Enolization Regioselectivity Involving Stereoisomeric 4a-Methyl-5-methoxyperhydrobenzo[7]annulen-2-ones

Efficient synthetic routes to the four isomers 17b−20b of the title ketone are described. Entry begins from the Wieland−Miescher homologue 3 whose pair of carbonyl groups are amenable to regiochemical manipulation. The compositions of the reaction mixtures generated under kinetic or thermodynamic control were defined by 1H NMR analysis subsequent to chromatographic purification. The regiochemical trends are correlated with B3LYP/6-31G* calculations, the results of which conform to the preferred introduction of a 1,2- or 2,3-double bond.