Racemization, amino acids, -proline

Some workers avoid delay. Pai)adium-on-carbon was used effectively for the reductive amination of ethyl 2-oxo-4-phenyl butanoate with L-alanyl-L-proline in a synthesis of the antihyperlensive, enalapril maleate. SchifTs base formation and reduction were carried out in a single step as Schiff bases of a-amino acids and esters are known to be susceptible to racemization. To a solution of 4,54 g ethyl 2-oxO 4-phenylbutanoate and 1.86 g L-alanyl-L-proline was added 16 g 4A molecular sieve and 1.0 g 10% Pd-on-C The mixture was hydrogenated for 15 hr at room temperature and 40 psig H2. Excess a-keto ester was required as reduction to the a-hydroxy ester was a serious side reaction. The yield was 77% with a diastereomeric ratio of 62 38 (SSS RSS)((55). 

However, it was not until the beginning of 1994 that a rapid (<1.5 h) total resolution of two pairs of racemic amino acid derivatives with a CPC device was published [124]. The chiral selector was A-dodecanoyl-L-proline-3,5-dimethylanilide (1) and the system of solvents used was constituted by a mixture of heptane/ethyl acetate/methanol/water (3 1 3 1). Although the amounts of sample resolved were small (2 ml of a 10 inM solution of the amino acid derivatives), this separation demonstrated the feasibility and the potential of the technique for chiral separations. Thus, a number of publications appeared subsequently. Firstly, the same chiral selector was utilized for the resolution of 1 g of ( )-A-(3,5-dinitrobenzoyl)leucine with a modified system of solvents, where the substitution of water by an acidified solution... 

Metal ion complexes. These classic CSPs were developed independently by Davankov and Bernauer in the late 1960s. In a typical implementation, copper (II) is complexed with L-proline moieties bound to the surface of a porous polymer support such as a Merrifield resin [28-30]. They only separate well a limited number of racemates such as amino acids, amino alcohols, and hydroxy acids. 

Since the proline residue in peptides facilitates the cyclization, 3 sublibraries each containing 324 compounds were prepared with proline in each randomized position. Resolutions of 1.05 and 2.06 were observed for the CE separation of racemic DNP-glutamic acid using peptides with proline located on the first and second random position, while the peptide mixture with proline preceding the (i-alamine residue did not exhibit any enantioselectivity. Since the c(Arg-Lys-0-Pro-0-(i-Ala) library afforded the best separation, the next deconvolution was aimed at defining the best amino acid at position 3. A rigorous deconvolution process would have required the preparation of 18 libraries with each amino acid residue at this position. 

The improvements in resolution achieved in each deconvolution step are shown in Figure 3-3. While the initial library could only afford a modest separation of DNB-glutamic acid, the library with proline in position 4 also separated DNP derivatives of alanine and aspartic acid, and further improvement in both resolution and the number of separable racemates was observed for peptides with hydrophobic amino acid residues in position 3. However, the most dramatic improvement and best selectivity were found for c(Arg-Lys-Tyr-Pro-Tyr-(3-Ala) (Scheme 3-2a) with the tyrosine residue at position 5 with a resolution factor as high as 28 observed for the separation of DNP-glutamic acid enantiomers. 

There have also been reports [36, 37] that racemization of amino acids occurs more rapidly using MW heating than conventional heating at the same temperature. Chen et al. [36] observed that racemization of amino acids in acetic acid the presence of benzaldehyde was accelerated by MW heating. Lubec et al. [37] reported that some D-proline and ris-4-hydroxy-D-proline were found in samples of infant milk formula when they were heated in a MW oven. On the other hand, conventionally heated samples did not contain these unnatural D-amino acids. This report caused concern, and received media attention because D-proline is neurotoxic and suggested that MW heating of some foods could have deleterious effects on their nutritional value and the health of the consumer. 

H Kuroda, S Kubo, N Chino, T Kimura, S Sakakibara. Unexpected racemization of proline and hydroxy-proline phenacyl ester during coupling reactions of Boc-amino acids. Int J Pept Prot Res 40, 114, 1992. 

Racemic mixtures of underivatized amino acids N-alkyl- and N-aryl-substitued derivates of amino acids (phenylalanine and proline) on graphitic carbon 0.001 M Cu(acetate)2 aqueous solution 229, 230... 

