Synthesis of monovinyl- and divinyl-chlorophyll analogs and their physical properties

Article abstract of DOI:10.1016/j.tet.2016.12.003

Chlorophyll-a was chemically modified to methyl pyropheophorbides-a possessing 3,8-diethyl, 3-vinyl-8-ethyl, 3-ethyl-8-vinyl, and 3,8-divinyl groups. Analogous 3-ethyl-7-vinyl- and 3,7-divinyl-chlorins were prepared by derivatization of chlorophyll-b. The synthetic free bases as well as zinc 3-vinyl-chlorins were dissolved in THF and the monomeric diluted solutions were characterized by optical spectroscopy including visible absorption, circular dichroism, and fluorescence emission spectra. The optical data indicated that the 3-vinylation bathochromically shifted the visible (Soret/Qx/Qy) absorption and fluorescence emission maxima, the 7-vinylation moved the Soret/Qx and Qy/emission bands to longer and slightly shorter wavelengths, respectively, and the 8-vinylation induced red shifts of the Soret/Qx maxima and no (or faint red) shifts of the Qy/emission maxima. Zinc complexes of 3,7- and 3,8-divinyl-chlorins showed almost the same optical properties including fluorescence emission quantum yields and lifetimes as well as the same first oxidation potentials, thus, 3,7-divinyl-chlorophyll-a could be considered an alternative pigment to the naturally occurring 3,8-divinyl analog.

Full text of DOI:10.1016/j.tet.2016.12.003

Tetrahedron 73 (2017) 313e321
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Synthesis of monovinyl- and divinyl-chlorophyll analogs and their
physical properties
,
, *
Kifa Kim a, Kazuki Tsuji a b, Yusuke Kinoshita a, Tomohiro Miyatake b, Hitoshi Tamiaki a
a Graduate School of Life Sciences, Ritsumeikan University, Kusatsu, Shiga 525-8577, Japan
b Department of Materials Chemistry, Faculty of Science and Technology, Ryukoku University, Otsu, Shiga 520-2194, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Chlorophyll-a was chemically modified to methyl pyropheophorbides-a possessing 3,8-diethyl, 3-vinyl-
8-ethyl, 3-ethyl-8-vinyl, and 3,8-divinyl groups. Analogous 3-ethyl-7-vinyl- and 3,7-divinyl-chlorins
were prepared by derivatization of chlorophyll-b. The synthetic free bases as well as zinc 3-vinyl-chlorins
were dissolved in THF and the monomeric diluted solutions were characterized by optical spectroscopy
including visible absorption, circular dichroism, and fluorescence emission spectra. The optical data
indicated that the 3-vinylation bathochromically shifted the visible (Soret/Qx/Qy) absorption and fluo-
rescence emission maxima, the 7-vinylation moved the Soret/Qx and Qy/emission bands to longer and
slightly shorter wavelengths, respectively, and the 8-vinylation induced red shifts of the Soret/Qx
maxima and no (or faint red) shifts of the Qy/emission maxima. Zinc complexes of 3,7- and 3,8-divinyl-
chlorins showed almost the same optical properties including fluorescence emission quantum yields and
lifetimes as well as the same first oxidation potentials, thus, 3,7-divinyl-chlorophyll-a could be consid-
ered an alternative pigment to the naturally occurring 3,8-divinyl analog.
Received 16 November 2016
Received in revised form
2 December 2016
Accepted 5 December 2016
Available online 8 December 2016
Keywords:
Fluorescence emission data
Photosynthesis
Pigment
Pyropheophorbide
Visible absorption spectra
© 2016 Elsevier Ltd. All rights reserved.
1. Introduction
monovinyl(MV)-Chl-c3 / Chl-c3 (Emiliania huxleyi),10 while DV-
Chl-f has not yet been found. Substitution of the 8-ethyl group with
Phototrophs have several pigments including cyclic tetrapyrrole
chlorophylls (Chls), linear tetrapyrrole bilins, and polyene carot-
enoids.1,2 In the initial stage of photosynthesis, Chls are important
for light absorption, excitation energy migration, and charge sep-
aration processes. Most Chl molecules in oxygenic photosynthetic
organisms have a vinyl group at the 3-vinyl group (left drawing of
Fig. 1) except Chl-d bearing the 3-formyl group in the limited cya-
nobacterial species, such as Acaryochloris marina.3e5 One more vi-
nyl group is found at the 8-position of the above 3-monovinylated
Chl molecules in some natural cells to give 3,8-divinylated Chl
the 8-vinyl group shifts the intense absorption band on the blue
side of the visible light region to a longer wavelength and induces
no (or only slight) change of the redmost maximum.3,10,11 The
former primarily controls the sunlight-harvesting ability and the
latter regulates the site energy in photosynthetic apparatuses.
