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The acid hydrolysis of phenyl β-d -xylopyranosides
Corbohydraie Research
277
Elwvicr PublishingCompany,Amsterdam Printedin Belgium
THE ACID HYDROLYSIS F. VAN W H-I-K.
IJNENDAELE
W.. Lab.
(Received
Anorg-
AND
OF PHENYL B-D-XYLOPYRANOSIDES
C. K. DE BRUYNE
Scheikunde
A, State
University,
Ghent (Belgium)
July 12th, 1968)
ABSTRACT
Twenty-eight substituted phenyl B-D-xylopyranosides were hydrolysed in hydrochloric acid. Rate coefiicients and kinetic parameters were determined. Application of the Hammett-Zucker, the Bunnett, and the entropy criteria indicate a unimolecular (A-l) mechanism. The linear relation between activation entropy and enthalpy, proved by the Exner method, indicates that some of the xylosides may be hydrolysed via a different mechanism. o&o-Substituents have a rather complex influence on the reaction. INTRODUCTION
In a previous paper’, we described the acid-catalysed hydrolysis of alkyl /?-D-xylopyranosides. The present investigation is concerned with the influence of substituents on the rate parameters of the hydrochloric acid-catalysed hydrolysis of phenyl /?-D-xyiopyranosides. The accepted mechanism, first suggested by Edward’, is analogous to the A-l mechanism for the hydrolysis of acetals34. The slow, rate-limiting step involves unimolecular heterolysis of the glycoside conjugate acid to form a cyclic carboniumoxonium ion, which then reacts with water. Similar investigations have been carried out for substituted phenyl cx-D-glucopyranosides5, phenyl B-D-glucopyranoside&‘, and phenyl fl-D-glucopyranosiduronic acids7. In these studies, the authors accepted the A-l mechanism with fission of the glucosyl-oxygen bond’, but without ring-opening. They showed that, for the #J-Dseries, electron-releasing substituents facilitate the reaction_ The Hammett reaction constant has a low value because the substituents have an influence on both the formation of the conjugate acid and its subsequent heterolysis, but affect these two processes in opposing manners, thus-partially cancelling each other. RESULTS AND DL5CUSSION
Twenty-eight phenyl B-D-xylopyranosides were hydrolysed in 0.1~ aqueous hydrochloric acid at different temperatures. Rate coefficients, energy and entropy Carbohyd.Res., 9 (1969)277-286
278
F. VAN WUNENDAEZE,
C. K. DE BRUYNE
of activation, and estimated standard deviations are presented in Table L All of the reactions are fxrstorder and In k is a linear function of l/T. TABLE
I
RATE COEFFI~S IN
o.lhf
AND
HYDROCHLORIC
KINETIC
PARAMI333tS
FOR
THE
Io5kl (set-l) 70”
60” 1 2
None
4 5 6 7 8 9
o-Chloro p-Methyl m-Methyl o-Methyl p-Nitro m-Nitro
4.44 z&o.03 14.9
10 c-Nitro p-Chloro-m-methyl 2,CDimethyl 3,CDimethyl 2,dDimethyI 2,3,STrimethyl Z+Dichloro p-Amino o-Amino
m-Amino p-Acetamido m-Acetamido o-Acetamido pMethoxy o-Methoxy P-Ethoxv
2.40 2.96 2.21 1.31 1.47 4.90 2.21 2.18 2.36 7.95 1.50 3.26 0.99 0.17 0.82 2.85 3.25 2.61 1.93 7.50 2.09
&0.04 &to.10 f0.03 10.03 &O.Ol f0.06 f0.04 f0.13 ho.14 SO.30 &to.01 &0.18 f0.03 *to.01 rtO.07 &O.OS 10.12 f0.03 10.02 &0.07 f0.05
8.51 10.7 7.98 4.34 4.93 14.6 8.08 8.28 9.09 30.2 6.83 11.4 3.40 0.57 2.92 10.4 10.5 9.66 7.43 24.8 8.31
26 b-Benzyioxy 27 p-Bromo
2.08 zO.07 7.49 1.94 &to.04 7.15
28
2.05 SO.06
m-Bromo
E kcal. mole-1
80”
2.59 &to.05 9.50 2.21 &0.03 8.00 2.19 &to.07 7.64
p-ChIoro 3 m-Chloro
11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
HYDROLYSIS
0~
PHENYL
/~-D-XYL~~YRAN~STC)?~~
ACID
32.5 26.9 25.3 46.3 27.9 36.3 26.9 14.7 15.6 39.9 27.3 29.1 32.6 106 28.7 37.3 10.9 1.8 9.8 35.5 31.6 31.7 26.7 76.8 30.7 25.1 24.6 21.7
6.92
InJruence of the acid concentration.
-
29.6 f0.5 29.2 &0.6 28.6 f0.3 27.4 f0.2 28.7 10.4 29.3 ho.6 29.2 f0.2 28.3 r0.3 27.6 &-to.3 24.7 50.2 29.4 (0.3 30.3 +0.7 30.7 f1.2 30.3 34.5 28.5 28.0 27.5 29.1 29.5 26.6 29.2 30.7 27.2 31.4 29.1 29.7 27.6
so.9 f0.3 il.2 f0.6 kO.8 *0.2 *o-7 f0.9 *to.2 so.3 f0.2 AO.4 50.4 f0.3 f0.4
Ass (60”) A Gt (60”) Cal.deg-1. mole-1 kcal. mole-l
f11.6 f1.5 +10.0 +8.3 +6.1 +9.0 + 10.0 +9.6 -t-6.3 -l-4.4 -1.7 +10.7 +13.8 + 14.7 +16.0 f25.1 +a.7 +4.s -0.2 f7.7 fll.4 f3.2 +10.5 + 14.3 +6.6 +X6.5 +9.7 fll.3 f5.1
*1.5 *1.5 &LO &LO h1.7 fl.0 AI.0 &-l-O &-l-O *1.0 &2.0 rt4.0 &3.0 &I.0 *3.0 h2.0 c2.3 *0.5 &2.0 12.0 *1.0 *o.s rtO.6 Al.0 -11.2 *1.0 &I.3
25.08 25.19 25.16 24.70 25.12 25.00 25.17 25.51 25.50 24.62 25.19 25.18 25.14 24.33 25.44 24.92 25.70 26.86 25.90 25.01 24.93 25.06 25.27 24.37 25.22 25.22 25.26 25.22
If the rate-limiting step involves the
unimolecular heterolysis of the conjugate acid to a glycosyl carbonium ion (A-l), a linear relation between the logarithm of the rate coefficient and the Hammett acidity function should exist’. For a number
of phenyl B-D-xylosides,
the pseudo-first-order
rate coefficient kl was determined at constant temperature and various concentrations of hydrochloric
acid (Tables
II and III).
