A Cycloisomerization Strategy for the De Novo Synthesis of Steroidal Alkaloids

A strategy that leverages a [2+2+2] cycloisomerization to open de novo access to the Veratrum family of alkaloids has been developed and deployed to achieve the total synthesis of steroidal alkaloids. The highly convergent approach described herein includes (i) the enantioselective synthesis of a diyne fragment containing the steroidal A/B rings, (ii) the asymmetric synthesis of a propargyl-substituted piperidinone (F ring) unit, (iii) the high-yielding union of the above fragments, and (iv) the intramolecular [2+2+2] cycloisomerization reaction of the resulting carbon framework to construct in a single step the remaining three rings (C/D/E) of the hexacyclic cevanine skeleton. From a common cevanine-type intermediate, two concise approaches toward other members of this important family of steroidal alkaloids have been devised, both of which rely on skeletal reorganization.