The 3-vinyl-8-ethyl-Chl molecules are biosynthesized through
hydrogenation of the 8-vinyl group by specific enzymes,
divinyl reductases.12 Suppression of the ability of the reductase
gives 3,8-divinylated analogs. As photosynthetically active mon-
ovinylated Chl pigments, regioisomeric 3-ethyl-8-vinyl-Chls are
not detected in oxygenic phototrophs. No 3,8-diethyl-Chls are
utilized in the initial events of photosynthesis. Furthermore, the
7-substituent of Chls-a/c1/c2/d/f is a methyl group, while a formyl
or methoxycarbonyl group is found in (DV-)Chl-b or (MV-)Chl-c3,
respectively.3,4,10 The functional groups at the 7-position are
limited to the three substitutes and 7-vinylated Chls have not
been observed. To address the reasons for the inaccessibility of
such Chl molecules, we prepared diethylated, monovinylated, and
divinylated Chl-a/b derivatives, 1a, 1b/1c/(Zn-)2a, and (Zn-)2b/
(Zn-)2c (right drawing of Fig. 1), and compared their optical and
electrochemical properties in THF solution.
molecules: Chl-a
chlorococcus),6,7 Chl-b / DV-Chl-b (Prochlorococcus sp. NIES-
2086),8 Chl-c1 Chl-c2 (Chaetoceros calcitrans),9 and
/
divinyl(DV)-Chl-a (Synechococcus/Pro-
/
Abbreviations: APCI, atmospheric pressure chemical ionization; BChl,
bacteriochlorophyll; Chl, chlorophyll; CD, circular dichroism; DV, divinyl; FCC, flash
column chromatography; FWHM, full width at half maximum; HRMS, high reso-
lution mass spectrum; LDI, laser desorption/ionization; MV, monovinyl; PDC, pyr-
idinium dichromate; 7V, 7-vinyl.
* Corresponding author.
E-mail address: tamiaki@fc.ritsumei.ac.jp (H. Tamiaki).
http://dx.doi.org/10.1016/j.tet.2016.12.003
0040-4020/© 2016 Elsevier Ltd. All rights reserved.

314
K. Kim et al. / Tetrahedron 73 (2017) 313e321
Fig. 1. Molecular structures of natural [divinyl(DV)-]chlorophylls(Chls)-a (left) and their synthetic analogs (right).
2. Results and discussion
with methylenetriphenylphosphorane prepared in situ from
methyltriphenylphosphonium iodide and potassium tert-but-
oxide5,23 to afford 3,7-divinyl-chlorin 2b [42%, step (vii)]. The new
compound was identified by its 1H NMR, mass, and visible spectra
(section 4.2.2). According to previously reported procedures,5 the
3-vinyl group of 2f was selectively hydrogenated despite the
presence of the reactive 7-hydroxymethyl group to 1f [step (viii)]
and oxidized by PDC to 1e [step (vi)], then methylenated by the
Wittig reaction to 1b [step (vii)]. Catalytic hydrogenation of 3,7-
divinyl-chlorin 2b gave a mixture of 3-vinyl-chlorin 2a and 7-
vinyl-chlorin 1b as the monohydrogenated products and 1a as
the doubly hydrogenated product. No regioselective reduction
occurred and the isolation of 1b from the reaction mixture was
difficult. The non-selective hydrogenation was comparable to the
less-selective hydrogenation (using rhodium on alumina as the
catalyst) of DV-Chl-a reported previously.25 It is noteworthy that
the enzymatic reduction of divinyl-chlorophyllide-a (the dephyty-
lated derivative of DV-Chl-a) proceeds regioselectively at the 8-
vinyl group to give solely the product monovinylated at the 3-
position, chlorophyllide-a.12
2.1. Synthesis of (zinc) methyl pyropheophorbides
Chl-a from cyanobacterial cells (spirulina) was modified to
methyl pyropheophorbide-a (2a, Scheme 1) according to reported
procedures.5,13 The 3-vinyl group of 2a was catalytically hydroge-
nated to give methyl mesopyropheophorbide-a (1a), as shown in
step (i) of Scheme 1.13,14 The C7¼C8 double bond of 1a was selec-
tively oxidized to afford cis-diol 1d [step (ii)], which was doubly
dehydrated to yield 8-vinyl-chlorin 1c [step (iii)].5,15
The 3-vinyl substituent in 2a was hydrated to a 31-epimeric
mixture of the corresponding secondary alcohol, methyl 8-ethyl-
12-methyl-bacteriopheophorbide-d ([E,M]BPhe-dM) [step (iv) of
Scheme 1]. The carbinol was also obtained from chemical alteration
of extracted bacteriochlorophyll(BChl)-a or an isolated BChl-d ho-
molog.13 Similarly to the synthesis of 1d from 1a, triol 2d was ob-
15
tained by the oxidation of [E,M]BPhe-dM [step (ii)] and was
treated with an acid, p-toluenesulfonic acid [step (v)]. After reflux
in THF for 1 h, bacteriochlorin 2d was transformed to chlorin
compounds through monodehydration of the cis-7,8-diol.16 The
crude alcoholic products were directly heated in 1,2-
dichlorobenzene under neutral conditions (reflux for 5 h) to
induce further dehydration and afford 3,8-divinyl-chlorin (2c,
methyl divinyl-pyropheophorbide-a) in a yield of 33% for the triple
dehydration (Scheme S1). The molecular structure of 2c was char-
acterized by its spectral data (section 4.2.3), which was also
confirmed by the reported data.17 The one-pot simple conversion of
2d to 2c proceeded by refluxing the mixed solvents (1:9
CH2Cl2eC6H6) with the same acid18 but the isolated yield was lower
(9%) than the above. Moreover, the Wittig reaction (vide infra)19e23
of 3-formyl-8-vinyl-chlorin, methyl 8-vinyl-pyropheophorbide-d,15
with CH2¼PPh3 gave 2c (46%). The transformation of 2a to 2c using
the Wittig reaction took four steps including the oxidative cleavage
of the 3-vinyl to formyl group and the overall yield was 6% (Scheme
S2). The synthetic process through the triple dehydration [step (v)]
is superior since it could be shortened to three steps and the total
yield of 2a to 2c via 2d increased twofold (13%). Alternatively, 2c
could be prepared by chemical modification of DV-Chl-a isolated
from genetically mutated cyanobacterial cells9,24 based on the same
procedures as in the aforementioned transformation of Chl-a to 2a.
