The solid state structure of the Rimazolium hydroxylation product I was determined by X-ray investigations, and stereostructure of 1,6,7,8,9,9a-hexahydro-4H-pyrido[1,2-a]pyrimidin-4-ones were studied by semiempirical quantum chem. calculations at the AM1 level.While 9a-unsubstituted 1-methyl-1,6,7,8,9,9a-hexahydro-4H-pyrido[1,2-a]pyrimidin-4-ones adopt a cis-fused conformation, I has a trans-fused conformation to avoid a serious non-bonding interaction between the 9a-ethoxy and 1-Me groups.