Enantiomers diastereomeric derivatives, using

I. Review on indirect separation of enantiomers as diastereomeric derivatives using UV, fluorescence and electrochemical detection, Biomed. Chromatogr., 6 163 (1992). 

An extremely important aspect in pharmaceutical research is the determination of drug optical purity. The most frequently applied technique for chiral separations in CZE remains the so-called dynamic mode where resolution of enantiomers is carried out by adding a chiral selector directly into the BGE for in situ formation of diastereomeric derivatives. Various additives, such as cyclodextrins (CD), chiral crown ethers, proteins, antibiotics, bile salts, chiral micelles, and ergot alkaloids, are reported as chiral selectors in the literature, but CDs are by far the selectors most widely used in chiral CE. 

Ketones. The lithio derivative of ( +)-(.S )-. Y,.S -dimethyl-S-phenylsulfoxiinine (16), or its enantiomer, may be used both to resolve racemic ketones164 and to determine absolute configurations of ketones. For example, rac-19 on addition of 16 formed the diastereomeric /j-hydroxy-sulfoximines 17 and 18 which were separated. The configuration of 17 was established by an X-ray analysis. The ketones can be regenerated from the /1-hydroxysulfoximines by thermolysis. Thus, heating of 17 and 18 to 80 °C (for 12 h) furnished 19 [a precursor of (+ )-modhephen] and ent-l9165. 

Enantiomers are derivatized with an optically pure chiral derivatization reagent to form a pair of diastereomers. The ability to resolve the diastereomeric derivatives on an achiral sorbent is enhanced when the chiral centers of the enantiomers and the derivatives are in close proximity [181]. Two different separation mechanisms have been proposed. One postulates that the diastereomers are separated by differences in molecular structure and polarity [182], The other possible mechanism is based on differences in the diastereomer energies of adsorption [183]. Table 5.7 lists the chiral reagents that have been used for separation of enantiomers as diastereomers. 

A stereoselective GC method for determination of etodolac enantiomers in human plasma and urine was first reported as a preliminary method [35], and then as a validated method [36]. Sample preparation involved addition of (S)-(+)-naproxen (internal standard) and sodium hydroxide to diluted plasma or urine. The samples were washed with diethyl ether, acidified with hydrochloric acid, and extracted with toluene. ( )-(+)-naproxen was used as a derivatizing agent to form diastereomeric derivatives of etodolac. The gas chromatograph system used in this work was equipped with fused-silica capillary column (12 m x 0.2 mm i.d.) coated with high-performance cross-linked methylsilicone film (thickness 0.33 pm) and a nitrogen-phosphorous detector. The operating conditions were injector 250°C detector 300°C column 100-260°C (32 °C/min). 

The other approach, resolution of a racemic mixture, is also used in many industrial processes. Here physical resolution (i.e., resolution by crystallization of a diastereomeric derivative) is in general the favored method. However, kinetic resolution in principle can also be used for separating out two enantiomers. Although kinetic resolution involving a soluble metal complex catalyst has yet to be widely practiced, the potential importance of such an approach is significant (see Section 9.3.5). 

The above forms the basis for the chromatographic separation of enantiomers via diastereomeric derivatives. A mixture of the enantiomers of A is reacted, before chromatographic analysis, with a pure enantiomer of B, for example, (-)-B, to produce, in this case, diastereomers [2] and [4]. In this theoretical example, A may represent a sample of a drug whose enantiomeric composition is to be determined, and B is the chiral deriva-tizing agent (CDA) used to convert the enantiomers of A to diastereomeric... 

