Synthesis of the Indolizidine Core of Virosinine A via a Microwave-Promoted Cascade Cyclization Involving Iminyl Radicals

The indolizidine core of virosinine
A was synthesized
by means
of a microwave-promoted cascade reaction featuring 5-exo-trig iminyl radical cyclization, thiyl radical
elimination, and intramolecular imine alkylation. The resulting bicyclic
iminium ion underwent stereoselective reduction by Red-Al to deliver
the target compound. DFT calculations suggested that both the radical
cyclization and thiyl radical elimination steps are reversible at
high reaction temperatures.