Synthesis of a functionalizable water-soluble cryptophane-111

Article abstract of DOI:10.1021/ol4012019

The development of optimized xenon host systems is of crucial importance for the success of molecular imaging using hyperpolarized ¹²⁹Xe MRI. Cryptophane-111 is a promising candidate because of its encapsulation properties. The synthesis of cryptophane-111-based biosensors requires both water-solubilizing and chemically activatable groups. An expeditious synthesis of a water-soluble and functionalizable cryptophane-111 is described.

Full text of DOI:10.1021/ol4012019

ORGANIC
LETTERS
XXXX
Vol. XX, No. XX
000–000
Synthesis of a Functionalizable
Water-Soluble Cryptophane-111
†
Emmanuelle Dubost, Naoko Kotera, Sebastien Garcia-Argote, Yves Boulard,
†
†
‡,§
ꢀ
§
§
§
†
ꢀ
ꢀ
Estelle Leonce, Celine Boutin, Patrick Berthault, Christophe Dugave, and
Bernard Rousseau*,†
CEA, iBiTec-S/SCBM, LabEx LERMIT, F-91191 Gif-sur-Yvette, France, CEA,
iBiTec-S/SBiGeM, CEA Saclay, 91191 Gif-sur-Yvette, France, and CEA, IRAMIS/
ꢀ
SIS2M, Laboratoire Structure et Dynamique par Resonance Magnetique, F-91191,
ꢀ
Gif-sur-Yvette, France
bernard.rousseau@cea.fr
Received April 29, 2013
ABSTRACT
The development of optimized xenon host systems is of crucial importance for the success of molecular imaging using hyperpolarized 129Xe MRI.
Cryptophane-111 is a promising candidate because of its encapsulation properties. The synthesis of cryptophane-111-based biosensors requires
both water-solubilizing and chemically activatable groups. An expeditious synthesis of a water-soluble and functionalizable cryptophane-111 is described.
Magnetic resonance imaging (MRI) is a powerful meth-
od for scanning deep tissues or opaque biological samples,
but its low sensitivity precludes its use for imaging mole-
cular targets. In this context, laser-polarized 129Xe NMR
spectroscopy is a highly attractive tool for in vitro and in
vivo MRI. Xenon is a nontoxic gas, soluble in biological
fluids, and readily delivered by inhalation. Moreover,
thanks to an NMR signal enhanced by 4 or 5 orders of
magnitude through optical pumping,1 small amounts of
gas dissolved in biological tissues can be easily detected
with an excellent signal-to-noise ratio. Development of
xenonbiosensors, which capturexenon atomsinmolecular
cages suitably functionalized to bind the desired target,
have been the subject of particular attention over the past
decade.2 Cryptophanes are molecular cages made by the
connection of two cyclotriveratrylene units through covalent
bridging (Figure 1). They exhibit outstanding properties
for xenon encapsulation. The cavity size, which directly
depends on the length of the molecular bridge, dramati-
cally affects both the affinity for xenon and the rate of in
and out exchange of xenon. Today, biosensors are only
constructed from cryptophane A.3 Although cryptophane-
111 displays the highest affinity for xenon in both organic
solvents4 and water,5 it has never been used for the synthe-
sis of biosensors. Indeed, no congener that is simulta-
neously water-soluble and functionalizable has been de-
scribed. In addition, the challenge of current chemical
developments is rendering the cryptophane moiety more
hydrophilic in order to avoid the biosensor’s uptake by
biological membranes3f or formation of self-assemblies.3d
Only two water-soluble cryptophanes-111 were de-
scribed independently by Travis Holman and our group.
The former, functionalized with six cationic electron-with-
drawing [(η5-C5Me5)RuII]þ ([Cp*Ru]) moieties, is soluble
† CEA, iBiTec-S/SCBM.
‡ CEA, iBiTec-S/SBiGeM.
§ CEA, IRAMIS/SIS2M, Laboratoire Structure et Dynamique par
at neutral pH (>30 ꢀ 10ꢁ3 mol dmꢁ3, 293 K) and exhibits a
3
ꢀ
ꢀ
Resonance Magnetique.
