Fabrications of potential imaging probes based on a β-alkyl substituted porphyrin with a terpyridine external coordination site

Article abstract of DOI:10.1016/j.tetlet.2011.10.132

We report synthesis and coordination properties of β-alkyl porphyrin derivatives bearing terpyridylphenyl substituents at the meso-position and propionate side chains at the β-positions. Rhenium(I) carbonyl ion was encapsulated not in the core but in the

Full text of DOI:10.1016/j.tetlet.2011.10.132

Tetrahedron Letters 52 (2011) 7164–7167
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Fabrications of potential imaging probes based on a b-alkyl substituted
porphyrin with a terpyridine external coordination site
⇑
⇑
Masaaki Suzuki , Tomoya Uehara, Yasushi Arano, Tyuji Hoshino, Saburo Neya
Graduate School of Pharmaceutical Sciences, Chiba University, 1-33, Yayoi-cho, Inage-ku, Chiba 263-8522, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
We report synthesis and coordination properties of b-alkyl porphyrin derivatives bearing terpyridylphe-
nyl substituents at the meso-position and propionate side chains at the b-positions. Rhenium(I) carbonyl
ion was encapsulated not in the core but in the external terpyridyl ligand. Such porphyrin–terpyridine
hybrid porphyrins can be potentially available for less-invasive imaging like SPECT.
Ó 2011 Elsevier Ltd. All rights reserved.
Received 19 August 2011
Revised 17 October 2011
Accepted 21 October 2011
Available online 28 October 2011
Keywords:
Porphyrin
Terpyridine
Metal complex
Rhenium tricarbonyl
SPECT
Noninvasive imaging techniques are perfect tools for the diag-
nosis of many diseases, like cancers. SPECT (Single Photon Emission
Computed Tomography) is more easily available than PET (Positron
Emission Tomography) because, unlike PET, it does not require a
large facility such as a cyclotron. Technetium-99m (99mTc) is one
of the most desirable radionuclides for external imaging in diag-
nostic nuclear medicine, due to the emission of gamma ray of opti-
mal energy (140 keV), a suitable half-life (6 h), and availability
from a 99Mo-99mTc generator system.1 Porphyrin is widely used
as an anchor structure for cancers because of its highly tumor-
selective aggregation nature and is combined with some therapeu-
tic techniques.2 However, a porphyrin core can not coordinate
99mTc ion under ambient conditions within a half-life time of
99mTc.3 On the other hand, terpyridine (2,20:60,200-terpyridine, tpy)
is a common ligand for a variety of transition metals owing to its
acyclic tridentate coordination site. Furthermore, tpy metal com-
plexes also have high affinity for G-quadruplex so that tpy can
be potentially available for an anti-cancer strategy.4 When tpy is
introduced to the porphyrin periphery, the resulting porphyrin
derivative can convey transition metal ions to a specific site in
body such as cancer. There are already many kinds of cancer-tar-
geting porphyrins and most of them are fabricated from protopor-
phyrin IX or chlorophyll that is naturally-occurring porhyrins
having alkyl side chains at the b-positions.5 However, very limited
works of tpy-embedded b-alkyl substituted porphyrins were re-
ported.6 It is probably because basicity of pyridine may quench
the acid catalyst used in the reaction of pyridine-containing start-
ing materials. Actually, the yields of reported procedures were
quite low.6 Herein, we report the syntheses of porphyrin deriva-
tives bearing tpy at the peripheral positions and their out-to-in
stepwise hetero- and homo-bismetal coordination behavior.
At first, we tried to synthesize P2-type porphyrin6c by the reac-
tion of terpyridylbenzaldehyde7 4 with dipyrromethane8 3 by the
means of simple acid-catalyzed condensation methods. But several
attempts (temperature: 0 °C–reflux, microwave, Adler-method9)
resulted in the formation of mixtures of only tarry products. Propi-
onate side chains were indispensable because it would be essential
for future biological study. So, we moved to the synthesis of P1-
type porphyrin (Fig. 1) as a target compound in the next step.
We attempted the [2+2] condensation by the way of 5-terpyridyl-
phenyl-dipyrromethane (8, Scheme 1). To a solution of 4 and pyr-
role 5 in acetonitrile was added p-toluenesulfonic acid. After 5 h,
saturated aqueous solution of sodium bicarbonate was added and
the resulting mixture was extracted twice with chloroform, fol-
lowed by drying over anhydrous sodium sulfate. Recrystallization
from chloroform/methanol afforded terpyridine-appended dipyr-
romethane 6 in a 70% yield and the ester-groups were hydrolyzed
by treatment with strong base to give dicarboxylic acid 7. Formyl
groups were introduced at the a-positions by decarboxylation with
excess trifluoroacetic acid (TFA) and subsequent addition of tri-
ethyl ortho-formate in a one-pot protocol. b-Propionate substituted
dipyrromethane 2 was synthesized from 1-acetoxymethylpyrrole 1
according to the reported procedure.8 The
a-carboxylic groups of 3
were removed by TFA followed by addition of a solution of 8 in
chloroform and then the mixture was stirred for 3 h.
⇑
Corresponding authors. Tel./fax: +81 43 226 2935 (M.S).
E-mail address: m-suzuki@faculty.chiba-u.jp (M. Suzuki).
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.10.132

