The thesis is divided in to five chapters. Chapter I: Synthesis of New thiolevomannosan derivatives In this chapter, a general and efficient method for the synthesis of conformationally locked thiosugars has been discussed. An unprecedented synthesis of a novel thioorthoester and its synthetic utility in glycosylation has been demonstrated. Chapter II: Studies on -Mannosidase Inhibitors The synthesis and evaluation of novel, conformationally locked glycomimic, thiolevomannosan and its analogs sulfoxide and sulfone starting from readily available D-mannose is discussed in this chapter. This is the first report of thiosugar derivatives with enhanced potency compared to kifunensine. Docking and biochemical studies have been discussed. Chapter III: Studies on Novel Cyclic Tetraselenides of Mannose In this chapter, the syntheses and structural properties of novel cyclic tetraselenides starting from mannose have been discussed. These tetraselenides are the first of their kind where all four selenium atoms are arranged in a cyclic manner as the backbone of mannose. X-ray structures have been reported for the tetraselenides. Chapter IV: Novel Chalcogenides of Uridine and Thymidine: Synthesis and Applications An efficient and simple method to synthesise disulfides and diselenides of thymidine and uridine derivatives has been demonstrated in this chapter. The utility of these disulfides in various ring opening reactions as well in Michael addition reactions has been demonstrated. Chapter V: Studies on New, Potent Urease Inhibitors In this chapter, a facile one-pot synthesis of thio and selenourea derivatives under mild conditions by the reaction of amines and commercially available Viehe’s iminium salt in the presence of benzyltriethylammonium tetrathiomolybdate as sulfur transfer reagent and tetraethylammonium tetraselenotungstate as selenium transfer reagent has been discussed. A few of the urea derivatives have shown potent inhibitor activity in the nanomolar range for jackbean urease.