Racemic leucine

Boc-L-Leucinal racemizes if stored at room temperature. Although it solidified In the cold it became liquid at room temperature. It is very soluble in pentane at room temperature, but crystallizes from it at -30°C. It is reported to melt at 63-66 C. 

The separation of the enantiomers of most amino acids can be ach ie ved by differential crystallisation of their JV-acetyl derivatives, such as that of leucine. Racemic A-acetyl leucine 88 is dissolved in the right solvent mix cooled and seeded with 4% by weight of natural (.S )-88. Pure (.S )-88 crystallises out in good yield.22... 

Ito et al. described the first application of pH-zone refining to chiral separation using A -dodecanoyl-L-proline-3,5-dimethylanilide as CS (40 mM) in the separation of DNB-( )-Leu enantiomers [59]. A binary solvent system composed by a mixture of MTBE-water has been used. Due to the acidic character of the analyte, ttifluo-roacetic acid was added as a retainer agent to the organic MTBE stationary phase and ammonia was added to the aqueous mobile phase as a displacer agent. In the described conditions the separation of 2 g of leucine racemate (CS/racemate molar ratio 1.30) in a single run was possible in a CCC device of 330 mL. 

If, however, the /7-nitrophenyl ester of iV-henzoyl-L-leucine is treated with 1-methyl-piperidine in chloroform for 30 min and then coupled with glycine ethyl ester, the dipeptide isolated is almost completely racemic. Furthermore, treatment of the p-nitrophenyl ester of iV-benzoyl-L-leucine with 1-methylpiperidine alone leads to the formation of a crystalline material, C13H15NO2, having strong IR bands at 1832 and 1664 cm . Explain these observations, and suggest a reasonable stmcture for the crystalline product. 

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... 

Fig. 3-1. Separation of racemic 3,5-dinitrobenzamido leucine Al.A -diallylamide on silica and polymer-based chiral stationary phases. Conditions column size 150 x 4.6 mm i.d. mobile phase 20 % hexane in dichloromethane flowrate 1 mL min injection 7 pg. Peaks shown are l,3,5-tri-rert.-butylbenzene (1), R-enantiomer (2) 5-enantiomer (2 ). (Reprinted with permission from ref. [8]. Copyright 1997 American Chemical Society.)...

However, the use of a HPLC separation step enabled a remarkable acceleration of the deconvolution process. Instead of preparing all of the sublibraries, the c(Arg-Lys-O-Pro-O-P-Ala) library was fractionated on a semipreparative HPLC column and three fractions as shown in Fig. 3-2 were collected and subjected to amino acid analysis. According to the analysis, the least hydrophobic fraction, which eluted first, did not contain peptides that included valine, methionine, isoleucine, leucine, tyrosine, and phenylalanine residues and also did not exhibit any separation ability for the tested racemic amino acid derivatives (Table 3-1). 

The two best selectors resulting from Li s screening, DNB-L-ala and DNB-L-leu, were then prepared on a larger scale, attached to silica beads modified with 3-amino-propyl-triethoxysilane, and the CSPs were packed into columns. Respective separation factors of 4.7 and 12 were found for the separation of racemic naphthyl leucine ester 17 using these CSPs. 

These two selectors terminated with a glycine were then prepared on a larger scale, their carboxyl groups reacted with 3-aminopropyltriethoxysilane, and the conjugate immobilized onto silica. Each CSP was packed into columns and used for the separation of racemic (l-naphthyl)leucine ester 17. Separation factors of 6.9 and 8.0 were determined for the columns with DNB-ala-gly and DNB-leu-gly selector respectively. These were somewhat lower than those found for similar CSPs using the parallel synthesis and attached through a different tether [87]. 

Figure 24 shows the ternary phase diagram (solubility isotherm) of an unsolvated conglomerate that consists of physical mixtures of the two enantiomers that are capable of forming a racemic eutectic mixture. It corresponds to an isothermal (horizontal) cross section of the three-dimensional diagram shown in Fig. 21. Examples include A-acetyl-leucine in acetone, adrenaline in water, and methadone in water (each at 25°C) [141].

It has been known for years that the activated residues of acyl- and peptidylamino acids enantiomerize during coupling (1.9). However, the racemization tests available (see section 4.9) did not allow for a valid comparison of the tendency of residues to isomerize because they incorporated a variety of aminolyzing residues and N-substituents. Valid demonstration of the different sensitivities of residues was provided by classical work on the synthesis of insulin. It was found that a 16-residue segment with O-tert-butyltyrosine at the carboxy terminus produced 25% of epimer in HOBt-assisted DCC-mediated coupling in dimethylformamide, and the same segment with leucine at the carboxy terminus produced no epimer. Only when series such as Z-Gly-Xaa-OH coupled with valine benzyl ester became available was it possible to compare many residues with confidence. Unfortunately, it transpires that the issue is extremely complex. 

Their studies involved the partial polymerization of NCAs of mixtures of specific amino adds having known e.e.s, followed by determination of the e.e.s of the amino adds in both the resulting polypeptides and in the residual unreacted NCA monomers. [94] In a typical experiment it was found that when an optically impure leucine NCA monomer having an l > d e.e. of 31.2% was polymerized to the extent of 52 % to the helical polyleucine peptide, the e.e. of the polymer was enhanced to 45.4 %, an increase of 14.2 %. In the same experiment the e.e. of the unreacted leucine NCA monomer was depleted to a similar extent. Analogous experiments with valine NCAs of known e.e.s, however, led to a reverse effect, namely, the preferential incorporation of the racemate rather than one enantiomer into the growing polyvaline peptide. This finding was interpreted to be the result of the fact that polyvaline consists of (3-sheets rather than a-helices, emphasizing that the Wald mechanism applies only to a-helix polymers. At about the same time Brach and Spach [95] showed that, under proper conditions, (3-sheet polymers could also be implicated in the amplification of amino add e.e.s. 

Novozymes, a subtilisin produced by Bacillus licheniformis, was used by Chen et al ° to carry out a dynamic kinetic resolution of benzyl, butyl, or propyl esters of DL-phenylalanine, tyrosine, and leucine. The hydrolysis was performed at pH 8.5 in 2-methyl-2-propanol/water (19 1) and the freed L-amino acids precipitated. The key feature bringing about continual racemization of the remaining D-amino acid esters was the inclusion of 20 mmol 1 pyridoxal phosphate. 

Circularly polarized light (CPL) has been proposed as one of the origins of the chirality of organic compounds.Asymmetric photolysis of racemic leucine by... 

Finally in this sechon, a novel approach has been developed for the enzyme-catalyzed synthesis of D-tert-leucine 55 from the corresponding racemate (Scheme... 

Scheme 2.25 Kinetic resolution of racemic ferf-leucine 55 using leucine dehydrogenase.

An elegant four-enzyme cascade process was described by Nakajima et al. [28] for the deracemization of an a-amino acid (Scheme 6.13). It involved amine oxidase-catalyzed, (i )-selective oxidation of the amino acid to afford the ammonium salt of the a-keto acid and the unreacted (S)-enantiomer of the substrate. The keto acid then undergoes reductive amination, catalyzed by leucine dehydrogenase, to afford the (S)-amino acid. NADH cofactor regeneration is achieved with formate/FDH. The overall process affords the (S)-enantiomer in 95% yield and 99% e.e. from racemic starting material, formate and molecular oxygen, and the help of three enzymes in concert. A fourth enzyme, catalase, is added to decompose the hydrogen peroxide formed in the first step which otherwise would have a detrimental effect on the enzymes. 

---