Chl-b from spinach was altered to methyl 71-hydroxy-pyro-
pheophorbide-a (2f)5 and oxidized by pyridinium dichromate
(PDC) to give methyl pyropheophorbide-b (2e) in a yield of 55%
[step (vi) of Scheme 1]. 7-Formyl-chlorin 2e was treated
Using the standard procedures shown in step (ix) of Scheme 2,
free bases 2aec were metalated to zinc complexes Zn-2aec. Syn-
thetic Zn-2a and Zn-2c are models of natural Chl-a and DV-Chl-a,
respectively, although the former two complexes lack the 132-
methoxycarbonyl group and are substituted at the central metal
(Mg / Zn) and esterifying group (phytyl / methyl). Zn-2b is an
analog of DV-Chl-a.
2.2. Optical properties of methyl mesopyropheophorbides 1aec
When 1a was dissolved in THF, the diluted solution (ca. 10 mM)
gave apparently five absorption peaks in the visible region (Fig. 2A).
The intense band in the purple light region is called a Soret band,
the weak bands in the blue to green region are Qx bands, and the
bands in the yellow to red region are Qy bands. All three visible
bands are composed of the main band at a longer wavelength and
the minor bands at shorter wavelengths. The Qx and Qy bands
afford apparently two maxima, (0,0) and (0,1), as shown in Table 1,
while the Soret band has two shoulders in the blue side of its main
maximum at 407 nm.
Substitution of the 7-methyl group of 1a with the vinyl group, as
in 1b, induced a slight blue shift of the Qy bands (1 nm) and red-
shift of the Soret band by 10 nm. The substitution largely affected
the shape and positions of the Qx bands. The relative Qx(0,0) band
intensity of 1b halved in comparison with that of 1a. The Qx(0,0)/

K. Kim et al. / Tetrahedron 73 (2017) 313e321
315
Scheme 1. Synthesis of methyl pyropheophorbides lacking a vinyl group 1a and possessing one vinyl group 1b/1c/2a and two vinyl groups 2b/2c: (i) H2, Pd-C/THF, Me2CO; (ii) OsO4,
C5H5N/CH2Cl2, then H2S/MeOH; (iii) p-MeC6H4SO3H$H2O/C6H6 (rt / reflux); (iv) HBr/AcOH, then H2O, followed by CH2N2/Et2O, CH2Cl2; (v) p-MeC6H4SO3H$H2O/THF (reflux), then
1,2-Cl2C6H4 (reflux) (vi) (C5H5NHþ)2Cr2O27ꢀ/CH2Cl2; (vii) CH3PPhþ3 ꢀ
I , tBuOK/CH2Cl2; (viii) H2, Pd-Fibroin/THF, Me2CO.
(0,1) maxima of 1b moved to wavelengths of about 5 nm longer
than those of 1a. These spectral changes can be explained as fol-
lows. The Qx bands are largely regulated by the transition moment
along the molecular x-axis, the BeD ring direction (Fig.1), while the
moment on the y-axis (AeC ring direction) perpendicular to the x-
axis primarily controls the Qy bands. Therefore, the p-conjugatable
substituents at the 7-position on the B-ring largely affect the Qx
bands and less the Qy bands. Since the present 7-vinyl group is
withdrawing effect of the 7-vinyl group in 1b slightly blue-shifted
the Qy bands: group electronegativities ( s) of the methyl and vi-
c
nyl groups are calculated to be 2.47 and 2.79, respectively.26 The
blue shifts were consistent with the observation that a more
strongly electron-pulling formyl group at the 7-position of Chl-b
and its synthetic analogs moved their Qy maxima to shorter
wavelengths than those of the corresponding 7-methylated com-
pounds including Chl-a.3,5,27 The Soret band is produced by a
combination of the Bx with By bands, so the Soret maxima of 1b
were bathochromically shifted by a large red-shifted Bx band based
directly conjugated to the chlorin p-system, the p-elongation along
the x-axis as in 1a / 1b red-shifted the Qx bands. A vinyl group is
more electronegative than a methyl group and the electron-
on an increase of the p-conjugation along the x-axis rather than a

316
K. Kim et al. / Tetrahedron 73 (2017) 313e321
Scheme 2. Synthesis of zinc methyl pyropheophorbides-a Zn-2aec: (ix) Zn(OAc)2$2H2O/MeOH, CH2Cl2.
Table 1
Visible absorption maxima (nm) of methyl pyropheophorbides 1aec, 2aec, and Zn-
2aec in THF (ca. 10ꢀ5 M).a
Compound
Soret
Qx
Qy
(0,1)
(0,0)
(0,1)
(0,0)
1a
1b
1c
2a
2b
2c
Zn-2a
Zn-2b
Zn-2c
407
417
416
412
421
421
427
436
436
502 (9)
507 (9)
507 (8)
508 (10)
512 (11)
513 (10)
526 (4)
531 (4)
532 (4)
533 (9)
539 (5)
539 (5)
537 (8)
541 (6)
542 (5)
568 (6)
eb
601 (7)
600 (6)
601 (6)
610 (7)
609 (7)
610 (6)
609 (11)
606 (9)
609 (9)
657 (44)
656 (34)
657 (39)
668 (46)
666 (36)
668 (36)
656 (67)
653 (60)
655 (61)
eb
a
The values in parentheses indicate the absorption intensities (%) relative to the
Soret maximum.
b
The band was so broad that the maximum could not be clearly confirmed.
positions to the transition moment along the y-axis. It is noted
that the absorption spectrum of 1c looks like that of 1b and the
differences in their maxima are at most 1 nm, thus a vinyl group on
the B-ring affects the visible spectra in a similar manner. The Qy
peak positions of 1aec did not change so dramatically, but the in-
tensity of the Qy(0,0) band relative to that of the redmost Soret
peak apparently decreased in the order of 1a > 1c > 1b. Further-
more, substitution effects on the Qy/Soret maxima and relative Qy
intensities were confirmed by molecular model calculations
(Table S1).
