Photoreduction of zinc 8-vinylated chlorophyll derivative to bacteriochlorophyll-b/g analog possessing an 8-ethylidene group

Article abstract of DOI:10.1016/j.bmcl.2013.02.050

When a pyridine solution of zinc methyl 8-vinyl-mesopyropheophorbide-a was irradiated with visible light in the presence of ethanol, ascorbic acid and diazabicylo[2.2.2]octane under nitrogen at room temperature, zinc (7R/S,8E)-8-ethylidene-bacteriochlorin was obtained via 1,4-hydrogenation. The 1,4-photoreduction is similar to the enzymatic reduction of 8-vinyl- chlorophyllides to (E)-8-ethylidene-bacteriochlorins in anoxygenic photosynthetic bacteria producing bacteriochlorophylls-b/g. The resulting zinc 8-ethylidene-bacteriochlorin was readily isomerized to the chemically more stable 8-ethyl-chlorin by further illumination. As a by-product, zinc 8-vinyl-7,8-cis-bacteriochlorin was slightly formed by photoinduced 1,2-hydrogenation of zinc 8-vinyl-chlorin.

Full text of DOI:10.1016/j.bmcl.2013.02.050

Bioorganic & Medicinal Chemistry Letters 23 (2013) 2377–2379
Contents lists available at SciVerse ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Photoreduction of zinc 8-vinylated chlorophyll derivative
to bacteriochlorophyll-b/g analog possessing an 8-ethylidene group
⇑
Hitoshi Tamiaki , Meiyun Xu, Takuya Tanaka, Tadashi Mizoguchi
Graduate School of Life Sciences, 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:
When a pyridine solution of zinc methyl 8-vinyl-mesopyropheophorbide-a was irradiated with visible
light in the presence of ethanol, ascorbic acid and diazabicylo[2.2.2]octane under nitrogen at room
temperature, zinc (7R/S,8E)-8-ethylidene-bacteriochlorin was obtained via 1,4-hydrogenation. The
1,4-photoreduction is similar to the enzymatic reduction of 8-vinyl-chlorophyllides to (E)-8-ethyli-
dene-bacteriochlorins in anoxygenic photosynthetic bacteria producing bacteriochlorophylls-b/g. The
resulting zinc 8-ethylidene-bacteriochlorin was readily isomerized to the chemically more stable
8-ethyl-chlorin by further illumination. As a by-product, zinc 8-vinyl-7,8-cis-bacteriochlorin was slightly
formed by photoinduced 1,2-hydrogenation of zinc 8-vinyl-chlorin.
Received 26 December 2012
Revised 5 February 2013
Accepted 9 February 2013
Available online 18 February 2013
Keywords:
Bacteriochlorin
Biosynthesis
Electronic absorption spectra
Hydrogenation
Photoreaction
Ó 2013 Elsevier Ltd. All rights reserved.
Bacteriochlorophyll(BChl)s-b1–3 and g4–6 are photosynthetically
active pigments and are characterized by the presence of an
E-ethylidene group at the 8-position (see Fig. 1).7 The unique moi-
ety enables the BChls to absorb longer wavelength lights at a near-
infrared region than the other BChls.8 The redmost (Qy) absorption
peaks of BChls-b and g in diethyl ether are positioned at 795 and
767 nm, respectively, as the monomeric species, while anoxygenic
bacteria utilizing BChls-b and g as the main photosynthetic pig-
ments in vivo gave Qy maxima at around 1020 (in Blastochloris vir-
idis, formerly called Rhodopseudomonas viridis) and 790 nm (in
Heliobacterium modesticaldum). The biosynthetic route of the 8-
ethylidene group had not been unraveled for a long time5,6 since
the discovery of anoxygenic photosynthetic bacteria producing
BChls-b and g in 19639 and 1983,10 but was very recently identified
to be produced by 1,4-reduction of 8-vinyl-chlorophyllide at the
B-ring (see lower drawing in Scheme 1).11 We earlier reported that
similar 1,4-addition (Diels–Alder reaction) of 8-vinyl-chlorophyll
(Chl) derivatives (dienes) with tetracyanoethylene (dienophile)
afforded chemically stable analogs of BChls-b and g.12 Here, we
examined photoinduced 1,4-hydrogenation of zinc 8-vinyl-Chl
derivative 1 to give the corresponding bacteriochlorin 2 possessing
the 8-ethylidene group (see Scheme 2).
