Oligopyridine ligands possessing multiple or mixed anchoring functionality for dye-sensitized solar cells

Article abstract of DOI:10.1016/j.tet.2015.06.029

This paper describes the synthesis and characterisation of targeted 2,2′-bipyridine-3,3′,4,4′-tetracarboxylic acid and 2,2′-bipyridine-4,4′,5,5′-tetracarboxylic acid via succinct synthetic pathways. Further, we report methods for producing asymmetric bipyridines bearing both carboxylate and phosphonate anchoring groups in the ester protected form, and the first reported synthesis of a group of new polyoxo oligopyridine ligands based on terpyridine and quaterpyridine. A robust synthetic strategy using Kr?nhke conditions was developed and demonstrated for synthesising oligopyridine moieties targeted for application in the dye-sensitized solar cell (DSSC). This class of novel ligands was designed to provide alternative anchoring functionality and to improve the metal oxide surface binding properties of coordination complexes in DSSC applications.

Full text of DOI:10.1016/j.tet.2015.06.029

Tetrahedron 71 (2015) 5238e5247
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Oligopyridine ligands possessing multiple or mixed anchoring
functionality for dye-sensitized solar cells
a
a
a
b
€
Clint P. Woodward , Campbell J. Coghlan , Thomas Ruther , Timothy W. Jones ,
Yanek Hebting c, Richard L. Cordiner c, Ryan E. Dawson c, Diane E.J.E. Robinson c,
,
Gregory J. Wilson b
*
a CSIRO Energy Flagship, Clayton Laboratories, Bayview Ave, Clayton 3168, Australia
b CSIRO Energy Flagship, Energy Centre, 10 Murray Dwyer Cct, Mayfield West 2304, Australia
c Dyesol Australia Pty Ltd, 3 Dominion Place, Queanbeyan, NSW 2620, Australia
a r t i c l e i n f o
a b s t r a c t
This paper describes the synthesis and characterisation of targeted 2,20-bipyridine-3,30,4,40-tetra-
carboxylic acid and 2,20-bipyridine-4,40,5,50-tetracarboxylic acid via succinct synthetic pathways. Further,
we report methods for producing asymmetric bipyridines bearing both carboxylate and phosphonate
anchoring groups in the ester protected form, and the first reported synthesis of a group of new polyoxo
oligopyridine ligands based on terpyridine and quaterpyridine. A robust synthetic strategy using
Article history:
Received 22 January 2015
Received in revised form 26 May 2015
Accepted 9 June 2015
Available online 14 June 2015
€
Kronhke conditions was developed and demonstrated for synthesising oligopyridine moieties targeted
Keywords:
for application in the dye-sensitized solar cell (DSSC). This class of novel ligands was designed to provide
alternative anchoring functionality and to improve the metal oxide surface binding properties of co-
ordination complexes in DSSC applications.
Oligopyridine
Dye anchoring
€
Kronhke synthesis
Crown Copyright Ó 2015 Published by Elsevier Ltd. All rights reserved.
€
Kronhke synthesis
DSSC
Solar
1. Introduction
In this work we considered that ligands bearing either more than
one anchoring functional group per pyridine unit or a hybrid an-
The use of d6 ruthenium coordination complexes as surface
adsorbed dyes namely cis-bis-diisothiocyanato bis(4,40-dicarbox-
ylato-2,20-bipyridine) ruthenium(II) (N3), di-tetrabutylammonium
choring system could result in enhanced surface binding proper-
ties. With this in mind a series of ligands were designed to improve
dye anchoring by introducing additional carboxylic acid groups at
the ligand periphery, or by substituting for a stronger binding group
at one site, as shown in Fig. 1. The proposed ligand structures retain
at least one carboxylic acid functionality in the para position on one
of the pyridine rings to ensure that effective electron injection to
a metal oxide semiconductor can still be achieved.
cis-bis(diisothiocyanato)
bis(4,40-dicarboxylato-2,20-bipyridine)
ruthenium(II) (N719) and (triisothiocyanato)(4,40,400-tricarbox-
ylato-2,20060,200-terpyridine) ruthenium(II) (N749) in the dye-
sensitized solar cell have remained a solid benchmark in terms of
cell performance since their development. However, the long term
durability of these complexes in photovoltaic devices may be im-
proved through modification of the surface anchoring functionality
of the dyes. Much work has been conducted to understand the
relative strength and binding modes of linker groups for anchoring
sensitisers to metal-oxide surfaces,1,2 It is now well understood that
increased surface adhesion can either be achieved through an in-
creased electron-withdrawing ability of the functional group2
(quantified by the Hammett parameter, s) or by the use of multi-
ple anchoring groups especially in tripodal binding configurations.1
Fig. 1. Concept for improved ligand anchoring ability.
In this work we present succinct methodologies for preparation
of polyoxo oligopyridines based on increased electron withdrawing
* Corresponding author. Tel.: þ61 2 4960 6017; fax: þ61 2 4960 6021; e-mail
address: greg.wilson@csiro.au (G.J. Wilson).
http://dx.doi.org/10.1016/j.tet.2015.06.029
0040-4020/Crown Copyright Ó 2015 Published by Elsevier Ltd. All rights reserved.

C.P. Woodward et al. / Tetrahedron 71 (2015) 5238e5247
5239
properties and substitutions for enhanced binding affinity. Prepa-
ration of ruthenium coordination complexes based on these com-
pounds will be reported in a subsequent publication.
Access to the free acid ligand 1 was achieved under basic hy-
drolysis conditions (1 M LiOH in THF/water, 5 h reflux), yielding the
deprotected product. After selective removal of the THF, the pH was
adjusted to 2 and the solution was allowed to stand overnight
resulting in the formation of the desired product as a white pre-
cipitate (w65e75%). Complimentary analytical structural charac-
terisation (1H NMR, 13C NMR, IR, MS and EA) was performed,
corresponding to the expected compound (1). The 1H NMR spec-
trum showed only two doublet resonances at 7.78 and 8.67 ppm in
deuterated methanol, whilst the 13C NMR spectrum displayed
seven carbon shifts including two distinct carboxylic acid reso-
nances at 168.1 and 170.1 ppm.
