Syntheses and optical properties of stable 8-alkylidene-bacteriochlorins mimicking the molecular structures of natural bacteriochlorophylls-b and g

Article abstract of DOI:10.1016/j.tet.2004.09.091

We prepared bacteriochlorophyll(BChl)-b and g models by Diels-Alder reactions of 8-vinyl-chlorophylls with tetracyanoethylene. The resulting 8-alkylidene-bacteriochlorins with various substituent groups at the 3-position had the same π-conjugate as BChls-b/g. While the natural pigments isomerized by addition of an acid to afford the corresponding chlorins, the synthetic models were stable under the acidic conditions due to dialkylation at the 7-position. These BChl-b/g models are useful for investigating the optical properties of relatively unstable BChls-b/g. Chemically stable 8-alkylidene-bacteriochlorins were systematically prepared as models of natural bacteriochlorophylls-b/g easily isomerizing to the corresponding chlorins, and their optical properties were investigated in detail.

Full text of DOI:10.1016/j.tet.2004.09.091

Tetrahedron 60 (2004) 11349–11357
Syntheses and optical properties of stable
-alkylidene-bacteriochlorins mimicking the molecular structures
of natural bacteriochlorophylls-b and g
8
*
Michio Kunieda, Tadashi Mizoguchi and Hitoshi Tamiaki
Department of Bioscience and Biotechnology, Faculty of Science and Engineering, Ritsumeikan University,
Noji-higashi 1-1-1, Kusatsu, Shiga 525-8577, Japan
Received 2 August 2004; revised 24 September 2004; accepted 27 September 2004
Available online 13 October 2004
Abstract—We prepared bacteriochlorophyll(BChl)-b and g models by Diels–Alder reactions of 8-vinyl-chlorophylls with tetracyanoethyl-
ene. The resulting 8-alkylidene-bacteriochlorins with various substituent groups at the 3-position had the same p-conjugate as BChls-b/g.
While the natural pigments isomerized by addition of an acid to afford the corresponding chlorins, the synthetic models were stable under the
acidic conditions due to dialkylation at the 7-position. These BChl-b/g models are useful for investigating the optical properties of relatively
unstable BChls-b/g.
q 2004 Elsevier Ltd. All rights reserved.
1. Introduction
do chlorophyll(Chl)s-a/d in higher plants or cyanobacteria
5
and (Zn-)BChl-a in photosynthetic bacteria. As compared
Bacteriochlorophyll(BChl)s-b and g are found in some
1
with any other photosynthetic (B)Chls (Chls-a/b/c/d, BChls-
a/c/d/e), BChls-b and g are quite unique in their molecular
structures possessing an ethylidene group at the 8-position,
and have the same bacteriochlorin p-conjugate (doubly
saturated porphyrin moiety at 7–8 and 17–18 bonds). BChl-
2
purple bacteria and heliobacteria, respectively, and their
molecular structures including the absolute configurations
3
,4
have already been determined (left drawing of Fig. 1).
They function as photosynthetic pigments in natural light-
harvesting, energy transfer and charge separation systems as
3
b possesses an acetyl group (R ) at the 3-position, which is
Figure 1. Molecular structures of natural BChls- and BPhes-b/g (left), their isomerized products (center) and synthetic stable model compounds 1–5 (right).
Keywords: Bacteriochlorin; Diels–Alder reaction; Isomerization; Tetracyanoethylene.
*
Corresponding author. Tel.: C81 77 566 1111; fax: C81 77 561 2659; e-mail: tamiaki@se.ritsumei.ac.jp
0040–4020/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2004.09.091

11350
M. Kunieda et al. / Tetrahedron 60 (2004) 11349–11357
the same function group as in BChl-a. BChl-g possesses a
3
vinyl group (R ) at the 3-position, which is the same
dienophile thus tended to give bacteriochlorin molecules
bearing a C8]C8 double bond as models of BChls-b/g as
1
substituent as in Chl-a. Only a few reports are available on
,7
in the right drawing of Figure 1. Additionally, the
stereochemistry of the double bond was fixed as an
E-form in the same stereoisomer with the natural BChls-
b/g, due to the C7–C8 fused six-membered ring. BChls-b
and g have a phytyl, farnesyl or geranylgeranyl group as the
6
in vitro investigation of BChls-b/g
because of their
instabilities, i.e. the easy isomerization of the bacterio-
chlorin moiety (two reduced pyrroles at rings B and D) to
the corresponding chlorin (one reduced pyrrole at ring D)
3
,4
8
,9
1
at the 13 -position. These moieties do not play any
7-propionate ester and also have a methoxycarbonyl group
even under ambient conditions as shown in Figure 1.
2
1
Stable bacteriochlorin models possessing a C8]C8 double
bond are thus necessary to elucidate the optical properties of
naturally occurring BChls-b/g. In this paper, we intended to
synthesize chemically stable 8-alkylidene-bacteriochlorins
1
7–19
significant roles in their monomeric absorption spectra,
and are here changed to 17 -COOCH and 13 -H for
preparing simple and stable model compounds.
2
2
3
2
1
–5 (right drawing of Fig. 1) as the BChl-b/g models.
Such Diels–Alder reactions would allow us to prepare a
series of 8-alkylidene-BChl analogues possessing various
substituents at the 3-position. Since the synthetic bacterio-
chlorins have two alkyl groups as peripheral substituents at
the 7-position, undesired rearrangement of the bacterio-
chlorin moiety to the chlorin can no longer take place. Here,
we report syntheses of stable bacteriochlorins possessing a
similar p-conjugate system with natural BChls-b/g as well
as their chemical stabilities and optical properties.
1
To form the C8]C8 double bond, we employed Diels–
Alder reaction of 8-vinyl-chlorin with tetaracyanoethylene
(
pyrroles have been studied.
