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Abstract: The synthesis and conformations of 30methylDMJ. 3-O-(u-D-glucopyranosyl)-DMJ (GlcaL3DMJ). and 3.G-(a-D-mannopyranosyl)-DMJ (Manal,3DMJ) are described. Manal,3DMJ and Glca1.3DMJ are shown to be very powerful inhibitors of an endomannosidase. The potential use of these compounds in both probing the pathways of N-linked glycoprotein pmcessin g and in the chemotherapy of some viral diseases is discussed. Polyhydroxylated mono- and bicyclic nitrogen heterocycles, in which nitrogen is substituted for oxygen in the carbohydrate ring, have provided an important class of glycosidase inhibitors;1 the properties of these aminosugar glycosidase inhibitors has maintained interest in the further isolation of new natural products,* such as 6-deoxyDMDP3 and in the design of analogues with better specificity for individual hydrolases.4 Although there are many potential applications of such materials,5 one area in which such compounds have made a major contribution has been in the investigation of the processing of N-linked oligosaccharides.6 The major route for N-hnked oligosaccharidc biosynthesis in most cell types is via the sequential hydrolysis of a cotranslationally transferred, triglucosylated oligosaccharide by exoglycosidases of the intracellular membrane compartmen