2. Results and discussion
2.1. Bipyridine synthesis
Synthesis of the 2,20-bipyridine-3,30,4,40-tetracarboxylic acid (1)
required oxidation of 1,10-dimethyl-2,7-phenanthroline at both
methyl groups as well as at the phenanthroline backbone, analo-
gous to a procedure detailed by Dawid et al.3 Initial oxidation at-
tempts (6 equiv of KMnO4) failed to effect complete oxidation
(Scheme 1), with preferential oxidation of the phenanthroline
backbone. Using an excess of KMnO4 (12 equiv) successfully oxi-
dised the 4,7-dimethyl-1,10-phenanthroline to 3,30,4,40-tetra-
carboxylic acid 2,20-bipyridine, indicated by the 1H NMR spectrum
(chemical shifts at 7.49 and 8.52 ppm) and LC-MS. Attempts to
purify at this point by selective pH driven precipitation (after MnO2
removal) proved difficult, as the product failed to precipitate upon
adjusting the pH to 2. Conversion to the tetraethyl ester (refluxing
EtOH, cat. H2SO4, 5 days) allowed for easier purification by selective
precipitation from water after removal of the salt byproducts car-
ried through from the oxidation step, giving the tetra-ethyl ester (2)
in a modest yield of 24% after recrystallisation from H2O/EtOH.
Recrystallisation was deemed necessary to remove minor
byproducts.
For the synthesis of the target 2,20-bipyridine-4,40,5,50-tetra-
carboxylic acid, several strategies were considered starting with
pyridine dimerisation. The regioselective dimerisation of pyridines
using Pd/C or Raney Ni have been reported previously.4e7 Of par-
ticular interest is the procedure reported by Newkome et al. where
ethyl isonicotinate is regioselectively dimerised at 200 ꢀC over Pd/C
under vacuum.5 Investigating this reaction methodology on diethyl
pyridine-3,4-dicarboxylate using conventional heating and no
vacuum (Scheme 2) gave regioselective dimerisation to the desired
tetraethyl-2,20-bipyridine-4,40,5,50-tetracarboxylate (4) in w10%
conversion.
Scheme 2. Attempted synthesis routes to tetraethyl-2,20-bipyridine-4,40,5,50-tetra-
carboxylate (4).
Scheme 1. Synthesis of 2,20-bipyridine-3,30,4,40-tetracarboxylic acid (1).
Our attempts to directly couple 3,4-pyridine dicarboxylic an-
hydride under similar conditions failed to yield any desired prod-
uct. However, results of subsequent reactions varied with
significantly lower conversion yields (<2%) observed. Due to the
variability of this approach a more reliable procedure was de-
veloped. We envisaged a more reliable procedure by dimerising
3,4-alkyl substituted pyridines prior to conducting the oxidation
and the esterification steps. The dimerisation of neat 3,4-lutidine
over Pd/C (180 ꢀC, 7 days) resulted in very low yields (1e3%) of
the bipyridine product 5, (Scheme 2). Switching the alkyl chain
length at the 3-position from methyl to ethyl (3-ethyl-4-methyl
pyridine) improved the dimerisation to w7% conversion (5,50-
diethyl-4,40-dimethyl-2,20-bipyridine (6)) and an isolated yield of
6.5%, collected as colourless crystals, which formed upon standing.
The tetraethyl 2,20-bipyridine-3,30,4,40-tetracarboxylate ligand
was characterised by several analysis techniques (1H NMR, 13C
NMR, IR, MS, EA), which were consistent with the expected struc-
ture. The oxidation/esterification process was modified using
methanol in the esterification stage to produce a less sterically
hindered ligand derivative, tetramethyl-2,20-bipyridine-3,30,4,40-
tetracarboxylate (3), as shown in Scheme 1. Although lower yields
were encountered (w20%), the esterified product was recovered as
a crystalline solid. The recovered product was characterised by 1H
NMR, 13C NMR, IR, MS and EA without further recrystallisation.
Crystals of the tetramethyl-2,20-bipyridine-3,30,4,40-tetracarbox-
ylate ligand (3) of suitable purity/quality for single-crystal X-ray
diffraction were grown via slow diffusion of diethyl ether into
a methanolic solution of the ligand. The two pyridine rings of the
ligand displayed a trans configuration, minimising the steric in-
teraction of the ester groups at the 3 and 30 positions, as shown in
Fig. 2 (see also CCDC 1056206).
Oxidation of
5 was attempted using several methodologies
(Scheme 2), however all gave unfavourable products. Oxidation
with KMnO4 and subsequent esterification yielded incomplete
oxidation products, the main cause being poor solubility of starting
reagent(s) (5 or 6). Further attempts with more polar solvent
mixtures (water/tBuOH (1:1)) were equally unsuccessful. Previous
attempts by Kelly et al. who reported oxidation of 4,40,5,50-tetra-
methyl-2,20-bipyridine using 4% aqueous HNO3 resulted in de-
carboxylation at 5,50positions.8
To circumvent the difficulties encountered in the generation of
the 2,20 bond an alternative synthesis pathway was considered
€
using the Krohnke pyridine synthesis to construct the second
pyridine ring (Scheme 3).
The initial step, starting from diethyl-3,4-pyridine dicarbox-
ylate, was a regioselective acetylation performed using an adapted
Fig. 2. Crystal structure of tetramethyl-2,20-bipyridine-3,30,4,40-tetracarboxylate (3).

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C.P. Woodward et al. / Tetrahedron 71 (2015) 5238e5247
Scheme 3. Synthesis of 2,20-bipyridine-4,40,5,50 tetracarboxylic acid (10).
literature method, giving the desired diethyl-6-acetylpyridine-3,4-
dicarboxylate product (7) as the major product after chromatog-
raphy (80% yield).9 The acetylated pyridine 7 was then converted to
the iodopyridinium salt 8 using the Ortoleva-King reaction,10
a product precipitated upon addition of Et2O to the reaction mix-
ture. The iodopyridinium salt 8 was typically accompanied by
pyridinium iodide (typically w20e30%), so due to the difficulty of
removing pyridinium iodide from the isolated material, the sub-
sequent reaction was carried out without further purification. The
reaction of the iodopyridinium salt 8 with 2-methyl-2-butenal and
ammonium acetate yielded the desired diethyl-40,50-dimethyl-
[2,20-bipyridine]-4,5-dicarboxylate (9), which was purified by silica
gel plug after aqueous reaction workup (39% yield). The purification
process was simplified by performing a selective crystallisation
after the ethanol had been removed, increasing the isolated yield to
66%. The oxidation of the bipyridine 9 was performed using KMnO4
(10 equiv) under basic conditions in order to carry out the ester
hydrolysis concurrently. After solvent concentration and pH ad-
justment using 4 M HCl, the desired product 2,20-bipyridine-4,40-
5,50-tetracarboxylic acid (10) flocculated out of solution as a white
solid in an average yield of 16%. The 1H NMR spectrum showed only
two singlets in the aromatic region at 9.04 and 9.43 ppm. In
a similar way to the 3,30,4,40 substituted ligand 1 the 13C NMR
spectrum displayed 7 resonances, with two distinct carbon envi-
ronments of the carboxylic acid groups found at 169.5 and
171.2 ppm. The ESI accurate mass spectrum obtained show a signal
at 331.0208 m/z in the negative mode corresponding to the value
expected for [MH]ꢁ.
