Electrophilic Cyclisation of Bis(4-methoxybenzylthio)acetylene − Competition Between Ar2-6 and Ar1-5 Routes, Yielding 1H-2-Benzothiopyrans or Spiro Derivatives of Cyclohexadienone

Abstract

Treatment of bis(4-methoxybenzylthio)acetylene (1) with iodine monochloride yields different products in the presence or absence of nucleophiles such as water or alcohols. Normally, the electrophilic cyclisation of bis(benzylthio)acetylenes produces 1H-2-benzothiopyrans 2 by intramolecular ortho attack on the aromatic ring by a vinyl cation formed in situ (Ar2-6 cyclisation). In the case of 1, however, the high electron density in the ipso position of the aromatic ring favours ipso attack (Ar1-5 route). The fate of the ipso σ complex is determined by the presence or absence of nucleophiles in the reaction medium. When nucleophiles are excluded, the σ complex is stabilised by 1,2-migration and formation of 1H-2-benzothiopyran 2a. In the presence of water, the σ complex yields spirocyclohexadienone dihydrothiophenes 3a and 3d. In the presence of 3-methylbenzyl alcohol, the methoxy substituent of the 2-benzothiopyran ring is exchanged by the 3-methylbenzyloxy group in product 2d. These findings are consistent with the formation of 2a, 2c, 2d and 2f by ipso − and not ortho − attack on the 4-methoxyphenyl ring. Similar results were obtained both with ICl and from a proton-induced cyclisation. In one-pot syntheses, 3a and 3d were transformed into 2-benzothiopyrylium salts 4a and 4b by tritylium tetrafluoroborate, and 3a and 3b were rearranged into the 6-hydroxy-substituted 2-benzothiopyrans 2b and 2g by proton catalysis. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)