O-ALKYLATION AT THE ANOMERIC CENTER .5. 1-O-ALKYLATION OF D-GLUCOPYRANOSE

Direct 1-O-methylation of 1-O-metalated 2,3,4,6-tetra-O-benzyl-D-glucose (1) with methyl trifluoromethanesulfonate (3a) as a strong alkylating agent led to the methyl glucopyranosides 4a, with the .alpha./.beta. anomer ratios varying from 4:1 to 1:11 depending on solvent and temperature. With the less electrophilic trifluoromethanesulfonates of D-glucose 3b and 3c, of D-ribose 3d, and D-glycerol (3e) as alkylating agents, 1 gave the .beta.-disaccharides and .beta.-glycosides 4b-.beta.-4e-.beta., exclusively. Product formation is discussed in terms of the various nucleophilicities of the starting materials and in terms of different internal complexation. Introduction of the sterically demanding (p-methoxyphenyl)diphenylmethyl protecting group in the 6-O-position of 2,3,4-tri-O-benzyl-D-glucose to form 8 and subsequent 1-O-metalation and reaction with trifluoromethanesulfonate 3c gave the .alpha.-connected isomaltoside 9c-.alpha.. However, from 1-O-metalated 8 and the trifluoromethanesulfonates of glycerol 3e and 3f under identical reaction conditions, only the .beta.-connected glycosides 9e-.beta. and 9f-.beta., respectively, were obtained. Structures were assigned by 1H NMR and by deprotection to known compounds.