Thermische Umlagerungen von halogensubstituierten Aryl‐propargyläthern

7‐Chloro‐ 2 ‐chloromethyl‐benzofuran (13) and 3, 8‐dichloro‐ 2 H ‐1‐benzopyran (12) are the main products from the thermal rearrangement (230–260°) of 2, 6‐dichlorophenyl propargyl ether (7) . Compounds 17 , 18 and 19 are also formed, but in much smaller amounts (scheme 2 and table 1). However, in the case of the bromo‐compounds 8 and 9 the rearrangement products are the benzofuran derivatives 21 and 22 , containing one bromine atom less per molecule (scheme 4). The corresponding naphthyl propargyl ethers 10 and 11 can be rearranged much more easily (180°) to the halogeno‐naphthofurans 24 and 26 respectively. In the case of the bromo‐ether 11 , 2‐methyl‐naphtho[2, 1‐ b ]furan (25) is also formed (scheme 5). If the propargylic hydrogen atoms in 7 and 11 are replaced by deuterium atoms, then after rearrangement the deuterium atoms in the products d‐ 13 and d‐ 26 are found in the β‐positions to the oxygen atom of the furan ring (schemes 3 and 5). It is suggested that initially a [3 s , 3 s ]‐sigmatropic rearrangement of the aryl propargyl ethers to the 6‐allenyl‐6‐halogeno‐cyclohexa‐ 2 , 4‐dien‐1‐ones ( e.g. a ) occurs and that from these the isolated products are formed via radical pathways (scheme 6). Under neutral conditions aryl propargyl ethers containing a free ortho ‐position give on heating benzopyran derivatives [ 2 ]. When this thermal reaction is carried out in sulfolane in the presence of powdered potassium carbonate, 2 ‐methyl‐benzofuran derivatives are formed (table 2 ). This leads to the possibility of preparing, depending on the conditions, either benzopyran or benzofuran derivatives by the Claisen rearrangement of aryl propargyl ethers. The mechanism for the formation of the benzofurans is given in scheme 9.