Y = D

The close structural relationship between akuammine and pseudoakuammigine, first suggested by Robinson and Thomas, has been confirmed by reduction of both 0-tosylakuammine and pseudoakuammigine by Raney nickel and hydrogen to dihydroisopseudoakuammigine, C22H~gN203,mp 264"-267". This base is almost certainly identical with Janot's dihydropseudoakuammigine (LI),mp 252", since it contains the same functional groups and exhibits the characteristic UV-spectra in neutral and acid solution of the eserine-like bases (49). The most important aspect of the chemistry of pseudoakuammigine which does not find a ready explanation in terms of the structure XLVIII (R = H) concerns the degradation to apopseudoakuammigine and the further transformations of this product. Apopseudoakuammigine itself is formed by prolonged treatment of pseudoakuammigine with hot mineral acid, and differs from its progenitor by the elements of a methylene group. Its IR-spectrum shows the presence of a y-lactone in which the carbonyl frequency (1746 cni-1) is lowered by hydrogen bonding, presumably with a hydroxyl group, since the 0-acetate exhibits normal carbonyl frequencies, a t 1773 and 1747 cm-1. Since the carbinolamine ether oxygen is situated /3 to the ester grouping, it cannot be involved in lactone formation, and must appear in apopseudoakuammigine as the free hydroxyl group ; apopseudoakuammigine therefore probably contains the part-structure LVIIa. The presence of this chromophore is confirmed by the UV-spectrum of apopseudoakuammigine, which is very similar to that of pseudoakuammigine, except that it is displaced slightly toward longer wavelengths ; however, the spectra of the corresponding ammonium ions (i.e. in concentrated acid) are identical. The formation of LVIIa from the proposed structure of pseudoakuammigine (XLVIII; R = H ) is clearly not a simple reaction, owing

140

J. E. SAXTON

to the steric impossibility of forming a lactone ring a t C-2 in this ring system. This, and other important features of the molecule, suggest very forcibly that a profound rearrangement occurs during the conversion of pseudoakuammigine into the apo base. Thus, apopseudoakuammigine gives no trace of pseudoakuammigol or stereoisomeric indoline base on reduction with lithium aluminum hydride ; instead, a mixture of indole derivatives is obtained. A further difficulty concerns the basicity of apopseudoakuammigine (pK, 5.6), which is a significantly weaker base than pseudoakuammigine. This is ascribed to the proximity of the carbon of the carbonyl group to N,, which results in a base-weakening field effect.

\. ./ 4

LVII~

LVIIb

LVIIId

LVIII~

LVIIIC

LVIIIb

7.

ALKALOIDS OF

Picralima nitida

141

I n contrast to pseudoakuammigine, which is not affected by alkali even under fairly vigorous conditions, apopseudoakuammigine is degraded a t room temperature, with the appearance of indole UVabsorption. With hot alkali, apopseudoakuammigine is converted in good yield into an indole base, C20H24N20 ; this change corresponds to the loss of carbon dioxide. The single oxygen atom in this product is contained in an aldehyde group (IR-absorption at 1726 and 2705 cm-I), since it gives an oxime which, on dehydration with acetic anhydride, gives a product exhibiting nitrile IR-absorption. These results can be explained by the reverse Mannich decomposition of a base (LVIIb) in which Nb is separated from the beta position of an indoline system by only one carbon atom. The product (LVIIIa) can then undergo an internal hydride shift with formation of a ,B-aldehydo acid (LVIIIb); decarboxylation to the indole aldehyde base (LVIIIc) then follows naturally. A related, but simpler, reaction, is the potassium borohydride reduction of apopseudoakuammigine to an indolic amino acid, which may be formulated as LVIIId. The unshared electrons on Nbare evidently involved in this reaction, since apopseudoakuammigine methiodide gives an indoline betaine, and not an indole derivative, on reduction with borohydride (49). CHz 'YHM~

LIXe

LIXC

The presence of an allylamine system in pseudoakuammigine, postulated in XLVIII, is confirmed by the platinum-catalyzed hydrogenolysis of pseudoakuammigine methiodide, which affords a dihydromethine base: C23H30N203, by fission of the C-21 to Nb bond; the apo base methiodide similarly yields a dihydromethine. Both bases still contain the ethylidene group, and contain one additional C-methyl group.

