By means of the mercury cyclization of the unsaturated amidals 3a-0, obtained from the reaction of 1, 3, 5-tris-[(S)-phenylethyl] hexahydrotriazine (1) and a,@-unsaturated acyl chlorides, diastereomeric mixtures of im-idazolidin-4ones 5-8 and perihydropyrimidin-4- ones 4-10 have been synthesized and easily separated by flash, chromatography. The subsequent hydrolysis under acid conditions of the separated heterocycles affords ...