Toward carboxylate group functionalized A4, A2B 2, A3B oxaporphyrins and zinc complex of oxaporphyrins

Article abstract of DOI:10.1016/j.tet.2011.04.034

Series of new oxaporphyrins were isolated from the reaction of furan-1,4-diol, pyrrole, and an aldehyde under Lindsey's conditions, which gives easy access to ester group functionalized oxaporphyrins. The ester substituents can be readily hydrolyzed to te

Full text of DOI:10.1016/j.tet.2011.04.034

Tetrahedron 67 (2011) 4680e4688
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Toward carboxylate group functionalized A4, A2B2, A3B oxaporphyrins and zinc
complex of oxaporphyrins
,
,
,
,
Ram Ambre a b, Chien-Yi Yu a b, Sandeep B. Mane a b, Ching-Fa Yao b, Chen-Hsiung Hung a
*
a Institute of Chemistry, Academia Sinica, Nankang, Taipei 115, Taiwan
b Department of Chemistry, National Taiwan Normal University, Taipei 11677, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history:
Series of new oxaporphyrins were isolated from the reaction of furan-1,4-diol, pyrrole, and an aldehyde
under Lindsey’s conditions, which gives easy access to ester group functionalized oxaporphyrins. The
ester substituents can be readily hydrolyzed to terminal carboxylic acid in the presence of KOH. The
Zn(II) oxaporphyrins have been synthesized from the reaction of free base with ZnCl2 and fully char-
acterized by variable temperature NMR, 2D NMR, and single crystal X-ray diffraction studies.
Ó 2011 Elsevier Ltd. All rights reserved.
Received 3 March 2011
Accepted 11 April 2011
Available online 20 April 2011
Keywords:
Porphyrin Synthesis
Core-Modified Porphyrin
Oxaporphyrin
Zinc Porphyrin
X-ray structure
1. Introduction
focusing on synthesis and characterization of heteroporphyrins,11
_ ꢀ
reports on their metal complexes are still limited. Latos-Grazynski
Porphyrins and their derivatives are one of the most important
classes of heterocycles attracting interdisciplinary research and
having broad range of application. Porphyrins play key roles in the
field of photodynamic therapy (PDT),1 DNA photocleavage,2 organic
light emitting diodes (OLED),3 dye sensitized solar cells (DSSC),4
organocatalysis,5 and asymmetric catalysis.6 In additional to the
studies using regular N4-porphyrins, replacement of pyrrolic nitro-
gen with a heteroatom to provide new physicochemical properties
is an attractive aspect of study. Alternation of pyrrolic nitrogen with
various heteroatoms, such as oxygen, sulfur, selenium, or phos-
phorus explores the notable changes in terms of electronic struc-
ture, physicochemical, spectroscopic, and metal binding properties.7
Newly developed water soluble thiaporphyrin and selenaporphyr-
ins found competitive candidates for photodynamic therapy
(PDT).1h Heteroporphyrins with one or more functional groups on
meso position is one of the important building blocks in synthesis of
hybrid porphyrin diad, triad, tetrad, pentad, nonad or arrays.8 Syn-
thesis, characterization, and various properties of thiaporphyrins
have been extensively studied by Ravikanth8,9aef and other
researchers.1h,9g,h Moreover, Matano and co-workers reported syn-
thesis, structure, reactivity, optical, and redox properties of new
phosphaporphyrins.8a,b,10 Although there are several articles
and co-workers explored the most promising and pioneering study
on metal binding properties of Ni(II) thiaporphyrin,12 Cu(II) thia-
porphyrin,13 Fe(II) thiaporphyrin,12d Pd(II) thiaporphyrin,14 Fe(II)
oxaporphyrin,15,16a Ni(II) oxaporphyrin,16 Ni(II) dioxaporphyrin,16b
and Ni(II) selenaporphyrin.17 Our group has reported the exclusive
metal complex of dithiaporphyrin, [Ru(S2TTP)Cl2].18 Metal binding
properties of Cu(II) with oxaporphyrin, dioxaporphyrin, and oxa-
thiaporphyrin were reported by Chandrashekhar and co-workers.19
Arnold and co-workers synthesized and characterized lithium 21-
thiaporphyrin complex.20 To the best of our knowledge, there is only
one report on Zn(II) oxaporphyrin with incomplete characteriza-
tions.21 Herein we report the synthesis and characterization of
newly designed oxaporphyrins with terminal carboxylic acid func-
tional groups and their Zn(II) oxaporphyrin complexes as a step
toward applying oxaporphyrins and their metal complexes to en-
ergy related research areas.
2. Results and discussion
Condensation of 1 equiv of 2,5-bis(arylhydroxymethyl)furan
with 2 equiv of aryl aldehyde and 3 equiv of pyrrole under regular
porphyrin forming conditions followed by separation of reaction
products by column chromatography resulted in symmetrical A4
and A2B2 ester functionalized oxaporphyrins while the A3B oxa-
porphyrin with a lower symmetry can be prepared using
a-aryl-
* Corresponding author. Tel.: þ0 886 2 27898570; fax: þ0 886 2 27834202;
e-mail address: chhung@chem.sinica.edu.tw (C.-H. Hung).
2,5-furandimethanol as the starting diol.10h Hydrolysis of ester
0040-4020/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2011.04.034

R. Ambre et al. / Tetrahedron 67 (2011) 4680e4688
4681
functionalized oxaporphyrins to free-base carboxylic acid oxapor-
phyrins can be easily achieved by using aqueous KOH.22 The zinc
metalation of these oxaporphyrins can be readily obtained from
reaction with molar excess of ZnCl2 by using literature methods.23
aldehyde and 3 equiv of pyrrole in CH2Cl2 in the presence of cata-
lytic amount of BF3$OEt2 followed by oxidation with DDQ. TLC as
well as mass spectrum analysis of the crude reaction mixture
showed the formation of only two porphyrins, the desired oxa-
porphyrin and N4-porphyrin. Formation of the dioxaporphyrin was
not observed, which is in agreement with the previous reports.10h
All six oxaporphyrins, 4e9, are achieved in reasonable yields and
were characterized by 1H NMR, IR, absorption, and mass
spectroscopy.
The benchmark compound oxaporphyrin 4 has been reported
by Lindsey10b and Lee10c in more than four steps from starting
material. In our current protocol oxaporphyrin 4 was achieved in
only two steps with reasonable yield. The characterization data of
the oxaporphyrin 4 is in agreement with the literature reports.10b
The 1H NMR spectra of the oxaporphyrins 4e9 recorded in CDCl3
show a C2-axis embedded porphyrinic pattern with some minor
2.1. Synthesis of 2,5-bis(arylhydroxymethyl)furan
The symmetrical diols 1, 2, and, 3 were synthesized with slight
modification of the literature method.10h Condensation of 1 equiv
of 2,5-dilithiated furan with 2.5 equiv of corresponding aryl car-
boxaldehyde in THF at 0 ꢀC yields required diols after usual work-
up and separation methods. The structures of studied aldehydes
and products with respective yields are cited in Table 1. TLC analysis
of the reaction mixture showed the exclusive formation of sym-
metrical diol with some unreacted aldehyde. The reaction mixtures
were purified by silica gel column chromatography using ethyl
acetate/hexane (30:70) as eluent to afford diols 1, 2, and 3 in 78%,
37%, and 35% yield, respectively. All the three diols were charac-
terized by 1H NMR, 13C NMR, IR, and mass spectrometry.
