4',4'-Dimethyl-2-methylsulfanyl-3,4,5,6,7,8-hexahydropyrido[2,3-d]pyrimidine-6-spiro-1'-cyclohexane-2',4,6'-trione, C(15)H(19)N(3)O(3)S, (I), has a markedly polarized molecular-electronic structure, and the molecules are linked into a three-dimensional framework by a combination of N-H.O, C-H.O and C-H.pi hydrogen bonds. 8-Hydroxymethyl-4',4'-dimethyl-2-methylsulfanyl-3,4,5,6,7,8-hexahydropyrido[2,3-d]pyrimidine-6-spiro-1'-cyclohexane-2',4,6'-trione, C(16)H(21)N(3)O(4)S, (II), where the hydroxymethyl substituent is disordered over two sets of sites, has a much less polarized structure than (I); the molecules are linked by a combination of O-H.O and N-H.O hydrogen bonds into chains containing alternating R(2)(2)(8) and R(2)(2)(16) rings, and these chains are linked into sheets by a combination of a pi-pi stacking interaction and a C-H.O hydrogen bond. 8-Ethoxymethyl-2-methoxy-4',4'-dimethyl-3,4,5,6,7,8-hexahydropyrido[2,3-d]pyrimidine-6-spiro-1'-cyclohexane-2',4,6'-trione, C(18)H(25)N(3)O(5), (III), has an unpolarized electronic structure, and a combination of N-H.O, C-H.O and C-H.pi hydrogen bonds links the molecules into sheets.