Synthetic zinc tetrapyrroles complexing with pyridine as a single axial ligand

Article abstract of DOI:10.1016/S0968-0896(98)00154-0

Zinc chlorins were prepared from chlorophyll-a. Visible spectra in benzene showed that synthetic zinc chlorins complexed with pyridine as an axial ligand to form the monopyridine adducts. The equilibrium constants for the complexation were dependent upon the chlorin structure: substitution of electron-withdrawing groups at the peripheral position enhanced the coordinated ability of the central zinc. ¹H NMR spectra in benzene-d₆ also indicated that single pyridine coordinated to the central zinc. Comparison of the equilibrium constant in a zinc chlorin with those of the corresponding zinc bacteriochlorin (7,8-dihydrochlorin) and porphyrin (17,18-dedihydrochlorin) led to an increase in the saturation and flexibility of the tetrapyrrole π-plane ligands making the central zinc more axial-ligated. All the zinc tetrapyrroles in benzene complexed with pyridine to form 5-coordinated (1:1) complexes, not 6-coordinated bis-adducts. The observed equilibrium constants were consistent with the energy changes of the complexation calculated from molecular modeling. Copyright (C) 1997 Elsevier Science Ltd.

Full text of DOI:10.1016/S0968-0896(98)00154-0

BIOORGANIC &
MEDICINAL
CHEMISTRY
Bioorganic & Medicinal Chemistry 6 (1998) 2171±2178
Synthetic Zinc Tetrapyrroles Complexing with Pyridine as a
Single Axial Ligand
Hitoshi Tamiaki,* Shiki Yagai and Tomohiro Miyatake
Department of Bioscience and Biotechnology, Faculty of Science and Engineering, Ritsumeikan University, Kusatsu, Shiga 525-8577, Japan
Received 25 June 1998; accepted 24 July 1998
AbstractÐZinc chlorins were prepared from chlorophyll-a. Visible spectra in benzene showed that synthetic zinc
chlorins complexed with pyridine as an axial ligand to form the monopyridine adducts. The equilibrium constants for
the complexation were dependent upon the chlorin structure: substitution of electron-withdrawing groups at the pe-
ripheral position enhanced the coordinated ability of the central zinc. 1H NMR spectra in benzene-d6 also indicated that
single pyridine coordinated to the central zinc. Comparison of the equilibrium constant in a zinc chlorin with those of
the corresponding zinc bacteriochlorin (7,8-dihydrochlorin) and porphyrin (17,18-dedihydrochlorin) led to an increase
in the saturation and ¯exibility of the tetrapyrrole p-plane ligands making the central zinc more axial-ligated. All the
zinc tetrapyrroles in benzene complexed with pyridine to form 5-coordinated (1:1) complexes, not 6-coordinated bis-
adducts. The observed equilibrium constants were consistent with the energy changes of the complexation calculated
from molecular modeling. # 1998 Elsevier Science Ltd. All rights reserved.
Introduction
tetrahydrofuran.2 However, no unequivocal evidence is
available for the presence of 4-coordinated Mg in
(B)Chls. Therefore, the complexation of an axial-ligand
free (B)Chl with a ligand was not directly observed and
the coordinated ability of the central Mg could only be
estimated from the exchange reaction, L1-MgN4+L2!
L2-MgN4+L1.3
In a photosynthetic apparatus, (bacterio)chlorophylls
(=(B)Chls) interact with peptides to make a variety of
complexes which play important roles including light-
harvesting and energy/electron transfer.1 One of the
major interactions is complexation of the central mag-
nesium with the imidazole moiety in histidine of pep-
tides as an axial ligand. The relationship between the
coordinated abilities and the naturally occurring tetra-
pyrrole ligands is of interest both chemically and bio-
logically but no report of work is available to our
knowledge.
On the other hand, synthetic zinc complexes prepared
by replacement of the central Mg in (B)Chls with Zn,
zinc (bacterio)pheophytins (=Zn-(B)Phes) are good
structural and functional models for naturally occurring
(B)Chls.4,5 Recently, it was found that Zn-BPhe-a is
used instead of BChl-a in a special photosynthetic bac-
terium.6 In zinc porphyrins, both the 4- and 5-coordi-
nated zincs (ZnN4 and L-ZnN4) were isolated and
direct measurements of the coordinated ability were
made, ZnN4 ‡ L ! L ZnN4.7,8 Moreover, a few 6-
coordinated zinc porphyrins (L1L2-MgN4) were pro-
posed in the solid state from the crystallographic ana-
lyses,9 but no experimental evidence was submitted for
the presence of 6-coordinated zinc in a solution.
In natural systems, magnesium of (B)Chls is found to be
5-coordinated, L-MgN4, from vibrational spectral and
X-ray crystallographic analyses.1 In vitro, (B)Chls are
usually coordinated with one axial ligand to give 5-
coordinated Mg; 6-coordinated Mg was also observed
in highly coordinatable solvents such as pyridine and
Key words: Complexation; NMR; porphyrins and analogues;
substituent e€ects.
