STUDIES ON THE CHEMISTRY OF PYRIMIDINE-DERIVATIVES WITH DIMETHYLDIOXIRANE - SYNTHESIS, CYTOTOXIC EFFECT AND ANTIVIRAL ACTIVITY OF NEW 5,6-OXIRANYL-5,6-DIHYDRO AND 5-HYDROXY-5,6-DIHYDRO-6-SUBSTITUTED URACIL DERIVATIVES AND PYRIMIDINE NUCLEOSIDES