Benzoannulation of Quinones by a Cycloaddition‐Fragmentation Approach. A Simple Synthesis of Methoxycarbonyl‐Substituted Polyacenoquinones

Abstract

The cycloadducts 2A‐5A obtained from the Diels‐Alder cycloadditions of 1,2,3,4‐tetrachloro‐4,5‐dimethoxycyclopentadiene (1) with p‐benzoquinone (2), 1,4‐naphthoquinone (3), 1,4‐anthraquinone (4), and 2,3‐dicyano‐1,4‐benzoquinone (5) were subjected to the reaction with triethylamine in dichloromethane at room temperature. Cycloadducts 2A and 5A enolized to give the corresponding hydroquinones 2B and 5B, which were oxidized with DDQ to afford naphthoquinone ester 2D and anthraquinone ester 5D, respectively. In the cases of cycloadducts 3A and 4A, the enolization occurred concurrently with oxidation and fragmentation to produce directly the polyacenoquinone esters 3D and 4D, respectively. Under the same reaction condition, the unsymmetrical cycloadduct 6A derived from naphthoquinone ester 2D and 1 yielded isomeric polyacenoquinone esters 6Da and 6Db in a ratio of about 8:1.