A mild conversion from 3-vinyl- to 3-formyl-chlorophyll derivatives

Article abstract of DOI:10.1016/j.bmcl.2011.02.054

The C3-vinyl group of a chlorophyll derivative, methyl pyropheophorbide-a, was converted into the formyl group by a novel one-pot reaction with thiophenol at room temperature. The mild reaction can provide insight into development of 'green' catalysts dis

Full text of DOI:10.1016/j.bmcl.2011.02.054

Bioorganic & Medicinal Chemistry Letters 21 (2011) 2489–2491
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Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
A mild conversion from 3-vinyl- to 3-formyl-chlorophyll derivatives
Toru Oba a, , Yuki Uda a, Kohei Matsuda a, Takanori Fukusumi a, Satoshi Ito a, Kazuhisa Hiratani a,
⇑
Hitoshi Tamiaki b,
⇑
a Department of Material and Environmental Chemistry, Graduate School of Engineering, Utsunomiya University, Utsunomiya, Tochigi 321-8585, Japan
b Department of Bioscience and Biotechnology, Faculty of Science and Engineering, Ritsumeikan University, Kusatsu, Shiga 525-8577, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
The C3-vinyl group of a chlorophyll derivative, methyl pyropheophorbide-a, was converted into the for-
myl group by a novel one-pot reaction with thiophenol at room temperature. The mild reaction can pro-
vide insight into development of ‘green’ catalysts displacing OsO4 or O3, and into elucidation of unknown
biosynthetic processes of chlorophyll-d.
Received 26 November 2010
Revised 9 February 2011
Accepted 14 February 2011
Available online 17 February 2011
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Chlorophyll-a
Chlorophyll-d
Formylation
Oxidative cleavage
Chlorophyll-d (Chl-d, 1 see Fig. 1) is a photosynthetic pigment
that possesses a formyl group at the C3-position instead of a vinyl
group of Chl-a (2). Chl-d absorbs sunlight at a longer wavelength
than Chl-a does, and functions in the light-harvesting antennas
and reaction centers of Acaryochloris (A.) marina, so broad attention
has been attracted to its unique structure,1 the evolution of such
photosynthetic systems,2 and so on. It was reported that A. marina
had close homologs to Chl-a biosynthesis genes,3 suggesting that
Chl-d would be synthesized by oxidation of the C3-vinyl of Chl-a
or chlorophyllide-a lacking the phytyl ester (R = H, Fig. 1).
It is also known that Chl derivatives are promising natural
photosensitizers applicable to photodynamic therapy4,5 and dye
sensitized solar cells.6 The C3-substituent of Chls is useful for fine
tuning of cell permeability and light absorption. The C3-[1-(1-hexyl-
oxy or 1-octyloxy)ethyl]-derivatives showed relatively better cell
permeability than the other compounds.4 Wittig and Knoevenagel
reactions on the C3-formyl group afforded a variety of the C3-
ethenyl derivatives with different absorption bands.7 These
compounds required multi-step synthesis from methyl pyropheo-
phorbide-a (3, see Scheme 1) through C3-(1-bromoethyl)- and
formyl-derivatives. The latter oxidation of the C3-vinyl of 3 to formyl
group of methyl pyropheophorbide-d (4) requires hazardous
oxidizing reagents such as KMnO4, OsO4 and O3.8–10
X
3
N
N
N
N
Mg
O
O
O
OMe
OR
Figure 1. Molecular structures of naturally occurring chlorophyll(Chl)s (R = phytyl).
Chl-d (1): X = O; Chl-a (2): X = CH2.
O
O
S
Ph
*
NH N
N HN
PhSH
TsOH
NH N
N HN
NH N
N HN
O
O
O
O
O
O
OMe
Direct but mild introduction of desired substituents at the
C3-moiety is important for Chl chemistry. Among these, a reaction
OMe
OMe
5
3
4
Scheme 1. Conversion of C3-vinyl group of 3 into C3-formyl group of 4.