Since Chl-a/b derivatives 1aec are chiral and have the same
stereochemistry at the 17- and 18-positions, (17S,18S), circular di-
chroism (CD) bands were observed in the visible region (Fig. 2B).
Negative CD peaks were seen at the Qy(0,0)/(0,1) positions and
positive peaks were visible at the Qx(0,0) and Soret maxima. The
CD peak intensity at the Qx(0,0) is dependent on the visible ab-
sorption intensity.
Excitation of a diluted THF solution of 1aec at the Soret maxima
gave fluorescence emission bands at around 660 nm (Fig. 3B). The
slight shifts of their emission peak positions (lem) were consistent
with those of the Qy(0,0) absorption peak positions (labs), as ex-
Fig. 2. Visible absorption (A) and CD spectra (B) of methyl mesopyropheophorbides
1aec in THF (ca. 10
mM). All the absorption spectra were normalized at the Soret
maxima.
slightly blue-shifted By band proposed from the electronegativity
of the 7-vinyl group.
Dehydrogenation of the 8-ethyl group of 1a to the 8-vinyl group
of 1c did not change the Qy maxima in THF but red-shifted the Qx
and Soret maxima. The bathochromic shifts are ascribable to the
aforementioned p-conjugation along the x-axis. No shifts observed
in the Qy maxima were different from the slight blue shifts by
pected (Fig. 3 and Table 2). The Stokes shifts (Ds) of 1aec were the
same and small at 3 nm, indicating that the molecular structures of
their ground states were similar to those of their first singlet
modification of 1a to 1b. Since ethyl and methyl groups have almost
the same electronegativity [
small difference is due to the substitution effect at the 7- and 8-
c
(Et) ¼ 2.48 and
c
(Me) ¼ 2.47],26 the

K. Kim et al. / Tetrahedron 73 (2017) 313e321
317
Table 2
excited states. Their main emission bands resembled the Qy(0,0)
absorption bands in a mirror image. The bandwidths of the former
were comparable to those of the latter, and the full widths at half
maxima (FWHMs) of the fluorescence bands were only about 10%
larger than those of the Qy(0,0) bands. The fluorescence emission
quantum yield (Fem) of 1a was larger than those of 1b and 1c. The
vinyl group at the B-ring partly suppressed the Fem values. The
decrease order of 1a > 1c > 1b is interestingly consistent with the
order of their relative Qy(0,0) intensities (vide supra). The fluo-
rescence emission of 1aec decayed in a single exponential manner
and their lifetimes (tems) were almost the same, hardly varying by
7,8-vinylation.
Qy absorption and fluorescence emission data of methyl pyropheophorbides 1aec,
2aec, and Zn-2aec in aerated THF at room temperature.a
Compound
labs/nm
lem/nm
D
/nm
Fem/%
tem/ns
1a
1b
1c
2a
2b
2c
Zn-2a
Zn-2b
Zn-2c
657 (327)
656 (351)
657 (299)
668 (408)
666 (433)
668 (401)
656 (412)
653 (407)
655 (392)
660 (355)
659 (375)
660 (325)
674 (416)
671 (421)
673 (403)
661 (427)
658 (417)
660 (414)
3
3
3
6
5
5
5
5
5
25
20
22
27
19
21
21
17
17
6.4
6.5
6.5
6.8
6.6
6.6
3.0
3.0
2.9
a
labs, Qy(0,0) absorption maximum; lem, emission maximum (excited at Soret
em e labs Fem, emission quantum yield (excited at Soret
maxima);
D
, Stokes shift ¼
l
;
2.3. Optical properties of methyl pyropheophorbides 2aec
maxima); tem, emission lifetime (excited at 403 nm); the values in parentheses
indicate full widths at half maxima of bands (cmꢀ1).
3-Vinyl-chlorin 2a in THF showed a similar visible spectral
shape as that of 3-ethyl-chlorin 1a but the dehydrogenation shifted
all the peak positions to longer wavelengths (Figs. 2A and 4A). Red
shifts of the Qy maxima were about 10 nm and larger than those of
the Qx and Soret maxima (ca. 5 nm) (Table 1). The 3-vinyl group
elongated the
p-conjugation with the chlorin moiety along the y-
axis and largely affected the Qy bands, which was consistent with
the substitution effect at the 3-position of chlorophyll pigments
previously reported.3 Similar shifts were observed in the 31,32-
dehydrogenation of 1b/c to 2b/c. Close inspection indicated that
the Qy(0,0) bands of 2aec were broader than those of 1aec. The
substitution of the 3-ethyl with 3-vinyl group increased FWHMs by
about 30% (Table 2). The broadening can be ascribable to the
rotational conformation of the 3-vinyl group around the C3eC31
single bond.23
In 3-vinyl-chlorins 2aec, introduction of a vinyl group at the 7-
or 8-position induced the same effects in their visible spectra as in
3-ethyl-chlorins 1aec. Typically, small blue or no shifts of the Qy
maxima were observed for the 7- or 8-vinylation, respectively,
although it apparently red-shifted the Qx and Soret maxima. All the
three 3-vinyl-chlorins 2aec gave negative CD peaks in the Qy(0,0)
regions and positive CD peaks in the Qx(0,0) and redmost Soret
regions (Fig. 4B), which was comparable to the CD spectra of 3-
ethyl-chlorins 1aec. The 3-vinylation as in 1aec / 2aec did not
disturb the substitution effect of the 7,8-vinylation on the visible
and CD spectra.
