Steroidal Ureas as Enantioselective Receptors for an N-Acetyl α-Amino Carboxylate

Article abstract of DOI:10.1021/ol027009l

(Matrix Presented) Cholic acid has been elaborated into three regioisomeric bis-carbamoylureas, which have been investigated as enantioselective receptors for N-acetyl phenylalanine. L/D selectivities, peaking at 5:1, have been determined by a sensitive a

Full text of DOI:10.1021/ol027009l

ORGANIC
LETTERS
2002
Vol. 4, No. 26
4639-4642
Steroidal Ureas as Enantioselective
Receptors for an N-Acetyl r-Amino
Carboxylate
Laura Siracusa,† Fionn M. Hurley,‡ Stefan Dresen,‡ Laurence J. Lawless,‡
,†
M. Nieves Pe´rez-Paya´n,‡ and Anthony P. Davis*
School of Chemistry, UniVersity of Bristol, Cantock’s Close,
Bristol BS8 1TS, United Kingdom, and Department of Chemistry, Trinity College,
Dublin 2, Ireland
anthony.daVis@bristol.ac.uk
Received October 1, 2002
ABSTRACT
Cholic acid has been elaborated into three regioisomeric bis-carbamoylureas, which have been investigated as enantioselective receptors for
N-acetyl phenylalanine. L/D selectivities, peaking at 5:1, have been determined by a sensitive and rapid MS-based extraction method that
should be generalizable to related systems.
Enantioselective recognition is a long-standing issue in
supramolecular chemistry.1 Despite much research, it con-
tinues to provide interesting challenges. Success for some
substrate classes remains elusive, and there are still few
systems of any type which exhibit selectivities of g90% ee.2
Moreover, good enantiodiscrimination often requires careful
matching of substrate and receptor. Effective discrimination
for a range of substrates may therefore require an equally
wide range of receptors.
Modular receptor-like architectures provide an attractive
approach to this area. Given suitable screening methods,3
they could be elaborated into libraries from which the best
variant for any particular substrate could be selected. Podand
structures are especially easy to vary, and “cholapod”4
architectures such as 1 have particular advantages. The
steroidal framework, derived from cholic acid 2, provides a
chiral scaffold that positions the codirected legs so as to
create a binding site for a small-to-medium size molecule.
The side chain may be used to control solubility, or to link
the scaffold to a polymer backbone for solid-phase synthesis.
Cholapod libraries prepared by “split-and-mix” solid-phase
† University of Bristol.
‡ University of Dublin.
(1) Webb, T. H.; Wilcox, C. S. Chem. Soc ReV. 1993, 22, 383. Zhang,
X. X.; Bradshaw, J. S.; Izatt, R. M. Chem. ReV. 1997, 97, 3313. Ogoshi,
H.; Mizutani, T. Acc. Chem. Res. 1998, 31, 81.
(2) Selected examples: Peacock, S. C.; Domeier, L. A.; Gaeta, F. C.
A.; Hegelson, R. C.; Timko, J. M.; Cram, D. J. J. Am. Chem. Soc. 1978,
100, 8190. Jeong, K. S.; Muehldorf, A. V.; Rebek, J. J. Am. Chem. Soc.
1990, 112, 6144. Hong, J.-I.; Namgoong, S. K.; Bernardi, A.; Still, W. C.
J. Am. Chem. Soc. 1991, 113, 5111. Yoon, S. S.; Still, W. C. J. Am. Chem.
Soc. 1993, 115, 823. Pirkle, W. H.; Murray, P. G.; Rausch, D. J.; McKenna,
S. T. J. Org. Chem. 1996, 61, 4769. Mart´ın, M.; Raposo, C.; Almaraz, M.;
Crego, M.; Caballero, C.; Grande, M.; Mora´n, J. R. Angew. Chem., Int.
Ed. Engl. 1996, 35, 2386. Botana, E.; Ongeri, S.; Arienzo, R.; Demarcus,
M.; Frey, J. G.; Piarulli, U.; Potenza, D.; Gennari, C.; Kilburn, J. D. Chem.
Commun. 2001, 1358.
(3) For a method applicable to one-bead-one-compound libraries, see:
Weingarten, M. D.; Sekanina, K.; Still, W. C. J. Am. Chem. Soc. 1998,
120, 9112.
(4) Ayling, A. J.; Pe´rez-Paya´n, M. N.; Davis, A. P. J. Am. Chem. Soc.
2001, 123, 12716.
10.1021/ol027009l CCC: $22.00 © 2002 American Chemical Society
Published on Web 12/05/2002

