The effect of electron withdrawing protecting groups at positions 4 and 6 on 1,2-cis galactosylation

Ethyl 2,3,4-O-tribenzyl-6-O-chloroacetyl-1-d-thiogalactoside, phenyl 4,6-O-diacetyl-2,3-dibenzyl-1-d-thiogalactoside and phenyl 2,3-O-dibenzyl-4,6-O-dichloroacetyl-1-d-thiogalactoside were employed in the study of the stereoselectivity of the glycosylation reaction with several acceptors, ranging from unhindered linear primary alcohols to other sugars, using NIS/TfOH as activator. Higher α-selectivities were obtained in the glycosylation reactions with phenyl 2,3-O-dibenzyl-4,6-O-dichloroacetyl-1-d-thiogalactoside as the donor, showing that a stronger electron withdrawing 4-O-ester group had an influence in the anomeric selectivity favouring the formation of 1,2-cis galactosides.