Toward fullerene-based X-ray contrast agents: Design and synthesis of non-ionic, highly-iodinated derivatives of C60

Article abstract of DOI:10.1016/S0040-4039(01)02119-0

An X-ray contrast agent precursor based on C₆₀ has been designed, synthesized, and characterized. The compound is the monoadduct of a malonodiamide containing six iodine atoms and eight acetal-protected alcohols with multiple hindered rotations

Full text of DOI:10.1016/S0040-4039(01)02119-0

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 561–564
Toward fullerene-based X-ray contrast agents: design and
synthesis of non-ionic, highly-iodinated derivatives of C60
Tim Wharton and Lon J. Wilson*
Department of Chemistry and the Center for Nanoscale Science and Technology, MS-60, Rice University, Houston,
TX 77251-1892, USA
Received 24 July 2001; revised 31 October 2001; accepted 7 November 2001
Abstract—An X-ray contrast agent precursor based on C60 has been designed, synthesized, and characterized. The compound is
the monoadduct of a malonodiamide containing six iodine atoms and eight acetal-protected alcohols with multiple hindered
rotations at ambient temperature. © 2002 Elsevier Science Ltd. All rights reserved.
3
The use of contrast agents has become routine in
contemporary medical imaging. Although technological
advances have increased the popularity of ultrasound
and magnetic resonance imaging techniques, X-ray
related imaging still accounts for 75–80% of all diag-
nostic imaging procedures. Water-soluble, non-ionic X-
tungsten clusters ), and specialized compounds for
2
niche applications (i.e. liver-specific and blood-pool
agents ). In view of the increasing interest in the medi-
cal application of fullerene materials, we report here
the synthesis and characterization of the first X-ray
contrast agent precursor, 5, based on the biologically
compatible and versatile C60 scaffolding (Fig. 2).
4
5
1
6
ray contrast agents such as Iohexol are based on the
1,3,5-triiodobenzene structure (Fig. 1). Administered
intravenously, the agents enhance radiographic image
contrast by increasing X-ray attenuation via their mul-
tiple electron-rich iodine atoms. In the US alone, iodi-
nated X-ray contrast agents are currently used in
Starting from 5-aminoisophthalic acid (Scheme 1),
diacid dichloride 1 was prepared in two steps as previ-
7
ously reported. Condensation of 1 with malonyl
dichloride in refluxing THF gave 2 as colorless micro-
2
8
approximately 20 million procedures annually.
crytals in 83% yield. Given the solubility limitations of
C , it was necessary for the 1,3-diols of 4 to be
60
Today’s X-ray contrast agents are reasonably safe and
effective. However, there is current interest in novel
compounds that could give superior performance (i.e.
Figure 1. Structure of Iohexol.
Keywords: X-ray contrast agent; fullerenes; fullerene derivatives.
*
Corresponding author. Fax: +1-713-348-5155; e-mail: durango@
rice.edu
Figure 2. Structure of the C -based X-ray contrast agent
precursor, 5.
60
0
040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)02119-0

5
62
T. Wharton, L. J. Wilson / Tetrahedron Letters 43 (2002) 561–564
Scheme 1. Synthesis of 5. Reagents and conditions: (i) KICl , H O, 55°C, 24 h; (ii) SOCl , reflux 6 h; (iii) CH (COCl) , THF, reflux
2
2
2
2
2
6
h; (iv) benzyl chloroformate, NEt , EtOH; (v) PhCHO, cat. H SO , toluene, reflux 6 h; (vi) 300 psi H , Pd/C, EtOH, 50 min;
3 2 4 2
(
vii) DMA, NEt , 24 h; (viii) C , CBr , DBU, toluene:pyridine 4:1.
3 60 4
protected so that a mutual solvent system could be used
when appending 4 to the fullerene. This was most easily
achieved by O,O%-protection of serinol (3-amino-1,2-
propanediol) prior to condensation with 2, and this was
carried out in three steps. Protection of the serinol
amine as the benzyl carbamate was followed by benzyl-
idene acetal formation with benzaldehyde (cat. H SO )
the cis intermediate gave O,O%-benzylidene protected
serinol, 3, as the cis isomer exclusively. The condensa-
tion of 2 with 3 proceeded smoothly in dimethyl acet-
1
1
amide (DMA) with NEt in 95% yield to give 4. For
3
the final addition of 4 to C , the method of Hirsch was
60
12
used.
Accordingly, DBU (1,8-diazabicyclo[5.4.0]-
undec-7-ene) was added to a solution of CBr , 4, and
4
2
4
C60 in 4:1 toluene:pyridine. After chromatographic
to give an equilibrium mixture of cis:trans substituted
,3-dioxane in a ratio of 8:2. The cis isomer selectivity
workup, 5 was obtained as a brown solid in 70% yield.
1
can be rationalized by the axial preference for the
Compounds 4 and 5 are interesting examples of steri-
N-benzyl carbamate group due to intramolecular Nꢀ
13
cally-congested hexasubstituted benzenes
with as
9
H···O hydrogen bonding. The cis isomer of 3 has also
many as ten hindered rotations at ambient temperature.
been shown to react as a nucleophile at twice the rate of
1
Variable-temperature 500 MHz H NMR experiments
10
the corresponding trans isomer. Thus, the intermedi-
ate cis isomer of N- and O,O%-protected serinol was
purified by fractional recrystallization and the trans
isomer was recycled in subsequent runs. Removal of the
benzoxycarbonyl group by catalytic hydrogenation of
of 4 (see Ref. 11) indicated that although most rota-
tions are free at 70°C, some rotations remain hindered
even at 130°C in DMSO-d6 as evidenced by a still
changing spectrum. However, thermal decomposition
of the benzylidene acetals prohibited even higher tem-