In all cases, log k, showed a linear dependence on H,,, but the requirement of unit slope was only approximately fulfilled. A least-squares fit of the data yields the values given in Table deviation
IV, where b represents
on b, spix the standard
Carbohyd. Res., 9 (1969) 277-286
the slope, sb the estimated
error of the estimate, R the number
standard
of points,
and
HYDROLYSIS OF XYLOPYRANOSIDES
279
TABLE II INFLUENCE
OF THE
ACID
ON
CONCENTRATION
THE
RATE
COEFFICIENT
(COLORIMETRIC
DEl-ERMMATIONS)
I@kl see-1 HCI (M) H:
0.1 +0.9s TtW1p. (degf ees)
Substituent
None p-Chloro p-Methyl p-Chloro-m-methyl p-Nitro o-N&o p-Ethoxy p-Benzyloxy Z&Dimethyl 3,4-Dimethyl
0.25 +0.55
. 2.59 2.19 2.70 2.23 0.18 4.20 2.09 2.11 4.33 4.71
60 60 60 so.2 45 60 60 59.9 65 65
0.30 +0.45
-
8.26 7.39 9.00 4.77 0.465 6.18 5.36 9.07 12.4 -
0.50 +0.20
1.0 -0.20
1.5 -0.47
20.7 1.76 20.9
40.0 32.1 38.0 32.8 2.45 55.7 32.0
65.8 57.6 66.8 164b -
12.5
-
28.8
-
25.4 27.3
-
62.5 69.9
-
15.3 13.3 15.4 13.3 0.98 ’ 23.6 13.6
0.75 - 0.03
QFrom Ref. 13. bin 2~ HCI (&IO= -0;69). TABLE III INFLUENCE
OF THE
ACID
CONCENTRATION
ON
THE
RATE
COEFFICIENT
(POLARIMETRIC
DEIERMINATIONS)
l@kl (se&) ut 45” HCl I-G
1 -0.20
2 -0.69
3 -1.05
4 -1.40
None
4.59
p-Methyl
4.89
14.4 14.3 12.3 7.5 26.6 12.4 13.4
36.3 37.0 30.2 18.1 62.7 30.7 34.I
79.4 81.6 65.9 37.6 119 65.2 73.9
(M)
Subsfituent
p-Chloro p-Nitro o-N&o m-Acetamido p-Ethoxy
3.93 2.45 8.75 4.33 4.31
5 - 1.76
176 186 137 79 142 168
“From Ref. 13. Q the intercept of the function log 105k, = a + bH,, as calculated by regression analysis. For the parameter b, however, a t-test indicates that, in most cases, the
deviation from unity is not significant. According to Bunnett’O, a better criterion is a plot of log k, + Ho ttersus log (activity water) (log a&o)_ The slope of the resulting straight line defines the new parameter zu. However,
are curved to calculate all cases
if this is done for the phenyl j3-D-xylosides, all of the plots
the slopes dependent on acid concentration. is thus w parameters rough graphical is given Table IV). the slope b of the Hammett-Zucker plot is greater than unity, the sign
of w is negative and thus in accordance with a unimolecular- mechanism. In the other Carbahyd. Res., 9
(1969) 277-286
280
F. VAN WDNJWDABLB,
C. K. DE BRUYNE
cases, w is positive, which should lead to the conclusion that the hydrolysis proceeds by an S,2 mechanism entailing a oucleopbilic attack of water on the conjugate acid. This change of mechanism is highly improbable. Moreover, if the sign of IOis determined by the value of 6, whose deviation from unity is statistically not significant, the sign itself is determined by the random variations, and the positive w-values cannot invalidate the conclusion from the Zucker-Hammett criterion. The only possible conclusion is that, in all cases, w approaches zero, suggesting the same A-l mechanism for all of the phenyl B-D-xylosides. For alkyl /?-D-xylosides’, all b-slopes are greater than unity and all w-values negative, but, in these cases, the deviations from unity are statistically significant. TABLE SLOPES
IV OF TIE
ZUCKER-HAh%hlETT
PLOTS;
W PARAhfETER
Substituent
Method
-b
wz
sb
None
p”::
0.985 1.021 1.022 0.980 0.997 1.00 0.991 1.025 0.975 0.982 0.987 0.969 0.952 0.980
0.023 0.018 0.020 0.009 0.023 0.048 0.025 0.017 0.016 0.002 0.008 0.015 0.006 0.018
0.020 0.015
p-Methyl p-Chloro
p-Chloro-m-methyl p-Ethosxy p-Nitro 2,,6Dimethyl 3&Dimethyl p-Benzyloxy o_Nitro m-Acetamido
P c P C c P c+p c C C C P
a
n
5 5
0.017
0.008 0.019 0.051 0.026 0.014 0.006
0.002 0.009 0.017 0.004 0.015
: 5 5 5 5 10 4 4 4 4 5
0.376 0.464 0.476 0.308 0.407 0.298 0.311 0.433 0.204
0.601 0.638 0.273 0.558 0.437
W
+os -2.0 -0.2 fO.8 +0.1 -0.2 +- 0.4 -0.2 +0.3 fl.O +0.7 1-l-7 +2.0 +0.2
When log k, - log [HCl] is plotted tlersus log a,,, the slope of this line delines the parameter w*. According to Bunnetti’, water acts as a nucleophile in the ratedetermining step if w* < -2. When the values of k, determined in the interval 1 to’ 5~ hydrochloric acid (Table III) are used, the plots are approximately linear and the w*-parameter takes the value - 6 to - 7 for all xylosides. Ifthe k,-values determined at lower acid concentration (Table IQ are used, the points are scattered, and an exact determination of w? is impossible. A rough estimation, however, gives a constant value of ca. --LO for all of the xylosides. Hence, in all cases, w* indicates an S,2 mechanism, in contrast to the w-parameter, the Hammett criterion, and the entropy criterion. Thus, it seems that, for the hydrolysis of glycosides, the w and w* parameters reflect more than g simple hydration change in the transition of the conjugate acid to an activated complex. Isokinetic relationship. - In this series of similar reactions, a linear relation Carbohyd- Res., 9 (1969) 277-286
HYDROLYSIS
281
OF XYLOPYRANOSIDES
between the activation enthalpy and entropy was proved by plotting two values of log kl, obtained at two different temperatures, ag‘ainst each other according to the Exner” method. Using the k, values from Table I, a plot of log 106k, (SOO)verszrs log lO”k, (60”) gives a linear relation. Regression analysis yields the equation: log 106k (SOa) = 1.102 + 0.982 log 106k (60”) with slope b = 0.982 +O.OSO, isokinetic temperature #l = -lSS’K, TJT, = 0.943, n = 28, the correlation coefficient r = 0.977, and the standard error of the estimate s,,,~ = 0.067. If the values of the derivatives 10, 15, and 18, which deviate slightly from the calculated line, are omitted, one finds the equation: log 106k(800) = 1.10+0.983 log 106k(600), with b =0.983 I = 0.983, s~,~ = 0.045, and n = 25.
