Protecting Group Dependence of Stereochemical Outcome of Glycosylation of 2-O-(Thiophen-2-yl)methyl Ether Protected Glycosyl Donors

A series of glycosyl donors possessing a (thiophen2-yl) methyl ether protecting group at position 2 were synthesised and the effect of the protecting group pattern of other hydroxyls on the stereochemical outcome of glycosylation was investigated. Studies revealed optimal a-selectivity for glycosylation using a fully armed tri-benzylated donor, whilst other protecting group patterns were significantly less effective. Lowtemperature NMR studies of both fully armed and fully disarmed donors revealed the intermediacy of cyclised sulfonium ion intermediates. Reaction conditions were developed which allowed removal of the (thiophen-2-yl) methyl ether protecting group either selectively, or together with benzyl ethers.