Synthesis of Linear Heterocycles: Thermal Sigmatropic Rearrangement of 4-(4-Aryloxybut-2-ynyloxy)[1]benzopyran-2-thiones

 

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Abstract

A number of 4-(4-aryloxybut-2-ynyloxy)[1]benzopyran-2-ones 6a-h were converted to corresponding thiones 4a-h. The thiones 4a-h in refluxing 1,2-dichlorobenzene gave 4-aryloxymethylthiopyrano[2,3-b][1]benzopyran-5(2H)-ones 5a-d and products 7a-d in 20-26% and 50-55% yield, respectively, whereas thiones 4e-h furnished exclusively products 7e-h in 80-85% yield. Addition of radical initiator azobisisobutyronitrile or radical scavenger hydroquinone or acid or base does not seem to have any effect on the rearrangement. All the butynyl ethers studied so far underwent sigmatropic rearrangements at the 4-coumarin-4-yloxypropynyl function of compounds 4a-h to give products 5a-d and or 7a-h.

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