differences depending upon the substituents. As expected
b-furan
protons appears as singlet in the most downfield region, followed
by
b
-pyrrolic protons opposite to furan ring and
b
b
-pyrrolic protons
-pyrrolic protons
adjacent to furan ring. In oxaporphyrin 6 and 8,
adjacent to furan ring give sharp doublets with a coupling constant
of 4.5 Hz and 4.8 Hz, respectively (see Supplementary data). The
Table 1
Synthesis of 2,5-bis(arylhydroxymethyl)furana
d
values of the protons on aromatic rings vary in a range of
O
H
R3
R3
R2
R1
7e8.5 ppm, whereas the aliphatic protons are observed in the
range of 1.5e4.5 ppm depending on the substituents. As reported in
literature the inner NH signal was not observed on 1H NMR spectra
when commercially supplied CDCl3 was used as the solvent due to
the more basic nature of oxaporphyrins in comparison with N4-
porphyrin.10b,e
R2
R1
nBuLi/TMEDA,
Hexane/THF,
+
O
R1
R3
O
HO
HO
R2
Product
R1
eH
R2
R3
Yield (%)b
The IR spectra of oxaporphyrin 6e9 show stretching frequency at
around 1723 cmꢁ1 supporting the presence of carbonyl group. The
UVevis absorption spectra of oxaporphyrin 4e9 in CH2Cl2 displayed
one Soret band and four indistinct Q bands. The Qx(0,0) band at
w672 nm is red-shifted significantly in comparison with w649 nm
for N4-porphyrins. The Soret band for oxaporphyrin 5 (426 nm) with
its electron donating groups experiences the highest degree of red-
shift in comparison to all oxaporphyrins. The oxaporphyrin 4
(421 nm) shows comparable Soret band with its N4-porphyrin
(w419 nm).24 Based on these observations, the oxaporphyrins
bearing identical substituents exhibit red-shifts absorption bands
compared with the parent N4-porphyrins. This is in consistent with
the literature report that, although not as significant as dioxapor-
phyrins or thiaporphyrins, the slight stabilization of the LUMO and
destabilization of HOMO results red shift on the absorption spectra of
oxaporphyrin.25
1
2
3
eCH3
eH
eOCH3
eH
78
37
35
eOCH3
eH
eOCH3
eCOOCH3
a
Reactions conditions: (i) 37 mmol nBuLi (2.5 M solution in hexane), 37 mmol
tetramethylethylenediamine (TMEDA), 50 mL hexane, 15 mmol furan, rt, 1 h; (ii)
37 mmol aldehyde, 30 mL hexane, 0 ꢀC, 1 h.
b
Yield of analytically pure product.
2.2. Synthesis of A4 and A2B2 oxaporphyrins
The usual condensation of symmetrical diol with aromatic al-
dehyde and pyrrole in the presence of catalytic amount of BF3$OEt2,
followed by oxidation with DDQ gives oxaporphyrins as repre-
sented in Table 2. The oxaporphyrins 4e9 were prepared by con-
densing 1 equiv of respective diols 1e3, 2 equiv of corresponding
2.3. Synthesis and spectral characterization of Zn(II)
complexes
Table 2
Synthesis of oxaporphyrina
Insertion of zinc ion has been readily achieved22 with high yields
by reacting the free base oxaporphyrins 4e9 with molar excess zinc
chloride in the presence of 2,6-lutidine as listed in Table 3. This
method is broadly applied for the metalation of regular N4-por-
phyrins but has never been applied for the preparation of zinc(II)
oxaporphyrins.8a,b,10 Monitoring on reaction progress showed rapid
change in solution color, which accompanied by splitting of Soret
band and red-shifting in Q bands on absorption spectra.26 After
work-up, the formation of Zn(II) complex was confirmed by the
observation of mass spectra with m/z corresponding to [MꢁCl]þ of
Zn(II) oxaporphyrin 10e14, or 15.
The NMR spectra of Zn(II) complexes show significant changes,
compared tothose of free base oxaporphyrins. The furan, pyrrole, and
aromatic protons shift downfield, whereas pyrrole and aromatic
protons show increased number of resonances resulted from less
symmetrical square-pyramidal coordination geometry. Interestingly,
except complex 12, the NMR spectra of Zn(II) oxaporphyrins
R1
BF3 OEt2 (10mol%),
O
N
R1
O
N
R1
HO
DDQ
CH2Cl2
R2
O
+
R1
+
H
N
H
R2
HO
HN
R2
Product
R1
4-MeC6H4
3,4,5-MeOC6H2
4-MeO2CC6H4
4-MeC6H4
R2
Yield (%)b
4
5
6
7
8
9
4-MeC6H4
10
10
7
3,4,5-MeOC6H2
4-MeO2CC6H4
4-MeO2CC6H4
4-MeC6H4
9
9
4-MeO2CC6H4
4-MeO2CC6H4
3,5-tBuC6H3
14
a
Reaction conditions: (i) 1 mmol diol, 3 mmol pyrrole, 2 mmol aldehyde,
10 mol % BF3$OEt2 (48%), 100 mL CH2Cl2, 1 h (ii) 1 mmol DDQ, 1 h.
b
Yield of analytically pure product.

4682
R. Ambre et al. / Tetrahedron 67 (2011) 4680e4688
temperature is decreased and, at ꢁ60 ꢀC, two of the most downfield
Table 3
Synthesis of zinc(II) complexesa
phenyl resonances merge into a triplet resonance at 8.215 ppm.
Similar dynamic behavior on NMR signals of meso tolyl rings in ru-
thenium dithiaporphyrin complex has been reported as a result of
R1
R1
ZnCl2
N
O
Cl
Zn
the reduced steric constrain between tolyl ortho-protons and b pyr-
N
N
O
N
2,6-lutidine
rolic protons from a highly twisted core.18 However, it is un-
anticipated to observe a dramatic difference on rotational energy
barriers for the tolyl groups adjacent to furan group in comparison
with those adjacent to the pyrrole ring.
R2
R2
R1
R1
CH3CN/MeOH
HN
N
R2
R2
The absorption spectra show distinctively different pattern for
free base oxaporphyrins and their metal complexes.25 A split Soret
band with lmax at 426 and 442 nm is observed for zinc oxapor-
phyrin 12 as shown in Fig. 2, which is in consistent with the
breaking of orbital degeneracy because of the formation of a less
symmetrical five coordinated zinc oxaporphyrin complex. Similar
split Soret band has been observed in the iron(II) and copper(II)
complex of oxaporphyrin. Noticeably, significant red-shifted with
an oxorhodo-type pattern are observed in the Q bands region of 12.
Near identical Soret and Q band patterns are observed among zinc
oxaporphyrins with an either A4 or A2B2 meso-substituents.
Product
R1
R2
Yield(%)b
10
11
12
13
14
15
4-MeC6H4
4-MeC6H4
80
90
96
80
78
99
3,4,5-MeOC6H2
4-MeO2CC6H4
4-MeC6H4
4-MeO2CC6H4
4-MeO2CC6H4
3,4,5-MeOC6H2
4-MeO2CC6H4
4-MeO2CC6H4
4-MeC6H4
3,5-tBuC6H3
a
Reaction conditions: 25 mg oxaporphyrin, molar excess ZnCl2, 2,6-lutidine,
20 mL CH3CN/MeOH (3:1), 1 h.
b
Yield of analytically pure product.
demonstrate broadened signals for the resonances on meso phenyl
protons at room temperature suggesting that the tolyl groups are
under dynamic process. For the Zn(II) oxaporphyrin 10, the eight
doublets from four tolyl meso-substituents can be classified into two
groups with significantly different half-widths. The temperature-
dependent 1H NMR study on Zn(II) oxaporphyrin 10 was per-
formed and the results are overlaid in Fig. 1 to demonstrate that the
tolyl resonances at 8.25, 7.93, 7.58, and 7.52 ppm completely coalesce
at60 ꢀC, while, atthesame temperature, resonances fromanotherset
of tolyl groups are still visible. All resonances are sharpened when the
Fig. 2. Absorption spectra of oxaporphyrin 6 ( ), Zn(II) oxaporphyrin 12 ( ), and
oxaporphyrin 16 ( ). The inset depicts the spectra in the Q band region under higher
concentration.
Structure of oxaporphyrin zinc(II) complex 12 has been confirmed
by the single crystal X-ray diffraction study. The molecular structure
with thermal ellipsoids is shown in Fig. 3. The zinc center adopted
a five coordinated square-pyramidal geometry with chloride sit in
ꢁ
theapicalposition. The bonddistanceof2.2078(16) A isinthe normal
Fig. 1. Variable temperature NMR spectra of Zn(II) oxaporphyrin 10.
Fig. 3. The X-ray crystal structure of Zn(II) oxaporphyrin 12.

