Reactions of thiamine and related thiazolium ylides with aryl isothiocyanates afforded zwitterionic adducts (Va-j) or cycloadducts (VIa-c). An electron attracting substituent in the aryl group favored the zwitterion formation, while an electron donating substituent favored the cycloadduct formation. A ring expansion reaction of 2-thiocarbamoylated thiazolium ring giving 1, 4-thiazine derivative was also described.
