Synthesis of an azasteroid using an acyl iminium ion-initiated tandem cyclization

An acyl iminium ion-initiated tandem cyclization gave an unexpected dienone product, a secoazasteroid (2). The factors governing the formation of 2 were investigated in an attempt to optimize its formation. The reaction was applied to a more elaborate system, resulting in the synthesis of the full steroid skeleton of 13-azaandrosta-1,4-diene-3,17-dione (3), which contains the unusual substitution of a chlorine atom for the axial 19-methyl.