Anal. Chem. 2000, 72, 5459-5465
Identification of Chiral Selectors from a
200-Member Parallel Combinatorial Library
Yan Wang, Louis H. Bluhm, and Tingyu Li*
Department of Chemistry, Vanderbilt University, Box 1822-B, Nashville, Tennessee 37235
Some of the more recent reports of the application of combi-
Selection of chiral selectors for the resolution of racemic
N-(1 -naphthyl)leucine ester 1 was studied with a 2 0 0 -
member parallel library prepared on polymeric synthesis
resin. Through this study, the library screening procedure
developed previously is improved and other pertinent
issues concerning the screening process are also ad-
dressed. The equilibration time required for screening is
reduced from 2 4 h to ∼3 h. Excellent correlation of the
outcome between the resin batch equilibration experiment
and chromatographic separation is further demonstrated.
It is also demonstrated that selectors with separation
factors as low as 1 .4 could be identified by this batch
screening process. In addition, a great deal of information
regarding enantioselective interactions was obtained in
this parallel library study. Such information should prove
useful in improving future library designs.
natorial libraries in enantioselective separations use mixture
libraries,7 while others use libraries of pure components (parallel
libraries).8 Certain advantages encompass each technique: Parallel
libraries often offer a more straightforward screening process,
while a mixture library approach allows a larger number of
compounds to be analyzed more readily. Of the published
examples of the application of parallel libraries to the development
of chiral selectors, one involves the parallel microscale synthesis
of potential chiral selectors onto silica gel, followed by screening
of the parallel library with HPLC.8a Another recently reported
method also involves the screening of chiral selectors on silica
gel.8b The third parallel example is a reciprocal chromatographic
assay of racemic library components.8c In this method, one
enantiomer of the racemic analyte was immobilized onto a
chromatographic support, and the resolution of individual racemic
library components was tested using this analyte stationary phase
by HPLC.
In recent years, combinatorial libraries have evolved consider-
ably for the development of selective binders for a given target
molecule.2 In these techniques, a large number of compounds (the
library) can be screened for a desired property. In a short period
of time, combinatorial libraries have found widespread applications
in both pharmaceutical research and material sciences.3 Applica-
tions of combinatorial libraries in enantioselective separations have
also been reported.4 In fact, some of the early works in chromato-
graphic enantioseparations, such as the rapid solution screening
of chiral selectors by NMR1 or by CE,5 and the reciprocal
development of chiral selectors,6 are combinatorial in nature.
We have also published a method to screen parallel combina-
torial libraries for chiral selectors based on a batch assay of
potential selectors on synthesis resin.9 In this procedure, a
potential chiral selector (one parallel library member) is attached
to a resin suitable for solid-phase synthesis. Racemic analyte in
the proper solvent is then allowed to equilibrate with this resin-
bound potential selector. After an equilibration time, the enantio-
meric ratio of the analyte in the supernatant is analyzed by circular
dichroism. An enantiomeric excess in the supernatant as indicated
by CD would suggest a selective adsorption of one of the two
enantiomers to the resin, which in turn is indicative of a chiral
selector. The suspected chiral selector would then be resynthe-
* To whom correspondence should be addressed: (phone/ fax) 615 343 8466;
(e-mail) tingyu.li@vanderbilt.edu.
(5) (a) Armstrong, D. W. Pittcon′98, New Orleans, LA, March 1-5, 1998. (b)
Armstrong, D. W.; Tang, Y.; Chen, S.; Zhou, Y.; Bagwill, C.; Chen, J.-R. Anal.
Chem. 1 9 9 4 , 66, 1473-1484. (c) Armstrong, D. W.; Rundlett, K. L.; Chen,
J.-R. Chirality 1 9 9 4 , 6, 496-509.
(6) (a) Pirkle, W. H.; Welch, C. J.; Lamm, B. J. Org. Chem. 1 9 9 2 , 57, 3854-
3860. (b) Welch, C. J. J. Chromatogr., A 1 9 9 4 , 666, 3-26.
