β-Aminocrotononitrile in heterocyclic synthesis: Synthesis of polysubstituted pyridines as precursors to bicycles and polycycles

β-aminocrotononitrile (1) reacted with either cyanothioacetamide to give (3) or malononitrile to afford an anion (5). Pyridine-2(1H)-thione (4) was obtained by boiling of (3) in ethanol and Et 3N or treatment of (5) with H 2S, respectively. The reaction of anion 5 with isothiocyanates (6) gave N-substituted pyridine-2(1H)-thiones (7). N-Substituted pyridine-2(1H)-thiones (7) can be used for the preparation of pyrido[2,3-d]pyrimidines (8a–e) and (10a–e), or the preparation of pyrido[1,2-a]pyrimidines (12a–d). 1,8-Naphthyridine derivatives (14a–d) and (16a–e) can also be obtained from pyridine-2(1H)-thione (7). Finally, 1,8-naphthyridine derivatives (16a–e) can be used for the preparation of tetracyclic compounds 17a–c and 18a,b.