Reaction of ketenes with N,N ‐disubstituted α‐aminomethyleneketones. XI. Synthesis of 2 H ‐pyrano[3,2‐ g ]benzothiazole derivatives

Cycloaddition of dichloroketene to N,N ‐disubstituted 6‐aminomethylene‐5,6‐dihydro‐2‐phenylbenzothiazol‐7‐(4 H )ones gave in good yield N,N ‐disubstituted 4‐amino‐3,3‐dichloro‐3,4,5,6‐tetrahydro‐8‐phenyl‐2 H ‐pyrano[3,2‐ g ]benzothiazol‐2‐ones II, which are derivatives of the new heterocyclic system 2 H‐ pyrano[3,2‐ g ]benzothiazole. Dehydrochlorination with triethylamine of II afforded N,N ‐disubstituted 4‐amino‐3‐chloro‐5,6‐dihydro‐8‐phenyl‐2 H ‐pyrano[3,2‐ g ]benzothiazol‐2‐ones III in good to moderate yield. The dimethylamino adduct was dehydrochlorinated in high yield by refluxing in toluene, whereas the diisopropylamino adduct gave in low yield 6‐(2,2‐dichloroethylidene)‐5,6‐dihydro‐2‐phenylbenzothiazol‐7‐(4 H )one with the triethylamine treatment. The dehydrochlorinated product IIId (NR 2 = pyrrolidino) was obtained directly in low yield by cycloaddition of dichloroketene to the corresponding enaminone. Full aromatisation of IIIa,g [NR 2 = N(CH 3 ) 2 and N(CH 3 )C 6 H 5 , respectively] to the corresponding N,N ‐disubstituted 4‐amino‐3‐chloro‐8‐phenyl‐2 H ‐pyrano‐[3,2‐ g ]benzothiazol‐2‐ones was accomplished with DDQ in refluxing benzene.