The ketonitrone 8 was prepared from D-fructose as an inexpensive starting material, and was used in a stereoselective synthesis of 1-deoxymannojirimycin (4), its previously unknown N-hydroxy analogue 15, as well as the polyhydroxylated ketonitrone 14. The latter were assayed as potential glycosidase inhibitors on a panel of 13 selected purified enzymes. N-Hydroxy DMJ (15) exhibited a moderate and non selective activity towards the snail - mannosidase EC 3.2.1.25.