Polycyclic hydroxyquinones. Part 22. Diels–Alder reaction with chloro derivatives of anthracene-1,4,9,10-tetraone

Abstract

The Diels–Alder reaction of 2-chloro- and 2,3-dichloro-anthracene-1,4,9,10-tetraones (8) and (9) with substituted buta-1,3-dienes occurred at the internal double bond and gave angular adducts. The reaction of quinizarin (4) with chlorine in glacial acetic acid afforded the 2,3,4a,9a-tetrachloro-4a,9a-dihydroanthracene-1,4,9,10-tetraone (17). The cycloaddition of (17) with 2,3-dimethylbuta-1,3-diene (10) gave the fully aromatised linear adduct (19) whereas a reactive diene such as 1-methoxy-3-trimethylsilyloxybuta-1,3-diene (12) afforded the ketone (22), originating from the angular adduct. An explanation for the last result involves the ready halogen elimination in compound (17) to give (9) in the presence of a strongly nucleophilic diene.