R. Ambre et al. / Tetrahedron 67 (2011) 4680e4688
4683
characterized by 1H NMR, IR, and mass spectroscopy. The absorption
spectrum of Zn(II) oxaporphyrin complex 20 (421 nm) shows
marked red-shift in Soret band compared to free base dicarboxylic
acid oxaporphyrin 19 (417 nm). (see Supplementary data).
ꢁ
range for ZneCl bond. The zinc metal ion deviates 0.59 A from the
four-atom-mean-plane defined by three inner core nitrogens and the
oxygen atom on the furan moiety. According to the thermal param-
eters, the oxygen atom of furan moiety has been assigned to disorder
with its trans pyrrolic nitrogen with 50% occupancy on each site. The
disordering precludes a precise comparison on the ZneO and ZneN
bond distances. The porphyrin core in the structure of 12 adopts
a significant S4 ruffling distortionwith an average atomic deviation of
ꢁ
0.153 A from the 24-atom-mean-plane on the oxaporphyrin core.
Noticeably, limited reports are available on the crystal structures of
metallooxaporphyrins15,16b and compound 12 is the first structurally
characterized zinc oxaporphyrin complex.
2.4. Base hydrolysis to the carboxylic acid functionalized
oxaporphyrin
The free base oxaporphyrins 16e18, and 19, all with two car-
boxylic acid functional groups, can be achieved in quantitative
yields by hydrolysis of their corresponding oxaporphyrin 6e8, and
9 using aqueous KOH as represented in Table 4.21 The obtained
product was characterized by 1H NMR, IR, absorption, and mass
spectroscopy. As per the expectations, 1H NMR spectrum shows
significant downfield shifts of all signals comparing to its starting
oxaporphyrin. For example in the case of oxaporphyrin 19 the furan
protons downshifted 0.26 ppm in comparison to its starting oxa-
porphyrin 9 supporting the presence of carboxylic acid groups (see
Supplementary data). Dramatic changes observed in absorption
spectroscopy are shown in Fig. 2. Dicarboxylic acid oxaporphyrin 16
experiences blue shift accompanied by splitting in Soret band,
whereas Q bands becomes broad and indefinite in comparison with
its oxaporphyrin 6 and zinc complex 12. The broadening of ab-
sorption bands presumably is a result of molecular aggregation
through intermolecular hydrogen bonding interactions and/or
Scheme 1. Synthesis of zinc(II) oxaporphyrin 20. Reaction conditions: 20 mg ox-
aporphyrin, molar excess ZnCl2, 2,6-lutidine, 20 mL MeOH 1 h. Analytically pure
product obtained in quantitative yield.
2.6. Synthesis of A3B type zinc(II) oxaporphyrin
monocarboxylic acid complexes
The A3B type zinc oxaporphyrion 26 was prepared as repre-
sented in Scheme 2. Furan-2-carbaldehyde was reduced to furan-2-
methanol 21 using NaBH4 in 94% yield. Furan-2-methanol on
treatment with 3,4,5-trimethoxybenzaldehyde in presence of nBuLi
and tetramethylethylenediamine (TMEDA) gives
a-(3,4,5-trime-
thoxyphenyl)-2,5-furandimethanol 22, which undergoes regular
porphyrin forming condensation with 3 equiv of pyrrole and
2 equiv of 3,4,5-trimethoxybenzaldehyde gives A3B type meso-
unsubstituted oxaporphyrin 23. Oxaporphyrin 23 on reaction with
NBS in CH2Cl2 gives meso brominated oxaporphyrin 24. The met-
alation of oxaporphyrin 24 using ZnCl2 gives Zn(II) oxaporphyrin
25. Zn(II) oxaporphyrin 25 was characterized by 1H NMR, IR, Mass,
absorption spectroscopy, and single crystal X-ray crystallography.
Zn(II) oxaporphyrin 25 undergoes Sonogashira coupling with
4-ethynylbenzoic acid under reported reaction conditions gives A3B
type zinc(II) oxaporphyrin monocarboxylic acid complexes 26 in
31% yield, which is successfully characterized by 1H NMR, IR, Mass,
and absorption spectroscopies. A noticeable difference between the
spectroscopic properties of oxaporphyrin 25 and ethynyl group
appended oxaporphyrin 26 is the significant red shift on the Soret
band from 434 to 451 nm and the Q band from 449 to 461 nm. The
porphyrin pep interactions.
Table 4
Hydrolysis of oxaporphyrina
R1
R3
N
N
O
N
O
N
KOH(aq)
THF
R2
R4
R1
R3
HN
HN
R4
presence of n
(C^C) stretching band at 2130 cmꢁ1 further signifies
the presence of ethynyl moiety.
R2
Product R1
R2
R3
R4
Yield(%)b
16
17
18
19
4-MeO2CC6H4 4-MeO2CC6H4 4-HO2CC6H4 4-HO2CC6H4 99
4-MeC6H4 4-MeO2CC6H4 4-MeC6H4 4-HO2CC6H4 97
3. Conclusion
4-MeO2CC6H4 4-MeC6H4
4-HO2CC6H4 4-MeC6H4
99
In conclusion the current article reveals facile and straight for-
ward synthesis of A4, A2B2, and A3B oxaporphyrins with carboxylic
acid as a terminal functional group. We have successfully in-
vestigated the preparation of functionalized Zn(II) oxaporphyrin
complexes with fully characterizations. Free base oxaporphyrin
dicarboxylic acids can be easily obtained from the hydrolysis of
their corresponding ester oxaporphyrin, which provide an impor-
tant functional group for exploring the potential to apply oxapor-
phyrins in material science or biological research areas.
4-MeO2CC6H4 3,5-tBuC6H3
4-HO2CC6H4 3,5-tBuC6H3 98
a
Reaction conditions: 25 mg oxaporphyrin, 500 mg aqueous KOH, 50 mL THF,
reflux, 10 h.
b
Yield of analytically pure product.
2.5. Synthesis of zinc(II) oxaporphyrin dicarboxylic acid
complexes
The conversion of free base oxaporphyrin 19 to zinc(II) oxapor-
phyrin 20 is represented in Scheme 1. Treatment of the free base
oxaporphyrin 19 in MeOH with excess ZnCl2 and drops of 2,6-luti-
dine gives zinc(II) oxaporphyrin 20. However, the Zn(II) metalation
on oxaporphyrin 16, 17, and 18 turned into highly insoluble pre-
cipitation in all common organic solvents and no further charac-
terization was attempted. The Zn(II) oxaporphyrin complex 20 was
4. Experimental section
4.1. General information
All chemicals were purchased from Acros Organics and Sigma
Aldrich and used directly without further purification. 1H NMR and

4684
R. Ambre et al. / Tetrahedron 67 (2011) 4680e4688
OMe
OMe
MeO
MeO
MeO
OMe
nBuLi/TMEDA,
+
O
Hexane/THF,
1h, 38%
O
OH
OH
OH
O
H
21
22
OMe
OMe
MeO
OMe
OMe
H
MeO
MeO
BF3 OEt2
DDQ,
CH2Cl2
MeO
MeO
MeO
OMe
+
N
NH
N
NBS
CH2Cl2
+
O
N
H
O
OH
OH
2 h,15%
MeO
15 min, 68%
O
22
MeO
OMe
OMe
23
OMe
OMe
OMe
OMe
OMe
MeO
MeO
N
MeO
OMe
OH
O
ZnCl2
CH3CN/MeOH
MeO
MeO
MeO
MeO
MeO
MeO
MeO
OH
O
Cl
N
Cl
Zn
N
O
N
N
NH
Br
Zn
Br
MeO
O
O
1 h, 97%
N
N
Pd2(dba)3/AsPh3
THF/Et3N
48 h, 31%
N
MeO
MeO
OMe
OMe
MeO
OMe
OMe
MeO
OMe
OMe
26
24
25
Scheme 2. Synthesis of Zn(II) oxaporphyrin 26. See Experimental section for reaction conditions and more synthetic details. Zn(II) oxaporphyrin 25 was characterized by single
crystal X-ray crystallography. Reported yields are of analytically pure products.
13C NMR spectra were recorded on a Bruker 300 or 400 MHz
spectrometer and performed in CDCl3
¼7.26 ppm) and CD3OD
residual electron density: þ0.965/ꢁ0.728 e Aꢁ3; H atoms were
calculated from the SHELXTL program and fixed for further
refinements.
