Design and synthesis of C60-benzenesulfonamide conjugates

Article abstract of DOI:10.1016/j.tetlet.2010.05.017

Synthesis of C₆₀-benzenesulfonamide conjugates is reported. The strategies for improving their water solubility, as required for binding to human carbonic anhydrase II, are discussed.

Full text of DOI:10.1016/j.tetlet.2010.05.017

Tetrahedron Letters 51 (2010) 3645–3648
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Design and synthesis of C60–benzenesulfonamide conjugates
*
Tatiana Y. Zakharian, David W. Christianson
Roy and Diana Vagelos Laboratories, Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104-6323, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 13 January 2010
Revised 2 May 2010
Accepted 7 May 2010
Available online 12 May 2010
Synthesis of C60–benzenesulfonamide conjugates is reported. The strategies for improving their water
solubility, as required for binding to human carbonic anhydrase II, are discussed.
Ó 2010 Elsevier Ltd. All rights reserved.
Since its discovery in 1985,1 the unique physical properties and
chemical reactivity of C60 (fullerene) have led to the exploration of
its utility in myriad applications ranging from enzyme inhibitors2 to
chemical sensors3 to superconductivity.4 To date, the use of C60 for
biological and medicinal purposes has largely been limited by its poor
aqueous solubility, but the development of soluble host-guest encap-
sulation complexes5,6 and covalent derivatization of the C60 scaffold-
ing itself with polar groups7 have facilitated the preparation of
aqueous C60 solutions. Importantly, the substituents used to cova-
lently modify C60 not only influence aqueous solubility but also influ-
ence and direct biological activity.8 Accordingly, C60 derivatives have
been utilized in numerous biological applications, for example, as
neuroprotective agents,9 photosensitizers for photodynamic ther-
apy,10 MRI contrast agents,11 and transfection agents.12
A number of C60 derivatives have been reported to inhibit phar-
maceutically important enzymes such as HIV-1 protease13 and
NH2
O
S
O
C
O
OH
b
O
NH2
S
O
NH2
O
O
S
O
H
N
N
Cl
O
a
C60, glycine
paraformaldehyde
c
1
2
Scheme 1. Reagents: (a) toluene, reflux; (b) SOCl2; (c) pyridine.
* Corresponding author. Tel.: +1 215 898 5714; fax: +1 215 573 2201.
E-mail address: chris@sas.upenn.edu (D.W. Christianson).
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.05.017

