Leucine ester

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]. 

As with the 5-amino-4-phenyl-l,3-dioxane auxiliary47 53, the rert-leucine ester group has to be removed by oxidative degradation, in this case by a regioselective decarboxylation using fe/7-butyl hypochlorite. The expense of this auxiliary, coupled with its destruction, limits the practical value of this interesting procedure. 

It was first shown byE. Fischer, in 1894, that enzymes were specific in their action thus maltase acts only upon a-glucosides and emulsin only upon /3-glucosides. Later, he found that trypsin acted asymmetrically upon inactive polypeptides, e.g., alanyl-leucine was hydrolysed in such a way that only the compound composed of d-alanineand I-leucine, the natural isomers, was split up into its constituents, whereas the compound composed of 1-alanine and d-leucine was unattacked. Again, inactive leucine ester was found by Warbui to be only partially hydrolysed by trypsin he obtained 1-leucine and d-leucine ester. 

In 2008, the same group developed an asymmetric version of this reaction (Scheme 10.10).24 Run under similar conditions, but with more silver oxide (1 equiv) and thus less reoxidant (benzoquinone 0.5 equiv) and in the presence of catalytic amounts of chiral ligand (20 mol%), the best enantiomeric excesses and yields were obtained with menthyl-L-leucine ester as the chiral ligand. 

The product was Step 1 (9g) was dissolved in 200 ml THE and IH-benzotriazolium-l-[bis(dimethylamino)methylene]hexafluoro-phosphate(l-)-3-oxide) (10 mmol) dissolved in 10 ml DMF and added over 10 minutes. Tetrahydrostatin L-leucine ester (525 mg) dissolved in 100 ml THE was added, the reaction pH adjusted to 8.5 using diisopropylethy-lamine, and the mixture stirred overnight. Thereafter, the reaction volume was reduced by 50% and the product precipitated in 500 ml hexane. The crude product was washed 3 times with ethyl alcohol/water, 3 1, and dried under a lyophilizer. 

Finally, libraries aimed to chiral resolution of racemates will be covered here in particular, the use of chiral stationary phases (CSPs) has recently been reported for the identification of materials to be used for chiral separation of racemates by HPLC. The group of Frechet reported the selection of two macroporous poly methacrylate-supported 4-aryl-1,4-dihydropyrimidines (DHPs) as CSPs for the separation of amino acid, anti-inflammatory drugs, and DHP racemates from an 140-member discrete DHP library (214,215) as well as a deconvolutive approach for the identification of the best selector phase from a 36-member pool library of macroporous polymethacrylate-grafted amino acid anilides (216,217). Welch and co-workers (218,219) reported the selection of the best CSP for the separation of a racemic amino acid amide from a 50-member discrete dipeptide iV-3,5-dinitrobenzoyl amide hbrary and the follow-up, focused 71-member library (220). Wang and Li (221) reported the synthesis and the Circular Dichroism- (CD) based screening of a 16-member library of CSPs for the HPLC resolution of a leucine ester. Welch et al. recentiy reviewed the field of combinatorial libraries for the discovery of novel CSPs (222). Dyer et al. (223) reported an automated synthetic and screening procedure based on Differential Scanning Calorimetry (DSC) for the selection of chiral diastereomeric salts to resolve racemic mixtures by crystallization. Clark Still rejxrrted another example which is discussed in detail in Section 9.5.4. 

Aminopeptidases are present in many tissues (Table III). Leucine aminopeptidase from intestinal mucosa is very effective in catalyzing the hydrolysis of leucine from the amino terminus of peptides, polypeptides, and proteins. It also hydrolyzes leucine amide and leucine esters (10). The designation leucine aminopeptidase is somewhat of a misnomer because activity is also observed when other amino acids replace leucine. Only the L-isomers of amino acids are substrates, and the presence of a D-amino acid residue or proline in the penultimate position will retard hydrolysis (10). Enzymes having the same specificity as the intestinal aminopeptidase have been identified and/or isolated from kidney, pancreas, muscle, lens, and various bacterial sources (10). The kidney... 