TCNE). Various Diels–Alder reactions of cyclic tetra-
1
0–15
In most cases, b-vinyl-
porphyrins or chlorins, C(b )]C(b)–CH]CH , reacted
0
2
with a dienophile to give the corresponding [4C2]
cycloadduct with one more reduced p-conjugate, –C(b )–
C(b)]CH–CH –. The resulting Diels–Alder products had
2
a six-membered ring attached to the b,b -positions and also
an exo-double bond on the b-position similar to the
0
0
2. Results and discussion
8-ethylidene group of BChls-b/g. In the chlorin p-conjugate
system, the C]C double bond on the pyrrole ring opposite
the reduced pyrolidine is more reactive than any other
2.1. Synthesis of 8-vinyl-chlorophyll derivatives
1
double bonds in the skeletal p-conjugate. The Diels–
6
Alder reaction of 8-vinyl-chlorophyll derivatives with a
As diene molecules, we prepared 8-vinyl-chlorins 6–10
Scheme 1. Synthesis of 3-substituted 8-vinyl-chlorins: (i) collidine, reflux; (ii) H
AcOH–THF for 16; (iii) N-methyl-morphorine-N-oxide–Pr
2
–Pd/C for 13, 30% HBr–AcOH, H
2
O, CH
2
N
S; (v) TsOH, CH Cl –benzene (rt to reflux), CH
2
for 14, cat. OsO
N
4 4
–NaIO in
4
; (vi) NaBH –
4
RuO ; (iv) OsO
4
4
-pyridine, H
2
2
2
2
2
2 2
MeOH; (vii) 30% HBr–AcOH, 1-hexanol, CH N ; (viii) 1,2-dichlorobenzene, 170 8C.

M. Kunieda et al. / Tetrahedron 60 (2004) 11349–11357
11351
possessing various substituent groups at the 3-position as
follows (Scheme 1). The 3-vinyl group of methyl pyro-
pheophorbide-a (12) was converted to ethyl, 1-hydroxy-
by reduction of the 3-acetyl group in 9. To synthesize 3,8-
divinyl-chlorin 8, we utilized reported thermal degradation
of 21 derivatized from methyl pheophorbide-a 11,
20
28
3
20
21
ethyl and formyl groups (R ) as in chlorins 13, 14 and
because the b-keto ester moiety on the E-ring promoted
an elimination reaction at the 3-position of the A-ring. As
2
6, respectively, according to reported procedures.
0
29
1
Oxidation of 1-hydroxyethyl group in 14 produced
described above, a series of 3-substituted 8-vinyl-chlorins
6–10 were prepared.
2
-acetyl-chlorin 15. Reaction of chlorins 13–16 with
2
3
OsO in the presence of pyridine followed by treatment of
4
2
3–25
H S gas
2
afforded the corresponding 7,8-cis-diols 17–20
in moderate yields. The resulting diols 17, 19 and 20 were
double-dehydrated by treatment of p-toluenesulfonic acid at
2.2. Diels–Alder reaction for preparing 8-alkylidene-
BChl derivatives
2
3,26,27
room temperature followed by reflux to give 3-ethyl,
3-acetyl and 3-formyl-8-vinyl-chlorins 6, 9 and 10 as a
Diels–Alder reactions of 8-vinyl-chlorins 6–10 as dienes
with TCNE were achieved by the following procedures. A
slight excess of TCNE (ca. 1.2 equiv) was added to 6–10 in
dry chloroform and refluxed for 30 min under nitrogen. The
Diels–Alder reaction was monitored by ultraviolet–visible–
near infrared (UV–VIS–NIR) spectral analysis; a specific
red-shift of the Qy absorption maximum based on a change
of p-conjugate moieties from chlorin to bacteriochlorin. All
the reactions afforded the corresponding 8-alkylidene-
bacteriochlorins 1–5 as a 1:1 7-epimeric mixture. Molecular
structures of all the Diels–Alder products 1–5 were
major product. After the similar acidic treatment of 18, 3-(1-
hydroxyethyl)-8-vinyl-chlorin 7 and 3,8-divinyl-chlorin 8
were produced in w1 and 3%, respectively, with some
degradation products. Desired 7 was successively prepared
1
1
1
determined by their 1D H NMR, 2D H– H correlation
and rotating-frame Overhauser effect spectroscopies
(
COSY/ROESY) and high resolution mass (HR-MS,
ionized by fast atomic bombardment (FAB)) spectral
analyses: the specific proton signals of the 8-alkylidene
group were situated around 7 and 4 ppm for 8-CH and
1
8
observed at the position of one-to-one adduct.
-CH , respectively, and the main mass ion peak was
2
In the reactions of 3-ethyl, 3-acetyl and 3-formyl-8-vinyl-
chlorins 6, 9 and 10 with TCNE, no significant side-reaction
occurred and desired [4C2] cycloadducts 1, 4 and 5 were
Scheme 2. Diels–Alder reactions of 3-ethyl-, 3-acetyl- and 3-formyl-8-
vinyl-chlorins 6, 9 and 10 with TCNE.
Scheme 3. Reactions of 3-(1-hydroxyethyl)-8-vinyl- and 3,8-divinyl-chlorins 7 and 8 with TCNE.

11352
M. Kunieda et al. / Tetrahedron 60 (2004) 11349–11357
isolated in good yields after purification of flash column
chromatography (FCC) and recrystallization (Scheme 2).
essentially acted as an ene and the 8-vinyl group could
function as a diene accompanying the C8]C7.
