Synthesis and reactivity of cyclic quinonimines of the 2H-1,4-benzothiazine series

Periodate oxidn. of cysteinylhydroquinone Et ester gave 2H-1,4-benzothiazine, evidently via rearrangement of the intermediate quinonimine I (R = CO2Et). The relatively more stable quinonimine I (R = H) could be obtained in cryst. form by oxidn. of the dihydrobenzothiazine. In soln., I (R = H) exhibits a marked tendency to undergo acid- or base-catalyzed isomerization to the highly unstable 7-hydroxy-2H-1,4-benzothiazine. Once formed, the latter can either give the trimer II by an imine-enamine type condensation, or undergo oxidn. with formation of the lactam III or of the Δ2,2'-bi(2H-1,4-benzothiazine) dye.