Carbohydrates chiral carbons

Identify the chiral carbon atoms in a carbohydrate or n-amino acid. 

Improved /V,/V-ligands, shown in Figure 103, have a chiral carbon closer to the coordinating atoms, i.e., a linkage between the carbohydrate Cl carbon and the N donor. This feature enhances the ability of the ligands to induce enantioselective coordination of prochiral olefins. 

Despite its efficiency in numerous cases optical resolution is by no means a trivial operation. In each case the optimum method has to be found by laborious trial and error procedures the optical purity of the material has to be secured and its absolute configuration has to be established before the compound can be used in a synthetic sequence. These drawbacks of optical resolution led chemists to start their syntheses from optically active natural products (the so-called chiral carbon pool ). A variety of suitable ex-chiral-pool compounds including carbohydrates, amino acids, hydroxy acids, and terpenoids are shown. 

Notice also that several of the carbons are chiral. Carbohydrates are labeled D or L as follows When in a Fischer projection as shown, if the hydroxyl group on the highest numbered chiral carbon points to the right, the carbohydrate is labeled D if to the left, then L. 

C is correct. The formula for the number of isomers of a carbohydrate is T, where n is the number of chiral carbons. 

C is correct. The general formula for a carbohydrate is Cn(H,0)n. Since this carbohydrate is in the Fischer projection with the aldehyde or ketone at the top, and the bottom chiral carbon is positioned to the right, it is of the D configuration. The only way to know about polarized light is to use a polarimeter. 

General Three Carbon Chiral Synthons from Carbohydrates Chiral Pool and Chiral Auxiliary Approaches... 

Glyceraldehyde contains a single chiral carbon, located in position 2. All carbohydrates contain chiral carbons. The number of possible isomers is given by 2 . For glyceraldehyde, only two isomers are possible, d and l. For a hexose, on the other hand, which in its open form (Figure 9.2) contains four chiral atoms, the number of isomers is 24 = 16, half of them in the d and half in the l series. The first and last carbons in a carbohydrate molecule, if the open structure (e.g., Figure 9.2) is viewed, are nonchiral. 

An additional complicating issue should be kept in mind when reading the seminal pre-1968 literature, myo-Inositol derivatives that are currently designated D configuration were assigned L configuration and vice versa in the literature published before 1968. The reasons for the pre-1968 nomenclature are as follows before 1968, rules of carbohydrate nomenclature dictated that the orientation of highest numbered chiral carbon, C-6 in this case, specify the... 

Since all carbohydrates have chiral carbon atoms, it was recognized long ago that a standard method of repreientation is needed to describe carbo hydrate steneochemtsSfy. The method most commonly used employ s Fischer projections for depicting chirality centers on a flat page. 

Of most concern in using the chiral carbon pool, however, is the enantiomeric homogeneity of the natural products themselves. Although it is generally accepted that most carbohydrates and amino acids are enandomerically pure, it is known that many terpenes are not. The small molecules may or not be enandomerically pure. The only sure method of avoiding a nasty surprise during the projected synthesis is to use a starting material, the enandomeric composition of which is known with certainty. 

In addition to the R and S designations, compounds with two chiral centers may also be identified by stereochemical nomenclature that describes the entire system. For example, the erythro and threo nomenclature derived from carbohydrate chemistry may be employed to describe the relative positions of similar groups on each chiral carbon. Thus, the ephedrines are designated as erythro forms since the similar groups (OH and NHCH3) are on the same side of the vertical axis of the Fischer projection, and the pseudo-ephedrines are designated as threo forms since like groups are on opposite sites of the vertical axis of the projection (Fig. 10). 

An important property of carbohydrates that was recognized in the 19th century was that they generally, but not always, rotated plane polarized light and that this was specific for each carbohydrate. This property is due to the presence of asymmetric or chiral carbons that have four different groups attached to the carbons. Those carbohydrates that rotate plane polarized light... 

Several diastereoselective HDA (hetero-Diels-Alder) reactions of a,/3-unsaturated carbonyl compounds and electron-rich alkenes have been exploited to gain carbohydrate derivatives with good diastereomeric excess. In HDA reaction, up to three chiral centers are formed with high stereoselectivity at each chiral carbon [78]. 

The structures and designations of D- and L-glyceraldehyde are defined hy convention. In fact, the D- and L-terminology is generally applied only to carbohydrates and amino acids. For organic molecules the D- and L- convention has been replaced by a new system that provides the absolute configuration of a chiral carbon. This system, called the (R) and (S) system, is described in Appendix D, Stereochemistry and Stereoisomers Revisited. 

Calculate If a carbohydrate has 2" possible isomers, where n is equal to the number of chiral carbon atoms in the structure, calculate the number of possible isomers for the following monosaccharides galactose, glucose, and fructose. 

We have already seen (in Section 3.1) that some molecules are not super-imposable on their mirror images and that these mirror images are optical isomers (stereoisomers) of each other. A chiral (asymmetric) carbon atom is the usual source of optical isomerism, as was the case with amino acids. The simplest carbohydrate that contains a chiral carbon is glyceraldehyde, which... 

The simplest carbohydrate is the aldotriose, glyceraldehyde (Fig. 3-1). The 4-carbon aldoses are related to D- and L-glyceraldehyde and can be viewed as having been stracturaUy derived from them by the introduction of a hydroxylated chiral carbon atom between C-1 and C-2. Thus there are four aldotetroses. Two simple aldopentoses can be derived stmcturally from each of the fom aldotetroses described, making a total of eight aldopentoses. 

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