Common side reactions of the glycosyl donor in chemical glycosylation

•Glycosyl donor hydrolysis is discussed.•Glycosyl donors may undergo elimination reactions in competition with substitution.•The sulfur atom of thioglycosides may act as a nucleophile in glycosylation.•Glycosyl trichloroacetimidates can undergo rearrangement to amides.

Chemical glycosylation is central to carbohydrate chemistry and is generally recognised as a challenging reaction. This review describes the most reoccurring side reactions of glycosyl donors in glycosylation and how scientists have attempted to explain their observations and in some cases succeeded in solving a particular encountered problem. The topics covered are donor hydrolysis, elimination to form glycals, intermolecular aglycon transfer of thioglycosides and glycosyl imidate rearrangement.

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