Ring-chain tautomerism of 2-aryl-substituted-hexahydropyrimidines and tetrahydroquinazolines

2-Aryl-substituted-1-isopropyl- and 1-phenylhexahydropyrimidines and 3-isopropyl- and 3-phenyl-1,2,3,4-tetrahydroquinazolines proved to be ring-chain tautomeric mixtures in CDCl3 at 300 K, whereas only ring-closed tautomers could be detected for the 1- or 3-methyl-substituted analogues. The ratios of the ring-chain tautomeric forms at equilibrium could be described by the equation log KX=ρσ++log KX=H.