A facile synthetic method for conversion of chlorophyll-a to bacteriochlorophyll-c

Article abstract of DOI:10.1021/jo0703855

(Chemical Equation Presented) A simple synthetic route for converting chlorophyll(Chl)-a to bacteriochlorophyll(BChl)-c is described. Methyl bacteriopheophorbide(MBPhe)-d, easily obtained from Chl-a, was selectively brominated at the 20-position with pyridinium tribromide, and the following Suzuki-coupling with methylboronic acid afforded ca. a 4:1 mixture of desired MBPhe-c (20-Me) and debrominated MBPhe-d (20-H) in quantitative yield. Separation of MBPhe-c, transesterification of the 17-propionate group (Me → farnesyl), and magnesium insertion successfully led to naturally occurring BChl-c.

Full text of DOI:10.1021/jo0703855

A Facile Synthetic Method for Conversion of
Chlorophyll-a to Bacteriochlorophyll-c
†
Shin-ichi Sasaki, Tadashi Mizoguchi, and Hitoshi Tamiaki*
Department of Bioscience and Biotechnology, Faculty of Science
and Engineering, Ritsumeikan UniVersity,
Kusatsu, Shiga 525-8577, Japan
tamiaki@se.ritsumei.ac.jp
ReceiVed February 26, 2007
FIGURE 1. Chemical structures of (a) chlorophyll-a (1) and (b)
homologues of bacteriochlorophyll-c (2) isolated from Chlorobium
tepidum.
5
precursor such as 3, we recently developed a direct methylation
6
technique of Zn-4 to Zn-5 as shown in Scheme 1. Since the
enzymatic reaction has been performed in microgram scale due
to the limited amount of the purified methyl transferase, His-
6
,7
tagged BchU,
we decided to investigate an alternative
synthetic method of 20-methylation toward a Chl-a derivative
to obtain one of the less abundant BChl-c homologues (2R[E,M])
in green sulfur bacteria, epimerically pure 3 R-BChl-c possess-
A simple synthetic route for converting chlorophyll(Chl)-a
to bacteriochlorophyll(BChl)-c is described. Methyl bacte-
riopheophorbide(MBPhe)-d, easily obtained from Chl-a, was
selectively brominated at the 20-position with pyridinium
tribromide, and the following Suzuki-coupling with meth-
ylboronic acid afforded ca. a 4:1 mixture of desired MBPhe-c
1
ing 8-ethyl and 12-methyl groups. Although the structural
differences between 3 and 5 are only two functional groups at
the 3- and 20-positions, traditional synthetic transformation
required multiple steps with a low total yield due to the necessity
8
(20-Me) and debrominated MBPhe-d (20-H) in quantitative
of various protection/deprotection processes. In light of recent
9
yield. Separation of MBPhe-c, transesterification of the 17-
propionate group (Me f farnesyl), and magnesium insertion
successfully led to naturally occurring BChl-c.
progress in metal-catalyzed cross-coupling reactions, here we
report an improved synthetic route of converting natural Chl-a
(1) to the corresponding BChl-c homologue (2).
As to the synthetic study of BChl-c derivatives, Smith et al.
examined various routes for obtaining 5 from Chl-a derivatives
including 3. Among several candidates for the 20-selective
introduction of a functional group, they reported the following
procedure as a successful method: (i) protection of the reactive
Chlorophyll(Chl)-a (1, see Figure 1a) is a representative dye-
pigment in oxygenic photosynthesis, while bacteriochlorophyll-
8
(
BChl)s-c (2, see Figure 1b) are found as main light-harvesting
1
pigments in green photosynthetic bacteria. Although the
synthesis and self-assembling ability of BChl-d derivatives, 20-H
analogues of BChl-c, have been extensively investigated, studies
(4) Studies of natural BChl-d: (a) Nozawa, T.; Ohtomo, K.; Takeshita,
N.; Morishita, Y.; Osawa, M.; Madigan, M. T. Bull. Chem. Soc. Jpn. 1992,
65, 3493-3494. (b) Mizoguchi, T.; Saga, Y.; Tamiaki, H. Photochem.
Photobiol. Sci. 2002, 1, 780-787. For model compounds of BChl-d: (c)
Cheng, P.; Liddell, P. A.; Ma, S. X. C.; Blankenship, R. E. Photochem.
