Synthesis of the 5'-fluoro-2'-β-methyl analogues of neplanocin was carried out. Key intermediate cyclopentenone 19 was prepared from methyl α-D-mannopyranoside by a new approach consisting of ring-closing metathesis, stereoselective introduction of the 2'-methyl group, and intramolecular oxyselenenylation of the double bond as representative steps. Subsequent introduction of a fluorine atom at the