Here we report on synthesis of zinc pheophytin analogues
and the relationship between the coordinated abilities and
*Corresponding author. Tel.: 0081 77 566 1111; fax: 81 77 561
2659; e-mail: tamiaki@se.ritsumei.ac.jp
0968-0896/98/$ - see front matter # 1998 Elsevier Science Ltd. All rights reserved
PII: S0968-0896(98)00154-0

2172
H. Tamiaki et al./Bioorg. Med. Chem. 6 (1998) 2171±2178
the tetrapyrrole ligands, which showed that the central
zinc was coordinated with pyridine to form 5-coordi-
nated (1:1) complexes, not 6-coordinated bis-adducts, in
benzene solution.
The values of the ratio (a) of the complexation
were determined from the change of the absorbance
(Abs) at any wavelength: a=(Abs Absinitial)/(Abs®nal
Absinitial),8 at ‰PyŠ
ˆ 0 mM and [Py] ®nal>200 mM.
initial
The values of K were calculated for all concentrations of
added pyridine at any wavelength. Use of n=1 a€orded
almost the same estimated K-value for each concentra-
tion (the error was within 10%), while use of n=2 gave
large deviation for the K-values. The results strongly
supported that all the synthetic zinc chlorins 1±4 were
bonded with a single pyridine in the solution to form 1:1
coordinated complexes. The K-values at several wave-
lengths and the average K are listed in Table 1. These
data show that addition of oxo group to the chlorin
ligand enhanced the equilibrium constant. The 3/13-keto
carbonyl group(s) withdrew the electron on the central
zinc through the chlorin p-plane to produce more Lewis
acidic and axial-ligated Zn. These substituents on the
peripheral position a€ected the coordinated ability of the
central zinc. The substituent e€ect was consistent with
previous reports on complexation of metalloporphyrins
with N-ligands.13
Results and discussion
Synthesis of zinc tetrapyrroles
Methyl pyropheophorbide-a (7) was prepared by mod-
ifying chlorophyll-a.10 Transformation4,10,11 of the 3-
vinyl group of 7 to acetyl or ethyl group and/or reduction
of the 13-keto carbonyl group of 7 to methylene group,
followed by zinc metallation a€orded zinc chlorins 1±4
possessing/lacking the 31/131-oxo group (Scheme 1).
Methyl bacteriopyropheophorbide-a (8) was prepared
from bacteriochlorophyll-a.11 The metal-free compound
8 was zinc-metallated to give zinc bacteriochlorin 5, which
was the 7,8-dihydrogenated form of 4 (Scheme 2). 17,18-
Oxidation12 of zinc chlorin 4 a€orded the corresponding
zinc porphyrin 6. All the synthetic tetrapyrroles were
1
characterized by H NMR, infrared, visible, mass spec-
tra and/or elemental analyses. Zinc tetrapyrroles 1±6
were puri®ed by several recrystallizations and the pure
samples were used for the following spectroscopic ana-
lyses.
1H NMR spectral change in complexation of zinc chlorin
3 with pyridine
In benzene-d6, zinc chlorin 3 was monomeric and 4-
1
coordinated zinc complex from the H NMR spectrum
([3]=1.7 mM). During the titration of pyridine, 1H
NMR spectra were measured at 25 ꢀC. The signals of
protons at the meso-positions (5-, 10-, and 20-positions)
of 3 were shifted to the lower ®eld in the course of the
titration (see Figure 2(A)). Addition of one equivalent
of pyridine almost induced saturation in the low ®eld
shift, indicating 1:1 complexation of 3 with pyridine.
The K-values were obtained from eq (2) [n=1]. The
values of a were determined from the change in the
chemical shifts (d) at the meso-positions, a ˆ ꢁd dinitial†=
ꢁdfinal dinitial†. Each K was calculated for all concentra-
tions of added pyridine. The average K-values were
26000, 23000, and 21000 from change of d5-H, d10-H, and
Visible spectral change in complexation of zinc chlorins
with pyridine
In dry benzene, synthetic zinc chlorins 1±4 (ˆ ZnN4)
showed several sharp absorption peaks in the visible
region, which were characteristic of monomeric species.
Benzene cannot coordinate to the central zinc and the
monomeric species in the solution were 4-coordinated
zinc complexes without axial ligand. To the solution (ca.
10 mM), highly coordinatable pyridine (=Py) was added
dropwise at 25 ꢀC, which induced visible spectral chan-
ges (see Fig. 1). In the initial stage of the titration, the
absorption spectra were greatly changed, but ®nally the
spectral change was so small that it was undetectable
(‰PyŠ > 200 mM). Clear isosbestic points indicated the
presence of two species in the visible spectral change:
either an equilibrium between ZnN4 and PyZnN4 (5-
coordination, n=1 in eq (1)) or between ZnN4 and
Py2ZnN4 (6-coordination, n=2). In the dry benzene
solution, equilibrium constant (K) in the complexation
of ZnN4 with Py was derived by eq (2).
d20-H, respectively, which were similar to the value from
visible spectral change described above (=29900).
