Synthesis of an isolable 4aH-benzocycloheptene, ethyl 4-methoxy-4a-methyl-4aH-benzocycloheptene-5-carboxylate, and a study of its thermal rearrangement

A synthesis of ethyl 4-methoxy-4a-methyl-4aH-benzocycloheptene-5-carboxylate (12), the first isolable 4aH-benzocycloheptene, is described. The skeleton is constructed by Cope rearrangement of the divinyl-cyclopropane (8). Further unsaturation is then introduced by the use of dichlorodicyanobenzoquinone to give the tetraenone (10), from which an extended enolate anion is generated by means of sodium hydride in 1,2-dimethoxyethane. O-Methylation of the anion gives the ether (12). At 138 °C this compound undergoes unimolecular skeletal rearrangement and gives a mixture of three products. It is proposed that these are formed from a common bisnorcaradiene intermediate (17). A new type of rearrangement, which involves the conversion of the bis-nor-caradiene (17) into a second bis-norcaradiene by a (σ2s+π4a+π4a) process, is suggested to explain the formation of the product (19). The methoxy-ester (12) also undergoes 4 + 2 cycloaddition with 4-phenyl-1,2,4-tri-azolinedione and an acid-catalysed methyl group migration.4-Methoxy-4a-methyl-4aH-benzocycloheptene (28) has been generated by an analogous route; at room temperature this undergoes a skeletal rearrangement, of the same type as observed with the ester (12).