The prepn. of 2,2,4,6,6-pentamethyl-2,3,4,5 -tetrahydro-pyrimidine (acetonine) from acetone and NH3 comprises reacting in the presence of 0.02-0.5 (0.05-0.15) mol/100 ml acetone of a phenolic cpd. with molecular cavity of type p-substd. Calix(n) are of formula (I) at 0.1-2(0.25-1.0)MPa and 30-100 (45-60) deg.C. R = SO3R3, NO2, COR4, halogen; R3 = H, (cyclo)alkyl, aryl, metal ion, pref. alkali metal ion, or NH44; R4 = H, OH, (cyclo)alkyl, aryl, OR5 with R5 = alkyl, cycloalkyl, aryl; R1, R2 = H or alkyl; X = -CR6R7, -CH2OCH2-, -CO-; and n = 3-20. USE/ADVANTAGE - Acetonine is produced in high yield and with high selectivity. It is used for the prepn. of triacetoneamine, in the prepn. of UV stabilisers for polymers and as an intermediate for the prepn. of pharmaceutical prods.