Isomeric 6H,8H-naphthyridino-1,3-benzothiazin-8-ones

Abstract

The synthesis of the ring system present in two of the four possible isomeric 6H,8H-naphthyridino-1,3-benzothiazines is reported here for the first time. 2,3-Dimethoxy-6H,8H-[2,7]naphthyridino[2,3-c][1,3]benzothiazin-8-one (5) and several related compounds have been prepared by both chemical and photochemical methods. 2,3-Dimethoxy-6H,8H-[1,6]naphthyridino[6,7-c][1,3]benzothiazin-8-one (9) has been isolated in 15% yield along with 5 in 38% yield on irradiation of 6,7-dimethoxy-3-nicotinoyl-4-methylidene-2H-1,3-benzothiazine (8).