During the titration, all the d-values of pyridine also
increased (see Figure 2(B)). Below one equivalent
([Py]0<1.7mM), all the signals of pyridine were shifted to
a higher ®eld than those of uncoordinated pyridine and
this high ®eld shift decreased in the order,
do > dm > dp H. These results led to the nitrogen
H
H
of pyridine coordinating to the central zinc and pyridine
protons being a€ected by the ring current of the chlorin
p-moiety. These titration curves in pyridine proton sig-
nals ®t the theoretical eq (2) [n=1], which proved to be
about 2 3Â104 for the K-value from data of the
o ; m , and p-H. These values were consistent with
ZnN4 ‡ nPy ! ꢀPy†nZnN4
ꢀ1†
ꢀ2†
Â
Á
à Â
ÃÂ
Ã
n
K ˆ Py ZnN4 = ZnN4 Py
n
ÁÈÂ
Ã
Â
à É
n
ˆ a= 1
a
Py
na ZnN4
0
0

H. Tamiaki et al./Bioorg. Med. Chem. 6 (1998) 2171±2178
2173
Scheme 1. i) HBr±AcOH (55ꢀC), CH2N2/Et2O; ii) Pr4NRuO4±Me(O)N(CH2CH2)2O/CH2Cl2; iii) NaBH4±TFA/CH2Cl2 (0 ꢀC); iv)
.
Zn(OAc)2 2H2O/MeOH±CH2Cl2; v) H2/Pd±C/CH3COCH3.
those obtained from visible and 1H NMR spectral
changes of zinc chlorin 3 (vide supra).
Complexation of zinc tetrapyrroles with pyridine
Visible absorption spectral change of dry benzene solu-
tion of zinc tetrapyrroles 4±6 by titration of pyridine led
to the equilibrium constant K for the 1:1 complexation
as described above. The values of K were estimated to
be 49600, 65300, and 21400 for 4, 5, and 6, respectively
Job's plots14 drawn from change of the chemical shifts
at the above six proton signals (see Fig. 3) clearly sup-
ported a 1:1 complex of 3 with pyridine in benzene-d6
([3]+[Py]=1.2 mM).

2174
H. Tamiaki et al./Bioorg. Med. Chem. 6 (1998) 2171±2178
Table 1. Equilibrium constant K for 1:1 complexation of zinc
tetrapyrroles 1±6 with pyridine in benzene at 25 ꢀC
X Y
7±8
17±18
K/M 1(l/nm ) Average K
1 H2 H2 C=C
CH±CH
(17S,18S)
12500 (396)
13100 (403)
14200 (503)
26400 (402)
26900 (414)
26100 (512)
27900 (641)
28600 (404)
31200 (511)
30500 (550)
29400 (645)
46200 (409)
49000 (432)
52700 (439)
50500 (672)
64000 (356)
64600 (394)
68300 (559)
64300 (578)
24800 (423)
19100 (430)
22300 (552)
19400 (621)
13300‹700
2 O H2 C=C
CH±CH
(17S,18S)
26800‹600
3 H2
O
C=C
CH±CH
(17S,18S)
29900‹900
49600‹2000
65300‹1500
21400‹2100
.
Scheme 2. i) Zn(OAc)2 2H2O/MeOH±CH2Cl2; ii) DDQ/
CH3 COCH3.
4 O O C=C
CH±CH
(17S,18S)
5 O O CH±CH CH±CH
(7R,8R) (17S,18S)
6 O O C=C
C=C
zinc of rigid porphyrin 6 was ®xed more tightly to give
the mono-adduct with more diculty than those of 4
and 5.
Figure 1. Absorption spectral change in dry benzene solution
of zinc chlorin
(=0 340 mM) at 25 ꢀC. Arrows show the direction of absor-
bance change with addition of pyridine.
3 (=12 mM) by addition of pyridine
Estimation of complexation constants by molecular
modeling
(see Table 1). The decrease in p-conjugation of the tet-
rapyrrole ligand (porphyrin 6>chlorin 4>bacterio-
chlorin 5) enhanced these values. The result is explained
by an electronic e€ect (vide supra) and/or the following
coordination process. Considering that all the sub-
stituents at the peripheral positions were the same in 4±
6, the ¯exibility of the tetrapyrrole p-plane a€ected the
coordinated ability of the central zinc. In ¯exible bac-
teriochlorin 5, the 4-coordinated zinc in the nearly
square planar complex (ZnN4) moved in an axial direc-
tion and formed the pyramidal 1:1 complex with pyr-
idine (PyZnN4) more easily than in 4 and 6. The central
Energy-minimized molecular structures of zinc tetra-
pyrroles (ZnN4) and the complexes (PyZnN4) with a
pyridine as an axial ligand were estimated by MM+/
PM3 calculation.15 To suppress ¯exibility in the sub-
stituents at the peripheral position, the structures of 1±6
for the molecular modeling were slightly changed; both
the 8-ethyl and the 17-propionate groups were sub-
stituted with a methyl group. The energy di€erence ÁE
between the optimized ZnN4 and PyZnN4 was calcu-
lated. Figure 4 shows that the calculated ÁE is linearly
correlated with log K induced by experimental results

H. Tamiaki et al./Bioorg. Med. Chem. 6 (1998) 2171±2178
2175
Figure 4. Dependency of ÁE calculated from molecular mod-
eling with log K observed from visible spectral change.