(
d
(d
¼3.31 ppm) solutions. Chemical shifts are reported in parts per
million. Coupling constants J are reported in hertz. The signals are
described as s: singlet; d: doublet; dd: doublet of doublet; m:
multiplet, and br: broad. FT-IR measurements were carried out by
4.3. Furan-2,5-diylbis(p-tolylmethanol) (1)
a
PerkineElmer Paragon 1000 spectrophotometer. The high-
Distilled dry n-hexane (50 mL) was added to a 250 mL two-
necked, round-bottomed flask fitted with a rubber septum and
nitrogen gas inlet tube. Tetramethylethylenediamine (TMEDA,
5.6 mL, 37.5 mmol) and nBuLi (14.8 mL of 2.5 M solution in hexane,
37 mmol) were added and solution was stirred under nitrogen for
10 min. Furan (1.08 mL, 15 mmol) was then added and stirred for
another 1 h at ambient temperature. As the reaction progressed,
turbid solution appeared indicating the formation of the 2,5-dili-
thiated salt of furan. Reaction mixture was cooled to 0 ꢀC in an ice
bath; to it an ice-cold solution of p-tolualdehyde (4.3 mL, 37 mmol)
in dry THF (30 mL) was added and stirred for 15 min. Reaction was
quenched by adding an ice-cold saturated NH4Cl aqueous solution.
Organic layer was washed with water and brine solution, and dried
over anhydrous MgSO4. Solvent was removed on rotary evaporator
to afford the crude compound. TLC analysis showed only two spots
corresponding to unchanged p-tolualdehyde and desired diol 1.
Crude product was purified on silica gel column chromatography
using 30% ethyl acetate/hexane to afford diol 1 as a yellow solid
(3.60 gm, 78% yield); mp: 110e112 ꢀC; 1H NMR (300 MHz, CDCl3)
resolution mass spectra and HR-FAB were conducted on a JMS-
700 double focusing mass spectrometer. UVevis spectra were
recorded with an Agilent 8453 spectrophotometer. Flash chroma-
tography was carried out by using silica gel (40e63 mm, Merck).
Analytical TLC was performed on Merck silica gel plates. Melting
points were recorded using an Electrothermal capillary melting
point apparatus.
4.2. Single crystal X-ray diffraction data collection for 12
The measurement was performed on a Burker APEX diffrac-
ꢁ
tometer using Mo K
a
radiation (
l
¼0.71073 A) at T¼100 K in
u scan
mode with 2qmax¼52.7ꢀ. Crystals were prepared by the slow dif-
fusion of hexane into a solution of 12 in CH2Cl2 to yield a brown
crystal of C52H36ClN3O9Zn. Crystal data for 12: triclinic space group
ꢁ
Pꢁ1, a¼11.525(2), b¼14.575(3), c¼16.811(3) A,
a
¼78.55(3),
3
ꢁ
b
¼70.61(3),
g
¼76.38(3)ꢀ, V¼2566.5(9) A , rcalcd¼1.226 Mg/m3,
Z¼2; total no. of reflections collected: 34,254; no. of independent
reflections: 10,090 [R(int)¼0.0883]. An absorption correction based
on a least-squares fit against jFcjꢁjFoj differences was applied with
d
¼7.27 (d, J¼8.1 Hz, 4H), 7.14 (d, J¼8.1 Hz, 4H), 5.89 (d, J¼2.7 Hz,
2H), 5.89 (d, J¼5.1 Hz, 2H), 3.29 (br s, 2H), 2.35 (s, 6H); 13C NMR
m
¼0.585 mmꢁ1, Tmin.¼0.9547, and Tmax.¼0.9713. The structure was
(300 MHz, CDCl3)
d¼156.07, 137.62, 129.00, 126.61, 126.57, 108.04,
solved by direct methods with SHELXTL v.6.14 and refined against
69.71, 21.11; IR (KBr, cmꢁ1): 3312, 3016, 2919, 1613, 1411, 1014, 856,
797, 767; HRMS-ESI calcd for C20H20O3Na ([MþNa]þ): 331.1310,
found 331.1308.
[F2] using SHELXTL.27 1997. The final R1 and wR2 indices (for
I>2s(I)) are 0.0730 and 0.1621, respectively, with max./min.

R. Ambre et al. / Tetrahedron 67 (2011) 4680e4688
4685
4.4. Furan-2,5-diylbis((3,4,5-trimethoxyphenyl)methanol) (2)
oxaporphyrin 5 and N4-porphyrin. Chromatographic separation of
the crude mixture in the same way as that of oxaporphyrin 4 gives
oxaporphyrin 5 as a solid product (97 mg, 10% yield). Mp>300 ꢀC;
The 2,5-dilithiofuran was prepared by treating furan (1.08 mL,
15 mmol) with nBuLi (14.8 mL of 2.5 M solution in hexane, 37 mmol)
in the presence of tetramethylethylenediamine (TMEDA, 5.6 mL,
37.5 mmol) in n-hexane (50 mL). The condensation of 2,5-dilithio-
furan with 3,4,5-trimethoxybenzaldehyde (7.35 gm, 37.5 mmol)
under the same experimental conditions and purification pro-
cedures as mentioned for diol 1, afford the diol 2 as a sticky yellow
solid (2.55 gm, 37% yield); mp 41e43 ꢀC; 1H NMR (300 MHz, CDCl3)
1H NMR (300 MHz, CDCl3)
d
¼9.31 (s, 2H), 8.99 (s, 2H), 8.73 (d,
J¼3.72 Hz, 2H), 8.67 (d, J¼4.2 Hz, 2H), 7.43 (d, J¼6 Hz, 8H), 4.18 (s,
6H), 4.17 (s, 6H), 3.99 (s, 12H), 3.96 (s, 12H); IR (KBr, cmꢁ1): 3451,
2932, 2827, 1578, 1502, 1409, 1234, 1126, 1004, 915, 816, 721; lmax
(CH2Cl2) [nm (log 3)]: 426 (5.25), 510 (4.30), 582 (3.95), 611 (3.92),
672 (3.83); HRMS-ESI calcd for C56H53N3O13 ([MþH]þ): 976.3657,
found 976.3660.
d
¼6.59 (d, J¼4.5 Hz, 4H), 5.96 (d, J¼3.0 Hz, 2H), 5.65 (s, 2H), 3.78 (s,
6H), 3.74 (s, 12H), 3.56 (br s, 2H); 13C NMR (300 MHz, CDCl3)
4.8. Oxaporphyrin (6)
d
¼155.81, 153.05, 137.40, 136.27, 108.04, 103.57, 69.90, 60.73, 55.97;
IR (KBr, cmꢁ1): 3430, 2940, 2839, 1594, 1506, 1462, 1419, 1331, 1232,
1124, 1006, 84, 772, 796; HRMS-ESI calcd for C24H28O9Na
([MþNa]þ): 483.1631, found 483.1632.
Methyl 4-[hydroxyl(5-{hydroxyl[3-(methoxycarbonyl)phenyl]
methyl}furan-2-yl)methyl]benzoate (3, 396 mg,1 mmol), methyl 4-
formylbenzoate (328 mg, 2 mmol), and pyrrole (206
were dissolved in CH2Cl2 (100 mL) under nitrogen. BF3$Et2 (48%)
(30 L 0.1 mmol) was added and the reaction mixture stirred at
mL, 3 mmol)
4.5. Methyl 4-[hydroxyl(5-{hydroxyl[3-(methoxycarbonyl)
phe-yl]methyl}furan-2-yl)methyl]benzoate (3)
m
room temperature for 1 h, followed by oxidation with DDQ
(227 mg, 1 mmol) in air for an additional 1 h resulting in a mixture
of two porphyrins, desired oxaporphyrin 6 and N4-porphyrin.
