Radical Cyclization by Ipso Substitution of the Methoxy Group: Considerable Effect of HMPA on Samarium-Mediated Cyclization.

Alkyl radicals generated by treatment of thiocarbamates of conformationally favorable 3-alkyl-3-arylpropan-1-ols with tris(trimethylsilyl)silane and AIBN efficiently undergo intramolecular ipso substitution of the methoxy group, yielding the corresponding cyclized products. In contrast, either conformationally favorable or flexible 1-arylalkan-3- or 4-ones easily cyclize into five- or six-membered condensed rings by treatment with SmI2 via ketyl radical intermediates. The addition of HMPA as cosolvent dramatically changes the cyclization mode of the SmI2-induced reaction, and the para-cyclization products are exclusively formed. This “HMPA effect” can be rationalized by the strong chelating ability of HMPA with the samarium atom.