Amino acids classical resolution

Early examples of enantioselective extractions are the resolution of a-aminoalco-hol salts, such as norephedrine, with lipophilic anions (hexafluorophosphate ion) [184-186] by partition between aqueous and lipophilic phases containing esters of tartaric acid [184-188]. Alkyl derivatives of proline and hydroxyproline with cupric ions showed chiral discrimination abilities for the resolution of neutral amino acid enantiomers in n-butanol/water systems [121, 178, 189-192]. On the other hand, chiral crown ethers are classical selectors utilized for enantioseparations, due to their interesting recognition abilities [171, 178]. However, the large number of steps often required for their synthesis [182] and, consequently, their cost as well as their limited loadability makes them not very suitable for preparative purposes. Examples of ligand-exchange [193] or anion-exchange selectors [183] able to discriminate amino acid derivatives have also been described. 

Classical electrostatic modeling based on the Coulomb equation demonstrated that the model system chosen could account for as much as 85% of the effect of the protein electric field on the reactants. Several preliminary computations were, moreover, required to establish the correct H-bond pattern of the catalytic water molecule (WAT in Fig. 2.6). Actually, in the crystal structure of Cdc42-GAP complex [60] the resolution of 2.10 A did not enable determination of the positions of the hydrogen atoms. Thus, in principle, the catalytic water molecule could establish several different H-bond patterns with the amino acids of the protein-active site. Both classical and quantum mechanical calculations showed that WAT, in its minimum-energy conformation,... 

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. ... 

One of the classical approaches of liquid chromatography, paper chromatography, was used for chiral resolution about 50 years ago but is not part of modem practice. In paper chromatography, the stationary phase is water bonded to cellulose (paper material), which is of course chiral and hence provides a chiral surface for the enantiomers. However, some workers used chiral mobile phase additives also in paper chromatography [73,74]. In 1951 some research groups independently [73,75-77] resolved the enantiomers of amino acids. Simultaneously, numerous interesting publications on chiral resolution by paper chromatography appeared [70]. 

It is well-known that catalytic amounts of aldehyde can induce racemization of a-amino acids through the reversible formation of Schiff bases.61 Combination of this technology with a classic resolution leads to an elegant asymmetric transformation of L-proline to D-proline (Scheme 6.8).62 63 When L-proline is heated with one equivalent of D-tartaric acid and a catalytic amount of n-butyraldehyde in butyric acid, it first racemizes as a result of the reversible formation of the proline-butyraldehyde Schiff base. The newly generated D-proline forms an insoluble salt with D-tartaric acid and precipitates out of the solution, whereas the soluble L-proline is continuously being racemized. The net effect is the continuous transformation of the soluble L-proline to the insoluble D-proline-D-tartaric acid complex, resulting in near-complete conversion. Treatment of the D-proline-D-tartaric acid complex with concentrated ammonia in methanol liberates the D-proline (16) (99% ee, with 80-90% overall yield from L-proline). This is a typical example of a dynamic resolution where L-proline is completely converted to D-proline with simultaneous in situ racemization. As far as the process is concerned, this is an ideal case because no extra step is required for recycle and racemization of the undesired enantiomer and a 100% chemical yield is achievable. The only drawback of this process is the use of stoichiometric amount of D-tartaric acid, which is the unnatural form of tartaric acid and is relatively expensive. Fortunately, more than 90% of the D-tartaric acid is recovered at the end of the process as the diammonium salt that can be recycled after conversion to the free acid.64... 

A variety of methods exists for the synthesis of optically active amino acids including asymmetric synthesis92-100 and classic and enzymatic resolutions.101-104 However, most of these methods are not readily applicable to the preparation of a,a-disubstituted amino acids as a result of poor stereoselectivity and lower activity at the a-carbon. Attempts to resolve the racemic 2-amino-2-ethylhexanoic acid and its ester through classic resolution failed. Several approaches for the asymmetric synthesis of the amino acid were evaluated including alkylation of 2-aminobutyric acid... 

A second example is the reciprocal DR of (R,S)-alaninol. Both (R)- and (,S )-alaninol are interesting chiral intermediates used in recent drug developments28 but are difficult to obtain by classical resolution.29 Alaninol is also one of the few examples that resisted the standard DR procedure. However, resolution of (f ,. S )-alaninol could be achieved with mandelic acid in isopro-panol/water (19 1) in the presence of enantiopure 2-amino-1-butanol.30 Whereas the latter amine... 

Solids that strongly attract water and other polar solvents are the common media for achieving classical column-chromatographic separation of amino acids and peptides, on the basis of the partition principle (Hearn, 1991 Hancock, 1984). Cellulose (i.e. paper in the form of sheets or powder), one of the media of this type used since the earliest days of chromatography, also has the capacity to bind, through adsorbed water, to one enantiomer of certain amino acids, e.g. tryptophan, more strongly than to the opposite enantiomer (chiral or enantioselective separation chromatographic resolution), because cellulose is homochiral (constructed purely of one enantiomer). 