Diels–Alder reactions of 7 and 8 afforded the corresponding
[
2.3. Chemical stabilities of synthetic models 3 and 4 in
comparison with BPhes-b/g
4C2] cycloadducts 2 and 3, respectively, and additional
by-products (Scheme 3). Complete consumption of 3-(1-
hydroxyethyl)-8-vinyl-chlorin 7 led to the formation of
desired 2 (55%) and its dehydrated product 3 (17%). In
refluxing chloroform, neither 3-(1-hydroxyethyl)-8-ethyl-
chlorin 14 in the presence of TCNE nor 3-(1-hydroxyethyl)-
Molecular structures and skeletal p-conjugates of synthetic
1
–5 were quite similar to those of natural BChls-b/g, all of
1
which had the C8]C8 double bond, so bacteriochlorins
–5 might be model compounds for BChls-b/g. In this
1
8
-vinyl-chlorin 7 in the absence of TCNE afforded their
section, we will compare the chemical stabilities between
the synthetic 3/4 and natural metal-free bacteriopheo-
phytin(BPhe)s-b/g (see left drawing of Fig. 1).
dehydrated chlorins 12 and 8, but 2 gave the corresponding
dehydrated 3, indicating that thermal dehydration of 3-(1-
hydroxyethyl) group occurred more easily in 2 than in 7 and
14. Under the above reaction conditions, such a bacterio-
chlorin p-conjugate promoted elimination of a water
molecule on the 3-position.
Figure 2 shows UV–VIS–NIR spectral change of BPhes-b/g
(upper) and the corresponding models 3/4 (lower) in acidic
acetone in the dark. After acidic treatment, absorption peaks
characteristic of BPhe-b at 777, 527 and 367 nm (red line of
Fig. 2A) decreased with a concomitant increase in new
peaks (blacks in Fig. 2A), and finally changed to peaks
typical of chlorin chromophore at 674 and 421 nm (blue in
Fig. 2A), which were similar to peaks of 3-acetyl-chlorin 15.
Normal-phase HPLC analysis showed that the products
were a simple isomerized chlorin, 3-Ac-pheophytin(Phe)-a
In the case of 3,8-divinyl-chlorin 8, desired Diels–Alder
adduct 3 was prepared predominantly after 30-min reflux.
When the reaction mixture was worked up by a standard
procedure, an undesired product was isolated besides 3
(
evaporation of the reaction mixture in the dark to produce
71%). The over-reaction of 3 with TCNE occurred during
1
1
1
1
[
(
2C2] cycloadduct 23 (11%). Addition of excess TCNE
5 equiv) to the chloroform solution of 3 resulted in
as a major product and its 8 -oxidized chlorins (8 -OH or
8 -OOH, not determined) as a minor product. These results
1
6
exclusive formation of 23, but no [4C2] cycloadducts at
around the 3-position could be isolated. In the chlorin
chromophore (17,18-dihydroporphyrin), the 3-vinyl group
indicated that the acidic treatment of metal-free BPhe-b
promoted isomerization of C7H–C8]C8 to C7]C8–
1
1
C8 H moiety to give 3-Ac-Phe-a (Fig. 1) as in the
Figure 2. UV–VIS–NIR spectral changes of BPhe-b (A) and g (B), and the corresponding models 4 (C) and 3 (D) in acetone (3.0 ml) by acidic treatment
(aqueous 3.5% HCl, 10 ml) at room temperature. Spectra were recorded at 3/2/30/30-min intervals for A/B/C/D, respectively. The red and blue lines are initial
(before acid treatment) and final spectra (after 213/38/300/300-min treatment), respectively.

M. Kunieda et al. / Tetrahedron 60 (2004) 11349–11357
11353
magnesium complex BChl-b (to 3-Ac-Chl-a) previously
6
reported.
2.4. Electronic absorption properties of BPhes-b/g and
their synthetic stable models 1–5
,8
Acidic treatment also induced isomerization of BPhe-g
absorption peaks at 748, 515 and 366 nm, red line of
Figure 3A shows UV–VIS–NIR spectra of 3-acetyl forms
BPhe-b and 4 (broken and solid lines, respectively) in
dichloromethane, and broken and solid lines in Figure 3B
are spectra of 3-vinyl forms BPhe-g and 3, respectively.
Synthetic 4/3 had similar spectra to BPhes-b/g except for
slight blue-shifted Qy maxima. In the Qy region (600–
(
Fig. 2B) to a chlorin chromophore (660 and 417 nm, blue in
Fig. 2B), resembling 3-vinyl-chlorin 12. HPLC analyses
supported that the major product was a chlorin chromophore
possessing 660 nm as the Qy maxima. Therefore, BPhe-g
isomerized by an action of an acid to give Phe-a (Fig. 1) as
850 nm), all four bacteriochlorins had an intense Qy(0,0)
6
,9
band at a longer wavelength with association of a small
Qy(1,0) band at a shorter wavelength. The Qy(0,0) maxima
lmax[Qy(0,0)]s of semi-natural BPhes-b/g were situated at
in BChl-g to Chl-a. The isomerization of bacteriochlorin
to chlorin chromophores was completed in 213 and 38 min
for BPhes-b and g, respectively, and their half lifetimes
were estimated to be 25 and 6 min, respectively, showing
that the isomerization of BPhe-g was catalyzed by an acid
more efficiently than that of BPhe-b, similar to the reported
7
6
7
83/751 nm with full widths at half maximum (FWHMs) of
10/720 cm , and those of synthetic 4/3 were situated at
K1
K1
52/730 nm with FWHMs of 430/500 cm , respectively
8
,9
(see Table 1). BPhe-b and 4 possessing the 3-acetyl group
had more red-shifted and sharper Qy(0,0) bands than BPhe-
g and 3 which possessed the 3-vinyl group, respectively.
The lmax[Qy(1,0)]s of BPhes-b/g and 4/3 were located at
isomerization of BChls-b/g.
We examined chemical stabilities of synthetic models 3 and
under the same acidic conditions (Fig. 2C and D). Almost
4
700/677 and 677/662 nm, respectively. Energetic difference
D between Qy(0,0) and Qy(1,0) peaks of BPhes-b/g and 4/3
no spectral changes were observed for 5 h. HPLC analyses
after acidic treatment did not show any other peaks than 3/4,
indicating that synthetic models 3/4 were quite inactive
for the acidic treatment compared to natural BPhes-b/g.