upper drawing in Scheme 1).13 We thus applied the photoreduction
in the A-ring to that in the B-ring. As the desired 8-vinyl-chlorin, zinc
methyl 8-vinyl-mesopyropheophorbide-a (1) was prepared by
modifying naturally occurring Chl-a.14 Zinc complex 1 (ca. 10À4 M)
in 8% (v/v) ethanol and pyridine was irradiated with visible light
of a halogen lamp in the presence of excess amounts of ascorbic acid
and DABCO under nitrogen at room temperature (Scheme 2).18 After
illumination for a few min, one product was isolated and was iden-
tified as zinc methyl mesopyropheophorbide-a (4) by comparison
with the authentic sample.13,15 The photoproduct 4 was the hydro-
genated form of the 8-vinyl group in the starting material 1.
During the photoreduction, four other peaks were observed by
HPLC analysis (Fig. 2). All the four HPLC bands gave the same
molecular ion peaks in the online mass spectra as that of 4:
Simpson and Smith reported that zinc 3-vinyl-chlorin in a deaer-
ated ethanol and pyridine solution of ascorbic acid and diazabicy-
lo[2.2.2]octane (DABCO) was 1,4-hydrogenated with illumination
of visible light to give zinc 3-ethylidene-isobacteriochlorin (see
⇑
Corresponding author. Fax: +81 77 561 2659.
E-mail address: tamiaki@fc.ritsumei.ac.jp (H. Tamiaki).
Figure 1. Molecular structures of naturally occurring bacteriochlorophyll(BChl)s-b
and g.
0960-894X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.bmcl.2013.02.050

2378
H. Tamiaki et al. / Bioorg. Med. Chem. Lett. 23 (2013) 2377–2379
Scheme 1. 1,4-Hydrogenation of 3- (upper) and 8-vinyl-chlorins (lower).
m/z = 613 (MH+) in APCI mode. These products were two hydrogen
adducts to 1 and photohydrogenated isomers. From Figures 2B and
C, two major peaks were observed at 12.5 and 13.5 min and the
third and fourth eluted fractions were denoted as #3 and #4. Both
the online electronic absorption spectra were identical to give in-
tense maxima at 728, 556, 401 and 359 nm in 10% (v/v) aqueous
methanol (see Fig. 3A). The spectral shape was similar to that of
bacteriochlorin 5 possessing the 8-methylene moiety (see left
drawing in Fig. 4)14 as shown in Figure 3B. The fractions #3/4 were
thus assigned to the stereoisomers of compound 2 possessing the
8-ethylidene group. Based on the predominant formation of
(E)-3-ethylidene-isobacteriochlorin in the photoreduction on the
A-ring (see upper drawing in Scheme 1),13 the resulting 8-ethyli-
dene-bacteriochlorin 2 should be E-isomer and give two epimers
at the chiral 7-position. The fractions #3/4 would be (7R,8E)- and
(7S,8E)-stereoisomers of 2. The further photoreaction of HPLC-iso-
lated 2 in aqueous methanol with or without the above additives
gave 8-ethyl-chlorin 4. The photoisomerization of 2 to 4 is
consistent with that of BChls-b/g (8-ethylidene-bacteriochlorins)
to (3-acetyl-)Chl-a (8-ethyl-chlorins),4,5,19 indicating that 2 has a
C7–C8@CHCH3 moiety in a molecule.