Scheme 4. Synthesis of ethyl 50-methyl-[2,20-bipyridine]-4-carboxylate (14) and ethyl
50-((diethoxyphosphoryl)methyl)-[2,20-bipyridine]-4-carboxylate (17).
4-carboxylate-[2,20-bipyridine] (14) failed to yield the desired
product effectively, and
a complex mixture was generated.
Replacing the crotonaldehyde with methacrolein and operating
under the same reaction conditions resulted in the successful for-
mation of ethyl-50-methyl-[2,20-bipyridine]-4-carboxylate (15). The
compound was easily purified by passage through a silica gel plug
(86% isolated yield). It was also found that the purity of the product
could be improved by trituration using hexane/ethyl acetate to
remove other reaction side products from the desired bipyridine.
Using an adapted literature procedure,11 the bromination of the
ethyl-5-methyl-40-carboxylate-[2,20-bipyridine] (15) with NBS in
CCl4 and using AIBN as the radical initiator was successfully per-
formed, resulting in the desired product in approximately 80%
conversion. The mono brominated product 16 could be purified by
column chromatography, with slight contamination by starting
material (>90% pure).
The brominated bipyridine 16 was then reacted with a tenfold
excess of triethyl phosphite in refluxing chloroform to install the
phosphonate ester functionality. After removal of the solvent and
excess triethyl phosphite by distillation, the 1H NMR spectrum of
the crude showed near complete conversion to the desired asym-
metric ligand, ethyl 50-((diethoxyphosphoryl)methyl)-[2,20-bipyr-
idine]-4-carboxylate (17). The mixture was then purified by column
chromatography giving the desired asymmetric ligand precursor,
17, in a 53% isolated yield. However, attempts to cleave the esters to
form the required ligand 18 using acid (2 M HCl/THF reflux 5 h) or
TMSBr (TMSBr 2 equiv in CHCl3, NEt3) were unsuccessful, yielding
complex mixtures. Selectivity of the hydrolysis reaction in attempts
to optimize reaction conditions proved difficult for isolating 18.
These were abandoned in favor of poly-carboxy analogues of the
terpyridine ligand.
2.2. Asymmetric bipyridine synthesis
€
Following our success with the Kronhke synthesis, a bipyridine
bearing dual anchoring functionalities was also considered using an
analogous strategy to the one detailed above. Firstly, ethyl 2-
acetylisonicotinate (11) was synthesised by ortho-acetylation of
ethyl isonicotinate using a literature procedure (Scheme 4).9
The reaction produced an undesirable diacetylated by-product
12, not reported previously. The separation of the by-product was
performed by column chromatography, giving the required mono
ortho-acetylated product 11 in a 60% isolated yield. Ethyl-2-
acetylisonicotinate (11) was further converted to the iodopyr-
idinium salt 13 using Ortoleva-King conditions, with the desired
compound simply filtered from the reaction mixture after a pro-
longed period of cooling at 4 ꢀC, however the desired compound 13
was again accompanied by residual pyridinium iodide. The amount
of pyridine used in the reaction could be reduced to 10 mL of pyr-
idine per gram of starting material without significantly affecting
the isolated yield. Reaction of the pyridinium salt 13 with croto-
2.3. Terpyridine synthesis
Utilizing the method developed for the synthesis of 10 we
considered that terpyridine and quaterpyridine derivatives pos-
sessing the 4,5-pyridyl ester substitution could also be accessed by
0
€
naldehyde under Kronhke conditions to produce ethyl-4 -methyl-

C.P. Woodward et al. / Tetrahedron 71 (2015) 5238e5247
5241
reacting a suitable chalcone with the previously prepared pyr-
idinium iodide salt 8. To investigate terpyridine preparation, (E)-
diethyl-6-(3-(furan-2-yl)acryloyl)pyridine-3,4-dicarboxylate (19)
was first synthesised by reaction of 7 with one equivalent of 2-
furaldehyde over Al2O3 (basic) in a grind reaction, giving w75%
conversion after 48 h, with an isolated yield of 46% after column
chromatography (Scheme 5).
yl)-[2,20:60,2ʺ-terpyridine]-4.4ʺ,5,5ʺ-tetracarboxylic acid (24) after
pH driven precipitation.
To allow for the ultimate carboxylic acid functionality in-
stallation, the furan ring system on terpyridine 21 was oxidised in
the presence of KMnO4 (Scheme 6).13
Scheme 6. Synthesis of pentacarboxy terpyridine 29.
The reaction applied a modified literature method14 using
refluxing t-butanol:water (2:1) with KMnO4 (10 equiv) in the ab-
sence of base for 14 h. This resulted in complete oxidation of the
furan ring however the ester groups were partially hydrolysed,
which lead to a poor mass recovery (w25%). Lowering the reaction
temperature to 70 ꢀC allowed for complete oxidation of the furan
without ester group cleavage (60% isolated yield). Ester cleavage
was then performed using LiOH,H2O in refluxing THF:water (1:1)
for 5 h. After removal of the THF, the solution was adjusted to pH 2
using 1 M H2SO4 to protonate the carboxylic acid groups although
substantial gelation of the reaction mixture was encountered at this
stage. Addition of water and gentle heating was required to disrupt
the gel sufficiently to allow continued pH adjustment. Further
disruption of the gel was performed through hot filtration allowing
the collection of [2,20:60,2ʺ-terpyridine]-4,40,4ʺ,5,5ʺ-pentacarbox-
ylic acid (26) as a white solid in isolated yields of 60e70%. The
resultant pentacarboxy terpyridine 26 showed limited solubility
and could only be dissolved in DMF, DMSO and basified aqueous
media. The 1H NMR spectrum of 26 displayed only three signals for
the pyridine protons at 8.75, 8.96 and 9.14 ppm, whilst the 13C NMR
clearly displayed three different CO resonances at 165.7, 166.3,
167.7 ppm for the three different carboxylic acid carbon environ-
ments. HRMS was also obtained, giving a m/z value of 454.0516,
which corresponded to the calculated m/z value of 454.0517 for the
expected [MH]þ peak (see Supplementary data). Elemental analysis
of the ligand (post-drying) indicated the inclusion of one water
molecule per ligand.
