Nitrogen bridgehead compounds. Part 32. Absolute configuration and circular dichroism of 6-methyl-tetrahydro-4H-pyrido[1,2-a]pyrimidin-4-ones

Abstract

From the resolved enantiomers of (1b), whose enantiomeric purities were determineed by ¹H n.m.r., three types of 6-methyl-tetrahydro-4H-pyrido[1,2-a]pyrimidin-4-one derivatives (1)–(3) were prepared in optically active form. The absolute configuration at the chiral centre C(6) was established on the basis of the anomalous X-ray scattering of the bromine atom of compound (–)-(5) synthesized from compound (+)-(5) synthesized from compound (+)-(1b). Analysis of the experimental u.v. and c.d. spectra led to the identification of two different types of chromophoric systems in molecules of types (1) and (2) on the one hand, and in those of type (3) on the other. The results of CNDO/S calculations for the energies, oscillator strengths, and rotational strengths due to the transitions of simple model molecules with geometries based on X-ray data were in good qualitative agreement with the experimental u.v. and c.d. spectra, and allowed the chiroptical properties of compounds (1)–(3) to be correlated with their known absolute geometries.