Journal of the Chemical Society. Perkin transactions I p. 3225 - 3233 (1981)
Update date:2022-08-04
Topics:
Bradbury, Robert H.
Gilchrist, Thomas L.
Rees, Charles W.
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 deg 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-norcaradiene (17) into a second bis-norcaradiene by a (?28 + ?4a + ?4a) 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-triazolinedione and an acid-catalysed methyl group migration. 4-Methoxy-4a-methyl-4aH-benzocycloheptene (28) has been generated by a analogous route; at room temperature this undergoes skeletal rearrangement, of the same type as observed with the ester (12).
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