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 (2Fmol(-1)) of bromides and substrates in a milieu containing an excess of chlorides (Br(θ)/1/Cl(θ)=1:1:6.8) and (ii) by anodic generation of free chlorine from chlorides (2Fmol(-1)) and subsequent addition of bromides and substrates in a ratio Br(θ)/1=1:1. The corresponding 2-bromo-2-deoxy-glycopyranosyl chlorides were obtained in high yields.