Photobiol. 1993, 58, 290-295. (d) Tamiaki, H. Photochem. Photobiol. Sci.
of BChl-c derivatives are relatively limited probably due to their
lesser accessibility from natural sources.2
-4
Aiming at the
introduction of 20-methyl group on an easily accessible chlorin
2
005, 4, 675-680.
(
5) Tamiaki, H.; Amakawa, M.; Shimono, Y.; Tanikaga, R.; Holzwarth,
*
To whom correspondence should be addressed. Fax: +81-77-561-2659,
Tel: +81-77-566-1111.
A. R.; Schaffner, K. Photochem. Photobiol. 1996, 63, 92-99.
(6) Harada, J.; Saga, Y.; Yaeda, Y.; Oh-oka, H.; Tamiaki, H. FEBS Lett.
2005, 579, 1983-1987.
†
Present address: Nagahama Institute of Bio-Science and Technology,
Nagahama, Shiga 526-0829, Japan.
(1) (a) Scheer, H. In Chlorophylls and Bacteriochlorophylls: Biochem-
(7) (a) Harada, J.; Wada, K.; Yamaguchi, H.; Oh-oka, H.; Tamiaki, H.;
Fukuyama, K. Acta Crystallogr. Sect. F 2005, 61, 712-714. (b) Wada, K.;
Yamaguchi, H.; Harada, J.; Niimi, K.; Osumi, S.; Saga, Y.; Oh-oka, H.;
Tamiaki, H.; Fukuyama, K. J. Mol. Biol. 2006, 360, 839-849.
(8) (a) Smith, K. M.; Bisset, G. M. F.; Tabba, H. D. Tetrahedron Lett.
1980, 21, 1101-1104. (b) Smith, K. M.; Bisset, G. M. F.; Bushell, M. J.
J. Org. Chem. 1980, 45, 2218-2224. (c) Smith, K. M.; Bisset, G. M. F.;
Bushell, M. J. Bioorg. Chem. 1980, 9, 1-26. (d) Smith, K. M.; Goff, D.
A.; Simpson, D. J. J. Am. Chem. Soc. 1985, 107, 4946-4954.
(9) (a) Miyaura, N.; Suzuki, A. Chem. ReV. 1995, 95, 2457-2483. (b)
Chemler, S. R.; Trauner, D.; Danishefsky, S. J. Angew. Chem., Int. Ed.
2001, 40, 4544-4568. (c) Hassan, J.; Sevignon, M.; Gozzi, C.; Schulz, E.;
Lemaire, M. Chem. ReV. 2002, 102, 1359-1469.
istry, Biophysics, Functions and Applications; Grimm, B., Porra, R. J.,
R u¨ diger, W., Scheer, H., Eds.; Springer: The Netherlands, 2006; Chapter
1
, pp 1-26. (b) Tamiaki, H.; Mizoguchi, T.; Shibata, R. Photochem.
Photobiol. 2007, 83, 152-162.
2) For reviews, see: (a) Tamiaki, H. Coord. Chem. ReV. 1996, 148,
(
1
83-197. (b) Miyatake, T.; Tamiaki, H. J. Photochem. Photobiol. C:
Photochem. ReV. 2005, 6, 89-107. (c) Balaban, T. S.; Tamiaki, H.;
Holzwarth, A. R. Topics Curr. Chem. 2005, 258, 1-38.
(3) (a) Tamiaki, H.; Takeuchi, S.; Tsuzuki, S.; Miyatake, T.; Tanikaga,
R. Tetrahedron 1998, 54, 6699-6718. (b) Tamiaki, H.; Omoda, M.; Saga,
Y.; Morishita, H. Tetrahedron 2003, 59, 4337-4350, and references cited
therein.
10.1021/jo0703855 CCC: $37.00 © 2007 American Chemical Society
4
566
J. Org. Chem. 2007, 72, 4566-4569
Published on Web 05/12/2007

SCHEME 1
SCHEME 2
FIGURE 2. HPLC charts of (a) synthetic epimerically pure 2R[E,M],
b) natural bacteriochlorophyll-c mixtures extracted from Chlorobium
(
tepidum, and (c) cochromatography of synthetic 2R[E,M] with natural
extracts. Absorbance was monitored at 435 nm.
