Electrochemical bromination of peracetylated d-glucal: Effect of DMSO on chemoselectivity

The electrochemical bromination of 3,4,6-tri-O-acetyl-d-glucal (1) has been investigated in dimethyl sulfoxide (DMSO) by cyclic voltammetry and preparative-scale electrolyses. In this solvent, the bromination involves a bromine–DMSO complex of the type [DMSO–Br]+Br− whose reactivity towards 1 was clearly evidenced by cyclic voltammetry. Importantly, only the monobrominated glucose and mannose derivatives were obtained from electrolyses. This specific behavior was attributed to the nucleophilicity of DMSO that may react with the oxocarbenium intermediate preventing thus a possible second bromination of the transient species. The gluco/manno ratio, roughly equal to 30/70, indicated an electrophilic attack by [DMSO–Br]+Br− preferentially from the β side of the starting glycal for steric reasons. The treatment of the crude product obtained after the end of the electrolyses with acetic anhydride allowed the preparation of derivatives in which the anomeric carbon bears an acetoxy group. Based on both the cyclic voltammetry experiments and preparative-scale electrolyses, a mechanism is proposed for the bromination of peracetylated d-glucal in DMSO. These new and original results are of high interest in carbohydrate chemistry and especially for the preparation of new valuable oligosaccharides.