Indian Journal of Chemistry Vol. 41B, September 2002, pp. 1907-1914

Oxidation of threose-series, pentose and hexoses by N-arylbromosulphonamides in alkaline medium

V Shashikala & K S Rangappa*

Department of Studies in Chemistry, University of Mysore, Manasagangotri, Mysore 570 006, India *Fax: 91-821-518835 or 421263 ; email: rangappaks@yahoo.com

Received 18 July 2001; accepted (revised) 4 December 2001

Kinetic studies of the oxidation of D-galactose, L-sorbose and D-xylose by bromamine-T (sodium-N-bromo-p­toluenesulphonamide or BAT) and bromamine-B (sodium-N-bromobenzene sulphonamide or BAB) in alkaline medium has been investigated at 303 K. The rate of the reaction is first order both with respect to oxidant and sugar, and second order with respect to [HOT The addition of the reaction product p-toluenesulphonamide (PTS) or benzenesulphonamide (BSA) and the variation of ionic strength of the medium have no effect on the rate. The rate decreases with the decrease in dielec­tric constant of the medium and values of dAB, the size of activated complex are calculated. Proton inventory studies in H20 . D20 mixtures suggest a single transition state. Product analysis for D-galactose, L-sorbose and D-xylose reveal that hexoses give mainly mixture of Iyxonic and threonic acids with minor proportions of hexonic, xylonic and glyceric acids, whereas xylose yields a mixture of Iyxonic, threonic and glyceric acids with minor amounts of xylonic and hexonic acids. From the results of kinetic studies, reaction stoichiometry and product analysis, a possible mechanism for the oxidation of threose-series sugars is suggested .

In our broad programme on the oxidation of mono­saccharides by N-haloamines recently, we have stud­ied the kinetics and mechanism of oxidation of eryth­rose-series sugars, with chloramine-T (CAT) and chloramine-B (CAB) in alkaline medium.1.2 The ob­served reaction stoichiometry of 2-3 moles of CAT or CAB per mole of sugar was significantly different from the previously reported sugar to oxidant stoichiometry of 1: 1 for aldoses and 1:2 for fructose. 3

.5

We have also shown by HPLC and GLC-MS analysis that the products of oxidation for erythrose-series sugars were mixtures of aldonic acids consisting of arabinonic, ribonic, erythronic and glyceric acids.' These product profiles were also different from those reported previously, the corresponding aldonic acids for aldoses and arabinonic acid for fructose. 3

.5

Our recent study on the oxidation of erythrose se­ries sugars with CAT and CAB gave two interesting results . ' First, the sugars that can exist in the furanose ring form in appreciable proportions reacted with oxi­dant much faster than those, which exist almost exclu­sively in the pyranose form. Thus, for hexoses rate of oxidation of fructose was higher than glucose and mannose. Similarly, ribose was oxidized faster than arabinose. Second, surprisingly the products formed from both pentoses and hexoses including keto­hexoses were strikingly similar for all erythrose-series

sugars studied. Based on these results, we proposed a novel pathway for the oxidation of erythrose-series sugars by CAT.' It was therefore of interest to study the oxidation of sugars by N-arylbromosulphonamides and make a preliminary comparison of the oxidative behaviour of the chlorine and bromine analogues. In the present study, the mechanism of oxidation of threose series sugars by N-arylbromosulphonamides was investigated by kinetic studies and product analy­sis. The results demonstrate that the kinetics and mechanism of oxidation of threose series sugars are generally similar to those observed for erythrose se­ries sugars.

Materials and Methods D-Galactose, L-sorbose and D-xylose were pur­

chased from Sigma chemicals. BAT and BAB were prepared by the method of Nair et aL. 6 The aqueous solutions of BAT and BAB were prepared, standard­ized by iodometric method and preserved in brown bottles to prevent their photochemical deterioration . The concentrated aqueous solution of sodium perchlo­rate was used to maintain the ionic strength of the re­action mixture. All other chemicals used were of ana­lytical grades of purity. Triply distilled water was used for preparing aqueous solutions. The solvent iso­tope studies were made with D20 (99.4%) supplied by Bhabha Ato·mic Research Center, Bombay (India).