T. Wharton, L. J. Wilson / Tetrahedron Letters 43 (2002) 561–564
563
1
3
perature experiments. An interpretable
C NMR
References
spectrum (500 MHz) of 4 was achieved at 70°C with
the expected 14 signals. However, the 500 MHz
spectrum of 5 (0.05 M in pyridine-d ) at 70°C was
still very broadened and the S/N ratio of most of the
signals was too small to interpret even after 17 h of
data collection. This indicates that some of the rota-
tions of the addend (4) are significantly more hin-
dered on addition of the malonodiamide to C .
1
1
13
C
1. Haavaldsen, J.; Nordal, V.; Kelly, M. Acta. Pharm. Suec.
1983, 20, 219–232; German Offen. 2,727,196, 1977.
2. Yu, S.-B.; Watson, A. D. Chem. Rev. 1999, 99, 2353–
2377.
3. (a) Yu, S.-B.; Droege, M.; Downey, S.; Segal, B.; New-
comb, W.; Sanderson, T.; Crofts, S.; Suravajjala, S.;
Bacon, E.; Earley, W.; Delecki, D.; Watson, A. D. Inorg.
Chem. 2001, 40, 1576–1581; (b) Yu, S.-B.; Droege, M.;
Segal, B.; Kim, S.-H.; Sanderson, T.; Watson, A. D.
Inorg. Chem. 2000, 39, 1325–1328.
5
6
0
Anilide and substituted anilide malonodiamide deriva-
tives of C60 have been reported to be paramagnetic.
14
However, solid-state Evan’s balance measurements
indicated that 5 is essentially diamagnetic, ruling out
paramagnetism as a major source of signal broaden-
ing in the NMR spectra. In the case of 5, it is likely
that the fully-substituted anilide, with it’s large iodine
atoms, is too sterically encumbered to allow a signifi-
cant charge-transfer type interaction between the
phenyl ring and the cage of C60 that are thought to
give rise to paramagnetism in related derivatives. In
fact, it is somewhat surprising that 4 reacts with C60
in such good yield, considering its significant steric
demands.
4
. Sovak. M.; Hoey, G. B.; Smith, K. R. In Handbook of
Experimental Pharmacology; Sovak, M., Ed. Radiocon-
trast Agents. Springer: Berlin, 1984; Vol. 73, pp. 1–125.
. Reviews: (a) Wilson, L. J. Interface; Medical applications
of fullerenes and metallofullerenes; 1999, 8, 24–28; (b)
Jensen, A. W.; Wilson, S. R.; Schuster, D. I. Bioorg.
Med. Chem.; Biological applications of fullerenes; 1996,
4, 767–779; (c) Da Rox, T.; Prato, M. Chem. Comm.
5
14
1999, 663–669.
6
. Moussa, F.; Pressac, M.; Hadchouel, M.; Arbeille, B.;
Chretien, P.; Trivin, F.; Ceolin, R.; Szwarc, H. In Proc.
Electrochem. Soc. Recent advances in the chemistry and
physics of fullerenes and related materials. 1997; Vol. 5,
pp. 332–336.
1
5
In vitro studies have shown that 5 provides X-ray
attenuation comparable to the commercial agent
Iohexol as expected based on its 30% iodine content.
However, a contrast agent that is based on C60 offers
potential advantages over conventional X-ray contrast
agents. For example, such fullerene-based agents
when used in vivo, could prove to be specific for liver
or kidney tissue and thus serve as tissue-specific imag-
ing agents. The versatility of the fullerene core allows
a ‘tuning’ of the properties of an X-ray contrast
agent by varying the type or degree of functionaliza-
tion. In addition, the larger, pseudo-spherical, molec-
ular volume of a C -based agent could increase
7. Pillai, K. M. R.; Diamantidis, G.; Duncan, L.; Ran-
ganathan, R. S. J. Org. Chem. 1994, 59, 1344–1350.
8. Pfeiffer, H.; Speck, U. US Patent 4,239,747, 1980; Chem.
Abstr., 88, 104953.
9
. Juaristi, E.; Diaz, F.; Cuellar, G.; Jimenez-Vazquez, H.
A. J. Org. Chem. 1997, 62, 4029–4035.
10. Buck, K. W.; Duxbury, J. M.; Foster, A. B.; Perry, A. R.;
Webber, J. M. Carbohydr. Res. 1966, 2, 122–131.
1
11. Spectral data: for 4; TLC R 0.54 in 1:1 THF:toluene; H
f
NMR (500 MHz, DMSO-d , 70°C) l (ppm, solvent std.)
6
3
.57 (s, 2H, -COCH CO-), 3.99 (distorted d, J=6.5 Hz,
2
60
4
4H, -NHCH(CH -) ), 4.22 (distorted q, J=8.4 Hz, 16H,
2 2
blood-pool retention time, and may be confined to
the vascular spaces, leading to a novel ‘blood-pool’
contrast agent.
-
CH(CH O-) ), 5.63 (s, 4H, PhCH(O-) ), 7.35 (m, 12H,
2
2
2
PhH), 7.50 (m, 8H, PhH), 7.98 (bs, 1H, NH), 8.7–9.3
13
(
bm, 3H, NH), 10.08 (s, 2H, NH); C NMR (500 MHz,
DMSO-d , 70°C) l (ppm, solvent std.) 41.95 (-CO-
6
In conclusion, the first X-ray contrast agent precursor
based on C60 has been synthesized and characterized.
Water solubilizing 5 with malonodiserinolamide moi-
CH CO-), 43.70 (-NHCH(CH -) , 68.45 (-CH(CH O-) ),
2
2
2
2
2
9
0.21, 98.10 (arom. CI), 100.40 (PhCH(O-) ), 126.14,
2
1
6
127.40, 128.23 (arom. CH), 138.27 (arom. CCH(O-)2),
eties,
followed by benzylidene acetal cleavage,
142.07 (arom. CNH-), 149.22 (arom. CCONH-), 164.55,
should lead to the first fullerene-based X-ray contrast
1
7
168.90 (CꢁO); FT-IR 1654 (s), 1103 (s); MALDI-TOF
agent for in vivo imaging. Efforts in this direction
are continuing.
−
MS calcd for C H N O I [M] 1829.8, found 1828.7
59 52
6
14 6
−
[
M−1] ; Anal. (C H N O I ) calcd: I, 41.60; C, 38.71;
59 52 6 14 6
H, 2.86; N, 4.59. Found: I, 41.86; C, 39.17; H, 3.34; N,
.41; UV umax 285; for 5, TLC R 0.75 1:1 THF:toluene;
4
f
1
H NMR (500 MHz, pyridine-d , 70°C) l (ppm, solvent
Acknowledgements
5
std.) 4.16–5.10 (bm, 20H, -NHCH(CH O-) , -NHCH-
2
2
(
CH O-) ), 5.78–6.05 (bs, 4H, PhCH(O-) ), 7.33–7.68,
2
2
2
The authors wish to thank Dr. Larry Alemany at
Rice University for his help with collection of the
NMR data and Dr. Ken C. Wright and Irene A.
Szwarc at the M.D. Anderson Cancer Center, Hous-
ton, Texas, for their assistance with the in vitro X-ray
experiments. This research was supported by The
Robert A. Welch Foundation (C-0627) and C Sixty,
Inc., Toronto, Canada.
7.68–7.96 (bm, 20H, PhH), 8.89 (1H, NH); MALDI-
−
TOF MS calcd for C119
2547.5; FT-IR 1654 (s), 1103 (s), 526 (m); UV umax (nm)
273, 328; Anal. (C119 ) calcd: I, 29.87; C, 56.07;
H N O I [M] 2547.8. Found:
50 6 14 6
H N O I
50 6 14 6
H, 1.98; N, 3.30; O, 8.79, found: I, 31.26; C, 54.76; H,
2.60; N, 3.05; O, 8.34 (O by subtraction).
12. Camps, X.; Hirsch, A. J. Chem. Soc., Perkin Trans. 1
1997, 1595–1596.