TABLE 1. Palladium Catalyzed Cross-Coupling Methylation of
2
0-Bromo-chlorin 6a
entry MeB(OH)3 (equiv) PdCl2(dppf) (equiv) ratio of products (4:5)b
1
2
3
4
10
10
100
100
1
1:2.7
1:4.0
1:4.2
c
0.1
0.05
0.01
a
All reactions were conducted in refluxed THF in the presence of
Cs2CO3. b Determined by H NMR. Reaction did not proceed probably
1
c
due to the deactivation of the small amount of catalyst.
1
1
)
1/1) with pyridinium tribromide in CH2Cl2 proceeded
1
smoothly to give 6 in 87% yield as a 1:1 3 R/S mixture. Among
three kinds of cross-coupling methylations examined, Suzuki-
coupling using MeB(OH)2 as a methyl source and PdCl2(dppf)
as a catalyst in the presence of Cs2CO3 in THF gave a better
result than the following two types of Pd-catalyzed conditions.
3
-vinyl group as the 3-(2-chloroethyl) group, (ii) introduction
of the methylthiomethyl group at the 20-position on the copper
II) complex, and (iii) reduction toward the 20-methyl group,
12
Negishi-coupling using ZnMe and PdCl (dppe) in 1,4-dioxane
2
2
(
gave a complex mixture containing unmethylated Zn-6. Me-
thylation of 4 using (AlMe )‚DABCO, Pd (dba) , and phosphine
demetalation, deprotection, and hydration of the 3-vinyl to 3-(1-
hydroxyethyl) group.8d Since the reaction scheme was reported
to be ca. 10 steps with 3% yield, we investigated cross-coupling
methylations leading to a more direct synthetic route for
3
2
3
13
ligand in THF gave 5 up to 10% which can be ascribable to
the presence of enolizable protons in 4. Thus, reaction conditions
of Suzuki-coupling were optimized, and the results are sum-
marized in Table 1. Although the debromination 6 to 4 was
converting 20-H to 20-Me group. In the first stage, selective
a,8b
14
2
0-bromination of 3-(1-hydroxyethyl)chlorin 43 was exam-
inevitable, this byproduct was less produced by increasing the
ined (Scheme 2) because it is necessary to avoid the bromination
of the 3-vinyl group and also because the hydration of this group
is a required step for the synthesis of BChl-c. Although the
bromination of 4 using HBr-H2O2 in aqueous THF did not work
ratio of MeB(OH) to PdCl (dppf), and all the reactions were
essentially quantitative to afford only a mixture of 4 and 5 after
2
2
(11) 20-Bromination of methyl mesopyropheophorbide-a possessing a
3
-ethyl group with pyridinium tribromide was reported to be 79%: Kelley,
R. F.; Tauber, M. J.; Wasielewski, M. R. J. Am. Chem. Soc. 2006, 128,
779-4791.
12) Herbert, J. M. Tetrahedron Lett. 2004, 45, 817-819.
(13) Cooper, T.; Novak, A.; Humphreys, L. D.; Walker, M. D.;
Woodward, S. AdV. Synth. Catal. 2006, 348, 686-690.
well in contrast to the successful 20-bromination of 1-hy-
droxymethyl-chlorin,10 it was found that treatment of 4 (31R/S
4
(
(
10) Kureishi, Y.; Tamiaki, H. J. Porphyrins Phthalocyanines 1998, 2,
1
59-169.
J. Org. Chem, Vol. 72, No. 12, 2007 4567

7
R17 in 88% yield (Scheme 3). Under the ester-exchange
1
conditions, the secondary hydroxy group at the 3 -position did
18
not react, presumably due to the steric hindrance. Magnesium
insertion was done by treating with excess Mg(ClO4)2 in
pyridine, and undesired dehydration of the 3-(1-hydroxyethyl)
group partially occurred to form magnesium 3-vinyl-chlorin 8.