20.042, j3= -138”K,
A t-test indicates that, in both cases, the slope 6 does not deviate significantly from unity. Since this deviation, however, causes a large uncertainty in the p-value, the calculations were repeated using log 106k at 70”. This yields: log 106k(70”) versus log 106k(60”): b = 0.99 kO.02, j? = -95”K, and log 106k(80”) versus log 106(700): b = 1.00 t0.02, fl = 0°K. The most probable b-value approaches unity, and hence j3 approaches zero. In the Exner classification, this represents case 2, with E constant and a rate controlled mainly by changes of d St, which means that purely electronic effects of the substituents are of minor importance. If, on the other hand, the /?-value is not zero, but negative, and 1 z=-b > T,/T,, this represents case 4 or the reverse of compensation. The same reaction series was analyzed by the Leffler12 method in which E is plotted versus &. Regression analysis yields the equation (60”); E(kca1) = 26.0+0.317&*, with p = 317”K, ss = 6”K, s,,,== 0.488, r = 0.964, and n = 28. In this case, the iso‘kinetic temperature is different from the value in the Exner plot and approaches the mean temperature (343°K) of the experiment. Since log k is approximately constant, this p-value probably represents an “error slope”. With the exception of the 2,3,5_trimethylphenyl and possibly the o-nitrophenyl and o-arninophenyl derivatives, all points fit the calculated Exner isokinetic line, which suggests that these compounds are hydrolyzed by the same A-l mechanism. Even normal ortfio effects do not deviate the points from the line. Alkyl B-D-xylopyranosidesl reacted in a somewhat different way. In contrast to the phenyl analogues, both activation parameters were variable, but in the sense that their effects partially compensated each other. The Leffler plot was quite real and resulted in an isokinetic temperature of the same order as found in the Exner plot. Moreover, the p-value was positive and higher than the temperature of the experiment. Free-energy relationship. - Graphical analysis indicates that several points deviate strongly in a log 106k,(60”) uersm cr plot. The values for the compounds 4, 7, 10, 12, 14, 15, 16, 17, 18, 19, 22, and 24 were thus omitted in the calculations. Carbohyd. Res., 9
(1969) 277-286
HYDROLYSIS
OF XYLOPYRANOSIDES
283
rate nor the activation parameters are changed signScantly. This also indicates that steric effects play a minor role. In fact, because of the equatorial position of the aglycon and the unimolecuhu nature of the rate-limiting heterolysis of the protonated reacta& steric hindrance factors in the transition state of the A-I mechanism are unlikely. TABLE LINEAR
V FREE-?SNERGY
Substituent
None p_Chloro m-Chloro o-Chloro p-Methyl m-Methyl o-M&y1 p-Nitro m-Nitro o_Nitro pChloro-m-methyl 2.6Dimethyl 3,4-Dimethyl 2,dDimethyl 2,3,5-Trlmethyl 2&Dichloro p-AIL&l0
o-Amino m-Amino p-Acetamido m-Acetamido o-Acetamido p-Methoxy o-Methoxy p-Ethoxy p_Bentyloxy p-Bromo m-Bromo
RELATtONSI3lP
e7
0.00 + 0.277 +0.373 +0.20 -0.170 - 0.069 -0.17 + 1.27 +0.710 Cl.22 +0.158 -0.34 -0.239 -0.34 -0.308 + 0.427 +0.60 -to.86 0.00 +0.21 0.00 - 0.268 -0.39 -0.24 - 0.03= +0.232 +0.391
log
AIog IO%
xm1
obseroed
calculated
1.412 1.344 1.340 1.647 1.380 1.471 I.344 1.117 1.167 1.690 1.344 1.338 1.373 1.900 1.176 1.513 0.996 0.230 0.911 1.454 1.512 1.417 1.286 1.875 1.320 1.318 1.288 1.312
1.361 1.328 1.307 1.332 1.386 1.371 1.386 1.176 1.258 1.183 1.338 1.411 1.396 1.411 1.406 1.298 1.273 -
+0.051 +0.016 -I-0.033 +0.315 -0.006 +0.100 -0.042 - 0.059 -0.091 +0.507 -IO.006 - 0.073 - 0.023 +0.489 -0.230 +0.215 -0.277 -
0.912 1.361 1.331 1.361 1.400 1.418 1.396 1.364 1.327 1.304
-0.001 +0.093 +0.1s1 + 0.056 -0.114 +0.457 - 0.076 - 0.046 - 0.039 + 0.008
aFrom Ref. 16.
These factors, however, can influence the initial state of the molecule. In the case of the 2,6-dimethyl derivative, the enhanced rate is probably due to a decrease of the entropy in the initial conformation, the activation energy remaining constant. A second o-methyl group, in contrast to the same group in the metu (3,44imethyl) or pura position (2,4-dimethyl), prevents the rotation of the aglycon. When only one o-methyl substituent is present, this is not the case, and the rate is not changed signi&antly. Carbohyd. Res., 9 (1969) 277-286
F. VAN WUNENDABLE,
284
C. K. DE BRUYNB
The other ortha grqups have a more complex influence, and neither polar nor steric factors alone can explain the rate changes. These derivatives deviate significantly from the Hammett plot; the effect of the orrho group does not parallel the effect of the same group in the para position, e.g., a nitro group increases the rate in the art/lo, but decreases it in the para position; a methoxyl group (g* = -0.39) and a chloro group (o* = t0.20) both increase the rate. Even if one considers the possibility that some of the ortho substituents interfere sterically or by hydrogen bonding with the glycon, so that the p-orbitals of the substituent and of the benzene ring are prevented from becoming parallel to each other, thus inhibiting delocalisation through the z-electrons, the remaining inductive effect16 (cJ cannot explain the differences in rate. In one case, the o-methoxyl group, inhibition of the resonance changesf6 the sign of G (cl = t0.33) and can thus explain why this substituent has the same effect as the cbloro group, but even then the point still deviates from the Hammett plot. From the values of Tables I and V, it follows that ortho groups capable of hydrogen bonding with the hydroxyl group at C-2 increase the rate by lowering the ener,T of activation.