5
64
T. Wharton, L. J. Wilson / Tetrahedron Letters 43 (2002) 561–564
1
1
3. Bradamante, S.; Vittadini, G. Magn. Res. Chem. 1987, 25,
83–292.
4. Wharton, T.; Alford, J. M.; Husebo, L. O.; Wilson, L. J.
wavelength (0.087–8.5 nm) X-rays in a Faxitron 43855A
2
imager. The image was captured on standard Kodak
MIN-R 2000 film. The density of the film was determined
for each of the samples with a Noritsu Model 810 densit-
ometer. The solution of 5 showed X-ray attenuation
comparable to that of the standard.
In Proc. Electrochem. Soc. Functionalized Fullerenes.
2
000; Vol. 9, pp. 258–266.
15. In 1 mL syringes, a 0.197 M (150 mg I/mL) solution of 5
in 5:1 DMF:toluene, 4 standard aqueous solutions of
Omnipaque at concentrations of 300 mg I/mL, 225 mg
16. Wharton, T.; Kini, V. U.; Mortis, R. A.; Wilson, L. J.
Tetrahedron Lett. 2001, 42, 5159–5162.
®
I/mL, 150 mg I/mL, and 75 mg I/mL (diluted with saline),
and 2 samples containing the saline (0.9% NaCl) and the
DMF:toluene backgrounds were exposed to diagnostic
17. Wilson, L. J.; Wharton, T.; Sagman, U. ‘Fullerene(C60)-
Based X-ray Contrast Agent for Diagnostic Imaging,’
Provisional Patent application, November 10, 2000.