Chromatographic separation of the crude mixture in a same way as
that of oxaporphyrin 4 gives oxaporphyrin 6 as a solid product
The 2,5-dilithiofuran was prepared by treating furan (1.08 mL,
15 mmol) with nBuLi (14.8 mL of 2.5 M solution in hexane,
37 mmol) in the presence of tetramethylethylenediamine (TMEDA,
5.6 mL, 37.5 mmol) in n-hexane (50 mL). The condensation of 2,5-
dilithiofuran with methyl 4-formylbenzoate (6.19 gm, 37.5 mmol)
under the same experimental conditions and purification pro-
cedures as mentioned for diol 1, afforded the diol 3 as a yellow solid
(2.1 gm, 35% yield). Mp 112e114 ꢀC; 1H NMR (300 MHz, CDCl3)
(66 mg, 7% yield). Mp>300 ꢀC; 1H NMR (300 MHz, CDCl3)
2H), 8.86 (s, 2H), 8.58 (d, J¼4.8 Hz, 2H), 8.51 (d, J¼4.8 Hz, 2H),
8.48e8.42 (m, 8H), 8.28e8.24 (m, 8H) 4.11 (s, 12H); IR (KBr, cmꢁ1):
3438, 2924, 2852, 1723, 1607, 1434, 1278, 1112, 1018, 810; lmax
d
¼9.17 (s,
(CH2Cl2) [nm (log 3)]: 421 (5.50), 509 (4.63), 541 (4.51), 620 (4.36),
d
¼7.96 (d, 8.4 Hz, 4H), 7.42 (d, 8.4 Hz, 4H), 5.90 (d, 2.1 Hz, 2H), 5.77
669 (4.40); HRMS-ESI calcd for C52H37N3O9 ([MþH]þ): 848.2608,
(d, 3.9 Hz, 2H), 3.89 (s, 6H), 3.47 (br s, 2H); 13C NMR (300 MHz,
found 848.2584.
CDCl3)
d
¼166.86, 155.33, 145.49, 129.48, 126.42, 126.37, 108.44,
69.14, 52.06; IR (KBr, cmꢁ1): 3286, 2953, 1719, 1609, 1432, 1278,
1181, 1018, 814, 738; HRMS-ESI calcd for C22H20O7Na ([MþNa]þ):
419.1107, found 419.1111.
4.9. Oxaporphyrin (7)
Furan-2,5-diylbis(p-tolylmethanol) (1, 308 mg, 1 mmol), methyl
4-formylbenzoate (328 mg, 2 mmol), and pyrrole (206
3.0 mmol) were dissolved in CH2Cl2 (100 mL) under nitrogen.
BF3$Et2 (48%) (30 L, 0.1 mmol) was added and the reaction mix-
mL,
4.6. Oxaporphyrin (4)
m
In a 250-mL round-bottom flask CH2Cl2 (100 mL) was added and
purged with nitrogen for 15 min. In this flask furan-2,5-diylbis(p-
ture stirred at room temperature for 1 h, followed by oxidation with
DDQ (227 mg, 1 mmol) in air for an additional 1 h resulting in
a mixture of two porphyrins, desired oxaporphyrin 7 and N4-por-
phyrin. Chromatographic separation of the crude mixture in a same
way as that of oxaporphyrion 4 gives oxaporphyrin 7 as a solid
product (68 mg, 9% yield). Mp>300 ꢀC; 1H NMR (300 MHz, CDCl3)
tolylmethanol) 1 (308 mg,1 mmol), p-touladehyde (236
mL, 2 mmol),
and pyrrole (206 L, 3 mmol) were dissolved. BF3$Et2 (48%) (30
m
mL,
0.1 mmol) was added and the reaction mixture was stirred at room
temperature for 1 h. DDQ (227 mg, 1 mmol) was added and stirred
for an additional 1 h in air. TLC analysis indicated the formation of
the desired oxaporphyrin 4 and N4-porphyrin. Solvent was evapo-
rated on rotary evaporator and crude compound was passed
through a short neutral alumina column using 10% acetone/CH2Cl2
as eluent. The obtained material was separated by silica gel column
chromatography using 20% acetone/CH2Cl2 as eluent gives oxapor-
phyrin 4 as a solid product (67 mg, 10% yield). Mp>300 ꢀC; 1H NMR
d
¼9.28 (s, 2H), 8.84 (s, 2H), 8.64 (d, J¼4.5 Hz, 2H), 8.60 (d, J¼4.2 Hz
2H), 8.46 (d, J¼8.1 Hz, 4H), 8.30 (d, J¼7.8 Hz, 4H), 8.10 (d, J¼7.2 Hz,
4H), 7.59 (d, J¼7.8 Hz, 4H), 4.13 (s, 6H), 2.73 (s, 6H); IR (KBr, cmꢁ1):
3438, 2924, 2852, 1723, 1607, 1434, 1275, 1109, 1021, 964, 804, 708;
lmax (CH2Cl2) [nm (log 3)]: 421 (5.48), 508 (4.47), 541 (4.08), 592
(3.86), 674 (4.00); HRMS-ESI calcd for C50H37N3O5 ([MþH]þ):
760.2811, found 760.2838.
(300 MHz, CDCl3)
d
¼9.21 (s, 2H), 8.90 (s, 2H), 8.66 (d, J¼3.4 Hz, 2H)
8.59 (d, J¼4.5 Hz, 2H), 8.09 (d, J¼7.5 Hz, 8H), 7.57 (d, J¼7.5 Hz, 8H),
4.10. Oxaporphyrin (8)
2.72 (s, 6H), 2.71 (s, 6H); IR (KBr, cmꢁ1): 3455, 3236, 3021, 2911,1510,
1181, 964, 810, 770; lmax (CH2Cl2) [nm (log
3)]: 421 (5.14), 509 (3.92),
Methyl 4-[hydroxyl(5-{hydroxyl[3-(methoxycarbonyl)phenyl]
576 (3.8), 604 (3.60), 672 (2.48); HRMS-ESI calcd for C48H37N3O
methyl}furan-2-yl)methyl]benzoate (3, 396 mg, 1 mmol), p-tol-
([MþH]þ): 672.3015, found 672.3000.
ualdehyde (236 mg, 2 mmol), and pyrrole (206
mL, 3.0 mmol) were
dissolved in CH2Cl2 (100 mL) under nitrogen. BF3$Et2 (48%) (30
mL,
4.7. Oxaporphyrin (5)
0.1 mmol) was added and the reaction mixture stirred at room
temperature for 1 h, followed by oxidation with DDQ (227 mg,
1 mmol) in air for an additional 1 h resulting in a mixture of two
porphyrins, desired oxaporphyrin 8 and N4-porphyrin. Chromato-
graphic separation of the crude mixture in a same way as that of
oxaporphyrion 4 gives oxaporphyrin 8 as a solid product (68 mg, 9%
The diol, furan-2,5-diylbis((3,4,5-trimethoxyphenyl)methanol)
(2, 461 mg, 1 mmol), 3,4,5-trimethoxybenzaldehyde (392 mg,
2 mmol), and pyrrole (206
(100 mL) under nitrogen. BF3$Et2 (48%) (30
and the reaction mixture was stirred at room temperature for 1 h,
followed by oxidation with DDQ (227 mg, 1 mmol) in air for an
additional 1 h resulting in a mixture of two porphyrins, desired
m
L, 3 mmol) were dissolved in CH2Cl2
mL 0.1 mmol) was added
yield). Mp>300 ꢀC; 1H NMR (300 MHz, CDCl3)
d¼9.15 (s, 2H), 8.92
(s, 2H), 8.68 (d, J¼4.5 Hz, 2H), 8.49 (d, 4.5 Hz, 2H), 8.45 (d, 8.7 Hz,
4H), 8.29 (d, J¼8.7 Hz, 4H), 8.08 (d, J¼7.8 Hz, 4H), 7.57 (d, J¼7.8 Hz,

4686
R. Ambre et al. / Tetrahedron 67 (2011) 4680e4688
4H), 4.12 (s, 6H), 2.71 (s, 6H); IR (KBr, cmꢁ1): 3403, 2924, 2852,
1723, 1605, 1434, 1272, 1118, 1016, 962, 789, 708; lmax (CH2Cl2) [nm
8.83 (s, 2H), 8.49e8.42 (m, 6H), 8.22e8.10 (m, 6H), 7.62 (m, 4H),
4.12 (s, 6H), 2.73 (s, 6H); IR (KBr, cmꢁ1): 2995, 2919, 1722, 1607,
1434, 1275, 1180, 1275, 1112, 1012, 993, 798; lmax (CH2Cl2) [nm
(log 3)]: 422 (4.79), 511 (4.06), 559 (4.96), 642 (3.90), 668 (3.92);
HRMS-ESI calcd for C50H37N3O5 ([MþH]þ): 760.2811, found
(log 3)]: 426 (5.11), 442 (5.04), 545 (4.06), 588 (4.03), 636 (3.82);
760.2810.
