Glycosyl Radical-Based Synthesis of C-Glycoamino Acids and C-Glycopeptides

Radical-based reactions usually exhibit excellent functional-group compatibilities due to their mild initiation conditions. Glycosyl radical involved C-glycosylation modifications are important strategies to achieve highly regio- and chemoselective constructions of C-glycosidic bonds or C-glycoside linkages of peptides and proteins. In this Concept, we cover recent developments in glycosyl radical-based synthesis of unnatural amino acids and late-stage modification of peptides and proteins, and provide a preliminary outlook on the possible development of this direction in the future.

S-Fluorenylmethyl protection of the cysteine side chain upon N(α)-Fmoc deprotection

Deprotection of an N(α)-Fmoc-protected glycocysteine derivative 7 with an excess of morpholine unexpectedly led to the fluorenylmethyl-protected thioether 8 in high yield. The suggested mechanism for this reaction comprises the addition of the cysteine thiolate on the exocyclic double bond of dibenzofulvene, which is formed during Fmoc deprotection. The influence of base concentration on this transprotection reaction was studied.