Dialkylation of the 7-position and/or the fused ring on the
K1
was 1490/1450 and 1460/1410 cm , respectively, showing
the Ds were slightly larger in BPhe-b and 4 than in BPhe-g
and 3. Relative intensities I s (based on the Soret peak
intensity) in Qy(0,0) peaks of BPhe-b/g and 4/3 were 0.58/
rel
7
,8-positions led to 3/4 being highly chemically stable.
0
0
.36 and 0.88/0.58, and their Qy(1,0) peak intensities were
.10/0.13 and 0.11/0.13, showing that BPhe-b and 4 had
more intense Qy(0,0) and less intense Qy(1,0) bands than
BPhe-g and 3, respectively.
In the Qx absorption region (460–550 nm) of all the above
four bacteriochlorins, large Qx(0,0) bands were associated
with small Qx(1,0) bands at shorter wavelength. The
lmax[Qx(0,0)]s of BPhe-b/g and 4/3 situated at 532/521
and 538/527 nm with FWHMs of 850/570 and 620/
4
K1
60 cm , respectively, while I s in Qx(0,0) peaks of
rel
BPhes-b/g and 4/3 were 0.24/0.25 and 0.28/0.27. BPhe-b
and 4 had more red-shifted and broadened Qx(0,0) bands
than BPhe-g and 3. The Ds between Qx(0,0) and Qx(1,0) of
K1
BPhe-b and 4 (1310 and 1330 cm ) were slightly larger
than those of BPhe-g and 3 (1260 and 1250 cm ) as
K1
observed in Qy bands. The same substituent effect on
3-acetyl/vinyl groups was thus clearly observed in both Qy
and Qx bands of semi-natural BPhes-b/g and synthetic 4/3.
As described above, BPhes-b/g were easily transferred to
their chlorin chromophores, so their visible spectra were
sensitive to the presence of a small amount of such chlorin
impurities; typically, small Qy(1,0) peaks of BPhes-b/g are
situated at around intense Qy(0,0) peaks of their altered
chlorins. It is noteworthy that the present spectral analyses
can be achieved in detail by using stable models containing
no chlorin-type impurities.
Next, we measured UV–VIS–NIR spectra of other bacte-
riochlorins 1/2/5 possessing the same skeletal p-conjugate
as BPhes-b/g. The lmax[Qy(0,0)]s of 1–5 as well as their
lmax[Qx(0,0)]s were red-shifted in the order of 1!2!3!
4
by model calculation using ZINDO/S
!5 (see Table 1). These observed orders were reproduced
30–32
(Table 2). All
the synthetic bacteriochlorins 1–5 are applicable to the
investigation of optical properties of a series of 3-substituted
8-alkylidene-bacteriochlorins as described above; they
Figure 3. UV–VIS–NIR spectra of BPhe-b and 4 (A), and BPhe-g and 3 (B)
in dichloromethane. Solid and broken lines were synthetic 4/3 and BPhes-b/
g, respectively. All the spectra were normalized at the Soret peak.

1
1354
M. Kunieda et al. / Tetrahedron 60 (2004) 11349–11357
Cl
Table 1. UV–VIS–NIR peak data of 1–5 and BPhes-b/g in CH
2
2
a
K1
]
b
a
K1
]
c
Compound
l
max/nm (Irel ) [FWHM/cm
D[Qy(0,0)–(1,0)]
l
max/nm (Irel ) [FWHM/cm
D[Qx(0,0)–(1,0)]
/cm
K1
K1
/cm
Qy(0,0)
Qy(1,0)
Qx(0,0)
Qx(1,0)
1
2
3
4
715 (0.50) [490]
722 (0.56) [480]
730 (0.58) [500]
752 (0.88) [430]
760 (1.30) [340]
783 (0.58) [610]
751 (0.36) [720]
651 (0.13)
656 (0.13)
662 (0.13)
677 (0.11)
686 (0.09)
701 (0.10)
677 (0.13)
1370
1410
1410
1460
1430
1490
1450
522 (0.25) [450]
524 (0.27) [460]
527 (0.27) [460]
538 (0.28) [620]
543 (0.39) [520]
532 (0.24) [850]
521 (0.25) [570]
490 (0.10)
492 (0.10)
494 (0.10)
502 (0.07)
507 (0.08)
497 (0.06)
489 (0.07)
1250
1260
1250
1330
1290
1310
1260
5
BPhe-b
BPhe-g
a
Relative peak intensity Irel was based on the Soret peak intensity.
7
b
c
D½Qyð0; 0ÞKð1; 0ÞꢀZð1=l ½Qyð1; 0ÞꢀK1=l ½Qyð0; 0ÞꢀÞ!10 :
max
max
7
D½Qxð0; 0ÞKð1; 0ÞꢀZð1=l ½Qxð1; 0ÞꢀK1=l ½Qxð0; 0ÞꢀÞ!10 :
max
max
Table 2. Absorption maxima (nm) observed for 1–5 in CH
2
2
Cl (exp) and estimated by ZINDO/S calculation (calcd)
Compound
Qy(0,0)
Qx(0,0)
Exp
Calcd
Exp
Calcd
1
2
3
4
5
715
722
730
752
760
727
727
739
745
751
522
524
527
538
543
551
552
560
562
563
2
pyropheophorbide-d (16) were prepared according to the
reported procedures.
0
would also be useful to elucidate optical properties of semi-
natural BPhes-b/g, as well as natural BChls-b/g in the
distorted forms in the polypeptide environment where the
3-substituents were conformationally restricted for rotation
around the 3–3 bond.