synthesis have already been used for sequence-selective
peptide recognition5 and for studies in enzyme modeling.6
Scheme 1. Preparation of 4-6a
As part of our general program on receptors derived from
cholic acid,4,7 we have recently shown that guanidinium
cations of form 3 can extract N-acetyl R-amino carboxylates
from an aqueous phase into chloroform with up to 80% ee.8
Moreover, a lipophilic analogue has been found to transport
N-acetyl phenylalanine through bulk liquid membranes with
high turnover numbers and moderate-to-good enantioselec-
tivities.9 These results suggest that the cholapod architecture
is intrinsically suitable for enantioselective recognition.
However, further exploration of this system was hampered
by problems in synthesizing the guanidinium centers and
handling the cationic products. We therefore decided to
investigate urea 4, an electroneutral analogue of 3, as a
potential model for receptor libraries. We now report the
synthesis and recognition properties of 4, and also of
regioisomers 5 and 6. The enantioselectivities of 4-6 have
been measured by using a straightforward, rapid, and accurate
MS-based procedure that may prove broadly useful in
screening for enantiodiscrimination by small quantities of
potential receptors.
a Reagents: (a) Zn, AcOH; (b) p-CF3C6H4NCO, Et3N, DCM;
(c) PhNCO, DCM, cat. TMSCl; (d) NaOMe, MeOH; (e) PhNCO,
CHCl3, cat. TMSCl; (f) TFA, DCM.
used for the synthesis of 3.8b 7R-Urea 5 was synthesized via
amine 8, previously reported by Kasal and co-workers.10
Finally 12R-urea 6 was prepared from diol 9, an intermediate
in an earlier synthesis of a differentially protected 3,12-
diamine.11 A p-trifluoromethyl substituent was incorporated
in the urea moieties to acidify the ArNH, hopefully promot-
ing the formation of doubly H-bonded urea-carboxylate
motifs.
The enantioselective recognition of carboxylates can be
studied through the extraction of racemic substrates from
aqueous into organic phases. Enantioselectivities are obtained
simply by measuring the ratios of substrate enantiomers
present in the organic phase. This procedure is convenient,
and directly relevant to potential applications in separation
systems. The method is more obviously applicable to cationic
(5) Boyce, R.; Li, G.; Nestler, H. P.; Suenaga, T.; Still, W. C. J. Am.
Chem. Soc. 1994, 116, 7955. Cheng, Y. A.; Suenaga, T.; Still, W. C. J.
Am. Chem. Soc. 1996, 118, 1813.
(6) De Muynck, H.; Madder, A.; Farcy, N.; De Clercq, P. J.; Pe´rez-
Paya´n, M. N.; O¨ hberg, L. M.; Davis, A. P. Angew. Chem., Int. Ed. 2000,
39, 145.
(7) Reviews: (a) Davis, A. P. Chem. Soc. ReV. 1993, 22, 243. (b) Davis,
A. P.; Bonar-Law, R. P.; Sanders, J. K. M. In ComprehensiVe Supra-
molecular Chemistry; Murakami, Y., Ed.; Pergamon: Oxford, UK, 1996;
Vol. 4 (Supramolecular Reactivity and Transport: Bioorganic Systems),
pp 257. (c) Davis, A. P. In Supramolecular Science: Where It Is and Where
It Is Going; Ungaro, R., Dalcanale, E., Eds.; Kluwer: Dordrecht, The
Netherlands, 1999; pp 125-146. Recent contributions: (d) Ayling, A. J.;
Broderick, S.; Clare, J. P.; Davis, A. P.; Pe´rez-Paya´n, M. N.; Lahtinen, M.;
Nissinen, N. J.; Rissanen, K. Chem. Eur. J. 2002, 8, 2197. (e) Lambert, T.
N.; Boon, J. M.; Smith, B. D.; Pe´rez-Paya´n, M. N.; Davis, A. P. J. Am.
Chem. Soc. 2002, 124, 5276-5277.
(8) (a) Davis, A. P.; Lawless, L. J. Chem. Commun. 1999, 9. (b) Lawless,
L. J.; Blackburn, A. G.; Ayling, A. J.; Pe´rez-Paya´n, M. N.; Davis, A. P. J.
Chem. Soc., Perkin Trans. 1 2001, 1329.
(9) Baragan˜a, B.; Blackburn, A. G.; Breccia, P.; Davis, A. P.; de
Mendoza, J.; Padro´n-Carrillo, J. M.; Prados, P.; Riedner, J.; de Vries, J. G.
Chem. Eur. J. 2002, 8, 2931.
(10) Kasal, A.; Kohout, L.; Lebl, M. Collect. Czech. Chem. Commun.
1995, 60, 2147.
(11) Barry, J. F.; Davis, A. P.; Pe´rez-Paya´n, M. N.; Elsegood, M. R. J.;
Jackson, R. F. W.; Gennari, C.; Piarulli, U.; Gude, M. Tetrahedron Lett.
1999, 40, 2849.
The syntheses of receptors 4-6 are summarized in Scheme
1. The 3R-urea 4 was prepared from azido-diol 7, previously
4640
Org. Lett., Vol. 4, No. 26, 2002

