Benzoquinones and related compounds. Part 4. Thermolysis of the Diels–Alder adduct of 2-acetyl-5,6-dichloro-1,4-benzoquinone and cyclopentadiene: evidence for a partial retro-diene reaction

Abstract

Addition of chlorine to (2-methyl-1,3-dioxolan-2-yl)-1,4-benzoquinone occurs at the unsubstituted double bond. Subsequent enolisation and cleavage of the acetal affords 2-acetyl-5,6-dichlorohydroquinone in 50% overall yield. Oxidation of this gives the corresponding 1,4-benzoquinone which with cyclopentadiene yields, predominantly, the 1:1 Diels–Alder adduct (6) by endo-addition to the 2,3-double bond. Thermolysis of this adduct in benzene results in disproportionation to cyclopentadiene and the spiro-acetal (13); thermolysis in acetic acid also yields (13), but the major product is the dihydrobenzofuran (14), an isomer of the Diels–Alder adduct. Mechanisms for the formation of these products are discussed.