(1) Desvaux, H.; Gautier, T.; Le Goff, G.; Petro, M.; Berthault, P.
high xenon binding constant in water (2.9 ꢀ 104 molꢁ1 dm3)
ꢀ
3
(Figure 2).6 However, the possible instability of the
organometallic moieties in biological fluids might be a
drawback to the use of such derivatives. Moreover, the
Eur. Phys. J. D 2000, 12, 289–296.
(2) Spence, M. M.; Rubin, S. M.; Dimitrov, I. E.; Ruiz, E. J.;
Wemmer, D. E.; Pines, A.; Yao, S. Q.; Tian, F.; Schultz, P. G. Proc.
Natl Acad. Sci. U.S.A. 2001, 98, 10654–10657.
r
10.1021/ol4012019
XXXX American Chemical Society

Figure 1. Structure of cryptophane-111 and cryptophane A.
Figure 2. Structures of water-soluble cryptophanes-111: (left)
metalated cryptophane-1115; (right) cryptophane-111 substi-
tuted by a trisulfonated group.6
vectorization of a biosensor built from this cage to a
biological target seems difficult from a chemical point of
view. In this context, we described the synthesis of a metal-
free water-soluble cryptophane-111 (Figure 2).7 This was
achieved by grafting an original trisulfonated linker onto
the cryptophane core. However, several functionalization
experiments were unsucessful in our laboratory. As a
result, we refocused on a completely different strategy
and we describe here a straightforward synthesis of a
water-soluble and functionalizable cryptophane-111.
A cryptophane-222 bearing six carboxylic acid groups
has been successfully monofunctionalized and led to a
Zn2þ biosensor.3l Analogously utilizing this approach, a
cryptophane-111 bearing six carboxylate groups would be a
perfect precursor for the design of new biosensors (Figure 3).
Such a cryptophane-111, 3, could be obtained from the
corresponding hexabrominated compound 2. The latter
could be synthesized from cryptophane-111 1 available at
the multigram scale (Figure 3).8
(3) (a) Spence, M. M.; Ruiz, E. J.; Rubin, S. M.; Lowery, T. J.;
Winssinger, N.; Schultz, P. G.; Wemmer, D. E.; Pines, A. J. Am. Chem.
Soc. 2004, 126, 15287–15294. (b) Hilty, C.; Lowery, T. J.; Wemmer,
D. E.; Pines, A. Angew. Chem. 2006, 118, 76–79. (c) Wei, Q.; Seward,
G. K.; Hill, P. A.; Patton, B.; Dimitrov, I. E.; Kuzma, N. N.;
Dmochowski, I. J. J. Am. Chem. Soc. 2006, 128, 13274–13283. (d) Roy,
V.; Brotin, T.; Dutasta, J.-P.; Charles, M.-H.; Delair, T.; Mallet, F.;
Huber, G.; Desvaux, H.; Boulard, Y.; Berthault, P. Chem. Phys. Chem.
2007, 8, 2082–2086. (e) Chambers, J. M.; Hill, P. A.; Aaron, J. A.; Han,
Z.; Christianson, D. W.; Kuzma, N. N.; Dmochowski, I. J. J. Am. Chem.
Soc. 2009, 131, 563–569. (f) Schlundt, A.; Kilian, W.; Beyermann, M.;
Sticht, J.; Gunther, S.; Hopner, S.; Falk, K.; Roetzschke, O.; Mitschang,
L.; Freund, C. Angew. Chem., Int. Ed. 2009, 48, 4142–4145. (g)
Berthault, P.; Desvaux, H.; Wendlinger, T.; Gyejacquot, M.; Stopin,
A.; Brotin, T.; Dutasta, J.-P.; Boulard, Y. Chem.;Eur. J. 2010, 16,
12941. (h) Meldrum, T.; Seim, K. L.; Bajaj, V. S.; Palaniappan, K. K.;
Wu, W.; Fancis, M. B.; Wemmer, D. E.; Pines, A. J. Am. Chem. Soc.
Figure 3. Retrosynthetic pathway.
