Sugars optical isomerism

Fig. 3.—The basic assumption for Fischer s research of the optical isomerism of sugars.

Many of the organic compounds found in nature are chiral. More importantly, most natural compounds in living organisms are not only chiral, but are present in only one of their optical isomeric forms. Such compounds include amino acids, proteins, enzymes and sugars. 

Optical isomerism is of paramount importance in the structure of simple sugars. Most of the important sugars found in nature have the D configuration, based on the standard compound D-glyceraldehyde. 

In the second method, which can be applied to compounds with an optically active center near the potentially tautomeric portion of the molecule, the effect of the isomerization on the optical activity is measured. In favorable cases both the rate of racemization and the equilibrium position can be determined. This method has been used extensively to study the isomerization of sugars and their derivatives (cf. reference 75). It has not been used much to study heteroaromatic compounds, although the very fact that certain compounds have been obtained optically active determines their tautomeric form. For example, oxazol-5-ones have thus been shown to exist in the CH form (see Volume 2, Section II,D,1, of article IV by Katritzky and Lagowski). 

A number of examples involving the stereochemistry of five membered rings are met in furanose sugars. An interesting example is that of 2, 5 dimethylcyclopentane 1, 1 dicarboxylic acid. This acid can exist in two geometrically isomeric forms which can be distinguished by decarboxylation. The cis xxvii isomer forms two monocarboxylic acids which are meso because they possess a vertical plane of symmetry. The trans isomer xxviii forms only one monocarboxylic acid and since it possesses no elements of symmetry, therefore, exists in optically active forms and a meso variety. 

The isomerism of a- and jS-glucose is to be attributed to the spatially different arrangement of the H and OH-groups attached to the asymmetric carbon atom 1. This atom is asymmetric in the cyclic lactol formula (Tollens). The mutarotation of the sugars, i.e. the gradual change to the final stationary value of the optical rotation, is to be explained by an equilibrium occurring in solution between the various... 

In 1894, Fischer wrote "It will probably be possible to obtain all riiem-bers of the sugar group by a combination of the cyanohydrin reaction with the reduction of lactones, as soon as we have succeeded in finding the two optically active forms of glyceraldchyde. All observations agree with the isomerisms foreseen by Van t Hoff, above all the disappearance of isomers if the molecule becomes constitutionally symmetric. This includes the transformation of different stereoisomers into one and the same substance if one of several asymmetric centers is abolished." An example of this is... 

Tnformation about the characteristics of keto-hexoses in solution has been - derived mainly from optical rotatory data (I, 2, 3, 4) and in recent years by application of the principles of conformational analysis (5, 6). In the current study an attempt is made to describe the conformation and composition of these sugars in solution by nuclear magnetic resonance (NMR) spectroscopy, a highly sensitive means for examining stereochemistry and for differentiating between isomeric species. 

Optical Activity.—We come now, in the case of the isomeric alcohols, to a new and inost interesting example of isomerism. The five carbon alcohol 2-methyl butanol-i, differs from the other seven structurally isomeric amyl alcohols not only in structure, but also in other striking ways. Three different amyl alcohols are known all of which have the constitution of 2-methyl butanol-i. Two of these three are known as optically active all the other amyl alcohols being inactive. Certain substances either in the crystalline form, as in the case of quartz in solution, as in the case of sugar or in the liquid form, as in the case of the alcohol we are considering possess this physical property of optical activity. This property is shown by the fact that the compound has the power to turn or rotate the plane of vibration of a ray of light that has been polarized. 

This sugar is also an aldo-pentose and is stereo-isomeric with arabinose. It is known as wood sugar because it is obtained by the hydrolysis of wood gum, i.e.f of the pentosans present in this gum. It is crystalline and melts at 140°- 60°. It is optically active, being dextrorotatory. Its osazone melts at 160°. By reduction it yields a penta-hydroxy alcohol and by oxidation it yields tri-hydroxy glutaric acid. 

Optical rotations have been recorded for most preparations of sugar sulfates but may be of little diagnostic value, since the values for isomeric sugar sulfates are often rather close to one another. In some cases, the values recorded in the literature are too numerous to mention individually, and some arbitrary choice of values for inclusion in Table IV has been necessary. 

Optical activity involving the ability to rotate the plane of polarized light was first observed by Biot in 1815-1835 in a number of naturally occurring organic compounds, such as turpentine, camphor, sugars, and tartaric acid. Since optically active compounds exhibited this property both in their crystalline form as well as in solutions, it was reasoned that this property is inherent in the molecules. Mitscher-lich in 1844 observed that although tartaric and racemic acids are isomeric, the former and its salt are optically active, while racemic acid is inactive. ... 

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