SUBJECTS racemates, racemic acid

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). 

A racemic alcohol may be converted into a racemic acid by reaction with one molar equivalent of phthalic anhydride the product is a half ester of a dicarboxylic acid (see Section 7.9.1). This can now be subjected to the resolution process for acids and, in due course, the alcohols can be regenerated by hydrolysis of the ester. 

In Section 7.3, the subject of amino acid analysis is covered including a brief description of hydrolytic techniques, column preparations, and data analysis 7,8 This is followed by a discussion of racemization assays (Section 7.4). These areas of analysis are critically important for researchers in the field of peptide synthesis 9-11 In Section 7.4, a systematic approach is outlined for the study of racemization and mechanisms are discussed for epi-merizations at asymmetric sites. A comparative study is presented for determining the intrinsic rates of racemization, based upon urethane-protected V-carboxy an hydrides (UN-CAs). These approaches are extremely important for understanding the tendencies of novel amino acids and other building blocks to racemize. 

Oxabicyclo[3.2.1]oct-6-en-2-ones are also available from this chemistry, as illustrated in Scheme 30. Treatment of 84 with sodium methoxide and then lithium hydroxide, followed by subjecting the resulting acid to the standard Curtius rearrangement conditions, results in the formation of 8-oxabicyclo[3.2.1]oct-6-en-2-one (85) in 52% yield. Racemic 85 has been used by Vogel in a stereoselective synthesis of the P-C-hexopyranoside 86. ... 

Reaction of a racemic acid or base with an optically active base or acid gives a pair of diastereomeric salts. Members of this pair exhibit different physicochemical properties (e.g., solubility, melting peint, boiling point, adsorbtion, phase distribution) and can be separated owing to these differences. The most important method for the separation of enantiomers is the crystallization. This is the subject of this chapter. 

Enzymatic hydrolysis is also used for the preparation of L-amino acids. Racemic D- and L-amino acids and their acyl-derivatives obtained chemically can be resolved enzymatically to yield their natural L-forms. Aminoacylases such as that from Pispergillus OTj e specifically hydrolyze L-enantiomers of acyl-DL-amino acids. The resulting L-amino acid can be separated readily from the unchanged acyl-D form which is racemized and subjected to further hydrolysis. Several L-amino acids, eg, methionine [63-68-3], phenylalanine [63-91-2], tryptophan [73-22-3], and valine [72-18-4] have been manufactured by this process in Japan and production costs have been reduced by 40% through the appHcation of immobilized cell technology (75). Cyclohexane chloride, which is a by-product in nylon manufacture, is chemically converted to DL-amino-S-caprolactam [105-60-2] (23) which is resolved and/or racemized to (24)... 

Resolution of Racemic Amines and Amino Acids. Acylases (EC3.5.1.14) are the most commonly used enzymes for the resolution of amino acids. Porcine kidney acylase (PKA) and the fungaly3.spet i//us acylase (AA) are commercially available, inexpensive, and stable. They have broad substrate specificity and hydrolyze a wide spectmm of natural and unnatural A/-acyl amino acids, with exceptionally high enantioselectivity in almost all cases. Moreover, theU enantioselectivity is exceptionally good with most substrates. A general paper on this subject has been pubUshed (106) in which the resolution of over 50 A/-acyl amino acids and analogues is described. Also reported are the stabiUties of the enzymes and the effect of different acyl groups on the rate and selectivity of enzymatic hydrolysis. Some of the substrates that are easily resolved on 10—100 g scale are presented in Figure 4 (106). Lipases are also used for the resolution of A/-acylated amino acids but the rates and optical purities are usually low (107). 

To obtain a better understanding of the reaction mechanism, some compounds that are considered to he intermediates were subjected to the reaction. Various reaction courses can be considered as illustrated in Fig. 21. Path A a-Methyltropic acid is oxidized to a-phenyl-a-methylmalonic acid. Then, the malonate is converted to optically active a-phenylpropionate hy arylmalonate decarboxylase. In order to confirm this assumption, incubation of the malonic acid with Rhodococcus sp. was carried out. The result obtained was the total recovery of the substrate, indicating that no decarboxylase is present in this bacterium. Path B a-Methyltropic acid is converted to racemic a-phenylpropionic acid, which is deracemized to optically active propionic acid. To examine the possibility of this route, racemic a-phenylpropionic acid was subjected to the reaction to observe... 

