â-C-galactopyranosides, respectively. The major reason for
the high stereoselectivity of these reactions is believed to
be the formation of cyclic sulfur-stabilized intermediates:
episulfonium ions and, in the case of vinyl ethers, five-
membered sulfonium ions (Scheme 2). Following our interest
Scheme 3a
Scheme 2
in the use of these and similar cyclic intermediates for
synthesis of C-glycosides and related compounds, we have
investigated the possibility of preparing these carbohydrate
derivatives by reacting exo-glucal (1) with the episulfonium
ions generated from â-(arylsulfanyl)alkyl chlorides. In this
paper, we report the initial results of our study.
a
Values with an asterisk are yields given for the mixtures of
diasteromers.
exo-Glucal7 (1, 2,6-anhydro-3,4,5,7-tetra-O-benzyl-1-
deoxy-D-glucohep-1-enitol) was synthesized from com-
mercially available methyl R-D-glucopyranoside using the
was isolated, and its cyclic sulfonium structure was proven
by X-ray crystallographic analysis. Reactions of the sulfo-
nium intermediates 8a,b with H
NaCNBH
tively.
8-13
reaction sequence
proposed by the R. J. K. Taylor group
1
and the Ramberg-B a¨ cklund rearrangement as a key step
2
O at 0 °C and MeOH and
(Scheme 3).
3
at -78 °C provided glycosides 10-12, respec-
The episulfonium ion 6 was prepared in situ from 1-chloro-
1
-methoxy-2-methyl-2-(p-tolylsulfanyl)propane (4) obtained
by the addition of p-TolSCl to 1-methoxy-2-methylpropene
2, Scheme 3). The episulfonium salt reacted with exo-glucal
at -78 °C to form another intermediate that has either the
Compounds 10-12 have two new chiral centers; thus,
mixtures of up to four diastereomers could be expected. The
(
1
1
13
analysis of H and C NMR spectra of 12 indicated that the
glycoside was obtained predominantly in one diastereomeric
form with traces (less than 5%) of another isomer. In the
case of derivatives 10 and 11, the reactions were less
oxonium (8a) or cyclic sulfonium (8b) structure or, most
likely, is a mixture of 8a and 8b in equilibrium. The salts
8a or 8b could not be isolated; however, the intermediate of
stereoselective and the mixtures of two isomers in a ratio of
a similar ArSCl-mediated dimerization of two vinyl ethers
1
9
1
:1 ( H NMR data) were obtained. Pure compounds 10a,
1a, 11b, and 12a were isolated using TLC.
(
6) Koikov, L. N.; Smoliakova, I. P.; Liu, H. Carbohydr. Res. 2002,
37, 1275-1283.
7) (a) Yang, W.-Y.; Yang, Y.-Y.; Gu, Y.-F.; Wang, S.-H.; Chang, C.-
It is known that, due to the anomeric effect, reactions of
3
(
pyranoxonium intermediates with nucleophiles occur through
attack of the latter from the bottom face furnishing predomi-
C.; Lin, C.-H. J. Org. Chem. 2002, 67, 3773-3782. (b) Cambell, A. D.;
Paterson, D. E.; Raynham, T. M.; Taylor, R. J. K. Chem. Commun. 1999,
1
4
nantly, or exclusively, R-glycosides. Since the oxonium
intermediate 8a has an alkyl group at C(1), its reaction with
a hydride ion must lead to the â-C-glycoside 12a. If a
nucleophile reacted with the sulfonium salt 8b, the other
anomer, R-C-glucopyranoside, would be the major product.
Cyclic sulfonium salt 8b is formed as a result of the ArS
group attacking from the bottom face, as directed by the
anomeric effect. This suggests that the absolute configuration
of the C(1) chiral center in the pyranose ring of the sulfonium
1
599-1600 and references therein. (c) Johnson, C. R.; Johns, B. A. Synlett
1
997, 1406-1408 and references therein.
(8) (a) Griffin, F. K.; Murphy, P. V.; Paterson, D. E.; Taylor, R. J. K.
Tetrahedron Lett. 1998, 39, 8179-8182. (b) Belica, P. S.; Franck, R. W.
Tetrahedron Lett. 1998, 39, 8225-8228.
(9) Perrine, T. D.; Glaudemnas, G. P.; Ness, R. K.; Kyle, J.; Fletcher,
H. G. J. Org. Chem. 1967, 32, 664-673.
(10) Glaudemnas, G. P.; Fletcher, H. G. 2,3,4,6-Tetra-O-benzyl-R-D-
glucopyranose. In Methods in Carbohydrate Chemistry; Whistler, R. L.,
BeMiller, J. N., Eds.; Academic Press: New York, 1972; pp 374-375.
(11) Li, P.; Sun, L.; Landry, D. W.; Zhao, K. Carbohydr. Res. 1995,
275, 179-184.
12) Jung, J. C.; Kim, K. S.; Kim, Y. H. Synth. Commun. 1992, 22,
1583-1587.
13) Meyers, C. Y.; Malte, A. M.; Matthews, W. S. J. Am. Chem. Soc.
1969, 91, 7510-7512.
(
(
(14) Postema, M. H. D. C-Glycoside Synthesis; CRC Press: Boca Raton,
FL, 1995; p 2.
3896
Org. Lett., Vol. 4, No. 22, 2002