Dihydro-3-(triphenylphosphoranylidene)-2,5-thiophendione: a convenient synthon for the preparation of substituted 1,4-thiazepin-5-ones and piperidinones via the intermediacy of thioacids

Reaction of thiomaleic anhydride with triphenylphosphine gives the title compound, which undergoes reaction with a variety of aldehydes to give a range of alkylidene thiomaleic anhydrides (substituted monothioitaconic anhydrides). Subsequent treatment with tert-butoxycarbonylamino-substituted thiols, or under radical conditions with tert-butoxycarbonylamino-substituted alkyl halides results in a series of substituted monothiomaleic anhydrides, that on exposure to trifluoroacetic acid and then base lead to thiocarboxyl substituted 1,4-thiazepin-5-ones and piperidinones, respectively, that are ultimately trapped by reaction with 2,4-dinitrobenzenesulfonamides to give the corresponding amides.