Scheme 5. Synthesis of heterocycle appended terpyridines.
As both of the reactants were liquids the grind mixing pro-
ceeded well, compared to mixing of solids under similar condi-
tions.12 It was later found that using additional furaldehyde
(3 equiv) enhanced the conversion to near quantitative (w100%).
This obviated the need for chromatographic separation of the
unreacted starting materials (excess furaldehyde removed by high
vacuum), yielding the desired chalcone, (E)-diethyl-6-(3-(furan-2-
yl)acryloyl)pyridine-3,4-dicarboxylate (19) in a 75% isolated yield.
(Note: Mass losses were attributed to the adsorption of organic
material to the alumina surface). Improved electron withdrawing
properties in the ligand can be installed through incorporation of
a thiophene moiety to the heterocycle structure. Using this opti-
mised grinding procedure, a thiophene analogue, (E)-diethyl-6-(3-
(thien-2-yl)acryloyl)pyridine-3,4-dicarboxylate (20) was syn-
thesised by reacting 7 with 3 equiv of thiophene-2-carboxaldehyde
over Al2O3 (basic) in a grind reaction with an isolated yield of 93%.
Reaction of chalcone 19 with the pyridinium salt 8 in the presence
of ammonium acetate in refluxing ethanol produced the desired
furan appended terpyridine, tetraethyl-40-(furan-2-yl)-[2,20:60,200-
terpyridine]-4,400,5,500-tetracarboxylate (21) in w80% conversion.
After aqueous workup the terpyridine 21 was purified by column
chromatography and isolated in a 69% yield. Purification was later
simplified by trituration using ethanol. Reaction of the thiophene
chalcone 20 with 8 under the same conditions used to prepare 19
resulted in the desired thiophene appended terpyridine, tetraethyl-
40-(thien-2-yl)-[2,20:60,200-terpyridine]-4,400,5,500-tetracarboxylate
(22) in 47% yield.
2.4. Quaterpyridine synthesis
Given the success of the terpyridine adducts, the preparation of
a quaterpyridine was also undertaken (Scheme 7).
An attempt was made to directly construct the furan appended
terpyridine 21 by reacting 2-furaldehyde with diethyl 6-acetyl-3,4-
pyridine dicarboxylate 7 in an aqueous KOH solution containing
ammonia (Scheme 5), as recently reported for the preparation of
the N749 ligand. Unfortunately this was unsuccessful, most likely
due to the basic conditions employed, resulting in ester hydrolysis.
Both of the synthesised terpyridine esters (21 and 22) were sub-
jected to the established LiOH hydrolysis conditions, resulting in
the clean generation of tetra-acids, 40-(furan-2-yl)-[2,20:60,200-ter-
pyridine]-4,400,5,500-tetracarboxylic acid (23) and 40-(thiophen-2-
Scheme 7. Synthesis of furan appended quaterpyridine 29.

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C.P. Woodward et al. / Tetrahedron 71 (2015) 5238e5247
First, a dichalcone (1E,5E)-1,6-difurylhexa-1,5-diene-3,4-dione
was collected by vacuum filtration, washed with water and dried in
a desiccator to give a white solid (3.91 g). The solid was recrystal-
lised with H2O/EtOH (w2:1 ratio, 5e10 mL) filtered by suction fil-
tration and dried to give the desired compound 2 as fine white
crystals (2.60 g, 24%); mp: 108e110 ꢀC; 1H NMR (400 MHz, CDCl3):
(27) was prepared according to a literature method.15 A double
€
Kronhke reaction between 27 and two equivalents of 8 was then
performed to give a difuran quaterpyridine, (tetraethyl-40,4ʺ-
di(furan-2-yl)-[2,20:60,2ʺ:6ʺ,2000-quaterpyridine]-4,4000,5.5000-tetra-
carboxylate (28), having four peripheral ester groups in a 31% iso-
lated yield. The ester hydrolysis was accomplished using the same
LiOH hydrolysis method used for the other oligopyridines afore-
mentioned to produce 40,4ʺ-di(furan-2-yl)-[2,20:60,2ʺ:6ʺ,2000-qua-
terpyridine]-4.4000,5.5000-tetracarboxylic acid (29) in 95% yield.
No attempt was made to synthesize an oxidised analogue of 29
as it was understood that the product would be extremely insoluble
given the relatively low solubility of 26 even in highly basic media.
Reduced solubility of a quaterpyridine analogue would prove im-
practical for a chelation ligand in DSSC applications.
d
1.22 (t, 6H, J¼7.1 Hz, CH3), 1.38 (t, 6H, J¼7.1 Hz, CH3), 4.28 (q, 4H,
J¼7.1 Hz, OCH2), 4.40 (q, 4H, J¼7.1 Hz, OCH2), 7.76 (d, 2H, J¼4.9 Hz,
ArH), 8.73 (d, 2H, J¼4.9 Hz, ArH). 13C NMR (100 MHz, CDCl3):
d 13.9,
14.1, 61.8, 62.5, 122.9, 129.2, 138.8, 149.4, 154.2, 164.9, 166.8.
IR(ATR): 678(w), 737(w), 815(w), 853(w), 880(w), 1009(m),
1058(m), 1074(m), 1009(s), 1129(m), 1197(m), 1258(s), 1365(w),
1389(w), 1448(w), 1466(w), 1562(w), 1717(s), 2937, (w), 2976(m),
3082(w). MS (ESI): Calculated m/z [MH]þ 445.2, Observed m/z
[MH]þ 445.1. Elemental Analysis for C22H24N2O8: C, 59.45; H, 5.44;
N, 6.30. Found C, 59.42; H, 5.38; N, 6.27.