By the 3-vinylation of 1aec to 2aec, the main fluorescence
Fig. 4. Visible absorption (A) and CD spectra (B) of methyl pyropheophorbides 2aec in
THF (ca. 10 mM). All the absorption spectra were normalized at the Soret maxima.
emission maxima moved to longer wavelengths (12e14 nm) and
their bands widened (50e80 cmꢀ1 for FWHM) (Fig. 5 and Table 2).
The Stokes shifts of 2aec were 5e6 nm and two times larger than
those of 1aec. Enhancement of the emission bandwidths and
Stokes shifts may be due to the fact that 2aec could take various
rotamers with different
p-conjugations of the 3-vinyl group with
the chlorin moiety (vide supra). The 3-vinylation slightly affected
the emission quantum yields and single-exponentially obeyed
lifetimes (Table 2).
The fluorescence emission quantum yields were reduced in the
order of 2a > 2c > 2b and their lifetimes were almost the same. The
substitution effects at the 7/8-vinylation of 2a to 2b/c were
consistent with those of 1a to 1b/c. An increase in the absorbance
ratio of the Qy(0,0) over Soret maxima enhanced the Fem values as
described in 1aec. The linear correlation of the relative Qy(0,0)
intensity with Fem in 1/2aec (Fig. S1) is in good agreement with the
Fig. 3. Visible Qy(0,0) absorption (A) and fluorescence emission spectra (B) of methyl
mesopyropheophorbides 1aec in aerated THF. All the spectra were normalized at their
most intense peaks and the emission spectra were measured by excitation at the Soret
maxima.

318
K. Kim et al. / Tetrahedron 73 (2017) 313e321
observation for natural Chls previously reported.27 Synthetic Chl
derivatives bearing a relatively more intense Qy band were more
fluorescent.
determined by cyclic voltammetry: Eo1x/2(Zn-2a) ¼ 340 mV and E1/
ox
ox(Zn-2b) ¼ E (Zn-2c) ¼ 360 mV (vs. Ag/Agþ). Substitution of a
2
1/2
methyl or ethyl group on the B-ring with a vinyl group increased
ox
the E
value by 20 mV, irrespective of the introduced position.
1/2
Since a vinyl group is more electron-withdrawing than alkyl groups
(vide supra), and the chlorin -system directly conjugated with a
2.4. Physical properties of zinc methyl pyropheophorbides Zn-2aec
p
vinyl group was less electrochemically oxidized. The substitution
effect was confirmed by the observation that Zn-1a possessing the
Zinc metalation of free bases 2aec to Zn-2aec induced blue
shifts and relative enhancement in intensity of the Qy bands, red
shifts and relative reduction of the Qx bands and red shift of Soret
maxima (15 nm) in THF (Figs. 4/6A and Table 1). The 7/8-vinylation
of Zn-2a to Zn-2b/c affected the visible spectra similarly to those of
1a to 1b/c and 2a to 2b/c: typically, small blue shifts of the Qy(0,0)
bands and large red shifts of the Soret maxima. The Qy(0,0)
bandwidths of Zn-2aec were not largely dependent on the 7,8-
substituents and their band shapes were similar to each other. In
Zn-2aec, negative, faint positive, and intense positive CD peaks
were observed in the Qy(0,0), Qx(0,0), and redmost Soret absorp-
tion regions, respectively (Fig. 6B). It is noteworthy that the zinc
complexes are axially ligated with a THF molecule and the resulting
five-coordinated species led to be monomeric in diluted THF
solution.
3-ethyl group was more oxidized by 20 mV than the 3-vinyl analog
ox
Zn-2a: Eo1x/2(Zn-1a) ¼ 320 mV. The change of E
by substitution
1/2
with a vinyl group was similar to the reported data of Eo1x/2(DV-Chl-
a) e Eo1x/2(Chl-a) ¼ ca. 10 mV.8
The above results provide the following effects by the 7,8-
vinylation. The intense Soret band moves to a longer wavelength
by the 7,8-vinylation and more largely overlaps with the solar
spectrum so the pigments can absorb sunlight more efficiently. The
7,8-vinylation affects the Qy(0,0) band only slightly so that the
energy-donating and accepting abilities of the Chl molecules and
their singlet excited energies are not so largely changed. Addi-
tionally, the oxidation potentials faintly increase by the 7,8-
vinylation and the electron transfer processes are primarily inde-
pendent of the substitution. In the initial stages of photosynthesis,
the divinylated Chls are more useful for absorbing and harvesting
sunlight than monovinylated Chl-a, and are also comparable in
excited energy and electron transferring events to Chl-a.
