A series of 2-aryl substituted hexahydropyrimidines and perimidines were synthesized from aromatic aldehydes and substituted 1, 3-propanediamines or 1, 8-naphthalenediamine. The 1H and 13C NMR spectra showed that 2-arylperimidines and 2-aryl-4, 4, 6- trimethylhexahydropyrimidines exist exclusively in ring forms even in DMSO solutions, whereas 2-aryl-4-methylhexahydropyrimidines undergo chain-ring-chain tautomerism ...