The requirements for homochirally pure a-amino acids have not ruled out any of these general synthetic methods (which all give racemic products), since resolution of DL-a-amino acids and their derivatives is a simple, albeit time-consuming, solution to this need. Classical methods for resolution include physical separation of the DL-amino acids themselves (by chromatography on a chiral phase e.g. resolution of DL-tryptophan over cellulose, see Section 4.15), fractional crystallisation of certain racemates or supersaturated solutions (through seeding with crystals of one enan-... 

Enzymic resolution is also generally useful. At first sight it is of restricted applicability, since most of the classical methods are based on the selectivity of a proteinase for catalysing the hydrolysis of the l enantiomer of an A-acyl derivative of a DL-amino acid (Equation (6.7)) or of a DL-amino acid ester. The normal substrates for these enzymes are derivatives of particular coded amino acids. 

CCCs may obtain chiral compounds by classical resolution, kinetic resolution using chemical or enzymatic metlrods, biocatalysis (enzyme systems, whole cells, or cell isolates), fermentation (from growing whole microorganisms), and stereoselective chemistry (e.g., asymmetric reduction, low-temperature reactions, use of chiral auxiliaries). CCCs may also be CCEs by capitalizing on a key raw material position and going downstream. Along with companies manufacturing chiral molecules primarily for other purposes, such as amino acid producers, these will be the key sources for the asymmetric center. 

Classical resolution by crystallization, the oldest method, is usually the development chemist s first approach to obtain a single enantiomer. It is attractive given the wealth of expertise that has been accumulated, much of it in amino acid production. 

Amoxicillin (7) is a semisynthetic penicillin antibiotic. The penicillin portion is derived from fermentation of either penicillin-V or -G, and then the side chain is removed chemically to afford 6-aminopenicillanic acid (6-APA) [3,16], The D- 7-hydroxyphenylglycine is then attached as the new side chain—chemical and enzymatic methods are available to achieve this [17-21]. This amino acid is obtained by a classical resolution or by enzymatic hydrolysis of a hydantoin (Chapter 8) [22-26],... 

A variety of methods exists for the synthesis of optically active amino acids, including asymmetric synthesis [85-93] and classic and enzymatic resolutions [94-97], However, most of these methods are not applicable to the preparation of a,a-disubstituted amino acids due to poor stereoselectivity and lower activity at the a-carbon. Attempts to resolve the racemic 2-amino-2-ethylhexanoic acid and its ester through classic resolution failed. Several approaches for the asymmetric synthesis of the amino acid were evaluated, including alkylation of 2-aminobutyric acid using a camphor-based chiral auxiliary and chiral phase-transfer catalyst. A process based on Schollkopf s asymmetric synthesis was developed (Scheme 12) [98]. Formation of piperazinone 24 through dimerization of methyl (5 )-(+)-2-aminobutyrate (25) was followed by enolization and methylation to give (35.6S)-2,5-dimethoxy-3,6-diethyl-3.6-dihydropyrazine (26) (Scheme 12). This dihydropyrazine intermediate is unstable in air and can be oxidized by oxygen to pyrazine 27, which has been isolated as a major impurity. 

In subsequent process generations, peniciUin G acylase derived enzymes were also used to couple the synthetic side chains, such as D-phenylglycine (ampicillin, cephalexin) and D-p-hydroxyphenylglycine (amoxicillin, cephadroxil) in the form of amino acid amides or esters to 6-APA and 7-ADCA (Scheme 4.6D). Biotransformation routes to the n-phenylglycine and n-p-hydroxyphenylglycine side chains were also developed (Scheme 4.6C), but the enzymatic process towards n-phenyl-glycine amide has been substituted by a classical resolution. 

A classic example of a typical enzymatic resolution on an industrial scale is the acylase-mediated production of L-methionine. This method has also been applied for the production of L-phenylalanine and L-valine. In addition to acylases, amidases, hydantoinases, and /i-lactam hydrolases represent versatile biocatalysts for the production of optically active L-amino acids. A schematic overview of the different type of enzymatic resolutions for the synthesis of L-amino acids is given in Fig. 2. 

Even more complicated reactions can be used to racemise during a resolution. The amino ketone 102 is needed for the synthesis of the analgesic and useful asymmetric reagent (see chapter 24) DARVON. Classical resolution with dibenzoyl tartaric acid 9 succeeds in crystallising the (+) enantiomer and racemising the mother liquors by reverse Mannich reaction.25... 

Attempts to resolve the racemic acid of 3 and its ester through classic resolution failed. In the early stages of development, a process based on SchoUkopf s asymmetric synthesis was developed (see Section 9.3). Large-scale development work was aimed at finding a biocatalytic process to resolve the amino acids. Racemic a,a-disubstituted a-amino esters were synthesized by standard chemistry through alkylation of the Schiff s bases formed from the amino esters (Scheme 9.5), or through formation of hydantoins. ... 

---