142

J . E. SAXTON

Hence, the part-structure LVIIb for apopseudoakuammigine may be expanded to LIXa. Whereas pseudoakuammigine dihydromethine, like the alkaloid itself, is unaffected by potassium borohydride, apopseudoakuammigine dihydromethine is reduced to an indolic amino acid. The latter was not isolated pure, but was converted, by reaction with methanolic hydrogen chloride, into a crystalline indolic hydroxy-y-lactone, which exhibits carbonyl absorption a t 1761 and 1726 cm-1. The formation of such a lactone can only occur if the double bond is involved; this, therefore, implies that the double bond must be situated in the P,y-or y,a-position with respect to the carbonyl group. The part-structure LIXa for apopsuedoakuammigine can therefore be expanded to the alternatives LIXb and LIXc. Since the NMR-spectrum of the lactone reveals that it contains a methyl and an ethyl group attached to quaternary carbon, the C-20 carbon atom must be the y-carbon atom with respect to the carbonyl function in both apopseudoakuammigine and the indolic y-lactone. The latter must therefore contain the part-structure LIXd and apopseudoakuammigine the part-structure LIXc (49, 50). One of the principal characteristics of apopseudoakuammigine is the ease with which it can be degraded to indolic products ; in this respect its behavior can be contrasted sharply with that of pseudoakuammigine. The part-structure LIXc explains this feature very neatly, and it is thus apparent that a complex rearrangement must have occurred during its formation from pseudoakuammigine. Although the complete structure of apopseudoakuammigine is not known with certainty, theconstitution LXII (R = H) has been tentatively proposed, and is consistent with all the experimental data a t present available (50). This structure can be obtained by acid treatment of pseudoakuammigine (XLVIII) by protonation to the corresponding ammonium ion (XLIX), followed by a double migration of the substituents at positions 2 and 7. Migration of the a substituent (C-6) at C-7 to C-2 yields the spirocyclic carbonium ion (LX); this is now capable of conversion by migration of the ,l3 substituent (C-3) at C-2 into an ammonium ion (LXI) with stereochemical inversion at C-7. Formation of the lactone ring at C-2 then yields apopseudoakuammigine (LXII; R =H). On the basis of this structure, most of the reactions of apopseudoakuammigine can be satisfactorily explained. The reduction with potassium borohydride can be accounted for, as postulated by Joule and Smith, by the reverse Mannich decomposition of LXII, followed by reduction of the intermediate (LXIII); the product of this reaction would then be the indolic amino acid LXIV, a structure which is consistent with the observation that this amino acid is extremely resistant to esteri-

7.

ALKALOIDS OF

Picralima nitida

143

fication. The mechanism by which LXIV is formed accords with the reported failure of apopseudoakuammigine methiodide to give an analogous product with potassium borohydride.

Pseudoakuammigine

COOMe

Me

Me

LX

LXI l4

19

18

N ’

LXII R = H ; Apopseudoakuammigine R = OMe; 0-Methylapoakuammine

The degradation with alkali, following the reaction path outlined earlier (part-structures LVIIb --f LVIITa+ b +c) would result in the formation of the aldehyde LXVI. The production of this compound can be explained very convincingly by reverse Mannich decomposition of apopseudoakuammigine to the intermediate zwitter ion LXIII. The alternative conformation of this molecule is one (LXV) in which the (2-17 hydrogen is situated in close proximity to the trigonal C-3; hydride transfer, as postulated by Joule and Smith, would then be expected to

144

J. E. SAXTON

&

Mc

LXIII

LXV

CH,OH

YHO

&HMe

Me

LXIV

HOCHz

& C. Me

MeLXVIII

Me

LXVI

m

CHzCHO

CH=CHz Me LXVII

HOCHa HOCHz

Me LXTX

Mt?

LXX

proceed readily, to give the aldehyde (LXVI) (50, 51). This structure is consistent with the properties of this aldehyde, which affords, on zinc dust distillation, a mixture of 3-ethylpyridine and N-methylcarbazole ;

7.

ALKALOIDS OF

Picralima nitida

145

the latter is considered to be formed by decomposition of LXVI into 3-ethylpyridine and (possibly) the aldehyde LXVII, which then undergoes cyclization and aromatization (compare the thermal decomposition of akuammicine to XXXV, via the unsaturated ester XXXIV) (50, 51). Finally, the indolic y-lactone, obtained from apopseudoakuammigine dihydromethine (LXVIII) by borohydride reduction followed by treatment with methanolic hydrogen chloride, is probably LXX ; the intermediate noncrystalline acid is presumably LXIX.

