Benzocyclobutenes. Part 4. Synthesis of benzocyclobutene-1,2-diones by pyrolytic methods

Oxidation of the cyclic hydrazides prepared from phthalic anhydrides in the presence of anthracene gives the corresponding Diels-Alder adducts which, on flash vacuum pyrolysis, give benzocyclobutene-1,2-dione (BBD) and its 4-chloro, 3,6- and 4,5-dichloro, 4,5-dibromo, and 4,5-dimethyl derivatives in 75–98% yield. Cyclobuta[b] and cyclobuta[a]-naphthalene-1,2-dione as well as cyclobuta[b]- and cyclobuta[a]-pyridine-1,2-dione have been prepared similarly; the last three of these diones are very unstable. Cyclobuta[b]naphthalene-1,2-dione has also been made by pyrolysis of benz[f]indene-1,2,3-trione. Attempts to make thiophen and furan analogues of BBD from appropriate anthracene adducts failed as did attempts to make tetrachloro- and tetrabromo-derivatives of BBD by the pyrolysis of tetrahalogenophthalimidosulphoximides.