Approaches to cyclopropazulenes: Transannular aldol reactions of some derivatives of 1,10-dibromobicyclo[8.1.0]undecane-3,8-dione

This paper describes synthetic approaches to potential precursors to cyclopropazulenes, a novel class of nonbenzenoid cycloproparenes. The acid-catalysed transannular aldol reaction of 1,10-dibromobicyclo[8.1.0]undecarie-3,8-dione (10) gives a single ketol 13 possessing the cyclopropane moiety fused to the five-membered ring. Similar behaviour is observed in the aldol reaction of 27, a benzo-fused analogue of 10. In the transannular aldol reaction of dione 45, the epoxy bridge forces the reaction in the desired sense to give a ring system (47 and 48) in which the cyclopropane ring is fused to the seven-membered ring. The alpha,beta-unsaturated ketone 47 was elaborated into the benzofulvene derivative 51 but conversion of this into the benzocyclopropazulene 58 by overall deoxygenation and debromination could not be achieved. The diones used in this study were generally obtained in good yield by ozonolysis of an appropriate cyclic alkene. In the case of 1a,9a-dibromo-1a,2,3,8,9,9a-hexahydro-1H-cycloprop[b]anthracene (26), an alkene possessing methylene groups that are both allylic and benzylic, ozone effected overall dehydrogenation instead of cleavage of the alkene. (C) Wiley-VCH Verlag GmbH & Co.