VII. Akuammine Akuammine, C22H26N204, mp 254"-259" (dec.), pK, 7.5, [a]= -66.7' (ethanol), is the principal base in Picralima nitida, and occurs to the extent of 0.56% in the seeds. The molecule contains methoxyl, methylimino, C-methyl, and hydroxyl groups, but no aldehydic or ketonic carbonyl groups (1, 5, 23); it is a tertiary base, since its methiodide is a true quaternary salt (1).No evidence is available for the presence of an imino group ; benzoylation and acetylation produce 0-acyl derivatives (5). Akuanimine is soluble in alkali, which converts it into "akuammine hydrate," an alkali-soluble, microcrystalline substance which does not melt below 310", and brown, amorphous by-products, similarly soluble in alkali (1). Akuammine is also converted into intractable, resinous materials by boiling dilute hydrochloric acid (5))and it has even been known to decompose during attempted recrystallization from boiling methanol (23). The color reaction of akuammine with nitric acid is blood-red, and treatment with nitrous acid yields a scarlet, crystalline product, which does not give Liebermann's nitrosamine reaction and which is very probably a nitroakuammine hydrochloride (5). The UV-spectrum of the alkaloid is typical of indoline bases (23, 47)) which, combined with the evidence from color reactions, was initially interpreted as indicating that akuammine is a methoxylated indoline base. However, a detailed examination of its coIor reactions reveals that akuammine is a 5hydroxydihydroindole derivative, since it behaves in an exactly analogous manner to p-methylaminophenol ; its sensitivity to alkali is therefore readily understandable. The position of the hydroxyl group is confirmed by the IR-spectrum (band at 811 cm-', characteristic of 1,2,4-trisubstituted benzenes) (23), and by comparison of its UVspectrum with that of 6-methoxy-9,11-dimethylhexahydrocarbazole (LXXI). The spectra of the methoxyl isomers of LXXI are significantly different (52).

146

J. E. SAXTON

Zinc dust distillation of the amorphous material obtained when akuamrnine decomposed in methanol solution gave an indolaceous substance (probably skatole), a volatile base identified as 3-ethylpyridine (23), and carbazole (31). Hydrogenation studies were inconclusive, and although evidence for the formation of a dihydro derivative was obtained, this was not fully characterized (23).

MeooTl ""&p I

Me

'

N ' Me

LXXI

Me

1

-O/

\-Me

MeOOC

LXXII

Of the three remaining oxygen atoms, two are present as an ester group (IR-band at 1736 cm-I), and the third, inert, oxygen is probably contained in an ether linkage. Taking cognizance of the fact that the oxidation color reactions and behavior of akuammine are characteristic of substances of type LXXI, and quite different from those of 2,3disubstituted indolines which readily suffer dehydrogenation to indole derivatives, akuammine must belong to the 8-series rather than the a-series of indole alkaloids. The constitution LXXII was proposed to explain all the available evidence; the double bond was provisionally located in the position adjacent to the methyl group, to account for the positive iodoform reaction (23). However, akuammine is not an enol ether, since its IR-spectrum does not exhibit an absorption band a t 1650 cm-1; the (highly strained) formula XLVII (R = OH) was therefore preferred (46). Much of the evidence obtained recently that has a bearing on the structure of akuammine has been derived from the degradation of the closely related alkaloid pseudoakuammigine, and has been discussed above. Robinson and Thomas (46) noted the very close similarity of the IR-spectra of akuammine and pseudoakuammigine, except for the hydroxyl band in the spectrum of akuammine ;they therefore formulated pseudoakuammigine as a deoxyakuammine. The correctness of this deduction has recently been demonstrated by Joule and Smith (49)' who converted 0-tosylakuammine and pseudoakuammigine into the same derivative, dihydroisopseudoakuammigine (LI ; Janot's dihydropseudoakuammigine), by boiling with Raney nickel in ethanol solution in an atmosphere of hydrogen. This close relationship is also confirmed by the similarity of the fragmentation patterns observed in the mass spectra of derivatives of 0-methylakuammine and pseudoakuammigine

7. ALKALOIDS

OF

Picralima nitida

147

(vide infra) (49). The structure proposed by Janot and his collaborators (48)for pseudoakuammigine is XLVIII (R = H) ; accordingly, akuammine is XLVIII (R = OH). The limited amount of experimental work that has been carried out recently on akuammine shows that its behavior is exactly analogous to that of pseudoakuammigine (49). Thus, the UV-spectrum of 0-methylakuammine in neutral and in dilute acid solution is typical of an indoline derivative ; a marked bathochromic shift is observed in concentrated hydrochloric acid, and the spectrum is now characteristic of the 3-Hindolium ion. The recovery of 0-methylakuammine from this solution shows that no rearrangement of the molecule has occurred, and suggests further that the readily reformed carbinolamine ether ring is five- or six-membered. I n accordance with the structure XLVIII (R = OMe), 0-methylakuammine is reduced by lithium aluminum hydride to 0-methylakuamminol (LVI; R = OMe), an indoline base which contains two alcoholic hydroxyl groups, and which, like pseudoakuammigol (LVI ; R = H), gives rise to an (amorphous) isopropylidene derivative. Although 0-methylakuammine (XLVIII; R = OMe) is stable to mineral acid under mild conditions, prolonged treatment with 3 N hydrochloric acid at 80" gives 0-methylapoakuammine by loss of the elements of a methylene group. The IR-spectrum of this product parallels that of apopseudoakuammigine in that it indicates the presence of a y-lactone grouping in which the carbonyl frequency (1756 cm-1) is lowered by hydrogen bonding with a hydroxyl group, since the carbonyl bands appear at the expected frequencies in the corresponding 0-acetate. By analogy with the structure proposed for apopseudoakuammigine (LXII ; R = H), 0-methylapoakuammine may be provisionally formulated as LXII, R = OMe (49, 50).