Naturally occurring Chl-a was dissolved in diethyl ether to form
its 5-coordinated species with a diethyl ether molecule as a single
axial ligand.28 Based on the reported data in diethyl ether, the
visible absorption maxima of Chl-a possessing the 3-vinyl group
were compared with those of synthetic 7V-Chl-a bearing the 3,7-
divinyl groups and natural DV-Chl-a carrying the 3,8-divinyl
groups (Table 3). The three Qy(0,0) maxima are almost the same
at around 660 nm, but the Soret bands of 7V- and DV-Chls-a were
red-shifted by ca. 5 nm. The reported tendency is comparable to the
present observation in Zn-2aec. Moreover, the relative Qy(0,0)
intensities to the Soret maxima decreased in the order of Chl-a >
DV-Chl-a > 7V-Chl-a, which is consistent with that for Zn-2aec.
Therefore, Zn-2aec are good models for the above Chls-a.
Zinc complexes Zn-2aec in THF were fluorescent with the same
Stokes shifts of 5 nm (Fig. 7 and Table 2). Zinc metalation of 2aec to
Zn-2aec decreased the fluorescence emission quantum yields and
lifetimes, which are ascribable to the heavy atom effect of the
central zinc. The 7/8-vinylation of Zn-2a to Zn-2b/c reduced the
quantum yields, which were dependent on the relative Qy(0,0)
intensities.
3. Concluding remarks
The present synthetic Chl derivatives showed characteristic
optical and electrochemical properties in THF which were depen-
dent on the peripheral substituents. The substitution of an alkyl
group with a vinyl group on the B-ring shifted the Soret and Qx
bands bathochromically and induced no or small blue shifts of the
The first oxidation potentials (Eo1x/2) of Zn-2aec in THF were
Fig. 5. Visible Qy(0,0) absorption (A) and fluorescence emission spectra (B) of methyl
pyropheophorbides 2aec in aerated THF. All the spectra were normalized at their most
intense peaks and the emission spectra were measured by excitation at the Soret
maxima.
Fig. 6. Visible absorption (A) and CD spectra (B) of zinc methyl pyropheophorbides Zn-
2aec in THF (ca. 10
mM). All the absorption spectra were normalized at the Soret
maxima.

K. Kim et al. / Tetrahedron 73 (2017) 313e321
319
controls their physical properties.30 The
p
-deconjugation makes
Table 3
Visible absorption maxima (nm) of chlorophylls in diethyl ether reported previously.
their properties similar to those of the corresponding ethyl-
chlorin.23 The conformational changes would be useful for the
regulation of photosynthetic activity in DV-Chl-a producing
organisms.
Compound
Soret
Qy(0,0)a
Reference
19
29
3
Chl-a
432
430
430
662 (76)
660 (80)
661 (77)
4. Experimental
19
7V-Chl-ab
DV-Chl-a
a
437
660 (65)
4.1. General
29
3
435
436
660 (71)
660 (71)
All melting points were measured with a Yanagimoto micro
melting apparatus and were uncorrected. Visible absorption and CD
spectra were measured with a Hitachi U-3500 spectrophotometer
and a Jasco J-720W spectropolarimeter, respectively. The fluores-
cence emission spectra and quantum yields were obtained by a
Hamamatsu Photonics C9920-03G spectrometer. Fluorescence
emission lifetimes were determined by a Hamamatsu Photonics
C7990S system. Cyclic voltammetry was performed by a BAS CV-
50W analyzer with a conventional three-electrode system (plat-
inum working/counter electrodes and aq. 0.1 M silver nitrate for
reference): a THF solution of tetraethylammonium and tetrabuty-
lammonium perchlorates (each 0.1 M) and scan rate ¼ 2000 mV/s.
FT-IR spectra were recorded on a Shimadzu IRAffinity-1 spectro-
photometer and a Shimadzu AIM-8000R microscope was used for
measurements of solid films on an aluminum coated glass (reflec-
tion mode). 1H and 13C NMR spectra were recorded on a JEOL ECA-
The values in parentheses indicate the absorption intensities (%) relative to the
Soret maximum.
b
7-Demethyl-7-vinyl-chlorophyll-a.
Qy maxima. The vinylation partly decreased the relative Qy(0,0)
peaks to the Soret maxima and fluorescence emission quantum
yields, but the emission lifetimes were independent of the modi-
fication. These substitution effects are ascribable to a
p-con-
jugatable and electron-withdrawing vinyl group along the
molecular x-axis. The substitution position of a vinyl group on the
B-ring hardly affected the optical properties, although Qy(0,0) ab-
sorption and fluorescence emission maxima of 7-vinyl-chlorins
were shifted more hypsochromically by 1e2 nm than those of the
corresponding 8-vinyl-chlorins. The oxidation potentials slightly
increased by the vinylation due to the electron-withdrawing vinyl
group, in which no substitution position effect was observed.
Synthetic 7-vinylated Chl-b derivatives exhibited similar optical
and electrochemical properties as those of the 8-vinylated Chl-a
derivatives. Similarly to DV-Chl-a, 7V-Chl-a would be useful for a
photosynthetic pigment but has never been found in any photo-
trophs. The unavailability is due to its biosynthetic inconvenience
including the complicated transformation of the 7-methyl to vinyl
group. In the biosynthesis of Chl-a, the 8-vinyl group of its pre-
cursor is hydrogenated to the 8-ethyl group, so the reductase is
inactivated to readily produce DV-Chl-a.12
600 (600 and 151 MHz) spectrometer; CHCl3
(
d
¼ 7.26 ppm) and
13CDCl3
(d
¼ 77.0 ppm) were used as internal references. Standard
mass data were obtained using laser desorption/ionization (LDI) by
a Shimadzu AXIMA-CFR plus spectrometer. High resolution mass
spectra (HRMS) were recorded on a Bruker micrOTOF II spec-
trometer: atmospheric pressure chemical ionization (APCI) and
positive mode in a methanol solution. TLC or flash column chro-
matography (FCC) was performed with silica gel (Merck, Kieselgel
60 F254 or Kieselgel 60, 40e63 mm, 230e400 mesh).
All the reactions were carried out in the dark under nitrogen.
