Preparation of alpha and beta anomers of 1,2,3,6-tetra-O-acetyl-4-chloro-4-deoxy-D-galactopyranose based upon anomerization and kinetic acetylation

摘要

4-Chloro-4-deOXY-alpha-D-galactopyranose, 1,2,3,6-tetra-O-acetyl-4-chloro-4-deoxy-alpha-D-galactopyranose and 1,2,3,6-tetra-O-acetyl-4-chloro-4-deoxy-beta-D-galactopyranose were readily prepared from 1,4:3,6-dianhydro-beta-D-fructofuranosyl 4-chloro-4-deoxy-alpha- D-galactopyranoside. In the study, we found an interesting anomerization phenomenon of 4-chloro-4-deoxy-D-galactose. The molar ratio of alpha and beta anomers in solution is about 1:2 when the anomerization reaches a dynamic equilibrium, and the beta anomer could completely convert to the alpha anomer in the process of crystallization and precipitation. The acetylation of 4-chloro-4-deoxy-D-galactopyranose is kinetically controlled, and the configuration of the starting galactose determines the configuration of the resulting acetates. The influence of the chloro group at C-4 and the O-acetyl group at the anomeric carbon on the galactopyranose ring conformations is discussed, based upon the crystallographic data for the alpha and beta anomers of 1,2,3,6-tetra-O-acetyl-4-chloro-4-deoxy- D-galactopyranose. (C) 2005 Elsevier Ltd. All rights reserved.