VIII. Picraline Picraline was first isolated by Thomas (15a) from the crude alkaloidal fraction of Picralima seeds by chromatography on alumina. Very recently its isolation has been reported by other investigators, and the molecule has been thoroughly characterized (15, 51a, 53, 54). Picraline, [a]=-124" (MeOH),pK, 5.65 (50% aqueous C~~HZ~N mp Z 180"-182", O~, ethanol), contains one methoxyl group, one active hydrogen, and two C-methyl groups. Its UV-spectrum exhibits maxima a t 231 and 289 mp and is unaffected by the addition of dilute acid or alkali ; in concentrated perchloric acid, however, a reversible change occurs, with the appearance

148

J . E. SAXTON

of maxima a t 241, 246, and 310 mp, characteristic of the protonated indolenine system. This behavior is clearly reminiscent of the carbinolamine ether grouping present in pseudoakuammigine. The IR-spectrum of picraline contains bands due to NH or OH (3400 cm-I), ester (1724 cm-I), and acetoxyl (1695 and 1250 cm-') groups, a double bond (1613 cm-l), and an ortho-disubstituted benzene nucleus ( - 750 cm-1); the ester group can be identified from the NMR-spectrum as a carbomethoxyl group. The presence of an acetoxyl group is also supported by the NMRspectrum, and confirmed by the formation of deacetylpicraline, C21H24N204, by acid hydrolysis of picraline. It is noteworthy that deacetylpicraline is also a constituent of Picralima seeds. The second C-methyl group and the double bond in picraline are present in an ethylidene group, since ozonolysis yields acetaldehyde ; the NMRspectrum also indicates clearly the presence of this group (15,51a, 53,54). Both picraline and deacetylpicraline react with aqueous alcoholic potassium hydroxide at 80" to give picrinine, CzoHzzNz03, which is formed by loss of the elements of formaldehyde from deacetylpicraline. Since picrinine still contains a carbomethoxyl group (IR- and NMRspectra) this reaction is very probably the retroaldol cleavage of formaldehyde from a 8-hydroxypropionic ester group, such as occurs in echitamine and akuammidine ; however, in the present instance, this reaction proceeds with unexpected ease. The third oxygen atom in picrinine must be present in an ether function, since the IR-spectrum affords no evidence for the presence of either a hydroxyl or a second carbonyl group. Since the UV-spectra of picrinine in neutral and in concentrated perchloric acid solution are very similar to those of picraline, this ether oxygen must be attached to the indoline a-position. The presence of the ,&hydroxypropionic ester unit in deacetylpicraline is established by oxidation with chromic acid in acetone, which yields an aldehyde base, picralinal, CzlHzzNz04;the latter is readily deformylated by short treatment with methanolic potassium hydroxide, which affords picrinine in quantitative yield. Reduction of picralinal with sodium borohydride regenerates deacetylpicraline. Vigorous treatment of deacetylpicraline with sodium borohydride gives a noncrystalline indoline base, which exhibits the UV-absorption of an anilinium ion in concentrated perchloric acid ; hence, the N,-carbinolamine ether function must have suffered reduction. Since acetylation of the noncrystalline base gives a product which exhibits acylaniline UV-absorption, picraline and its derivatives must contain an N,H group (53, 54).

'

1 Deacetylpicraline (burnamine) also occurs in the bark of Hunteria eburnea Pichon (56).

7.

ALKALOIDS OF

Picralima nitida

149

Reduction of picraline or deacetylpicraline with lithium aluminum hydride gives picralinol, C2,,H2,N202 (15, 51a), which, as expected from the reduction of a p-hydroxypropionic ester residue, behaves as a 1,3-diol since i t gives rise t o an isopropylidene derivative (53, 54). The UV-spectrum of picralinol is that of an indoline base, changed to anilinium ion in concentrated acid, which indicates that the Na-carbinolamine ether function has been reduced. The total loss of this oxygen atom suggests that its other point of attachment in picraline is also a carbon atom adjacent t o nitrogen, which can only be Nb;picraline must therefore be a biscarbinolamine ether. This deduction is supported by the comparatively weak basicity of picraline and the increased basicity (pK, 8.15) of picralinol (53, 54). The structural features present in picraline are, therefore, given by the part-structure LXXIII. Assuming a close structural relationship with pseudoakuammigine (XLVIII ; R = H), which is evident from the similarity of the IR-spectra of picralinol and pseudoakuammigol (LVI; R =H), this can be expanded to the complete structure (LXXIV) for picraline. The carbinolamine ether oxygen atom is attached to C-2 from the UV-evidence; the other point of attachment, adjacent to Nb, cannot be C-21, since this is sterically prohibited. Attachment at C-5 is favored, since the NMR-spectra of picraline, deacetylpicraline, and picrinine exhibit a signal (51, 53, 54) at -5.20 T , characteristic of a carbinolamine hydrogen : >N-CH-0-.

