Penta-Substituted Pyridines from Alkynes and Isoxazolines via Consecutive [2+2]-Cycloadditions

Herein we report the formation of penta-substituted pyridines from alkynes and isoxazolines facilitated by visible-light-mediated energy transfer. This unique reactivity was initially observed as an unexpected side reaction in our previously reported methodology accessing 1-azetines. Experimental results support a mechanism involving two sequential [2+2]-cycloadditions that form a highly strained 4-4-5 fused ring intermediate, followed by an electrocyclic ring-opening/aromatization sequence.