Synthesis and Properties of Methano-Bridged Bisdehydro[17]-, -[19]-, -[21]- annulenones and the Related Compounds

14-Methyl-10,12-bisdehydro-4,9-methano-1H-[17]annulen-1-one, 14-methyl-10,12-bisdehydro-4,9-methano-1H-[19]annulen-1-one, and 16-methyl-12,14-bisdehydro-6,11-methano-1H-[21]annulen-1-one, as well as their methylated and benzannelated derivatives, were synthesized and their tropic properties were studied by NMR and UV spectra under neutral and acidic conditions. The tropicity in these compounds was found to be lower than in their non-bridged monocyclic counterparts. Thus the methano-bridge works to decrease the planarity of the peripherally conjugated system.