Dehydrobromination of αα-dibromotetrahydrobenzocyclohepten-5-ones: a convenient synthesis of benzocyclohepten-5-ones

Abstract

Reaction with Lithium chloride in boiling dimethylformamide has been shown to give high yields of benzocyclohepten-5-ones (9–11) from 6,6-dibromo-6,7,8,9-tetrahydrobenzocyclohepten-5-ones (6–8). Similar eliminations occur from 6,9-dibromo-6,7,8,9-tetrahydrobenzocyclohepten-5-ones (12) and (14). The mechanism of the elimination has been investigated by preparation of possible intermediates, an x-bromo-1,2,2a,7a-tetrahydrocyclobut[a]inden-7-one (22) or (23) and 6-bromo-8,9-dihydrobenzocyclohepten-5-one (28). The available evidence favours an acid-catalysed double bond isomerisation.