OTJo>dN<

HOCH2,

,COOMe C

/ \

H

LXXIII 19

I8

CHMe

a

LXXIV Pioralme

b

+ cn

TABLE I

0

MASSSPECTRA OF PICRALINOL AND RELATED COMPOUNDS;M/e VALUESOF PRINCIPAL PEAKS MQ- CHzOH or

Compound Picralinol (LXXV) Tetradeuteropicralinol

(LXXVII)

M"

MQ- O H MQ-CDzOH LXXVIII" LXXIX

LXXIX - HzO

LXXIX - CHzO or

- CDzO LXXX" LXXXI' LXXXII'

326

309

295

25 1

196

178

166

144

143

130

330

313

297 299

253

199

181

167 169

146

145

131

Pseudoakuammigol (LVI; R = H)

340

323

309

265

196

178

166

158

157

144

Trideuteropseudoakuammigol (LVIa)

343

326

310 312

266

198

180

166 168

59

158

145

Tetradeuteropseudoakuammigol (LVIb)

344

327

311 313

267

199

181

167 169

60

159

145

0-Methylakuamminol (LVI; R = OMe)

370

353

295

196

178

166

188

a

M

+ s G ra

174

Picralinol series: R = R' = H ; pseudoakuammigol series: R = Me,R' = H; 0-methylakuamminol series: R = Me, R' = OMe.

w

7.

ALKALOIDS OF

Picralima nitida

151

On the basis of this structure (LXXIV) for picraline, picralinol must be the diol LXXV. The latter differs from the proposed structure for pseudoakuammigol (LVI; R = H) only in that it possesses a hydrogen atom attached to N, instead of a methyl group ; accordingly, N,-methylation of picralinol should yield pseudoakuammigol. This methylation has been realized by the lithium aluminum hydride reduction of the triformyl derivative (LXXVI) of picralinol, which affords pseudoakuammigol directly (51a).

LXXV

LXXVII

LXXVI

The structures proposed for picralinol (LXXV) and pseudoakuammigol (LVI ; R = H),and also for 0-methylakuamminol (LVI; R = OMe), receive impressive support from a study of their mass spectra, together with those of tetradeuteropicralinol [the lithium aluminum deuteride reduction product (LXXVII) of picraline], trideuteropseudoakuammigol (LVIa), and tetradeuteropseudoakuammigol (LVIb). The fragmentation processes exhibited by these bases on electron impact are summarized in Table I, which also indicates possible structural assignments for the various fragments (50, 51, 51a, 53, 54). The deuterated positions in the fragments derived from LVIa, LVIb, and LXXVII are indicated by asterisks in the formulas LXXVIII-LXXXII. These data leave little room for doubt that picralinol has the constitution LXXV; the structure for picraline, however, is not so well established. Smith’s view that picraline is best represented by LXXIV is convincingly supported by the UV-evidence, but it is not accepted by other workers. The typically indoline spectrum of picraline undergoes a bathochromic shift in concentrated acid, and deacetylpicraline can be recovered on basification. This observation would seem to exclude the

152

J. E. SAXTON

possibility of the presence of a methoxyl group at C-2. Further, it may be deduced that the oxygen atom attached to C-2 must be so situated in the ring system that reformation of the C-2 to oxygen linkage is a very facile process. The structure LXXIV accords with this behavior, and is also consistent with the NMR-spectrum (51, 5 3 , 54).

LXXVIII

LXXIX

LXXXI

LXXX

LXXXII

A different view has been expressed by Janot, Djerassi, and their collaborators (51a),who reject the structure LXXIV on the grounds that an ether bridge between C-2 and C-5 is sterically impossible. However, the diagram LXXIVb was drawn from a Dreiding model of this structure, which would appear, as far as it is pessible to judge from the use of models, not to be severely strained. In lieu of LXXIV, Janot, Djerassi, and their co-workers consider the possibilities for picraline based on the structure LXXV for picralinol. The mass spectrographic evidence indicates that in picraline oxygenated substituents are attached to C-2, C-5, (2-17, and C-22, i.e., picraline has the partial formula LXXXIII. The various possibilities for picraline are consequently given by the structures LXXXIV-LXXXVI ; a fourth structure containing a ylactone function can be ignored, since it is eliminated by the IR-evidence. The first of these structures (LXXXIV) was eliminated by Janot and co-workers, who did not observe a bathochroniic shift of the UVspectrum of picraline in strong acid; a structure analogous to that of pseudoakuammigine thus appeared to be very improbable. The constitution LXXXV was similarly rejected since the UV-spectrum of deacetylpicraline was also reported not to exhibit a bathochromic shift

7.

