Syntheses and properties of a trimethyltetradehydro[21]annulenone and its benzannulated derivatives

The syntheses of 2,9,14-trimethyl-10,11,12,13-tetradehydrocycloheneicosenone (3), 19-methyl-20,21,22,23-tetradehydrobenzocycloheneicosen-11-one (4), and 10-methyl-22,23,24,25-tetradehydredibenzo[a,g]cycloheneicosen-11-one (5) are described. The influence of α-methyl substitution and benzannulatien on the structure and tropicity of the tetradehydro[21]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 dimethylannulenone (2).