Carbohydrates isomerization

Another Fischer achievement was the synthesis of small peptides via the condensation of amino acids. Fischer suggested that there was a common linkage that held pairs of amino acids together in all proteins—the peptide bond. He understood that proteins were tremendously complex, owing to the large number of constituents and the fact of stereoisomerism. By 1916 Fischer had synthesized and characterized 100 peptides, but knew they represented a tiny fraction of what was possible, see also Amino Acid Carbohydrates Isomerism van t Hoff, Jacobus. 

It should also be noted that some pathways to be discussed below depend on highly unusual chemical reactions. To give just one example, the formation of the pyridine ring system of vitamin Be depends on a protein that catalyzes a complex series of reactions, including carbohydrate isomerization, imine formation, ammonia addition, aldol-type condensation, cycliza-tion, and aromatization (9). 

Inulin.—Inulin is a carbohydrate isomeric with starch which has the chemical formula of C12H20O10. It is found dissolved in the cell sap of many plants, especially those of the Composite. If pieces of a plant part containing this substance be placed directly in alcohol for at least a week, then sectioned and mounted in alcohol, sphaero-crystals of inulin will be seen applied to the walls of the cells. When these sections are treated with a 25 per cent, solution of alpha naphthol and 2 or 3 drops of strong H2SO4, the sphaerocrystals will dissolve with a violet color. Fehling s solution is not reduced by inulin. 

Carbohydrates undergo a number of isomerization and degradation reactions under both laboratory and physiological conditions For example a mixture of glucose fructose and mannose results when any one of them is treated with aqueous base This reaction can be understood by examining the consequences of enohzation of glucose... 

Mutarotation (Section 25 8) The change in optical rotation that occurs when a single form of a carbohydrate is allowed to equilibrate to a mixture of isomeric hemiacetals... 

Complications arising from other types of isomerism. Positional and geometrical isomerism, also described in Sec. 1.6, will be excluded for simplicity. In actual polymers these are not always so easily ignored. Polymerization of 1,2-disubstituted ethylenes. Since these introduce two different asymmetric carbons into the polymer backbone (second substituent Y), they have the potential to display ditacticity. Our attention to these is limited to the illustration of some terminology which is derived from carbohydrate nomenclature (structures [IX]-[XII]) ... 

High fmctose com symps (HFS, HFCS, isosymp, isoglucose) are concentrated carbohydrate solutions containing primarily fmctose and dextrose as well as lesser quantities of higher molecular weight saccharides. A 42 wt % fmctose symp is produced by partial enzymatic isomerization of dextrose hydrolyzate. 

Structural Identification of Isomeric O-Trimethylsilyl Derivatives of Some Hexuronic Acids, J. F. Kennedy, S. M. Robertson, and M. Stacey, Carbohydr. Res., 57 (1977) 205-214. 

S. Ravaud, X. Robert, H. Watzlawick, R. Haser, R. Mattes, and N. Aghajari, Trehalulose synthase native and carbohydrate complexed structures provide insights into sucrose isomerization, /. Biol. Chem., 282 (2007) 28126-28136. 

The isomerism existing between the pairs of nucleotides was attributed to the different locations of the phosphoryl residues in the carbohydrate part of the parent nucleoside,49 63 since, for instance, the isomeric adenylic acids are both hydrolyzed by acids to adenine, and by alkalis or kidney phosphatase to adenosine. Neither is identical with adenosine 5-phosphate since they are not deaminated by adenylic-acid deaminase,68 60 and are both more labile to acids than is muscle adenylic acid. An alternative explanation of the isomerism was put forward by Doherty.61 He was able, by a process of transglycosidation, to convert adenylic acids a" and 6 to benzyl D-riboside phosphates which were then hydrogenated to optically inactive ribitol phosphates. He concluded from this that both isomers are 3-phosphates and that the isomerism is due to different configurations at the anomeric position. This evidence is, however, open to the same criticism detailed above in connection with the work of Levene and coworkers. Further work has amply justified the original conclusion regarding the nature of the isomerism, since it has been found that, in all four cases, a and 6 isomers give rise to the same nucleoside on enzymic hydrolysis.62 62 63 It was therefore evident that the isomeric nucleotides are 2- and 3-phosphates, since they are demonstrably different from the known 5-phosphates. The decision as to which of the pair is the 2- and which the 3-phosphate proved to be a difficult one. The problem is complicated by the fact that the a and b" nucleotides are readily interconvertible.64,64... 

