Acetylenes as potential antarafacial components in concerted reactions. Formation of pyrroles from thermolyses of propargyl amines, of a dihydrofuran from a propargylic ether, and of an ethylidenepyrrolidine from a .beta.-amino acetylene

Abstract

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 35% 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 ] Mobius process, wherein the acetylenic unit is the antarafacial component