Some novel aspects of the synthesis of oxygen heterocycles

The reactions of benzyne and tetrachlorobenzyne with αβ-unsaturated aldehydes which form 2-H-chromen derivatives have been investigated. The mechanism was established by [14C] and [2H] labelling experiments and proceeds initially through a benzoxeten intermediate. Subsequent opening of these intermediates followed by thermal cyclisation of an o-quinone methide gave the products. A degradation of 5,6,7,8-tetrachloro-2-H-flaven to a tetrachlorosalicylic acid derivative was carried out, to confirm the position of the [14C] radiolabel. Also described are a number of rearrangement reactions of 5,6,7,8-tetrachloro-4-H-flaven in which 2-benzoyl-dihydrobenzofuran, or 2-benzoyl-benzofuran derivatives were obtained if bromine was present in the reaction mixture. [Continues.]