3.2. Estimation of electronic absorption peaks in 1–5
using ZINDO/S
1
33,34
Initial molecular structures of bacteriochlorins 1–5 were
made using MMC and PM3 in HYPERCHEM version 6.0
3. Experimental
3
0–32
according to the literature.
In the calculation, configu-
rations of the 7-methyl group in 1–5 were fixed in the same
direction as natural BChls- and BPhes-b/g. Their energy
minimized structures were made by repeating ZINDO/S and
MMC calculations until the MMC calculation was finished
with one cycle. In the ZINDO/S calculation, were used
HOMO and LUMO whose energy gaps were less than 7 eV.
Their electronic absorption peaks were estimated from the
ZINDO/S calculation based on their energy minimized
structures.
3
.1. General
UV–VIS–NIR spectra were measured in air-saturated
solvents at room temperature on a Hitachi U-3500
spectrophotometer. H NMR spectra in chloroform-d were
recorded at room temperature with a JEOL JNM-A400
Fourier transform NMR spectrometer; tetramethylsilane
was used as an internal standard. H– H COSY and H– H
3
0
1
1
1
1
1
ROESY (t Z400 ms) were recorded to determine the
m
molecular structure of synthetic compounds. FAB-MS
spectra were recorded on a JEOL GCmate II spectrometer;
FAB-MS samples were dissolved in chloroform and
m-nitrobenzyl alcohol and polyethylene glycol were used
as a matrix and an internal standard, respectively. HPLC
was carried out with a Shimadzu LC-10AD pump and an
SPD-M10A photodiode array detector. A packed silica gel
column (Cosmosil 5SL-II, Nacalai Tesque, 6.0f 250 mm)
was used for normal-phase HPLC. Dichloromethane and
acetone for UV–VIS–NIR spectra were purchased from
Nacalai Tesque (grade for spectroscopy). FCC was carried
out on silica gel (Merck Kieselgel 60, 9358). All procedures
including syntheses and spectral measurements were
performed in the dark.
2
3,25
3.3. Oxidation of C7–C8 double bond
1.4 Equivalent of OsO and a small amount of pyridine were
4
added to a dichloromethane solution of chlorin (1 equiv).
After stirring at room temperature for 16 h under nitrogen,
the reaction was quenched by addition of methanol and
10-min bubbling of H S gas. The resulting OsS was
removed and the filtrate was evaporated in vacuo. The
residue was purified by FCC and recrystallization from
dichloromethane and hexane to give the corresponding 7,8-
cis-dihydroxy-chlorin.
2
4
2
3
Oxidation of 13 gave 17 in 69% yield (lit., 52%).
2
Oxidation of 14 gave 18 in 57% yield (lit., 56%).
5
3
Methyl pheophorbide-a (11), methyl pyropheophorbide-a
5
27
Oxidation of 15 gave 19 in 75% yield (lit., 57%).
2
0
24
Oxidation of 16 gave 20 in 71% yield (lit., 74%).
(
(
3
12),
13), methyl bacteriopheophorbide-d (14), methyl
methyl 3-devinyl-3-ethyl-pyropheophorbide-a
2
0
21
28
Oxidation of 21 gave 22 and the crude product without
FCC was used for the following pyrolysis due to instability
2
-acetyl-3-devinyl-pyropheophorbide-a (15) and methyl
2

M. Kunieda et al. / Tetrahedron 60 (2004) 11349–11357
11355
2
of 22 which possessed the 13 -methoxycarbonyl group for
FCC.
3.5. Diels–Alder reaction
TCNE (1.2 equiv) was added to a 8-vinyl-chlorin (ca.
0 mmol) in dry chloroform (25 ml) and refluxed for 30 min
under nitrogen. The solution was poured into water, washed
twice, extracted with dichloromethane and dried over
5
3
3
.4. Preparation of 3-substituted 8-vinyl-chlorins
2
3,27
.4.1. Double dehydration of 7,8-cis-diol
. To a
Na
FCC (Et
2
SO
4
. After evaporation, the residue was purified by
O/CH Cl ) and recrystallization from dichloro-
dichloromethane and benzene solution (1:4) of 7,8-cis-
diol, p-toluenesulfonic acid was added and the reaction
mixture was stirred at room temperature for 2 h under
nitrogen. After consumption of the diol was completed to
give singly dehydrated 8-(1-hydroxyethyl)-chlorin by
monitoring blue-shifted Qy maximum to 650–690 from
2
2
2
methane and hexane to give the corresponding bacterio-
chlorin as a 1:1 7-epimeric mixture.
3.5.1. TCNE adduct of 3-ethyl-8-vinyl-chlorin (1). Diels–
Alder reaction of 3-ethyl-8-vinyl-chlorin 6 with TCNE
afforded bacteriochlorin 1 as a dark green solid in 91% yield
7
00–760 nm, the reaction mixture was refluxed for 30 min.
(
3–4% Et O/CH Cl for FCC); UV–VIS–NIR (CH Cl )
2 2 2 2 2
The solution was poured into ice-cold water and extracted
with dichloromethane. The organic phase was washed with
lmaxZ715 (rel. 0.50), 651 (0.13), 598 (0.04), 522 (0.25),
4
1
90 (0.10), 460 (0.06), 393 (0.85), 370 nm (1.0); H NMR
4
% KHSO4 and water, and dried over Na SO . After
2 4
(
CDCl ) dZ8.76/75 (1H, s, 10-H), 8.62/61 (1H, s, 5-H),
evaporation in vacuo, the residue was dissolved in a small
amount of dichloromethane and subsequently an ethereal
diazomethane solution was added and stirred for 30 min.
After removal of the solvents in vacuo, the residue was
purified with FCC and recrystallization from dichloro-
methane and hexane to give pure 8-vinyl-chlorin.