The entropy change becomes less favorable.
The hydrogen
bond
influences the electron density around the glycosyl-oxygen atom more than is accounted for by the a-value, but because of the two opposing effects on protonation and heterolysis, the overall effect on the rate remains speculative_ Although this explanation
cannot be ruled out, it becomes improbable if the influence on protonation is dominant (Q is negative), because hydrogen bonding should then decrease the rate. Hydrogen bonding may also result in a decrease
in entropy
of the initial state of the molecule,
caused by restriction imposed upon rotation of the aglycon. Even the conformation of the glycon may be changed, either by steric strain imposed by bulky groups or by hydrogen
bonding. These factors
may alter the change of molecular
order on passing
to the transition state and/or the energy necessary to reach a half-chair conformation. Probably several of these effects co-operate, and for the moment it seems impossible to separate them. Three of the phenyl j&II-xylosides show exceptional activation parameters. The 2,3,5_trimethylphenyl derivative has a high activation ener,oy and entropy. For the o-nitrophenyl and o-aminophenyl compounds, both parameters are small.
It is difficult to imagine that the high entropy in the case of the 2,3,5_trimethylphenyl xyloside is a result of a decrease in entropy of the initial conformation because none of the dimethylphenyl xylosides show this effect. It is also noteworthy that these compounds do not fit the isokinetic relationship. This, together with the unusual value of AS*, suggests a change in mechanism. One possibility is the open-chain mechanism, with protonation of the ring-oxygen atom (in this case owing to the baseweakening effect of the strong electron-withdrawing substituents on the glycosidic oxygen), and heterolysis with ring opening. Since, in this case, the aglycon remains attached to the g!ycosyl oxygen-atom in the heterolysis step, steric effects can play a dominant role and markedly influence&. In the open-chain mechanism, electronwithdrawing groups impede both protonation and heterolysis, and the rate should Carbohyd. Res,
9 (1969) 277-286
HYDROLYSIS
285
OF XYLOPYRANOSIDES
thus decrease. This is, however, not the case, and in the present state of knowledge this expIanation must remain specuIative. EXPERWENTAL
The synthesis of the substituted pheoyl B-D-xylopyranosides was performed as described previously’7-1g. The hydrolyses were carried out ip lO-ml, glass-stoppered tubes, immersed in a thermostat bath (accurate to within 0.05”). Exactly measured portions of the xyloside solution (0.001~) in hydrochloric acid were transferred into the tubes. These were stoppered and placed simultaneously in the thermostat bath.
When they had reached thermostat
temperature,
the first tube was withdrawn and
time taken as zero. After measured intervals, the other tubes were withdrawn, cooled, and neutralized with the calculated amount of sodium hydroxide. The concentration of the phenol was then determined calorimetrically. For nitrophenol, the yellow color developed in alkali was measured with a Beckman C calorimeter at 400 nm (Jenaer Schott lL/PIL firter). For the other phenols, the concentration was determined by the method of Folin and Ciocalteu “, the blue colours being measured with a Klett-Summerson calorimeter with filter 42 (40@-465 nm). For each phenol, a calibration curve had to be constructed, using the absorption of known concentrations of the phenol in similar conditions. The first-order rate coefficients (ln k, set-‘) were calculated from least-squares, straight-line fits of the usual log plots: log S, = log So - kt, which were always linear. The polarimetric measurements, carried out with a Perkin-Elmer Model 141 photoelectric polarimeter, and the calculations of the thermodynamic activation functions with their estimated standard deviation, were performed as described previously’. this
ACKNOWLEDG-
We thank Prof. Dr. L. Massart for his interest in this work, and Miss J. De Lat for technical assistance. REFERENCES 1 2 3 4 5 6 7 8 9 10
C.K.DE BRUYNEAND F.Vsr~w~~~~~~~~~,ccrrboltyd.Res.,6(1968)36?. J. T. EDWARD, Chem. Ind. (London), (1955) 1102. J. M. O'GORMAN AND H. J. LUCAS, J. Amer. Chem. Sue., 72 (1950) 5489. A. M. WENTHE AND E. H. CORDES, J. Amer. Chem. Sot., 87 (1965) 3173. A.N. HALL,~. HOLLINGSHEAD, AND H.N.RYDoN,J. Chem. Sot., (1961)4290. R. L. NATH AND H. N. RYDON, Biochem. J., 57 (1954). L. K. SEMKE, N. S. THOMPSON, AND D. G. WILLIALIS, J. Org. Chem., 29 (1964)1041. C. ARMOUR, C. A. B~NTON,S.PATAI,L.H.SELMAN,ANDC.A.VERNON,J. Chem.Soc, L. ZUCKER AND L. P. HAMLIETT, J. Amer. Chem. Sot., 61 (1939) 2791. J. F. Bmmm, J. Amer. Chem. Sot., 83 (1961) 4956, 4968,4973,4978. 11 0. WER, Coffection Czech. Chem. Cornmun., 29 (1964) 1094. 12 J. E. LEFFJJS, J. ZMg. Clzem., 20 (1955) 1202.
(1961)412.
Carbohycf. Res., 9 (1969) 277-286
286 13 14 15 16 17 18 19 20
F. VAN
WIJNENDAELE,
C. K. DE BRUYNE
M. A. PAUL AND F. A. LONG, Chem. Reo., 57 (1957) I. J. HINE, Physical Organic Chemistry, McGraw-Hill, New York, 1962, pp_ 87 and 98. J. HONE, J. Amer. Chem. Sue., 82 (1960) 4877. 0. EXNER, Collection Czech. Chem. Commun., 31 (1966) 65. C. K. DE BRUI-NE, H. VERSELE, AND M. CLAEYSENS, Nature, 205 (1965) 900. C. K. DE BRUYNE AND H. VERVOORT, Nature, 211 (1966) 1292. C. K. DE BRUYNE AND F. VAN W IJNENDAELE,Curbohyd. Res., 4 (1967) 102. 0. Form AND V. CIOCALTEU, J. Biol. Chem., 73 (1927) 627.
