Configurational and conformational isomerism of 9,14-dimethyl-, 2,9,14-trimethyl- and 9,14,19-trimethyl-10,11,12,13-tetradehydro[19]annulenones in neutral and acidic solutions

Abstract

All of the title compounds (1–3) exist as single configurational isomers with ‘all-E’ geometry, when synthesized, and have been shown to be products formed under kinetic control which isomerize in trifluoroacetic acid to more stable isomers with ‘2-Z’ and ‘4-Z’ geometry. Most of these configurational isomers exist as a rapidly equilibrating mixture of two (or more) conformers. The results of ¹H NMR studies on the isomer equilibria and the isomerization rates in CDCl₃ and CF₃CO₂D–CD₂Cl₂ solutions including variable temperature experiments are reported. Complete isomer equilibria are not attained because some of the isomerization barriers are too high (> 27 kcal mol^–1).