Valine resolution

Based on chiral functional monomers such as (15), MICSPs can be prepared using a racemic template. Thus, using racemic A-(3,5-dinitrobenzoyl)-a-methylbenzy-lamine (16) as template, a polymer capable of racemic resolution of the template was obtained [67]. Another chiral monomer based on L-valine (17), was used to prepare MIPS for the separation of dipeptide diastereomers [68]. In these cases the configu-... 

The elimination of turbulent mixing in the flow chamber and the short residence time of the reaction mixture in the detection chamber provided a high separation efficiency of 100,000 theoretical plates for labeled amino acids, obtaining good resolution in comparison with previous CL detectors [79, 82], For the isoluminol thiocarbamyl derivative of valine, a detection limit of 500 amol was reported however, it was not possible to separate all 20 isoluminol-deriva-tized amino acids. 

Total resolution of [269] into its two enantiomers was achieved by liquid-liquid chromatography through complexation to L-valine adsorbed initially on diatomaceous earth (Timko et al., 1978). On the basis of comparative chromatographic studies, the separation factors (a) and the EDC values were correlated (Cram et al., 1975) (Table 59). 

Table 1 Gas chromatographic resolution factors of N-pentafluoropropionyl amino acid isopropyl esters on polysiloxanes carrying chiral L-valine t-butylamide groups (Val-NtBu)... 

Racemic modifications may be resolved. There are very few examples of this approach having been employed successfully. The racemic cylic ether (RS)-36, which contains two CH2OCH2CO2H arms attached to the 3 and 3 positions on the axially chiral binaphthyl units, has been resolved (48-50, 93, 94) to optical purity in both its enantiomers by liquid-liquid chromatography using a chiral stationary phase of either (R)- or (S)-valine adsorbed on diatomaceous eaitii. Very recently, the optical resolution of crown ethers (/ S)-37 and (/ 5)-38, incorporating the elements of planar chirality in the form of a rron -doubly bridged ethylene unit, has been achieved (95) by HPLC on (+)-poly(triphenyl-methyl methacrylate). 

The enzymatic formation of 4 from isobutylamine, which is derived from valine, during the biosynthesis of valanimycin, was confirmed by high-resolution MS analysis The importance of the pathway shown in equation 5 is the notion that naturally occurring N—N bond products are likely to involve the condensation of HA and an amine to yield a hydrazine that is subsequently transformed to an azoxy moiety such as that occurring in valanimycin. 

The numerous preparations of mono-, di-, tri-, and hexafluoro derivatives of valine, norvaline, leucine, norleucine, and isoleucine, using classical methods of amino acid chemistry (e.g., amination of an a-bromoacid, " azalactone, Strecker reaction, amidocarbonylation of a trifluoromethyl aldehyde, alkylation of a glycinate anion are not considered here. Pure enantiomers are generally obtained by enzymatic resolution of the racemate, chemical resolution, or asymmetric Strecker reaction. ... 

With chiral racemic oxiranes one enantiomer reacts faster than the other the degree of kinetic resolution is very high for L-valine/alanine-based dialkoxydihydropyrazines. For example, in the reaction of one equivalent of (2.S )-2,5-dihydro-2-isopropyl-3,6-dimethoxy-5-methyl-pyrazine (1, R1 = CH3) with two equivalents of fW-(//,/ )-2,3-dimethyloxirane (R2,R4 = CH3 R = H) virtually only the (2//,3/ )-oxirane enantiomer reacts with the lithiated dihydropyrazine to give exclusively the (l /, 2/, 2 / )-configuratcd adduct i.e., (2/ ,5S)-2,5-dihydro-5-isopropyl-3,6-dimethoxy-2-[(l/ ,2/ )-2-(2-methoxyethoxymethoxy)-l-methylpropyl]-2-methylpyrazine, entry 7. Likewise, kinetic resolution (intramolecular) occurs upon reaction with rac-7-oxabicy-clo[4.1.0]heptane (entry 8). 

Finally, it is noted that suitable enantiomers of helicenes have been applied as selector for non-helical compounds. The disodium salt of P-( +)-7,10-dicarboxy hexahelicene coated on silicagel was successfully used in resolving the N-(2,4-dinitro-phenyl)-a-amino acid esters. Good resolutions were found for alanine, isoleucine, valine, phenylalanine and phenylglycine 94). 

Trifluoroacetyl derivatives of amines were resolved on trifluoro-acetylated-L-valyl-L-valine isopropyl ester coated in capillary columns. Better efficiency was obtained when a ureide, which was formed by the condensation of phosgene with L-Valine isopropyl ester, was coated on glass capillaries of about 70 meters in length and 0.25 mm i.d. Purity of the reagent (column coating) will affect resolution but not the relative peak areas. 

The resolution of optically active compounds by gas chromatography with chiral phases is a well-established procedure, and the separation of IV-perfluoro-acetylated amino acid ester enantiomers in 1967 was the first successful application of enantioselective gas-liquid chromatography [39] Amino acids have been resolved as their A-trifluoroacetyl esters on chiral diamide phases such as N-lauroyl-L-valine rerr-butylamide or iV-docosanoyl-L-valine fert-butylamide [40,41,... 

FIGURE 5 Effect of the concentration of acetonitrile on (a) the retention (k) and the (b) separation (a) factors for the chiral resolution of (O) dansyl asparagine and ( ) dansyl valine on a Chiralpak WH column using 0.25 M ammonium acetate as the major component of the mobile phase. (From Ref. 53.)... 

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