9- tetrahydro-47/-pyrido conformations

The relative stereostructure of 9-acetyl-7-hydroxy-l,2-dimethyl-7-meth-oxycarbonyl-4-phenyl-6-oxo-l, 4,7,8-tetrahydro-6/7-pyrido[l, 2-u]pyri-midine-3-carboxylate 122 was justified by an X-ray diffraction analysis (97JOC3109). The stereochemistry and solid state structure of racemic trans-6,9-//-l, 6-dimethyl-9 z-ethoxy-9-hydroxy-4-oxo-l,6,7,8,9,9 z-hexahydro-4//-pyrido[l,2- z]pyrimidine-3-carboxylate (123), adopting a cw-fused conformation, were determined by X-ray investigations (97H(45)2175). 

Conformational analysis of 4-oxo-6,7,8,9-tetrahydro-4//-pyrido[l,2-a]pyrimidine-3-acetates and -3-carboxylates 30 (R = H) and their mono-methylated (R = Me, R1 = H) and 6,9-, 7,9-, and 8,9-dimethylated derivatives were carried out by H and l3C NMR spectroscopy (86JOC394). At ambient temperature the 6-methyl derivatives predominantly adopt the energetically most favorable half-chair conformation with a pseudoaxial methyl group. In the other half-chair conformation a serious 1-3 allylic strain exists between the pseudoequatorial methyl group and the adjacent carbonyl group. At the 7- and 8-methyl derivatives the half-chair conformations with equatorial methyl group occur almost exclusively, but the 9-... 

For 9-phenylamino-6,7,8,9-tetrahydro-4//-pyrido[l,2-a]pyrimidin-4-ones 32 (R = NHPh R1 = H R2 = H, COOH) the half-chair conformation with a pseudoaxial 9-phenylamino group was the predominant one. But with the 6-methyl-9-phenylamino derivatives 32 (R = NHPh, R1 = Me) the cis-trans ratio was near 1 1 in the imine form. The presence of a methyl group on the nitrogen atom of the 9-anilino group 32 (R = NMePh) increased the amount of the cis form which contained the 9-substituent in a pseudoequatorial position [85JCS(P1)1015]. The 6-methyl group was in a pseudoaxial position in all tautomers. 

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