6‐Methyl‐6,7,8,9‐tetrahydro‐4H‐pyrido[1,2‐a]pyrimidin‐4‐ones 1‐5 were subjected to Vilsmeier‐Haack acylation with complexes of phosphoryl chloride and different amides. Acylation at position 9 of the pyridopyrim‐idines was successful with the iminium salt formed in situ from N‐formylpiperidine, N‐methylformanilide or N,N‐diethylbenzamide, but unsuccessful with the iminium salt formed from N,N‐diethylacetamide or N,N‐di‐ethylisobutyramide, respectively. The iminium salt formed from formanilide, N‐methylpyrrolidinone or formamide reacted only with those tetrahydropyridopyrimidinones which contain a strongly electronegative substituent (e.g. CN or CO2Et) in position 3. With the latter derivatives, the 9‐phenylaminomethylene group could be introduced using N,N‐diphenylformamide or in a “one‐pot” procedure with aniline and triethyl orthoformate. Ethanolysis of 9‐N‐methyl‐N‐phenylaminomethylene derivatives 15 and 19 afforded 9‐ethoxy‐methylene compounds 26 and 27 in the presence of hydrogen chloride. The structures of the 9‐substituted 6‐methyltetrahydropyridopyrimidin‐4‐ones 14‐25 were elucidated by means of uv, 1H and 13C nmr spectroscopy. 9‐Piperidinomethylene 14, 9‐(N‐methyl‐N‐phenylaminomethylene 15‐19 and 9‐(N‐methyl‐2‐pyrrolidinylidene) 21 derivatives exist as E geometric isomers. 9‐Phenylaminomethylene‐6‐methyl‐4‐oxo‐6,7,8,9‐tetra‐hydro‐4H‐pyrido[1,2‐a]pyrimidine‐3‐carbonitrile 20 displays a solvent‐dependent E‐Z isomerism. The bis‐compound 25 contains both E and Z geometric exo C CH double bonds. 9‐Benzoyl derivatives 23 and 24 exist predominantly as the 1,6,7,8‐tetrahydropyridopyrimidin‐4‐one tautomer.
ASJC Scopus subject areas
- Organic Chemistry