Electrochemical bromochlorination of peracetylated glycals

Peracetylated glycals—3,4,5-tri-O-acetyl-d-glucal (1a), 3,4,5-tri-O-acetyl-d-galactal (1b) and 3,4-di-O-acetyl-6-deoxy-l-glucal (1c)—have been bromochlorinated by a suitable halogenating agent, generated electrochemically from a mixture of bromides and chlorides in dichloromethane. The reaction was performed in two ways: (i) by a constant current electrolysis (2 F mol−1) of bromides and substrates in a milieu containing an excess of chlorides (Br⦵Br⦵/1/Cl⦵Cl⦵ = 1:1:6.8) and (ii) by anodic generation of free chlorine from chlorides (2 F mol−1) and subsequent addition of bromides and substrates in a ratio Br⦵Br⦵/1 = 1:1. The corresponding 2-bromo-2-deoxy-glycopyranosyl chlorides were obtained in high yields.

Graphical abstractFigure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Cyclovoltammetric investigations of reaction conditions for the electrochemical halogenation of peracetylated glycals. ► Electrochemical generation of halogenating species. ► Synthesis of 2-bromoglycopyranosyl chlorides. ► Hemoselectivity in halogenation of peracetylated glycals.