Monosaccharides stereogenic centers

Do not rotate a Fischer projection formula in the plane of the page, because you might inadvertently convert a compound into its enantiomer. When using Fischer projections it is usually best to convert them to structures with wedges and dashes, and then manipulate them. Although a Fischer projection formula can be used for the stereogenic center in any compound, it is most commonly used for monosaccharides. 

B Monosaccharides with More Than One Stereogenic Center... 

The number of possible stereoisomers of a monosaccharide increases exponentially with the number of stereogenic centers present. An aldohexose has four stereogenic centers, and so it has 2 = 16 possible stereoisomers, or eight pairs of enantiomers. 

Although Fischer projections are commonly used to depict monosaccharides with many stereogenic centers, care must be exercised in using them since they do not give a true picture of the three-dimensional structures they represent. Because each stereogenic center is drawn in the less stable eclipsed conformation, the Fischer projection of glucose really represents the molecule in a cylindrical conformation, as shown in Figure 27.2. 

The letters D and L are used to label all monosaccharides, even those with multiple stereogenic centers. The configuration of the stereogenic center farthest from the carbonyl group determines whether a monosaccharide is D- or L-. 

How many stereogenic centers are present in each type of monosaccharide (a) an aldotetrose (b) a ketohexose ... 

The common name of each monosaccharide indicates both the number of atoms it contains and the configuration at each of the stereogenic centers. Because the common names are firmiy entrenched in the chemicai literature, no systematic method has ever been estabiished to name these compounds. 

The family of D-ketoses, shown in Figure 27.5, is formed from dihydroxyacetone by adding a new carbon (bonded to H and OH) between C2 and C3. Having a carbonyl group at C2 decreases the number of stereogenic centers in these monosaccharides, so that there are only four D-ketohexoses. The most common naturally occurring ketose is D-fructose. 

D-Sugar A monosaccharide with the OH bonded to the stereogenic center farthest from the carbonyl group drawn on... 

Anomeric carbon (Section 27.6) The stereogenic center at the hemiaeetal carbon of a cyclic monosaccharide. 

Dihydroxyacetone phosphate-dependent aldolases (DHAP-aldolases) have been used widely for preparative synthesis of monosaccharides and sugar analogs (Fessner and Walter 1997 Wymer and Toone 2000 Silvestri et al. 2003). Among them, RAMA RhuA and FucA from E. coli are the most available aldolases, especially the former which was one of the first to be commercialized (Fessner and Walter 1997 Takayama et al. 1997). In many of the chemo-enzymatic strategies they are involved, the biocatalytic aldol addition to the configuration of the newly stereogenic centers is fixed by the enzyme. However, pertinent examples have been reported in which... 

Aldolases have been studied as catalysts for monosaccharide synthesis for nearly 40 years. The best studied member of the group is a fructose-1,6-diphosphate (FDP) aldolase from rabbit muscle (RAMA, E.C. 4.1.2.13) (20), In vivo, this enzyme catalyzes the reversible condensation of D-glyceraldehyde-3-phosphate and dihydroxy acetone phosphate (DHAP) to generate FDP (Scheme 1). In the synthetic direction, the enzyme catalyzes the formation of two new stereogenic centers with absolute stereospecificity the stereochemistry of the new vicinal diol is always D-... 

In a D monosaccharide, the OH group on the stereogenic center farthest from the carbonyl group is on the right in a Fischer projection. The d, on the other hand, is an abbreviation for dextrorotatory that is, a d-compound rotates the plane of polarized light in the clockwise direction. A D-sugar may be dextrorotatory or it may be levorotatory. There is no direct correlation between D and don and 1. 

---