HRMS (FAB) calcd for C50H36ClN3O5Zn ([M]þ): 857.1635, found
857.1633.
4.11. Oxaporphyrin (9)
4.12.5. Zn(II) oxaporphyrin (14). Mp>300 ꢀC; 1H NMR (300 MHz,
Methyl 4-[hydroxyl(5-{hydroxyl[3-(methoxycarbonyl)phenyl]
methyl}furan-2-yl)methyl]benzoate (3, 396 mg, 1 mmol), 3,5-Di-
CDCl3)
d
¼9.32 (s, 2H), 9.06 (d, J¼4.8 Hz, 2H), 8.88 (s, 2H), 8.78 (d,
J¼4.8 Hz, 2H), 8.51 (s, 2H), 8.48 (s, 2H), 8.42e8.31 (m, 4H), 8.25 (br
s, 2H), 7.94 (br s, 2H), 7,56 (br s, 4H), 4.13 (s, 6H), 2.71 (s, 6H); IR
(KBr, cmꢁ1): 2919, 1722, 1607, 1434, 1275, 1181, 1111, 1012, 993, 798,
tert-butylbenzaldehyde (436 mg, 2 mmol), and pyrrole (206
3.0 mmol) were dissolved in CH2Cl2 (100 mL) under nitrogen.
BF3$Et2 (48%) (30 L, 0.1 mmol) was added and the reaction mixture
mL,
m
757; lmax (CH2Cl2) nm (log 3): 427 (5.56), 444 (5.47), 548 (4.45), 589
stirred at room temperature for 1 h, followed by oxidationwith DDQ
(227 mg,1 mmol) in air for an additional 1 h resulting in a mixture of
two porphyrins, desired oxaporphyrin 9 and N4-porphyrin. Chro-
matographic separation of the crude mixture in a same way as that
of oxaporphyrion 4 gives oxaporphyrin 9 as a solid product (14%
(4.44), 634 (4.10); HRMS (FAB) calcd for C50H36ClN3O5Zn ([M]þ):
857.1635, found 857.1633.
4.12.6. Zn(II) oxaporphyrin (15). Mp>300 ꢀC; 1H NMR (300 MHz,
CDCl3)
J¼4.8 Hz, 2H), 8.51 (br s, 2H), 8.48 (br, s 2H), 8.41e8.33 (m 4H) 8.22
(br s, 2H), 7.91 (br s, 2H), 7.83 (br s, 2H), 4.12 (s, 6H), 1.55 (s, 18H),
1.50 (s, 18H); IR (KBr, cmꢁ1): 2957, 1724, 1609, 1274, 1194, 1113, 1021,
d
¼9.31 (s, 2H), 9.07 (d, J¼4.5 Hz, 2H), 8.89 (s, 2H), 8.79 (d,
yield, 133 mg). Mp>300 ꢀC; 1H NMR (300 MHz, CDCl3)
2H), 8.94 (s, 2H), 8.67 (d, J¼4.5 Hz, 2H), 8.48 (d, J¼4.5 Hz, 2H), 8.44
(d, J¼7.8 Hz, 4H), 8.28 (d, J¼7.8 Hz, 4H), 8.00 (s, 4H), 7.79 (s, 2H), 4.11
(s, 6H), 1.52 (s, 36H); IR (KBr, cmꢁ1): 3434, 2957, 1724, 1605, 1592,
d¼9.12 (s,
970, 818, 710; lmax (CH2Cl2) [nm (log 3)]: 428 (5.18), 445 (5.10), 549
1478, 1386, 1274, 1113, 974, 819, 710; lmax (CH2Cl2) [nm (log
3
)]: 424
(4.12), 588 (5.11), 632 (3.83); HRMS (FAB) calcd for C64H64ClN3O5Zn
(5.36), 515 (4.74), 546 (4.60), 559 (4.60) 674 (4.67); HRMS-ESI calcd
([M]þ): 1051.3826, found 1051.3833.
for C64H65N3O5 ([MþH]þ): 956.5002, found 956.4991.
4.12. General procedure of Zn(II) insertion (10e14, or 15)
4.13. General procedure of hydrolysis of free base
oxaporphyrin (16e18, or 19)
25 mg of the respective oxaporphyrin (4e8, or 9) was dissolved
in 20 mL CH3CN/MeOH (3:1). To this solution molar anhydrous
excess zinc chloride (20 equiv) and a drop of 2,6-lutidine were
added. Reaction progress was monitored by TLC and UVevis
spectroscopy. The solvent was removed under reduced pressure
after 1 h. Dried residue was dissolved in CH2Cl2 added water and
extracted with dichloromethane, organic layer was collected and
dried with anhydrous MgSO4 and evaporated to dryness. The res-
idue contains practically pure Zn(II) oxaporphyrin complexes
(10e14, or 15).
25 mg of the respective oxaporphyrin (6e8, or 9) was dissolved
in 50 mL of THF, and 500 mg KOH in 2 mL water was added to it and
refluxed for 10 h. After cooling, the reaction mixture was treated
with 1 N HCl. The precipitation formed were filtered off and
washed with distilled water. The residue remained is dissolved in
methanol and dried in vacuum to yield practically pure free-base
oxaporphyrin dicarboxylic acid or tetracarboxylic acid (16e18, or
19) in quantitative yield.
4.13.1. Oxaporphyrin (16). Mp>300 ꢀC; 1H NMR (300 MHz, CD3OD)
4.12.1. Zn(II) oxaporphyrin (10). Mp>300 ꢀC; 1H NMR (300 MHz,
d
¼9.89 (s, 2H), 9.26 (d, J¼5.1 Hz, 2H), 9.15 (d, J¼5.7 Hz, 2H), 8.91 (s,
CDCl3)
d
¼9.37 (s, 2H), 9.03 (s 1H), 9.01 (s, 1H), 8.87 (s, 2H), 8.85 (s,
2H), 8.63e8.56 (m, 8H), 8.55e8.46 (m, 8H); IR (KBr, cmꢁ1): 3430,
3130, 1710, 1609, 1502, 1223, 1113, 1016, 981, 789, 717; lmax (MeOH)
2H), 8.25 (br s, 2H) 8.20 (d, J¼7.5 Hz, 2H), 8.10 (d, J¼7.5 Hz, 2H), 7.95
(br s, 2H), 7.62e7.58 (m, 8H), 2.72 (s, 6H), 2.71 (s, 6H); IR (KBr,
[nm (log 3)]: 410 (4.53), 435 (4.32), 574 (3.53); HRMS-ESI calcd for
cmꢁ1): 2918, 1262, 1178, 1013, 993, 800; lmax (CH2Cl2) [nm (log
3
)]:
C48H29N3O9 ([MþH]þ): 792.1982, found 792.1979.
426 (4.82), 443 (4.76), 538 (4.19), 551 (4.22), 661 (4.15); HRMS-ESI
calcd for C48H36N3OZn ([MꢁCl]þ): 734.2150, found 734.2142.
4.13.2. Oxaporphyrin (17). Mp>300 ꢀC; 1H NMR (300 MHz, CD3OD)
d
¼9.87 (s, 2H), 9.14 (d, J¼5.1 Hz, 2H), 9.02 (d, J¼4.8 Hz, 2H), 8.85 (s,
4.12.2. Zn(II) oxaporphyrin (11). Mp>300 ꢀC; 1H NMR (300 MHz,
2H), 8.59 (m, 8H), 8.37 (d, J¼8.1 Hz, 4H), 7.85 (d, J¼7.8 Hz, 4H), 2.79
CDCl3)
d
¼9.49 (s, 2H), 9.10 (d, J¼5.1 Hz, 2H), 8.96 (s, 2H), 8.95 (d,
(s, 6H); IR (KBr, cmꢁ1): 3400, 2917, 1707, 1606, 1277, 1186, 1017, 822,
J¼4.5 Hz, 2H), 7.61 (br s, 2H), 7.55 (br s, 2H), 7.47 (br s, 2H), 7.33 (br
s, 2H), 4.19 (s, 6H), 4.00 (s, 6H), 3.97e3.93 (m, 24H); IR (KBr, cm-1):
2932, 2831, 1582, 1504, 1410, 1235, 1126, 1002, 943, 799, 723; lmax
714; lmax (MeOH) [nm (log 3)]: 412 (4.67), 435 (4.50), 575 (5.42);
HRMS-ESI calcd for C48H33N3O5 ([MþH]þ): 732.2498, found
732. 249.