We started our investigation by studying the bromi-
nation step. As reported in a previous communication,
cryptophane-111 1 can be readily monobrominated using
N-bromosuccinimide NBS in chloroform.9 Using an ex-
cess of this reagent and longer times of reaction, we
observed the formation of a mixture of polybrominated
compounds (Table 1, entry 1). After 7 days at room tem-
perature, the hexabrominated compound 2 was isolated in
35% yield (Table1, entry 2). Heating to reflux did not
improve this result (Table 1, entry 3). The use of a large
excess (120 equiv) of NBS accelerated the formation of
compound 2 but complicated its purification due to the
formation of di-, tri-, and tetrabrominated compounds
(Table 1, entry 4). After 13 days of reaction at room tem-
perature, the hexabrominated cryptophane-111 2 was iso-
lated in 60% yield (Table 1, entry 5). This product precipitated
during the reaction and was isolated by simple filtration.
We were pleased to observe that the hexabromination
takes place in a perfectly regioselective manner. Indeed, the
1H NMR spectrum of compound 2 shows only two doublets
in the aromatic area at 6.92 and 8.06 ppm (J = 6.0 Hz).10
This demonstrates that compound 2 was regioselectively
€
€
€
2010, 132, 5936–5937. (i) Schilling, F.; Schroder, L.; Palaniappan, K. K.;
Zapf, S.; Wemmer, D. E.; Pines, A. ChemPhysChem 2010, 11, 3529–
3533. (j) Seward, G. K.; Bai, Y.; Khan, N. S.; Dmochowski, I. J. Chem.
Sci. 2011, 2, 1103–1110. (k) Boutin, C.; Stopin, A.; Lenda, F.; Brotin, T.;
Dutasta, J.-P.; Jamin, N.; Sanson, A.; Boulard, Y.; Leteurtre, F.; Huber,
ꢁ
G.; Bogaert-Buchmann, A.; Tassali, N.; Desvaux, H.; Carriere, M.;
Berthault, P. Bioorg. Med. Chem. 2011, 19, 4135–4143. (l) Kotera, N.;
ꢀ
Tassali, N.; Leonce, E.; Boutin, C.; Berthault, P.; Brotin, T.; Dutasta, J.-
P.; Delacour, L.; Traore, T.; Buisson, D.-A.; Taran, F.; Coudert, S.;
ꢀ
Rousseau, B. Angew. Chem., Int. Ed. 2012, 51, 1–5. (m) Stevens, T. K.;
Palaniappan, K. K.; Ramirez, R. M.; Fancis, M. B.; Wemmer, D. E.;
Pines, A. Magn. Reson. Med. 2012, 69, 1245–1252.
(4) Fogarty, H. A.; Berthault, P.; Brotin, T.; Huber, G.; Desvaux, H.;
Dutasta, J.-P. J. Am. Chem. Soc. 2007, 129, 10332–10333.
ꢀ
(5) (a) Jacobson, D. R.; Khana, N. S.; Colle, R.; Fitzgerald, R.;
Laureano-Perez, L.; Bai, Y.; Dmochowski, I. J. Proc. Natl Acad. Sci.
ꢀ
U.S.A. 2011, 108, 10969–10973. (b) Huber, G.; Brotin, T.; Dubois, L.;
Desvaux, H.; Dutasta, J.-P.; Berthault, P. J. Am. Chem. Soc. 2006, 128,
6239–6246.
(6) Fairchild, R. M.; Joseph, A. I.; Holman, K. T.; Fogarty, H. A.;
Brotin, T.; Dutasta, J.-P.; Boutin, C.; Huber, G.; Berthault, P. J. Am.
Chem. Soc. 2010, 132, 15505–15507.
ꢀ
(8) Traore, T.; Delacour, L.; Kotera, N.; Merer, G.; Buisson, D.- A.;
Dupont, C.; Rousseau, B. Org. Process Res. Dev. 2011, 15, 435–437.
ꢀ
(9) Traore, T.; Delacour, L.; Garcia-Argote, S.; Berthault, P.; Cintrat,
ꢀ
ꢀ
(7) Traore, T.; Clave, G.; Delacour, L.; Kotera, N.; Renard, P.-Y.;
Romieu, A.; Berthault, P.; Boutin, C.; Tassali, N.; Rousseau, B. Chem.