M. Suzuki et al. / Tetrahedron Letters 52 (2011) 7164–7167
7165
9: R = Me, M1 = H2, M2 = none
PR
N
N
10: R = Me, M1 = Zn, M2 = ZnCl2
N
N
N
N
1
R = H, M = Zn, M2 = ZnCl2
11:
12:
13:
M1
N
M2
1
R = Et, M = H2, M2 = none
R = Me, M = H2, M2 = ReBr(CO)3
1
PR
14: R = Me, M1 = Zn, M2 = ReBr(CO)3
(PR = CH2CH2CO2R)
P1-type
PMe
PMe
N
N
N
N
NH
N
N
N
N
HN
PMe
PMe
P2-type
Figure 1.
PMe
PMe
9
11
12
13
14
PMe
1.0
0.8
2 :R = Bn
(b)
(a)
AcO
NH HN
CO2Bn
N
H
3: R = H
0.6
x 5
RO2C
CO2R
1
(PMe = CH2CH2CO2Me)
0.4
0.2
0.0
N
N
N
N
N
N
300
400
500
600
700
wavelength / nm
(c)
+
Figure 2. UV–visible absorption spectra in CH2Cl2.
CO2Et
N
H
played the upfield-shifted signals due to the inner NH protons at
ꢀ3.28 and ꢀ3.16 ppm while the downfield-shifted signals due to
the peripheral protons (meso-protons at 10.19 and 9.80 ppm in a
2:1 ratio, those of b-alkyl substituents between 2.52 and 4.42 ppm,
Figure S1, Supplementary data). The signals due to terpyridylphenyl
group appeared between 7.43 and 9.07 ppm. These results indicated
the existence of aromatic ring-current effect of porphyrin, appar-
ently showing that one terpyridylphenyl group was successfully
introduced at the meso-position of a porphyrin ring.
5
CHO
NH HN
4
R
R
6: R = CO2Et
7: R = CO2H
(d)
(e)
8:
R = CHO
Since hydrolysis of the propionate groups of 9 under basic aque-
ous conditions was not successful because of the poor solubility of 9,
zinc ion was inserted into the porphyrin core in order to improve the
solubility. Addition of zinc chloride into a solution of chloroform and
methanol mixed solvent gave biszinc complex 10 quantitatively. In
the 1H NMR spectrum, signals due to the inner NH protons disap-
peared and the peak pattern of the tpy group also changed by coor-
dination of zinc ion. In the UV/vis spectrum, Soret-band was
observed at 408 nm and Q-bands were observed at 536 and
572 nm along with the band derived from tpy complex at 327 and
336 nm (Fig. 2), suggesting formation of zinc–porphyrin along with
zinc–terpyridine.4 10 was dissolved in tetrahydrofuran and refluxed
in the presence of aqueous sodium hydroxide, giving probably dee-
sterificated product 11, which was precipitated by the addition of
acetic acid and filtered. Although further characterization of 11
was not possible because of the very poor solubility for any common
solvents, subsequent acid catalyzed ethylation of the propionic acid
chains in ethanol was successfully carried out, affording demetallat-
ed diethyl ester 12. Positive mode ESI-TOF mass spectrum demon-
strated a parent ion peak at m/z = 902.4340 corresponding to that
of 12 (calcd for C57H56N7O7 = 902.4388: [M+H]+). This reaction
seems to be an efficient way for future control of solubility.
Scheme 1. (a) AcOH, H2O (b) H2, Pd/C, THF (c) p-TsOH, MeCN (d) NaOHaq, THF,
MeOH (e) TFA, then CH(OEt)3.
Usually, porphyrin synthesis was quenched at the same time by
oxidation with such as 2,3-dichloro-5,6-dicyano-1,4-benzoquinone
(DDQ) or chloranil, ferric chloride and so on. In this case, however,
any porphyrin-related products were not obtained only by the addi-
tion of an oxidant. So, we employed a neutralizing process with
triethylamine (TEA) before the oxidation step (Scheme 2). When
the reaction was monitored by TLC, UV irradiation (365 nm) illumi-
nated a red fluorescent spot on the TLC plate, indicating formation of
a porphyrinic product. Silica gel column chromatography was car-
ried out with 3% methanol in chloroform as an eluent and subse-
quent recrystallization from chloroform/methanol gave purple
crystalline solids in an 8.3% yield and they were identified as the tar-
get porphyrin 9. UV/vis absorption spectrum of 9 in dichlorometh-
ane showed Soret-band at 405 nm and Q-bands at 503, 536, 572,
and 625 nm (Fig. 2). High-resolution positive mode ESI-TOF mass
spectrum of 9 exhibited a parent ion peak at m/z = 874.3962 and
896.3973
C
(calcd
for
C
55H52N7O4 = 874.4075:
[M+H]+;
55H51N7NaO4 = 896.3895: [M+Na]+). 1H NMR spectrum of 9 dis-