(CH2Cl2) [nm (log 3)]: 443 (4.95), 446 (4.96), 550 (3.91), 590 (3.93),
635 (3.64); HRMS-ESI calcd for C56H52N3O13Zn ([MꢁCl]þ):
4.13.3. Oxaporphyrin (18). Mp>300 ꢀC; 1H NMR (300 MHz, CD3OD)
1038.2792, found 1038.2782.
d
¼9.85 (s, 2H), 9.53 (d, J¼3 Hz, 2H), 8.91 (s, 2H), 8.80 (s, 2H), 8.69 (d,
J¼8.1 Hz, 4H) 8.63 (d, J¼8.1 Hz, 4H), 8.50 (d, J¼7.5 Hz, 4H), 7.94 (d,
4.12.3. Zn(II) oxaporphyrin (12). Mp>300 ꢀC; 1H NMR (300 MHz,
J¼7.5 Hz, 4H), 2.82 (s, 6H); IR (KBr, cmꢁ1): 3436, 2918, 2851, 1717,
CDCl3)
d
¼9.38 (s, 2H), 8.98 (d, J¼4.8 Hz, 2H), 8.84 (s, 2H), 8.83 (s,
1606,1223,1112, 807, 720; lmax (MeOH) [nm (log 3)]: 415 (5.21), 437
2H), 8.52e8.32 (m. 14H), 8.14 (br d, J¼7.2 Hz, 2H), 4.13 (s, 6H), 4.12
(5.04), 577 (4.22); HRMS-ESI calcd for C48H33N3O5 ([MþH]þ):
(s, 6H); IR (KBr, cmꢁ1): 2945, 1723, 1607, 1434, 1276, 1118, 1021, 867,
732.2498, found 732.2494.
763, 719; lmax (CH2Cl2) [nm (log 3)]: 426 (5.24), 442 (5.18), 547
(4.04), 588 (4.02), 656 (4.12); HRMS (FAB) calcd for C52H36ClN3O9Zn
4.13.4. Oxaporphyrin (19). Mp>300 ꢀC; 1H NMR (300 MHz, CD3OD)
([M]þ): 945.1432, found 945.1429.
d
¼9.38 (s, 2H), 8.95 (s, 2H), 8.63 (br s, 2H), 8.61 (br s, 2H), 8.41 (d,
J¼7.8 Hz, 4H), 8.26 (d, J¼7.8 Hz, 4H), 8.10 (br s, 4H), 7.92 (br s, 2H),
4.12.4. Zn(II) oxaporphyrin (13). Mp>300 ꢀC; 1H NMR (300 MHz,
1.55 (s, 36H); IR (KBr, cmꢁ1): 3395, 2961, 2868, 1591, 1544,
CDCl3)
d
¼9.45 (s, 2H), 8.96 (d, J¼5.1 Hz, 2H), 8.93 (d, J¼5.1 Hz, 2H),
1384,1284, 975, 818, 715; lmax (MeOH) [nm (log 3)]: 417 (4.56), 439

R. Ambre et al. / Tetrahedron 67 (2011) 4680e4688
4687
(4.36), 577 (3.55); HRMS-ESI calcd for C62H61N3O5 ([MþH]þ):
(KBr, cmꢁ1): 3400, 2940, 2836, 1594, 1506, 1462, 1420, 1331, 1235,
1126, 1009, 779, 703; HRMS-ESI calcd for C15H18O6Na ([MþNa]þ):
317.1001, found 317.1005.
928.4689, found 928.4681.
4.14. Synthesis of Zn(II) oxaporphyrin (20)
4.17. Oxaporphyrin (23)
Free base oxaporphyrin 19 (20 mg) was dissolved in 20 mL
MeOH. To this solution molar excess anhydrous zinc chloride
(20 equiv) and a drop of 2,6-lutidine were added and stirred for 1 h.
Excess MeOH was removed under reduced pressure. Reaction
mixture was then treated with distilled water. The crystals formed
were filtered off and washed with distilled water. The residue
remained is dissolved in methanol and dried to vacuum to yield
practically pure Zn(II) oxaporphyrin 20 in quantitative yield.
In a 250-mL round-bottom flask 100 mL CH2Cl2 was taken and
purged with nitrogen for 15 min, To this [5-(hydroxymethyl)furan-2-
yl](3,4,5-trimethoxyphenyl)methanol 22 (294 mg, 1.0 mmol), 3,4,5-
trimethoxybenzaldehydealdehyde (392 mg, 2 mmol) and pyrrole
(209 mL, 3 mmol) were dissolved. BF3$Et2 (48%)(30 mL, 0.1 mmol) was
added and the reaction mixture was stirred at room temperature for
1 h. DDQ (227 mg, 1 mmol) was added and the reaction mixture was
stirred for an additional 1 h in air. TLC analysis of the reaction mixture
showed the formation of four porphyrins. The solvent was removed
on a rotary evaporator and reaction mixture was separated by silica
gel column chromatography using CH2Cl2 as eluent to afford oxa-
porphyrin 23 as a solid product (121 mg, 15% yield). Mp>300 ꢀC; 1H
Mp>300 ꢀC; 1H NMR (400 MHz, CD3OD)
d
¼9.61 (s, 2H), 9.10 (d,
J¼4.8 Hz, 2H), 8.93 (m, 4H), 8.52 (d, J¼8.0 Hz, 4H), 8.39 (d, J¼8.0 Hz,
4H), 8.11 (d, J¼1.5 Hz, 4H), 7.98 (m, 2H), 1.55 (s, 36H); IR (KBr,
cmꢁ1): 3561, 2957, 2869, 1609, 1248, 1044, 1012, 907, 801, 721; lmax
(MeOH) [nm (log 3)]: 421 (3.80), 437 (3.74), 546 (2.19), 587 (2.77),
675 (2.62); HRMS (FAB) calcd for C64H64N3O5Zn ([MꢁCl]þ):
NMR (300 MHz, CDCl3)
d
¼10.18 (s,1H), 9.73 (d, J¼4.8 Hz,1H), 9.47 (d,
990.3824, found 990.3813.
J¼4.8 Hz, 1H), 9.15 (d, 4.5 Hz, 1H), 9.05e9.04 (m. 2H), 8.93 (d,
J¼4.5 Hz, 1H) 8.76 (d, J¼4.8 Hz, 1H), 8.71 (d, J¼4.5 Hz, 1H), 7.44 (d,
4.8 Hz, 4H), 7.41 (s, 2H), 4.19 (s, 6H), 4.17 (s, 3H), 4.00 (s, 6H), 3.97 (s,
12H); IR (KBr, cmꢁ1): 3439, 2928, 2849, 1580, 504, 1436, 1409, 1384,
4.15. Furan-2-methanol (21)
2-Furancarboxaldehyde (9.6 mL, 100 mmol) was added to
100 mL methanol in a 250 mL round-bottom flask under N2 at-
mosphere. NaBH4 (5.70 g, 150 mmol) was added to it dropwisely
and stirred for 1 h. Product formation was checked by TLC. The
reaction was quenched by adding 50 mL of water and extracted
with ethyl acetate. The collected organic layer was dried over
MgSO4 and concentrated, and the crude product was purified by
silica gel column chromatography using 20% ethyl acetate/hexane
as eluent. The furan-2-methanol 21 was collected as a yellow oily
1236, 1126, 1004, 919, 819, 817, 718; lmax (CH2Cl2) [nm (log 3)]: 419
(4.63), 503 (3.78), 597 (3.26), 603 (3.32), 685 (2.97); HRMS-ESI calcd
for C47H43N3O10 ([MþH]þ): 810.3027, found 810.3024.
