Chiral Separations of Guaifenesin, Methocarbamol, and Racemorphan IULIA DEMIAN Boehtingw Ingelheim Chemicals, Inc., Petersburg, Virginia

CHIRALITY 5238-240 (1993)

High-Performance Liquid Chromatography (HPLC) Chiral Separations of Guaifenesin, Methocarbamol, and Racemorphan IULIA DEMIAN Boehtingw Ingelheim Chemicals, Inc., Petersburg, Virginia

ABSTRACT HPLC chiral separations on silica gel coated with derivatized cellulose stationary phases are described. Most examples make use of the Chiralcel OD column from Daicel, Inc. With a judiciously chosen mobile phase, baseline separations of the enantiomers can be achieved. If those separations are used as a base for enantiomeric purity determination, detectable limits of 0.1% of the minor enantiomer are routinely accessible. Examples are given concerning separations of gmfenesin, methocarbamol, and racemorphan. 0 1993 Wiley-Liss, Inc.

KEY WORDS: chiral HPLC, Chiralcel OD, Chiralcel OD-R, guaifenesin, methocarbamol, dextromethorphan, levomethorphan, racemorphan

Three pharmaceuticals, guaifenesin (I), methocarbamol tector, and the Spectra Physics Model 4270 integrator for (II), and racemorphan (111) were analyzed by chiral HPLC for data acquisition and processing. The columns used, from Dai- the determination of their enantiomeric purity. Their chemical cel, Inc., were Chiralcel OD and Chirakel OD-R, both 25 structures are presented in Figure 1. The chiral centers are cm x 4.6 mm i.d. and 10 pm particle size. The packing of the marked with an asterisk. Chiralcel columns is derivatized micro crystalline cellulose, Guaifenesin [3-(0-methoxyphenoxy)-l,2-propanediol, coated on silica gel. The Chiralcel OD column is recom- Merck Index #4465, synonym: guaiacol glyceryl ether, glyc- mended for normal phase work, while Chiralcel OD-R is rec- eryl guaiacolate] has expectorant properties and is widely ommended for reverse phase applications. used in cough remedy formulations. ' Methocarbamol [3-(0- The optical rotations were determined with a Rudolph methoxyphenoxy)-1,2-propanediol-1-carbamate, Merck In- AUTOPOL I11 polarimeter. dex #5903, synonym: guaiacol glyceryl ether carbamatel is a Au chemicals and reagents used were analytical grade. skeletal muscle relaxant.' Both guaifenesin and methocar- Statistical analysis of experimental data was performed with bamol are used as racemic mixtures, and there are no pub- the RS1 (BBN, Cambridge, MA) software package on a Pow- lished reports about the pharmacological properties of the erMate 386/20 (NEC Corp.) microcomputer operating under enantiomers. The biological activity of guaifenesin and metho- MS-DOS. carbamol enantiomers is a current object of study. The chromatographic parameters are listed in Table 1. Racemorphan' is the trivial name of the racemic mixture of the enantiomers dextromethorphan and levomethorphan. Dextromethorphan (3-methoxy-N-methylmorphinan,Merck RESULTS AND DISCUSSION Index #8116) is widely used as a hydrobromide in antitussive fonn~lations.~Its enantiomer, levomethorphan, has analgesic The optical rotations of guaifenesin enantiomers are properties. Traces of dextromethorphan markedly enhance [a]DZ2 = -9.73" (I, MeOH), and [a]D2' = +8.57" (I, the analgesic potency of levomethorphan. MeOH).

