Peptides chiral resolution

The cryptophane is typical of the chiral resolution of methane derivatives (eg, CHFClBr) (146) and the basket-shaped host of Figure 23d exhibits extremely high enantioselectivity for various peptides (144). 

See Section IV.1 for alternative methods of chiral resolution. Partial chemical hydrolysis of proteins and peptides with hot 6 M HC1, followed by enzymatic hydrolysis with pronase, leucine aminopeptidase and peptidyl D-amino acid hydrolase, avoids racemiza-tion of the amino acids281. The problems arising from optical rotation measurements of chiral purity were reviewed. Important considerations are the nonideal dependence of optical rotation on concentration and the effect of chiral impurities282. 

Aboul-Enein, H.Y. and Imran, A., Chiral resolution of clenbuterol, cimaterol, and mabuterol on CHIROBIOTIC V, T and TAG columns, J. Sep. Set, 25, 851,2002. Berthod, A. et al.. Facile RPLC enantioresolution of native amino-acids and peptides using a teicoplanin chiral stationary phase, J. Chromatogr. A, 731, 123, 1996. 

In view of the importance of chiral resolution and the efficiency of liquid chromatographic methods, attempts are made to explain the art of chiral resolution by means of liquid chromatography. This book consists of an introduction followed by Chapters 2 to 8, which discuss resolution chiral stationary phases based on polysaccharides, cyclodextrins, macrocyclic glyco-peptide antibiotics, Pirkle types, proteins, ligand exchangers, and crown ethers. The applications of other miscellaneous types of CSP are covered in Chapter 9. However, the use of chiral mobile phase additives in the separation of enantiomers is discussed in Chapter 10. 

The application of antibiotics as chiral selectors has resulted in the successful resolution of almost all types of neutral, acidic, and basic racemic molecule. These antibiotics have been used for the enantiomeric resolution of amino acids, their derivatives, peptides, alcohols, and other pharmaceuticals. The selectivities of the most commonly used antibiotic-based (vancomycin, teicoplanin, and ristocetin A) CSPs varied from one racemate to another and are given in Table 1. Vancomycin was used for the chiral resolution of amino acids, amines, amides, imides, cyclic amines, amino alcohols, hydantoins, barbiturates, oxazolidinones, acids, profens, and other pharmaceuticals. Teicoplanin was found to be excellent chiral selector for the enantiomeric resolution of amino acids, amino alcohols, imides, peptides, hydantoins, a-hydroxy and halo acids, and oxazolidinones, whereas ristocetin A is capable of chiral resolution of amino acids, imides, amino... 

Based on chiral functional monomers such as (15) in Table 5.6C MIPs capable of chiral resolution can be prepared using a racemic template. Thus, using racemic N-(3,5-dinitrobenzoyl)-a-methylbenzylamine (16 in Table 5.6C) as template, a polymer capable of racemic resolution of the template was obtained [86]. Another chiral monomer based on L-valine (17) was used to prepare MIPs for the separation of dipeptide diastereomers [94]. In these cases, the configurational chirality inherent in the pendant groups of the polymer are to some extent themselves chiral selectors and the effect of imprinting is merely to enhance the selectivity. Alternative approaches to imprint peptides via strong monomer-template association have recently been... 

CARBON-13 NMR SPECTRA SMALL PEPTIDES DIHYDROFOLATE REDUCTASE ANTIBIOTICS CHIRAL RESOLUTION DRUG METABOLISM... 

The type of structure of chiral compounds, as well as that of the stationary phase, could strongly affect the separation. LeFevre et al. [28] investigated the effect of racemic dansyl-amino acids structure on the resolution using p-CD as a CMPA in the RP-TLC system. The same authors confirmed stereochemistry of the peptide-derived amino acids and detected a partial racemization of several amino acids occurring during hydrolysis of enkephalin, gramicidin, achatin-I, and cyclosporin by using /3-CD as a CMPA and RP-TLC [25]. In some cases, /3-CD as a CMPA can improve chiral resolution of cellulose and can be used for enantioseparation of some flavanones [16], amino acids, and oxazolidinone [15], as well as of budesonide in pharmaceuticals [29], and some aromatic amino acids, and aromatic amino alcohols [19]. 

Amino acid separations represent another specific application of the technology. Amino acids are important synthesis precursors - in particular for pharmaceuticals -such as, for example, D-phenylglycine or D-parahydroxyphenylglycine in the preparation of semisynthetic penicillins. They are also used for other chiral fine chemicals and for incorporation into modified biologically active peptides. Since the unnatural amino acids cannot be obtained by fermentation or from natural sources, they must be prepared by conventional synthesis followed by racemate resolution, by asymmetric synthesis, or by biotransformation of chiral or prochiral precursors. Thus, amino acids represent an important class of compounds that can benefit from more efficient separations technology. 