ALKALOIDS OF

Picralima nitida

153

in acid solution; evidently, the presence of a hydroxyl group at C-2 would be expected to result in the ready formation of an indolenine derivative in acid solution. The remaining structure (LXXXVI) is supported, as the others are invalidated, by the mass spectrum of picraline, which shows a peak at M+ - 73, owing to loss of the acetoxymethyl group. In contrast, the mass spectrum of deacetylpicraline does not contain this peak, but instead exhibits one at M+ - 31, resulting from loss of the hydroxymethyl group. Hence, it was concluded that pieraline has the structure LXXXVI ; the methoxyl group at C-2 was tentatively assigned the cc-configuration,since this resulted in the least steric strain (51a).

* LXXXIII

H

co I kH3 LXXXV

CHMe

c~

LXXXIV

0

H

bfi’ Me

H

%HMe

LXXXVI

The two structures which have been firmly proposed for picraline are, therefore, LXXIV and LXXXVI. Both proposals account satisfactorily for the chemical reactions discussed above, and are consistent with its IR- and NMR-spectra; hence, a firm distinction between the two structures cannot be made on the basis of these data. The correction of the contradictory reports concerning the UV-absorption of picraline in acid solution would not materially assist the arguments. Both LXXIV and LXXXVI should exhibit indoline absorption in neutral and dilute acid solution, and both would be expected to show 3H-indolium ion absorption in the presence of concentrated acid. However, the recovery of deacetylpicraline from the solution of picraline or deacetylpicraline in

154

J. E. SAXTON

concentrated acid is a vital observation which is only consistent with the structure LXXIV, and effectively excludes the structure LXXXVI (55). Further evidence in support of LXXIV is provided by the mass spectrum of picraline, which, in addition to the peak at M+ - 73 (M/e 337), owing to loss of the acetoxymethyl group, contains two further prominent peaks, at M/e 351 (Mf - 59) and M/e 239 (M+ - 171). These are due, respectively, to loss of (a)the acetoxyl or carbomethoxyl group, and (b) C-16 and its substituent groupings (CH3COOCH,-C-COOMe) together with CO derived from C-5. There is no peak owing to loss of COz, and the peak owing to loss of the methoxyl group, at M+ - 31, is only very weak ;these characteristics of the mass spectrum would indeed be surprising if LXXXVI were correct. That the peak at M/e 351 is the result of loss of a carbomethoxyl group, and not an acetoxyl group, is evident from the mass spectrum of picrinine (LXXIV, with H in place of CHsCOOCHzat C-l6), which also exhibits a peak at M+ - 59, owing to loss of the carbomethoxyl group. Here the loss of an acetoxyl group does not come into consideration (55). The sodium borohydride reduction of picraline furnishes a (noncrystalline) base which exhibits typically indoline UV-absorption, and which must therefore arise by reduction of the N,-carbinolamine ether linkage. Since this product still contains amethoxyl group, observed in the NMR-spectrum as a singlet at 6.287, picraline does not possess a methoxyl group at C-2, and can therefore not have the structure LXXXVI. All these data, however, are consistent with the structure LXXIV (55) In addition to the formation of deacetylpicraline, the acid hydrolysis of picraline yields a yellow base, flavopicraline, CzoH,oNzOs,which exhibits UV-absorption at 245 and 390 m p , with a shoulder at 305-320 mp. In acid solution the long-wavelength maximum is shifted to 438 mp. The IR-spectrum of flavopicraline exhibits carbonyl absorption at 1761 cm-l, characteristic of a y-lactone, while the NMR spectrum discloses the presence of ethyl and CH=CH-N groups; there is no evidence for the presence of an ethylidene group. Sodium borohydride reduction affords a colorless indole base (UV-spectrum) which still contains the presumed y-lactone grouping (IR-absorption at 1760 cm-l) (15, 51, 53). The structure proposed for flavopicraline (LXXXVII) can be derived from deacetylpicraline by a mechanism which is formally analogous to that postulated for the conversion of pseudoakuammigine (XLVIII ; R = H ) into apopseudoakuammigine (LXII ; R = H) ; the closure of the lactone ring at C-20 implies that the stereochemistry at C-16 in picraline is the same as that in pseudoakuammigine (55) Finally the colorless indole base obtained on sodium borohydride

7.

