The two Upases, Novozyme®-435 and Lipozyme® TL IM has been used for diastereoselective acetylation of 4-C-hydroxymethyl-l,2-O4sopropylidene-3-O-alkyl-β-L-threo-pentofuranose in toluene. It has been observed that Novozyme*-435 mediates acetylation reaction of the dihydroxy compound with vinyl acetate leading to the formation of 5-O-acetyl-4-C-hydroxymethyl-l,2-O4sopropylidene-3-O-alkyl-β-L-threo-pentofuranose in 76 to 80 % de, whereas Lipozyme® TL IM mediates acetylation reaction leading to the formation of 4-C-acetoxymethyl-l,2-0-isopropylidene-3-0-alkyl-β-L-threo-pentofuranose in 60 to 77 % de. The de of the acetylated compound has been calculated on the basis of integration of anomeric protons from the 'H NMR spectrum of the diastereomeric mixture. Further, the site of acetyaltion on C-5 CH 2OH or C-l' CH 2OH in the dihydroxy sugar derivative was established on the basis of NOE and 1H- 1H COSYNMR spectral studies of the compounds.