3. Conclusion
4.3. [2,20-Bipyridine]-3,30,4,40-tetracarboxylic acid (1)
A novel series of new oligopyridine ligands possessing novel
anchoring group motifs have been synthesised for the preparation
of further ruthenium complexes of increased surface stability. The
synthesis of the respective complexes of these ligands and their
evaluation of efficiency and stability in dye-sensitized solar cells
will be the subject of future research articles from our laboratories.
Tetraethyl 2,20-bipyridine-3,30-4,40-tetracarboxylate (2) (2.60 g,
5.85 mmol) and lithium hydroxide monohydrate (2.45 g,
58.50 mmol) were combined in 40 mL of THF/water (1:1 ratio) and
the resulting mixture was heated to reflux whilst stirring for 5 h.
The reaction mixture was cooled to room temperature and THF was
removed by rotary evaporation. The solution was adjusted to pH
1.5e2 with 4 M HCl then the solvent volume was reduced in volume
by 1/3rd. The mixture was allowed to stand at room temperature,
resulting in the precipitation of a solid. The solid was collected by
filtration, washed with cold diethyl ether and dried to give a white
solid (952 mg). The collected solid was found to be not completely
soluble in methanol. The solid was taken up in methanol, filtered
and solvent removed by rotary evaporation to give the desired
compound 1 as a white solid, Crop 1 (559 mg). A second crop was
obtained by taking the filtrate and removing the diethyl ether by
rotary evaporation. Once the diethyl ether had been removed an
additional 1/3rd volume of water was removed and the solution
was then allowed to stand at room temperature, resulting again in
the precipitation of a solid. The solid was collected by filtration,
washed with cold diethyl ether and dried to give the desired
compound 1 as a white solid, Crop 2 (921 mg), which was com-
pletely soluble in methanol. Characterisation data obtained was
4. Experimental section
4.1. General
Reagents were acquired from the CSIRO chemical store with the
following exceptions: paraldehyde purchased from Merck and
diethyl-pyridine-3,4-dicarboxylate purchased from Sigma Aldrich.
All solvents used in the synthetic procedures were commercial high
purity (HPLC or analytical grade) commercial products. 1H and 13C
NMR spectra were recorded on a Bruker BioSpin Av400 spec-
trometer operating at 400 MHz and 100 MHz respectively. Chem-
ical shift values are reported relative to residual solvent signal (1H,
13C) as a reference. Infrared spectra were recorded on a Perkin
Elmer Spectrum 400 FTIR spectrometer fitted with a Specac ATR
crystal plate. Low resolution mass spectra were acquired on a Shi-
madzu Liquid ChromatographyeMass Spectrometer. High resolu-
tion mass spectra were acquired on Bruker Bioapex 47e Fourier
Transform mass spectrometer. Elemental analyses were conducted
at the microanalytical unit of the Research School of Chemistry
Australian National University, Acton, ACT.
identical to previous preparations. Overall yield 76%; mp: >190 ꢀ
C
(decomp.); 1H NMR (400 MHz, d4-MeOD):
d
7.78 (d, 2H, J¼4.9 Hz,
ArH), 8.67 (d, 2H, J¼4.9 Hz, ArH).13C NMR (100 MHz, d4-MeOD):
d
123.9, 130.6, 141.3, 150.7, 155.9, 168.1, 170.1.MS (ESI): Selected IR
bands (cmꢁ1): 3509 br m, 3384 br w, 3155 br m, 3098 br m, 1848 br
m, 1777 m, 1705 m, 1674 m, 1620 m, 1602 m, 1407 m, 1273 m,
1246 m, 1120 w, 1103 w, 897 w, 849 w, 727 w, 667 w, 638 w, 568 w.
Calculated m/z [MH]þ 333.0, Observed m/z [MH]þ 333.0. Calculated
m/z [MH]ꢁ 331.0, Observed m/z [MH]ꢁ 330.9. Elemental Analysis for
4.2. Tetraethyl 2,20-bipyridine-3,30,4,40-tetracarboxylate (2)
A
suspension of 4,7-dimethylphenathroline (5.00 g,
24.01 mmol) in sodium hydroxide (1.92 g, 48.02 mmol) dissolved in
100 mL of distilled water was treated with KMnO4 (45.53 g,
288.10 mmol). The resulting mixture was heated to reflux and
stirred for 18 h. The reaction mixture was cooled to room tem-
perature and filtered. Activated charcoal pellets (10 g) were added
to the filtrate and the resulting mixture was stirred and heated at
50 ꢀC for 30 min then filtered through a Celite pad (2ꢂ20 mL). The
filtrate solution was reduced to half the original volume, pH ad-
justed to 2 using 4 M HCl and the remaining water was removed
under reduced pressure to give a solid mass. The solid was then
suspended in ethanol (dry, 300 mL) and treated with 9 mL of conc.
sulfuric acid. The resulting mixture was heated to reflux and stirred
for 5 days. The white solid in the reaction mixture was collected via
filtration, washed with ethanol (2ꢂ10 mL) and the filtrate solvent
volume reduced by rotary evaporation down to 10e15 mL. The
residue was then treated with ice/water slurry, resulting in pre-
cipitation of a white solid. Once the ice had melted the precipitate
C
14H8N2O8: C, 50.61; H, 2.43; N, 8.43. Found C, 47.60; H, 2.70; N, 7.7.
4.4. Tetramethyl 2,20-bipyridine-3,30,4,40-tetracarboxylate (3)
A
suspension of 4,7-dimethylphenathroline (5.00 g,
24.01 mmol) in sodium hydroxide (1.92 g, 48.02 mmol) dissolved in
100 mL of distilled water was treated with KMnO4 (45.53 g,
288.10 mmol). The resulting mixture was heated to reflux and
stirred for 18 h, cooled to room temperature and finally filtered.
Activated charcoal pellets (10 g) were added to the filtrate and the
resulting mixture was stirred and heated at 50 ꢀC for 30 min then
filtered through a Celite pad (2ꢂ20 mL). The filtrate solution was
reduced to half the original volume, the pH was adjusted to 2 using
4 M HCl and the remaining water was removed under reduced
pressure to give a solid mass. The solid was then suspended in
methanol (dry, 250 mL) and treated with 9 mL of conc. sulfuric acid.
The resulting mixture was heated to reflux and stirred for 5 days.

