Ru(0)-Catalyzed Synthesis of Conjugated Iminotrienes and Subsequent Intramolecular Cyclization Giving Polysubstituted Pyrroles

A reliable method for preparing polysubstituted
pyrroles from conjugated
iminohexatrienes has been discovered. Ru(0)-catalyzed cross-dimerization
of iminoalkyne with conjugated dienes provides a series of conjugated
iminohexatrienes. Subsequent treatment with a catalytic amount of
acetic acid (7 mol %) leads to an unexpected cyclization yielding
2-alkenylpyrroles. The overall reaction can be considered as a formal
(4 + 1) annulation that involves the formation of a conjugated iminohexatriene
followed by an intramolecular aza-Michael-type 5-exo-trig cyclization and subsequent proton migration.