Neighbouring group participation in isoxazole ring bromination. Part I. Spirobi-isoxazolines from 5-phenacyl-3-phenylisoxazole oxime

Abstract

Bromination with N-bromosuccinimide or bromine of the (E)-oxime of 5-phenacyl-3-phenylisoxazole (2) gives (4RS,5SR)-4-bromo-3,8-diphenyl-1,6-dioxa-2,7-diazaspiro[4,4]nona-2,7-diene (3), which results from cis-addition of the neighbouring oxide group to the highly stabilized isoxazolium ion derived from the electrophilic attack of bromine upon the isoxazole ring. (4RS,5SR,9RS)-4,9-Dibromo-3,8-diphenyl-1,6-dioxa-2,7-diazaspiro-[4,4]nona-2,7-diene (4) is also formed by –CH₂– group radical bromination of oxime (2) before the electrophilic attack on the isoxazole ring.