Phenylthiocarbamoyl derivatives of 18 chiral amino acids were separated on a C8 column connected in series to a phenylcarbamoylated (3-cyclodextrin column (Iida et al., 1997). The Cg column separated the derivatized amino acids from one another entering the chiral column. Under this configuration several enantiomers of adjacent amino acids coeluted resulting in poor resolution. However, this configuration was successful in determining the amino acid sequence and chirality of the amino acids in a D-amino acid containing peptide. 

The application of CPO, HRP and CiP is limited to sterically unencumbered substrates and all these peroxidases produce the same absolute configuration of the chiral hydroperoxide. To overcome this limitation, the semisynthetic enzyme selenosubtilisin, a mimic for glutathione peroxidase, with the peptide framework of the serine protease subtilisin was developed by Bell and Hilvert. This semisynthetic peroxidase catalyzes the reduction of hydrogen peroxide and hydroperoxides in the presence of 5-mercapto-2-nitrobenzoic acid. It was utilized by Adam and coworkers and Schreier and coworkers for the kinetic resolution of racemic hydroperoxides (equation 17) . The results obtained were very promising. 

It is more difficult to prepare a chiral a,a-dialkylammo acid. Nevertheless, when such analogues are incorporated into the backbone of a peptide chain, analogues with modified properties are obtained. In this context, such residues have been evaluated as a new type of conformational constraint for the synthesis of enzyme-resistant agonists and antagonists of bioactive peptides. Here, the asymmetric synthesis or the resolution of the chiral quaternary amino acid is necessary and numerous procedures, which have recently been reviewed, were developed to produce the requisite amino acids in enantiomerically pure form. 

Schering Plough demonstrated the kinetic resolution of a secondary amine (24) via enzyme-catalyzed acylation of a pendant piperidine (Scheme 7.13) [32]. The compound 27 is a selective, non-peptide, non-sulfhydryl farnesyl protein transfer inhibitor undergoing clinical trials as a antitumor agent for the treatment of solid tumors. The racemic substrate (24) does not contain a chiral center but exists as a pair of enantiomers due to atropisomerism about the exocylic double bond. The lipase Toyobo LIP-300 (lipoprotein lipase from Ps. aeruginosa) catalyzed the isobu-tylation of the (+) enantiomer (26), with MTBE as solvent and 2,2,2-trifluoroethyl isobutyrate as acyl donor [32]. The acylation of racemic 24 yielded (+) 26 at 97% and (-) 25 at 96.3% after 24h with an E >200. The undesired enantiomer (25)... 

To test the feasibility of enzyme-catalyzed enantiosective reactions in solid/gas reactors and to evaluate the efficiency of the resolution obtained in the gas phase compared to liquid systems, resolution of racemic 2-pentanol, catalyzed by CALB, through alcoholysis with methyl propanoate as acyl donor has been investigated in both liquid media and the gas phase [24]. As CALB has an enantiopreference for R enantiomers of secondary alcohols, this last reaction leads to S-2-Pentanol. This compound is a chiral intermediate in the synthesis of several potential anti-Alzheimer s drugs that inhibit 3-amyloid peptide release and/or its synthesis [25]. The degree of enantioselectivity was measured by using the enantiomeric ratio E, which is defined as the ratio of the specificity constants kcat/KM for the enantiomers (R/S in this case). E can be determined from the enantiomeric excess of... 

A number of methods for the synthesis of piperazic acid (7) and related derivatives are currently available as a result of growing interest in natural product chemistry and in their potential in medicinal chemistry. Their chemistry and conformational properties have been comprehensively reviewed. 2451 Racemic piperazic acid is obtained by condensation of penta-2,4-dienoic acid with phthalazinedione and subsequent reductive deprotection of the resulting A,A -bis(phthaloyl)-l,2,3,6-tetrahydropyridazine-3-carboxylic acid.12431 Resolution of racemic piperazic acid is achieved by fractional crystallization of the ephedrine salt of Nl-(benzyloxycarbonyl)piperazic acid from ethyl acetate. 246,2471 A typical route to enantiomerically pure (3S)-piperazic acid 56 starts from chiral 2-amino-5-hydroxyvaleric acid 55 as shown in Scheme 12.1248 Convenient stereoselective syntheses have been reported for 5-hydroxy- and 5-chloropiperazic acids as important constituents of natural cyclic peptides and depsipep-tides.1249,2521... 

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