(
)
66
K.S. Rangappa et al.rCarbohydrate Research 306 1998 57–67
proposed in Scheme 2. This mechanism accounts for
the observed kinetics and products of oxidation.
chromatography was used for the identification of the
w
x
oxidation products 4–6 . Because of poor resolution
under the conditions employed, the product identifi-
cation was not definitive. In the present study, the
stoichiometry was estimated after an incubation pe-
riod of 24 h, and the products were rigorously ana-
lyzed using HPLC and GLC–MS. Thus, the results of
Ž
.
In the proposed mechanism Scheme 2 , the anions
y
Ž
.
Ž
E
of sugars keto-isomer in the case of hexoses
and aldo-isomer in the case of pentoses react with
.
CAT to form intermediates X1–X3. In the case of
y
Ž
.
anions E from hexoses, the loss of hydrogen can
occur at either C-1 or C-3 to form C-1–C-2 or
C-2–C-3 enediols containing chloroxyl group at C-2.
Since these enediols contain polarized double bonds,
hydroxide ion can add at C-2 to form intermediates
X1 and X2; the formation of X2 accompanies
epimerization at C-2 and C-3. X1 and X2 then can
undergo cleavage of C–C bonds between C-1 and
C-2, the former giving arabinonic acid and the latter
forming a mixture of arabinonic and ribonic acids. In
the case of Ey from pentoses, hydrogen can be
removed only from C-2 to form the C-1–C-2 enedio-
late anion, which in the presence of CAT and alkali
forms intermediate X3 with epimerization at C-2. The
cleavage of C-1–H bonds from X3 gives a mixture of
arabinonic and ribonic acids. The cleavage of C–C
bonds between C-2 and C-3 in X1 and X2, and the
breaking of C–C bonds between C-1 and C-2 in X3
yield aldo-tetrose without epimerization at C-4
Ž
the present study suggest a novel pathway Scheme
.
2 for the oxidation of sugars by CAT.
5. Graphic summary
N-Chloro-p-toluenesulfonamide oxidizes erythro-
series pentoses and hexoses to a mixture of arabi-
nonic, ribonic, erythronic, and glyceric acids through
enediol-anion intermediates. Based on kinetics stud-
ies and on product profiles, a plausible mechanism is
suggested.
Acknowledgements
We thank Professor Derek Horton, The American
University, Washington, DC, for suggestions regard-
ing the mechanism, Dr. Roberta Merkle, Complex
Carbohydrate Research Center, University of Geor-
gia, and Dr. Lewis Pannell, NIDDK, NIH, for GLC–
MS analyses. M.P.R. is grateful to the University of
Ž
.
Ž
.
hexoses or at C-3 pentoses . The aldotetrose fur-
ther oxidizes to yield erythronic acid and a minor
Ž
.
proportion of threonic acid Table 5 . 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.
Ž
.
Mysore for the University Grants Commission UGC ,
India, research fellowship. D.S.M. thanks the UGC,
India, for awarding the emeritus Professorship.
Although in the proposed mechanism, CAT is
shown to react with preformed enediol-anion inter-
mediate, it is possible that the ring opening of sugars
is assisted by CAT. Since the furanosidic form of
sugars is expected to be less energetic compared with
the pyranosidic form, CAT may preferentially react
with the furanosidic form rather than pyranosidic
form. Consistent with this suggestion, it is known
that a significant proportion of fructose exists in the
furanose configuration, whereas glucose and mannose
exist almost exclusively in the pyranose configuration
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