Amino acid, protecting groups resolution

Patrikeev et al. also employed imprinted silicas for thin layer chromatography. The silica was mixed with plaster and immobilised on a plate for use in the separation and identification of gramines [31] and for the resolution of amino acid derivatives [29]. In the latter experiment it was found that the influence of the amino acid protective group, dinitrophenol, was too dominant to allow the separation of... 

A conceptually straightforward approach to the preparation of pure enantiomers of amino acids would be resolution of their diaslereomeric salts. Typically, the amine group is first protected as an amide and the resulting prodnct is then treated with an optically active amine, such as the inexpensive alkaloid brucine (Section 25-8 and margin). The two diastereomers formed can be separated by fractional crystallization. Unfortunately, in practice, this method can be tedious and can suffer from poor yields. 

This procedure is restricted mainly to aminodicarboxyhc acids or diaminocarboxyhc acids. In the case of neutral amino acids, the amino group or carboxyl group must be protected, eg, by A/-acylation, esterification, or amidation. This protection of the racemic amino acid and deprotection of the separated enantiomers add stages to the overall process. Furthermore, this procedure requires a stoichiometric quantity of the resolving agent, which is then difficult to recover efficiendy. Practical examples of resolution by this method have been pubUshed (50,51). 

The resolution was then based on the enzymatic propanolysis of this derivative in dioxane as solvent. Lip Novozyme 435 selectively cleaves the L-form of the oxazolone producing an L-enriched (81-87% ee) 2-acetamido-3-(heteroaryl)propionic acid propyl ester, the dynamic aspect of the process being based on the continual racemization of the residual oxazolone. The propyl group was then removed with alkali and a second selective enzymatic step to remove the acetyl protecting group with Fluka Acylase 1 produced the L-amino acid at better than 99% ee (Scheme 13). 

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. 

As shown in Tables 1 and 2, iV-protectcd amino acids have been widely used as CDAs mainly because of their ready availability. However, it was noticed that chromatographic resolution of the diastereomeric amides increased when the space-filling protecting group (e.g., Boc) was cleaved, indicating that the bulky Boc group shields the diastereodifferentiation and the specific adsorption sites of the analyte with the stationary phase94. 

Isowa et al. developed an alternative strategy synthesis of protected N5-hydroxy-ornithine from achiral building blocks and enzymatic resolution of the racemic product to its enantiomers 77). The hydroxylamines were protected with tosyl- and benzyl-groups because these protecting groups are stable to the reaction conditions employed in the peptide formation and in the deprotection of the a-amino group. With this key compound, rhodotorulic acid 78>, its enantiomer 78) and ferrichrome 79) were synthesized by straightforward peptide synthesis (Fig. 4a).. ... 

An t-triazine scaffold bearing a free and a protected amino group was synthesized and used for connecting two different and differently derivatized amino acids (Figure 23). Two diastereoisomeric chiral systems were obtained and, once linked to silica gel, they were used in the chromatographic resolution of structurally and electronically different racemic analytes, chosen among the racemates resolved by the isolated amino acid derivatives. 

The synthesis of benzyloxycarbonyl (31) derivatives by the amino-lysis of various active esters of benzyl carbonate has been reported The use of JV-hydroxypiperidyl methoxybenzyl carbonate and JV-hydroxypiperidyl f-butyl carbonate for the synthesis of -methoxy-benzyloxycarbonyl and f-butyloxycarbonyl (45) derivatives, respectively, has been described The benzhydryloxycarbonyl group (35) could be removed by the action of boron trifluoride in glacial acetic acid . The use of easily cleaved, optically active, protecting groups of the urethane type for the resolution of racemic primary and secondary amines was reported . 

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