BF3·OEt2-Mediated Tandem Annulation: A Strategy To Construct Functionalized Chromeno- and Pyrano-Fused Pyridines.

A simple and efficient one-pot annulation of arylidenones, alkynes, and nitriles in the presence of BF3·OEt2 is described. A highly functionalized variety of N-substituted pyridine-fused chromeno and pyrano derivatives were obtained with satisfactory yields under mild reaction conditions. The method was proven to be valid for the synthesis of a diverse library of chromeno[3,4-c]pyridines, thiochromeno[3,4-c]pyridines, pyrano[3,4-c]pyridines, and thiopyrano[3,4-c]pyridine derivatives from readily accessible substrates. This experimentally simple protocol provides structurally complex, biologically relevant heterocycles in a one-pot operation.