It may be argued that if the actual extent of enantiomeric contamination of a CDA is known accurately, the reagent may be safely used, because the appropriate correction in diastereomeric peak ratios can be made. An objection (5) to this argument is that if the enantiomerically impure CDA is present in excess, differences, if any, between the CDA enantiomers in their reaction rates with the analyte enantiomers (i.e., diastereoselective kinetics) will stUl result in an error in the determination of the enantiomeric ratio. In practice, however, such kinetic differences are usually negligible. A more precise but cumbersome solution to this problem is to separate the four stereoisomeric derivatives using chiral chromatographic conditions, for example, a chiral stationary phase. Under such conditions, four distinct peaks are obtainable as a matter of principle (whether the four stereoisomers are actuaUy resolved depends, of course, on the chromatographic conditions chosen). A review of the literature indicates that small (1-2%) enantiomeric contamination of a CDA may not necessarily render the CDA useless in many applications. It is clear, nevertheless, that the CDA used should be enantiomerically pure whenever possible. This simplifies the analysis and eliminates any uncertainty associated with enantiomeric contamination. There is, in fact, an application in which enantiomerically impure CDAs cannot be used safely the determination of trace enantiomeric impurity in an analyte. If the CDA used is itself enantiomerically contaminated, the accurate determination of the extent of trace enantiomeric contamination of the analyte may be difficult if not impossible. 

There are several important considerations in the design of preparative separations via derivatization with CDAs. First, the CDAs should produce diastereomeric derivatives that, after the chromatographic separation, can be cleaved chemically to yield the original drug enantiomers. The components resulting from the reaction used to cleave the derivatives should be separable without undue difficulty. An important factor is the enantiomeric purity of the CDA, since enantiomeric contamination of the CDA... 

TLC has also been used for the separation of diastereomeric derivatives of enantiomers, but this form of chromatography has not attained widespread use in indirect resolutions. Other chromatographic techniques, for example, supercritical fluid chromatography, capillary electrophoresis, countercurrent chromatography, etc., have not received much attention in indirect enantioseparation. 

Several derivatives of mandelic add have been used as CDAs for the resolution of chiral alcohols as the corresponding esters. Thus, in a study of the steric course of the cytochrome P-450-mediated hydroxylation of phenylethane (ethylbenzene), White et al. derivatized the enzymatically formed 1-phenylethanols with (S)-O-propionylmandelyl chloride, [36], and separated the diastereomeric derivatives by GLC (153). Comber and Brouillette synthesized the enantiomers of carnitine via derivatization of a racemic precursor alcohol with (R)- or (S)-a-methoxyphenylacetic [(0)-... 

The natural product gossypol, [45], a polyphenolic binaphthyl, is a male antifertility agent of considerable current interest. The available evidence suggests that the antifertility action of [45] may reside only in the (-) enantiomer (225). Procedures have been devised for the chromatographic separation of the enantiomers of [45] as diastereomeric derivatives of optically active (i-amino alcohols (226,227). L-Phenylalaninol was used in one procedure (227), whereas (—)-norepinephrine was employed in... 

The application of liquid chromatography on nonracemic ( optically active ) sorbents has become a well-developed and successful method for the analytical and preparative separation of enantiomers.25,26 It is particularly useful for racemates that are difficult to resolve via diastereomeric derivatives, i.e., for molecules lacking functional groups for reaction or interaction. This is the case for many pyrans and oxazines, the first separations of which were published in... 

Reactive crystallization or precipitation processes of industrial relevance include liquid-phase oxidation of pura-xylene to terephthalic acid, acidic hydrolysis of sodium salicylate to salicylic acid, and the absorption of ammonia in aqueous sulfuric acid to form ammonium sulfate. " A special type of reactive crystallization is the diastereomeric crystallization, widely applied in the pharmaceutical industry for the resolution of the enantiomers. Here, the racemate is reacted with a specific optically active material (resolving agent), to produce two diastereomeric derivatives (usually salts), which are separated by crystallization. Diastereomeric crystallization is commonly used in... 

As Louis Pasteur proposed in his time, enantiomers can be separated as diastereomeric derivatives after a chemical reaction with a chiral selector using fractional recrystallization (see Chapter 5). However, separation techniques such as chromatography and electrophoresis are now recognized for fhe separation of enantiomers. Even if chromatography is considered as fhe method of choice for chiral analytical purposes (see Chapter 7), capillary electrophoresis (CE) has recently gained importance in fhe field of stereoselective analysis. 