C.P. Woodward et al. / Tetrahedron 71 (2015) 5238e5247
5243
The white solid in the reaction mixture was removed by filtration,
4.8. Diethyl 6-acetyl-3,4-pyridine dicarboxylate (7)
washed with methanol (2ꢂ20 mL) and the filtrate solvent volume
reduced by rotary evaporation to 10e15 mL. The residue was then
treated with ice/water slurry, resulting in precipitation of a white
solid. Once the ice had melted the precipitate was collected by
vacuum filtration, washed with water and dried in a desiccator to
give a white solid (1.20 g,13%); mp: 140e145 ꢀC; 1H NMR (400 MHz,
Synthesis conducted following a modified literature method.9
Diethyl pyridine-3,4-dicarboxylate (10.00 g, 44.8 mmol) was
added to a solution of paraldehyde (29.6 g, 672 mmol) in CH3CN
(140 mL) at room temperature. Addition of FeSO4,7H2O (0.6 g), TFA
(6.8 g) and 70% tBuOOH (20 g) was then performed and the resulting
mixture was heated to reflux for 18 h. Once cooled to room tem-
perature the acetonitrile was removed in vacuo and the residue was
taken up in water (150 mL) and extracted with toluene (4ꢂ100 mL).
The combined organic fractions were dried over MgSO4, filtered and
solvent remove by rotary evaporation. The brown residue was pu-
rified by passage through multiple silica gel plugs (SiO2, heptane/
ethyl acetate, 3:1, w5 g of crude material per column) with the de-
sired compound eluting first, collected as a pale yellow oil 7 (8.51 g,
72% yield) after solvent removal. NB: Able to recover additional
amounts by conducting further chromatography on impure frac-
CDCl3):
d
3.81 (s, 6H, 2ꢂOCH3), 3.93 (s, 6H, 2ꢂOCH3), 7.78 (d, 2H,
J¼5.0 Hz, ArH), 8.75 (d, 2H, J¼5.0 Hz, ArH). 13C NMR (100 MHz,
CDCl3):
d 52.8, 53.4, 123.2, 129.2, 138.5, 149.6, 153.8, 165.1, 167.5.
Selected IR bands (cmꢁ1): 3009.9 br w, 2958.9 w, 2841.0 w, 1742.3 s,
1728.2, s,1561.6 m,1453.7 m,1430.5 m,1323.8 w,1323.8 w,1260.1 s,
1196.5 s, 1125.4 s, 1101.3 s, 1074.4 s, 1058.5 m, 956.4 s, 892.8 w,
862.6 m, 834.7 w, 810.4 s, 767.6 m, 750.5 m, 709.8 w, 680.4 m. High
resolution MS (þve mode ESI): Calculated m/z [MþNa]þ 411.0799,
Observed m/z 411.0806. Elemental Analysis for C18H16N2O8: C,
55.67; H, 4.15; N, 7.21. Found C, 55.63; H, 4.24; N, 7.25; X-ray
Crystallographic Data:
C
18H16N2O8, M¼388.33 Triclinic, space
tions. 1H NMR (400 MHz, CDCl3):
d
1.27e1.41 (m, 6H, CH3ꢂ2), 2.74 (s,
group P-1. Unit Cell Dimensions: a¼7.699(3), b¼8.123(3),
3H, CH3), 4.39e4.45 (m, 4H, OCH2ꢂ2), 8.20 (d, 1H, J¼0.6 Hz, ArH),
ꢀ
3
ꢀ
ꢀ
c¼16.632(6) A,
a
¼99.928(11)ꢀ,
b
¼90.596(11)ꢀ,
g
¼117.481(9)ꢀ A,
9.05 (d, 1H, J¼0.6 Hz, ArH). 13C NMR (100 MHz, CDCl3):
d 14.2, 26.0,
ꢁ3
V¼904.1(5) A , Z¼2, Dc¼1.426 g cm
, m
¼0.114 mmꢁ1, F000¼404,
62.6, 62.7,120.1,128.6,141.5,150.0,155.6,165.0,165.7,198.7. MS (ESI):
Calculated m/z [MH]þ 266.1, Observed m/z [MH]þ 266.0. Calculated
m/z [MþNa]þ 288.1, Observed m/z [MþNa]þ 288.0.
T¼199 K, 2qmax¼50.1ꢀ, 3150 reflections collected, 2542 unique
(Rint¼0.0343). R1¼0.0390, wR2¼0.1201 for 2542 data with I>2
sI;
Largest diff. peak and hole 0.946 and ꢁ0.984 e.Aꢁ3
.
4.9. Pyridinium iodide salt of diethyl 6-acetyl-3,4-pyridine
dicarboxylate (8)
4.5. Tetraethyl 2,20-bipyridine-4,40,5,50-tetracarboxylate (4)
Diethyl pyridine-3,4-dicarboxylate (1.00 g, 4.48 mmol) was
treated with palladium on carbon (200 mg, Pd/C with 50% H2O) and
heated to 200 ꢀC under an argon atmosphere and left to stir for 7
days. The mixture was hot filtered through a Celite pad and washed
with portions of CHCl3 (3ꢂ50 mL). The CHCl3 was removed by
distillation and the collected residue purified by column chroma-
tography (EtOAc), giving the desired product as a colourless crys-
talline solid 4 (120 mg, 12%). Note: Procedure could not be
Synthesis conducted following a modified literature method.9
A solution of diethyl 6-acetyl-3,4-pyridine dicarboxylate (7)
(4.00 g, 15.08 mmol), and iodine (4.21 g, 16.59 mmol), in dry pyri-
dine (40 mL) was heated to reflux for 3 h under an argon atmo-
sphere. After this time the reaction mixture was cooled, treated
with diethyl ether (40 mL), and then stored at 4 ꢀC overnight whilst
remaining under argon. The precipitate formed was filtered by
suction filtration, washed with diethyl ether (3ꢂ40 mL) then dried
in a vacuum desiccator, giving the desired product (8) as a brown
solid (8.12 g, 29% purity by NMR, 33% Yield). Product used without
consistently repeated. 1H NMR (400 MHz, CDCl3):
d 1.36e1.43 (m,
12H, CH3), 4.37e4.47 (m, 8H, OCH2), 8.66 (s, 2H, ArH), 9.12 (s, 2H,
ArH). 13C NMR (100 MHz, CDCl3): 14.2, 62.2, 62.5, 120.0, 125.8,
142.3,150.6,157.4,165.1,166.5. Selected IR bands (cmꢁ1): 3453 br w,
3082 w, 2976 w, 2937 w, 1717 s, 1562 w, 1466 w, 1448 w, 1389 w,
1365 w, 1258 s, 1197 m, 1129 w, 1099 m, 1074 m, 1058 m, 1009 m,
880 w, 853 w, 815 w, 771 w, 750 w, 737 w, 678 w. MS (ESI): Cal-
culated m/z [MH]þ 445.1605, Observed m/z [MH]þ 445.1592.