receptors such as 3, but can be employed for electroneutral
receptors provided a cation of suitable lipophilicity is also
present. In our previous work on 3, the measurements were
performed by 1H NMR, effectively employing the cation as
a chiral shift reagent. However, for the present work, we
wanted to develop a method that would be more general,
and could be applied routinely to receptors synthesized on
very small scales.
residues were dissolved in acetonitrile for analysis by LCMS
(for details, see the Supporting Information). In some cases
quantitative analysis by NMR, after addition of an internal
standard, was used to obtain estimates of the total amounts
extracted.
Mass spectroscopy on isotopically labeled “pseudorace-
mates” provides a sensitive and general method for measur-
ing enantioenrichments.12 This technique seemed readily
adaptable to the study of enantioselective extractions. A
receptor in one phase may be equilibrated with an excess of
pseudoracemate in the second phase. The mass-differentiated
pseudoenantiomers are extracted in a ratio that reflects the
enantioselectivity, and a single MS analysis of the receiving
phase can yield this figure. A potential complication is that
receptor-substrate association might perturb the analysis.
However, use of LCMS, under conditions which separate
receptor from substrate, removes this possibility.
For our studies on 4-6, the phenylalanine derivatives 10
(L-deuterated) and 11 (D-undeuterated) were employed as
substrates. Aqueous solutions for the extraction experiments
were prepared by dissolving a 1:1 mixture of the corre-
sponding acids (to 30 mM), Et4NOH (to 3 mM), and NaOH
(to 27 mM) in a pH 7.4 phosphate buffer (0.1 M). This
concentration of tetraethylammonium countercation was
found to give acceptable levels of extraction without promot-
ing background (nonselective) phase transfer. Aliquots of
these aqueous solutions (2 mL) were added to receptors 4-6
in chloroform (1 mL, various concentrations; see Table 1).
The results of these experiments are summarized in Table
1. All three receptors extracted ∼17 mol % of 10/11 under
these conditions, and each showed a significant preference
for the L-derivative 10. The selectivities ranged from ∼5:1
for 4 to ∼3:1 for 6. The validity and reliability of the method
were confirmed as follows. First, control mixtures of 10 and
11 were made up in known ratios and analyzed by LCMS.
The analyses gave the predicted results to good accuracy.13
Second, repeat measurements of enantioselectivity gave
consistent results to within (1%. Third, the measured
enantioselectivities were independent of receptor concentra-
tion (see Table 1). This confirmed the absence of significant
background extraction, which should degrade the apparent
selectivity at low receptor concentrations. Finally, the
complementary pseudoracemate 12 + 13 was prepared and
tested against receptor 5. The measured L/D ratio of 3.9
corresponds closely to those given in Table 1, confirming
that isotope effects are negligible in this system.
Table 1. Extraction of 10 + 11 by Receptors 4-6 into
