X-ray crystallographic study of substituted perhydropyrimidinones.: Extreme changes in ring conformation

X-ray crystal structures of 20 differently substituted perhydropyrimidin-4-ones are presented. Analysis of these data reveal a remarkable conformational sensitivity of a six-membered ring to substitution. Thus half-chair; envelope, boat, twist-boat, and intermediate conformations are found for the six-membered heterocycle, providing evidence for a relatively flat conformational energy surface in this ring, interpretation of the preferred conformations is advanced in terms of steric interactions among substituents and, in some cases, as the result of particular conformational (A(1,3) strain, anomeric) effects.