UV-mediated anomeric activation

Abstract

We report the UV-mediated activation of various glycosyl α-trichloroacetimidates (TCAs) to form glycosyl α-chlorides or O-glycosides depending on the carbohydrate, protecting groups, and solvent. In THF, Et₂O, and CH₂Cl₂, most of the glycosyl TCAs studied underwent conversion to their respective α-chlorides. In the case of per-acetylated glucosyl TCA, a reaction with THF was observed which caused formation of the 4′-chlorobutyl β-glucoside. O-Glycosylation with primary and secondary alcohols present in large excess was found to take place in acetonitrile, which was chosen as solvent because it does not facilitate competing glycosyl chloride formation. Fluorescent byproducts were formed in THF, Et₂O, and in reactions with benzyl-protected TCAs. We propose that this is a result of decomposition of 2,3-dichloromaleimide formed from eliminated trichloroacetamide via a series of radical reactions.