Two minor peaks at 10.5 and 11 min were found in Figures 2B
and C and the fractions #1/2 were eluted faster than #3/4. The iso-
mers #1/2 gave the same online electronic absorption spectra hav-
ing 704-, 553- and 342-nm maxima (Fig. 3C). The spectra were
almost the same as that of 7,8-trans-bacteriochlorin 6 (Figs. 3D
and 4 right),14 showing that the isomers #1/2 were 7,8-dihydroge-
nated products of 1. Considering that similar photoinduced
1,2-hydrogenation initially proceeded in a cis-fashion,13,20,21 the
fractions #1/2 would be assigned to 7,8-cis-isomers of 3, that is,
(7R,8S)- and (7S,8R)-stereoisomers. When the aerated HPLC-
elutions of #1/2 were allowed to stand in the dark at room temper-
ature, the starting material 1 was gradually produced. The facile
air-oxidation of #1/2 to 1 showed that #1/2 were chemically labile
7,8-cis-forms and possessed a vinyl group at the 8-position of bac-
Figure 2. HPLC of the photoreaction mixture of 1 to 2–4: Cosmosil 5C18-AR-II, 4.6 /
 150 mm, 10% (v/v) H2O–MeOH, 1.0 mL/min, detection at 648 (A), 704 (B) and
728 nm (C): #1/#2 = cis-3; #3/#4 = 8E-2.
ity for the photoreduction in the B-ring is consistent with that in
the A-ring reported earlier,13 where 1,2-hydrogenation was favor-
able to 1,4-one in a benzene-based solvent. Under the same condi-
tions, the photoreaction on the B-ring of 1 in pyridine proceeded
about 10-fold more slowly than that on the A-ring (see Scheme 1).
It is noteworthy that the relationship of electronic absorption max-
ima between zinc 3-ethyl-8-ethylidene-bacteriochlorin 2 and zinc
3-ethyl-8-vinyl-bacteriochlorin 3 is well correlated with that be-
tween BChl-b (magnesium 3-acetyl-8-ethylidene-bacteriochlorin)
and BChl-a (magnesium 3-acetyl-8-ethyl-bacteriochlorin) as
shown in Table S1.
In conclusion, photoreduction of zinc 3-ethyl-8-vinyl-chlorin 1
occurred in the B-ring possessing a vinyl group, not in the A-ring
bearing sole alkyl groups, to give 8-ethylidene-bacteriochlorin 2
via 1,4-hydrogenation of C7@C8–CH@CH2 and 8-vinyl-bacterio-
chlorin 3 via 1,2-hydrogenation of C7@C8. Further illumination of
the reaction mixture induced isomerization of 2 to chemically
more stable zinc 8-ethyl-chlorin 4. The 1,4-photoreduction of 1
to 2 is similar to the enzymatic reduction in 8-vinyl-chlorophyl-
lide-a and its 3-acetyl analog to bacteriochlorophyllide-g and b
possessing the 8-ethylidene group (BChls-g/b H, see Graphical
abstract),11 which were transformed to BChls-g/b via esterification
at the 17-substituent.
teriochlorin p-system as in 3.
The photoreduction of 1 in benzene instead of pyridine men-
tioned above also gave 2–4 as products but the formation ratio of
2 over 3 largely decreased (see Fig. S1). The change of regioselectiv-
Scheme 2. Photoreduction of zinc 8-vinyl-chlorin 1 to 8-ethylidene-bacteriochlorins 2 as 1,4-adducts as well as 8-vinyl-bacteriochlorins 3 and 8-ethyl-chlorin 4 as 1,
2-adducts.

H. Tamiaki et al. / Bioorg. Med. Chem. Lett. 23 (2013) 2377–2379
2379
Figure 3. Electronic absorption spectra of HPLC elutions of #3 (=8E-2) (A), 5 (B), #1 (=cis-3) (C) and 6 (D) in 10% (v/v) H2O–MeOH.
2. Risch, N. J. Chem. Res. 1981, 116.
3. Deisenhofer, J.; Epp, O.; Sinning, I.; Michel, H. J. Mol. Biol. 1995, 246, 429.
4. Brockmann, H., Jr.; Lipinski, A. Arch. Microbiol. 1983, 136, 17.
5. Michalski, T. J.; Hunt, J. E.; Bowman, M. K.; Smith, U.; Bardeen, K.; Gest, H.;
Norris, J. R.; Katz, J. J. Proc. Natl. Acad. Sci. U.S.A. 1987, 84, 2570.
6. Mizoguchi, T.; Oh-oka, H.; Tamiaki, H. J. Photochem. Photobiol., A: Chem. 2005,
81, 666.
7. Tamiaki, H.; Shibata, R.; Mizoguchi, T. J. Photochem. Photobiol., A: Chem. 2007,
83, 152.