⇑
Corresponding authors. Tel./fax: +81 28 689 6147 (T.O.); fax: +81 77 561 2659
(H.T.).
with thiols, good nucleophiles, seems promising. There have been
a few reports on the introduction of thio-substituents to the
E-mail addresses: tob_p206@cc.utsunomiya-u.ac.jp (T. Oba), tamiaki@se.ritsumei.
ac.jp (H. Tamiaki).
0960-894X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2011.02.054

2490
T. Oba et al. / Bioorg. Med. Chem. Lett. 21 (2011) 2489–2491
Table 1
•SPh
–H
•
Conversion of 3 to 4 and 5a determined by 1H NMR
3-CH=CH2
3-CH–CH2SPh
3-CH=CHSPh (7)
(3)
Thiol
Solvent
3 (%)
4 (%)
5 (%)
+O, –H
3-CO–CH2SPh (6)
+ O2
PhSH
PhSH
PhSH
PhSH
4-MeOPhSH
4-NO2PhSH
PhCH2SH
PhSH
CHCl3
THF
0
0
57
51
68
47
52
3
6
39
0
15
10
25
31
36
0
0
14
0
3-CH(O–O•)–CH2 SPh
3-CHO (4)
MeOH
0
DMSO-d6
DMSO-d6
DMSO-d6
DMSO-d6
CDCl3
0
0
97
90
0
Scheme 2. A possible radical mechanism for oxidative cleavage of the C3-vinyl
group.
PhSMe
CDCl3
100
even though the whole genome of A. marina producing Chl-d has
been reported.3 Kobayashi and his colleagues reported that Chl-a
was oxidized to give Chl-d in less yield by using papain.22 Recently,
Chen and her colleagues suggested that Chl-a and O2 are the biosyn-
thetic precursors of Chl-d.23 Here, we speculate that the C3-vinyl
group could be converted to the formyl group by action of thio-
functionalized components including a cystein residue in an enzy-
matic reaction pocket and an oxygen molecule. Oxidoreductase
using stable radical species or molecular oxygen, such as cycloxy-
genase, P450, or peroxidase may also be candidates.
In summary, we have developed a novel, one-pot reaction that
can convert the vinyl group of Chl-a derivative to a formyl group.
Such a mild and efficient conversion from a vinyl group to a formyl
group may be included in the yet unclear biosynthesis of Chl-d. Our
findings can provide insight into elucidation of unknown biosyn-
thetic processes of Chl-d, as well as to a novel ‘green’ catalyst dis-
placing strong oxidizing reagents. Further investigations are now
underway.
a
Reaction of
3
(1 equiv) with thiol (5 equiv) in the presence of TsOHÁH2O
(4 equiv) at room temperature under N2 for 18–24 h.
C31-position of Chl derivatives.11,12 We examined one-pot addition
of thiols to the C3-vinyl of 3, and unexpectedly found that the vinyl
group was converted into a formyl group to afford 4 (see Scheme
1). Here, we report on the mild conversion method of the vinyl
to formyl group at the chlorophyll peripheral position and also
the possible route of Chl-d biosynthesis.13
Typically, compound 3 (10 lmol), prepared from Chl-a as previ-
ously described,9 was dissolved in CHCl3. To this solution was
added thiophenol (PhSH, 5 equiv) and hydrated p-toluenesulfonic
acid (TsOHÁH2O, 4 equiv). The reaction mixture was stirred over-
night (18–24 h) in the dark at room temperature under N2 atmo-
sphere.14 After work-up, the products were isolated by silica gel
flash column chromatography, and analyzed by NMR, MS, and
VIS spectroscopies. An initially desired thiol-adduct was obtained
in 15% yield as the form of the C31-sulfoxide derivative 5,15 while
the unoxidized Markovnikov adduct of thiophenol (the 31-SPh
derivative) was rarely obtained. Surprisingly, C3-formylated chlo-
rin 49,10 was obtained through oxidative cleavage of the C3-vinyl
group as the main product in 57% yield.16
Acknowledgments
We thank Dr. Tomohiro Miyatake of Ryukoku University for
HRMS analysis and Dr. Min Chen of Sydney University for giving use-
ful information of Chl-d biosynthesis. This work was partially
supported by Grants-in-Aid for Scientific Research (C) (No.
