Steric and Electronic Factors Which Effect the Thermal Cyclization of Meta-Substituted Aryl Propargyl Ethers. Synthesis of 5- and 7-Substituted 3-Chromenes

The thermal cyclization of meta-substituted phenyl propargyl ethers (1 and 4) proceeded to yield a mixture of 5- and 7-substituted 3-chromenes. The ratio of chromene isomers was somewhat dependent upon the nature of the starting materials. Thus, terminal acetylenes (1) gave a mixture of 2 and 3 (resulting from para and ortho cyclization, respectively) with the latter product usually in slight excess. Nonterminal acetylenes (4) also gave a mixture of ortho- and para-cyclized products (6 and 5, respectively); however, para cyclization was found to be favored. Regioselective cyclization was greatest for 4d, which gave a mixture of 4,7- and 4,5-dimethyl-3-chromene in a ratio of 2:1. The cyclization of 3-(3-methoxyphenyloxy)-3-methylbutyne (16) also proceeded with little regioselectivity to give a mixture of 17 and 18. The effects of electron-donating and electron-withdrawing meta substituents were also studied.