Enantiomers of benzothiadiazine diuretics by direct chromatographic resolution of the racemic drugs

Racemisation in Chemistry and Biology

The two enantiomers of a compound often have profoundly different biological properties and thus their liability to racemisation in aqueous solutions is an important piece of information. The authors reviewed the available data concerning the process of racemisation in vivo, in the presence of biological molecules (e.g., racemase enzymes, serum albumin, cofactors and derivatives) and under purely chemical but aqueous conditions (acid, base and other aqueous systems). Mechanistic studies are described critically in light of reported kinetic data. The types of experimental measurement that can be used to effectively determine rate constants of racemisation in various conditions are discussed and the data they provide is summarised. The proposed origins of enzymatic racemisation are presented and suggest ways to promote the process that are different from processes taking place in bulk water. Experimental and computational studies that provide understanding and quantitative predictions of racemisation risk are also presented.

Studies of enantiomerization of chiral 3,4-dihydro-1,2,4-benzothiadiazine 1,1-dioxide type compounds

An on-column HPLC procedure using a chiral stationary phase (CSP) was developed for the determination of rate constants and free energy barriers of enantiomerization of (+/-)IDRA21. Subsequently, the HPLC method was applied for investigation of two structurally related chiral compounds. The individual enantiomers of the studied compounds were isolated in parallel by preparative HPLC and rate constants and free energy barriers of enantiomerization were determined in different solvents. The on-column enantiomerization data revealed that CSP induces different rate constants for the two enantiomers. The results generated off-line were used to determine the influence of solvents on the racemization of (+) and (-) IDRA21 and to gain further insight into the enantiomerization mechanism of chiral 3,4-dihydro-1,2,4-benzothiadiazine 1,1-dioxide type compounds.

Chiral resolution of the enantiomers of 7-chloro-3-methyl-3,4-dihydro-2H-1,2,4-benzothiadiazine 1,1-dioxide using high-performance liquid chromatography on cellulose-based chiral stationary phases

Analytical high-performance liquid chromatography (HPLC) methods using derivatized cellulose chiral stationary phases (CSPs) were developed for the separation of the enantiomers of 7-chloro-3-methyl-3,4-dihydro-2H-1,2,4-benzothiadiazine 1,1-dioxide ((+/-) IDRA21). In previous studies, (+/-) IDRA21 has been found to have an interesting inhibitory effect on the desensitization of alpha-amino-2,3-dihydro-5-methyl-3-oxo-4-isoxazolepropanoic acid (AMPA) receptor and improve cognition in animals. This compound possess one chiral carbon atom, but very little information has been reported on the stereoselectivity of his activity. Therefore resolution of the enantiomers of this compound and subsequent identification of stereospecificity in his pharmacological actions are clearly matters of interest. The resolution were made under normal- and reversed-phase conditions using a mobile phase consisting of n-hexane:2-propanol (70/30, v/v) and water:acetonitrile (60/40, v/v) respectively, and a CSP of silica-based cellulose tris-3,5-dimethyl-phenylcarbamate (Chiralcel OD and Chiracel OD-R). The enantiomeric nature of eluates was confirmed by circular dichroism (CD) spectra. A baseline separation (R(S) > 1.5) was obtained in both cases. Furthermore the isolation of optical isomers of (+/-) IDRA21 was performed using a semipreparative column packed with the same cellulose OD CSP.

Liquid chromatographic separation of the stereoisomers of thiazide diuretics

A number of racemic thiazide diuretics and analogues were resolved on two diastereomeric chiral stationary phases (CSPs) prepared from (S)- or (R)-alpha-[1-(6,7-dimethyl)naphthyl]-10-dodecenylamine and (S)-2-phenylpropanoic acid. Of the two diastereomeric CSPs, the (S,S) and the (R,S), the former is found to be better than the latter in separating the enantiomers of the racemic thiazide diuretics and their analogues with complete separation being observed on the (S,S)-CSP. Chiral recognition is controlled principally by the (R)- or (S)-alpha-[1-(6,7-dimethyl)naphthyl]-10-dodecenylamine portion of the CSPs. The second stereogenic center of the CSP provides but secondary effects on the chiral recognition presumably involving, in the case of the (S,S)-CSP, face-to-edge pi-pi interaction between the aromatic ring of the analytes and the phenyl on the second stereogenic center.