The use of covalent diastereomeric derivatives (e g. amides) is generally precluded by the strong conditions required for their cleavage to obtain the resolved enantiomers, but recently the resolution of 3-amino-l,4 benzodiazepines has been achieved by means of the preparation and separation of diastereomeric phenylalanyl amides [20]. This procedure, which is based upon the reaction of the racemic amine with N-Boc protected phenylalanine, followed by separation of the two diastereomeric amides and subsequent Edman degradation, was also applied to our series [21] and the desired enantiomers were obtained with excellent enantiomeric excess. However the total yield of the process was quite low due to a difficult chromatographic separation of the two diastereomers and the low conversion during the Edman degradation step. 

The triazole fungicides of general structure 64 such as hexaconazole 65 and flutriafol 66 are a rare case of human and plant medicine using similar compounds. They were initially used as racemates but it was soon essential to discover the active enantiomers. Conventional resolution by crystallisation of diastereomeric derivatives proved difficult. 

However, a source of the non-natural 9S isomer (2) was first required. The ready availability of natural crinitol made a racemization/resolution route, as illustrated in Scheme 1, attractive. Racemization was accomplished by Collins oxidation (16,25) to the dicarbonyl compound (14), followed by lithium aluminum hydride (LAH) reduction to give the racemic mixture (1 + 2). Resolution via diastereomeric derivatives seemed plausible. Esterification with enantiomerically pure a-methoxy-a-(trifluoromethyl) phenylacetic acid (MTPA) (17), followed by separation of diastereomers by recycle-HPLC (R-HPLC), had earlier been used to purify enantiomers of ipsenol and ipsdienol (26). A model system, the resolution of -3-nonen-2-ol, a secondary allylic alcohol naturally occurring in Rooibos tea (16,27), also worked satisfactorily. Therefore, the route using the bis-(MTPA) esters was selected for crinitol. 

A NMR study on the formation of diastereoisomeric inclusion complexes between fluorinated amino acid derivatives and a-CD in 10% D2O solution shows that the chemical shifts of the D-amino acid derivatives included by a-CD are upheld from those of their L analogues [77]. The shift difference between the diastereoisomers formed with D and L enantiomers can be used for chiral analysis and optical purity determinations. For example, the interaction of -CD with propanolol hydrochloride produces diastereomeric pairs. The protons of the antipode give NMR signals which differ in chemical shifts in D2O solution at 400 MHz. The intensity of the resonance signals for each diastereoisomer has been used for optical purity determination. By adding racemate to pure (—) isomer, this technique is able to measure optical purity of propanolol hydrochloride in water down to the level of 1%. 

Esters have historically been the most commonly used diastereomeric derivatives for the separation of the enantiomers of optically active carboxylic acids, particularly where recovery of the individual enantiomers is required. The reactions used to form diasteromeric esters can be summarized as follows. 

GC separation of enantiomers can be performed either direct (use of a chiral stationary phase, CSP) or indirect (off-column conversion into diastereomeric derivatives and separation by non-chiral stationary phases). The direct method is preferred as being simpler and minimizing losses during sample preparation. The key, of course, is to find a chiral stationary phase with both selectivity and temperature stability. 

Because the stationary phases originally used in LC were achiral, much research was devoted to the separation of enantiomers as diastereomeric derivatives produced by reaction with an optically pure reagent (Ar). The resultant diastereomers could, because of their different physicochemical properties, then be separated on conventional stationary phases. 

Racemic aryl-3-phospholene oxides could be separated into enantiomers with TADDOL derivatives, and also via diastereomeric complexation using calcium hydrogen 0,0-dibenzoyl-(2R,3R)-tartarate derivatives (Scheme 41). ... 

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