3
61
(
(
A)
B)
#
(
C)
0
10
20
30
40
50
60
Retention time / min
Figure 3. HPLC chromatogram of reaction products in the
supernatant of a CH Cl suspension of pheophorbide a methyl
ester (1) by bubbling air for 4 h. The products were eluted on
1
800 1700 1600- 1500
2
2
1
Wavenumber / cm
5
1
C -AR-II (6 mmº © 250 mm) with methanol at a flow rate of
.0 mL min . The fraction denoted by # was the 700-nm
18
¹
1
Figure 4. FT-IR spectra of the 700-nm absorbing compound
purified from products after incubation of pheophorbide a
methyl ester (1) with TiO2 particles in CH2Cl2 by bubbling
air (A), synthesized purpurin-18 methyl ester (2) (B), and
pheophorbide a methyl ester (1) (C) in the region between 1500
absorbing compound.
were positioned at 413 and 667 nm, respectively, before stirring
with TiO2 particles. When the suspension of 1 and TiO2 particles
was bubbled with air for 4 h, a new Qy absorption band appeared
at 700 nm. In contrast, the absorption band around 700 nm was
hardly observed by bubbling nitrogen gas for 4 h. No spectral
change of 1 was detected by bubbling air without TiO2 particles
¹
1
and 1850 cm
.
purified compound, two singlet peaks at 6.28 and 3.88 ppm,
which were assigned to protons at the 13 -position (1H) and in
the 13 -methoxycabonyl group (3H), respectively, of original 1,
disappeared (Figure S1). All the signals in H and C NMR
spectra of the purified compound corresponded with those of
2
4
(data not shown). These indicate that the 700-nm absorbing
2
2
1
13
product was generated from 1 in the presence of TiO particles
2
and molecular oxygen.
2
2
The supernatant of the CH2Cl2 suspension of 1 and TiO2
particles by bubbling air for 4 h was analyzed by reverse-phase
HPLC as shown in Figure 3. Fractions at 16.5 and 17.4 min were
purpurin-18 methyl ester (2) by conventional synthesis.
Figure 4 shows expanded FT-IR spectra of the purified com-
pound and the synthesized 2 as well as pheophorbide a
methyl ester (1) in the wavenumber region between 1500 and
1850 cm . The purified compound exhibited no stretching
vibrational peak of the 13 -carbonyl group, which was clearly
assigned to unreacted pheophorbide a methyl ester (1) and the
2
¹1
1
3 -stereoisomer of 1, namely pheophorbide a¤ methyl ester,
1
respectively. A new fraction, which had Soret and Qy bands at
¹
1
26
4
2
3
07 and 697 nm, respectively, in the HPLC eluent, appeared at
8.5 min (denoted by # in Figure 3). Other fractions at 10.5 and
detected at 1701 cm in the FT-IR spectrum of 1. New
¹
1
vibrational bands were observed at 1713 and 1730 cm in the
IR spectrum of the purified compound instead. These bands were
identical to those of the synthesized 2, and were ascribable to the
anhydride group of purpurin-18 type pigments. The vibrational
2.3 min exhibited Q absorption bands around 665 nm. There-
y
fore, the compound eluted at 28.5 min was the origin of the
00-nm absorption band formed by incubation of 1 with TiO2
particles.
The 700-nm absorbing compound, which was detected at
8.5 min in the chromatogram in Figure 3, was isolated by
7
¹
1
band at 1603 cm , which was assigned to a skeletal vibration
characteristic of the chlorin macrocycle, of the purified
compound was present at the same position as that of the
synthesized 2, and was shifted to lower wavenumber than that of
2
reverse-phase HPLC. The purified compound was characterized
by means of UVvis, NMR, ESI-MS, and FT-IR measure-
¹
1
3
1 at 1617 cm . The vibrational stretching band of 17 -carbonyl
group was preserved at 1744 cm in the spectrum of the purified
compound after incubation of 1 with TiO particles. All the
vibrational bands of the purified compounds were the same as
2
2
¹1
ments. In addition, the spectroscopic data of this product were
compared with those of purpurin-18 methyl ester (2) (Figure 1B)
that was conventionally synthesized under an alkaline condition
with molecular oxygen according to previous reports.2
2
325
22
those of the synthesized 2 (Figure S3). These characterizations
The compound purified from the supernatant of 1 after
treatment with TiO particles exhibited Soret and Q bands at
revealed that the new compound formed by treatment of 1 with
TiO particles was puruprin-18 methyl ester.
2
y
2
4
11 and 700 nm, respectively, in CH2Cl2. The Qy band was red-
The conversion efficiency from pheophorbide a methyl ester
(1) to purpurin-18 methyl ester (2) under the present conditions
for 4 h was estimated to be 32% (the average of three
independent measurements). This value was smaller than the
shifted by 33 nm compared with 1, and was characteristic of
purpurin-18-type pigments, which had a six-membered anhy-
dride exo-ring instead of an original five-membered exo-ring in
2
5
1
. This compound showed the molecular-ion peak at m/z 579.4
overall yield from 1 to 2 (50%) reported by Lee et al. In the
present reaction, CH Cl was appropriate compared with other
2
2
in the ESI-MS, which was identical to the molecular weight
of purpurin-18 methyl ester. In the H NMR spectrum of the
2
2
1
solvents such as methanol, which was used as a cosolvent
Chem. Lett. 2012, 41, 360362
Saga, Yoshitaka
