Proline racemization

F Dick, M Schwaller. SPPS of peptides containing C-terminal proline racemization free anchoring of proline controlled by an easy and reliable method, in HLS Maia, ed. Peptides 1994. Proceedings of the 23rd European Peptide Symposium. Escom, Leiden, 1995, pp 240-241. 

L-selective) or red color (D-selective), while beads with low selectivity assumed a brown coloration. Among the selected beads, two enantiomeric structures 9.148a,b (Fig. 9.59) showed a significant enantioselectivity that was confirmed upon resynthesis. Their enantioselectivity was also confirmed with the separation ofiV-acyl proline racemates lacking the dye moieties, thus highlighting the possible usefulness of 9.148a,b as chiral selectors. 

Rudnick and Abeles purified proline racemase to 95% homogeneity from Clostridium sticklandii, and characterized it 92. The enzyme is composed of two identical subunits with a molecular weight of about 38000, and is independent of any cofactors or metals. Most amino acid racemases require pyridoxal 5 -phosphate, which labilizes the bond between the a-hydrogen and the chiral center by aldimine formation with the a-amino group of the substrate. However, PLP is not involved in the reaction of proline racemase acting on an a-imino acid. The enzyme also acts on 2-hydroxy-L-proline and 2-allo-hydroxy-D-proline although slowly they are epimer-ized at a rate of 2 and 5% of the rate of L-proline racemization, respectively. L-Proline and D-proline showed Km values of 2.9 and 2.5 mti, respectively1119. ... 

One of the homochiral starting materials (45) for the acetylcholinesterase (ACE) inhibitor captopril [62571 -86-2] (47) is produced by a Hpase enzyme-catalyzed resolution of racemic 3-methyl-4-acetylthiobutyric acid (44) and L-proline (46) (65). 

L-Glutamic acid does not racemize in neutral solution, even at 100°C. Deviation of pH from neutral to greater than 8.5 results in thermal racemization with loss of taste characteristics. Racemization in neutral solution occurs at 190 °C after formation of the lactam, 5-oxo-L-proline, pyroglutamic acid [98-79-3]. 

BuOH, EDCI (EDCI = l-ethyl-3-[3-(dimethylamino)propyl]carbodi-imide hydrochloride), DMAP, CH2CI2, 88% yield.Cbz-Proline was protected without racemization. 

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. 

Kinetic resolution of the racemic aziridine-2-carboxylate 82 (Scheme 3.26) was reported by Iqbal and co-workers [74], When 82 was allowed to react with N-cinna-moyl-L-proline (81) under mixed anhydride coupling conditions, the N-acyl azir-idine 83 was obtained in optically pure form along with aziridine 84. 

Thus the product in such cases can exist as two pairs of enantiomers. In a di-astereoselective process, one of the two pairs is formed exclusively or predominantly as a racemic mixture. Many such examples have been reported. In many of these cases, both the enolate and substrate can exist as (Z) or (E) isomers. With enolates derived from ketones or carboxylic esters, (E) enolates gave the syn pair of enantiomers (p. 146), while (Z) enolates gave the anti pair. Addition of chiral additives to the reaction, such as proline derivatives, or (—)-sparteine lead to product formation with good-to-excellent asynunetric induction. Ultrasound has also been used to promote asymmetric Michael reactions. Intramolecular versions of Michael addition are well known. ... 

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. 

Synthesis of the common intermediate C (4), and its further conversion to 2 and 3 is illustrated in Scheme 7-3. Two racemic compounds, ( )-7 and ( + )-10, are prepared from readily available starting materials 5 and 8, respectively (Scheme 7-2). Coupling of 7 and 10 gives a mixture of diastereomers 11. An intramolecular aldol reaction of 11 catalyzed by D-proline yields diastereomers 12 and 13 in equal molar ratios (about 36% ee for each diastereomer). Compound 12, the desired ketone, is converted to 14, which is further purified by crystallization to give the compound in the desired stereochemistry in sterically pure form. Reduction of the ketone carbonyl group and subsequent methoxy... 

Nakajima et al. (128) found that a number of diamines functioned as moderately effective ligands for Cu(II) in the catalytic aerobic oxidation of 2-naphthols. A series of proline derivatives were evaluated with the aniline ligand 187 providing optimal selectivities. Unfortunately, 2-naphthol affords only racemic binaphthol. With an isolated catalyst, formulated as 187 Cu(OH)Cl in analogy to the known TMEDA Cu(OH)Cl (TMEDA = A,A,A,A-tetramethylethylenediamine), oxidative... 

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. 

Selenoxide syn-eliminations are another reaction type favoured by less polar solvents (Reich, 1979). The planar 5-membered, pericyclic transition state for syn-elimination of [39] was mimicked by the racemic proline-based cis-hapten [39] to give 28 monoclonal antibodies (Appendix entry 8.5) (Zhou et al., 1997). Abzyme SZ-cts-42F7 converted substrate [40] exclusively into... 

Since the oxidation of (96) proceeds through the acyliminium ions (97) (Scheme 36), where the positive charge is stabilized by the adjacent nitrogen atom, it should lead to racemic acetals (98) from enantioenriched acids (96). In fact, the electrolysis of A/-benzoylated L-proline (113) gave racemic N, O-acetal (114) regardless of the anode material used (Scheme 41) [129]. 

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... 

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