Phenyliodine(III) diacetate-mediated dearomatization of 2-(2-hydroxyaryl)benzoxazoles and 2-(2-hydroxyaryl)benzothiazoles: Regio- and stereoselective synthesis of tetramethoxycyclohexenones and bicyclo[2.2.2]octenones

Abstract

A range of stereochemically defined oxidative dearomatization products formed by treating 2-(2-hydroxyaryl)benzoxazoles and 2-(2-hydroxyaryl)benzothiazoles with phenyliodine(III) diacetate have been described, demonstrating remarkable regiochemical and stereochemical dictated by the nature of the phenol substituents. Benzoxazole-substituted phenols undergo dearomatization leading to the regio- and stereoselective synthesis of tetramethoxy-substituted cyclohexenones (4,4,5,6-tetramethoxycyclohex-2-en-1-ones). However, benzothiazole-substituted phenols readily undergo regio- and stereoselective [4+2] dimerization to bicyclo[2.2.2]octenone. Further, halogenated 2-(2-hydroxyaryl)benzoxazoles lead to formation of a different tetramethoxy-substituted cyclohexenones (2,4,5,6-tetramethoxycyclohex-2-en-1-ones) whereas halogenated 2-(2-hydroxyaryl)benzothiazoles afforded 4-methoxycyclohexa-2,5-dien-1-ones. The structure of all the compounds were established using NMR, FT-IR and HRMS techniques, and the structure of two compounds were confirmed by X-ray crystallography.