An Asymmetric Total Synthesis of D-Homo Steroids Involving a Lewis Acid Directed Diels-Alder Reaction

Abstract In recent years, applications of asymmetric, amino acid mediated aldol cyclizations1,2 have led to major advances in steroid total synthesis.3 Typical of such processes is the conversion of prochiral triketone la 4 to the steroid CD-synthon, (S)-(+)-enedione 2a in high chemical and optical yield, under the influence of (S)- proline.1,2 This approach to the introduction of chirality is the foundation upon which several enantiospecific and highly efficient syntheses of biologically important steroids (19-norsteroids, A-ring aromatic steroids, androstanes) or their precursors are based.3