Carbohydrates absolute configuration

Carbohydrate Components of Antibiotics. Part III. Synthesis of 3.6-Dideoxy-3-dimethylamino- 3-D-glucose Hydrochloride Monohydrate the Absolute Configuration of Mycaminose, A. B. Foster, T. D. Inch, J. Lehmann, M. Stacey, and J. M. Webber, /. Chem. Soc., (1962) 2116-2118. 

In spite of the former widespread use of d and l to denote absolute configuration, the method is not without faults. The designation of a particular enantiomer as d or l can depend on the compounds to which it is related. Examples are known where an enantiomer can, by five or six steps, be related to a known d compound, and by five or six other steps, be related to the l enantiomer of the same compound. In a case of this sort, an arbitrary choice of d or l must be used. Because of this and other flaws, the DL system is no longer used, except for certain groups of compounds such as carbohydrates and amino acids. 

Fischer s Convention. Initially, the absolute configurations of optical isomers were unknown to chemists working with optically active compounds. Emil Fischer, the father of carbohydrate chemistry, decided to relate the possible configurations of compounds to that of glyceraldehyde of which the absolute configuration was yet unknown but was defined arbitrarily. 

Furanoside diphosphinite ligands 10 and 11 (Fig. 11) were applied in the Ir-catalyzed asymmetric hydrogenation of several dehydroaminoacid derivatives [25]. The best enantioselectivities (ee s up to 78%) were obtained in the reduction of methyl W-acetamidoacrylate with ligand 10. These results using the lr/10-11 catalysts precursor show that enantiomeric excesses are strongly dependent on the absolute configuration of the C-3 stereocenter of the carbohydrate backbone. The best enantioselectivity were therefore obtained with ligand 10 with an... 

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. 

The authors found that the configuration of the anomeric C-atom, to which the hydroper-oxy group is bound, determines the absolute configuration of the epoxide formed. Remarkably, the allylic alcohol 142p, which could not be epoxidized under Sharpless conditions, was oxidized by the carbohydrate hydroperoxides in combination with i i(OPr-i)4, albeit... 

H. Redlich, W. Bruns, W. Francke, V. Schurig, T. L. Payne, and J. P. Vit6, Chiral building units from carbohydrates. XIII. Identification of the absolute configuration of emfo-brevicomin from Dendroclonus frontalis and synthesis of both enantiomers from D-ribose, Tetrahedron 43 2029 (1987). 

A serious ambiguity arises for compounds such as the active tartaric acids. If the amino-acid convention is used, (+)-tartaric acid falls in the d series by the sugar convention, it has the l configuration. One way out of this dilemma is to use the subscripts, v and g to denote the amino-acid or carbohydrate conventions, respectively. Then the absolute configuration of (+)-tartaric acid can be designated as either Ds-(+)-tartaric acid of lb-(+)-tartaric acid. 

Abstract Now an incisive probe of biomolecular structure, Raman optical activity (ROA) measures a small difference in Raman scattering from chiral molecules in right- and left-circularly polarized light. As ROA spectra measure vibrational optical activity, they contain highly informative band structures sensitive to the secondary and tertiary structures of proteins, nucleic acids, viruses and carbohydrates as well as the absolute configurations of small molecules. In this review we present a survey of recent studies on biomolecular structure and dynamics using ROA and also a discussion of future applications of this powerful new technique in biomedical research. 

Optical rotatory dispersion studies on lactones of aldonic acids and other carbohydrate acids were made by Hirst et al.11 and later by Okuda et al.7S The latter workers related the absolute configuration at C-2 of aldono-1,4-lactones to the sign of the Cotton effect. 

Use small capital letters d and l to indicate absolute configuration with amino acids and carbohydrates. 

The syntheses of sphingosines just described have a substantial advantage in comparison with those previously reported, because carbohydrates offer a variety of starting materials having known absolute configuration and, therefore, the necessity of stereoisomer separation does not arise. 

K. Leontein, B. Lindberg, and J. Lonngren, Assignment of absolute configuration of sugars by g.l.c. of their acetylated glycosides formed from chiral alcohols, Carbohydr. Res., 62 (1978) 359—362. 

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