Chapter 3 - The Reactions of Monosaccharides

Abstract

This chapter discusses the chemical reactions that cause functional group transformations within monosaccharides, either for the purpose of elaboration into oligosaccharides or for the synthesis of a derivative of the monosaccharide. Such derivatives include the deoxy and amino deoxy sugars. The major advances have been concerned with the discovery or invention, as some have called it, of chemoselective reagents for oxidation of the various hydroxyl groups within a carbohydrate. The well-established Jones' (CrO3, H2SO4) and Collins' (CrO3, pyridine) reagents soon gave way to milder and more easily handled oxidizing agents, namely pyridinium chlorochromate and pyridinium dichromate. These reagents oxidize both primary and secondary alcohols and can produce either the aldehyde or the acid from a primary alcohol. Another major class of oxidizing agents is based on dimethyl sulfoxide. Since the initial report by Pfitzner and Moffatt (dimethyl sulfoxide, dicyclohexylcarbodiimide), a string of reagents has been developed that activate the dimethyl sulfoxide for oxidation of, in particular, secondary alcohols. The reduction of an aldose to an alditol is now more safely done with sodium borohydride than with the more toxic sodium amalgam. With the advent of sodium borohydride and lithium aluminium hydride, many of the reductions performed on carbohydrates became easier and, with sodium borohydride, safer. A modern Kiliani-Fischer synthesis now uses sodium borohydride for reduction of the lactone to the aldose and, if necessary, further reduction to the alditol. Alkenes derived from carbohydrates are rarely found naturally and usually result from some chemical transformation in the laboratory, e.g., dehydration, dehydrohalogenation, or de-dihydroxylation. Anomeric alkenes, termed “glycols,” are unique and their chemistry is discussed separately. Carbocycles form the core of several important classes of biologically important molecules, e.g., the cyclitols.