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10.1.1.1.1.8.2 Variation 2: By Claisen Rearrangement of Phenyl Propargyl Ethers Using Cesium Fluoride

DOI: 10.1055/sos-SD-010-00002

Dell, C. P.Science of Synthesis, (20011033.

A more recent procedure allows efficient synthesis of 7-alkoxy-2-methylbenzofurans by a cesium fluoride mediated rearrangement of 2-alkoxyphenyl propargyl ethers (Scheme 30).[‌252‌‌254‌] The success here is attributed to the o-alkoxy effect.[‌253‌] In the absence of cesium fluoride, the predominant product is the isomeric benzopyran. Yields are much reduced in the absence of the o-alkoxy substituent. Interestingly, if the phenyl propargyl ether bears the electron-withdrawing methoxycarbonyl group in the o-, m-, or p-position, exclusive rearrangement to the benzofuran 78 occurs (Scheme 31).[‌252‌] With the m-ester, rearrangement gives two regioisomers where Claisen rearrangement has occurred via each of the adjacent free ortho positions.[‌252‌] Those compounds bearing a m-methoxy substituent give a mixture of all four regioisomeric benzofuran and benzopyran products.[‌252‌]

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