6-(4-Fluorophenyl)-8-phenyl-2,3-dihydro-4H-imidazo[5,1-b][1,3]-thiazin-4-one:: an unusual [6-5] fused-ring system

The title compound, C18H13FN2OS, is the first structural example of a [6-5] fused ring incorporating the 2,3-dihydro-4H- imidazo[5,1-b][1,3]thiazin-4-one molecular scaffold. The six-membered 2,3-dihydro-1,3-thiazin-4-one ring adopts an envelope conformation, with the S-CH2 C atom displaced by 0.761 (2) angstrom from the five-atom plane (all within 0.05 angstrom of the mean plane). The imidazole ring is planar. The phenyl ring is twisted from coplanarity with the imidazole ring by 23.84 (5)degrees and the 4-fluorophenyl ring is twisted by 53.36 (6)degrees, due to a close C(aryl)-(HO)-O-...=C contact with the thiazin-4-one carbonyl O atom. The primary intermolecular interaction involves a CH2 group with the F atom [(CF)-F-... =3.256 (2) angstrom and C-(HF)-F-... = 137 degrees].