A series of novel 1, 3, 4-thiadiazine derivatives were synthesized via chemical optimization on phthiobuzone. Their anti-herpes simplex virus (HSV) activities in vitro were also tested. Several compounds exhibited more highly potent anti-HSV activity and much higher selectivity index (SI) values than those of phthiobuzone. The most potent anti-HSV compound was 4f, which showed marked inhibition against HSV-1 (IC 50= 77.04 μg/ml) ...