21510132 to TO) and (A) (No. 22245030 to HT) from the Japan Soci-
ety for the Promotion of Science (JSPS), and by research project of
Utsunomiya University Center for Optical Research & Education.
It is noted that alcohol as the solvent improved the yield of
compound 4 greatly (nearly 70% in methanol), compared with
those in CHCl3, THF and DMSO (ca. 50%) shown in Table 1.17 In
alcohols, the corresponding acetals were partially produced and
acidic treatment was necessary for isolation of 4, where no
transesterification at the propionate residue occurred. Conversion
of the C3-vinyl group of 3 to the C3-formyl group of 4 was achieved
in one-pot reaction without hazardous oxidizing reagents (vide su-
pra); to our knowledge, this is the first report on such an oxidative
cleavage of chlorophyll peripheral substituents by a thiol.18
The unique oxidation mechanism has not yet been determined,
but the radical route was proposed by the following experimental
results. The adduct 5 was not a precursor of 4, because isolated 5
remained unchanged even when stirred with thiophenol and TsOH
for 24 h. Compound 5 was also not the oxidation catalyst, because
3 remained unchanged when incubated with 5. Two by-products
were detected from the reaction mixture in CHCl3 and were deter-
mined to be C3-COCH2SPh and C3-CH@CHSPh derivatives of 3
(compound 6 and 7, respectively).19 The thio-substitution at the
C32-position indicated that a radical PhSÅ initially attacked at the
C32-position of 3 to give 3-CHÅ–CH2SPh.20 The resulting C31-radia-
cal species was oxidatively cleaved to afford 4 and oxidized to pro-
duce the above two by-products (Scheme 2). The oxidizing reagent
would be oxygen molecules dissolved in solvents for the reaction
as shown by time courses of the UV and NMR spectra (data not
shown). The above radical mechanism is also supported by the re-
sults that the electron-rich thiol (4-MeOPhSH) is more reactive
(sensitive to oxidation) than the electron-poor thiol (4-NO2PhSH)
to give 4 smoothly (see Table 1). A similar oxidation reported ear-
lier supports the mechanism too.21
References and notes
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Miyashita, H.; Page, L.; Ramakrishna, P.; Satoh, S.; Sattley, W. M.; Shimada, Y.;
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12. Pickering, M. D.; Keely, B. J. Org. Geochem. 2008, 39, 1046.
13. Oba, T.; Uda, Y.; Ito, S.; Hiratani, K. Abstract of Papers, 89th Annual Meeting of
Chemical Society of Japan; Tokyo, March, 2009; Abstract 1J3-32.
The enzyme for oxidation of the C3-vinyl group of Chl-a (or chlo-
rophyllide-a) to the formyl group has not yet been determined,

T. Oba et al. / Bioorg. Med. Chem. Lett. 21 (2011) 2489–2491
2491
14. Used nitrogen gas (99.9%) contained O2 as an impurity, which could influence
the reaction.
(2H, 2s, NH Â 2); HRMS (FAB, m-nitrobenzyl alcohol) found: m/z 551.2656.