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T. Y. Zakharian, D. W. Christianson / Tetrahedron Letters 51 (2010) 3645–3648
NH2
O
S
O
CN
O
b
NH2
S
O
NH2
O
S
O
H
N
N
H
O
a
C60, glycine
paraformaldehyde
c
1
3
Scheme 2. Reagents and conditions: (a) toluene, reflux; (b) Raney Ni/HCOOH; (c) NaBH(OAc)3, AcOH.
acetylcholinesterase.14 In these examples, C60 is used as a scaffold
to which appropriate functional groups are attached to make spe-
cific noncovalent interactions in enzyme active sites based on
molecular modeling studies. Remarkably, while molecular models
of such protein–fullerene complexes have been described,13,15 an
X-ray crystal structure determination of a protein–fullerene com-
plex has never been reported.
With an eye toward the ultimate crystal structure determina-
tion of a protein–fullerene complex, we now describe the synthesis
of C60–benzenesulfonamide conjugates designed to bind to human
carbonic anhydrase II (CAII). This zinc metalloenzyme serves as a
useful model system for studying protein–ligand interactions.16
Importantly, CAII contains a cone-shaped active site capable of
accommodating C60 tethered to a benzenesulfonamide moiety that
confers specificity of binding to the Zn2+ ion at the base of the ac-
tive site; thus, the conjugation of C60 to the benzenesulfonamide
moiety is expected to anchor C60 in the enzyme active site. The
preparation of C60–benzenesulfonamide conjugates described be-
low represents a series of new chemical entities in the functional-
the aqueous solubility of C60–benzenesulfonamide conjugates, we
explored further derivatization of 7 with polar groups.
Direct derivatization of 7 with the protected water-solubilizing
groups, generated by the reaction of malonyl chloride with tert-bu-
tyl 3-hydroxypropionate, was attempted under Bingel conditions23
but there was no evidence for product formation. To improve the
solubility of compound 7 in the nonpolar solvents commonly used
for Bingel reactions, the sulfonamide was protected with 2,4-dime-
thoxybenzyl (DMB) groups.
The new sequence began with compound 10 which itself was
available in 3 steps from 2,4-dimethoxybenzylamine and 2,4-
dimethoxybenzaldehyde (Scheme 5).
Compound 10 was derivatized with the protected water-solubi-
lizing groups under Bingel conditions to form a mixture of regioi-
somers (Scheme 6). The sulfonamide and the carboxylic acid-
protecting groups were then removed under acidic conditions to
yield 11.
Unfortunately, the solubility of compound 11 in DMSO/water
mixtures was only slightly improved in comparison with that of
compound 7. At this stage it became evident that addition of multi-
ple water-solubilizing groups would be desirable. Our efforts along
these lines will be reported in due course.
ization of C60
.
The first C60–sulfonamide conjugate (compound 2) was ob-
tained by acylation of the previously reported fulleropyrrolidine
117 (Scheme 1), which was handled in dilute solutions to prevent
polymerization.
An alternative C60–benzenesulfonamide conjugate (2) was pre-
pared by reductive amination of 4-formylbenzenesulfonamide18
with fulleropyrrolidine 1 (Scheme 2). Reductive amination using
NaBH(OAc)3 has been previously reported as an excellent method
to generate N-alkylated fulleropyrrolidines.19
O
NH2
S
O
H
NH3 TFA
N
O
A C60–benzenesulfonamide conjugate with a longer linker be-
tween C60 and the benzenesulfonamide moiety (compound 5)
was prepared by the acylation of aminofullerene 420 with 4-amin-
osulfonylbenzoyl chloride21 (Scheme 3).
N
N
Treatment of aminofullerene 622 with 4-aminosulfonylbenzoyl
chloride (Scheme 4) gave a conjugate with a different angle of
attachment between the benzenesulfonamide moiety and C60
.
The C60–benzenesulfonamide conjugate 3 was insoluble in
water-miscible solvents. While all other derivatives 2, 5, and 7
were soluble in dimethylsulfoxide (DMSO) and dimethylformam-
ide (DMF), our attempts to produce stable aqueous solutions with
organic cosolvent content <50% (vol/vol) failed. Thus, to improve
4
5
Scheme 3. Reagents and conditions: 4-aminosulfonylbenzoyl chloride, triethyl-
amine (TEA).

T. Y. Zakharian, D. W. Christianson / Tetrahedron Letters 51 (2010) 3645–3648
3647
O
O
O
O
O
O
O
O
NH3
O
O
N
H
S
TFA
NH2
O
6
7
Scheme 4. Reagents and conditions: 4-aminosulfonylbenzoyl chloride, TEA.
O
NH2
O
O
O
N
H
O
a
O
O
8
O
O
b
O
O
O
O
O
O
O
O
N
H
S
N(DMB)2
N
S
O
O
O
O
O
c
O
OH
10
9
Scheme 5. Reagents and conditions: (a) NaBH(OAc)3; (b) 4-(chloro-sulfonyl)benzoic acid, TEA; (c) 2-ethoxy-1-ethoxycarbonyl-1,2-dihydroquinoline (EEDQ), TEA,
aminofullerene 6.
O
O
O
O
O
O
O
O
O
O
N
O
O
N
H
H
S
S
N(DMB)2
NH2
O
O
a, b
HO
O
O
O
O
10
O
OH
O
11
Scheme 6. Reagents and conditions: (a) 1,3-bis[3-(1,1-dimethylethoxy)-3-oxopropyl]propanedioic acid ester, I2, 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU); (b) trifluoro-
acetic acid (TFA)/CH2Cl2.
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Acknowledgment
We thank the American Asthma Foundation for supporting this
research.
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