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M. Suzuki et al. / Tetrahedron Letters 52 (2011) 7164–7167
Figure 3. Crystal structure of 12. Thermal ellipsoids are set at the 50% probability level.
CO2H OHC
PMe
PMe
N
N
N
N
N
NH
N
NH
NH
HN
HN
1) TFA
+
N
N
2) NEt3, DDQ
HN
PMe
PMe
CO2H OHC
3
8
9
Scheme 2.
Luckily, a nice single crystal of 12 for X-ray diffraction analysis
was obtained by vapor diffusion method of methanol into dichloro-
methane solution of 12.10 Figure 3 shows a crystal structure of 12.
The mean-plane deviation of the core 24 atoms (C20N4) was
0.038 Å indicating that the planar structure of its porphyrin ring
was not altered by the steric bulkiness of meso-terpyridylphenyl
substituent. The phenyl linker adopted almost perpendicular
geometry against the porphyrin plane (84°) while the dihedral an-
gle between the terpyridyl group and the phenyl group was 28°.
In the next step, metallation behavior of 9 was investigated. Be-
fore using radioisotopes, we have to make sure that 9 can chelate
stable isotopes among some metals used in SPECT such as 99mTc.
Generally, it is known that rhenium ion shows similar chemical
properties to technetium ion.1 So the Re(I) species was investigated
instead of 99mTc(I). As expected, Re+ could not be encapsulated in a
porphyrin core under ambient conditions but could be coordinated
by tpy ligand, when bromotricarbonyl(tetrahydrofuran)rhenium(I)
dimer ([ReBr(CO)3(C4H8O)]2) was added to the solution of 9 in chlo-
roform/methanol mixed solvent at room temperature.4b This
behavior was firstly characterized by positive mode ESI-TOF mass
spectrum, which exhibited a parent ion peak at m/z = 1144.3346
(calcd for C58H51N7O7Re = 1144.3407: [MꢀBr]+, Figure S2, Supple-
mentary data), indicating formation of 13. When 13 was treated
with methanol solution of zinc acetate, the solution color immedi-
ately changed from brown to pink that was very similar to that of 10
and positive mode ESI-TOF mass spectrum exhibited a parent ion
peak at m/z = 1206.2497 (calcd for C58H49N7O7ReZn = 1206.2542:
[MꢀBr]+, Figure S3, Supplementary data). In 1H NMR spectra of 13
and 14, signals due to the inner NH protons of 13 disappeared by
insertion of the zinc ion into the inner cavity of 13, revealing that
the rhenium ion at the tpy coordination site was not replaced by
the zinc ion under such conditions.
tually regarded as tpy-99mTc(I) complex. Furthermore, a variety of
tpy-99mTc-appended metaloporphyrins can be prepared quickly un-
der mild conditions because of the high affinity of porphyrin core for
many kinds of metal ions, so that porphyrin-related SPECT probes
may be combined to other therapeutic protocols like PDT (Photody-
namic Therapy) or MRI (Magnetic Resonance Imaging).
Acknowledgments
This work was supported by the Special Funds for Education
and Research (Development of SPECT Probes for Pharmaceutical
Innovation) from the Ministry of Education, Culture, Sports, Sci-
ence and Technology, Japan.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2011.10.132.
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In summary, we synthesized b-alkyl substituted porphyrinderiv-
atives bearing a terpyridine substituent at the meso-position. These
products demonstrated stepwise coordination properties depend-
ing on metal species so that it formed free base or zinc porphyrins
with an embedded tpy-Re(I) complex side structure, which was vir-

M. Suzuki et al. / Tetrahedron Letters 52 (2011) 7164–7167
7167
9. Lindsey, J. S. In The Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard, R.,
Dcalcd. = 1.347 g/cm3, T = ꢀ100 °C, R1 = 0.0829 (I > 2
r
(I)), RW = 0.2273 (all
Eds.; Academic: New York, 2000; Vol. 1, pp 49–55. Chapter 2.
data), GOF = 1.013, CCDC-840293. These data can be obtained free of charge
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data_request/cif.
ꢀ
10. Crystal data for 12: C57H55N7O4ꢁ(CH2Cl2)ꢁ(O) = 1003, triclinic, space group p1
(No. 2), a = 12.9609 (8) Å, b = 14.3426 (9) Å, c = 16.2138 (10) Å,
a = 64.5600
(10)°, b = 67.2800 (10)°,
c
= 88.6390 (10)°, V = 2473.7(3) Å3, Z = 2,