Coupling of Heteroaryldiazonium Tetrafluoroborates with 1,3-Dicarbonyl Compounds—regioselective Synthesis of Alkyl 1-Heteroaryl-4-hydroxy-1H-pyrazole-3-carboxylates

Coupling of 1-cyano-4-oxo-4H-quinolizine- (2), 1-ethoxycarbonyl-4-oxo-4H-quinolizine- (3), and 4-oxo-4H-pyridino[1,2-b]pyrimidine-3-diazonium tetrafluoroborates (4) with 1,3-dicarbonyl compounds (la-i) afforded the corresponding hydrazones (5-7) in 55-96% yields. The orientation around the C=N double bond in unsymmetrically substituted hydrazones (7a,c) was determined by NMR (NOESY) spectrometry. Heating of hydrazones (5a,b-7a,b), derived from alkyl 4-chloro-3-oxobutanoates (1a,b), furnished 1-(1-substituted quinolizin-3-yl)- (8, 9) and 1-(pyridino[1,2-a]-pyrimidin-3-yl)-4-hydroxy-1H-pyrazole-3-carboxylates (10) in 87-96% yields.