coordination were estimated in the following orders:
BChl-a/b (3-acetyl-bacteriochlorin)>BChl-g (3-vinyl-
bacteriochlorin)>Chl-a (3-vinyl-chlorin)>Chl-c1 (3-
vinyl-porphyrin); Chl-b (3-vinyl-7-formyl)>Chl-d (3-
Figure 2. Change of chemical shifts (d) of (A) 5-H (&), 10-H
(~) and 20-H (*) of zinc chlorin 3 (=1.7 mM) and (B) o-H
(&), m-H (~), and p-H (&) of additive pyridine (=0 65 mM)
in benzene-d6 at 25 ꢀC.
formyl-7-methyl)>Chl-a
(3-vinyl-7-methyl)>BChl-d
(3-(1-hydroxyethyl)-7-methyl-chlorin); BChl-f (7-for-
myl-20-hydrogen)>BChl-d (7-methyl-20-hydrogen)=
BChl-e (7-formyl-20-methyl)>BChl-c (7-methyl-20-
methyl).
Experimental
All melting points were measured with a Yanagimoto
micro melting apparatus and were uncorrected. Visible
and infrared absorption spectra were measured with
Hitachi U-3500 and Shimadzu FTIR-8600 spectro-
1
photometers, respectively. H NMR spectra were mea-
sured with a Bruker AC-300 spectrometer; ds are
expressed in parts per million relative to CHCl3
(7.26 ppm) as an internal reference. Mass spectra were
recorded on a JEOL HX-100 spectrometer; FAB-MS
samples were dissolved in CHCl3 and m-nitrobenzyl
alcohol was used as the matrix. The elemental analyses
were performed at the Microanalysis Center of Kyoto
University.
.
Figure 3. Job's plot of [3 Py] estimated from chemical shift
change of the ortho-proton in pyridine ([3]+[Py]=1.2 mM, in
benzene-d6, at 25 ꢀC).
Methyl pyropheophorbide-a (7),10 methyl mesopyro-
pheophorbide-a
(3-Et-7),16
methyl
bacterio-
pheophorbide-d (3-CH(OH)Me-7)10 and methyl
bacteriopyropheophorbide-a (8)11 were prepared
according to the reported procedures. All synthetic
procedures were done in the dark. Flash column chro-
matography (FCC) was performed with silica gel
(Merck, Kieselgel 60, 9385). Dry benzene for visible
spectral analysis was prepared as follows. After stirring
with H2SO4 overnight, benzene was separated, washed
with water, re¯uxed with CaH2 under nitrogen, distilled
from the visible spectral change, which is expected from
the equation of ÁG= RT ln K. As a result, the equi-
librium constants of the coordination are well estimated
by the molecular modeling. The same molecular model-
ing was performed in naturally occurring BChls-a/b/c/d/
e/f/g and Chls-a/b/c1/d. The order of ÁE in these
(B)Chls (=magnesium complexes) was similar to that in
zinc complexes 1±6 as expected, and the K-values in the

2176
H. Tamiaki et al./Bioorg. Med. Chem. 6 (1998) 2171±2178
1
and stored over molecular sieves 3A. Benzene-d6 for H
NMR spectral analysis was purchased (99.6% d, CEA)
and used without further puri®cation.
Na2SO4 and evaporated. The resulting residue was
recrystallized from CH2Cl2±hexane to give the corre-
sponding zinc complex. Several recrystallizations a€or-
ded an analytically pure sample to serve for
spectroscopic analyses.
General procedures
Hydration of 3-vinyl group. 3-Vinyl-chlorin (0.1 mmol)
was dissolved in 30% hydrogen bromide in acetic acid
(25 mL) and then stirred at 55 ꢀC under nitrogen for 4 h.
The solution was poured into cold H2O, extracted with
CHCl3, washed with aq. 4% NaHCO3 and H2O, dried
over Na2SO4, and evaporated in vacuo. The residue
was treated with an ethereal solution of excess diazo-
methane without moisture. The solution was stirred for
2 h and evaporated and the resulting residue was pur-
i®ed by FCC with 7±10% Et2O±CH2Cl2 and recrystalli-
zation from CH2Cl2±hexane to give 31-epimeric
mixtures (31R/31S=1/1) of the corresponding chlorin
possessing 1-hydroxyethyl group at the 3-position.
Synthesis of zinc methyl 131-desoxo-mesopyropheophor-
bide-a (1)
Methyl mesopyropheophorbide-a16 was reduced by
NaBH4±TFA in CH2Cl2 at 0 ꢀC17 to give the 131-desoxo
compound as dark purple crystals in 82% yield; mp
204±208 ꢀC (lit.18 197 ꢀC). MS (FAB) found: m/z 536.
Calcd for C34H40N4O2: M+, 536.
