Amino acids optical isomerism

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. 

Asymmetric induction during the reduction of 4-(48) was observed when a surface-modified carbon cathode was used.70 Optical yields were low but the effect of the chiral amino acid bound to the carbon surface was proved to be a true surface phenomenon. Induction of chirality by homogeneous rather than surface-bound agents has also been studied.71 All the isomeric acetylpyridines (48) were reduced in the presence of three different chiral alkaloids. Both carbinol products 2- and 4-(49) were shown to possess induced chirality, but the 3-carbinol (49) had none under any of the conditions tried. More rapid protonation of the intermediate was proposed to account for the lack of induced chirality. Optimization of optical yields was done.72 The pinacols (50) formed along with 49 were found to have no induced chirality. Optical yields have been as high as 50%.73 The role of electroabsorption was found to be important in the reduction of 2-(48).74 Product distributions were noted as a function of surfactant present in the electrolyte, carbinol 49 being favored... 

Additives that specifically interact with an analyte component are also very useful in altering the electrophoretic mobility of that component. For example, the addition of copper(II)-L-histidine (12) or copper(II)-aspartame (54) complexes to the buffer system allows racemic mixtures of derivatized amino acids to resolve into its component enantiomers. Similarly, cyclodextrins have proven to be useful additives for improving selectivity. Cyclodextrins are non-ionic cyclic polysaccharides of glucose with a shape like a hollow truncated torus. The cavity is relatively hydrophobic while the external faces are hydrophilic, with one edge of the torus containing chiral secondary hydroxyl groups (55). These substances form inclusion complexes with guest compounds that fit well into their cavity. The use of cyclodextrins has been successfully applied to the separation of isomeric compounds (56), and to the optical resolution of racemic amino acid derivatives (57). 

Rhamnaceous plant Discaria pubescens, differs from melonovine A only in its melting point and its optical rotation, and is thus considered to be diastereo-isomeric with respect to the amino-acid chain.19... 

Cleavage of imidazoles dehydroamino acids,3 2,4-Disubstituted imidazoles4 undergo a Diels-Alder-like reaction with singlet oxygen in the presence of DBU (1-2 equiv.) to provide imine diamides, which isomerize to dehydroamino acid derivatives in the presence of base. Hydrogenation in the presence of catalysts with chiral phosphine ligands results in optically active amino acid diamides. The overall process is illustrated for the synthesis of the N-benzylamide of N-acetylleucine (equation I). 

Changing the transition-metal from palladium to rhodium (equation 62) makes possible, in addition to the straight-chain alkylation product (243), the regio- and stereoselective synthesis of amino acid derivatives with a terminal double bond (242), starting from optically active branched allylic substrates 241 (Table 21)" . Remarkably, the substitution products were obtained with high enantiomeric excesses, what might result from a slow isomerization of the intermediary formed allyl rhodium complexes ". 

Many organosulfur compounds can be resolved into optically active forms (enantiomers) owing to the presence of a chiral (asymmetric) sulfur atom 5 important examples include sulfoxides and sulfonium salts. Chiral sulfoxides containing amino or carboxylic acid groups have been resolved by formation of the diastereoisomeric salts with d-camphor-10-sulfonic acid or d-brucine. The salts can then be separated by fractional crystallisation and the free optically isomeric sulfoxides liberated by acid hydrolysis. However, a more convenient synthetic procedure for the preparation of chiral sulfoxides of high optical purity is Andersen s method (see p. 30). 

Write the structural formulas of the two amino acids that are combined to make aspartame (ignoring optical isomerism). 

Chirality has been an area of enormous interest and significance for chemists. The identification of an enantiomeric excess in some amino acids found in a meteorite (Science 1997, pp. 275, 951) has generated a lot of excitement among chemists because chirality even figures in discussions of the origin of life. Robin Le Poidevin, in the chapter "Space and the Chiral Molecule," provides an instance of a reciprocally beneficial relationship between chemistry and philosophy. He examines some implications of the phenomenon of chirality in chemistry—optical isomerism in particular—for standard philosophical positions about space. At the same time, he shows how an understanding of "spatial realism" allows us to better appreciate the often subtle differences between different kinds of isomerism. 

This amino acid which is synthesized by muscle and excreted into urine without recycling has been methylated with [Djj-methyl iodide exclusively at N site and applied as tracer in vivo and as an isotopic dilution standard in vitro, by protecting the N and N positions in a cyclic urea derivative 34. The latter was obtained by treating the (5)-histidine methyl ester 35 with carbonyl diimidazole and hydrolysing the product 36 in boiling dilute hydrochloric acid to give isomerically and isotopically pure title compound 37 retaining at least 95% of its optical rotation (equation 5). 37 has been applied in clinical research. 

Optical isomerism of phenoxycarboxylic acids also plays a decisive role in their activity. Of the amino acid derivatives of 2,4-dichlorophenoxyacetic acid and 2,4-dichlorophenoxypropionic acid only the derivatives of the dl- and L-amino acids are active, while the respective D-amino acid derivatives are completely inactive. This can be attributed to the fact that plants are unable to hydrolyse the peptide bond of the D-derivatives (Wood and Fontaine, 1952 Krewson et al., 1956). 

Optical isomerism plays a vital role in living cells. Nearly all carbohydrates and amino acids are optically active, but only one of the isomers is biologically usable. For example, r/-glucose is metabolized for energy, but /-glucose is excreted unused. Similarly, /-alanine is incorporated naturally into proteins, but r/-alanine is not. Many drugs are chiral molecules of which one optical isomer is biologically active and the other has either a different type of activity or none at all. 

We have already seen (in Section 3.1) that some molecules are not super-imposable on their mirror images and that these mirror images are optical isomers (stereoisomers) of each other. A chiral (asymmetric) carbon atom is the usual source of optical isomerism, as was the case with amino acids. The simplest carbohydrate that contains a chiral carbon is glyceraldehyde, which... 

Lastly, the lactone was formed from the amino acid by Reaction (4). The resolution of the amino ester was done via the formation of diastereo-isomeric salts with (+) or (-)-dibenzoyltartaric acid, as described by Testa et al. The amino ester chiral precursor was shown to have an enantiomeric excess of 80%. It was then assumed that racemizatlon did not occur during the following steps of the monomer synthesis or during the polymerization. This assumption was indirectly verified by the measurements of a melting point of 260 C for the optically active polymer as compared to 11 0 C for the racemic polymer. 

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