The reaction mixture was purified by HPLC, and finally pure
2
R[E,M] was identified by comparison with the authentic
sample from naturally occurring BChls-c. Extracts from a green
sulfur bacteria, Chlorobium tepidum, contain several BChl-c
homologues as represented in Figure 2b. Thus the absoption
and CD spectra (Figure 3) as well as the HPLC peak (Figure
2c) of synthetically derived 2R[E,M] are shown to be coincident
with those of the natural component. It is noteworthy that no
racemization occurred during the conversion from 6R to
2
R[E,M].
In conclusion, we have developed a simple and efficient
synthetic route for converting natural Chl-a (1) to BChl-c (2).
Sequential procedures demonstrated in this study should be
useful for the synthesis of natural BChl-c analogues possessing
FIGURE 3. (a) Absorption and (b) CD spectra of synthetic epimeri-
cally pure 2R[E,M] (solid line) and natural product (dotted line). Each
-
5
sample was measured in Et
2
O at ca. 1 × 10 M.
1
b,19
various 17-propionate terminals.
SCHEME 3
Experimental Section
1
1
Proton peaks were assigned by H- H COSY and NOESY
spectra, and carbon peaks except for quaternary peaks were assigned
by DEPT and 13C- H COSY spectra. Methyl bacteriopyropheophor-
1
5
bide-d (4) was prepared from methyl pyropheophorbide-a (3) by
slight modifications of the previously reported procedure in 86%
yield.3
a
Methyl 20-Bromobacteriopheophorbide-d (6). To a solution
1
of 4 (3 R/S ) 1:1, 101 mg, 0.18 mmol) in CH
2
Cl
2
(50 mL) was
added pyridinium tribromide (75 mg, 0.23 mmol), and the mixture
was stirred for 30 min at room temperature. The mixture was poured
into 2% aqueous HCl and extracted with CH
washed with brine, dried over anhydrous Na
concentrated. The crude product was purified by silica gel chro-
matography (Et O-CH Cl , 1:9) to give 6 (100 mg, 87%) as a 1:1
R/S mixture. The epimeric mixture was separated by HPLC
2
Cl
2
. The extract was
2
SO
4
, filtered, and
2
2
2
1
3
(
1
6
(
Cosmosil 5SL-II, 10 mm φ × 250 mm, acetone/1,2-dichloroethane
:19, 2 mL/min, t
R
) 19 and 21 min for 6R and 6S, respectively).
Cl ) λmax 673 (ꢀ, 53800), 550
R: mp 144-146 °C; vis (CH
2
2
1
15600), 518 (8840), 414 nm (114000); H NMR (CDCl
3
, 600
MHz) δ ) 9.83 (1H, s, 5-H), 9.08 (1H, s, 10-H), 6.34 (1H, q, J )
1
7
(
(
Hz, 3-CH), 4.92, 4.74 (each 1H, d, J ) 19 Hz, 13 -CH
1H, q, J ) 7 Hz, 18-H), 3.92 (1H, dd, J ) 3, 10 Hz, 17-H), 3.66
3H, s, COOCH ), 3.58 (2H, q, J ) 7 Hz, 8-CH ), 3.57 (3H, s,
-CH ), 3.33 (3H, s, 12-CH ), 3.20 (3H, s, 7-CH ), 3.05 (1H, br,
OH), 2.57, 2.20 (each 1H, m, 17 -CH ), 2.09, 1.76 (each 1H, m,
2
), 4.70
consuming 6. Regarding the yield of 6 to 5 as 80% (entry 2),
the present scheme from 3 to 5 could be much improved
compared to the reported procedure,8d because reaction steps
were shortened to one-third and the yield was increased 20-
fold (from the previous 3% for ten steps to the present 86% ×
3
2
2
3
3
3
1
2
1
17-CH ), 2.07 (3H, d, J ) 7 Hz, 3 -CH ), 1.63 (3H, d, J ) 7 Hz,
2
3
1
8
7% × 80% ) 60% for three steps).
8 -CH
3
), 1.35 (3H, t, J ) 7 Hz, 18-CH
3
), 0.20, -2.39 (each 1H, s,
NH); 13C NMR (CDCl
, 150 MHz) δ ) 196.1 (C13 ), 173.4, 171.0,
1
3
To complete the synthesis of natural compound 2R[E,M],
1
1
60.6, 152.8, 151.7, 147.3, 144.2, 143.1, 139.1, 137.6, 137.1, 132.2,
32.2, 130.9, 129.3, 106.0, 94.6 (C1, 2, 3, 4, 6, 7, 8, 9, 11, 12, 13,
31
R-epimerically pure compound 6R was subjected to the
Suzuki-coupling, and the obtained 20-H/Me mixture was
1
5
separated in zinc-metalated forms by reversed-phase HPLC.
