Preparation of New Nitrogen-Bridged Heterocycles. 15. Synthesis and Reaction of 1,9a-Dihydropyridothiazines

Article abstract of DOI:10.1246/bcsj.60.1867

Alkaline treatment of 1-<2-(substituted methylthio)vinyl>pyridinium bromides, readily obtainable from the S-alkylation of pyridinium 1-(thiocarbonyl)methylides with various alkyl bromides such as bromoacetonitrile, ethyl bromoacetate, and some phenacyl bromides, gave smoothly 1,9a-dihydropyrido<2,1-c><1,4>thiazine derivatives as almost isomeric mixtures.These dihydropyridothiazines are very unstable and decomposed rapidly at ordinary temperature, but by treating them with a dehydrogenating agent such as lead tetraacetate (LTA) or 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) at 0 deg C the desulfurized and the rearrangement aromatic indolizine derivatives were formed in moderate to good yields.