Synthesis of a carotenobenzoporphyrin from a meso-diphenylporphyrin

Article abstract of DOI:10.1016/S0040-4039(00)01740-8

A carotenobenzoporphyrin has been prepared by covalently joining a benzoporphyrin derivative and an anilinocarotenoid by an amide linkage. A synthetic meso-diphenylporphyrin is the precursor of the benzoporphyrin moiety. The carotenobenzoporphyrin is highly fluorescent and does not sensitize measurable amounts of singlet oxygen. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)01740-8

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 9661–9665
Synthesis of a carotenobenzoporphyrin from a
meso-diphenylporphyrin†
Paul A. Liddell, Xristo Za´rate, Ana L. Moore,* Thomas A. Moore and Devens Gust
Department of Chemistry and Biochemistry and Center for the Study of Early Events in Photosynthesis,
Arizona State University, Tempe, AZ 85287-1604, USA
Received 25 August 2000; revised 2 October 2000; accepted 3 October 2000
Abstract
A carotenobenzoporphyrin has been prepared by covalently joining a benzoporphyrin derivative and an
anilinocarotenoid by an amide linkage. A synthetic meso-diphenylporphyrin is the precursor of the
benzoporphyrin moiety. The carotenobenzoporphyrin is highly fluorescent and does not sensitize measur-
able amounts of singlet oxygen. © 2000 Elsevier Science Ltd. All rights reserved.
Carotenoporphyrins have been synthesized as prototypes of imaging agents for the detection
of malignancy.1–3 The carotenoid moiety functions to suppress the phototoxicity associated with
porphyrin-based compounds by eliminating the formation of singlet oxygen, the principal agent
responsible for the phototoxic effects of porphyrins. Singlet oxygen is produced by energy
transfer from the porphyrin triplet species to ground state oxygen in a bimolecular process. The
role of the carotenoid is to interdict this bimolecular process by rapidly quenching the porphyrin
triplet species. Rapid quenching requires close proximity of the carotenoid moiety to the
porphyrin and effective electronic coupling between the chromophores to promote extremely
rapid energy transfer from the porphyrin triplet state to the carotenoid.4 Quenching the
porphyrin triplet state to <200 ns renders it kinetically incapable of singlet oxygen sensitization
at physiological concentrations of O2 (ꢀ10−3 M). On the other hand, the electronic coupling
provided by the linkage between the carotenoid and the porphyrin necessary for singlet oxygen
suppression often results in quenching of the porphyrin fluorescence. This is an undesirable
property for a fluorescence-based imaging agent. Fortunately, in carotenobenzoporphyrin 1 the
carotenoid does not excessively quench the fluorescence of the porphyrin because the linkage
that joins both chromophores limits the magnitude of their electronic interaction.
* Corresponding author.
†
This paper is dedicated to Professor Harry H. Wasserman on the occasion of his 80th birthday.
0040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)01740-8