1908 INDIAN 1. CHEM., SEC B, SEPTEMBER 2002

Kinetic measurements Pseudo-first order conditions were maintained with

respect to the oxidant (BAT or BAB) concentration. Stock solutions of sugars, oxidants, alkali and sodium perchlorate were thermostated for 30 minutes at 303K. The reaction was initiated by the rapid addition of BAT or BAB to the mixture and its progress was monitored by iodometric estimation of unconsumed BAT or BAB at regular intervals of time. The reaction was studied for more than two half lives. Pseudo-first order rate constants kobs were calculated from the plots of log [OX]o vs time, where OX represents the oxi­dant and these were reproducible within ± 5% error.

Stoichiometry and product analysis The reaction mixture containing sugar, alkali and

an excess of BATor BAB were kept for 24 hr at 303 K. The :.tnconsumed oxidant was determined iodomet­rically. From these data, the amount of the oxidant consumed per mole of sugar was found to be 3 moles for hexoses and 2 moles for pentoses.

The oxidation products were analyzed by Dionex BioLC HPLC (high performance liquid chromatogra­phy) coupled with pulsed amperometric detection us­ing a Carbo Pac PAl high - pH anion exchange col­umn (4x250 mm).6 An isocratic elution with 0.2 M NaOH was used. The products were identified by comparison of the HPLC retention times with those of the standard aldonic acids and by GC-MS (Table I).

For GC-MS characterization, the reaction mixture was extracted with diethyl ether to remove toluenesul­fonamide or benzenesulfonarnide and then passed through Ag 50W-X12(H+) and Ag 4-X4 (base) resins. The bound sample from Ag 4-X4 (base) resins were eluted with 1M pyridinell M acetic acid, pH 5.2 and lyophilized. The products were converted into their trimethylsilyl derivatives and then analyzed by GC-MS.

Results Effect of / reactant] on reaction rate

The reactions were carried out with varying con­centrations of oxidant (BAT or BAB) using constant

[HO'] and [S]o with substrate (S) in excess. Plots of log [OX] vs. time were linear(r>.9995, 5<.02) indicat­ing a first order dependence of reaction rate on [BAT]o or [BAB]o. The pseudo-first order rate con­stants, kobs obtained with dl ifferent rOX]o were simi lar (Table II), confirming the first order dependence of the rate on [OX].

The kobs values increased (Table II) with the in­crease in [S]o (O.lM-0.6M). The: plots of log kobs VS log [S]o were linear (r>.9998, s<.OI) with unit slopes. The second order rate constants (k2 = kobs / [S]o) were con­stant within the experimental errors demonstrating a first order dependence of the rate on [S]o' Further­more, the plots of kobs vs [S]o gave linear plots passing through the origin (Figure 1) indicating that the in­termediates formed with oxidant are only of the tran­sient existence.

Effect of [alkali] on reaction rate At constant [OX]o and [S]o, values of kobs increased

with an increase in [NaOH] (Table III) and the plots of log kobs VS log [HO'] were linear (r:>.9993, s<.04) with slopes of 2 thus indicating the second order de­pendence of the rate on [HOl

Effect of products, other ions and ionic strength on reaction rate

Addition of reaction products, p -toluenesulphon­amide (PTS) in the case of BAT and benzenesul­phon amide (BSA) in the case of BAB (0-0.008 M) and Br' and cr ions had negligible influence on the rate of the reaction. The rate of oxidation of sugars was unaltered when ionic strength (1) of the medium was changed using sodium perchlorate.

Effect of solvent composition on reaction rate When the solvent composition of the medium was

varied by adding methanol (0- 40%.), the reaction rate decreased with methanol content of the medium. Plots of log kobs VS 110. (D-dielectric constant of the me­dium) were linear (r>.9989, s<.04) with negative slopes.

