The synthesis and antiviral evaluation of 2',3'-dideoxy and 2',3'-didehydro-2',3'-dideoxy 4' thionucleosides in both enantiomeric series are described. Enantiomeric 4-O-silylated-4 hydroxymethyl-4-thiobutyrolactones, produced in high yield from chiral glycidols, provided suitable chiral synthons: phenylselenation of these intermediates resulted in anomeric selectivity in nucleobase glycosidations. L-d4-cytidine analogues showed marked anti-HBV and anti-HIV activity.