(7) (a) Murer, P.; Lewandowski, K.; Svec, F.; Frechet, J. M. J. Anal. Chem. 1999,
71, 1278-1284. (b) Weingarten, M. D.; Sekanina, K.; Still, W. C. J. Am.
Chem. Soc. 1 9 9 8 , 120, 9112-9113. (c) Vries, T.; Wynberg, H.; Echten, E.
V.; Koek, J.; Hoeve, W. T.; Kellogg, R. M.; Broxterman, Q. B.; Minnaard,
A.; Kaptein, B.; Sluis, S. V. D.; Hulshof, L.; Kooistra, J. Angew. Chem., Int.
Ed. Engl. 1 9 9 8 , 37, 2349. (d) M. Chiari, V. Desperati, E. Manera, R. Longhi
Anal. Chem. 1 9 9 8 , 70, 4967-4973. (e) Jung, G.; Hofstetter, H.; Feiertag,
S.; Stoll, D.; Hofstetter, O.; Wiesmuller, K.-H.; Schurig, V. Angew. Chem.,
Int. Ed. Engl. 1 9 9 6 , 35, 2148-2150.
(1) Abbreviations: NMM, N-methylmorpholine; PyBop, benzotriazolylox tris-
(pyrrolidino)phosphonium hexafluorophosphate; Fmoc, 9-fluorenylmethoxy-
carbonyl; DIC, diisopropylcarbodiimide; HOBt, 1-hydroxybenzotriazole; TFA,
trifluoroacetic acid; DMAP, 4-(dimethylamino)pyridine; DIPEA, N,N-diiso-
propylethylamine; DCM, dichloromethane; IPA, 2-propanol; Fmoc-Osu,
9-Fluorenylmethyloxycarbonylhydroxysuccinimide. Z, benzyloxycarbonyl.
(2) For some examples, see: (a) Lam, K. S.; Salmon, S. E.; Hersh, E. M.; Hruby,
V. J.; Kazmierski, W. M.; Knapp, R. J. Nature 1 9 9 1 , 354, 82-84. (b)
Houghten, R. A.; Pinilla, C.; Blondelle, S. E.; Appel, J. R.; Dooley, C. T.;
Cuervo, J. H. Nature 1 9 9 1 , 354, 84-86. (c) Janda, K. D. Proc. Natl. Acad.
Sci. U.S.A. 1 9 9 4 , 91, 10779-10785. (d) Bunin, B. A. The Combinatorial
Index; Academic Press: New York, 1998; pp 5-8. See also: Thompson, L.
A.; Ellman, J. A. Chem. Rev. 1 9 9 6 , 96, 555-600.
(3) For a general review, see: Borman, S. Chem. Eng. News 1 9 9 8 , 76 (14),
47-67.
(8) (a) Welch, C. J.; Protopopova, M. N.; Bhat, G. Enantiomer 1 9 9 8 , 3, 471-
476. (b) Tobler, E.; Lammerhofer, M.; Oberleitner, W. R.; Maier, N. M.;
Lindner, W. Chromatographia 2 0 0 0 , 51, 65-70. (c) Lewandowski, K.;
Murer, P.; Svec, F.; Frechet, J. M. J. Chem. Commun. 1 9 9 8 , 2237.
(9) Wang, Y.; Li, T. Anal. Chem. 1 9 9 9 , 71, 4178-4182.
(4) Examples in chiral separation are cited in refs 7 and 8. For examples in
protein purification, see: (a) Huang, P. Y.; Carbonell, R. G. Biotechnol Bioeng.
1 9 9 9 , 63, 633-641. (b) Palombo, G.; De Falco, S.; Tortora, M.; Cassani,
G.; Fassina, G. J. Mol. Recognit. 1 9 9 8 , 11, 243-246.
10.1021/ac000529e CCC: $19.00 © 2000 American Chemical Society
Published on Web 10/03/2000
Analytical Chemistry, Vol. 72, No. 21, November 1, 2000 5459