8. Tamiaki, H.; Kunieda, M. Handb. Porphyrin Sci. 2011, 11, 223.
9. Eimhjellen, K. E.; Aasmundrud, O.; Jensen, A. Biochim. Biophys. Res. Commun.
1963, 10, 232.
10. Gest, H.; Favinger, J. L. Arch. Microbiol. 1983, 136, 11.
11. (a) Tsukatani, Y.; Tamiaki, H. Japan Patent 141,370, 2012; (b) Tsukatani, Y.;
Yamamoto, H.; Harada, J.; Yoshitomi, T.; Nomata, J.; Kasahara, M.; Mizoguchi,
T.; Fujita, Y.; Tamiaki, H. Sci. Rep. 2013, 3, 1217. http://dx.doi.org/10.1038/
srep01217.
Figure 4. Molecular structures of synthetic zinc bacteriochlorins possessing the 8-
methylene group 5 (left) and with the 7,8-trans-fashion 6 (right).
12. Kunieda, M.; Mizoguchi, T.; Tamiaki, H. Tetrahedron 2004, 60, 11349.
13. Simpson, D. J.; Smith, K. M. J. Am. Chem. Soc. 1988, 110, 2854.
14. Zinc complexes
1 and 5 were prepared by standard metallation of the
corresponding chlorin15 and bacteriochlorin free bases16 and zinc complex 6
was afforded by modifying 3-vinyl-bacteriochlorin17 synthesized from BChl-a:
see ESI for the synthetic procedures and spectroscopic data.
15. Yagai, S.; Miyatake, T.; Tamiaki, H. J. Photochem. Photobiol., B: Biol. 1999, 52, 74.
16. Tamiaki, H.; Xu, M.; Kinoshita, Y. J. Photochem. Photobiol., A: Chem. 2013, 252,
60.
Acknowledgments
We thank Dr. Taichi Yoshitomi of Ritsumeikan University for
measurements of LC–MS. This work was partially supported by a
Grant-in-Aid for Scientific Research (A) (No. 22245030) from the
Japan Society for the Promotion of Science (JSPS).
17. Sasaki, S.; Tamiaki, H. J. Org. Chem. 2006, 71, 2648.
18. Zinc 8-vinyl-chlorin 1 (2.3 mg, 3.8 lmol) was dissolved in 8% (v/v) EtOH–
C5H5N (37.5 mL) and bubbled with N2 for 20 min. Ascorbic acid (263 mg,
2.3 mmol) and DABCO (150 mg, 0.85 mmol) were added to the solution and
bubbled with N2 for 5 min. The stirred solution in a sealed 50 mL round flask
was irradiated with a 250 W halogen lamp (ELMO slide projector OMNI 253 AF,
350–700 nm, 5400 lx). Photoreaction was monitored by electronic absorption
spectroscopy: appearance of a new peak at about 728 nm. After irradiation for
about 1 min, the reaction mixture was evaporated in vacuo, then flushed with
nitrogen gas to remove all the solvents. The residue was purified by HPLC
(Cosmosil 5C18-AR-II, 4.6 / Â 150 mm, 10% (v/v) H2O–MeOH, 1.0 mL/min):
Supplementary data
Supplementary data (synthetic procedures of 1, 5 and 6, HPLC of
photoreduction of 1 in benzene or pyridine and electronic absorp-
tion data of bacteriochlorins) associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/
j.bmcl.2013.02.050.
retention times of
1 and zinc 8-ethyl-chlorin 4 were 14.5 and 16 min,
respectively (see Fig. 2A). The separated HPLC fractions were poured into
water and extracted with CH2Cl2. The organic layer was evaporated in vacuo to
give pure samples.
References and notes
19. Steiner, R.; Cmiel, E.; Scheer, H. Z. Naturforschung 1983, 38c, 748.
20. Tamiaki, H.; Xu, M.; Mizoguchi, T. Tetrahedron Lett. 2012, 53, 3210.
21. Tamiaki, H.; Xu, M.; Machida, S. Chem. Lett. 2012, 41, 820.
1. Scheer, H.; Svec, W. A.; Cope, B. T.; Studier, M. H.; Scott, R. G.; Katz, J. J. J. Am.
Chem. Soc. 1974, 96, 3714.