Isomerization, 12.T63, 404 of n-butane, 13 697 butylenes, 4 409-410, 410t carbohydrate hydroxyl groups, 4 712 carboxylic acids, 5 44 catalytic aerogels for, J 763t of cyclohexane, 13 706 facilitation of, 20 100 maleic, 20 99-100 maleic anhydride 492, 15 493 paraffin, 16 844... 

Recently Cavaleiro et al. described an easy synthetic approach to glycoporphyrins from zinc(n) protoporphyrin-IX dimethyl ester 4 and O-allyl carbohydrate acetonides 5A-E (D-ribose (A), D-galactose (B), D-glucose (C), and two isomeric derivatives (D) and (E) of D-fructose) by cross-metathesis (Scheme 2).12 Two equivalents of each carbohydrate and the Grubbs catalyst were used, giving the carbohydrate derivatives 6 in a range of 74% to 93% yields. 

Several other types of photochemical reactions involving unsaturated carbohydrates have been reported. One of these is38 photochemical, E -Z isomerization of the groups attached to a double bond (see Scheme 5). A second is the internal cycloaddition between two double bonds connected by a carbohydrate chain.39-41 Although the carbohydrate portion of the molecule is not directly involved in this cycloaddition, its presence induces optical activity in the cyclobutane derivatives produced photochemically. Finally, a group of acid-catalyzed addition-reactions has been observed for which the catalyst appears to arise from photochemical decomposition of a noncarbohydrate reactant.42-44... 

The reactivity of carbohydrates is dominated by the reactivity of the aldehyde group and the hydroxyl on its next-neighbor (/ ) carbon. As illustrated by the middle row of Fig. 2.3, the aldehyde can be isomerized to the corresponding enol or be converted into its hydrate (or hemiketal) form upon reaction with water (or with an hydroxyl-group). These two reactions are responsible for the easy cycliza-tion of sugars in five- and six-membered rings (furanose and pyranose) and their isomerization between various enantiomeric forms and between aldehyde- and ketone-type sugars (aldose and ketose). 

The choice of a stationary phase will depend upon the nature of the carbohydrates to be separated and whereas an OV-17 column (phenylmethyl polysiloxy gum) may give satisfactory isomeric separations, a non-polar phase such as OV-1 (methylpolysiloxy gum) may be more useful for a wider range of carbohydrates (Figure 9.23). 

Treatment of the alcohol ( ) with trifluoromethylsulfonic anhydride (triflic anhydride) at -78 C afforded the ester (1 ) which could be isolated and characterized. We knew from previous experience (2J that sulfonyl esters vicinal to an isopropylidene acetal are relatively stable. The triflate T,) reacted cleanly with potassium azide and 18-crown-6 in dichloromethane at room temperature. The crystalline product [68% overall from (1 )] was not the azide ( ) but the isomeric A -triazoline ( )- Clearly the initially formed azide (18) had undergone intramolecular 1,3-cyclo-addition to the double bond of the unsaturated ester (21- ). The stereochemistry of the triazoline (1 ), determined by proton nmr spectroscopy, showed that the reaction was stereospecific. There are several known examples of this reaction ( ), including one in the carbohydrate series ( ). When the triazoline was treated with sodium ethoxide ( ) the diazoester ( ) was rapidly formed by ring-opening and was isolated in 85% yield, Hydrogenolysis of the diazo group of (M) gave the required pyrrolidine ester ( ) (90%). 

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