(9E)-9-benzylidene-2-methylsulfanyl-5-phenyl-6,7,8,9-tetrahydropyrimido[4,5-b]quinolin-4(3H)-one: a hydrogen-bonded R2(2)(8) dimer.

In the molecule of the title compound, C25H21N3OS, which was prepared by mild oxidation of the corresponding 5,6,7,8,9,10-hexahydro analogue, the fused carbocyclic ring adopts an envelope conformation. Pairs of molecules are linked into cyclic centrosymmetric dimers by pairs of inversion-related N-H···O hydrogen bonds.