9662
Carotenobenzoporphyrin 1 is highly fluorescent, approximately 1.3 times more fluorescent
than tetrakis-5,10,15,20-(4-methylphenyl)porphyrin (TTP), does not sensitize measurable
amounts of singlet oxygen at typical physiological O2 concentrations and because of the
extended chromophore of the benzoporphyrin moiety, its absorption is red shifted compared to
simple octaalkyl porphyrins.5
Diels–Alder reactions have been frequently employed to extend the chromophore of por-
phyrins. The vinyl group of compounds such as protoporphyrin IX in conjunction with the ring
A b,b-double bond mimics a diene unit which undergoes the cycloaddition reaction with a
number of dienophiles. Benzoporphyrin derivatives (BPDs) are examples of compounds
obtained by this [4+2] cycloaddition reaction which are presently being used in a number of
biomedical applications including the treatment of cancer by photodynamic therapy (PDT).6 A
synthetic problem associated with BPDs prepared from protoporphyrin IX and acetylenedicar-
boxylate is the isolation of the desired compound from a complex mixture. To avoid these
problems several laboratories have used other naturally occurring porphyrins instead of proto-
porphyrin IX.7–9 In the present report we describe the synthesis of a BPD from a synthetic
meso-diphenylporphyrin (6).
The synthesis of porphyrin 6, requisite for the preparation of 1, started with the coupling of
two dipyrromethanes 3, obtained by catalytic hydrogenation of 2, and 4 (Scheme 1). The
coupling reaction to form porphyrin 5 was effected in a solvent mixture of methanol and
dichloromethane (CH2Cl2) with an excess of p-toluenesulfonic acid, at room temperature. The
reaction mixture was treated with 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ), and after
column chromatography porphyrin 5 was isolated in 54% yield. Dehydrohalogenation to
introduce the vinyl group at the b-position of the macrocycle was carried out by treatment of
5 with 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) in dimethylformamide at 120°C (81% yield).
Four other steps were followed to complete the synthesis of 6. They consisted of base hydrolysis
of the methyl ester, metallation of the porphyrin with zinc acetate, esterification of the free acid
with benzyl bromide and lithium carbonate, and removal of the central metal by acid treatment.
Quantitative yields were obtained in each of these steps.
The fused six-membered ring of porphyrin 7 was built by the standard Diels–Alder reaction
between 6 and dimethyl acetylenedicarboxylate (DMAD).6 The reaction was carried out in a
sealed tube, in toluene at 110°C for 4 days. At this point, TLC indicated that although starting
material was still present, the major component was the more polar Diels–Alder adduct. The
solvent and unreacted dimethyl acetylenedicarboxylate were removed under vacuum and the
crude residue was redissolved in CH2Cl2. The solution was treated with an excess of DBU to
isomerize the initial adduct of the cycloaddition reaction to the chlorin structure of 7. After
stirring for 2 h, TLC showed that only a single major product had formed. After column
chromatography on silica gel (CH2Cl2 containing 2–2.5% acetone) pure benzoporphyrin 7 was
obtained in 63% yield.
For the preparation of 1 the carboxylic acid masked by the benzylic ester of 7 had to be
deprotected. The deprotection was effected by catalytic hydrogenation of zinc benzoporphyrin 7
dissolved in tetrahydrofuran with 10% palladium on carbon at a hydrogen pressure of ꢀ1.1
atm. After 2 h TLC showed that all the starting material was converted to a much more polar
compound. The catalyst was removed by filtration (Celite) and the CH2Cl2 solution of the
monoacid zinc-benzoporphyrin was shaken vigorously with 1 M hydrochloric acid to remove the
zinc metal from the macrocycle.

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Scheme 1.
The coupling reaction of the anilinocarotenoid (7%-apo-7%-(4-aminophenyl)-b-carotene)10 with
the benzoporphyrin monoacid acid was carried out with 2-chloro-4,6-dimethoxy-1,3,5-triazine
(CDMT) and N-methylmorpholine (NMM) in CH2Cl2. The solution of the benzoporphyrin
monoacid was cooled to 0°C and a slight excess of the triazine derivative was added. The
reaction was allowed to proceed at 0°C under a nitrogen atmosphere for 15 min and then at
room temperature for 4 h. TLC of the reaction mixture indicated that most of the benzopor-

