The Optically Active Amino Acids

The reaction can be used ia the large-scale production of the optically active amino acid detivatives. The chiraUty of the a-carbon is substantially retained and resolution of the product is avoided. 

The third method of separating optically active substances by combining them with optically active bases or acids had not been employed with any success until E. Fischer took up this question, the study of the optically active amino acids being his first work upon the chemical constitution of the proteins. The non-success of this method was in all probability due to the small affinity which the simple amino acids themselves have for combining with acids and bases even the attempts to separate the monoaminodicarboxylic acids, which are fairly strong acids, were not successful. 

The co-polymerization of D-alanine-derived A-propargylamide 22, L-valine-derived 23, and pyrene-based monomer 24 gives helical poly(22 -< o-23-c -24) carrying pyrene. The secondary structure of the co-polymer is tunable by the composition of the optically active amino acid units and solvent, which makes it possible to control the direction of the pyrene groups in the side chain. The interaction between the pyrene groups is small when the co-polymer takes a helical structure. The pyrene groups are regularly positioned in the polymer side chain. The co-polymer emits weak... 

The protoberberine alkaloid, xylopinine, has been synthesized in an optically active form by Kametani et al.22 ). A key reaction in this synthesis was the photochemical cyclization of the optically active amino acid derivative 1,2,3,4-tetrahydro-6,7-dimethoxy-3-methoxycarbonyl-1 -methylene-2-veratroylisoquinoline with 1,3... 

One example is the optically active amino acid derivative (S)-20n which contains a bipyridyl substituent (Scheme 3.14). The alkylation reaction in the presence of the cinchona alkaloid catalyst 33 proceeds with 53% ee (83% yield of (S)-20n) and gave the desired enantiomerically pure a-amino acid ester (S)-20n in >99% ee after re-crystallization [43]. Subsequent hydrolysis of the optically pure (S)-20n furnished the desired unprotected a-amino acid 35. A different purification method, subsequent enzymatic resolution, reported by Bowler et al., furnished the a-amino acid product 35 with enantioselectivity of 95% ee [44],... 

The interesting asymmetric synthesis of the optically active -amino acid 101 (Fig. 39), as well as of intermediates in alkaloid chemistry,may also be included in the same group of reactions. 

Phenylalanine, synthesized from [ C]cyanide by the Bucherer modification of the Strecker synthesis, was resolved into its l- and D-iso-mers by the action of the d- and L-amino acid oxidases, respectively. The optically active amino acid was separated from phenylpyruvic acid by cation exchange chromatography (3). Similarly, DL-[ F]acyl-p-fluoro-phenylalanine has been subjected to stereospecific deacylation with the fungal enz)nne, L-amino acylase enz)nnatically generated L-[ F]p-fluoro-phenylalanine was separated from the D-acyl amino acid by column chromatography (4). 

All the optically active amino acids (i.e., all except glycine) have an Lconfiguration. In addition, all amino acids exist as zwitterions. 

The DL-forms of nearly all of the amino acids have been prepared by racemization. The optically active amino acid is heated with a strong base, strong acid or an alkaline solution of acetic anhydride or ketene. Practical difficulties are that only a few of the optically active amino acids are readily obtainable, high temperatures and pressures may be required to effect rapid racemization and some amino acids are partly or wholly decomposed by treatment with hot acid or alkali. 

The optical isomers of DL-histidine have been separated by the fractional crystallization (spontaneous resolution) procedure described by Duschinsky (227) but no other applications of this method have been reported. nt-Amino acids are commonly resolved by fractional crystallization from water, ethanol or methanol of the salts formed from the DL-amino acid or a derivative and an optically active form of an organic acid or base. The more insoluble salt is removed and hydrolyzerl, and the optically active amino acid (A) is purified by recrystallizing it from... 

Thus, more than half the optically active amino acids occurring in proteins have been shown to be configurationally related to (-i-)alanine. The chemical method has not yet been applied to leucine, threonine, methionine, glutamic acid, lysine, and tryptophan, but the physical and biological properties of these amino acids leave no doubt whatsoever that they too have the same configuration as (-l-)alanine. In the reviewer s opinion, the conclusion that all these compounds have l configuration with respect to d( —)lactic acid is not established with absolute... 

On the other hand, the optically active amino acids can be converted into a racemic mixture by racemization reaction [2]. In reality, racemization reaction, especially when occurring in fossil or teeth [3], takes a lot of time, but it has also become established that racemization takes place with higher reaction rates, at natural pH as well as in dilute acid or base [2]. 

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