The Influence of Anomeric Configuration and Aglycone Structure on the Outcome of Acid-Promoted Ring Contraction in 2,3-Di-O-Silylated S-Galactopyranosides

摘要

Acid-promoted rearrangement of a series of 1-thio-alpha/beta-d-galactopyranosides with bulky triisopropylsilyl (TIPS) or tert-butyldiphenylsilyl (TBDPS) groups at both O-2 and O-3 and bearing various substituents at the S atom in anomeric position leading to ring contraction has been studied. The reaction rates of pyranose ring contraction in S-galactopyranosides were found to be strongly influenced by anomeric configuration and aglycone structure. Ethyl 1-thio-beta-d-galactopyranosides underwent ring contraction faster than ethyl 1-thio-alpha-d-thiogalactopyranosides. 1-Thio-beta-d-galactopyranosides with TIPS groups bearing electron-donating 5-tert-butyl-2-methylphenyl group in aglycone could be involved into rearrangement to furanosides under milder acidic conditions (aq TFA in CH2Cl2) than the corresponding 1-thio-beta-d-galactopyranosides with phenyl or 4-chlorophenyl groups in aglycone (anhydrous TFA in CH2Cl2). On the contrary, 1-thio-beta-d-thiogalactopyranoside bearing electron-withdrawing 4-nitrophenyl group in aglycone did not undergo ring contraction under identical conditions.