Methyl mesopyropheophorbide-a (1a),14 methyl 7-demethyl-7-
vinyl-mesopyropheophorbide-a (1b),5 methyl 8-deethyl-8-vinyl-
mesopyropheophorbide-a (1c),5 methyl pyropheophorbide-a (2a),5
methyl cis-7,8-dihydroxy-bacteriopheophorbide-d (2d),15 and
methyl 71-hydroxy-pyropheophorbide-a (2f)5 were prepared ac-
cording to reported procedures. Solvents and reagents for prepa-
ration of the compounds were obtained from commercial suppliers
and utilized as supplied. THF for optical spectroscopy was pur-
chased from Nacalai Tesque as reagent prepared specially for high
performance liquid chromatography and used without further
purification.
The vinyl groups on the B-ring are coplanar, with the chlorin p-
system taking an energetically stable conformation in solution
since they can be freely rotated around the C7eC71 or C8eC81
single bond. In photosynthetic apparatuses, the vinyl groups are
fairly fixed by specific interaction of environmental peptides and
their rotation from coplanar to perpendicular conformation
4.2. Synthesis of methyl pyropheophorbides
4.2.1. Synthesis of methyl pyropheophorbide-b (2e)
A dry dichloromethane solution (30 ml) of primary alcohol 2f
(47.6 mg, 84.4
(200 mg, 532
m
mol) was treated with an excess amount of PDC
m
mol) and stirred at room temperature for 5 h. After
disappearance of 2f on a TLC plate, the reaction mixture was filtered
on Celite and the solvent was evaporated. The residue was purified
by recrystallization (CH2Cl2 and hexane) to give aldehyde 2e
(26.0 mg, 46.3 mmol, 55%): see the spectral data in Ref. 31.
4.2.2. Synthesis of methyl 7-demethyl-7-vinyl-pyropheophorbide-a
(2b)
Potassium tert-butoxide (84.5 mg, 753
methyltriphenylphosphonium iodide (310 mg, 763
added to dry dichloromethane (12 ml) and stirred at room tem-
perature under argon for 1 min. After the mixture had turned to a
m
mol) and
Fig. 7. Visible Qy(0,0) absorption (A) and fluorescence emission spectra (B) of zinc
methyl pyropheophorbides Zn-2aec in aerated THF. All the spectra were normalized at
their most intense peaks and the emission spectra were measured by excitation at the
Soret maxima.
mmol) were

320
K. Kim et al. / Tetrahedron 73 (2017) 313e321
brilliant yellow color, a dry dichloromethane solution (1 ml) of
aldehyde 2e (10.0 mg, 17.8 mol) was added and stirred for 1 min.
4.3. Synthesis of zinc methyl pyropheophorbides-a
m
The reaction mixture was quenched with water and extracted with
dichloromethane several times. The combined dichloromethane
phases were washed with water, dried over sodium sulfate, and
filtered. After the solvent was evaporated, the residue was purified
by FCC (3% Et2O and CH2Cl2) and recrystallization (CH2Cl2 and
4.3.1. Zinc metalation
A methanol solution saturated with zinc acetate dihydrate
(1 ml) was added to a dichloromethane solution (5 ml) of free bases
1a and 2aec (2 mmol) and the mixture was stirred at room tem-
perature for 30 min. The reaction mixture was washed with an
aqueous 4% sodium bicarbonate solution and water, dried over
sodium sulfate, and filtered. After the solvents were evaporated, the
residue was reprecipitated from dichloromethane and hexane to
give the corresponding zinc complexes Zn-1a and Zn-2aec nearly
quantitatively.
hexane) to give olefin 2b (4.2 mg, 7.5 mmol, 42%): black solid; mp
104e105.5 ꢁC; VIS (CH2Cl2) lmax 664 (relative intensity, 0.16), 597
(0.07), 569 (0.07), 521 (0.08), 425 nm (1.00); IR (film) nmax 1737
(172-C¼O), 1692 cmꢀ1 (13-C¼O); 1H NMR (CDCl3)
d 9.59 (1H, s, 10-
H), 9.58 (1H, s, 5-H), 8.57 (1H, s, 20-H), 7.99 (1H, dd, J ¼ 18, 12 Hz, 3-
CH), 7.89 (1H, dd, J ¼ 18, 12 Hz, 7-CH), 6.29 (1H, dd, J ¼ 18, 1 Hz, 31-
CH trans to 3-CeH), 6.17 (1H, dd, J ¼ 12, 1 Hz, 31-CH cis to 3-CeH),
6.13 (1H, dd, J ¼ 18, 1 Hz, 71-CH trans to 7-CeH), 5.98 (1H, dd, J ¼ 12,
1 Hz, 71-CH cis to 7-CeH), 5.28, 5.13 (each 1H, d, J ¼ 19 Hz, 131-CH2),
4.49 (1H, dq, J ¼ 2, 7 Hz, 18-H), 4.30 (1H, dt, J ¼ 8, 2 Hz, 17-H), 3.83
(2H, q, J ¼ 8 Hz, 8-CH2), 3.68 (3H, s, 12-CH3), 3.61 (3H, s, 172-
COOCH3), 3.41 (3H, s, 2-CH3), 2.71 (1H, m, 17-CH), 2.57 (1H, m, 171-
CH), 2.30 (2H, m,17-CHCH),1.82 (3H, d, J ¼ 7 Hz,18-CH3),1.76 (3H, t,
J ¼ 8 Hz, 81-CH3), 0.36, ꢀ1.71 (each, 1H, s, NH x 2); MS (APCI) found:
m/z 561.2048. Calcd. for C35H37N4O3: MHþ, 561.2060.
