Preparation of New Nitrogen-Bridged Heterocycles. 15. Synthesis and Reaction of 1,9a-Dihydropyrido[2,1-<i>c</i>][1,4]thiazines

Abstract

Abstract 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 °C the desulfurized and the rearranged aromatic indolizine derivatives were formed in moderate to good yields.