9664
phyrin acid had been converted to the reactive intermediate. To the reaction mixture were added
an equimolar amount of anilinocarotenoid and two equivalents of 4-dimethylaminopyridine
(DMAP). After stirring the reaction mixture for 3 days, TLC indicated that the only major
product was the desired carotenobenzoporphyrin. The reaction mixture was applied to a silica
gel column and eluted with CH2Cl2 containing 2–3.5% acetone. A final recrystallization from
CH2Cl2/methanol gave pure carotenobenzoporphyrin 1 in 70% yield.11
Briefly summarizing the photophysics of compounds 1 and the trimethyl ester of 7 (8), their
fluorescence quantum yields have been determined in toluene using steady state measurements
by the comparative method with TTP as the standard. The fluorescence quantum yield of 1 is
0.14 while that of model benzoporphyrin 8 is 0.18. In agreement with the steady-state
fluorescence data, the fluorescence lifetime of 1 is 4.6 ns in toluene and that of 8 is 6 ns in the
same solvent. In 1, the rate constant for triplet energy transfer from the porphyrin to the
carotenoid measured by the rise time of the carotenoid triplet is 5.6×106 s−1.
The amide linkage used to connect the benzoporphyrin and the carotenoid chromophores
allows partial conjugation between the two. However in 1, the attachment of the carotenoid is
through an amide linkage at the meso-aromatic position of the macrocycle. For steric reasons,
the presence of the b-pyrrolic ethyl groups of the rings flanking the meso-aromatic substituent
limit the p–p overlap between the benzoporphyrin (fluorophore) and the carotenoid. This
structural arrangement serves to isolate to some extent the chromophores and consequently
reduces the fluorescence quenching by the nearby carotenoid. As a result, carotenobenzopor-
phyrin 1 is approximately three times more fluorescent than a related carotenopurpurin bearing
the same amide linkage but lacking the conformationally restricted meso-aromatic spacer.12
Acknowledgements
This work was supported by the US Department of Energy (DE-FG03-93ER14404). It is
publication 438 from the ASU Center for the Study of Early Events in Photosynthesis. The mass
spectra data was obtained at the Nebraska Center for Mass Spectrometry.
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L. J.; DeGraziano, J. M.; Gouni, I. J. Am. Chem. Soc. 1992, 114, 3590–3603.
5. The red-most absorption band of 1 is at 684 nm while the red-most band of 6 is at 632 nm in CH2Cl2. The
fluorescence spectrum maxima of 1 are at 688 and 771 nm in toluene.
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9665
11. Selected spectroscopic data for 1. NMR (500 MHz, CDCl3) l: −1.54 (1H, s, ꢀNH), −0.64 (1H, brs, ꢀNH), 1.03
(6H, s, Car 16-CH3, Car 17-CH3), 1.04 (3H, t, J=8 Hz, ꢀCH3), 1.11 (3H, t, J=8 Hz, ꢀCH3), 1.16 (3H, t, J=8
Hz, ꢀCH3), 1.46–1.48 (2H, m, Car 2-CH2ꢀ), 1.61–1.63 (2H, m, Car 3-CH2ꢀ), 1.73 (3H, s, Car 18-CH3), 1.78 (3H,
s, ꢀCH3), 1.98 (3H, s, Car 19-CH3), 1.99 (3H, s, Car 20-CH3), 2.01 (3H, s, 20%-CH3), 2.01–2.03 (2H, m, Car
4-CH2ꢀ), 2.09 (3H, s, Car 19%-CH3), 2.46–2.50 (2H, m, ꢀCH2ꢀ), 2.66–2.70 (2H, m, ꢀCH2ꢀ), 2.81 (1H, m, ꢀCH2ꢀ),
2.94–2.98 (1H, m, ꢀCH2ꢀ), 2.97 (3H, s, ꢀCO2CH3), 3.32 (3H, s, ꢀCH3), 3.38 (3H, s, ꢀCH3), 3.47 (3H, s, ꢀCH3),
3.90 (3H, s, ꢀCO2CH3), 4.07 (3H, s, OꢀCH3), 5.03 (1H, s, ꢀCHꢀCO2CH3), 5.38 (1H, d, J=6 Hz, vinyl-H),
6.12–6.94 (14H, m, Car vinyl-H), 7.18 (1H, dd, J=9 and 3 Hz, Ar-H), 7.29 (1H, dd, J=9 and 3 Hz, Ar-H), 7.41
(1H, d, J=6 Hz, vinyl-H), 7.52 (2H, d, J=8 Hz, Car Ar-H), 7.71 (1H, dd, J=8 and 2 Hz, Ar-H), 7.79 (2H, d,
J=8 Hz, Car Ar-H), 8.01 (1H, dd, J=8 and 2 Hz, Ar-H), 8.06 (1H, d, J=8 Hz, Ar-H), 8.09 (1H, d, J=8 Hz,
Ar-H), 8.19 (2H, m, Ar-H and ꢀNH), 8.29 (1H, d, J=8 Hz, Ar-H), 9.00 (1H, s, meso-H), 9.88 (1H, s, meso-H);
HRFAB-MS m/z calcd for C89H97N5O6: 1331.7438; found: (M+H)+ 1332.7461; UV–vis (CH2Cl2) umax nm (m M−1
cm−1): 362 (96,598), 470 (203,487), 509 (125,618), 585 (19,026), 625 (11,978), 684 (35,066).
12. Unpublished data.
.