4.3.2. Spectral data of zinc complexes Zn-1a/2aec
Zinc methyl mesopyropheophorbide-a (Zn-1a): VIS (THF) lmax
644 (relative intensity, 0.63), 602 (0.10), 565 (0.06), 520 (0.04), 423
(1.00), 403 nm (0.59); MS (LDI) found: m/z 612.4. Calcd. for
C
34H36N4O3Zn: Mþ, 612.2: see also the spectral data in Refs.32e34
Zinc methyl pyropheophorbide-a (Zn-2a): VIS (THF) lmax 656
(relative intensity, 0.67), 609 (0.11), 568 (0.06), 526 (0.04), 427
(1.00), 406 (0.55), 380 nm (0.33); MS (LDI) found: m/z 610.1. Calcd.
for C34H34N4O3Zn: Mþ, 610.2: see also the spectral data in
Refs.32,35,36
Zinc methyl 7-demethyl-7-vinyl-pyropheophorbide-a (Zn-2b):
VIS (THF) lmax 653 (relative intensity, 0.60), 606 (0.09), 531 (0.04),
436 (1.00), 378 nm (0.24); MS (LDI) found: m/z 622.1. Calcd. for
35H34N4O3Zn: Mþ, 622.2.
4.2.3. Synthesis of methyl 8-deethyl-8-vinyl-pyropheophorbide-a
(2c)
C
Zinc methyl 8-deethyl-8-vinyl-pyropheophorbide-a (Zn-2c):
p-Toluenesulfonic acid monohydrate (138.3 mg, 727
added to a THF solution (40 ml) of triol 2d (222.2 mg, 370
m
mol) was
mol).
VIS (THF) lmax 655 (relative intensity, 0.61), 609 (0.09), 532 (0.04),
436 (1.00), 380 nm (0.25); MS (LDI) found: m/z 608.3. Calcd. for
m
34H32N4O3Zn: Mþ, 608.2.
The mixture was refluxed for 1 h and cooled down to room tem-
perature. The reaction mixture was poured into an aqueous 4%
sodium bicarbonate solution and extracted with dichloromethane
several times. The combined dichloromethane phases were washed
with water, dried over sodium sulfate, and filtered. After the sol-
vents were evaporated, the residue was dissolved in 1,2-
dichlorobenzene (50 ml) and refluxed for 5 h. The solvent was
removed in vacuo and the residue was purified by FCC (3e5% Et2O
and CH2Cl2) and recrystallization (CH2Cl2 and hexane) to give olefin
C
Acknowledgements
We would like to thank Dr. Meiyun Xu of Ritsumeikan University
for her experimental assistance. This work was partially supported
by a Grant-in-Aid for Scientific Research on Innovative Areas
"Artificial Photosynthesis (AnApple)" (No. 24107002) from the
Japan Society for the Promotion of Science (JSPS).
2c (66.7 mg, 122 m
mol, 33%): dark green solid; mp 194e195 ꢁC; VIS
(CH2Cl2) lmax 667 (relative intensity, 0.37), 610 (0.07), 543 (0.05),
513 (0.09), 422 (1.00), 404 (shoulder, 0.71), 322 nm (18); IR (film)
Appendix A. Supplementary data
nmax 1737 (172-C¼O), 1695 cmꢀ1 (13-C¼O); 1H NMR (CDCl3)
d 9.66
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.tet.2016.12.003.
(1H, s, 5-H), 9.45 (1H, s, 10-H), 8.58 (1H, s, 20-H), 8.02 (1H, dd,
J ¼ 18, 12 Hz, 8-CH), 7.92 (1H, dd, J ¼ 18, 11 Hz, 3-CH), 6.30 (1H, dd,
J ¼ 18, 1 Hz, 81-CH trans to 8-CeH), 6.19 (1H, dd, J ¼ 12, 1 Hz, 81-CH
cis to 8-CeH), 6.15 (1H, dd, J ¼ 18, 1 Hz, 31-CH trans to 3-CeH), 6.00
(1H, dd, J ¼ 11, 1 Hz, 31-CH cis to 3-CeH), 5.27, 5.12 (each 1H, d,
J ¼ 19 Hz,131-CH2), 4.50 (1H, dq, J ¼ 2, 8 Hz,18-H), 4.30 (1H, dt, J ¼ 7,
2 Hz, 17-H), 3.66 (3H, s, 7-CH3), 3.62 (3H, s, 172-COOCH3), 3.42 (3H,
s, 2-CH3), 3.36 (3H, s, 12-CH3) 2.74e2.69, 2.61e2.55 (each 1H, m,
171-CH2), 2.34e2.28 (2H, m, 17-CH2), 1.82 (3H, d, J ¼ 8 Hz, 18-CH3),
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