Preparation and trapping of 1-Acetoxyisobenzofuran. The structure of a novel phthalide formed in the attempted generation of 1-Acetoxyisobenzofuran in the presence of quinones

Abstract

A new potential synthesis of anthraquinones based upon 1-acetoxyisobenzofuran (4) is reported. The key reaction used to generate isobenzofurans, namely the s-tetrazine reaction, proceeds smoothly in the presence of N-methylmaleimide to yield the expected adduct (16). In contrast, the reaction takes a different path in the presence of quinones to yield a novel phthalide (15). These reactions are considered to involve a non-concerted fragmentation of the intermediate dihydro-pyridazine (3) to yield the stabilized zwitterionic species (13). The zwitterion leads on to the phthalide (15) in a two step sequence involving the quinone as oxidant, while normal fragmentation is the sole reaction in the absence of oxidant.    

The structure of 3-[3',6'-di(pyridin-2''-yl)pyridazin-4'- yl]isobenzofuran-1(3H)-one (15) has been determined by single crystal X-ray diffraction and refined by full-matrix least squares to a residual of 0.107 for 1389 'observed' reflections. Crystals are monoclinic, P2₁/c, a 8.271(2), b 19.183(4), c 11.814(2)Ǻ, β 110.0(1)β.