New isonucleosides [methyl 5-(1-pyrimidinyl)furanosides] are prepared by nucleophilic opening of the oxetane ring of methyl 3,5-anhydro-2-O-(2-fluorobenzyl)-D-xylofuranoside with silylated pyrimidine bases in the presence of trimethylsilyl triflate. Structures, configurations and conformations were determined by NMR techniques and several X-ray diffraction analyses, seven of the isonucleosides were tested for cytotoxicity and activity against HIV, HSV and several other viruses.