Corbohyd. Res., 9 (1969) 2-l-286
277
Elwvicr PublishingCompany,Amsterdam Printedin Belgium
THE ACID HYDROLYSIS F. VAN W H-I-K.
IJNENDAELE
W.. Lab.
(Received
Anorg-
AND
OF PHENYL B-D-XYLOPYRANOSIDES
C. K. DE BRUYNE
Scheikunde
A, State
University,
Ghent (Belgium)
July 12th, 1968)
ABSTRACT
Twenty-eight substituted phenyl B-D-xylopyranosides were hydrolysed in hydrochloric acid. Rate coefiicients and kinetic parameters were determined. Application of the Hammett-Zucker, the Bunnett, and the entropy criteria indicate a unimolecular (A-l) mechanism. The linear relation between activation entropy and enthalpy, proved by the Exner method, indicates that some of the xylosides may be hydrolysed via a different mechanism. o&o-Substituents have a rather complex influence on the reaction. INTRODUCTION
In a previous paper’, we described the acid-catalysed hydrolysis of alkyl /?-D-xylopyranosides. The present investigation is concerned with the influence of substituents on the rate parameters of the hydrochloric acid-catalysed hydrolysis of phenyl /?-D-xyiopyranosides. The accepted mechanism, first suggested by Edward’, is analogous to the A-l mechanism for the hydrolysis of acetals34. The slow, rate-limiting step involves unimolecular heterolysis of the glycoside conjugate acid to form a cyclic carboniumoxonium ion, which then reacts with water. Similar investigations have been carried out for substituted phenyl cx-D-glucopyranosides5, phenyl B-D-glucopyranoside&‘, and phenyl fl-D-glucopyranosiduronic acids7. In these studies, the authors accepted the A-l mechanism with fission of the glucosyl-oxygen bond’, but without ring-opening. They showed that, for the #J-Dseries, electron-releasing substituents facilitate the reaction_ The Hammett reaction constant has a low value because the substituents have an influence on both the formation of the conjugate acid and its subsequent heterolysis, but affect these two processes in opposing manners, thus-partially cancelling each other. RESULTS AND DL5CUSSION
Twenty-eight phenyl B-D-xylopyranosides were hydrolysed in 0.1~ aqueous hydrochloric acid at different temperatures. Rate coefficients, energy and entropy Carbohyd.Res., 9 (1969)277-286
278
F. VAN WUNENDAEZE,
C. K. DE BRUYNE
of activation, and estimated standard deviations are presented in Table L All of the reactions are fxrstorder and In k is a linear function of l/T. TABLE
I
RATE COEFFI~S IN
o.lhf
AND
HYDROCHLORIC
KINETIC
PARAMI333tS
FOR
THE
Io5kl (set-l) 70”
60” 1 2
None
4 5 6 7 8 9
o-Chloro p-Methyl m-Methyl o-Methyl p-Nitro m-Nitro
4.44 z&o.03 14.9
10 c-Nitro p-Chloro-m-methyl 2,CDimethyl 3,CDimethyl 2,dDimethyI 2,3,STrimethyl Z+Dichloro p-Amino o-Amino
m-Amino p-Acetamido m-Acetamido o-Acetamido pMethoxy o-Methoxy P-Ethoxv
2.40 2.96 2.21 1.31 1.47 4.90 2.21 2.18 2.36 7.95 1.50 3.26 0.99 0.17 0.82 2.85 3.25 2.61 1.93 7.50 2.09
&0.04 &to.10 f0.03 10.03 &O.Ol f0.06 f0.04 f0.13 ho.14 SO.30 &to.01 &0.18 f0.03 *to.01 rtO.07 &O.OS 10.12 f0.03 10.02 &0.07 f0.05
8.51 10.7 7.98 4.34 4.93 14.6 8.08 8.28 9.09 30.2 6.83 11.4 3.40 0.57 2.92 10.4 10.5 9.66 7.43 24.8 8.31
26 b-Benzyioxy 27 p-Bromo
2.08 zO.07 7.49 1.94 &to.04 7.15
28
2.05 SO.06
m-Bromo
E kcal. mole-1
80”
2.59 &to.05 9.50 2.21 &0.03 8.00 2.19 &to.07 7.64
p-ChIoro 3 m-Chloro
11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
HYDROLYSIS
0~
PHENYL
/~-D-XYL~~YRAN~STC)?~~
ACID
32.5 26.9 25.3 46.3 27.9 36.3 26.9 14.7 15.6 39.9 27.3 29.1 32.6 106 28.7 37.3 10.9 1.8 9.8 35.5 31.6 31.7 26.7 76.8 30.7 25.1 24.6 21.7
6.92
InJruence of the acid concentration.
-
29.6 f0.5 29.2 &0.6 28.6 f0.3 27.4 f0.2 28.7 10.4 29.3 ho.6 29.2 f0.2 28.3 r0.3 27.6 &-to.3 24.7 50.2 29.4 (0.3 30.3 +0.7 30.7 f1.2 30.3 34.5 28.5 28.0 27.5 29.1 29.5 26.6 29.2 30.7 27.2 31.4 29.1 29.7 27.6
so.9 f0.3 il.2 f0.6 kO.8 *0.2 *o-7 f0.9 *to.2 so.3 f0.2 AO.4 50.4 f0.3 f0.4
Ass (60”) A Gt (60”) Cal.deg-1. mole-1 kcal. mole-l
f11.6 f1.5 +10.0 +8.3 +6.1 +9.0 + 10.0 +9.6 -t-6.3 -l-4.4 -1.7 +10.7 +13.8 + 14.7 +16.0 f25.1 +a.7 +4.s -0.2 f7.7 fll.4 f3.2 +10.5 + 14.3 +6.6 +X6.5 +9.7 fll.3 f5.1
*1.5 *1.5 &LO &LO h1.7 fl.0 AI.0 &-l-O &-l-O *1.0 &2.0 rt4.0 &3.0 &I.0 *3.0 h2.0 c2.3 *0.5 &2.0 12.0 *1.0 *o.s rtO.6 Al.0 -11.2 *1.0 &I.3
25.08 25.19 25.16 24.70 25.12 25.00 25.17 25.51 25.50 24.62 25.19 25.18 25.14 24.33 25.44 24.92 25.70 26.86 25.90 25.01 24.93 25.06 25.27 24.37 25.22 25.22 25.26 25.22
If the rate-limiting step involves the
unimolecular heterolysis of the conjugate acid to a glycosyl carbonium ion (A-l), a linear relation between the logarithm of the rate coefficient and the Hammett acidity function should exist’. For a number
of phenyl B-D-xylosides,
the pseudo-first-order
rate coefficient kl was determined at constant temperature and various concentrations of hydrochloric
acid (Tables
II and III).