Magnesium or lithium enolates of thiol carboxylic esters can be acylated with suitable reagents (c/ Volume 2, Part 1). Highly functionalized new derivatives are obtained. The example shown in equation (35)33,102 J5 closely related to the C-acylation of malonyl-CoA with acetyl-CoA, which plays an important role in the biosynthesis of polypeptides and fatty acids. (S)-4-Hydroxy-5-methyl-3-oxo-hexanoyl-L-leucine esters (41), 2-demethyl analogs of the Hip-Leu moiety of didemnin antibiotics, can be prepared via the 3-keto thiol carboxylic esters (40 equation 36). ° ... 

Orlistat. Orlistat acts in the GI tract, where it inhibits pancreatic lipases located in the lumen. Systemic exposure of the drug is not required for pharmacological activity. With negligible oral bioavailability (F < 5%), orlistat is primarily excreted unchanged in the feces. However, several metabolites including (40) and (41) have been identified in the plasma of both normal and obese volunteers. These compounds are formed by hydrolysis cf both the N-formyl leucine ester moiety and the lactone ring and do not exhibit any inhibitory activities toward pancreatic lipases (68) (see Fig. 15.1). 

Figure 32-16 Chromatogram of a racemic mixture of N-( 1 -Naphthyl)leucine ester 1 on a dinitrobenzene-leucine chiral stationary phase. The R and S enantiomers are seen to be well separated. Column 4.6 X 50 mm mobile phase, 20% 2-propanol in hexane flow rate 1.2 mL/min UV detector at 254 nm. (Reprinted with permission from L. H. Bluhm, Y. Wang, and T. Li, Anal.

Enantiomerically pure dipeptide is obtained when the 4-nitrophenyl ester of N-benzoyl-L-leucine is coupled with ethyl glycinate in ethyl acetate. If, however, the leucine ester is treated with 1-methylpiperidine in chloroform for 30 min prior to coupling, the dipeptide in nearly completely racemized. Treatment of the leucine ester with 1-methylpiperidine leads to formation of a crystalline material of composition C13H15NO2, which has strong IR bands at 1832 and 1664 cm Explain how racemization occurs and suggest a reasonable structure for the crystalline material. 

Carbethoxyl-glycyl-glycine ester, when heated with leucine ester, yielded carbethoxyl-glycyl-glycyl-leucine e.ster,... 

Carbetho.xyl-glycyl-glycinc amide and carbethoxyl-glycyl-glycyl-leucine ester give the biuret reaction as would be expected from the researches of Schiff in 1900, who found that glycine amide NH2. CH2. CO. NHj also gave the reaction. 

Leucyl-l-leucine (1-leucyl chloride + 1-leucine ester). d-TryptOphyl-glycine (d-tryptophyl chloride + glycine ester). 

Leucyl-glycyl-leucine (leucyl-glycyl chloride + leucine ester). 

The amplitude is considerable (a = 100-270), and it is positive for all the L-amino acid derivatives. A second, negative Cotton effect is observed between 245 and 225 nm. The iV-acetyl derivatives of the selenophenyl esters of L-phenylalanine and L-leucine esters show a negative Cotton effect at 290-295 nm. The reversal is explained by the drastic redistribution of electron density about the asymmetrically substituted a-carbon atom when the charged ammonium group is replaced by a neutral amide group. 

The complexation of the amino esters to Ru(II) did not give any chiral recognition. For example, reaction of Ru(apap-P)(MeCN)i with 10 equiv. of rac-valine or -leucine esters yielded a racemic mixture of the corresponding bis (amino ester) complexes DD DL LL = I 2 1) however, chiral recognition to 52% (for leucine methyl ester) was observed for the complexation of the rac-leucine esters to Ru(apap-P)CO , and the oxidation of the amino esters by the chiral Ru(apaP P)(0)2 (or perhaps, more precisely, coordination of amino ester to the Ru imino ester complex)... 

The procedure is illustrated with the chiral resolution of racemic N- -naphthyl)leucine ester 1 (Fig. 1) with a 200-member library. 

Add an equal amount of the racemic A-(l-naphthyl)leucine ester (1) (1.2 mg, 0.0030 mmol) in a mixture of chloroform and heptane (2 8, 0.60 mL) to each of the 200 wells that contained 0.03 mmol selector. 

Inject racemic A-(l-naphthyl)leucine ester 1 onto this column connected to a Beckman HPLC analytical gradient system. Chromatographic conditions mobile phase, 20% chloroform in heptane flow rate, 1.2 mL/min UV detection at 254 nm. 

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