further purification. 1H NMR (400 MHz, DMSO-d6):
d 1.29e1.35 (m,
6H, CH3), 4.35e4.42 (m, 4H, OCH2), 6.51 (s, 2H, CH2), 8.25 (s, 1H, Py-
H), 8.30 (t, 2H, J¼7.0 Hz, Py-H), 8.75 (t, 1H, J¼7.8 Hz, Py-H), 9.00 (d,
2H, J¼6.0 Hz, Py-H), 9.26 (s, 1H, Py-H). LR-MS (ESI): Calculated m/z
[MH]þ 343.1, Observed m/z [MH]þ 343.
4.10. Diethyl-40,50-dimethyl-[2,20-bipyridine]-4,5-
dicarboxylate (9)
4.6. 4,40,5,50-Tetramethyl-2,20-bipyridine (5)
Bipyridine prepared according to a literature procedure by
Mines et al.4
Procedure adapted from a literature method.9
A mixture of pyridiumium salt (8) (6.06 g, 7.48 mmol), 2-
methyl-2-butenal (629.18 mg, 7.48 mmol) and ammonium ace-
tate (5.69 g, 74.80 mol) was dissolved in ethanol (90 mL) and heated
to reflux for 3 h. The ethanol was then removed by rotary evapo-
ration and the oil was stored at 4 ꢀC overnight. The mixture was
then filtered and washed with cold ethanol (3ꢂ20 mL) giving the
desired product 9 as a yellow solid (1.62 g, 66%); mp: 264 ꢀC
4.7. 5,50-Diethyl-4,40-dimethyl-2,20-bipyridine (6)
3-Ethyl-4-methylpyridine (50.0 g, 412.61 mmol) was treated
with palladium on carbon (10 g, Pd/C with 50% H2O) and heated to
180 ꢀC under an argon atmosphere and left to stir for 7 days. The
mixture was hot filtered through a Celite pad and washed with
portions of CHCl3 (3ꢂ50 mL). The CHCl3 was removed by distilla-
tion and the remaining mixture left to cool to room temperature,
which resulted in the formation of colourless crystals. The crystals
were collected by suction filtration and washed with heptanes until
the washings were clear, giving the desired product as a colourless
crystalline solid 6 (3.20 g, 6.5%); mp: 143e144 ꢀC; 1H NMR
(decomp.); 1H NMR (400 MHz, CDCl3):
d
1.29e1.40 (m, 6H, CH3ꢂ2),
2.32 (s, 3H, CH3), 2.37 (s, 3H, CH3), 4.31e4.44 (m, 4H, OCH2ꢂ2), 8.22
(d, 1H, J¼0.6 Hz, ArH), 8.41 (d, 1H, J¼0.6 Hz, ArH), 8.53 (d, 1H,
J¼0.6 Hz, ArH), 9.10 (d, 1H, J¼0.6 Hz, ArH). 13C NMR (100 MHz,
CDCl3): 14.1, 16.4, 19.3, 61.9, 62.2, 118.7, 122.6, 123.9, 133.9, 142.1,
146.7, 149.9, 150.2, 152.1, 159.6, 165.2, 166.8. Low Resolution MS
(ESI): Calculated m/z [MH]þ 329.1, Observed m/z [MH]þ 329.0.
(400 MHz, CDCl3):
d
1.24 (t, 6H, J¼7.6 Hz, CH3), 2.38 (s, 6H, ArCH3),
2.68 (q, 4H, J¼7.6 Hz, OCH2), 8.12 (s, 2H, ArH), 8.38 (s, 2H, ArH). 13
C
4.11. 2,20-Bipyridine-4,40,5,50-tetracarboxylate (10)
NMR (100 MHz, CDCl3):
d 14.6, 18.9, 23.7, 122.2, 137.9, 146.0, 148.8,
154.2. MS (ESI): Calculated m/z [MH]þ 241.2, Observed m/z [MH]þ
Diethyl 40,50-dimethyl-[2,20-bipyridine]-4,5-dicarboxylate (9)
241.1.
(2.50 g, 7.61 mmol) and sodium hydroxide (609 mg, 15.23 mmol)

5244
C.P. Woodward et al. / Tetrahedron 71 (2015) 5238e5247
was added to water (100 mL) then potassium permanganate
(12.03 g, 76.14 mmol) was added in small portions over a 10 min
period. The resulting mixture was heated to reflux and stirred for
18 h. The reaction mixture was cooled to room temperature, filtered
through a Celite pad and washed with water (3ꢂ20 mL). The so-
lution was concentrated to approximately half its original volume.
The pH of the solution was adjusted to pH 2 using 4 M HCl and the
solution left at room temperature to precipitate. The mixture was
filtered and washed with copious amounts of water to remove any
traces of salt. The precipitate was dried in a desiccator overnight
giving the product 10 as a white precipitate (400 mg, 16%); mp:
4.14. Ethyl 50-methyl-[2,20-bipyridine]-4-carboxylate (15)
Synthesis conducted following a modified literature method.17
A mixture of pyridinium salt (13) (5.75 g, 14.44 mmol), meth-
acrolein (3.04 g, 43.32 mmol) and ammonium acetate (10.99 g,
144.40 mmol) dissolved in dry ethanol (40 mL) was heated to reflux
for 3 h. The ethanol was then removed by rotary evaporation, water
(100 mL) was added and the crude mixture was extracted with
ethyl acetate (3ꢂ100 mL). The combined organic fractions were
dried over MgSO4, filtered and solvent removed in vacuo to give
a crystalline orange solid, which was further purified by column
chromatography (SiO2, heptane/ethyl acetate, 2:1, Rf¼0.22) to give
the desired product 15 as a light yellow solid (3.04 g, 86% yield);
282 ꢀC (decomp.); 1H NMR (400 MHz, d4-MeOH):
ArH), 9.43 (s, 2H, ArH). 13C NMR (100 MHz, d4-MeOH):
d
9.04 (s, 2H,
122.1,
d
129.0, 145.8, 159.8, 169.5, 171.2. MS (ESI, -ve mode): Calculated m/z
mp: 69e70 ꢀC; 1H NMR (400 MHz, CDCl3):
d
1.42 (t, 3H, J¼7.1 Hz,
[M]ꢁ 331.0208, Observed 331.0208 [MH]ꢁ. Elemental Analysis for
CH3), 2.39 (s, 3H, CH3), 4.43 (q, 2H, J¼7.1 Hz, OCH2), 7.62 (ddd, 1H,
J¼8.1, 2.2 and 0.7 Hz, ArH), 7.82 (dd, 1H, J¼5.0 and 1.5 Hz, ArH), 8.28
(d, 1H, J¼8.1 Hz, ArH), 8.52 (d, 1H, J¼1.5 and 0.7 Hz, ArH), 8.78 (dd,
C
14H8N2O8: C, 50.61; H, 2.43; N, 8.43. Found C, 50.50; H, 2.38; N,
8.74.
