Carbohydrates configurational carbon

Four-carbon sugars are called tetroses. There are four possible aldotetroses, which are four-carbon sugars with an aldehyde (Fig. 22.6). The aldotetroses are the enantiomeric pair D- and L-erythrose and the enantiomeric pair D- and L-threose. Remember, for a carbohydrate to be D, the OH of the configurational carbon must be on the right-hand side of a Fischer projection. 

Carbohydrate names specify (1) the length of the carbon chain, (2) whether the molecule is an aldehyde or ketone, (3) whether the configurational carbon is D or L, (4) whether the structure is a cyclic six-membered ring or five-membered ring. 

Configurational carbon (Section 22.2) The stereogenic carbon of a carbohydrate that is furthest from C(l) or the carbonyl carbon. This is the carbon whose configuration determines whether the sugar is of the D or L family. 

For carbohydrates of the d series the configuration of the anomeric carbon is a if Its hydroxyl group is down p if the hydroxyl group at the anomeric carbon IS up... 

The carbohydrate 2,3,4,6-tetra-O-acetyl-a-D-glucopyranosyl bromide reacts with NaCo(CO)4 and PPh3 under 10 atm of CO to yield the expected acetyl (XXIX) and the corresponding alkyl 210). The former decarbonylates readily on storage or chromatography. Retention of configuration at carbon accompanies CO elimination. 

All carbohydrates can exist in either of these two forms and the prefix of D or L only refers to the configuration around the highest numbered asymmetric carbon atom. Enantiomers have the same name (e.g. D-glucose and L-glucose) and are chemically similar compounds but have different optical properties. The majority of naturally occurring monosaccharides, whether they be aldoses or ketoses, are of the D configuration. 

Figure 9.1 Enantiomers of glyceraldehyde. Enantiomers are mirror images of each other and are chemically similar but will cause a beam of plane polarized light to rotate in opposite directions. Glyceraldehyde has only one asymmetric centre ( ) and the designation of D or L is determined by the orientation of the O and OH groups about this carbon atom. For carbohydrates with more than one asymmetric carbon atom, the prefix d or l refers only to the configuration about the highest numbered asymmetric carbon atom, although in such enantiomers the configuration about all the asymmetric centres will also be reversed.

Hoffmann, H. M. R. Dunkel, R. Mentzel, M. Reuter, H. Stark, C. B. W., The total synthesis of C-glycosides with completely resolved seven-carbon backbone polyol stereochemistry Stereochemical correlations and access to L-configured and other rare carbohydrates. Chem. Eur. J. 2001,7, 4771-4789. 

Relative configurations at two contiguous carbon atoms in main chains bearing, respectively, substituents a and b (a b), are designated by the prefix erythro or threo, as appropriate, by analogy with the terminology for carbohydrate systems [6] in which the substituents are -OH. 

For molecules with two stereogenic centers, the traditional descriptors are erythro and threo to which, henceforth, a c (short for carbohydrate) is added for a reason that will become apparent (Table 10). They are unambiguously defined for simple sugars. The backbone is given by the carbon chain the reference conformation is the conformation with all carbon atoms eclipsed. This statement is identical with the requirement of a Fischer projection. If two identical substituents X are located on the same side/opposite sides of the plane traced by the backbone, then the c-erythrojc-threo configuration is assigned. 

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. 

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