EVIDENCE FOR CYCLIC BROMONIUM ION TRANSFER IN ELECTROPHILIC BROMINATION OF ALKENES - REACTION OF OMEGA-ALKENYL GLYCOSIDES WITH AQUEOUS N-BROMOSUCCINIMIDE

Evidence is provided to support the theory that intermolecular Br+ tra nsfer from a cyclic bromonium ion to an alkene occurs readily and can indeed overwhelm alternative reaction pathways. In the course of a stu dy to determine which omega-alkenyl glycosides could serve as glycosyl donors, it was found that upon treatment with N-bromosuccinimide (NBS ) in aqueous acetonitrile, under conditions in which an n-pentenyl gly coside underwent oxidative hydrolysis to the corresponding hemi-acetal , allyl, butenyl, and hexenyl analogs gave bromohydrin addition produc ts. It was further found that when pentenyl and hexenyl analogs were m ade to compete for an insufficient amount of NBS, the former reacted w hile the latter was apparently recovered unchanged. However, both reac ted independently at similar rates. In addition, the phenomenon was fo und to be concentration dependent. These results are consistent with t he intermolecular, non-degenerate transfer of Br+ from cyclic bromoniu m ion to alkene. Copyright (C) 1996 Elsevier Science Ltd