Another possible mechanism for the racemization of amino acid esters involves the in situ, transient, formation of Schiff s bases by reaction of the amine group of an amino acid ester with an aldehyde. Using this approach, DKR of the methyl esters of proline 5 and pipecolic acid 6 was achieved using lipase A from C. ant-arclica as the enantioselective hydrolytic enzyme and acetaldehyde as the racemiz-ing agent (Scheme 2.4). Interestingly, the acetaldehyde was released in situ from vinyl butanoate, which acted as the acyl donor, in the presence of triethylamine. The use of other reaction additives was also investigated. Yields of up to 97% and up to 97% e.e. were obtained [6]. 

Cyclic dipeptides, especially when N-alkylated, undergo extremely fast epimerization (79JA1885). For example, cyclo(L-Pro-L-Phe) is rapidly converted to its diastereomer, cyclo(D-Pro-L-Phe) (80% conversion), by treatment with 0.5 N NaOH at 25°C for 15 min. This diastereomer is the one in which the proline residue has epimerized and not the more activated phenylalanine. CNDO/2 calculations seem to provide a rationale for this. It is not yet completely clear why such base-catalyzed epimerizations of piperazinediones are so easy the conformation of the molecule may play a role in this (79MI1). It is also worth noting that even in linear peptides, rm-amides of N-alkyl-amino acids, which consist of s-trans and s-cis rotamers of almost equal energy, are more prone to racemization than the sec-amides, which exist only in the s-trans configuration. Of course, the amide functions of piperazine-2,5-diones are obliged to assume the s-cis conformation. 

They further observed the stereoselective effect in several other systems containing other amino acids as the polymer ligands74, and identified certain prerequisites for effective resolution (i) mixed complexes are more stable and it is better to use Cu(II) or Ni(II), (ii) the sum of the dentation of the polymer ligands and the mobile ligands (amino acids) should be equal to the coordination number of the central metal ion. The difference in free energy of formation of the two dia-stereomeric complexes with L- and D-proline was estimated to be 0.4 to 0.5 kcal/mol for the L-proline-Cu resin. It was emphasized that the stereoselectivity of this process was sufficient for the complete resolution of racemic a-amino acids74. ... 

Careful inspection of the reported photocatalytic reactions may demonstrate that reaction products can not be classified, in many cases, into the two above categories, oxidation and reduction of starting materials. For example, photoirradiation onto an aqueous suspension of platinum-loaded Ti02 converts primary alkylamines into secondary amines and ammonia, both of which are not redox products.34) ln.a similar manner, cyclic secondary amines, e.g., piperidine, are produced from a,co-diamines.34) Along this line, trials of synthesis of cyclic imino acids such as proline or pipecolinic acid (PCA) from a-amino acids, ornithine or lysine (Lys), have beer. successfuL35) Since optically pure L-isomer of a-amino acids are available in low cost, their conversion into optically active products is one of the most important and practical chemical routes for the synthesis of chiral compounds. It should be noted that l- and racemic PCA s are obtained from L-Lys by Ti02 and CdS photocatalyst, respectively. This will be discussed later in relation to the reaction mechanism. 

Intramolecular imine formation between a coordinated aminate ion and a 2-oxo acid has been utilized to synthesize the two racemic amino acids 2-cyclopropylglycine and proline.463 Thus anation of [Co(NH3)5OH2]3+ by Br(CH2)3C0C02H at pH 5 gives two major products (145) and (146). Both are converted to tetraammineiminocarboxylato chelates by attack of an adjacent deprotonated ammonia. The cyclopropylimine complex can, for example, be reduced by alkaline BH4 to give the (RS)-2-cyclopropylglycine complex. 

The best preventive measure against racemization in critical synthetic steps (e.g. fragment condensation, see p. 239) is to use glycine (which is achiral) or proline (no azlactone) as the activated carboxylic acid component. The next best choice is an aliphatic monoamino monocarboxylic acid, especially with large alkyl substituents (valine, leucine). Aromatic amino acids (phenylalanine, tyrosine, tryptophan) and those having electronegative substituents in the /7-position (serine, threonine, cysteine) are, on the other hand, most prone to racemization. Reaction conditions that inhibit azlactone formation and racemization are non-polar solvents, a minimum amount of base, and low temperature. If all precautions are taken, one still has to reckon with an average inversion of 1 % per condensation reaction. This means, for example, that a synthetic hectapeptide contains only 0.99100 x 100% = 37% of the fully correct diastereomer (see p. 233 f.). 

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