After demetalation to 5R, transesterification of the 17-propionate
was performed with excess farnesol and (Bu2ClSn)2O to give
(17) Structure of 7R was reported but the spectral data of the isolated
product were not described in the literature: Kenner, G. W.; Rimmer, J.;
1
6
Smith, K. M.; Unsworth, J. F. J. Chem. Soc., Perkin Trans. 1 1978, 845-
8
52.
(
18) Sasaki, S.; Tamiaki, H. Tetrahedron Lett. 2006, 47, 4965-4968.
(
14) The debromination reaction under Pd-catalyzed coupling conditions
(19) (a) Fages, F.; Griebenow, N.; Griebenow, K.; Holzwarth, A. R.;
is known to occur as an undesired side reaction. For example, see Balaban,
T. S.; Goddard, R.; Linke-Schaetzel, M.; Lehn, J.-M. J. Am. Chem. Soc.
Schaffner, K. J. Chem. Soc., Perkin Trans. 1 1990, 2791-2797. (b) Balaban,
T. S.; Holzwarth, A. R.; Schaffner, K. J. Mol. Struct. 1995, 349, 183-186.
(c) Chiefari, J.; Griebenow, K.; Griebenow, N.; Balaban, T. S.; Holzwarth,
A. R.; Schaffner, K. J. Phys. Chem. 1995, 99, 1357-1365. (d) Larsen, K.
L.; Miller, M.; Cox, R. P. Arch. Microbiol. 1995, 163, 119-123. (e)
Steensgaard, D. B.; Cox, R. P.; Miller, M. Photosynth. Res. 1996, 48, 385-
393.
2
003, 125, 4233-4239.
15) The mixture of Zn-4R/5R was separated more easily than the free-
base mixture of 4R/5R.
16) Otera, J.; Dan-oh, N.; Nozaki, H. J. Org. Chem. 1991, 56, 5307-
311.
(
(
5
4
568 J. Org. Chem., Vol. 72, No. 12, 2007

3
1
1
5
2
(
6
4, 15, 16, 17, 19, 20), 103.1 (C10), 99.1 (C5), 65.3 (C3 ), 51.7,
3.69 (2H, q, J ) 8 Hz, 8-CH
2-CH ), 3.27 (3H, s, 7-CH ), 2.65 (1H, br-s, OH), 2.52, 2.17 (each
2
1H, m, 17-CH ), 2.46, 2.14 (each 1H, m, 17 -CH ), 2.12 (3H, d, J
2 3
), 3.63 (3H, s, 12-CH ), 3.48 (3H, s,
5
2
2
1
1.5, 51.5 (C17, 17, 18), 48.1 (C13 ), 31.0 (C17 ), 29.7 (C17 ),
5.4 (C3 ), 20.6 (C18 ), 19.3 (C8 ), 17.3 (C2, 8 ),11.8 (C12 ), 11.2
3
3
2
1
1
1
2
1
1
2
1
+
1
7
8
11
12
C7 ); HRMS (FAB) m/z 645.2058 (MH ), calcd for C34
45.2076. 6S: mp 144-146 °C; vis (CH Cl ) λmax 673 (relative
intensity, 47%), 550 (14), 518 (8), 414 nm (100); H NMR (CDCl
00 MHz) δ ) 10.09 (1H, s, 5-H), 9.02 (1H, s, 10-H), 6.31 (1H,
H
38BrN
O
4 4
) 7 Hz, 3 -CH
3
), 2.04-1.90 (8H, m, 17 -, 17 -, 17 -, 17 -CH
2
),
),
1
7
15
2
2
1.69 (3H, d, J ) 8 Hz, 8 -CH ), 1.62 (6H, s, 17 -, trans-17 -CH
3
3
1
15
11
3
,
1.54 (3H, s, cis-17 -CH
3 3
), 1.52 (3H, s, 17 -CH ), 1.47 (3H, t, J
), 1.04, -1.91 (each 1H, s, NH); C NMR (CDCl ,
