Chirality, monosaccharides aldoses

Synthesis of chiral monosaccharides. Masamune, Sharpless, and coworkers2 have developed a general, iterative sequence for addition of two chiral hydroxymethy-lene units to an aldose. The key step involves regio- and stereoselective ring opening... 

Rare or unnatural monosaccharides have many useful applications as nonnutritive sweeteners, glycosidase inhibitors and so on. For example, L-glucose and L-fructose are known to be low-calorie sweeteners. In addition, rare or unnatural monosaccharides are potentially useful as chiral building blocks for the synthesis of biologically active compounds. Therefore, these compounds have been important targets for the development of enzymatic synthesis based in the use of DHAP-dependent aldolases alone or in combination with isomerases. Fessner et al. showed that rare ketose-1-phosphates could be reached not only by aldol addition catalyzed by DHAP-dependent aldolases, but by enzymatic isomerization/ phosphorylation of aldoses [35]. Thus, for example, L-fructose can be prepared... 

All the monosaccharides except dihydroxyacetone contain one or more asymmetric (chiral) carbon atoms and thus occur in optically active isomeric forms (pp. 17-19). The simplest aldose, glyceraldehyde, contains one chiral center (the middle carbon atom) and therefore has two different optical isomers, or enantiomers (Fig. 7-2). 

Most monosaccharides are aldoses, and almost all natural monosaccharides belong to the D series. The family of d aldoses is shown in I Figure 7.8. D-glyceraldehyde, the smallest monosaccharide with a chiral carbon, is the standard on which the whole series is based. Notice that the bottom chiral carbon in each compound is directed to the right. The 2" formula tells us there must be 2 trioses, 4 tetroses, 8 pentoses, and 16 hexoses. Half of those are the d compounds shown in Figure 7.8. The other half (not shown) are the enantiomers or l compounds. 

A potential limitation on the use of FDP aldolases for the synthesis of monosaccharides is that the products are always ketoses with fixed stereochemistry at the newly generated chiral centers on C-3 and C-4. There are, however, methods for establishing this stereochemistiy at other centers and for obtaining aldoses instead of ketoses. This technique makes use of a monoprotected dialdehyde as the acceptor... 

Monosaccharides are polyhydroxy aldehydes (aldoses) or polyhydroxy ketones (ketoses). They are classified as trioses, tetroses, pentoses, hexoses, etc. according to the number of carbon atoms. With the exception of dihydroxy acetone, monosaccharides contain one or more chiral C-atoms. Most monosaccharides carry an oxygen-containing group at each carbon atom. However, there are also deoxy sugars which have one or several oxygen-free carbon atoms. 

Figure 4.1 shows the formulas and names for D-aldoses using simplified Fischer projections. The occurrence of aldoses of importance in food is compiled in Table 4.1. Epimers are monosaccharides which differ in configuration at only one chiral C-atom. D-Glucose and D-mannose are 2-epimers. D-glucose and D-galactose are 4-epimers. 

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. 

---