Enantiomeric resolution with a new chiral stationary phase of 7-chloro-3-methyl-3,4-dihydro-2H-1,2,4-benzothiadiazine S,S-dioxide, a cognition-enhancing benzothiadiazine derivative

The direct analytical and semipreparative high-performance liquid chromatographic (HPLC) resolution of the enantiomers of IDRA 21 [1,7-chloro-3-methyl-3,4-dihydro-2H-1,2,4-benzothiadiazine S,S-dioxide] is reported. (+/-)-IDRA 21 administered orally to rats subjected to a water maze cognition test elicited a performance enhancing effect. Between the two enantiomers, (+)-IDRA 21 was identified as being pharmacologically active in the water maze performance test, whereas (-)-IDRA 21 was completely devoid of activity when given in doses comparable to those of the dextrorotatory (+)-enantiomer. The design and preparation of a new chiral stationary phase (CSP) employed for the liquid chromatographic resolution of the enantiomers of racemic IDRA 21 is presented. This brush-type CSP, which has not been described before, is a "mixed" (pi-donor pi-acceptor) type and is derived from (R)-N-(3,5-dinitrobenzoyl) allylglycine 2,6-dimethylanilide. It is easily prepared and possesses a relatively broad scope of applicability, as determined by its ability to resolve the enantiomers of both pi-acidic and pi-basic compounds.

Preparative chromatographic separation of enantiomers

The potential of the chromatographic separation of enantiomers on a preparative scale as a tool for the isolation of optically pure compounds is gaining increasing recognition. This review surveys the different chiral stationary phases (CSPs) used for preparative chromatography, emphasizing the advantages and drawbacks of each. The strategy to be followed for preparative separations is discussed and tables summarizing separations reported in the literature give an overview of practical applications. Cellulose triacetate has been used most frequently, probably because of its broad application range and its low production costs in comparison with more recently introduced CSPs. Nevertheless, the high efficiency of some of the novel CSPs is likely to contribute to the further development and expansion of the method.

Excavations in drug chirality: 1. Cyclothiazide

There is a great deal of current interest in the role and importance of chirality in the development of new drugs, but little attention is being paid to the stereochemistry of older drugs. Indeed, many older chiral drugs were introduced without adequate information on their stereochemical identity or composition. We have examined one such drug, the antihypertensive diuretic agent cyclothiazide. Standard sources of drug information and the research literature do not provide data on the stereochemical composition of clinically used cyclothiazide, although scattered reports indicate that the drug may consist of "several stereoisomers." Inspection of the chemical structure of the drug, 6-chloro-3,4-dihydro-3-(5-norbornen-2-yl)-2H-1,2,4-benzothiadiazin e-7- sulfonamide 1,1-dioxide, shows that it can exist as eight stereoisomers that may form four racemates. Using synthesis, fast-atom-bombardment mass spectrometry, gas-liquid chromatography, chiral and nonchiral high-performance liquid chromatography, and nuclear magnetic resonance spectroscopy, we determined that pharmaceutical cyclothiazide is in fact a mixture of the eight stereoisomers in the form of the four racemates. The two racemates with endo configuration at the norbornene moiety predominate over the exo racemates, and small but significant differences in isomer distribution between different batches of the drug were observed. We urge that in studies of older drugs the stereochemical details be considered.

Chromatographic resolutions. XIV. Optical resolution of the racemic anticancer drug ifosfamide and other chiral oxazaphosphorines

Ifosfamide, a chiral anticancer drug with phosphorus as chiral centre, was resolved into its enantiomers by chromatography on polyamides with bonded optically active substituents. Both enantiomers were isolated and characterized, the enantiomeric purity being at least 95% (31P nuclear magnetic resonance in the presence of an optically active shift reagent) and 99% (analytical chromatography), respectively. Experiments on the resolution of other oxazaphosphorines including the therapeutic drugs cyclophosphamide and trofosfamide are described.