The design and application of chiral, non-racemic Lewis acids for the asymmetric Diels-Alder reaction has recently been a subject of considerable interest.9 Several methods have been developed in many laboratories1 2 3 4 5 6 7 10 but catalysts are still needed that are more efficient in governing the stereochemical course of the cycloaddition reaction. 

The unsaturated amides (RCH=CHCONH2, where R = aryl or heteroaryl) in the presence of sodium acetate and NBS gave 3-bromoazetidin-2-ones 67 in moderate yield, probably by cyclization of 68 <99JCS(P1)2435>. The mesylate 69 cyclized in the presence of base to 70 and, after deprotection, the racemic P-lactam was subjected to lipase-mediated resolution to yield 71 (R = Et, ee 99%) and the amino acid 72 (R = Et, ee 98%) . 

The hydrolysis of racemic non-natural amides has led to useful products and intermediates for the fine chemical industry. Thus hydrolysis of the racemic amide (2) with an acylase in Rhodococcus erythrolpolis furnished the (S)-acid (the anti-inflammatory agent Naproxen) in 42 % yield and > 99 % enantiomeric excess1201. Obtaining the 7-lactam (—)-(3) has been the subject of much research and development effort, since the compound is a very versatile synthon for the production of carbocyclic nucleosides. An acylase from Comamonas acidovor-ans has been isolated, cloned and overexpressed. The acylase tolerates a 500 g/ litre input of racemic lactam, hydrolyses only the (+)-enantiomer leaving the desired intermediate essentially optically pure (E > 400)[211. 

To elucidate the metabolic pathway of phenylmalonic acid, the incubation broth of A. bronchisepticus on phenylmalonic acid was examined at the early stage of cultivation. After a one-day incubation period, phenylmalonic acid was recovered in 80% yield. It is worthy of note that the supposed intermediate, mandelic acid, was obtained in 1.4% yield, as shown in Eq. (8). The absolute configuration of this oxidation product was revealed to be S. After 2 days, no metabolite was recovered from the broth. It is highly probable that the intermediary mandelic acid is further oxidized via benzoylformic acid. As the isolated mandelic acid is optically active, the enzyme responsible for the oxidation of the acid is assumed to be S-specific. If this assumption is correct, the enzyme should leave the intact l -enantiomer behind when a racemic mixture of mandelic acid is subjected to the reaction. This expectation was nicely realized by adding the racemate of mandelic acid to a suspension of A. bronchisepticus after a 4-day incubation [4]. 

In a different approach, fluorescence-based DNA microarrays are utilized (88). In a model study, chiral amino acids were used. Mixtures of a racemic amino acid are first subjected to acylation at the amino function with formation of A-Boc protected derivatives. The samples are then covalently attached to amine-functionalized glass slides in a spatially arrayed manner (Fig. 10). In a second step, the uncoupled surface amino functions are acylated exhaustively. The third step involves complete deprotection to afford the free amino function of the amino acid. Finally, in a fourth step, two pseudo-Qn nX. om.Qx c fluorescent probes are attached to the free amino groups on the surface of the array. An appreciable degree of kinetic resolution in the process of amide coupling is a requirement for the success of the ee assay (Horeau s principle). In the present case, the ee values are accessible by measuring the ratio of the relevant fluorescent intensities. About 8000 ee determinations are possible per day, precision amounting to +10% of the actual value ((S(S). Although it was not explicitly demonstrated that this ee assay can be used to evaluate enzymes (e.g., proteases), this should in fact be possible. So far this approach has not been extended to other types of substrates. 

The Pirrung synthesis is notable for its brevity and clever amalgamation of [2 + 2] photocycloaddition and Wagner-Meerwein rearrangement chemistry Enol ether 757 was reacted with the Grignard rea nt from 5-bromo-2-methyl-l-pentene, subjected to acid hydrolysis, and irradiated to generate the tricycle 738. Wittig olefination of this ketone and treatment with p-toluenesulfonic acid provided racemic isocomene. 

Following an initial resolution step with 0.5 mol equivalents (R)-mandelic acid in TBME, the crystalline product was filtered and tlie waste isomers in the mother liquors (39% ee) were washed with base and then subjected to racemization with the SCRAM catalyst. Upon completion, the catalyst precipitated and was screened, fresh racemic amine was added, and the whole was resolved a second time. The process was repeated several times, giving the results summarized in Table 13.2. 

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