Enantiomeric purity of amino acids

The chiral purity of amino acids at large enantiomeric excess can be determined automatically by derivatization with 4-fluoro-7-nitro-2,l,3-benzoxadiazole (127b) followed by CE with cyclodextrin chiral selectors and detection of the LIF excitation at 488 nm. Lod 140 ppm of L-phenylalanine in D-phenylalanine324. 

This technique is now used extensively to assess enantiomeric excesses in organic reactions and separate small quantities of enantiomers. Closely related chiral corands are particularly useful in assessing the optical purity of amino acids, although modern chiral columns for HPLC may cost in excess of US 2000. 

Cellulose was the first sorbent for which the resolution of racemic amino acids was demonstrated [23]. From this beginning, derivatives such as microcrystalline triacetylcellulose and /3-cyclodextrin bonded to silica were developed. The most popular sorbent for the control of optical purity is a reversed-phase silica gel impregnated with a chiral selector (a proline derivative) and copper (II) ions. Separations are possible if the analytes of interest form chelate complexes with the copper ions such as D,L-Dopa and D.L-penicillamine [24], Silica gel has also been impregnated with (-) brucine for resolving enantiomeric mixtures of amino acids [25] and a number of amino alcohol adrenergic blockers were resolved with another chiral selector [26]. A worthwhile review on enantiomer separations by TLC has been published [27],... 

Pawlowska, M. and Armstrong, D. W. (1994). Evaluation of enantiomeric purity of selected amino acids in honey. Chirality 6, 270-276. 

Direct hydrogenation of amino acids to amino alcohols was first examined by Adkins et al. (3) via the esters, and recently studied by Antons and Beitzke in patents (4). Using Ru/C catalysts at high pressures (>14 MPa) and mild temperatures (70-150 °C), Antons demonstrated the conversion of carboxylic acids and amino acids with retention of optical activity in the product alcohols. High yields (>80%) and high enantiomeric purity (>97% in many cases) were achieved. Broadbent et al. had demonstrated earlier that under certain conditions hydrogenation of amino acids can be accompanied by deamination (8). 

Ekgorg-Ott, K.H., Armstrong, D.W. (1996). Chirality evaluation of the concentration and enantiomeric purity of selected free amino acids in fermented malt beverages (beers). Chirality 8, 49-57. 

In determination of the enantiomeric purity of aliphatic primary amines, the use of (S-aminoalcohols and ot-amino acids 1R(—)-myrtenal as a derivatising agent has been tested.1... 

Syntheses of naphthyridone derivatives follow the same procedures as those of quinolones, except that substituted 2-aminopyridines (Gould-Jacobs modification) or substituted nicotinic ester/nicotinoyl chloride are used instead of anilines or o-halobenzoic acid derivatives. Most of the recently introduced quinolone antibacterials possess bicyclic or chiral amino moieties at the C-7 position, which result in the formation of enantiomeric mixtures. In general, one of the enantiomers is the active isomer, therefore the stereospecific synthesis and enantiomeric purity of these amino moieties before proceeding to the final step of nucleophilic substitution at the C-7 position of quinolone is of prime importance. The enantiomeric purity of other quinolones such as ofloxacin (a racemic mixture) plays a major role in the improvement of the antibacterial efficacy and pharmacokinetics of these enan-... 

The enantiomeric purity of protected amino acids used in peptide synthesis can be determined by pre-column partial deprotection followed by derivatization with Marfey s reagent (116). The Marfey diastereoisomers can be easily resolved and determined by RP-HPLC using an ODS-Hypersil column288. Fifteen amino acids collected from mammalian tissues were derivatized with Marfey s reagent and subjected to two-dimensional TLC. Each individual spot (enantiomeric mixture of a diasteroisomer) was then resolved by RP-HPLC. Except for tyrosine (46) and histidine (117), subnanomole quantities of enantiomers could be analyzed289,290. 

A major advantage that nonenzymic chiral catalysts might have over enzymes, then, is their potential ability to accept substrates of different structures by contrast, an enzyme will select only its substrate from a mixture. Striking examples are the chiral phosphine-rhodium catalysts, which catalyze die hydrogenation of double bonds to produce chiral amino acids (10-12), and the titanium isopropoxide-tartrate complex of Sharpless (11,13,14), which catalyzes the epoxidation of numerous allylic alcohols. Since the enantiomeric purities of the products from these reactions are exceedingly high (>90%), we might conclude... 

Underivatized a-amino acids have successfully been separated on almost all the gly-copeptides CSPs teicoplanin [30, 84, 87], ristocetin A [33], TAG [45], A-40,926 [41], CDP-1 [55], and eremomycin [22, 23] in different chromatographic conditions. Vancomycin and avoparcin CSPs are not optimal for such substrates. A commercially available teicoplanin CSP was successfully employed for the determination of the enantiomeric purity of a sample of L-arginine [129]. For the lack of chromophore groups in the majority of them (i.e., aliphatic a-amino acids), UV detection at 205-210 nm usually yielded to loss of detection sensitivity (see Section 2.3.1.4). This problem was circumvented by the recent interfacing to the MS detection [116,117]. 

The enantiomeric purities of the (S)-amino acids were checked preparing the corresponding N-trifluoroacetylamino acid methyl esters, which are resolved into enantiomers by gas liquid chromatography on glass capillary... 

This type of interesting phenomenon has also been observed in non-organometallic reactions. The Hajos-Wiechert intramolecular aldol reaction of the triketone to the bicyclic aldol exhibits a nonlinear relation between the enantiomeric purity of the (S)-proline catalyst and the en-antioselectivity (Scheme 44) (75). With the partially resolved amino acid, the cyclization affords the product in an ee lower than anticipated. The reaction occurring via an enamine intermediate again may be interpreted in terms of participation of two proline molecules in the productdetermining transition state. 

Just like the common amino acids, the enantiomeric purity of y-amino-P-hydroxy acids is established by formation of diastereomeric derivatives and their analysis by HPLC or NMR spectrometric techniques. In addition to derivatization of the y-amino group, the P-hydroxy moiety of this class of compounds may also possibly be derivatized. 

In the Horner-Emmons reaction (Scheme 3), the sulfonylphosphonate carbanion 5 is formed in the presence of NaH and then reacts with an aldehyde to produce the intermediate 6 that undergoes in situ elimination to yield the vinyl sulfones and phosphonate anion. The sulfonyl group can stabilize the anion in the sulfonylphosphonate 5. The vinyl sulfones that are produced by this method using aldehydes as starting materials are exclusively the E (trans) isomers. The E-isomers of the vinyl sulfones are shown in the NMR spectra based on the coupling constants of the vinylic protons. Although strongly basic conditions are used in the Horner-Emmons reaction and a-amino aldehydes are easily racemized, the amino acid vinyl sulfones prepared by this method still show substantial optical activity. However, the enantiomeric purity of these compounds has not been determined. 5 ... 

The very short reaction times required for the alkylation of substrate 11a with benzylic bromides using Nobin as an asymmetric phase-transfer catalyst are important for the synthesis of 18F-fluorinated amino adds for use in positron-emission tomography (PET)-imaging studies. Thus, Krasikova and Belokon have developed a synthesis of 2-[18F]fluoro-L-tyrosine and 6-[18F]fluoro-L-Dopa employing a (S)-Nobin-catalyzed asymmetric alkylation of glycine derivative 11a as the key step, as shown in Scheme 8.14 [29]. The entire synthesis (induding semi-preparative HPLC purification) could be completed in 110 to 120 min, which corresponds to one half-life of18 F. Both the chemical and enantiomeric purity of the final amino acids were found to be suitable for clinical use. 

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