Acetylenes as Potential Antarafacial Components in Concerted Reactions. Formation of Pyrroles from Thermolyses of Propargylamines, of a Dihydrofuran from a Propargylic Ether, and of an Ethylidenepyrrolidine from a β-Amino Acetylene

Article abstract of DOI:10.1021/jo00071a015

A thermal cyclization of acetylenic compounds provides evidence for the ability of acetylenic links to act as antarafacial components in <2 + 2> processes.The cyclization competes with the normally favored acetylenic retro-ene reaction.Propargylic amines, without substituents whose presence would hinder a tight cyclic transition state, yield intermediate pyrrolines whose subsequent hydrogen elimination affords pyrroles in small amounts.The same process in 2-ethynyltetrahydropyran affords 8-oxabicyclo<3.2.1>octane in 35percent yield.A related thermal reaction of N-methyl-3-hexyn-1-amine provides a quantitative transformation to N-methyl-2-ethylidenepyrrolidine in a nominal <2_s + 2_a + 2_s + 2_s> Moebius process, wherein the acetylenic unit is the antarafacial component.Evidence for concertedness in these reactions is discussed.