Selective benzylation of some D-galactopyranosides. Unusual relative reactivity of the hydroxyl group at C-4

Abstract

Partial benzylation of methyl 2,3-di- O-benzyl-α- D-galactopyranoside gave methyl 2,3,6-tri- O-benzyl-α- D-galactopyranoside as the major product, whereas the isomeric 2,6-di- O-benzyl ether gave a mixture of products in which the ratio of methyl 2,4,6- to methyl 2,3,6-tri- O-benzyl-α- D-galactopyranoside was ≈4:1. The proportion of unreacted starting-material was low in both cases, whereas after a similar reaction of methyl 2,6-di- O-benzyl-β- D-galactopyranoside more than 50% of the dibenzyl ether was recovered unchanged. In this case also, considerably higher reactivity was exhibited by the hydroxyl group at C-4 than that at C-3. Acid hydrolysis of the methyl glycosides of the tribenzyl ethers afforded crystalline 2,4,6-tri- O-benzyl-α- D-galactose and syrupy 2,3,6-tri- O-benzyl- D-galactose. Structures of intermediates were established by acetylation, examination of their n.m.r. spectra, and conversion into the known 3- O and 4- O-methyl- D-galactose.