The embodiments described herein concern 5-substituted pyrimidine derivatives of conformationally locked nucleoside analogues and to the use of these derivatives as anti-viral and anti-cancer agents. The compounds are of the formula: ##STR1##

wherein B is uracil-1-yl or cytosin-1-yl having a 5-substituent selected from halogen, alkyl, alkenyl, and alkynyl, with the proviso that where B is uracil-1-yl, the 5-substituent is not methyl. The compounds are useful in the treatment of Herpes simplex virus (HSV).