Thermal Cyclization of Substituted Aryl Propargyl Ethers. the Scope and Regioseleetivity of the Reaction in the Synthesis of Substituted 3-Chromenes

The thermal cyclization of substituted aryl propargyl ethers was examined. Simple 3-aryloxypropynes (12, 15a-c) were cyclized to the corresponding chromenes in ∽60% yield. The cyclization of 12 was not regiospecific and two isomeric chromenes (13 and 14) were obtained. The thermal cyclizations of C-l and/or C-3 methyl-substituted 3-aryloxypropynes (16d-g) have been shown to proceed in a much higher yield than the corresponding unsubstituted compounds. The influence of water, reaction temperature, and solvent on the cyclization was also studied.