Nitrogen bridgehead compounds. Part 76 Synthesis and stereochemistry of ethyl 9‐benzylidene‐6,7,8,9‐tetrahydro‐2‐oxo‐2H‐ and 4‐oxo‐4H‐pyrido[1,2‐a]pyrimidine‐3‐carboxylates and their homologues

Abstract Ethyl 4‐oxo‐6,7,8,9‐tetrahydro‐4 H ‐pyrido[1,2‐ a ]pyrimidine‐3‐carboxylates 1–4 , their piperidine ring homologues 5–6 and their 2‐oxo isomers 7–9 were reacted with benzaldehyde. At low temperature, kinetically stable addition products were formed. Thermodynamically stable condensation products were obtained at higher temperature, which were also formed when the addition products were refluxed in benzene. The 9‐benzyl derivatives were prepared by the hydrogenation of the condensation products over Pd/C. The stereochemical features of the new compounds were determined via 1 H and 13 C nmr chemical shift and coupling constant analysis and NOE measurements.