Development of an Intramolecular Aryne Ene Reaction and Application to the Formal Synthesis of (±)-Crinine

A general and high yielding annulation strategy for the
synthesis
of various carbo- and heterocycles, based on an intramolecular aryne
ene reaction is described. It was found that the geometry of the olefin
is crucial to the success of the reaction, with exclusive migration
of the trans-allylic-H taking place. Furthermore,
the electronic nature of the aryne was found to be important to the
success of the reaction. Deuterium labeling studies and DFT calculations
provided insight into the reaction mechanism. The data suggests a
concerted asynchronous transition state, resembling a nucleophilic
attack on the aryne. This strategy was successfully applied to the
formal synthesis of the ethanophenanthridine alkaloid (±)-crinine.