Reactivity of steroidal 1-azadienes toward enamines: an approach to novel chiral penta- And hexacyclic steroids

Abstract

The chemical behavior of steroidalN-sulfonyl-1-azadienes toward carbonyl compounds, in the presence of pyrrolidine, is described. With aldehydes, these azadienes participate in hetero-Diels-Alder reactions with thein situgenerated enamines. The stereoselectivity results from the approach of the dienophiles from the less hindered α-face of the steroid, with the pyrrolidine moietyendoand retention of the enaminetransgeometry. This diastereoselective synthetic methodology led to a new class of chiral pentacyclic steroids. Interestingly, the studied steroidal scaffolds follow a different mechanistic pathway with cyclic ketones. They undergo a diastereoselective annulation reaction, under enamine catalysis, affording chiral hexacyclic steroids.