Table I- HPLC analysis of the products by the oxidation of sugars by BAT and BAB in alkcJine medium

Sugar Mole of BAT/BAB

o-Galactose

L-Sorbose

o-Xylose

consumed per mole of sugar

2.8

2.7

2.2

, based on the peak areas.

Glyceric acid

9

44

20

Products (approximate percentage)' Threonic acid Xylonic Ly}[onic Hexonic

erythronic acid acid a,: id acid

44 3 32 12

44 9 3

37 7 36

RANGAPPA el 01.: OXIDATION OF THREOSE-SERIES PENTOSE AND HEXOSES 1909

Table II-Effect of [reactant] on the rate of oxidation of sugars by BAT and BAB at 303K. ([HO·)=0.02 mol dm·). I =0.1 mol dm·3

)

1041 BATloII04[BAB lo 103[S)o 104koh, (S- I)

(mol dm·3) (mol dm') D-Galactose L-Sorbose D-Xylose

BAT BAB BAT BAB BAT BAB

15 30 3.49 3.89 14.39 18.62 6.58 6.89

20 30 3.45 3.84 14.35 18.55 6.48 6.82

25 30 3.40 3.75 14.30 18.40 6.59 6.91

30 30 3.51 3.8 1 14.4 1 18.62 6.25 6.76

35 30 3.41 3.78 14.39 18.54 6.48 6.83

40 30 3.45 3.83 14.20 18.35 6.29 6.68

20 10 1.15 1.28 4.78 6. 18 2.16 2.27

20 20 2.3 1 2.56 9.57 12.37 4 .32 4.55

20 30 3.45 3.84 14.35 18.55 6.48 6.82

20 40 4.62 5. 12 19. 14 24.73 8.64 9. 12

20 50 5.76 6.43 23.93 30.92 10.8 1 11.37

20 60 6.90 7.68 28.72 37.10 12.96 13 .60

Table III-Effect of [NaOH] on the rate of oxidation of sugars by BAT and BAB at 303K ([OX]o=0.002 mol dm'), [S]0=0.03 mol dm-), 1=0.1 mol dm') )

10JINaOH] D-Galactose (mol dm') BAT BAB

3 0.08 0.09

5 0.22 0.24

10 0.86 0.96

15 1.94 2. 16

20 3.45 3.84

25 5.38 6.01

30 7.74 8.64

35 10.57 11.76

o 10 a> :J) 40 !i) ED 70 70 ED !i) 40 :J) a> 10 o:~

1<t[S1(M)

Figure I-Plots of kd;y" vs [S)o; ([BATlo = (BABlo = 0.002 mol dm'), [Slo= 0.03 mol dm'), [HO·]=O.02 mol om') , I = 0.1 mol dm·)).

104 kobs ( S - I )

L-Sorbose D-Xylose BAT BAB BAT BAB

0.3 0.42 0.1 5 0.16

0.9 1.16 0.4 1 0.43

3.59 4.64 1.63 1.71

8.07 10.42 3.65 3.84

14.35 18.55 6.48 6.82

22.42 28.96 10.13 10.66

32.28 41.74 14.58 15 .35

43.88 56.40 19.85 20.88

Effect of temperature on reaction rate The reactions were studied at different tempera­

tures (298 to 318K) and the Arrhenius plot of log kobs vs Iff were found to be linear (r>.999, s<.03). The activation parameters for the composite reaction were calcu lated (Table IV).

Solvent isotope studies The solvent isotope studies were made in 0 20 and

the ratios kobs(H20) / kobs(020) were between 0.43 and 0.53 (Table V). Proton inventory studies were made in H20-020 mixture.

Test for free radicals Addition of aqueous solution of acry lamide to the

reaction mixture did not cause polymerization showing the absence of free radical species during oxidation.