3
8
5
1
3
.27/26 (1H, s, 20-H), 6.74–6.79 (1H, m, 8-CH), 5.05/4.89,
1
.02/4.86 (each 1H, d, JZ20 Hz, 13 -CH ), 4.26 (1H, m,
2
1
8-H), 4.08 (1H, m, 17-H), 3.82–3.99 (2H, m, 8 -CH ),
2
.64–3.68 (2H, m, 3-CH ), 3.63/62 (3H, s, COOCH ), 3.43
2
3
(3H, s, 12-CH ), 3.17 (3H, s, 2-CH ), 2.48–2.58, 2.17–2.33
3 3
(each 2H, m, 17-CH CH ), 2.24/23 (3H, s, 7-CH ), 1.67–
2 2 3
1
.74 (3HC3H, m, 3 -CH , 18-CH ), 0.64/62 and K0.69/71
2
Dehydration of 17 gave 6 in 60% yield (lit., 40%).
3
1
3 3
2
7
(each 1H, s, NH). MS (FAB) found: m/z 676.2920. Calcd for
C H N O : M , 676.2910.
Dehydration of 19 gave 9 in 28% yield (lit., 44%). The
other double dehydration of 22 including removal of
hexanol and hydrolysis-de(carbon dioxide) of methoxycar-
bonyl group by heating in 1,2-dichlorobenzene at 160 8C
C
40 36
8 3
3.5.2. TCNE adduct of 3-(1-hydroxyethyl)-8-vinyl-
chlorin (2). Diels–Alder reaction of 3-(1-hydroxyethyl)-8-
2
8
was done according to the reported procedures to give 8 in
4
2
0% total yield based on 21 (lit., 45%).
8
vinyl-chlorin 7 with TCNE afforded bacteriochlorin 2 as a
3
1
/7-diastereomeric mixture as a green solid in 55% yield
5–6% Et O/CH Cl for FCC); UV–VIS–NIR (CH Cl )
(
Compound 10. Dehydration of 20 (52.0 mg) in dichloro-
methane and benzene (8/24 ml) afforded methyl 8-deethyl-
2 2 2 2 2
lmaxZ722 (rel. 0.56), 656 (0.13), 598 (0.04), 524 (0.27),
4
1
92 (0.10), 461 (0.06), 394 (0.87), 371 nm (1.0); H NMR
8
brown solid after FCC (3–4% Et O/CH Cl ) and recrys-
-vinyl-pyropheophorbide-d (10, 11.7 mg, 24%) as dark
(CDCl ) dZ9.33/32/19/15 (1H, s, 5-H), 8.82/81 (1H, s,
3
2
2
2
1
8
0-H), 8.36/35/34/33 (1H, s, 20-H), 6.71–6.80 (1H, m,
-CH), 6.23–32 (1H, m, 3-CH), 5.02–5.14, 4.87–4.93 (each
tallization (CH Cl /hexane); UV–VIS–NIR (CH Cl )
2
2
2
2
lmaxZ693 (relative intensity, 0.60), 631 (0.07), 557
1
1
0.09), 526 (0.14), 436 (1.00), 390 nm (0.60); H NMR
1H, m, 13 -CH ), 4.29 (1H, m, 18-H), 4.11 (1H, m, 17-H),
2
3
4
2.63, 2.17–2.32 (each 2H, m, 17-CH CH ), 2.25/24/23 (3H,
2 2
s, 7-CH ), 2.08–2.15 (3H, m, 3 -CH ), 1.67–1.75 (3H, m,
3 3
(
(
(
1
.81–3.99 (2H, m, 8 -CH ), 3.63/62 (3H, s, COOCH ), 3.49/
2 3
CDCl ) dZ11.57 (1H, s, 3-CHO), 10.41 (1H, s, 5-H), 9.80
3
8/45 (3H, s, 12-CH ), 3.30/29/27/26 (1H, s, 2-CH ), 2.49–
3
3
1H, s, 10-H), 8.85 (1H, s, 20-H), 7.97 (1H, dd, JZ11,
1
1
6
(
8 Hz, 8-CH), 6.65 (1H, d, JZ11 Hz, 8 -CH-cis-to 8-CH),
1
1
.20 (1H, d, JZ18 Hz, 8 -CH -trans-to 8-CH), 5.35, 5.20
each 1H, d, JZ19 Hz, 13 -CH ), 4.59 (1H, dq, JZ2, 8 Hz,
2
1
18-CH ), 0.36/33 and K0.96/99 (each 1H, s, NH). MS
3
2
C
FAB) found: m/z 692.2888. Calcd for C H N O : M ,
(
40 36 8 4
1
3
7
1
8-H), 4.49 (1H, dt, JZ8, 2 Hz, 17-H), 3.79 (3H, s, 2-CH3),
6
92.2860.
.73 (3H, s, 12-CH ), 3.62 (3H, s, COOCH ), 3.46 (3H, s,
-CH ), 2.69–2.78, 2.56–2.65, 2.25–2.39 (1HC1HC2H, m,
3
3
3
3
.5.3. TCNE adduct of 3,8-divinyl-chlorin (3). Diels–
7-CH CH ), 1.85 (3H, d, JZ8 Hz, 18-CH ), K0.19 and
2
2
3
Alder reaction of 3,8-divinyl-chlorin 8 with TCNE afforded
bacteriochlorin 3 as a dark green solid in 71% yield (3–4%
Et O/CH Cl for FCC); UV–VIS–NIR (CH Cl ) lmaxZ730
K2.05 (each 1H, s, NH). MS (FAB) found: m/z 548.2417.
Calcd for C H N O : M , 548.2424.
