120
M. Li et al. / Carbohydrate Research 338 (2003) 117–121
the desired 16 as a major product, albeit in only 29%
yield. In addition, methylation of the monol 19 was not
successful under many of the usual conditions, e.g.,
MeI/Ag2O/DMF, MeOTf/DTBMP, due to the neigh-
boring acyl group migrations. Methylation was finally
effected by means of CH2N2 in the presence of
BF3·OEt2, giving 20 in 87% yield. Glycosylation with
monomethylated rhamnopyranosyl trichloroacetimi-
dates 23 and 24 and thioglycoside 25 under the promo-
tion of TMSOTf and NIS/AgOTf, respectively,
afforded the expected products (26–28 and 36–38) in
satisfactory yields (66–96%). Especially, glycosylation
with donor 23, in the absence of neighboring group
participation at the C-2 position, provided only the a
products (26 and 36), reflecting a strong anomeric effect
National Natural Science Foundation of China
(29925203).
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Acknowledgements
This work is supported by the Ministry of Science
and Technology of China (G1998051104), the Shang-
hai/Hong Kong/Anson Research Foundation, and the
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† The structures of 1–8 were unambiguously determined by
extensive 2D NMR analysis and were further confirmed by
ESIMS and elemental analysis.