Intercellular communication is governed by interactions between surface oligosaccharides and glycoproteins. The biosynthesis of these molecules involves glycosidations catalyzed by glycosyltransferases and hydrolysis of O-glycosyl linkages catalyzed by glycosidases. Monosaccharide and disaccharide mimics such as carbasugars, iminosugars and C-linked disaccharides can be glycosidase or glycosyltransferase inhibitors. They are leads as new drugs to treat infective diseases and cancer. The preparation of a conduramine, a pentahydroxyindolizidine, and two C-linked disaccharides are outlined.