Terminal Alkynes as One-Carbon Donors in [5+1] Heteroannulation: Synthesis of Pyridines via Ynimine Intermediates and Application in the Total Synthesis of Anibamine B.

Transition-metal-catalyzed [4+2] heteroannulation of α,β-unsaturated oximes and their derivatives with alkynes has been developed into a powerful strategy for the synthesis of pyridines. It nevertheless lacks regioselectivity when unsymmetrically substituted alkynes are used. We report herein the unprecedented synthesis of polysubstituted pyridines by a formal [5+1] heteroannulation of two readily accessible building blocks. A copper-catalyzed aza-Sonogashira cross-coupling between β,γ-unsaturated oxime esters and terminal alkynes affords ynimines, which, without isolation, undergo an acid-catalyzed domino reaction involving ketenimine formation, 6π-electrocyclization and aromatization to afford pyridines. Terminal alkynes served as a one-carbon donor to the pyridine core in this transformation. Di- through pentasubstituted pyridines are accessible with complete regioselectivity and excellent functional-group compatibility. The first total synthesis of anibamine B, an indolizinium alkaloid with potent antiplasmodial activity, was accomplished featuring this reaction as a key step.