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(10) See Supporting Information
B
Org. Lett., Vol. XX, No. XX, XXXX

Table 1. Bromination of Cryptophane-111 1
Table 2. Optimization of Lithiation of Cryptophane
NBS
reaction
time
temp
entry (equiv)
(°C)
compounds (yield, %)a
temp
1
9
6 days
rt
mixture of polybrominated
compounds
entry
electrophile
(°C)
resulta
b
1
2
3
4
5
CO2
ꢁ78
ꢁ15
ꢁ15
0
ꢁ
2
3
12
12
7 days
4 days
rt
2 (35%)
CO2
1b
reflux hexabromocryptophane 2
(50%) and
ClCO2Et
ClCO2Et
ClCO2Bn
1 and 4a
1 and 4a
4b and 5
pentabromocryptophane
(50%)b
ꢁ15
a Monitoring by LC/MS. b Traces of mono- and dicarboxylic acid
compounds.
4
5
120
12
4 days
rt
rt
2 (22%)
2 (60%)
13 days
a Isolated yield. b Monitoring by LC/MS.
of 2.0 mg mLꢁ1. These compounds reversibly encapsulate
3
xenon in D2O at pH 6 and display a specific 129Xe NMR
signal for the caged noble gas of 87 and 84 ppm at 293 K,
respectively.
brominated on the six ortho positions of the phenoxy rings,
as shown in Figure 3.
We then turned our attention to the lithiation of com-
pound 2 in order to obtain cryptophane-111 hexacar-
boxylic acid 3. We started our investigations by using
n-BuLi in THF and quenching lithiated species with
CO2. The reactions were monitored by LC/MS. After
performing the reaction at ꢁ78 °C, the starting material
was recovered along with small amounts of mono- and
dicarboxylic acid compounds (Table 2, entry 1). This result
demonstrated the possibility of performing the bromine/
lithium exchange and of trapping the lithiated intermedi-
ate. In contrast, at ꢁ15 °C, all of the brominated precursor
disappeared. However, cryptophane-1111 wasobtainedas
the major product with small amounts of mono- and
dicarboxylic acid compounds (Table 2, entry 2). This
proved that the lithium/bromine exchange occurred, but
the lithiated species were not efficiently trapped by CO2.
More reactive ethyl chloroformate, used as an electrophile,
gave a mixture of cryptophane-111 hexaethyl ester 4a and
cryptophane-111 1in the same proportions (Table 2, entry 3).
Similar results were obtained at 0 °C, demonstrating the
stability of lithiated intermediates (Table 2, entry 4).
Finally, using benzyl chloroformate as an electrophile led
to hexa- and pentabenzyl ester compounds 4b and 5 in the
same proportions (Table 2, entry 5). In order to skip
laborious purification steps, complete debenzylation by
hydrogenolysis over Pd/C in tetrahydrofuran/methanol
1/1 was carried out on the crude mixture of products,
leading to cryptophane-111 hexacarboxylic acid 3 in 18%
isolated overall yield. The pentacarboxylic acid analog 6
was obtainedin 23% overallyield asthe main side product.
As expected, cryptophane-111 hexacarboxylate 3 and
pentacarboxylate 6 are soluble in water at a concentration
In summary, we propose a strategy for the easy synthesis
of a functionalizable and water-soluble cryptophane-111.
Compound 3 can be readily obtained in three steps from
cryptophane-111 1 in moderate yields along with the
corresponding pentacarboxylic acid 6. As expected, cryp-
tophane-111 hexacarboxylate 3 encapsulates xenon and
therefore allows us to envision the development of new
cryptophane-based bioprobes for 129Xe MRI. Our on-
going research project aims to graft compound 3 onto
various biomolecules through amide bond formation ac-
cording to our previous work.6
Acknowledgment. We gratefully acknowledge Profes-
sor Yves Fort (Institut Jean Barriol, France) for his advice
ꢀ
on the lithiation of cryptophane. We are grateful to Celine
Puente and David-Alexandre Buisson (CEA/Saclay)
for LC/MS analysis and purifications. Financial sup-
ꢀ
port of the Fondation pour la Recherche Medicale is
gratefully acknowledged (Dossier DCM20111223065).
This work has benefited from the facilities and expertise
of the Small Molecule Mass Spectrometry platform of
IMAGIF (Centre de Recherche de Gif: www.imagif.
cnrs.fr).
Supporting Information Available. Detailed experimen-
tal procedures, full characterization, and copies of 1H and
13C NMR spectra of all new compounds are available.
This material is available free of charge via the Internet at
http://pubs.acs.org.
The authors declare no competing financial interest.
Org. Lett., Vol. XX, No. XX, XXXX
C