The Variety of Monosaccharides

The most general methods for the syntheses of 1,2-difunctional molecules are based on the oxidation of carbon-carbon multiple bonds (p. 117) and the opening of oxiranes by hetero atoms (p. 123fl.). There exist, however, also a few useful reactions in which an a - and a d -synthon or two r -synthons are combined. The classical polar reaction is the addition of cyanide anion to carbonyl groups, which leads to a-hydroxynitriles (cyanohydrins). It is used, for example, in Strecker s synthesis of amino acids and in the homologization of monosaccharides. The ff-hydroxy group of a nitrile can be easily substituted by various nucleophiles, the nitrile can be solvolyzed or reduced. Therefore a large variety of terminal difunctional molecules with one additional carbon atom can be made. Equally versatile are a-methylsulfinyl ketones (H.G. Hauthal, 1971 T. Durst, 1979 O. DeLucchi, 1991), which are available from acid chlorides or esters and the dimsyl anion. Carbanions of these compounds can also be used for the synthesis of 1,4-dicarbonyl compounds (p. 65f.). 

When sugars are treated with aqueous ammonia for a short time at low temperature in the absence of a catalyst, the reaction is arrested before heterocyclic compounds can be formed in appreciable proportion, and the products are mainly epimerization products of the sugars, probably formed by way of their 2,3-enediols. These epimerization products are summarized in Table I which shows the reactions of D-glucose, D-fructose, lactose, maltose, and melibiose with aqueous ammonia for a short time at low temperature. A dark-colored, high polymer is also formed in some instances (the browning reaction). In the ammoniacal solution, the monosaccharides are epimerized the disaccharides are epimerized and, in addition, may be hydrolyzed to monosaccharides that can also be epimerized hence, the variety of products obtained may be considerable. 

From among the variety of non-carbohydrate precursors, acetylenes and alkenes have found wide application as substrates for the synthesis of monosaccharides. Although introduction of more than three chiral centers having the desired, relative stereochemistry into acyclic compounds containing multiple bonds is usually difficult, the availability of such compounds, as well as the choice of methods accessible for their functionalization, make them convenient starting-substances for the synthesis. In this Section is given an outline of all of the synthetic methods that have been utilized for the conversion of acetylenic and olefinic precursors into carbohydrates. Only reactions leading from dialkenes to hexitols are omitted, as they have already been described in this Series.7... 

Naturally occurring polysaccharides are made up of a variety of monosaccharides connected by glyco-sidic bonds. Often, these polysaccharides are linked to proteins and lipids. The function of such polysaccharides is often critically dependent on the composition and sequence of their monomeric units. Determination of the structure of polysaccharides is a multistep process. In the early stages, it is crucial to ascertain both the relative amount and the chemical identity of the monomers. After a polysaccharide has been broken down to its monomeric units by hydrolytic or enzymatic digestion, a variety of analytical techniques can be employed to identify and quantify each monomer. Three of these techniques will be demonstrated in this laboratory exercise. 

Pyrolysis of a variety of monosaccharides has been shown to afford the corresponding... 

A variety of monosaccharide sugars and polyols is employed in the food industry. The use of these materials reflects availability, costs, and properties. A brief review of monosaccharide structure and nomenclature will follow, though the reader is directed to other sources for a greater depth of information [e. g., 3-5]. 

Hemicellulose Polysaccharides with a random, amorphous structure that are components of plant cell walls. Umelated to cellulose structurally, they are composed of a variety of monosaccharides, including some acidic sugars, with xylose being the most prevalent. 

Structural polysaccharides, also referred to as nonstarch polysaccharides (NSP), in SB are diverse, and some have complex structures. Total NSP concentration is the sum of water-insoluble and water-soluble NSP fractions. Nonstarch polysaccharides also can be divided into cellulosic and noncellulosic polysaccharides. The noncel-lulosic polysaccharides consist of a variety of monosaccharides (arabinose, galactose, glucose, mannose, xylose, and uronic acids Table 9.3) that are arranged in complex combinations. As the difference in fiber fractions between SBM and SH (Table 9.2) indicates, the NSP composition is rather different in SB cotyledons compared to SH. Purified SB cotyledon cell walls contain approximately 73% NSP, and small amounts of noncarbohydrate matter consisting of protein, minerals, and phenolics (BriUouet Carre, 1983). 

The formose reaction is principally an aldol reaction (anionic mechanism) of formaldehyde in an aqueous solution. By a complex repetition of the reaction, the carbon chain grows, to yield a variety of monosaccharides from C2 to Cs. 

Along with other carbohydrates, the oxidation of monosaccharides by supported metal catalysts has been reviewed [16]. Glucose is the most investigated substrate for the synthesis of a variety of products and intermediates (Figure 21.6). 

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