Calcd for C33H35N4O4: [M+H]+, 551.2658; VIS (CHCl3) kmax 697 nm (relative
intensity, 0.78), 428 (1.00), 384 (0.92). These data were in good agreement
with those of 4 previously reported.9,10
15. Methyl 31-phenylsulfinyl-mesopyropheophorbide-a (5):
A
31-epimeric 1:1
mixture; 1H NMR (CDCl3, 500 MHz) d 9.55 (1H, s, CH-10), 9.46/9.44 (1H, s,
CH-5), 8.54 (1H, s, CH-20), 7.65 (2H, d, J = 8 Hz, o-phenyl-H Â 2), 7.43 (2H, dt,
J = 2.5, 8 Hz, m-phenyl-H Â 2), 7.36 (1H, dd, J = 2.5, 8 Hz, p-phenyl-H), 6.25 (1H,
q, J = 8 Hz, CH-31), 5.26, 5.12 (2H, 2d, J = 20 Hz, CH2-132), 4.57 (1H, dq, J = 2,
7 Hz, CH-18), 4.30 (1H, dt, J = 7, 2 Hz, CH-17), 3.89/3.88 (3H, d, J = 7 Hz, CH3-32),
3.70 (2H, q, J = 8 Hz, CH2-81), 3.69 (3H, s, CH3-121), 3.61 (3H, s, CH3-175), 3.38/
3.37 (3H, s, CH3-21), 3.15/3.14 (1H, s, CH3-71), 2.73–2.52, 2.39–2.17 (4H, m,
CH2-171, 172), 1.77 (3H, t, J = 7 Hz, CH3-181), 1.69 (3H, t, J = 8 Hz, CH3-82), 0.28,
À1.85 (2H, 2s, NH Â 2); HRMS (FAB, matrix : m-nitrobenzyl alcohol) found: m/z
17. The reaction in MeOH achieved 76% as the best isolated yield.
18. Professor Keely of York Univ. independently found
Pickering, M. D.; Keely, B. J. Personal communication.
a similar oxidation;
19. 6 (3-COCH2SPh): partial 1H NMR (CDCl3) d 9.69 (1H, s, CH-5), 9.62 (1H, s, CH-
10), 8.75 (1H, s, CH-20), 7.50 (2H, d, J = 7.5 Hz, o-phenyl-H Â 2), 7.30 (2H, t,
J = 7.5 Hz, m-phenyl-H Â 2), 7.23 (1H, d, J = 7.5 Hz, p-phenyl-H), 5.33, 5.19 (2H,
2d, J = 20 Hz, CH2-132), 4.87 (2H, s, CH2-32), À0.11, À2.02 (2H, 2s, NH Â 2); MS
(MALDI) found: m/z 673.58 ([M+H]+); VIS (CHCl3) kmax = 680 nm.
7 (3-
675.3004. Calcd for C40H43N4O4S: [M+H]+ 675.3005; VIS (CHCl3) kmax 665 nm
CH@CHSPh): MS (MALDI) found: m/z 656.41 (M+); VIS (CHCl3) kmax = 672 nm.
20. The reaction with radical scavenger (BHT) inhibited the production of
,
(relative intensity, 0.53), 412 (1.00); IR (film) v 1035 cmÀ1 (S@O).
a
16. Methyl pyropheophorbide-d (4): 1H NMR (CDCl3, 500 MHz) d 11.57 (1H, s, CHO-
3), 10.33 (H, s, CH-5), 9.65 (H, s, CH-10), 8.85 (H, s, CH-20), 5.35, 5.20 (2H, 2d,
J = 20 Hz, CH2-132), 4.58 (1H, dq, J = 2, 7 Hz, CH-18), 4.38 (1H, dt, J = 7, 2 Hz,
CH-17), 3.79 (3H, s, CH3-21), 3.75 (2H, q, J = 8 Hz, CH2-81), 3.74 (3H, s, CH3-12),
3.62 (3H, s, CH3-175), 3.34 (3H, s, CH3-71), 2.8–2.5, 2.4–2.2 (4H, m, CH2-171,
172), 1.85 (3H, d, J = 7 Hz, CH3-181), 1.73 (3H, t, J = 8 Hz, CH3-82), À0.12, À2.03
formylated derivative 4, which supports the thiyl radical mechanism.
21. Baucherel, X.; Uziel, J.; Jugé, S. J. Org. Chem. 2001, 66, 4504.
22. Ohashi, S.; Iemura, T.; Okada, N.; Itoh, S.; Furukawa, H.; Okuda, M.; Ohnishi-
Kameyama, M.; Ogawa, T.; Miyashita, H.; Watanabe, T.; Itoh, S.; Oh-oka, H.;
Inoue, K.; Kobayashi, M. Photosynth. Res. 2010, 104, 305.
23. Schliep, M.; Crossett, B.; Willows, R. D.; Chen, M. J. Biol. Chem. 2010, 285, 28450.