Monosaccharide diastereomer

Classification of Carbohydrates 1102 23-3 Monosaccharides 1103 23-4 Erythro andThreo Diastereomers 1106 23-5 Epimers 1107... 

Of these 16 isomers, only one is the (glucose that we have described as the most abundant monosaccharide. A second isomer is ( f glucose, the enantiomer of the naturally occurring compound. The other 14 isomers are all diastereomers... 

The above methodology proved very successful for accessing pure standards of DFAs 5, 7, 10, and 14, which are among the most abundant DFAs in kinetic mixtures of diastereomers as well as in caramel. It is, however, intrinsically limited to compounds having identical ring size at both monosaccharide moieties. Moreover, the di-p-D-fructofuranose l,2 2,l -dianhydride isomer 12 remained elusive. 

Monosaccharides have one or more chiral carbon centers and can thereby form enantiomers and diastereomers. Most common monosaccharides are in the D-family. This means that, using D-glyceraldehyde as a starting point, other chiral carbons can be inserted between the carbonyl group and the D- carbon, producing families of... 

See also Monosaccharide Nomenclature, Diastereomers, Saccharides, Mannitol... 

See also Saccharides, Oligosaccharides as Cell Markers, Biosynthesis of Glycoconjugates (from Chapter 16), Diastereomers, Monosaccharide Nomenclature... 

See also Galactose Operon, Galactose Metabolism, Diastereomers, Saccharides, cAMP Receptor Protein, Monosaccharide Nomenclature... 

Organoleptic Properties. Stereochemical differences of enantiomeric excipients may influence perception by sensory organs. Kutti [17] reported as early as 1886 that the interaction of stereoisomer with chiral receptors led to chiral discrimination as a consequence of the formation of diastereomers. He observed that the dextrorotatory asparagine has a sweet taste whereas the levorotatory form is tasteless. Greenstein and Winitz [18] and Solms et al. [19] reported such differences for many amino acids. Shallenberger et al. [20] reported that for some monosaccharides, both isomers have similar sweetness. In contrast, aspartame (A-aspartylalanine methyl ester) is marketed as the l,l isomer because it is more than 100 times as sweet as sucrose. However, the l,d diastereomer of aspartame is bitter [11], It should be noted that the individual differences of perception of these properties could vary. 

How do we know that monosaccharides exist mainly as cyclic hemiacetals There is direct physical evidence. For example, if D-glucose is crystallized from methanol, the pure a form is obtained. On the other hand, crystallization from acetic acid gives the f3 form. The a and )3 forms of D-glucose are diastereomers. Being diastereomers, they have different physical properties, as shown under their structures in eq. 16.3 note that they have different melting points and different specific optical rotations. 

There are obviously many aldose monosaccharides and many ketose monosaccharides, but the discussion will focus only on the d diastereomers, as with the aldoses. The triose is l,3-dihydroxy-2-propanone (41 also called glycerone) and the tetrose is d-glycero-tetrulose (42). Two pentoses are named d-ribulose (43) and d-xylulose (44) and four hexoses are named d-psicose (45), d-fructose (46), d-sor-bose (47), and d-tagatose (48). All of these compounds are ketoses and are further classified according to the number of carbon atoms, as noted in Section 28.1. For example, 42 is a ketotetrose, 43 is a ketopentose, and 45 is a ketohexose. For 43-48, cyclization is possible to form a hemiketal. Just as aldehydes and alcohols react to form a hemiacetal, ketones and alcohols react to form a hemiketal (see Chapter 18, Section, 18.6). For 43, a five-membered ring (a furanose) is formed if the terminal CH2OH unit (in violet) reacts with the ketone carbonyl. The two anomers formed are 49 (a-d-ribulofuranose) and 50 (P-d-ribulofuranose). 

The Kiliani—Fischer synthesis extends chains of monosaccharides using the formation of a cyanohydrin to generate the additional stereogenic center. In the first step, one enantiomeric form of an aldose reacts with HCN to give a mixture of diastereomeric cyanohydrins. We recall that the formation of an additional stereogenic center in a chiral compound results in some stereoselectivity. A mixture results, but because diastereomers have different physical properties, the reaction mixture can be separated to give two cyanohydrins. 

---