Pyridine-2(1H)-thione in Heterocyclic Synthesis: Synthesis of Some New Nicotinic Acid Ester, Thieno[2, 3-b]pyridine, Pyrido[3′, 2′: 4, 5]thieno [3, 2-d]pyrimidine, and Thiazolylpyrazolo[3, 4-b]pyridine Derivatives

Nicotinic acid esters 3a–c were prepared by the reaction of pyridine-2(1H)-thione derivative 1 with α-halo-reagents 2a–c. Compounds 3a–c underwent cyclization to the corresponding thieno[2, 3-b]pyridines 4a–c via boiling in ethanol/piperidine solution. Compounds 4a–c condensed with dimethylformamide-dimethylacetal (DMF-DMA) to afford 3-{[(N,N-dimethylamino)methylene]amino}thieno[2, 3-b]- pyridine derivatives 6a–c. Moreover, compounds 4a–c and 6a–c reacted with different reagents and afforded the pyrido[3′,2′:4, 5]thieno[3, 2-d]pyrimidine derivatives 10a–d, 11a–c, 12a,b, 14a,b, 17, and 19. In addition, pyrazolo[3, 4-b]pyridine derivative 20 (formed via the reaction of 1 with hydrazine hydrate) reacted with ethylisothiocyanate yielded the thiourea derivative 21. Compound 21 reacted with α-halocarbonyl compounds to give the 3-[(3H-thiazol-2-ylidene)amino]-1H-pyrazolo[3, 4-b]pyridine derivatives 23a–c, 25, and 27a,b.