1910 INDIAN J. CHEM., SEC B, SEPTEMBER 2002

Table IV- Thermodynamic parameters for oxidation of sugars by BAT and BAS at 303K (lOX]0=0.002 mol dm·),[S]0=0.03 mol dm·), I HO·I=0.02 mol dm·), 1=0. 1 mol dm·)]

Thermodynamic D-Galactose L-Sorbose D-Xylose parameter BAT BAB BAT BAB BAT BAB

Eo k J mor l 122.1 120. 1 107 106 108 107.4

I'lil k J mor l 119.6 117.5 104 103 105.5 104.8

I'lC# kJmor l 93.8 93.6 90 90 92 .8 92.5

I'lS J K· I mor l 83.6 77.4 45 44 41.2 40. 1

Log A 22.5 22.2 2 1 20 20.4 20.3

Table V-Proton inven tory studies for the oxidation of sugars by BAT and BAB in water-deuterium oxide mixture at 303K ([OXlo =0.002 mol dm·), [HO·]=0.02 mol dm·), [S]0=0.03 mol dm·), 1=0.1 mol dm·)]

Atom fraction of Deuterium D-Galactose

(n) BAT BAB

0.00 3.45 3.84

0.25 3.91 4.40

0.50 4.43 5.07

0.75 5.52 6. 14

0.93 7.94 8.26

Analysis of products High pH anion exchange chromatography revealed

the formation of similar oxidation products for the threose-series sugars studied. A comparison of HPLC and GLC-MS retention times of the reaction products with those of the standards, indicated that Iyxonic, xy­Ionic, threonic and glyceric acids were the products of oxidation for all threose series sugars studied, for hexoes, besides these acids, small proportions of hex­onic acids were observed (Table I). The identities of all the oxidation products were confirmed by their mass fragmentation patterns. Xylose gave major pro­portions of Iyxonic, threonic and glyceric acids and minor proportions of xylonic acid. The oxidation prod­ucts were analyzed at 0.5, I, 2, 4, 8, 20 and 24 hr for all the sugars. The relative propOltions of various aldonic acids formed were similar at all time points analyzed.

All the aldohexoses studied here were oxidized mainly to pentonic and tetronic acids rather than to hexonic acids. This finding is in agreement with our earlier work I and confirms that threose-series hexoses are oxidized by bromamines in the keto-enolic form.

Furthermore for all the hexoses studied here, theronic and pentonic acids were formed within 30 minutes, hexonic acids were detectable in minor pro­portions only after significant amounts of the major products are formed. This demonstrates that the lower arbon aldonic acids were not derived from initially

formed six carbon aldonic acids.

104 k obs (s - I )

L-Sorbose D-Xylose BAT BAB BAT BAS

14.35 18.55 6.48 6.82

17.27 22.62 7.60 7.88

19.66 26. 14 8.87 9.05

23.12 29.96 10.37 10.90

27.08 35.40 14.26 15.32

Formation of high proportions of pentonic acids from xylose suggests that pentoses react with BAT or BAB predominantly in the aldo-enolic form. The formation of glyceric acid also in high proportion suggests that appreciable amounts of xylose react in the keto-enolic form to give glyceric and threonic acids.

Discussion The identical orders with respect to both oxidant

(BAT or BAB) and sugars suggest a common mecha­nism for the oxidation of sugars by oxidants (BAT and BAB). The organic haloamines behave as strong electrolytes in aqueous solution and the several equi­libria present are predominantly pH dependent.7

.9 Al­

though the oxidizing species in acidic solutions of BAT or BAB are RNBrH, RNBr2 and hypobromous acid, it has been established that in alkaline medium RNB( is the active oxidant (R = p-CH3C6H4S02 for BAT and R = C6H5S02 for BAB).,o-12

In alkaline solution sugars undergo enolization to form enediolate anions '4 in the absence of other reac­tants, these anions undergo epimerizaton and isomeri­zation (Lobry de Bruyn - Alberda Van Ekenstein transformation) to form a mixture of isomeric aldoses and ketoses.' 3 However in the presence of BAT or BAB the enediolate anions (E-) react with RNBr . to form intermediate (X) which in turn undergoes cleav­age to form products (Table I).

