Rearrangement reactions of bicyclic systems. Part III. Acid-catalysed rearrangements of 1,4-dihydro-1-methoxy-1,4-ethenonaphthalene (1-methoxybenzobarrelene) and its 5,6,7,8-tetrahalogeno-derivatives

Abstract

Rearrangements of the title compounds in sulphuric acid gave mixtures of isomeric ketones of structural types (4)[3,4-dihydro-1,4-ethenonaphthalen-2(1H)-one], (7)(5,8-dihydro-5,8-methanobenzocyclohepten-9-one), and (10)(5,9-dihydro-5,9-methanobenzocyclohepten-6-one). The mechanisms of the reactions have been clarified by studies in deuteriosulphuric acid and by using specifically deuteriated substrates. The compounds of type (4) arise by two independent pathways from a C-3 cation, and those of type (7) and (8) arise from a C-2 cation. The products do not arise from an equilibrating series of cations. The mechanistic interpretation is supported by the results of solvolysis studies with a series of toluene-p-sulphonates and rearrangements of methylsubstituted 1-methoxybenzobarrelenes.