SYNTHESIS OF 2-ARYL-5, 6-DIHYDRO-(1)BENZOTHIEPINO[5, 4-C]PYRIDAZIN-3(2H)-ONE BY A NOVEL DEHYDROGENATION REACTION

Treatment of 2-(4-methoxyphenyl)-4, 4a, 5, 6-tetrahydro-(1)benzothiepino[5, 4-c]pyridazin-3(2H)-one 7-oxide (1a) in methanesulfonic acid (MSA) gave 2-(4-methoxyphenyl)-5, 6-dihydro-(1)benzothiepino[5, 4-c]pyridazin-3(2H)-one (2a) in 80% yield. Compound 2a was also obtained by reating 4a, a deoxidized compound of 1a, with dimethylsulfoxide (DMSO) in MSA, trifluoroacetic acid or HBr-AcOH. Apparently this novel dehydrogenation reaction was derived from an acid catalyzed intermolecular redox reaction, that is, a concerted elimination of 4-hydrogen, 4a-hydrogen on 1a or 4a and sulfinyl oxygen on 1a or DMSO.