RANGAPPA et af.: OXIDATION OF THREOSE-SERIES PENTOSE AND HEXOSES 1911

In vi~w of the observed first order dependence of rate on oxidant ([BAT]o or [BAB]o) and [S]o and sec­ond-order dependence on [HO·]o. the following reac­tion sequence (Scheme I) is proposed for the oxida­tion of sugars by BAT or BAB in alkaline solutions.

K1 ..., S + HO' ~ E' + H;P

fast

K2 ..., F + RNSr' ~

fast X

k3 X + HO' ----=----l.~

slow, rate determining step

k4 -----'J.~ products

fast

Scheme I

... ( i)

... ( ii )

... ( iii)

... ( iv)

". (I)

where [OX]( = [BA T]( or [BAB](. K, and K2 represents the equilibrium constants for steps (i) and (ii) respec­tively , and k3 represents the specific reaction rate for the rate limiting step. It is assumed that [H20] > I + K2 [E'] and the rate law Eq.(l) is reduced to Eq. (2).

K, K2 k3 [SHOX], [HO - ]2 Rate= .

[H 20] ". (2)

which agrees with the observed rate law, rate = kobs [S] [Ho'f [OX].

The following results support the above conclu­sion.

(i) For reaction involving a fast pre-equilibrium of H+ or HO' ion transfer, the rate increases in deuterium oxide medium, because 0 30 + is a stronger acid than H30 + and DO' is a stronger base than HO·. Therefore, the observed increase of oxidation rate in deuterium oxide agrees with the fast pre-equilibrium transfer of W to HO' ion (step i).' 4 The dependence of rate con­stant (knob,) on ' n' (n = the atom fraction of deuterium oxide in a solvent mixture of water and deuterium oxide) is given by the Gross-Butler Eq.(3).'5.'6

1Z'(1- n + n<f>.) k O /k"=TS I

obs obs 1Z'(l- n + n(<f> .) RS J

". (3)

where <t>j and <t>j are isotopic fractionation factors for the isotopically exchangeable hydrogen sites in the

transitIOn state (TS) and reactant site (RS) respec­tively. If the reaction proceeds through a single transi ­tion state, then the Eq.(3) becomes Eq. (4).

... (4)

A comparison of the plots of kobs vs n (not shown) with the standard curves'? suggested a single proton exchange in the transition state. Furthermore, the plots of (kOobJk"obs) vs 'n' were linear (r>.9992, s<.02) with slopes (<t>j-I) from which <t>j, the fractionation factor of HO' ion can be calculated. The <f>j for the oxidation of threose-series sugars by BAT or BAB is about 0.4. Hence the formation of a single transition state with the active participation of HO- ion is indicated in the present studies.

(ii) Addition of methanol to the reaction mixture decreased the rate. The plots of log kobs vs lID were linear with negative slopes. Assuming a double sphere model'6 for the reaction, the effects of solvent compo­sition on the rate of a reaction involving two negati ve ions is given by the Eq.(5).' 8

log k = log ko - ZAZBe2/DKTdAB . .. (5 )

where ko is the rate constant in a medium of infinite dielectric constant, ZAe and ZBe are the charges on ions, dAB is the size of the activated complex, K is the Boltzmann constant and T is the absolute temperature. From the slope of the linear plots (slope = -ZAZBe2/ KT dAB) dAB values are calculated. The values are 4.02 A, 4.8 A and 4.6A in the case of BAT and 3.4 A, 3.9 A and 3.8 A in the case of BAB for D-galactose, L-sorbose and D-xylose, respectively. These values are comparable with those obtained for similar reactions.' 9

(iii) Scheme I shows that the rate-determining step, step (iii) involves interaction among two negatively charged ions, which requires a very high activation energy. The observed high activation energies (Table IV) agree with this prediction. Nearly constant !1G"# values (Table IV) suggest that a common mechanism is operative for the oxidation of sugars.

