Carbohydrates, branched-chain

Carbohydrates that have a carbon substituent attached to the main chain are said to have a branched chain. o-Apiose and L-vancosamine are representative branched-chain 

D-Apiose can be isolated from parsley and is a component of the cell wall polysaccharide of various marine plants. Among its novel stmctural features is the presence of only a single stereogenic center. L-Vancosamine is but one portion of vancomycin, a powerful antibiotic that is reserved for treating only the most stubborn infections. L-Vancosamine is not only a branched-chain carbohydrate, it is a deoxy sugar and an amino sugar as well. 

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The Fischer projection of the branched chain carbohydrate D apiose has been presented in Section 25 12... 

Branched chain carbohydrate (Section 25 12) Carbohydrate in which the main carbon chain bears a carbon substituent in place of a hydrogen or hydroxyl group Bromohydrin (Section 6 17) A halohydnn in which the halo gen IS bromine (see halohydnn)... 

N,0-Bis(trimethylsilyl)acetamide (3) is an alternative trimethylsilylating reagent that has been found to trimethylsilylate L-ascorbic acid completely,139 and to react with tertiary hydroxyl groups in steroids.140,141 The latter observation is of interest, in view of the isolation from antibiotic substances of branched-chain carbohydrates possessing tertiary hydroxyl groups.142 Some authors have recommended that the highly reactive bis(trimethylsilyl)acetamide (3) be used in the absence of pyridine, in preference to a mixture of hexamethyldisilazane and chlorotrimethylsilane in this solvent.143 In the tri-... 

From the determination of the end groups, the long-chain molecules of amylopectin seem to be composed of 24-30 glucose units, from which it is concluded that the substance is a branched-chain carbohydrate. 

Occurrence in nature of branched-chain carbohydrates has prompted interest in the syntheses of these complex structures and stimulated the preparation of analogues for biological evaluation. Consequently, new methods for the construction of these particular skeletons have been devised [1]. The use of carbohydrates as a cheap source of chiral starting materials [2-4] for the synthesis of complex, nonsugar molecules has prompted the emergence of new imaginative methods for formation of carbon-carbon bonds adapted to the particular reactivity of sugar moieties. 

This technique has been successfully used with 2,3- 3,4-, and 4,5- unsaturated sugars to form C-2, C-3, and C-4 branched-chain carbohydrates. Oxygen-substituted double bonds are also good radical acceptors [74,78]. Radicals also add to a glycal double bond to form C-2 branched-chain sugars [79]. 

The synthesis of vicinal doubly substituted carbohydrates can be achieved, as mentioned earlier, by successive formation of simple branched-chain carbohydrates. Other methods have emerged that are mostly based on the formation of 3-, 4-, 5-, or 6-membered ring fused to a pyranose-furanose ring. This process allows high stereocontrol on the two newly... 

H. Paulsen, V. Sinnwell, and J. Thiem, Applications of the 1,3-dithiane procedure for the synthesis of branched-chain carbohydrates. Methods Carbokydr. Chem. 8 185 (1980). 

E. J. Enholm, S. Jiang, and K. Abboud, Branched-chain carbohydrate lactones from a samar-ium(U) iodide promoted serial deoxygenation-carbonyl addition reaction, J. Org. Chem. 58 4061 (1993). 

Structurally modified carbohydrates include deoxy sugars, amino sugars, and branched-chain carbohydrates. 

A.-M. Sepulchre, G. Lukacs, G. Vass, and S. D. Gero, Synthesis and pulse Fourier transform 13C NMR spectra of branched-chain carbohydrates, Angew. Chem. Int. Ed., 11 (1972) 148. 

In most cases, the stereochemistry of the quaternary carbon atom in branched-chain carbohydrates cannot be elucidated from H-n.m.r. spectra, but 13C-chemical shifts, or long-range, 13C- H coupling-constants, may often yield valuable information.114-118 Likewise, the stereochemistry of acetal carbon atoms of benzylidene derivatives,103, 104 and of acetals derived from pyruvic acid,105,106 may be determined from l3C-chemical shifts. 

Similar application of the oxo reaction, as described in Section VI,2, to tetra-0-acetyl-3-deoxy-a-D-threo-hex-2-enopyranose (61) afforded a complex mixture of branched-chain carbohydrates. The separation and characterization of these products are still under investigation. 

Chapleur, Y, Chretien, F, Selected methods for branched-chain carbohydrates synthesis. In Modern Synthetic Methods in Carbohydrate Chemistry, Hanessian, S, Ed., Marcel Dekker, New York, pp. 207-252, 1996. 

Horton and coworkers84 established the potential value of euro-pium(III)-induced shifts for determining the configuration at tertiary alcohol centers in acetals of branched-chain carbohydrates. In their approach, sets of shift gradients for the unknown and a reference compound are compared. 

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