In all cases, log k, showed a linear dependence on H,,, but the requirement of unit slope was only approximately fulfilled. A least-squares fit of the data yields the values given in Table deviation
IV, where b represents
on b, spix the standard
Carbohyd. Res., 9 (1969) 277-286
the slope, sb the estimated
error of the estimate, R the number
standard
of points,
and
HYDROLYSIS OF XYLOPYRANOSIDES
279
TABLE II INFLUENCE
OF THE
ACID
ON
CONCENTRATION
THE
RATE
COEFFICIENT
(COLORIMETRIC
DEl-ERMMATIONS)
I@kl see-1 HCI (M) H:
0.1 +0.9s TtW1p. (degf ees)
Substituent
None p-Chloro p-Methyl p-Chloro-m-methyl p-Nitro o-N&o p-Ethoxy p-Benzyloxy Z&Dimethyl 3,4-Dimethyl
0.25 +0.55
. 2.59 2.19 2.70 2.23 0.18 4.20 2.09 2.11 4.33 4.71
60 60 60 so.2 45 60 60 59.9 65 65
0.30 +0.45
-
8.26 7.39 9.00 4.77 0.465 6.18 5.36 9.07 12.4 -
0.50 +0.20
1.0 -0.20
1.5 -0.47
20.7 1.76 20.9
40.0 32.1 38.0 32.8 2.45 55.7 32.0
65.8 57.6 66.8 164b -
12.5
-
28.8
-
25.4 27.3
-
62.5 69.9
-
15.3 13.3 15.4 13.3 0.98 ’ 23.6 13.6
0.75 - 0.03
QFrom Ref. 13. bin 2~ HCI (&IO= -0;69). TABLE III INFLUENCE
OF THE
ACID
CONCENTRATION
ON
THE
RATE
COEFFICIENT
(POLARIMETRIC
DEIERMINATIONS)
l@kl (se&) ut 45” HCl I-G
1 -0.20
2 -0.69
3 -1.05
4 -1.40
None
4.59
p-Methyl
4.89
14.4 14.3 12.3 7.5 26.6 12.4 13.4
36.3 37.0 30.2 18.1 62.7 30.7 34.I
79.4 81.6 65.9 37.6 119 65.2 73.9
(M)
Subsfituent
p-Chloro p-Nitro o-N&o m-Acetamido p-Ethoxy
3.93 2.45 8.75 4.33 4.31
5 - 1.76
176 186 137 79 142 168
“From Ref. 13. Q the intercept of the function log 105k, = a + bH,, as calculated by regression analysis. For the parameter b, however, a t-test indicates that, in most cases, the
deviation from unity is not significant. According to Bunnett’O, a better criterion is a plot of log k, + Ho ttersus log (activity water) (log a&o)_ The slope of the resulting straight line defines the new parameter zu. However,
are curved to calculate all cases
if this is done for the phenyl j3-D-xylosides, all of the plots
the slopes dependent on acid concentration. is thus w parameters rough graphical is given Table IV). the slope b of the Hammett-Zucker plot is greater than unity, the sign
of w is negative and thus in accordance with a unimolecular- mechanism. In the other Carbahyd. Res., 9
(1969) 277-286
280
F. VAN WDNJWDABLB,
C. K. DE BRUYNE
cases, w is positive, which should lead to the conclusion that the hydrolysis proceeds by an S,2 mechanism entailing a oucleopbilic attack of water on the conjugate acid. This change of mechanism is highly improbable. Moreover, if the sign of IOis determined by the value of 6, whose deviation from unity is statistically not significant, the sign itself is determined by the random variations, and the positive w-values cannot invalidate the conclusion from the Zucker-Hammett criterion. The only possible conclusion is that, in all cases, w approaches zero, suggesting the same A-l mechanism for all of the phenyl B-D-xylosides. For alkyl /?-D-xylosides’, all b-slopes are greater than unity and all w-values negative, but, in these cases, the deviations from unity are statistically significant. TABLE SLOPES
IV OF TIE
ZUCKER-HAh%hlETT
PLOTS;
W PARAhfETER
Substituent
Method
-b
wz
sb
None
p”::
0.985 1.021 1.022 0.980 0.997 1.00 0.991 1.025 0.975 0.982 0.987 0.969 0.952 0.980
0.023 0.018 0.020 0.009 0.023 0.048 0.025 0.017 0.016 0.002 0.008 0.015 0.006 0.018
0.020 0.015
p-Methyl p-Chloro
p-Chloro-m-methyl p-Ethosxy p-Nitro 2,,6Dimethyl 3&Dimethyl p-Benzyloxy o_Nitro m-Acetamido
P c P C c P c+p c C C C P
a
n
5 5
0.017
0.008 0.019 0.051 0.026 0.014 0.006
0.002 0.009 0.017 0.004 0.015
: 5 5 5 5 10 4 4 4 4 5
0.376 0.464 0.476 0.308 0.407 0.298 0.311 0.433 0.204
0.601 0.638 0.273 0.558 0.437
W
+os -2.0 -0.2 fO.8 +0.1 -0.2 +- 0.4 -0.2 +0.3 fl.O +0.7 1-l-7 +2.0 +0.2
When log k, - log [HCl] is plotted tlersus log a,,, the slope of this line delines the parameter w*. According to Bunnetti’, water acts as a nucleophile in the ratedetermining step if w* < -2. When the values of k, determined in the interval 1 to’ 5~ hydrochloric acid (Table III) are used, the plots are approximately linear and the w*-parameter takes the value - 6 to - 7 for all xylosides. Ifthe k,-values determined at lower acid concentration (Table IQ are used, the points are scattered, and an exact determination of w? is impossible. A rough estimation, however, gives a constant value of ca. --LO for all of the xylosides. Hence, in all cases, w* indicates an S,2 mechanism, in contrast to the w-parameter, the Hammett criterion, and the entropy criterion. Thus, it seems that, for the hydrolysis of glycosides, the w and w* parameters reflect more than g simple hydration change in the transition of the conjugate acid to an activated complex. Isokinetic relationship. - In this series of similar reactions, a linear relation Carbohyd- Res., 9 (1969) 277-286
HYDROLYSIS
281
OF XYLOPYRANOSIDES
between the activation enthalpy and entropy was proved by plotting two values of log kl, obtained at two different temperatures, ag‘ainst each other according to the Exner” method. Using the k, values from Table I, a plot of log 106k, (SOO)verszrs log lO”k, (60”) gives a linear relation. Regression analysis yields the equation: log 106k (SOa) = 1.102 + 0.982 log 106k (60”) with slope b = 0.982 +O.OSO, isokinetic temperature #l = -lSS’K, TJT, = 0.943, n = 28, the correlation coefficient r = 0.977, and the standard error of the estimate s,,,~ = 0.067. If the values of the derivatives 10, 15, and 18, which deviate slightly from the calculated line, are omitted, one finds the equation: log 106k(800) = 1.10+0.983 log 106k(600), with b =0.983 I = 0.983, s~,~ = 0.045, and n = 25.