ALKALOIDS OF

Picralima nitida

155

reduction of flavopicraline may provisionally be formulated as the indolic y-lactone (LXXXVIII). 19.18

Et

1

2 0 b '

LXXXVII

LXXXVIII

Flavopicreline

IX. Akuammiline Akuammiline, CzzH24Nz04, forms translucent prisms from ethanol, mp 160°, [u]$'" +47. 9 (EtOH), and contains methoxyl and two Cmethyl groups (5, 23). Its UV-spectrum is similar to that of 3,3-dimethylindolenine, but shows a small shift to longer wavelengths (46). The IR-absorption discloses the presence of hydroxyl or imino groups (3450 ern-I), an unconjugated ester (1736 cm-l), and possibly an orthodisubstituted benzene nucleus. The base gives a characteristic crimson Otto reaction, and a possible relationship to akuammigine has been suggested (23). (See note added in proof, p. 157.)

X. Akuammenine Akuammenine, CzoHzzNz04, is the least abundant alkaloid of this group, and is contained in the seeds to the extent of only 0.0006~0.As yet it has only been obtained as its scarlet picrate, mp 225", and no information regarding its constitution is available beyond the fact that it contains a methoxyl group ( 5 ) . REFERENCES 1. 2. 3. 4. 5. 6.

T. A. Henry and T. M. Sharp, J. Ghem. SOC.p. 1950 (1927). Sir Robert Robinson and A. F. Thomas, J. Chem. Soe. p. 3479 (1954). E. Clinquart, Bull. Acad. Roy. &fed. Belg. [5]6,492 (1926); Chem. dbstr. 21,151 (1927). E. Clinquart, J . Phann. Belg. 9, 187 (1927); Chem. Abstr. 22, 136 (1928). T. A. Henry, J . Chem. SOC.p. 2759 (1932). J. A. Goodson, T. A. Henry, and J. W. S. MacFie, Biochem. J . 24, 874 (1930).

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J. E. SAXTON

7. Raymond-Hamet, Compt. Rend. Acad. Sci. 211, 125 (1940). 8. Raymond-Hamet, Arch. Exptl. Pathol. Phurmakol. 199, 399 (1942); Chem. Abstr. 37, 5782 (1943). 9. Raymond-Kamet, Compt. Rend. SOC.Biol. 137, 404 (1943). 10. Raymond-Hamet, Compt. Rend. Soc. Biol. 138, 199 (1944). 11. Raymond-Hamet, Rev. Intern. Botan. Appl. Agr. Trop. 31, 465 (1951). 12. Raymond-Hamet, Compt. Rend. SOC.Biol. 148, 458 (1964). 13. Raymond-Hamet, Compt. Rend. Scad. Sci. 255, 1482 (1962). 14. Raymond-Hamet, Compt. Rend. Acad. Sci.221, 699 (1945). 15. L. Olivier, J. LQvy,J. Le Men, and M.-M Janot, Ann. Pharm. Franc. 20, 361 (1962). 18a. A. F. Thomas, D. Phil. Thesis, Oxford Univ. 1954; and Personal communication (1963). 16. M.-M. Janot and J. Le Men, Compt. Rend. Acad. Sci. 238, 2550 (1954). 17. M.-M. Janot and J . Le Men, Compt. Rend. Acad. Sci. 240, 909 (1955). 18. M.-M. Janot, J. Le Men, K. Aghoramurthy, and Sir Robert Robinson, Ezperientia 11, 343 (1955). 19. M.-M. Janot and J . Le Ken, Contpt. Rend. Acad. Sci. 243, 1789 (1956). 20. A. Chatterjee, C. R. Ghosal, N. Adityachaudhury, and S. Ghosal, C'hem. I n d . (London) p. 1034 (1961). 21. S. Silvers and A. Tulinslry, Tetrahedron Letters p. 339 (1962); Acta Cryst. 16, 579 (1963). 22. J. Gosset, J . Le Men, and M.-M. Janot, Ann. Pharm. Franc. 20, 448 (1962). 22a. M. F. Bartlett, B. Korzun, R. Sklar, A. F. Smith, and W. I. Taylor, J . Org. Chem. 28, 1445 (1963). 23. M. F. Millson, Sir Robert Robinson, and A. F. Thomas, Experienlia 9, 89 (1953). 24. Raymond-Hamet, Compt. Rend. Acad. Sci.233,560 (1951). 25. E. Wenkert and D. K. Roychaudhuri, J . Am. Chem. SOC.78, 6417 (1956). 26. E. Wenkert and D. K. Roychaudhuri, J . Am. Chem. Soc. 80,1613 (1958). 27. W. E. Rosen, Tetrahedron Letters p. 481 (1961). 28. E. Wenkert and D. K. Roychaudhuri, J. Am. Chem. SOC.79, 1519 (1957). 29. E. Wenkert, R. Wickberg, and C. L. Leicht, J. Am. Chem. Soc. 83, 5037 (1961). 30. Sir Robert Robinson and A. F. Thomas, J . Chem. SOC.p. 2049 (1955). 31. K. Aghoramurthy and Sir Robert Robinson, Tetrahedron 1, 172 (1957). 32. G. F. Smith and J. T. Wr6be1, J . Chem. Soc p. 792 (1960). 33. K. Bernauer, W. Arnold, C. Weissmann, H. Schmid, and P. Karrer, Helv. Chim. Acta 43, 717 (1960). 34. C. Weissmann, H. Schmid, and P. Karrer, Helv. Chim. Acta 44, 1877 (1961). 35. P. N. Edwards and G. F. Smith, J . Chem. SOC.p. 152 (1961). 36. J. LBvy, J . Le Men, and M.-M. Janot, Bull. SOC.Chim. France p. 979 (1960). 37. M.-&I.Janot, J. Le Men, A. Le Hir, J. LBvy, and F. Puisieux, Compt. Rend. Acad. Sci. 250, 4383 (1960). 37a. M.-M. Janot, Pure Appl. Chern. 6,635 (1963). 38. K. Bernauer, F. Berlage, W. von Philipsborn, H. Schmid, and P. Karrer, Helv. Chim. Acta 41, 2293 (1958). 39. P. N. Edwards and G. F. Smith, J . Chem. SOC.p. 1458 (1961). 40. P. N. Edwards and G. F. Smith, Proc. Chem. SOC.p. 215 (1960). 41. Raymond-Hamet, Compt. Rend. Acad. Sci. 236, 319 (1953); Bull. Soc. PhuTm. Bordeaux 90, 178 (1952); Chem. Abstr. 48, 8794 (1954). 42. J. LBvy, J. Le Men, and M.-M. Janot, Compt. Rend. Acad. Sci.253, 131 (1961). 43. M.-M. Janot, J. Le Men, J. Gosset, and J. LBvy, Bull. SOC.Chim.France p. 1079 (1962).

