Acid-Catalyzed Diels-Alder Reactions of Cycloalkenones

Whereas the Diels-Alder reaction has been in vogue as a facile method of six-membered ring construction for many years, its application to a one-step cis-octalone synthesis was precluded by discouraging, early reports on the conditions required for the interaction of dienes with 2-cyclohexenones and on the yields of the resultant adducts.^1,2 It is quite conceivable that these observations were responsible in part for the minimization of use of an otherwise attractive route toward angularly alkylated six-membered polycycles in the field of steroid total synthesis. The discovery of Lewis acid catalysis of the cycloaddition of α, β-unsaturated carbonyl compounds³ has rekindled interest in the above reactions.⁴ The following discussion illustrates octalone