Racemates derivatization

Another CDA that has been similarly applied (139-143) in the separation of dihydrodiol derivatives of PAHs is the add chloride [32] of (—)-menthoxyacetic add. This acid is commercially available from several sources. CDA [32] has also been used in the separation of the isomers of the new antihypertensive agent nipradilol. The chemical structure of this drug includes two asymmetric centers, and therefore the drug is a mixture of four stereoisomers, that is, two diastereomeric racemates. Derivatization of nipradilol, an amino alcohol, with [32] produced the four N,0-bis-menthoxyacetyl derivatives, which could be cleanly separated using silica gel LC, with detection at 275 nm (144). 

Figure 22.4 Separation of racemic propranolol as oxazolidone derivative with a dinitrobenzoylphenylglycine phase [reproduced with permission from I.W. Wainer, T.D. Doyle, K.H. Donn and J.R. Powell, J. Chromatogr., 306, 405 (1984)]. Conditions sample, whole-blood extract, 2.5 h after administration of an 80 mg dose of propranolol racemate, derivatized to oxazolidone with phosgene column, 25 cm X 4.6 mm i.d. stationary phase, 3,5-DNB-phenylglycine, 5 [xm mobile phase, hexane-isopropanol-acetonitrile (97 3 1), 2mlmin fluorescence detector, 290-335 nm IS, internal standard (oxazolidone derivative of pronethalol).

By choosing the chirality of the derivatization reagent the elution order can be controlled this is an advantage for trace analysis of one enantiomer in the presence of an excess of a second, where it is preferred for the minor enantiomer to elute first Little chance of racemization Derivatization is usually necessary to enhance volatility... 

Aromatic racemates derivatized with 3,5- DNB yielded better resolutions than non-aromatic racemates in the polar organic mode. This was just the opposite of the teicoplanin CSP indicating that ir-bonding is the primary interaction leading to resolution when using the polar organic mode on the CB-RN phase. Further evidence of the utility of the n-basic phases in the polar organic mode was presented for the HPLC enantioseparation of bicalutamide and its related compounds [23],... 

Three general methods exist for the resolution of enantiomers by Hquid chromatography (qv) (47,48). Conversion of the enantiomers to diastereomers and subsequent column chromatography on an achiral stationary phase with an achiral eluant represents a classical method of resolution (49). Diastereomeric derivatization is problematic in that conversion back to the desired enantiomers can result in partial racemization. For example, (lR,23, 5R)-menthol (R)-mandelate (31) is readily separated from its diastereomer but ester hydrolysis under numerous reaction conditions produces (R)-(-)-mandehc acid (32) which is contaminated with (3)-(+)-mandehc acid (33). 

Derivatization with Optically Active Reagents and Separation on Achiral Columns. This method has been reviewed (65) a great number of homochiral derivatizing agents (HD A) are described together with many appHcations. An important group is the chloroformate HD As. The reaction of chloroformate HD As with racemic, amino-containing compounds yields carbamates, which are easily separated on conventional hplc columns, eg (66),... 

Since this original synthesis, a great number of improvements (191—201) have been made in the stereoselective preparation and derivatization of the CO-chain precursor, in cuprate reagent composition and preparation, in protecting group utilization, and in the preparation and resolution of hydroxycyclopentenones. Illustration of some of the many improvements are seen in a synthesis (202) of enisoprost, a PGE analogue. The improvements consist of a much more efficient route to the enone as well as modifications in the cuprate reactions. Preparation of the racemic enone is as follows ... 

The advantage of this ketal is that unlike the THP group, only a single isomer is produced in the derivatization. Conditions used to hydrolyze the THP group can be used to hydrolyze this acetal. This group may also find applications in the resolution of racemic alcohols. 

