Tetracyclic phenothiazines. III. . Intermolecular hydride transfer in acid induced disproportionation of 1,2-dihydro-3-hydroxy-3H-pyrido[3,2,1-kl] phenothiazines

1,2-Dihydro-3-hydroxy-3H-pyrido3,2,1-kl]phenothiazines and 1H-pyrido3,2,1-kl]phenothiazines undergo acid catalyzed disproportionation with intermolecular hydride transfer to form pyrido3,2,1-kl]phenothiazinium salts and 1,2-dihydro-3H-pyrido3,2,1-kl]phenothiazines. Sodium borohydride reduction of 3-alkyl- or 3-arylpyrido3,2,1-kl]phenothiazinium salts gives 3-alkyl- or 3-aryl-1H-pyrido3,2,1-kl]phenothiazines. In the presence of a proton source, borohydride reduction of pyrido3,2,1-kl]phenothiazinium fluoroborate or 3-chloropyrido-3,2,1-kl]phenothiazinium perchlorate gives 1,2-dihydro-3H-pyrido3,2,1-kl]phenothiazine, while 1H-pyrido3,2,1-kl]phenothiazine is formed in aprotic solvents with pyridine present.