In our previous study on erythrose-series hexoses it was observed that each of the sugars studied was oxi­dized by CAT predominantly to pentonic and threonic acids. The observed reaction stoichiometry (Table I) agrees with the formation of mixtures of pentonic, threonic and erythronic acids. In the case of galactose, the major products are formed by the loss of one or two carbon atoms with the cleavage of C-I-C-2 and C-2-C-3 bonds respectively from the keto-enolic

19 12 INDIAN J. CH EM .• SEC B. SEPTEM BER 2002

OH OH OH CH H~t~H

t=O tHCH tHCH tHCH tHPH

CH C-H ~ t ;N, ~~6J Br R

H - tcY~ ~

H~t- H H ~ t - H cx:.c.H

l-O-ttO~r B? Jl-CH tHCH CH tHCH - ~

tHCH -RN 2-

tHCH t HPH

Keto-hexose (sarbose)

Keto-enolic anion

!OH (8

OH R, CH H-t -H N H ~ t - H

t -OJB(~ ~_

tHCH 0;

tHCH -tHCH tHPH

tHCH + tHCH tHCH

tHCH I -CHCH - Br tHCH I I -CHpH CHCH CHCH CH20H

tHPH tHPH Penton ic acid

Xl (Xylonic acid or Lyxoni acid)

cx:.c.H cx:.c.H

HO-~ • HO- ~')H~ ~ CHOH CHOH - Br

wt- OH

tHCH I Ct-O~

+

1-0-6-H

tHCH tHCH tHPH tHOH tHCH

6HPH 6H OH Keto-enolic aniol'l 2

(Ej

CH H ~ t - H

l-O- tJo~r H"""bio;H •

I-O-t -'H -CHpH, -B~ w 6-CH - cx:.c.H , - H

tH20H

X1 and X2

H

HO-t -O~r H@[;tH wt-CH

tHPH

H

6=0 wt- OH

tHCH tHCH tHPH

- HOOCH, + -

- H ,-Br

H

~O I-O-t -H

H"""t-CH t HPH

Threose

Pentoses (Xylose) Aldo-enolic anion

(E )

RNBr,CH •

RNBr,CH • -RN 2-

X2

H

l-O-t,.Or:-r I-O-t -H wt-CH

tHPH

H

1-0-61 oS:; wt- OH

tHPH

- CH,,()-l tHPH

Isomeric penton ic acids (Xylon ic acid + Lyxoni acid)

+ -- H , -Br

CCO-l

I-O-t -H wt-· CH

tHPH

+ -- H ,-Br

+

H

Threon ic acid

cx:.c.H

to- t -·H tH"OH

Glyceric acid

l-O -t"O~r I-O-t -H

tHCH I CHO-:

tHPH

cx:.c.H

f~, cx:.c.H

+

I-O- t -H tHCH ttO-i tHPH

cx:.c.H cx:.c.H

wt- OH tHCH tHCH t HPH

Lyxonic acid Xylonic acid I-O-t -H + I-O-t -H wt- CH tH OH

2

tHPH Threonic acid Glyceric acid

Scheme II

RANGAPPA el al.: OXIDATION OF THREOSE-SERIES PENTOSE AND HEXOSES 191 3

anion intermediates. The predominance of lyxonic acid and minor amounts of xylonic acid suggest that, for these sugars, the cleavage of the C-I-C-2 bond occurs without appreciable epimerization at C-3. The formation of significant amounts of hexonic acid from these sugars, suggest that appreciable proportions of the sugars react with oxidant in the aldo-enolic forms, which on cleavage of the C-I-H bond form hexonic acids. Minor amounts of pentonic acids are formed by the cleavage of the C-I-C-2 bond from the aldo-enolic anions of hexoses but sorbose gave mainly threonic and glyceric acids and minor proportions of pentonic acids (xylonic and negligible amount of lyxonic acid). The predominance of threonic and glyceric acids indi­cates the preferential cleavage of C-2-C-3 and C-3-C-4 bonds compared with C-I-C-2 bonds. Since xylonic acid was formed in higher proportion com­pared with lyxonic acid, the cleavage of the C-I-C-2 bond in xylose-series hexoses must occur without ap­preciable epimerization at C-3, as in the case of Iyxose-series hexoses. Formation of only a trace amount of hexonic acid from sorbose is due to negli­gible isomerization of sorbose to glucose and idose.