The above metal-free compound was zinc-metallated for
10 min to give the title compound as metallic dark pur-
ple crystals in 82% yield; mp 213±216 ꢀC; VIS (C6H6)
l
max=615 (e, 59900), 570 (6120), 542 (3520), 504 (6390),
403 (239000), 382 (55200) nm and (CH2Cl2) lmax=613
(relative intensity, 18), 567 (2.6), 541 (0.65, sh), 504
(2.9), 401 (100), 378 (23, sh) nm; IR (KBr) 1730 (172-
Oxidation of 31-hydroxyl group. 4-Methylmorpholine-
N-oxide (24 mg) and tetrapropylammonium perruthenate
(10 mg) were added to a dry CH2Cl2 (20 mL) solution of
chlorin possessing 1-hydroxyethyl group at the 3-posi-
tion (0.1 mmol) and stirred at room temperature under
nitrogen. After disappearance of the 31-hydroxy-chlorin
from VIS and TLC analyses (ca. 1.5 h), the reaction
mixture was poured into H2O. The aqueous layer was
extracted with several portions of CH2Cl2 and the com-
bined organic layers were dried over Na2SO4, and eva-
porated in vacuo. The residue was puri®ed by FCC with
Et2O±CH2Cl2 and recrystallization from CH2Cl2±hex-
ane to give the corresponding chlorin possessing 3-
acetyl group.
1
COO), 1616 (C=C) cm 1; H NMR (CDCl3) d=9.62,
9.57, 8.70 (each 1H, s, 5-, 10-, 20-H), 4.73, 4.68 (each
1H, dt, J=17, 5 Hz, 131-CH2), 4.58 (1H, dq, J=2, 7 Hz,
18-H), 4.40 (1H, dt, J=8, 2 Hz, 17-H), 3.90, 3.89 (each
2H, q, J=8 Hz, 3-, 8-CH2), 3.88 (2H, m, 13-CH2), 3.54,
3.46, 3.43, 3.36 (each 3H, s, 2-, 7-, 12-CH3, COOCH3),
2.66±2.75, 2.51±2.61, 2.31±2.46, 2.11±2.24 (each 1H, m,
17-CH2CH2), 1.83 (3H, d, J=7 Hz, 18-CH3), 1.763,
1.735 (each 3H, t, J=8 Hz, 31-, 81-CH3). MS (FAB)
found: m/z 598. Calcd for C34H38N4O264Zn: M+, 598.
Anal found: C; 67.21, H; 6.39, N; 9.14. Calcd for
.
C34H38N4O2Zn 1/2H2O: C; 67.04, H; 6.45, N; 9.19.
Reduction of 13-keto carbonyl group. Sodium borohy-
dride (87 mg) was added to ice-chilled tri¯uoroacetic
acid (3.4 mL) under nitrogen. To the stirred mixture was
Synthesis of zinc methyl 3-acetyl-3-devinyl-131-desoxo-
pyropheophorbide-a (2)
added
a
CH2Cl2 solution of
a
131-oxo-chlorin
The synthetic route of the title compound (7!!!!2)
was preliminarily reported by Tamiaki et al.21 and the
detailed procedures and the spectral data are described
here.
(0.1 mmol). After stirring overnight at room tempera-
ture, the reaction mixture was poured into H2O. The
aqueous layer was extracted with several portions of
CH2Cl2 and the combined organic layers were washed
with aq. 4% NaHCO3 and H2O, dried over Na2SO4,
and evaporated in vacuo. The residue was puri®ed by
FCC with CH2Cl2 and recrystallization from CH2Cl2±
hexane to give the corresponding 131-desoxo-chlorin.
Methyl pyropheophorbide-a (7)4,10,19 was reduced by
NaBH4±THF in CH2Cl2 at 0 ꢀC22 to give the 131-desoxo
compound as a black solid in 81% yield (lit.22 71 %);
mp 240±244 ꢀC. MS (FAB) found: m/z 534. Calcd for
C34H38N4O2: M+, 534.
Zinc metallation. A saturated methanol solution with
.
Zn(OAc)2 2H2O (1 mL) was added to a CH2Cl2 (10 mL)
The above 3-vinyl compound was hydrated to give 31-
hydroxy compound as a black solid in 71% yield; mp
165±170 ꢀC; VIS (CH2Cl2) lmax=640 (rel. 25), 586 (2.5),
541 (1.5, sh), 526 (1.8), 498 (8.6), 490 (7.5, sh), 405
(81, sh), 395 (100) nm; H NMR (CDCl3) d=10.18/17,
9.61, 8.93 (each 1H, s, 5-, 10-, 20-H), 6.68/65 (1H, q,
J=7 Hz, 3-CH), 4.95, 4.82 (each 1H, ddd, J=4, 7,
solution of metal-free chlorin (10 mg) and stirred at
room temperature under nitrogen. After VIS and TLC
analyses showed the metal-free chlorin had disappeared,
the reaction mixture was poured into aq. 4% NaHCO3,
and stirred for 10 min. White precipitates produced were
removed by ®ltration and the ®ltrate was dried over
1

H. Tamiaki et al./Bioorg. Med. Chem. 6 (1998) 2171±2178
2177
16 Hz, 131-CH2), 4.67 (1H, dq, J=2, 7 Hz, 18-H), 4.50
(1H, dt, J=9, 2 Hz, 17-H), 4.09 (2H, m, 13-CH2), 3.88
(2H, q, J=8 Hz, 8-CH2), 3.60, 3.58, 3.52, 3.45 (each
3H, s, 2-, 7-, 12-CH3, COOCH3), 2.76±2.87, 2.53±2.68,
2.32±2.45, 2.13±2.28 (each 1H, m, 17-CH2CH2), 2.23
(3H, d, J=7 Hz, 31-CH3), 1.86/1.84 (3H, d, J=7 Hz, 18-
CH3), 1.77 (3H, t, J=8 Hz, 81-CH3), 3.45 (1H, s, NH
(another NH was too broad to be observed)). MS
(FAB) found: m/z 552. Calcd for C34H40N4O3: M+,
552.
