J . Org. Chem. 1997, 62, 8221-8224
8221
Sch em e 1
En a n tioselective a n d Dia ster eoselective
Bin d in g Stu d y of Silica Bou n d
Ma cr obicyclic Recep tor s by HP LC
Francesco Gasparrini,*,† Domenico Misiti,†
W. Clark Still,*,‡ Claudio Villani,† and
Helma Wennemers‡
Dipartimento di Studi di Chimica e Tecnologia delle
Sostanze Biologicamente Attive, Universita´ “La Sapienza”,
P.le A. Moro 5, Roma, 00185 Italy, and Department of
Chemistry, Columbia University,
New York, New York 10027
Received December 5, 1996
Systematic investigation of enantioselective binding
processes can be conveniently carried out by direct
chromatographic methods after immobilization of a given
chiral host on a suitable inert matrix. In a previous
paper,1 we found close agreement between HPLC reten-
tion data gathered with a silica-bound, macrotricyclic
chiral stationary phase (CSP) and association constants
measured in free solution by NMR for a range of small
molecule peptidic guests. CSP’s with highly preorga-
nized, medium-sized hosts are attractive since they often
afford high levels of enantioselectivity and yet have
relatively low molecular complexity, thus facilitating the
understanding of operative chiral recognition mecha-
nisms from easily collected chromatographic data.
Here we describe the properties of two novel, chemi-
cally bonded chiral stationary phases (CSP -A and CSP -
B) prepared from synthetic receptor systems that com-
bine a semirigid framework with multipoint hydrogen-
bonding sites (Scheme 1). Both phases incorporate
previously described C2-symmetric, two-armed receptors2
covalently linked to LiChrosorb 5µ silica. These receptors
are readily prepared from two identical macrocyclic
armlike substructures connected by a trans-diaminopy-
rrolidine linker, the ring nitrogen of which carries an
allyl-terminated chain for grafting to the silica support.
The macrocyclic arms are tetraamides prepared from
chiral 1,2-diaminocyclohexane and isophthalic acid de-
rivatives. They serve as conformationally restricted
surfaces that carry an array of spatially defined hydrogen
bond donors and acceptors. In conjunction with the
linker, the macrocyclic arms fold to form a concave
binding site made up of aromatic rings and amide
functional groups that can make associative interactions
with guests having complementary shapes and function-
alities. These chiral hosts were grafted to γ-mercapto-
propyl silica microparticles by AIBN free radical addition
and produced CSP’s with receptor loading between 0.07
and 0.09 mmol/g of silica.
tions involved in the chiral recognition process. Among
the peptidic guests, the largest enantioselectivities were
found for amino acid derivatives that were N-acylated
by 3,5-dinitrobenzoyl (DNB) groups. N-Acylation by
other aromatic acids was also effective, with the enan-
tioselectivities varying as DNB > pentafluorobenzoyl >
naphthoyl ∼ benzoyl. Aliphatic N-acylation (e.g. by Boc,
Ac) led to no significant enantioselection.
Among DNB-substituted amino acid derivatives, both
phases showed a preference for S-configured guests and
a dependence of the degree of enantioselectivity on the
nature of the side chain and the C-terminal functionality.
Among the various amino acids studied, the best enan-
tioselection was found with Leu, Ile, and Met. Interest-
ingly, all of these amino acids have the same number of
heavy atoms in their side chains and are probably being
selected because their side chains are of similar size and
make a good fit to the concave binding site of the receptor.
The best C-terminal functionality for high enantioselec-
tion was a secondary amide, and the extent of enanti-
oselection did not depend on the bulk of the amide
substituent (entries 2, 12-14). Amino acid esters gener-
ally resolved poorly (entries 3 vs 11) while the tertiary
amides we studied did not resolve at all (entry 2 vs 15).
Comparison of retention data for the more retentive S
enantiomers of different ArCO-Leu-hexylamides (entries
3, 8, 9, and 10) revealed that the DNB derivatives were
always the most strongly retained. The increased affinity
of the DNB derivatives for the CSP’s contributed ∼0.5
kcal/mol in enantioselectivity over the other aroyl Leu
derivatives, suggesting that aromatic-aromatic interac-
tions are playing a role in stabilizing the host-(S)-guest
complexes. Although the exact nature of these interac-
tions is unclear at this time, it could be interpreted
according to the electrostatic model proposed for π-stacked
porphyrin4 and π-stacked phenyls in 1,8-diarylnaphtha-
lenes.5
Enantioselectivity and retention data collected from a
diverse set of simple racemic guests on CSP -A and
CSP -B (Table 1)3 were used to probe the binding interac-
* For Dr. Gasparrini: Phone: +39
6 49912776. Fax: +39 6
49912780. E-mail: gasparrini@axrma.uniroma1.it.
† Universita´ “La Sapienza”.
Other 3,5-dinitrobenzoylated mono- or difunctional
amines, alcohols, and amino alcohols were also effectively
‡ Columbia University.
(1) Gasparrini, F.; Misiti, D.; Villani, C.; Borchardt, A.; Burger, M.
T.; Still, W. C. J . Org. Chem. 1995, 60, 4314.
(2) For a description of related dye-labeled receptors, see: Wenne-
mers, H.; Yoon, S. S.; Still, W. C. J . Org. Chem. 1995, 60, 1108.
(3) Chromatographic data were collected with chlorinated solvents
for comparison purposes; other eluting systems (acetone, AcOEt, CH3-
CN, MBTE, or water-based eluents) can be used as well, although with
reduced enantioselectivities.
(4) Hunter, C. A., Sanders, J . K. M. J . Am. Chem. Soc. 1990, 112,
5525.
(5) Cozzi, F.; Cinquini, M.; Annunziata, R.; Siegel, J . S. J . Am. Chem.
Soc. 1993, 115, 5330. Cozzi, F.; Ponzini, F.; Annunziata, R.; Cinquini,
M.; Siegel, J . S. Angew. Chem., Int. Ed. Engl. 1995, 34, 1019.
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