C
3
3 32 4 4
2
2
2
2
2
(
rel. 0.58), 662 (0.13), 608 (0.04), 527 (0.27), 494 (0.10),
1
3
.4.2. Methyl 8-deethyl-8-vinyl-bacteriopheophorbide-d
7). To a dichloromethane solution (15 ml) of 3-acetyl-
463 (0.06), 394 (0.88), 374 nm (1.0); H NMR (CDCl ) dZ
3
2
7
(
chlorin 8 (13.3 mg), a methanol solution (200 ml) saturated
8.83 (1H, s, 5-H), 8.82 (1H, s, 10-H), 8.40 (1H, s, 20-H),
7.75–7.80 (1H, m, 3-CH), 6.77 (1H, m, 8-CH), 6.19–6.27
1
(2H, m, 3 -CH ), 5.03–5.10, 4.88–4.94 (each 1H, m, 13 -
1
with NaBH was added and the reaction mixture was stirred
4
2
for 10 min under nitrogen. The reaction mixture was poured
into ice-cold water and the separated organic phase was
washed with 2% HCl and water, and dried over Na SO .
CH ), 4.30 (1H, m, 18-H), 4.12 (1H, m, 17-H), 3.82–3.98
2
1
(2H, m, 8 -CH ), 3.63/62 (3H, s, COOCH ), 3.46 (3H, s,
2
3
2
4
12-CH ), 3.29 (3H, s, 2-CH ), 2.50–2.66, 2.21–2.40 (each
3
3
After removal of the solvent in vacuo, the residue was
purified with FCC (8% Et O/CH Cl ) and recrystallization
from dichloromethane and hexane to give pure 7 (10.7 mg)
2H, m, 17-CH CH ), 2.24/2.23 (3H, s, 7-CH ), 1.74/71 (3H,
2 2 3
2
2
2
d, JZ8 Hz, 18-CH ), 0.34/32 and K0.91/93 (each 1H, s,
3
NH). MS (FAB) found: m/z 674.2765. Calcd for
C H N O : M , 674.2754.
2
7
C
in 80% yield (lit., 82%).
4
0 34 8 3

11356
M. Kunieda et al. / Tetrahedron 60 (2004) 11349–11357
3
.5.4. TCNE adduct of 3-acetyl-8-vinyl-chlorin (4).
(second elution): UV–VIS–NIR (CH Cl ) lmaxZ743 (rel.
2 2
Diels–Alder reaction of 3-acetyl-8-vinyl-chlorin 9 with
TCNE afforded bacteriochlorin 4 as a dark brown solid in
1.23), 671 (0.14), 614 (0.06), 529 (0.36), 497 (0.12), 465
1
(0.09), 397 (1.0), 374 nm (0.91); H NMR (CDCl ) dZ9.10
3
8
2% yield (3–4% Et O/CH Cl for FCC); UV–VIS–NIR
2
(1H, s, 10-H), 8.99/98 (1H, s, 5-H), 8.76 (1H, s, 20-H), 6.94–
7.01 (1H, m, 8-CH), 6.28–6.38 (1H, m, 3-CH), 5.18–5.27,
2
2
(
(
(
CH Cl ) l Z751 (rel. 0.88), 677 (0.11), 620 (0.03), 538
2
2
max
1
0.28), 502 (0.07), 469 (0.06), 376 nm (1.0); H NMR
1
5.04–5.13 (each 1H, m, 13 -CH ), 4.77–4.85, 4.02–4.07
2
1
(each 1H, m, 3 -CH ), 4.45 (1H, m, 18-H), 4.27 (1H, m, 17-
CDCl ) dZ9.63 (1H, s, 5-H), 8.99/98 (1H, s, 10-H), 8.71/
3
2
1
H), 3.78–4.09 (2H, m, 8 -CH ), 3.67 (3H, s, 2-CH ), 3.64/63
7
4
0 (1H, s, 20-H), 6.83–6.88 (1H, m, 8-CH), 5.13–5.21,
.98–5.05 (each 1H, m, 13 -CH ), 4.41 (1H, m, 18-H), 4.22
2
3
1
(3H, s, COOCH ) 3.57 (3H, s, 12-CH ), 2.53–2.71, 2.18–
3 3
2.36 (each 2H, m, 17-CH CH ), 2.37/36 (3H, s, 7-CH ),
2 2 3
2
1
(1H, m, 17-H), 3.83–4.02 (2H, m, 8 -CH ), 3.63/62 (3H, s,
COOCH ), 3.57 (3H, s, 2-CH ), 3.53 (3H, s, 12-CH ), 3.28
2
1.82/77 (3H, d, JZ7 Hz, 18-CH ), K0.58/61 and K1.62/64
3
3
3
3
(
3H, s, 3-COCH ), 2.52–2.68, 2.18–2.34 (each 2H, m, 17-
3
(each 1H, s, NH). MS (FAB) found: m/z 802.2884 Calcd for
C H N O : M , 802.2877.
C
CH CH ), 2.29 (3H, s, 7-CH ), 1.74–1.79 (3H, d, JZ8 Hz,
2
2
3
46 34 12 3
1
(
8-CH ), K0.37/41 and K1.54/56 (each 1H, s, NH). MS
3
C
FAB) found: m/z 690.2696. Calcd for C H N O : M ,
4
0 34 8 4
3.6. Preparation of BPhes-b/g
6
90.2703
An acetone solution (10 ml) of extracted pigments from
cultured Blastochloris viridis was demetallated by 5-min
1
3
.5.5. TCNE adduct of 3-formyl-8-vinyl-chlorin (5).
Diels–Alder reaction of 3-formyl-8-vinyl-chlorin 10 with
TCNE afforded bacteriochlorin 5 as a dark brown solid in
stirring with an aqueous diluted HCl solution (3 ml). The
reaction mixture was poured into ice-cold water and
extracted by dichloromethane. After evaporation in vacuo,
the crude mixture including BPhe-b and some carotenoids
was separated and purified by normal-phase HPLC
(acetone/hexaneZ1/4) to give pure BPhe-b.
