Further reactions of furans with trithiazyl trichloride; mechanistic considerations

The reaction of 2,5-diarylfurans with trithiazyl trichloride 1 to give 5-aroyl-3-arylisothiazoles in a useful one-step synthesis of isothiazoles has been extended to both weakly and strongly polarised unsymmetrical 2,5-diarylfurans, These react in an entirely analogous manner; the more electron releasing aryl group becomes incorporated into the 5-aroyl group of the isothiazole as the exclusive (strong polarisation) or the major (weak polarisation) product, However, with 3-bromo-2-(4-methoxyphenyl)-5-(4-nitrophenyl)-furan 7, where the more reactive furan beta-position is now substituted, this regiospecificity is reversed (to give isothiazole 8), When one of the alpha-aryl groups in the furan is replaced by methyl the same regiospecific isothiazole formation is now accompanied by some ring and side chain chlorination (15-->16 + 17 + 18), All of these results can be explained by mechanisms (Schemes 2 and 5) which involve initial electrophilic attack of the furan to give a beta-thiazyl derivative, This highly reactive (nitrenoid) substituent then induces a novel opening of the furan ring 21 to give a highly delocalised intermediate 22 which recyclises to the isothiazole.