X-ray crystallographic study of substituted perhydropyrimidinones. Extremechanges in ring conformation

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