It was shown that the halogen atom occupies the quasi‐axial position in the predominant conformer of the 9‐halo derivatives of tetrahydro‐4H‐pyrido[1,2‐a]pyrimidin‐4‐ones. When R3 = Me, the conformational equilibrium is determined by the latter substituent which is always quasi‐axial. The effects of the methyl group and the halogen atoms on the 13C chemical shifts (SCS values) were used for the identification of cis and trans isomers. Interesting non additivity of substituent effects was found in derivatives bearing quasi‐axial substituents at C‐6 and C‐9: and this was caused by the ring flattening.
ASJC Scopus subject areas
- Organic Chemistry