Further reactions of furans with trithiazyl trichloride; mechanistic considerations

Abstract

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 β-position is now substituted, this regiospecificity is reversed (to give isothiazole 8). When one of the α-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 β-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.