20.042, j3= -138”K,
A t-test indicates that, in both cases, the slope 6 does not deviate significantly from unity. Since this deviation, however, causes a large uncertainty in the p-value, the calculations were repeated using log 106k at 70”. This yields: log 106k(70”) versus log 106k(60”): b = 0.99 kO.02, j? = -95”K, and log 106k(80”) versus log 106(700): b = 1.00 t0.02, fl = 0°K. The most probable b-value approaches unity, and hence j3 approaches zero. In the Exner classification, this represents case 2, with E constant and a rate controlled mainly by changes of d St, which means that purely electronic effects of the substituents are of minor importance. If, on the other hand, the /?-value is not zero, but negative, and 1 z=-b > T,/T,, this represents case 4 or the reverse of compensation. The same reaction series was analyzed by the Leffler12 method in which E is plotted versus &. Regression analysis yields the equation (60”); E(kca1) = 26.0+0.317&*, with p = 317”K, ss = 6”K, s,,,== 0.488, r = 0.964, and n = 28. In this case, the iso‘kinetic temperature is different from the value in the Exner plot and approaches the mean temperature (343°K) of the experiment. Since log k is approximately constant, this p-value probably represents an “error slope”. With the exception of the 2,3,5_trimethylphenyl and possibly the o-nitrophenyl and o-arninophenyl derivatives, all points fit the calculated Exner isokinetic line, which suggests that these compounds are hydrolyzed by the same A-l mechanism. Even normal ortfio effects do not deviate the points from the line. Alkyl B-D-xylopyranosidesl reacted in a somewhat different way. In contrast to the phenyl analogues, both activation parameters were variable, but in the sense that their effects partially compensated each other. The Leffler plot was quite real and resulted in an isokinetic temperature of the same order as found in the Exner plot. Moreover, the p-value was positive and higher than the temperature of the experiment. Free-energy relationship. - Graphical analysis indicates that several points deviate strongly in a log 106k,(60”) uersm cr plot. The values for the compounds 4, 7, 10, 12, 14, 15, 16, 17, 18, 19, 22, and 24 were thus omitted in the calculations. Carbohyd. Res., 9
(1969) 277-286
HYDROLYSIS
OF XYLOPYRANOSIDES
283
rate nor the activation parameters are changed signScantly. This also indicates that steric effects play a minor role. In fact, because of the equatorial position of the aglycon and the unimolecuhu nature of the rate-limiting heterolysis of the protonated reacta& steric hindrance factors in the transition state of the A-I mechanism are unlikely. TABLE LINEAR
V FREE-?SNERGY
Substituent
None p_Chloro m-Chloro o-Chloro p-Methyl m-Methyl o-M&y1 p-Nitro m-Nitro o_Nitro pChloro-m-methyl 2.6Dimethyl 3,4-Dimethyl 2,dDimethyl 2,3,5-Trlmethyl 2&Dichloro p-AIL&l0
o-Amino m-Amino p-Acetamido m-Acetamido o-Acetamido p-Methoxy o-Methoxy p-Ethoxy p_Bentyloxy p-Bromo m-Bromo
RELATtONSI3lP
e7
0.00 + 0.277 +0.373 +0.20 -0.170 - 0.069 -0.17 + 1.27 +0.710 Cl.22 +0.158 -0.34 -0.239 -0.34 -0.308 + 0.427 +0.60 -to.86 0.00 +0.21 0.00 - 0.268 -0.39 -0.24 - 0.03= +0.232 +0.391
log
AIog IO%
xm1
obseroed
calculated
1.412 1.344 1.340 1.647 1.380 1.471 I.344 1.117 1.167 1.690 1.344 1.338 1.373 1.900 1.176 1.513 0.996 0.230 0.911 1.454 1.512 1.417 1.286 1.875 1.320 1.318 1.288 1.312
1.361 1.328 1.307 1.332 1.386 1.371 1.386 1.176 1.258 1.183 1.338 1.411 1.396 1.411 1.406 1.298 1.273 -
+0.051 +0.016 -I-0.033 +0.315 -0.006 +0.100 -0.042 - 0.059 -0.091 +0.507 -IO.006 - 0.073 - 0.023 +0.489 -0.230 +0.215 -0.277 -
0.912 1.361 1.331 1.361 1.400 1.418 1.396 1.364 1.327 1.304
-0.001 +0.093 +0.1s1 + 0.056 -0.114 +0.457 - 0.076 - 0.046 - 0.039 + 0.008
aFrom Ref. 16.
These factors, however, can influence the initial state of the molecule. In the case of the 2,6-dimethyl derivative, the enhanced rate is probably due to a decrease of the entropy in the initial conformation, the activation energy remaining constant. A second o-methyl group, in contrast to the same group in the metu (3,44imethyl) or pura position (2,4-dimethyl), prevents the rotation of the aglycon. When only one o-methyl substituent is present, this is not the case, and the rate is not changed signi&antly. Carbohyd. Res., 9 (1969) 277-286
F. VAN WUNENDABLE,
284
C. K. DE BRUYNB
The other ortha grqups have a more complex influence, and neither polar nor steric factors alone can explain the rate changes. These derivatives deviate significantly from the Hammett plot; the effect of the orrho group does not parallel the effect of the same group in the para position, e.g., a nitro group increases the rate in the art/lo, but decreases it in the para position; a methoxyl group (g* = -0.39) and a chloro group (o* = t0.20) both increase the rate. Even if one considers the possibility that some of the ortho substituents interfere sterically or by hydrogen bonding with the glycon, so that the p-orbitals of the substituent and of the benzene ring are prevented from becoming parallel to each other, thus inhibiting delocalisation through the z-electrons, the remaining inductive effect16 (cJ cannot explain the differences in rate. In one case, the o-methoxyl group, inhibition of the resonance changesf6 the sign of G (cl = t0.33) and can thus explain why this substituent has the same effect as the cbloro group, but even then the point still deviates from the Hammett plot. From the values of Tables I and V, it follows that ortho groups capable of hydrogen bonding with the hydroxyl group at C-2 increase the rate by lowering the ener,T of activation.