MATERIALS AND METHODS TheHPLC was a LcSTAR system' Received for publication October 30 1992 accepted December 9, 1992. consisting of Model 9001 isocratic pump, 6 Port Valve with 10 Address reprint requests to i& demian: Bwhringer Ingeheh Chemicals. d sample ~OOP, and Model 9050 variable wavelength UV de- hc., 2820 North Normandy Drive, P. 0. Box 1658, Petersburg, VA 23805. 6 1993 Wiley-Liss, hc. HPLC CHIRAL SEPARATIONS 239

OCH, Chiralcel OD and Chiralcel OD-R columns allows the routine I * determination of the minor enantiomer concentrations as low OCH,-CH-CH,-OH as 0.1%. Several representative chromatograms are pre- I sented in Figure 2. In the inset of Figure 2, the separation OH factor, a,and the peaks’ resolution, R,, are listed, i.e., 1. a = k’(l)/kf(2) OH I where kf(l),k’(2) are capacity factors.

where t,, t, are retention times and W,, W, are baseline bandwidths. As part of a validation study, the linearity of the enantiomeric purity determination of methocarbamol was checked with synthetic mixtures having (R)-methocarbamolin the con- centration range from 0.1 to 10%.The representation:

%(R) = K x [lo0 x Area(R)]/[Area(R) + Area(S)]

is a straight line through the origin, with a slope (K> of 1.06028 and a correlation coefficient squared 8 = 0.999. Fig. 1. Chemical structures of yaifensin (I), methocarbamol(II), and racemor- Good separations of the enantiomeric pair dextromethorphan (111). phan-levomethorphan could be achieved only on the Chiralcel OD column. An uncontrollable factor is column-to-column re- producibility. On a “good“ column (see a and R, in Fig. 2) the quantitation of 0.1% levomethorphan in dextromethorphan is not difficult. On a “bad” column, while a is only slightly smaller, resolution is sigruficantly decreased (R, = 0.65) and only about 2% levomethorphan in dextromethorphan are de- The optical rotations of the methocarbamol enantiomers tectable. are even lower: [a]DZ2 = -0.8” (I, MeOH) and [(WIDz2 = In summary, separation of the enantiomers of guaifenesin, +0.5” (I, MeOH). methocarbamol, and racemorphan by HPLC on Chiralcel OD Such small optical rotations preclude precise determination and/or Chiralcel OD-R columns is suitable to the routine de- of enantiomeric purity by polarimetry, especially for samples termination of concentrations as low as 0.1% of the minor with high enantiomeric excess. In contrast, chiral HPLC on enantiomer.

TABLE 1. Chromatographic parameters Flow rate Detection UV Run time Analyte Column Mobile phase (dmin) (nm) (min)

~~~ Guaifenesin, methocarbamol Chiralcel OD 1:l abs. ethanokhexane 1.0 275 10 0.5% diethyl amine Guaifenesin, methocarbamol Chiralcel OD-R 2080 acetonitde: water 0.75 275 12 0.4 M NaCIO, Racemorphan Chiralcel OD Heme with 0.5% vol. of a 1.0-2.0 280 8-16 mixture of 1.5 p. vol. CH,CH and 5 p. vol. diethylamine 240 DEMIAN

GUAIFENESIN duatOWaflW LITERATURE CITED -1. R.urrJc on -2Rsumlcon + 1. Murphy, R.S. U.S. Pat. 2,770,649 (1956). Q*.lul co QJnW CV-R on olhlul co -388 -127 el30 Rrl.27 2. Schneider, O., Gruessner, A. U.S. Pat. 2,676,649 (1954). 3. Haefliger, 0. Brossi, A., Chopard-dit-Jean, L.H., Walter, M., Schneider, 0. Ueber ein photooxydationsprdukt von (+) -3-Methoxy-N-methyl-mor- phinan. Helv. Chim. Acta 392053,1956. 4. Comdi, H., Hellerbach, J., Zuest, A., Hardegger, E., Schneider, 0. Die kontiguration der morphinane. Helv. Chim. A& 42215, 1959. 5. Testa, B. Chiral aspects of drug metabolism. Trends Pharmacol. Sci. 760, 1986. 6. Daicel Chemical Industries, LTD. Application Guide for Chiral Column Selection, 1991.

RACEMORPHAN. duomatograms Rncmlc on a 'd -2 Rocdc Q) a bsrf olhkel 00 colvm cYlimlC.1 co coh or120 -1.17 -1.13 f3~0.65

Fig- 2. Example chromatograms.