Monosaccharides optical isomers

The monosaccharides have a couple of characteristics that prove to be important in terms of their structure and function. They all have at least one carbon that is bonded to four different groups (a chiral carbon), and most form five-and six-membered rings easily. The presence of chiral carbons allows these compounds to exist as two different optical isomers that are nonsuperimposable mirror images of each other. (Your feet are nonsuperimposable mirror images. Try putting a left shoe on a right foot )... 

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). 

Stereoisomers Isomers in which the atoms are linked together in the same atom-to-atom order, but with different arrangements in space. See Geometric isomers, Optical isomers. Trisaccharide A molecule consisting of three monosaccharides joined together by glycosidic bonds. 

The carbonyl and alcohol groups within the same monosaccharide may react together if the carbon chain is long enough. The result is a cyclic hemiacetal. A new chiral center is formed at the carbon which was previously the carbonyl. The two optical isomers that can result are called anomers. Five- and six-membered cyclic structures predominate with the alcohol oxygen as the last member of the ring. These are referred to as furanoses and pyranoses, respectively. Cyclic structures exist in equilibrium with the open-chain form. 

The structure shown below is that of a monosaccharide. How many optical isomers are possible for this structure ... 

The synthesis of more complex monosaccharides requires the construction of more complex and highly functionalized dienes and aldehydes. Furyl diene (124) serves as an important starting material in syntheses of KDO and NeuAc. This diene is easily synthesized from the condensation of benzoyloxy acetyl chloride with acetylfuran, followed by treatment with diazomethane. Cyclocondensation of diene (124) with chiral aldehyde (125a) using BFa-OEta as a catalyst gives the CF-type products (126) in a S l ratio of syn and anti isomers (126a and 126b). Aldehyde (125) is also available in optically pure form from... 

In case of compounds with a heterocyclic ring, more substituents at the sp carbon atom, change in the location of one of them leads to altered rotation of polarized light. A typical example of such situation includes spontaneous change of rotation of polarized light that takes place in aqueous solutions of optically pure monosaccharides it is called mutarotation and individual interconverting isomers are called anomers. 

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