Short Communications

Retention Tuning in the Enantioseparation of Naproxen Esters I

and Free Acid Christopher J. Welch", Ted Szczerba, and Scott R. Perrin Regis Technologies, Inc., 8210 Austin Avenue, Morton Grove, IL 60053

Keywords: Enantiomers Chiral stationary phase Whelk-0 Coelution Peak overlap

1 Introduction

The chromatographic determination of enantiopurity in multi- component samples is often plagued by the problem of peak coelution. This problem is frequently addressed by sample clean- up procedures to remove coeluting impurities, although this ap- proach should be used with caution, since any fractionation of an enantioenriched material, even with an achiral stationary phase, can potentially lead to further enantioenrichment [ 11. The use of coupled columns is also frequently used to improve reso- lution of overlapping peaks [2]. The simplest solution to the problem of coeluting peaks relies on adjusting mobile phase components to improve peak resolution. Such methods are by nature empirical, and method development may be quite slow.

This communication describes a recent study aimed at develop- ing a method for simultaneous determination of the enantiomeric

purity of naproxen free acid, 1, as well as ester derivatives 2 and 3 (Figure 1). Such methods are often useful for monitoring the progress of enzymatic kinetic resolutions, where the enantiopu- rity of both ester feedstock and hydrolyzed product are most conveniently monitored in a single assay [3]. The enantiomers of individual analytes 1-3 are well resolved using the Whelk-0 1 chiral stationary phase (CSP) [4,5], however when a mixture containing all three racemates was analyzed using a standard mobile phase for this separation (80:20:0. I hexane/2-pro- panoVacetic acid) some coeluting peaks were observed (Fig- ure la). In this study an unexpected influence of the nature of the alcohol polar modifier on the relative retention of naproxen free acid and esters is described. This mobile phase effect is further shown to be useful for adjusting retentions so as to afford simultaneous resolution of the enantiomers of these compounds.

(3R, 4s) Whelk-0 1 CSP

v

R = H 1 (naproxen) R = M e 2 R=Et 3

Figure 1. Separation of the enantiomers of the analytes 1-3 using the (3R, 4s)-Whelk-0 1 CSP. a) 80:20:0.1 hexane/lPA/HOAc; b) 80:20:0.1 hexaneiEtOWHOAc; c) 80: 11.25:8.75:O.1 hexanelIPA/EtOH/HOAc. Flow = 1 ml/min; detection = UV 254 nm.

J. High Resol. Chromatogr. VOL. 20, MARCH 1997 179

Short Communications

U m

0 to some degree by an improvement in peak shape-

phases contain 0.1 % acetic acid.

2 Experimental

2.1 Materials

The analytical (4.6 mm i.d. x 25 cm length) (3R,4S) Whelk-0 CSP (#786101) Whelk-0 1 CSP was obtained from Regis Tech- nologies, Inc., Morton Grove, IL. Solvents were HPLC grade.

2.2 Apparatus

Chromatographic analysis was performed using a Kratos ABI Spectoflow 400 pump, a Rheodyne Model 7010 injector fitted with a 10 pl sample loop, a Kratos Spectroflow 783 variable wavelength absorbance monitor, and a Hewlett-Packard HP 3396 integrating recorder. Chromatographic analysis was carried out at a flow rate of 1 .O ml/min. Detection was by UV at 254 nm.

which could give simultaneous resolution of all six peaks. One such composition was used in Figure l c to provide quite good separation of all peaks.

By analogy with the present description, researchers investigat- ing kinetic resolutions of naproxen or similar NSAIDs, may find this technique useful for method development when peak coelu- tion is a problem.

References

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W.H. Pirkle and C.J. Welch, J. Chromatogr. 731 (1996) 322.

J.J. Lalonde, C. Govardhan, N. Khalaf, A.G. Martinez, K Visuri, and A. Margolin, J. Amer. Chem. Soc. 117 (1995) 6845.

W.H. Pirkle and C.J. Welch, J. Liq. Chromatogr. 15 (1992) 1947.

W.H. Pirkle, C.J. Welch, and S.R. Wilson, Chirality 6 (1994) 615-622.

W.H. Pirkle and C.J. Welch, J. Chromatogr. 689 (1995) 189-193. Ms received: November 10, I996

180 VOL. 20, MARCH 1997 J . High Resol. Chromatogr.