Chloroform from Aqueous “Pseudoracemate"
The enantioselectivities obtained for these receptors are
quite encouraging. Certainly, receptor 4 compares quite well
with its cationic analogues 3. Arguably, the greater ease of
synthesis and handling of ureas vs guanidinium cations
provides sufficient compensation for the modest drop in
selectivity. As it stands receptor 4 has the potential for
enantioselective transport through nonpolar membranes,9
while elaboration into libraries of general form 14 should
be feasible and could yield high levels of enantiodiscrimi-
nation. The selectivities obtained for 5 and 6 may be thought
slightly disappointing. There were grounds for hoping that
these receptors might out-perform 4. The urea groups are
axial in 5 and 6, restricting rotation about the C(7/12)-N
bonds and directing the NH groups inward beneath the R-face
of the steroid.4 These compounds might therefore appear
better preorganized for binding chiral carboxylates, and likely
to show higher selectivities. In fact, both showed lower
enantiodiscrimination than 4. However, their selectivities are
still appreciable, and there is every reason to suppose that
the corresponding libraries 15 and 16 will contain highly
selective receptors.
[receptor]
(mM)
ratio extracted
amount
extracteda (%)
receptor
(L-10/D-11)
4
4
4
5
5
5
5
6
6
6
1
3
6
1
3
6
12
1
3
6
5.0
5.0b
4.9c
3.7
17b
17
3.6b
3.8b
3.7
3.1b
3.1b
3.1b
16
a Mol %, with respect to receptor. b Average of two measurements.
c Average of three measurements.
The mixtures were stirred vigorously for 1 h, then allowed
to separate. The organic phases were collected, passed
through hydrophobic filter paper, then evaporated, and the
(12) Recent examples: (a) Reetz, M. T.; Becker, M. H.; Klein, H. W.;
Stockigt, D. Angew. Chem., Int. Ed. Engl. 1999, 38, 1758. (b) Welch, C.
J.; Pollard, S. D.; Mathre, D. J.; Reider, P. J. Org. Lett. 2001, 3, 95. (c)
Diaz, D. D.; Yao, S. L.; Finn, M. G. Tetrahedron Lett. 2001, 42, 2617.
(13) For a similar study giving parallel results, see ref 12b.
Org. Lett., Vol. 4, No. 26, 2002
4641

organic phase with significant enantioselectivities. These
receptors should serve as models for more complex podands,
which may ultimately yield greatly improved performance.
The selectivities were measured by using a MS-based method
that is readily applicable to small quantities of electroneutral,
organic-soluble receptors.
Acknowledgment. We thank the European Commission
(TMR Network contract ERB-FMRX-CT98-0233, and Marie
Curie Fellowship to M.N.P.), Enterprise Ireland, Schering
Plough (Avondale) Ltd., the University of Bristol, and the
EPSRC (GR/R42757) for financial support of this work.
Supporting Information Available: Experimental details
for the synthesis of 4-6 and for the enantioselectivity
measurements. This material is available free of charge via
the Internet at http://pubs.acs.org.
In conclusion, we have shown that steroidal ureas 4-6
can extract an N-acetyl amino acid salt from aqueous into
OL027009L
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Org. Lett., Vol. 4, No. 26, 2002