7.

ALKALOIDS OF

Picralima nitida

157

44. L. D. Antonaccio, N. a. Pereira, B. Gilbert, H. Vorbrueggen, H. Budzikiewicz, J. M. Wilson, L. J. Durham, and C. Djerassi,J. Am. Chem. Soc. 84, 2161 (1962). 45. E. Clayton, R. I. Reed, and J. M. Wilson, Tetrahedron 18, 1449 (1962); M. Ohashi, H. Budzikiewicz, J. M. Wilson, C. Djerassi, J. Levy, J . Gosset, J. Le Men, and M.-M. Janot, Tetrahedron 19, 2241 (1963). 46. Sir Robert Robinson and A. F. Thomas, J . Chem. SOC.p. 3522 (1954). 47. Raymond-Hamet, Compt. Rend. Acad. Sci. 230, 1183 (1950). 48. J . LBvy, J. Le Men, and M.-M. Janot, Bull. SOC.Chim. France p. 1658 (1961). 49. J. ,4. Joule and G. F. Smith, J. Chem. Soc. p. 312 (1962). 50. A. Z. Britten, P. N. Edwards, J. A. Joule, G. F. Smith, and G. Spiteller, Chem. I n d . (L ondon)p. 1120 (1963). 51. G. E’. Smith, Personal communication (1963). 51a. L. Olivier, J. LBvy, J. Le Men, If.-M. Janot, C. Djerassi, H. Budzikiewicz, J. M. IVilson, and L. J . Durham, Bull. Soc. Chim. Prance p. 646 (1963). 52. M. F. Millson and Sir Robert Robinson, J . Chem. Soc. p. 3362 (1955). 53. A. Z. Britten and G. F. Smith, J. Chem. Soc. p. 3850 (1963). 54. G. F. Smith, Lecture delivered a t Anniversary Meeting, Chem. SOC.,Cardiff, March 1963. 55. A . Z. Britten, G. F. Smith, and G. Spiteller, Chem. I n d . ( L o n d o n )p. 1492 (1963). 56. M. F. Bartlett, R. Sklar, A. F. Smith, and W. I. Taylor,J. Org. Chem. 28, 2197 (1963); W. I. Taylor, M. F. Bartlett, L. Olivier, J. Lplvy, and J. Le Men, Bull. Soc. Chim. France p. 392 (1964). 57. L. Olivier, J. LBvy, J. Le Men, M.-M. Janot, H. Budzikiewicz, and C. Djerassi, Ann. Phorm. Franc . 22, 35 (1964).

NOTEADDEDIN PROOF Section IX, Akuammiline (see p. 155): I n a recent study of this minor base, Olivier et al. (57) have deduced a close structural affinity with picraline, and have tentatively proposed the structure LXXXIX.

LXXXIX