Preparation of α and β anomers of 1,2,3,6-tetra- O-acetyl-4-chloro-4-deoxy- d-galactopyranose based upon anomerization and kinetic acetylation

Abstract

4-Chloro-4-deoxy-α- d-galactopyranose, 1,2,3,6-tetra- O-acetyl-4-chloro-4-deoxy-α- d-galactopyranose and 1,2,3,6-tetra- O-acetyl-4-chloro-4-deoxy-β- d-galactopyranose were readily prepared from 1,4:3,6-dianhydro-β- d-fructofuranosyl 4-chloro-4-deoxy-α- d-galactopyranoside. In the study, we found an interesting anomerization phenomenon of 4-chloro-4-deoxy- d-galactose. The molar ratio of α and β anomers in solution is about 1:2 when the anomerization reaches a dynamic equilibrium, and the β anomer could completely convert to the α 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 α and β anomers of 1,2,3,6-tetra- O-acetyl-4-chloro-4-deoxy- d-galactopyranose.