Acid Catalysis of the Claisen Rearrangement. 2. Formation of the Benzofurobenzopyran and Benzofuro[3,2-b]benzofuran Skeletons from 1,4-bis(aryloxy)-2-butynes

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1, 4-Bis(aryloxy)-2-butynes (1) can be selectively converted into 4-(aryloxymethyl)-2H-chromenes (2), 6H-benzofuro[3, 2-c]-6a, 11a-dihydro-lla-methylbenzopyrans (3), or 5a, 10b-dihydro-5a,10b-dimethylbenzofuro[2, 3-b]benzofurans (4) by treating a dichloromethane solution of 1 with mercuric trifluoroacetate, silver tetrafluoroborate, or anhydrous aluminum chloride, respectively. A mechanism involving charge-induced Claisen rearrangement triggered by π complex formation between the heavy metal ion and the C-C multiple bond (in 1 and 2) is postulated for formation of 2 and 3. Sequential charge-induced Claisen rearrangement of 1 into 3 by coordination of AlCl3 with the oxygen atoms of 1 and 2 followed by ionic rearrangement of 3 into 4 is also postulated. The differing efficacy of metal ions in promoting isomerization of 1, 2, 3 is discussed. © 1978, American Chemical Society. All rights reserved.

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Journal of Organic Chemistry