Formation of Ethyl 1‐Thiomannopyranosides from 2‐ O ‐Chloroacetylated and 2‐ O ‐Levulinoylated Synthons

With the aim to prepare key glycosyl donors for the synthesis of fragments of the O‐specific polysaccharides of Vibrio cholerae O:1, 1‐ O ‐acetyl derivatives of perosamine bearing participating 2‐ O ‐chloroacetyl or 2‐ O ‐levulinoyl groups were treated with EtSH and BF 3 · Et 2 O. While poor stereoselectivity of the formation of ethyl 1‐thioglycosides (α:β = 3:2) was observed with 2‐ O ‐chloroacetylated intermediates, the same products can be obtained with good stereoselectivity (α:β = 7:1) from 2‐ O ‐levulinoylated intermediates. Selective regeneration of the carbonyl group from dithioketals in the presence of S,O‐acetals by treatment with AgNO 3 and Ag 2 O in MeCN/H 2 O is also described. The conversion allows direct preparation of 1‐thioglycosides from carbohydrates protected with the levulinic ester group. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)