3
13
6
) 7 Hz, 18-CH
3
1
1
q, J ) 7 Hz, 3-CH), 4.80, 4.56 (each 1H, d, J ) 19 Hz, 13 -CH
2
),
.68 (1H, q, J ) 7 Hz, 18-H), 3.82 (1H, dd, J ) 3, 10 Hz, 17-H),
.72 (3H, s, COOCH ), 3.64 (2H, q, J ) 8 Hz, 8-CH ), 3.56 (3H,
), 3.36 (1H, br, OH), 3.33 (3H, s, 12-CH ), 3.26 (3H, s,
150 MHz) δ ) 196.4 (C13 ), 173.1, 172.4, 159.4, 152.8, 151.6,
4
3
147.6, 144.2, 142.6, 142.1, 140.5, 139.3, 136.7, 135.4, 132.7, 131.4,
3
2
131.3, 131.1, 128.5, 106.0, 105.7 (C1, 2, 3, 4, 6, 7, 8, 9, 11, 12,
3
s, 2-CH
3
3
13, 14, 15, 16, 17, 19, 20, C
F
3, 7, 11), 124.2 (C
F
10), 123.5 (C
F
6),
1
1
7
3
8
-CH
3
), 2.65, 2.35 (each 1H, m, 17 -CH
2
), 2.14 (3H, d, J ) 7 Hz,
117.9 (C
F
2), 102.3 (C10), 97.5 (C5), 65.7 (C3 ), 61.4 (C
(C17), 48.8 (C13 ), 48.4 (C18), 39.6 (C 8), 39.4 (C 4), 31.2 (C17 ),
30.1 (C17 ), 26.6 (C 9), 26.1 (C 5), 25.6 (C3, C 12), 21.0 (C18 ),
F F F
F
1), 51.6
1
2
2
-CH
-CH
3
), 2.10, 1.76 (each 1H, m, 17-CH
), 1.27 (3H, t, J ) 7 Hz, 18-CH
2
), 1.64 (3H, d, J ) 8 Hz,
F
F
1
1
2
1
3
3
), -0.11, -2.55 (each 1H,
13
1
1
1
2
1
1
s, NH); C NMR (CDCl
3
, 150 MHz) δ ) 196.3 (C13 ), 173.5,
70.9, 160.4, 152.8, 151.7, 147.0, 144.2, 143.7, 139.0, 137.5, 137.1,
32.1, 131.5, 130.6, 129.2, 105.7, 94.6 (C1, 2, 3, 4, 6, 7, 8, 9, 11,
F
20.5 (C20 ), 19.5 (C8 ), 17.6 (C8 ), 17.4 (C 11 ), 16.7 (C2 ), 16.4
1
1
1
1
1
1
1
5
(
1
F F
(C 3 ), 15.9 (C 7 ), 12.1 (C12 ), 11.3 (C7 ); HRMS (FAB) m/z
+
771.4857 (MH ), calcd for C H N O 771.4849.
49
63
4
4
3
1
2, 13, 14, 15, 16, 17, 19, 20), 103.1 (C10), 100.1 (C5), 65.8 (C3 ),
1.8, 51.6, 51.3 (C17, 17, 18), 48.1 (C13 ), 31.2 (C17 ), 30.0
C17 ), 25.9 (C3 ), 20.6 (C18 ), 19.4 (C8 ), 17.3, 17.1 (C2, 8 ),-
1.8 (C12 ), 11.3 (C7 ); HRMS (FAB) m/z 645.2088 (MH ), calcd
for C34H38BrN O
4 4
Farnesyl Bacteriopheophorbide-c (7R). To a solution of 6R
32 mg, 0.050 mmol) in THF (10 mL) were added methyl boronic
acid (60 mg, 1.0 mmol), Cs
Bacteriochlorophyll-c (2R[E,M]). To a solution of 7R (4.0 mg,
.2 µmol) in pyridine (5 mL) was added Mg(ClO (223 mg, 1.0
mmol), and the mixture was stirred for 30 min at 80 °C. The mixture
was concentrated, diluted with CH Cl , washed with phosphate
5
2
2
5
4 2
)
1
2
1
1
1
2
1
1
+
2
2
645.2076.
buffer (pH ) 6.8) and water, and concentrated. The residue was
purified by HPLC (Cosmosil 5C18-AR-II, 10 mm φ × 250 mm,
(
MeOH 2 mL/min) to give titled compound 2R[E,M] (t
R
) 8 min),
) 20 min).
