The acid-catalysed oligomerisation of p-benzoquinone

p-Benzoquinone is transformed by mineral acids into a series of complex dibenzofuran derivatives. The phenolic trimer 5a is the dominant crystalline species. Small amounts of a helical tetrameric phenol (11) and a non-phenolic, cyclic tetramer the furo-[8]-circulene (6a) are also formed together with compounds of higher molecular weight. The furan rings are not generated by dehydration of 2,2'-dihydroxybiphenyl elements. A mechanism involving quinone hemiketals is suggested.