Recent advances in the development of unnatural oligosaccharides - Conformation and bioactivity

The synthetic method, conformation, and bioactivity of the unnatural oligosaccharides whose conformational properties are different from those of natural ones, with or without intention are outlined. In the first section of each chapter, interesting synthetic methods which have been developed recently are introduced. In the second section, the following conformational features are detailed. - 1)Oligosaccharide analogs built up of C-glycoside, S-glycoside, or N-glycoside have a more flexible conformation than natural ones. 2) Conformational properties are almost equal to each other between natural oligosaccharides and their analogs containing a pseudosaccharide, such as carbasugar or 5-thiosugar, in which the ring oxygen atom has been replaced with an other atom or atomic group at the nonreducing monosaccharide. 3) Conformationally fixed oligosaccharide analogs have been synthesized so that the global minimum conformation of the natural oligosaccharide was frozen by a cross-bridge, such as a methylene chain. 4) Recent trends are illustrated by the development of oligosaccharide analogs having unnatural conformation.- The binding abilities to receptor proteins are investigated in some of these oligosaccharide analogs. However, the abilities are equivalent to or lower than those of natural oligosaccharides in most cases. Though the bound-state conformations of oligosaccharide analogs in receptor proteins are equal to those of natural ones in most cases, some analogs bind to a receptor with a different conformation. The knowledge gained from these studies will provide very important information for the design of oligosaccharide-based drugs.