4.18. Oxaporphyrin (24)
In a 50 mL round-bottomed flask, a solution of the oxaporphyrin
23 (24 mg, 0.030 mmol) in dichloromethane (15 mL) was treated
with N-bromosuccinimide (7.12 mg, 0.040 mmol) at room tem-
perature for 15 min. The progress of the reaction was monitored by
TLC. After a complete consumption of the oxaporphyrin 23, the
reaction was stopped and the solvent was removed on a rotary
evaporator. The crude compound was subjected to silica gel column
chromatography with 30% CH2Cl2/hexane as eluent. Oxaporphyrin
24 was afforded as a solid product (18 mg, 68% yield). Mp>300 ꢀC;
compound (7.30 gm, 74% yield). 1H NMR (300 MHz, CDCl3)
d
¼7.38
(d, J¼1.5 Hz, 1H), 6.32 (dd, J¼1.8 Hz, 1H), 6.27 (d, J¼2.7 Hz, 1H), 4.27
(d, J¼5.7 Hz, 2H), 2.29 (br s, 1H); 13C NMR (300 MHz, CDCl3)
d
¼153.96, 142.52, 110.31, 107.70, 57.35; IR (KBr, cmꢁ1): 3435, 2926,
1638, 1570, 1262, 1017, 914, 800, 735; HRMS-ESI calcd for C5H6O2Na
([MþNa]þ): 98.0368, found 98.3453.
1H NMR (400 MHz, CDCl3)
d¼10.09 (d, J¼4.8 Hz, 1H), 9.53 (d,
4.16. [5-(Hydroxymethyl)furan-2-yl](3,4,5-trimethoxyphenyl)
methanol (22)
J¼4.71 Hz, 1H), 9.41 (d, J¼4.98 Hz, 1H) 8.96 (s, 2H), 8.78 (d,
J¼4.56 Hz, 1H), 8.68 (d, 4.71 Hz, 1H), 8.61 (d, J¼4.74 Hz, 1H), 7.42 (s,
2H), 7.39 (d, J¼5.67 Hz, 4H), 4.19e4.17 (m, 9H), 4.00e3.97 (m, 18H);
IR (KBr, cmꢁ1): 3452, 2933, 2831, 1508, 1501, 1463, 1409, 3161, 1331,
Distilled dry n-hexane (50 mL) was added into a 250-mL two-
necked round-bottom flask fitted with a rubber septum and ni-
trogen gas inlet tube. Tetramethylethylenediamine (TMEDA)
(3.48 mL, 30 mmol) and nBuLi (2.5 M solution in hexane) (12 mL,
30 mmol) were added and the solution was stirred under nitrogen
for 10 min. Furan-2-methanol 21 (1.96 mL, 20 mmol) was added to
the solution and stirred for 1 h at ambient temperature. The re-
action mixture was cooled to 0 ꢀC in an ice bath and then an ice-
cold solution of 3,4,5-trimethoxybenzalaldehyde (5.88 gm,
30 mmol) in dry THF (30 mL) was added. The reaction mixture was
stirred for another 15 min and quenched by adding an ice-cold
saturated NH4Cl solution. The organic layer was washed with water
and brine solution, and dried over anhydrous MgSO4. Solvent was
removed on a rotary evaporator to afford the crude compound. TLC
analysis showed three spots corresponding to unreacted furan-2-
methanol 21, 3,4,5-trimethoxybenzalaldehyde, and desired prod-
uct. The crude product was purified by silica gel column chroma-
tography using 20% ethyl acetate/hexane as eluent to afford
[5-(hydroxymethyl)furan-2-yl](3,4,5-trimethoxyphenyl) methanol,
22, as a yellow solid (2.23 gm, 38% yield). Mp 60e62 ꢀC; 1H NMR
1235, 1127, 919, 816, 723; lmax (CH2Cl2) [nm (log 3)]: 427 (5.06), 513
(4.05), 619 (3.40), 653 (3.17), 677 (3.19); HRMS-ESI calcd for
C47H42BrN3O10 ([MþH]þ): 888.2132, found 888.2139.
4.19. Zn(II) oxaporphyrin (25)
The oxaporphyrin 24 (10 mg) was dissolved in 20 mL CH3CN/
MeOH (3:1). To this solution molar excess anhydrous zinc chloride
(20 equiv) was added. Reaction progress was monitored by TLC and
UVevis spectroscopy. After 1 h stirring, the solvent was removed by
rotary evaporator. Dried residue was dissolved in CH2Cl2 added water
and extracted with dichloromethane, organic layer separated and
dried over anhydrous MgSO4, and evaporated to dryness to obtainpure
Zn(II) oxaporphyrin 25 in quantitative yield.1H NMR (300 MHz, CDCl3)
d
¼10.20 (d, J¼5.1 Hz, 1H), 9.83 (d, J¼5.1 Hz, 1H), 9.61 (d, J¼5.1 Hz, 1H),
9.19 (d, J¼4.8 Hz, 1H), 9.06 (d, 4.8 Hz, 1H), 8.94e8.89 (m, 3H), 7.60 (s,
1H), 7.56e7.55 (m, 2H), 7.43 (d, J¼1.5 Hz, 1H), 7.30e7.28 (m, 2H),
(300 MHz, CDCl3)
J¼3 Hz, 1H), 5.69 (s, 1H), 4.52 (s, 2H), 3.82 (s, 3H), 3.81 (s, 6H), 2.46
(br s, 2H); 13C NMR (300 MHz, CDCl3)
¼155.86, 153.94, 153.11,
137.45, 136.29, 108.38, 108.19, 103.60, 69.92, 60.77, 57.24, 56.04; IR
d¼6.64 (s, 2H), 6.17 (d, J¼3 Hz, 1H), 6.00 (d,
4.20e4.17 (m, 9H), 4.00e3.90 (m,18H); lmax (CH2Cl2) [nm (log 3)]: 434
(4.97), 449 (4.97), 552 (3.95), 595 (3.94), 641 (3.50); IR (KBr, cmꢁ1):
2935, 2844, 1581,1504,1463, 1410,1336,1236, 1126, 1007, 804; HRMS-
ESI calcd for C47H41BrN3O10Zn ([MꢁCl]þ): 950.1267, found 950.1257.
d

4688
R. Ambre et al. / Tetrahedron 67 (2011) 4680e4688
Macromolecules 2010, 43, 709e715; (d) Kwong, R. C.; Sibley, S.; Dubovoy, T.;
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A mixture of Zn(II) oxaporphyrin 25 (60 mg, 0.06 mmol),
[Pd2(dba)3] (21.98 mg, 0.024 mmol), 4-ethenylbenzoic acid
(13.15 mg, 0.09 mmol), and triphenylarsine (45.94 mg, 0.15 mmol)
added in two-neck 50 mL flask and kept under vacuum for 30 min
was flushed with N2, then added THF/triethylamine (12 mL,1:1 v/v)
and stirred under N2 for 48 h. After the complete consumption of
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d
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J¼4.8 Hz, 1H), 8.90 (m, 3H), 8.12e8.00 (m, 4H), 7.62 (s, 1H), 7.58 (d,
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4.02e3.92 (m, 18H); lmax (CH2Cl2) [nm (log 3) 451 (5.2), 461 (5.3),
563 (4.2), 613 (4.3), 657 (2.0); IR (KBr, cmꢁ1): 2925, 2853, 2130,
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Crystallographic data (excluding structure factors) for the
structures in this paper have been deposited with the Cambridge
Crystallographic Data Center as supplementary publication nos.
CCDC 809694. Copies of the data can be obtained, free of charge, on
application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK, (fax:
þ44 (0)1223 336033 or e-mail: deposit@ccdc.cam.ac.Uk).
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Acknowledgements
Authors greatly acknowledged the financial supports from Na-
tional Science Council (Taiwan) and Academia Sinica. Mass spec-
troscopy analyses were performed by Mass Spectrometry facility of
the Institute of Chemistry, Academia Sinica, Taiwan. The Assistance
from Dr. Yun-Sheng Wen and Miss Cheng-Ying Yu for X-ray data
collection is highly appreciated.
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