Enantiomer L and

The natural amino acids are mainly a-amino acids, in contrast to (3-amino acids such as p-alanine and taurine. Most a-amino acids have four different substituents at C-2 (Ca). The a atom therefore represents a chiral center—I e., there are two different enantiomers (L- and D-amino acids see p. 8). Among the proteinogenic amino acids, only glycine is not chiral (R = H). In nature, it is almost exclusively L-amino acids that are found. D-Amino acids occur in bacteria—e. g., in murein (see p.40)—and in peptide antibiotics. In animal metabolism, D-Amino acids would disturb the enzymatic reactions of L-amino acids and they are therefore broken down in the liver by the enzyme D-amino add oxidase. 

A significant ability to discriminate between chiral amines based on the quenching of S-di-2-naphthylprolinol fluorescence emission was reported by Diamond et al. [32], fl-Phenylethylamine (PEA) was seen to have a much greater efficiency as a quencher than the S-enantiomer. l- and D-norephedrine, which have structural conformation similarities to PEA, were also observed to have an enantiomeric selectivity. The mechanism of chiral recognition is proposed to be a combination of hydrogen bonding and 3D chirally restricted space. 

Step 7. Formation of L-Malate In Step 7, which is catalyzed hy the enzyme fumarase, water is added across the double bond of fumarate in a hydration reaction to give malate. Again, there is stereospecihcity in the reaction. Malate has two enantiomers, L- and D-malate, but only L-malate is produced. 

Supersaturated racemic solutions are unstable or metastable depending on the degree of supersaturation. For a system having a phase diagram as shown in Figure 3, the initial levels of the driving force for the crystallization are the same to the both enantiomers, i.e. AM for the L-isomer and BM for the D-isomer. Accordin y at the end of the complete crystallization when the true equilibrium is established, i.e. when no resolution results, the saturated solution ( ) and two crystal phases of the enantiomers (L and D) of the equal amounts coexist. There may be no need to mention that the problem of the purity drop is attributed to the crystallization of the undesired enantiomer, often it is called spontaneous crystaUization and that the... 

Lactic acid (2-hydro)QTJropionic acid), CH3-CHOHCOOH, is a simple chiral molecule which exists as two enantiomers, l- and o-laetie aeid, differing in... 

As a result, the baseline enantioseparation was obtained, as shown in Figure 19.1a. Monomeric Pro is one of these rare amino acids, which develop yellow (and not bluish) color when visualized with ninhydrin. l-Pto used as an external standard (Figure 19.1b) confirmed the identity of the lower yellow spot number 2 as enantiomer L and the upper yellow spot number 3 as enantiomer d, as shown in Figure 19.1a. The respective Rp values were 0.57 0.02 and 0.74 0.02. Brownish-purple spot number 1 (Rp = 0.32 0.02 Figure 19.1) apparently originates from the Pro-derived peptides and it is also fully separated from the monomeric l-Pto spot number 2. The presence of peptides in the two freshly prepared Pro samples wimesses to rapid peptidization of this amino acid (although contamination of the commercial monomeric DL-Pro and L-Pro samples with peptides cannot be excluded). 

The predominance of L-amino acids in biological systems is one of life s most intriguing features. Prebiotic syntheses of amino acids would be expected to produce equal amounts of L- and D-enantiomers. Some kind of enantiomeric selection process must have intervened to select L-amino acids over their D-connterparts as the constituents of proteins. Was it random chance that chose L- over D-isomers ... 

Among the J ,J -DBFOX/Ph-transition(II) metal complex catalysts examined in nitrone cydoadditions, the anhydrous J ,J -DBFOX/Ph complex catalyst prepared from Ni(C104)2 or Fe(C104)2 provided equally excellent results. For example, in the presence of 10 mol% of the anhydrous nickel(II) complex catalyst R,R-DBFOX/Ph-Ni(C104)2, which was prepared in-situ from J ,J -DBFOX/Ph ligand, NiBr2, and 2 equimolar amounts of AgC104 in dichloromethane, the reaction of 3-crotonoyl-2-oxazolidinone with N-benzylidenemethylamine N-oxide at room temperature produced the 3,4-trans-isoxazolidine (63% yield) in near perfect endo selectivity (endo/exo=99 l) and enantioselectivity in favor for the 3S,4J ,5S enantiomer (>99% ee for the endo isomer. Scheme 7.21). The copper(II) perchlorate complex showed no catalytic activity, however, whereas the ytterbium(III) triflate complex led to the formation of racemic cycloadducts. 

The substantial difference between these two chromatograms was a clear proof that CSP 13 interacted differently with the mixtures of l and d enantiomers. This also indicated the presence of at least one pair of enantiomers that interacted selectively with the CSP. Unfortunately, a tedious synthesis of 16 sublibraries (eight l and eight d) containing decreasing numbers of blocks had to be prepared to deconvolute the best selector. A comparison of the chromatograms obtained from these sublibraries in each deconvolution step was used again, and those selectors for which no difference was observed were eliminated. This procedure enabled the identification... 

The operating conditions and model parameters used in simulation for the TMB approach are presented in Table 9-1. The feed concentration of each enantiomer is 2.9 g L and columns were 2.6 cm wide and 10.5 cm long. The section length was... 

