The present invention concerns the synthesis of substantially anomerically pure 2,3-dideoxy-3- fluoro-5-0-(4-phenylbenzoyl)-a-D-erythropentofuranosyl chloride from an α/β-mixture of the corresponding methyl glycoside. The method of the invention provides the pure a-chloride without the need of anomeric separation. The α-chloride thus achieved is suitable for use as glycosyl donor in the preparation of 2',3'-dideoxy-3'-fluoro nucleosides. In particular, the preparation of 2',3'-dideoxy-3 '-fluoroguanosine, FLG, using an α/β-mixture of the methyl glycoside without the need of anomeric separation is disclosed.