66.21, H; 6.04, N; 8.83. Calcd for C34H36N4O3Zn: C;
66.50, H; 5.91, N; 9.12.
Synthesis of zinc methyl 3-acetyl-3-devinyl-
pyropheophorbide-a (4)
Methyl bacteriopheophorbide-d10,19 was oxidized by
Pr4RuO4 Me(O)N(CH2CH2)2O in CH2Cl211 to give the
3-acetyl compound as a black solid in 75% yield (5%
Et2O±CH2Cl2 for FCC); mp 264±267 ꢀC (lit.20 264±
265 ꢀC); VIS (CH2Cl2) lmax=683 (rel. 52), 623 (7.4), 547
(11), 514 (12), 417 (100), 384 (79) nm; 1H NMR (CDCl3)
d=10.00, 9.61, 8.78 (each 1H, s, 5-,10-, 20-H), 5.34, 5.18
(each 1H, d, J=20 Hz, 131-CH2), 4.56 (1H, dq, J=2,
7 Hz, 18-H), 4.37 (1H, dt, J=9, 2 Hz, 17-H), 3.73 (2H,
q, J=8 Hz, 8-CH2), 3.72, 3.66, 3.61, 3.30
(3H+3H+3H+6H, s, 2-, 31-, 7-, 12-CH3, COOCH3),
2.54±2.78, 2.25±2.37 (each 2H, m, 17-CH2CH2), 1.84
(3H, d, J=7 Hz, 18-CH3), 1.72 (3H, t, J=8 Hz, 81-
CH3), 1.75 (1H, br, NH (another NH was too broad
to be observed)). MS (FAB) found: m/z 564. Calcd for
C34H36N4O4: M+, 564.
The above 31-hydroxy compound was oxidized to give
3-acetyl compound as a black solid in 71% yield (1%
Et2O±CH2Cl2 for FCC); mp 221±225 ꢀC; VIS (CH2Cl2)
l
max=657 (rel. 27), 603 (4.0), 544 (3.9), 508 (11), 411
1
(100) nm; H NMR(CDCl3) d=10.42, 9.47, 8.98 (each
1H, s, 5-, 10-, 20-H), 4.91, 4.79 (each 1H, ddd, J=3, 7,
16 Hz, 131-CH2), 4.62 (1H, dq, J=2, 7 Hz, 18-H), 4.46
(1H, dt, J=7, 2 Hz, 17-H), 4.05 (2H, m, 13-CH2), 3.82
(2H, q, J=8 Hz, 8-CH2), 3.76, 3.59, 3.47, 3.42, 3.36
(each 3H, s, 2-, 31-, 7-, 12-CH3, COOCH3), 2.73±2.84,
2.57±2.67, 2.17±2.40 (1H+1H+2H, m, 17-CH2CH2),
1.84 (3H, d, J=7 Hz, 18-CH3), 1.84, (3H, d, J=7 Hz, 18-
CH3), 1.75 (3H, t, J=8 Hz, 81-CH3), 2.89 (1H, br, NH
(another NH was too broad to be observed)). MS (FAB)
found: m/z 550. Calcd for C34H38N4O3: M+, 550.
The above metal-free compound was zinc-metallated for
2 h to give the title compound as green crystals in 95%
yield; mp 137±140 ꢀC; VIS (C6H6) lmax=672 (e, 51900),
620 (7300), 569 (4000), 526 (2760), 432 (68000), 384
(31400, sh) nm and (CH2Cl2) lmax=669 (rel. 73), 618
(11), 568 (6.0), 525 (3.7), 429 (100), 404 (66, sh), 380 (54,
The above metal-free compound was zinc-metallated for
1.5 h to give the title compound as metallic purple crys-
tals in 84% yield; mp 132±135 ꢀC; VIS (C6H6)
sh) nm; IR (KBr) 1736 (172-COO), 1668 (3/13-C=O),
1
lmax=641 (e, 46200), 596 (6100), 515 (5300), 514 (5400,
1615 (C=C) cm
;
1H NMR (CDCl3) d=9.44, 8.93,
sh), 414 (123000), 382 (28300, sh) nm and (CH2Cl2)
lmax=639 (rel. 25), 595 (4.7, sh), 550 (1.4, sh), 511 (6.3),
8.52 (each 1H, s, 5-,10-, 20-H), 4.90, 4.87 (each 1H, d,
J=20 Hz, 131-CH2), 4.49 (1H, dq, J=2, 7 Hz, 18-H),
4.28 (1H, dt, J=8, 2 Hz, 17-H), 3.65 (2H, q, J=8 Hz, 8-
CH2), 3.54, 3.37, 3.25, 3.02, 2.70 (each 3H, s, 2-, 31-, 7-,
12-CH3, COOCH3), 2.42±2.61, 2.13±2.36 (each 2H, m,
17-CH2CH2), 1.87 (3H, d, J=7 Hz, 18-CH3), 1.67 (3H,
t, J=8 Hz, 81-CH3). MS (FAB) found: m/z 626. Calcd
for C34H34N4O64Zn: M+, 626. Anal found: C; 64.40, H;
411 (100) nm; IR (KBr) 1736/1712 (172-COO), 1649 (3-
1
C=O), 1616 (C=C) cm 1; H NMR (CDCl3) d=9.89,
9.38, 8.77 (each 1H, s, 5-, 10-, 20-H), 4.75, 4.65 (each
1H, dt, J=16, 6 Hz, 131-CH2), 4.58 (1H, dq, J=2, 7 Hz,
18-H), 4.41 (1H, dt, J=7, 2 Hz, 17-H), 3.91 (2H, m, 13-
CH2), 3.76 (2H, q, J=8 Hz, 8-CH2), 3.55, 3.53 3.40,
3.26, 3.05 (each 3H, s, 2-, 31-, 7-, 12-CH3, COOCH3),
2.66±2.78, 2.53±2.63, 2.32±2.42, 2.18±2.27 (each 1H, m,
17-CH2CH2), 1.86 (3H, d, J=7 Hz, 18-CH3), 1.71 (3H,
t, J=8 Hz, 81-CH3). MS (FAB) found: m/z 612. Calcd
for C34H36N4O364Zn: M+, 612. Anal found: C; 60.00,
.