2
O) λmax 660 (relative intensity, 63%), 624 (11),
2 3 2
CO (163 mg, 0.50 mmol), and PdCl -
byproduct 8 (t
R
) 14 min), and unreacted 7R (t
R[E,M]: vis (Et
31 nm (100); HRMS (FAB) m/z 793.4528 (MH ), calcd for
R
(
dppf) (8.2 mg, 0.010 mmol), and the mixture was refluxed for 2
h. Completion of the reaction was monitored by vis and MS spectra.
The mixture was poured into 2% aqueous HCl and extracted with
2
4
+
C
49
H
61MgN
4
O
4
793.4543. 8: vis (Et
2
O) λmax 666 (relative intensity,
CH
Na
by silica gel chromatography (Et
Cl , 2:98) to give a free-base chlorin mixture (5R:4R ) 3.8:1).
The mixture was zinc-metalated, separated by HPLC (Cosmosil
C18-AR-II, 10 mm φ × 250 mm, H O-MeOH 1:19, 2 mL/min,
) 41 and 45 min for 4R and 5R, respectively), and then shaken
with diluted aqueous HCl in CH Cl for demetalation. The spectral
2
Cl
2
. The extract was washed with brine, dried over anhydrous
, filtered, and concentrated. The crude product was purified
O-CH Cl , 1:9 to MeOH-CH
+
58%), 626 (11), 435 nm (100); HRMS (FAB) m/z 775.4430 (MH ),
2
SO
4
4 3
calcd for C49H59MgN O 775.4438.
2
2
2
2
-
2
Acknowledgment. We thank Drs. Jiro Harada and Michio
Kunieda of Ritsumeikan University for their helpful discussion.
This work was partially supported by Grants-in-Aid for Scien-
tific Research (No. 17029065) on Priority Areas (417) and
Young Scientists (Nos. 17750167 and 18750124) from the
Ministry of Education, Culture, Sports, Science and Technology
(MEXT) of the Japanese Government and for Scientific
Research (B) (No. 15350107) from the Japan Society for the
Promotion of Science (JSPS), as well as by the “Academic
Frontier” Project for Private Universities: matching fund subsidy
from MEXT, 2003-2007.
5
2
t
R
2
2
data of the obtained 5R was coincident with that of the reported
data.8b To a solution of 5R (7.0 mg, 0.012 mmol) in toluene (10
2 2
mL) was added farnesol (53 mg, 0.24 mmol) and (Bu ClSn) O (2.0
mg, 3.6 µmol), and the mixture was refluxed for 3 h. The mixture
was cooled to room temperature and subjected to silica gel
chromatography (Et
8%) as a black solid: mp 86-88 °C; vis (CH
relative intensity, 49%), 551 (15), 519 (10), 415 nm (100); H
2
O-CH
2 2
Cl , 0:10 to 1:9) to give 7R (8.2 mg,
8
(
2
Cl
2
) λmax 670
1
NMR (CDCl
3
, 600 MHz) δ ) 9.88 (1H, s, 5-H), 9.43 (1H, s, 10-
Supporting Information Available: 1H and 13C NMR spectra
5
H), 6.47 (1H, q, J ) 7 Hz, 3-CH), 5.22 (1H, t, J ) 7 Hz, 17 -CH),
of 6R, 6S, and 7 in CDCl (Figures S1-S6). This material is
3
1
5
.20, 5.21 (each 1H, d, J ) 19 Hz, 13 -CH
2
), 5.03 (2H, t, J ) 7
available free of charge via the Internet at http://pubs.acs.org.
9
13
Hz, 17 -, 17 -CH), 4.57 (1H, q, J ) 7 Hz, 18-H), 4.51 (2H, m,
4
17 -CH
2 3
), 4.16 (1H, dd, J ) 4, 9 Hz, 17-H), 3.86 (3H, s, 20-CH ),
JO0703855
J. Org. Chem, Vol. 72, No. 12, 2007 4569