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 BETA-AMINO ACETYLENE

A thermal cyclization of acetylenic compounds provides evidence for th e ability of acetylenic links to act as antarafacial components in [2 + 2] processes. The cyclization competes with the normally favored ace tylenic retro-ene reaction. Propargylic amines, without substituents w hose presence would hinder a tight cyclic transition state, yield inte rmediate pyrrolines whose subsequent hydrogen elimination affords pyrr oles in small amounts. The same process in 2-ethynyltetrahydropyran af fords 8-oxabicyclo[3.2.1]octane in 35% yield. A related thermal reacti on of N-methyl-3-hexyn-1-amine provides a quantitative transformation to N-methyl-2-ethylidenepyrrolidine in a nominal [2s + 2a + 2s + 2s] M obius process, wherein the acetylenic unit is the antarafacial compone nt. Evidence for concertedness in these reactions is discussed.