Benzocyclobutenes. Part 5. Synthesis of 4-hydroxy-, 4,5-dihydroxy-, and 3,6-dihydroxy-benzocyclobutene-1,2-dione (benzologues of semisquaric and squaric acid)

Abstract

4-Methoxy- and 4,5-dimethoxy-benzocyclobutene-1,2-dione have been made by flash vacuum pyrolysis of the anthracene adducts of the corresponding phthalazine-1,4-diones which were prepared from the appropriate methoxyphthalic anhydrides. 3,6-Dimethoxybenzocyclobutene-1,2-dione has been prepared from 2-amino-3,6dimethoxybenzoic acid via the addition of 3,6-dimethoxybenzyne to vinylidene chloride followed by hydrolysis of the resulting 1,1-dichloro-3,6-dimethoxybenzocyclobutene, bromination, and further hydrolysis. The three methoxy-diones and 4,5-dimethoxyphthalaldehyde have been demethylated by heating them with hydrobromic acid and the pK_a values of the four hydroxy-compounds have been measured. Preliminary experiments on the synthesis of 4,5-dimethoxybenzocyclobutene are recorded.