REARRANGEMENT REACTIONS OF BICYCLIC SYSTEMS .6. A [C-14] TRACER STUDYOF THE ACID-CATALYZED SKELETAL REARRANGEMENT OF 1,4-DIHYDRO-1-METHOXY-1,4-ETHENO-NAPHTHALENE (1-METHOXYBENZOBARRELENE) AND ITS 5,6,7,8-TETRAHALOGENO-DERIVATIVES

A degenerate skeletal rearrangement occurs during the acid catalysed c onversion of 1-methoxybenzobarrelene and its tetrachloro- and tetraflu oro-derivatives into the corresponding 3,4-dihydro-1,4-ethenoonaphthal en-2(1H)-ones (benzobarrelenones). A [C-14]-tracer study has confirmed that two pathways operate. In both pathways the methoxylated bridgehe ad C-1 of the benzobarrelene becomes the carbonyl C-2 of the benzobarr elenone. The pathways are distinguished by the fate of the benzobarrel ene bridgehead C-4: in one pathway aryl migration leaves this formerly bridgehead C-4 as the olefinic C-5 of the newly formed benzobarreleno ne; in the other pathway the bridgehead C-4 remains bonded to the aryl ring while a double migration of other skeletal bonds leaves it as th e bridgehead C-4 of the benzobarrelenone. The pathway involving aryl m igration is favoured in each case examined. The preference for aryl mi gration is greater in concentrated(> 98% sulphuric acid than in aqueou s (80%) sulphuric acid and in both cases the preference increases as t he aryl substitution varies from F-4 to Cl-4 to H-4.