Preparation and ring fission of some benzocyclobuten-1(2H)-one derivatives

Abstract

2,5-Dihydroanisole and its 3-methyl homologue react with dibromocarbene to give 1:2 adducts (I; R = H or Me) which, when boiled with pyridine, give pyridinium salts (II). These are converted by the Kröhnke reaction into the corresponding 3-bromo-6-methoxybenzocyclobutenones (III). The structure of the ketone (III; R = H) is proved by its fission under alkaline conditions and by oxidation of its pyridinium salt precursor, to give known compounds. Alkaline fission products of two isomeric bromomethoxybenzocyclobutenones are compared and the results are retionalised in terms of the –I and +E effects of the methoxy-group.