Syntheses and properties of trimethyltetradehydro[15]annulenones and the benzannelated derivatives

Syntheses of 5,10,15-(8) and 2,5,10-trimethyl-6,7,8,9-tetradehydrocyclopentadecenone (12), and 8,13-(9) and 6,13-dimethyl-14,15,16,17-tetradehydrobenzocyclopentadecen-7-one (13), 8,13-(10) and 10,13-dimethyl-14,15,16,17-tetradehydrobenzocyclopentadecen-9-one (14), and 8-methyl-(11) and 6-methyl-16,17,18,19-tetradehydrodibenzo[a,g]cyclopentadecen-7-one (15) are described. The influence of α-methyl substitution and benzannelation upon the structure and tropicity of the tetradehydro[15]annulenone ring system is discussed in view of the 1H n.m.r. and u.v. spectra of these annulenones, as well as those of the α-methyl unsubstituted annulenones (4)–(7).