Cyclization of Ynamide-Tethered 1,3,8-Triynes

A facile thermal cyclization of ynamide-tethered 1,3,8-triynes to form a 3,5,6,7-tetrahydro-1H-pyrano[3,4c]pyridine skeleton is described. Although the mechanism of this unusual reaction has yet to be defined, the formation of either a strained keteniminium or a biradical intermediate followed by a 1,5-hydride or -hydrogen shift is tentatively proposed as the key elementary steps in the reaction sequence. Appropriate electronic activation at the carbon center donating a hydride or hydrogen is crucial for successful cyclization.