Enantiomers (M)- and (P)-helicenebisquinones [32] 93 have been synthesized by high pressure Diels-Alder reaction of homochiral (+)-(2-p-tolylsulfo-nyl)-l,4-benzoquinone (94) in excess with dienes 95 and 96 prepared from the common precursor 97 (Scheme 5.9). The approach is based on the tandem [4 + 2] cycloaddition/pyrolitic sulfoxide elimination as a general one-pot strategy to enantiomerically enriched polycyclic dihydroquinones. Whereas the formation of (M)-helicene is explained by the endo approach of the arylethene toward the less encumbered face of the quinone, the formation of its enantiomeric (P)-form can be the result of an unfavourable interaction between the OMe group of approaching arylethene and the sulfinyl oxygen of 94. 

In a first report [24], the enantioselectivities of various proteases were evaluated by comparing the biocatalyzedhydrolysis of2-chloroethyl esters of N-acetyl-i- and D-amino acids in water and their transesterification with w-propanol in butyl ether. By comparing the ratio of the kc t/Ku values for the l- and D-enantiomers in the two reactions, a remarkable relation of the proteases enantioselectivity was observed apparently, in this case, the organic solvents destroyed the selectivity of the tested enzymes. This finding... 

Homochiral (5)- and (f )-l-(2-furyl)ethanols were prepared from 21 by lipase-catalyzed transesterification with vinyl acetate. The pure enantiomers are preciusors for the syntheas of L-and D-daunomycin <96TA907>. 

Since amines react more readily than alcohols in noncatalyzed reactions with anhydrides, the reaction is more difficult and initially required stoichiometric catalyst loadings [107], but could be performed in a catalytic sense with an O-acylated azlactone as acylating agent, which does not react with a benzylic amine at —50°C, but is capable of acylating the catalyst [108, 109]. Depending on the buUdness of the substrate, selectivities ranged from S = 11 to 27 (s = [ enantiomer l]/[ enantiomer 2])-... 

Cyclodextrins, toroidal molecules composed of 6, 7 and 8 D-gJucose units, are now commercially available at reasonable cost. They form inclusion compounds with a variety of molecules and often differentially include sulfoxide enantiomers . This property has been used to partially resolve some benzyl alkyl, phenyl alkyl and p-tolyl alkyl sulfoxides. The enantiomeric purities after one inclusion process ranged from 1.1% for t-butyl p-tolyl sulfoxide to 14.5% for benzyl t-butyl sulfoxide. Repeating the process on methyl p-tolyl sulfoxide (10) increased its enantiomeric purity from 8.1 % to 11.4% four recrystallizations raised the value to 71.5%. The use of cyclodextrins in asymmetric oxidations is discussed in Section II.C.l and in the resolution of sulfinate esters in Section II.B.l. 

A simple and rapid method of separating optical isomers of amino acids on a reversed-phase plate, without using impregnated plates or a chiral mobile phase, was described by Nagata et al. [27]. Amino acids were derivatized with /-fluoro-2,4-dinitrophenyl-5-L-alanine amide (FDAA or Marfey s reagent). Each FDAA amino acid can be separated from the others by two-dimensional elution. Separation of L- and D-serine was achieved with 30% of acetonitrile solvent. The enantiomers of threonine, proline, and alanine were separated with 35% of acetonitrile solvent and those of methionine, valine, phenylalanine, and leucine with 40% of acetonitrile solvent. The spots were scraped off the plate after the... 

Many compounds are less soluble as racemates than as their pure enantiomers. It thus appears probable that evaporation of an amino acid solution with a low ee should cause selective precipitation of the racemate crystals, which in turn should lead to an increase of the ee. Extremely simple manipulations, carried out in the chemistry department of Columbia University, led to a drastic increase in enantiomeric excess of phenylalanine 500 mg phenylalanine (with a 1 % ee of the L-component) was dissolved in water, and the resulting solution slowly evaporated until about 400 mg had crystallised out. The remaining solution contained a few mg of phenylalanine with 40% ee of the L-component (i.e., a 70 30 ratio of l to d). If 500 mg of such a solution (40% ee in water) is allowed to evaporate and is separated from the racemate, the result is about 100 mg, with 90% ee of the L-enantiomer (Breslow and Levine, 2006). 

The kind of enantiomer [d-(-)- or l-(+)-] synthesized in the formation of the C4 intermediate varies. The acetoacetyl-CoA reductase (EC 1.1.1.36), which is NADPH-dependent, stereoselectively reduces acetoacetyl-CoA to d-(-)-3-hydroxybutyryl-CoA (R. eutropha [15]). The NADH-dependent reductase catalyzes the reduction of acetoacetyl-CoA to L-(+)-3-hydroxybutyryl-CoA. Afterwards two stereospecific crotonyl-CoA hydratases, l-(+)- and D-(-)-speci-fic, convert the L-(+)-3-hydroxybutyryl-CoA into the D-(-)-isomer (Rhodo-spirillum rubrum [16]). 

Amino acids are the building blocks of protein. Except for glycine, all amino acids come into two different chiral forms, laevorotatory (L) and dextrorotatory (D) these forms are called enantiomers. In living organisms, the amino acids in protein are almost exclusively... 

In Fig. 24 points A and A represent the equal solubilities at the temperature T0 of the pure enantiomers, while E represents the solubility of the eutectic mixture. AEA is the solubility-composition curve, above which undersaturated solutions exist and below which a saturated solution is in equilibrium with the two solid phases, D and L. Figure 21 shows that, with an increase in temperature, A and A move toward the pure enantiomers D and L, while E moves toward the pure eutectic. All of these trends indicate an increase in solubility upon increasing the temperature. 

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