5.45, N; 8.87. Calcd for C34H34N4O4Zn 1/2H2O: C;
64.10, H; 5.54, N; 8.79.
Synthesis of zinc methyl bacteriopyropheophorbide-a (5)
Methyl bacteriopyropheophorbide-a (8, 20 mg)11 was
zinc-metallated in re¯uxing CHCl3 (10 mL) and MeOH
.
H; 5.35, N; 7.98. Calcd for C34H36N4O3Zn CH2Cl2: C;
60.10, H; 5.48, N; 8.01.
.
saturated with Zn(OAc)2 2H2O (10 mL) for 3 h (no
metallation occurred by stirring at room temperature as
described above) to give the title compound as a dark
blue solid in 66% yield; mp 193±196 ꢀC; VIS (C6H6)
lmax=774 (e, 144000), 706 (12400, sh), 561 (28800), 435
(4450, sh), 394 (62700), 356 (95200) nm and (CH2Cl2)
Synthesis of zinc methyl mesopyropheophorbide-a (3)
Methyl mesopyropheophorbide-a16 was zinc-metallated
for 1.5 h to give the title compound as purple crystals in
76% yield; mp 144±147 ꢀC (lit.4 144±148 ꢀC); VIS
(C6H6) lmax=645 (e, 76100), 599 (9410), 551 (3250), 423
(96300), 402 (60200), 380 (30400, sh) nm; see ref. 4 for
IR and 1H NMR data. MS (FAB) found: m/z 612.
Calcd for C34H36N4O364Zn: M+, 612. Anal found: C;
l
max=771 (rel. 100), 700 (13, sh), 553 (27), 433 (9.3, sh),
391 (58), 351 (91) nm; IR (KBr) 1736 (172-COO), 1650
1
;
1H NMR (CDCl3)
(3/13-C=O), 1604 (C=C) cm
d=8.82, 8.41, 8.26 (each 1H, s, 5-,10-, 20-H), 5.03, 4.88

2178
H. Tamiaki et al./Bioorg. Med. Chem. 6 (1998) 2171±2178
(each 1H, d, J=20 Hz, 131-CH2), 4.21 (2H, dq, J=3,
7 Hz, 7-, 18-H), 4.03 (2H, m, 8-, 17-H), 3.57, 3.47, 3.45,
3.11 (each 3H, s, 2-, 31-, 12-CH3, COOCH3), 2.42±2.54,
2.17±2.39, 1.89±2.07 (1H+3H+2H, m, 8-CH2, 17-
CH2CH2), 1.70 (6H, d, J=7 Hz, 7-, 18-CH3), 0.94 (3H,
t, J=7 Hz, 81-CH3). MS (FAB) found: m/z 628. Calcd
for C34H36N4O464Zn: M+, 628.
The Porphyrins; Dolphin. D., Ed.; Academic Press: New York,
1978; Vol. V, pp. 444.
4. Tamiaki, H.; Amakawa, M.; Shimono, Y.; Tanikaga, R.;
Holzwarth, A. R.; Scha€ner, K. Photochem. Photobiol. 1996,
63, 92.
5. Tamiaki, H.; Miyatake, T.; Tanikaga, R.; Holzwarth, A. R.;
Scha€ner, K. Angew. Chem. Int. Ed. Engl. 1996, 35, 772.
6. Wakao, N.; Yokoi, N.; Isoyama, N.; Hiraishi, A.; Shimada,
K.; Kobayashi, M.; Kise, H.; Iwaki, M.; Itoh, S.; Takaichi, S.;
Sakurai, Y. Plant Cell Physiol. 1996, 37, 889.
Synthesis of zinc methyl 3-acetyl-3-devinyl-
protopyropheophorbide-a (6)
7. Miller, J. R.; Dorough, G. D. J. Am. Chem. Soc. 1952, 74,
3977.
