Generation and interception of isobenzofurans from 2-(α-bromoalkyl)benzophenones

Abstract

2-(Bromomethyl)benzophenone, prepared by side-chain bromination of 2-methylbenzophenone, is converted into naphthalene derivatives [(2), (4), and (5)] by heating in tetrachloromethane with olefinic or acetylenic dienophiles. A mechanism is suggested in which 1-phenylisobenzofuran is generated by displacement of bromide by the neighbouring carbonyl group; the isobenzofuran then undergoes cycloaddition and the cycloadducts are aromatised by the hydrogen bromide present. Similar reactions are observed with 2-(2-bromoethyl)benzophenone, but in this case the primary Diels–Alder adducts (7) and (8) can be isolated. Similarly, (Z)-4-bromo-1,3-di-phenylbut-2-en-1-one (9) is converted into 2,4-diphenylfuran by heating in ethanol.