Crystal structures of (E)-2-amino-4-methylsulfanyl-6-oxo-1-(1-phenylethylideneamino)-1,6-dihydropyrimidine-5-carbonitrile and (E)-2-amino-4-methylsulfanyl-6-oxo-1-[1-(pyridin-2-yl)ethylideneamino]-1,6-dihydropyrimidine-5-carbonitrile

The title compounds 3a, C₁₄H₁₃N₅OS, and 3b, C₁₃H₁₂N₆OS, both show an E configuration about the N=C bond and a planar NH₂ group. The mol-ecules, which only differ in the presence of a phenyl (in 3a) or pyridyl (in 3b) substituent, are closely similar except for the different orientations of these groups. The amino hydrogen atoms form classical hydrogen bonds; in 3a the acceptors are the oxygen atom and the cyano nitro-gen atom, leading to ribbons of mol-ecules parallel to the b axis, whereas in 3b the acceptors are the oxygen atom and the pyridyl nitro-gen, leading to a layer structure perpendicular to (01).