Synthetic chiral adsorbents are usually prepared by tethering a chiral molecule to a silica surface. The attachment to the silica is through alkylsiloxy bonds. A study which demonstrates the technique reports the resolution of a number of aromatic compoimds on a 1- to 8-g scale. The adsorbent is a silica that has been derivatized with a chiral reagent. Specifically, hydroxyl groups on the silica surface are covalently boimd to a derivative of f -phenylglycine. A medium-pressure chromatography apparatus is used. The racemic mixture is passed through the column, and, when resolution is successful, the separated enantiomers are isolated as completely resolved fiactions. Scheme 2.5 shows some other examples of chiral stationary phases. 

The educt, a racemate, is derivatized before the separation with an agent which might be achiral or unichiral (Fig. 7-1), and afterwards is passed through a chromatographic system which is equipped with a stationary phase. This stationary phase may also be achiral or unichiral in nature. 

This strategy is the one most commonly used for the analytical determination of ena-tiopurity. A given racemate is reacted with a unichiral derivatizing agent, and the resulting pair of diastereomers is separated on an achiral stationary phase, in most of the cases on a reversed-phase type (Fig. 7-2). 

Most of the reactions applied to amines can also be transferred to alcohols (Eig. 7-5). One large group of chiral alcohols are the (i-adrenoreceptor blockers, for which a variety of derivatization agents was developed. One highly versatile reagent for the separation of (i-blockers is A-[(2-isothiocyanato)cyclohexyl]3,5-dinitrobenzoyl-amide (DDITC) [11]. Alternatively, unichiral drugs such as (3-blockers or (S)-naproxen [12] may be used in a reciprocal approach to derivatize racemic amine compounds. 

While amino compounds can be derivatized with acids and acid chlorides, it is possible to separate racemic acids (vice versa) with unichiral amino compounds... 

The mixture of free amino acids is reacted with OPA (Fig. 7-8) and a thiol compound. When an achiral thiol compound is used, a racemic isoindole derivative results. These derivatives from different amino acids can be used to enhance the sensitivity of fluorescence detection. Figure 7-9 shows the separation of 15 amino acids after derivatization with OPA and mercaptothiol the racemic amino acids may be separated on a reversed-phase column. If the thiol compound is unichiral, the amino acid enantiomers may be separated as the resultant diastereomeric isoindole compound in the same system. Figure 7-10 shows the separation of the same set of amino acids after derivatization with the unichiral thiol compound Wisobutyryl-L-cysteine (IBLC). 

Derivatization of a racemic compound with an achiral group may play an important role in the analysis of a chiral compound (Fig. 7-15). In the case of substances with low or no UV-activity, the compounds can be rendered detectable by introducing an UV-absorbing or fluorescent group. If the racemate itself shows selectivity on a chiral stationary phase (CSP), this method can be applied to reduce the limit of detection. Examples have been reported in the literature, especially for the derivatization of amino acids which are difficult to detect using UV detection. Different derivatization strategies can be applied (Fig. 7-16). 

It is important not only that a multiplicity of compounds in the sample mixture may be selectively derivatized - as was shown for Type III reactions - but also that one racemate may be derivatized with a multiplicity of derivatizing agents (Fig. 7-17). Although this approach can be used to optimize the analogues of a compound [28, 29], it is of special interest when a compound is required to be separated on a preparative scale. 

Merino-Merino et al. [32] used the OPA reagent (o-phthaldehyde condensed with 2-mercaptoethanol) to separate penicillamine enantiomers after their derivatization. Racemic and (/q-penicillamine were dissolved in aqueous 0.5 M NaOH, and treated with the derivatizing solution (methanolic o-phthaldehyde and 2-mercaptoethanol in 0.4 M potassium borate buffer solution of pH 10). The reaction mixture was set aside for 2 min at room temperture, whereupon a portion of solution was analyzed by HPLC. The method used a Cyclobond column (25 cm x 4.6 mm) maintained at 5 °C, a mobile phase of ethanol/1% triethylammonium acetate (1 1 pH 4.5) eluted at... 

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