The entropy change becomes less favorable.
The hydrogen
bond
influences the electron density around the glycosyl-oxygen atom more than is accounted for by the a-value, but because of the two opposing effects on protonation and heterolysis, the overall effect on the rate remains speculative_ Although this explanation
cannot be ruled out, it becomes improbable if the influence on protonation is dominant (Q is negative), because hydrogen bonding should then decrease the rate. Hydrogen bonding may also result in a decrease
in entropy
of the initial state of the molecule,
caused by restriction imposed upon rotation of the aglycon. Even the conformation of the glycon may be changed, either by steric strain imposed by bulky groups or by hydrogen
bonding. These factors
may alter the change of molecular
order on passing
to the transition state and/or the energy necessary to reach a half-chair conformation. Probably several of these effects co-operate, and for the moment it seems impossible to separate them. Three of the phenyl j&II-xylosides show exceptional activation parameters. The 2,3,5_trimethylphenyl derivative has a high activation ener,oy and entropy. For the o-nitrophenyl and o-aminophenyl compounds, both parameters are small.
It is difficult to imagine that the high entropy in the case of the 2,3,5_trimethylphenyl xyloside is a result of a decrease in entropy of the initial conformation because none of the dimethylphenyl xylosides show this effect. It is also noteworthy that these compounds do not fit the isokinetic relationship. This, together with the unusual value of AS*, suggests a change in mechanism. One possibility is the open-chain mechanism, with protonation of the ring-oxygen atom (in this case owing to the baseweakening effect of the strong electron-withdrawing substituents on the glycosidic oxygen), and heterolysis with ring opening. Since, in this case, the aglycon remains attached to the g!ycosyl oxygen-atom in the heterolysis step, steric effects can play a dominant role and markedly influence&. In the open-chain mechanism, electronwithdrawing groups impede both protonation and heterolysis, and the rate should Carbohyd. Res,
9 (1969) 277-286
HYDROLYSIS
285
OF XYLOPYRANOSIDES
thus decrease. This is, however, not the case, and in the present state of knowledge this expIanation must remain specuIative. EXPERWENTAL
The synthesis of the substituted pheoyl B-D-xylopyranosides was performed as described previously’7-1g. The hydrolyses were carried out ip lO-ml, glass-stoppered tubes, immersed in a thermostat bath (accurate to within 0.05”). Exactly measured portions of the xyloside solution (0.001~) in hydrochloric acid were transferred into the tubes. These were stoppered and placed simultaneously in the thermostat bath.
When they had reached thermostat
temperature,
the first tube was withdrawn and
time taken as zero. After measured intervals, the other tubes were withdrawn, cooled, and neutralized with the calculated amount of sodium hydroxide. The concentration of the phenol was then determined calorimetrically. For nitrophenol, the yellow color developed in alkali was measured with a Beckman C calorimeter at 400 nm (Jenaer Schott lL/PIL firter). For the other phenols, the concentration was determined by the method of Folin and Ciocalteu “, the blue colours being measured with a Klett-Summerson calorimeter with filter 42 (40@-465 nm). For each phenol, a calibration curve had to be constructed, using the absorption of known concentrations of the phenol in similar conditions. The first-order rate coefficients (ln k, set-‘) were calculated from least-squares, straight-line fits of the usual log plots: log S, = log So - kt, which were always linear. The polarimetric measurements, carried out with a Perkin-Elmer Model 141 photoelectric polarimeter, and the calculations of the thermodynamic activation functions with their estimated standard deviation, were performed as described previously’. this
ACKNOWLEDG-
We thank Prof. Dr. L. Massart for his interest in this work, and Miss J. De Lat for technical assistance. REFERENCES 1 2 3 4 5 6 7 8 9 10
C.K.DE BRUYNEAND F.Vsr~w~~~~~~~~~,ccrrboltyd.Res.,6(1968)36?. J. T. EDWARD, Chem. Ind. (London), (1955) 1102. J. M. O'GORMAN AND H. J. LUCAS, J. Amer. Chem. Sue., 72 (1950) 5489. A. M. WENTHE AND E. H. CORDES, J. Amer. Chem. Sot., 87 (1965) 3173. A.N. HALL,~. HOLLINGSHEAD, AND H.N.RYDoN,J. Chem. Sot., (1961)4290. R. L. NATH AND H. N. RYDON, Biochem. J., 57 (1954). L. K. SEMKE, N. S. THOMPSON, AND D. G. WILLIALIS, J. Org. Chem., 29 (1964)1041. C. ARMOUR, C. A. B~NTON,S.PATAI,L.H.SELMAN,ANDC.A.VERNON,J. Chem.Soc, L. ZUCKER AND L. P. HAMLIETT, J. Amer. Chem. Sot., 61 (1939) 2791. J. F. Bmmm, J. Amer. Chem. Sot., 83 (1961) 4956, 4968,4973,4978. 11 0. WER, Coffection Czech. Chem. Cornmun., 29 (1964) 1094. 12 J. E. LEFFJJS, J. ZMg. Clzem., 20 (1955) 1202.
(1961)412.
Carbohycf. Res., 9 (1969) 277-286
286 13 14 15 16 17 18 19 20
F. VAN
WIJNENDAELE,
C. K. DE BRUYNE
M. A. PAUL AND F. A. LONG, Chem. Reo., 57 (1957) I. J. HINE, Physical Organic Chemistry, McGraw-Hill, New York, 1962, pp_ 87 and 98. J. HONE, J. Amer. Chem. Sue., 82 (1960) 4877. 0. EXNER, Collection Czech. Chem. Commun., 31 (1966) 65. C. K. DE BRUI-NE, H. VERSELE, AND M. CLAEYSENS, Nature, 205 (1965) 900. C. K. DE BRUYNE AND H. VERVOORT, Nature, 211 (1966) 1292. C. K. DE BRUYNE AND F. VAN W IJNENDAELE,Curbohyd. Res., 4 (1967) 102. 0. Form AND V. CIOCALTEU, J. Biol. Chem., 73 (1927) 627.
Corbohyd. Res., 9 (1969) 2-l-286