In contrast to hexoses, xylose gave high pro­portions of pentonic acids. Clearly, these major product~ are formed by the cleavage of the C-I-H bond and also gave hi gh propOltions of threonic and glyceric acids, which are formed by cleavage of the C-I-C-2 and C-2-C-3 bonds respectively. Since the later type of bond-cleavage facilitates through the in volvement of the keto-enolic form, it is possible that portions of pentoses react in the keto-enolic form . However, xylose was not isomerized to xylulose to a signi ficant level. Together, these data suggest that pentoses undergo oxidation by BAT or BAB mainly through aldo-enolic intermediates and only minor proportions may be oxidized via the keto-enolic form. Under similar conditions, xylose was also completely oxidized.

In view of these considerations, a plausible mecha­ni sm for the oxidation of therose-series sugars by BAT or BAB is proposed (Scheme II). This mecha­ni sm accounts for the observed kinetics, reaction stoichiometry and products formed.

In the proposed mechanism (Scheme II) the anions (E') of sugars react with oxidant to form intermediates XI-X3. For threose-series hexoses, the anions (E') intermediates are predominantly the keto-enolic forms and minor proportions of aldo-enolic forms. However, for pentoses, the major reacting species are the aldo­enolic anions ; probably minor proportions of keto-

isomer may also be involved. In the case of anions (E) from hexoses, the loss of hydrogen can occur at either C-l or C-3 to form C-I-C-2 or C-2-C-3 enediols containing a hypobromite group at C-2. Since epimerization at C-3 was limited, as evidenced by the formation of only very minor proportions of epimeric pentonic acids from hexoses, it can be concluded that cleavage of the C-I-H bond occurs preferentially as compared with cleavage of the C-3-H bond to form C­I-C-2 enediols. The enediols thus formed contain polarized double bonds to which hydroxide ion can add at C-2 to form intermediates XI (major) and X2

(minor). XI and X2 then can undergo cleavage of C-C bonds between C-I and C-2, the former giving Iyxonic acid and the latter forming a mixture of Iyxonic and xylonic acids.

In the case of aldo-enolic anions from pentoses, hydrogen can be removed only from C-2 to form the C-J-C-2 enediol-anion, which in the presence of BAT or BAB and alkali forms intermediate X3 with epim­erization at C-2. The cleavage of C-I-H bonds from X3 gives a mixture of Iyxonic and xylonic acids. The cleavage of C-C bonds between C-2 and C-3 in XI and X2 and the breaking of C-C bonds between C- I and C-2 in X3 yields aldo-tetrose without epimeriza­tion at C-4 (hexoses) or at C-3 (pentoses). The aldo­tetrose further oxidizes to yield threonic acid and a minor proportion of erythronic acid (Table I). The reaction can proceed further with the cleavage of C-C bonds between C-3 and C-4 of hexoses and the break­ing of C-C bonds between C-2 and C-3 of pentoses, to form glyceric acid. Minor proportions of threonic and glyceric acids could also be formed by the cleavage of C-I-C-2 and C-2-C-3 bonds, respectively, from the keto-enolic form of pentoses through the reaction se­quences.

Acknowledgement

One of the authors (VS) is grateful to the UGC, New Delhi, for awarding the Teacher Fellowship. Author (KSR) thanks the Depaltment of Science and Technol­ogy (DST), New Delhi for the financial support.

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1914 INDIAN J . CHEM., SEC B, SEPTEMBER 2002

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