8
4% yield (3–5% Et O/CH Cl for FCC); UV–VIS–NIR
2 2 2
(
(
(
CH Cl ) l Z760 (rel. 1.30), 686 (0.09), 627 (0.03), 543
2 2 max
1
0.39), 507 (0.08), 474 (0.07), 381 nm (1.0); H NMR
CDCl ) dZ11.44 (1H, s, 3-CHO), 9.98/97 (1H, s, 5-H),
3
9
m, 8-CH), 5.17–5.25, 5.03–5.10 (each 1H, m, 13 -CH ),
.04/03 (1H, s, 10-H), 8.79/78 (1H, s, 20-H), 6.89–6.93 (1H,
1
Pure BPhe-g was also prepared by the above procedures
except that pigment extracts from Heliobacterium
modesticaldum were used.
2
4
8
3
.44 (1H, m, 18-H), 4.26 (1H, m, 17-H), 3.87–4.04 (2H, m,
-CH ), 3.70 (3H, s, 2-CH ), 3.65/63 (3H, s, COOCH )
.56 (3H, s, 12-CH ), 2.56–2.70, 2.18–2.30 (each 2H, m, 17-
3
1
36
2
3
3
CH CH ), 2.32 (3H, s, 7-CH ), 1.77/80 (3H, d, JZ8 Hz, 18-
3.7. Acidic treatment of natural BPhes-b/g and synthetic
4/3
2
2
3
CH ), K0.50/54 and K1.58/60 (each 1H, s, NH). MS
(
3
C
FAB) found: m/z 676.2564. Calcd for C H N O : M ,
39 32 8 4
6
76.2547.
To an acetone solution (3 ml) of sample (ca. 10 mmol), an
aqueous 3.5% HCl solution (10 ml) was added allowed to
stand in the dark. UV–VIS–NIR spectra were recorded at
2/3 min intervals for BPhes-b/g and every 30 min for 4 and
3. When Qy maxima of BPhes-b/g had completely
disappeared, the acidic acetone solution was diluted with
water and extracted with dichloromethane. In 4/3, the same
work-up was done after 5-h standing. After evaporation in
vacuo, the residue was analyzed by normal-phase HPLC
(acetone/hexaneZ1/4 for BPhes-b/g and 3/7 for 4/3).
Chromatograms given from BPhe-b or g showed some
chlorin chromophores, 3-Ac-Phe-a or Phe-a at 11 or 9 min,
respectively, which was eluted faster than BPhe-b or g at 13
3
.5.6. [2C2] TCNE adduct at the 3-vinyl group of 3 (23).
The title compound 23 was isolated in 11% yield as a by-
product in the Diels–Alder reaction of 3,8-divinyl-chlorin 8
with TCNE described above. Alternative preparation of 23
as a major product was achieved as follows. TCNE
(
3.0 equiv) was added to 3,8-divinyl-chlorin 8 (50 mmol)
in dry chloroform (20 ml) and refluxed for 30 min under
nitrogen. The solution was repeatedly evaporated in vacuo
after addition of chloroform (each 15 ml). When the Qy
maximum was completely red-shifted (730–740 nm), the
reaction mixture was worked up in a similar manner with
the above Diels–Alder reaction to give pure 23 as a 3 /7-
diastereomeric mixture as a dark brown solid in 78% yield
(6% Et O/CH Cl for FCC). The mixture was separated into
2 2 2
two fractions on normal-phase silica gel HPLC (acetone/
1
1
or 11 min, and a small amount of 8 -oxidized chlorin at over
50 min. HPLC from 4 or 3 showed a single peak at 10 or
8 min, which was consistent with the elution time of 4 or 3.
1
hexaneZ3:7) to afford each 3 -epimerically pure sample as
a 7-epimeric mixture, but the absolute configurations could
not be determined. Fraction 1 (first elution): UV–VIS–NIR
Acknowledgements
(
(
CH Cl ) l Z743 (rel. 1.23), 671 (0.14), 614 (0.06), 529
We thank Prof. Hirozo Oh-oka of Osaka University and
Prof. Keizo Shimada of Tokyo Metropolitan University for
the provision of Heliobacterium modesticaldum and
Blastochloris viridis, respectively. Technical assistance of
MS-measurements by Dr. Tomohiro Miyatake of Ryukoku
University, and helpful discussions with Dr. Shin-ichi
Sasaki and Mr. Kazunori Hamada of Ritsmeikan
University are also acknowledged. This work was partially
supported by Grants-in-Aid for Scientific Research
(No. 15033271) on Priority Areas (417) from the Ministry
of Education, Culture, Sports, Science and Technology
2
2
max
1
0.36), 497 (0.12), 465 (0.09), 397 (1.0), 374 nm (0.91); H
NMR (CDCl ) dZ9.10 (1H, s, 10-H), 8.96/92 (1H, s, 5-H),
3
8
.77/76 (1H, s, 20-H), 6.92–6.99 (1H, m, 8-CH), 6.04–6.11
1H, m, 3-CH), 5.17–5.29, 5.03–5.12 (each 1H, m, 13 -
1
(
CH ), 4.94–5.04, 4.06–4.15 (each 1H, m, 3 -CH ), 4.47
1
2
2
1
1H, m, 18-H), 4.27 (1H, m, 17-H), 3.88–4.07 (2H, m, 8 -
(
CH ), 3.65/64 (3H, s, 2-CH ), 3.62 (3H, s, COOCH ) 3.57
(
CH CH ), 2.30 (3H, s, 7-CH ), 1.81/77 (3H, d, JZ7 Hz, 18-
CH ), K0.76/78 and K1.78/80 (each 1H, s, NH). Fraction 2
2
3
3
3H, s, 12-CH ), 2.54–2.72, 2.16–2.38 (each 2H, m, 17-
3
2
2
3
3

M. Kunieda et al. / Tetrahedron 60 (2004) 11349–11357
11357
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MEXT) of the Japanese Government and for Scientific
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17. Tamiaki, H.; Miyata, S.; Kureishi, Y.; Tanikaga, R.
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