To an acetone (30 mL) solution of zinc methyl 3-acetyl-
3-devinyl-pyropheophorbide-a (1, 21 mg) was added 2,3-
dichloro-5,6-dicyano-p-benzoquinone (11 mg).12 After
stirring for 5 min, the solution was poured into H2O,
extracted with CH2Cl2 and washed with aq. 4%
NaHCO3. The organic phase was dried over Na2SO4,
and evaporated in vacuo. The residue was puri®ed by
FCC with 13±14% Et2O±CH2Cl2 and recrystallization
from CH2Cl2±MeOH to give the title compound as a
purple solid in 63% yield; mp 267±270 ꢀC; VIS (C6H6)
8. Kirksey, C. H.; Hambright, P.; Storm, C. B. Inorg. Chem.
1969, 8, 2141; Vogel, G. C.; Searby, L. A. Inorg. Chem. 1973,
12, 936.
9. Schauer, C. K.; Anderson, O. P.; Eaton, S. S.; Eaton, G. R.
Inorg. Chem. 1985, 24, 4082; Scheidt, W. R.; Eigenbrot, C. W.;
Ogiso, M.; Hatano, K. Bull. Chem. Soc. Jpn. 1987, 60, 3529;
Bhyrappa, P.; Krishnan, V.; Nethaji, M. J. Chem. Soc. Dalton
Trans. 1993, 1901; Bhyrappa, P.; Wilson, S. R.; Suslick, K. S. J.
Am. Chem. Soc. 1997, 119, 8429; Taylor, P. N.; Wylie, A. P.;
Huuskonen, J.; Anderson, H. L. Angew. Chem. Int. Ed. Engl.
1998, 37, 986.
lmax=621 (e, 32700), 568 (7700), 430 (234000), 418
(110000, sh) nm and (CH2Cl2) lmax=619 (rel. 12), 568
(2.4), 425 (100), 414 (53, sh) nm; IR (KBr) 1736 (172-
COO), 1695/1670 (3/13-C=O) cm 1; 1H NMR (CDCl3)
d=10.56, 9.79, 9.60 (each 1H, s, 5-,10-, 20-H), 5.37 (2H,
s, 131-CH2), 4.02 (2H, q, J=8 Hz, 8-CH2), 3.89 (2H, t,
J=8 Hz, 17-CH2), 3.79, 3.78, 3.72, 3.63, 3.40, 3.31 (each
3H, s, 2-, 31-, 7-, 12-, 18-CH3, COOCH3), 2.85 (2H, t,
J=8 Hz, 171-CH2), 1.84 (3H, t, J=8 Hz, 81-CH3). MS
(FAB) found: m/z 624. Calcd for C34H32N4O464Zn:
M+, 624.
10. Tamiaki, H.; Takeuchi, S.; Tsudzuki, S.; Miyatake, T.;
Tanikaga, R. Tetrahedron 1998, 54, 6699.
11. Tamiaki, H.; Kouraba, M.; Takeda, K.; Kondo, S.;
Tanikaga, R. Tetrahedron: Asymmetry 1998, 9, 2101.
12. Tamiaki, H.; Watanabe, T.; Miyatake, T. J. Porphyrins
Phthalocyanines 1998, 2, in press.
13. Baker, E. W.; Storm, C. B.; McGrew, G. T.; Corwin, A. H.
Bioinorg. Chem. 1973, 3, 49; Vogel, G. C.; Beckmann, B. A.
Inorg. Chem. 1976, 15, 483; McDermott, G. A.; Walker, F. A.
Inorg. Chim. Acta 1984, 91, 95.
14. Tsukube, H.; Furuta, H.; Odani, A.; Takeda, Y.; Kudo, Y.;
Inoue, Y.; Liu, Y.; Sakamoto, H.; Kimura, K. In Comprehen-
sive Supramolecular Chemistry; Davies, J. E. D.; Ripmeester,
J. A., Eds.; Pergamon Press: Oxford, 1996; Vol. 8, pp 425±482.
Acknowledgements
We thank Dr. Toru Oba of Ritsumeikan University for
his helpful discussion. This work was partially sup-
ported by a research grant from Human Frontier Sci-
ence Program, Grants-in-Aid for Scienti®c Research
(Nos. 09480144 and 10146252) from the Ministry of
Education, Science, Sports and Culture, Japan and the
Sasakawa Scienti®c Research Grant from The Japan
Science Society. TM is grateful for his JSPS Research
Fellowship for Young Scientists.
15. Kureishi, Y.; Tamiaki, H. J. Porphyrins Phthalocyanines
1998, 2, 159.
16. Smith, K. M.; Go€, D. A.; Simpson, D. J. J. Am. Chem.
Soc. 1985, 107, 4946.
17. Smith, N. W.; Smith, K. M. Energy Fuels 1990, 4, 675.
18. Maruyama, K.; Yamada, H.; Osuka, A. Photochem. Pho-
tobiol. 1991, 53, 617.
19. Smith, K. M.; Bisset, G. M. F.; Bushell, M. J. J. Org.
Chem. 1980, 45, 2218.
20. Inho€en, H. H.; Jager, P.; Mahlhop, R.; Mengler, C. D.
Liebig Ann. Chem. 1967, 704, 188.
References and Notes
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