1H, J¼5.0 and 0.8 Hz, ArH), 8.87 (dd, 1H, J¼1.5 and 0.8 Hz, ArH). 13
C
4.12. Ethyl 2-acetyl-isonicotinate (11)
NMR (100 MHz, CDCl3): d 14.5, 18.6, 61.9, 120.1, 120.2, 121.0, 122.6,
134.1, 137.7, 139.0, 150.0, 153.1, 157.6, 165.5. Selected IR bands
(cmꢁ1): 3425 br w, 2976 w, 2937 w, 1726 s, 1719, s, 1552 m, 1414 m,
1369 m, 1279 m, 1243 s, 1224 s, 1102 m, 1017 s, 837 s, 770 m, 751 s,
727 s, 680 m, 647 w, 579 w. MS (ESI): Calculated m/z [MH]þ 243.1,
Observed m/z [MH]þ 243.0. Anal. calcd for C14H14N2O2; C, 69.41; H,
5.82; N, 11.56. Found C, 68.67; H, 6.46; N, 11.20.
Synthesis conducted following a previously reported method.9
Methyl isonicotinate (9.07 g, 60.0 mmol) was added to a solu-
tion of paraldehyde (45.0 g, 1.50 mol) in CH3CN (150 mL) at room
temperature. Addition of FeSO4,7H2O (300 mg, 1.08 mmol), TFA
(7.0 g) and 70% tBuOOH (16.0 g) was then performed and the
resulting mixture was heated to reflux for 4 h. Once cooled to
room temperature the acetonitrile was removed in vacuo and the
residue was taken up in water (100 mL) and extracted with tol-
uene (3ꢂ75 mL). The combined organic fractions were dried over
MgSO4, filtered and solvent removed by rotary evaporation. The
brown residue was purified by passage through a silica gel plug
(SiO2, heptane/ethyl acetate, 4:1) with the desired compound 11
eluting first (Rf¼0.26), collected as a yellow solid (6.90 g, 60%
yield) after solvent removal; mp: 48e49 ꢀC (lit. 56 ꢀC);16 1H NMR
4.15. Ethyl 50-(bromomethyl)-[2,20-bipyridine]-4-carboxylate
(16)
Procedure adapted from a literature method.11
A mixture of bipyridine (0.50 g, 2.06 mmol), NBS (404 mg,
2.27 mmol) and AIBN (40 mg) was dissolved in carbon tetrachloride
(10 mL) was heated to reflux for 2 h. The reaction mixture was
filtered and the solvent was then removed by distillation. The crude
mixture was purified further by column chromatography (SiO2,
CHCl3/ethyl acetate, 4:1) to give the desired product 16 as a col-
(400 MHz, CDCl3):
d
1.42 (t, 3H, J¼7.1 Hz, CH3), 2.74 (s, 3H, COCH3),
4.43 (q, 2H, J¼7.1 Hz, OCH2), 8.02 (dd, 1H, J¼4.9 and 1.6 Hz, ArH),
8.54 (dd, 1H, J¼1.6 and 0.8 Hz, ArH), 8.82 (dd, 1H, J¼4.9 and 0.8 Hz,
ourless solid (310 mg, 47% yield). 1H NMR (400 MHz, CDCl3):
d 1.44
ArH). 13C NMR (100 MHz, CDCl3):
d 14.4, 26.1, 62.6, 121.1, 126.3,
(t, 3H, J¼7.1 Hz, CH3), 4.45 (q, 2H, J¼7.1 Hz, OCH2), 4.55 (s, 2H,
CH2Br), 7.85e7.89 (m, 2H, ArH), 8.42 (d, 1H, J¼8.2 Hz, ArH), 8.72 (d,
1H, J¼2.0 Hz, ArH), 8.82 (dd, 1H, J¼4.9 and 0.6 Hz, ArH), 8.93 (d, 1H,
J¼0.6 Hz, ArH). MS (LR-ESI): Calculated m/z [MH]þ 331, Observed m/
z [MH]þ 331.
139.1, 150.0, 154.7, 164.7, 199.5. Characterisation data for the
byproduct ethyl 2,5-diacetylisonicotinate (12) Eluted second, 4:1
heptane/ethyl acetate (Rf¼0.13). 1H NMR (400 MHz, CDCl3):
d 1.39
(t, 3H, J¼7.2 Hz, CH3), 2.61 (s, 3H, COCH3), 2.73 (s, 3H, COCH3), 4.41
(q, 2H, J¼7.2 Hz, OCH2), 8.34 (d, 1H, J¼0.8 Hz, ArH), 8.77 (d, 1H,
J¼0.8 Hz, ArH).
4.16. Ethyl 50-((diethoxyphosphoryl)methyl)-[2,20-bipyr-
idine]-4-carboxylate (17)
4.13. Pyridinium iodide salt of ethyl 2-acetyl-isonicotinate
(13)
Synthesis conducted following a modified literature method.18
Ethyl 50-(bromomethyl)-[2,20-bipyridine]-4-carboxylate (16)
(200 mg, 0.62 mmol) was added to a solution of CHCl3 (10 mL)
containing triethylphosphite (1.0 g, 6.23 mmol) and the resulting
mixture was heated to reflux for 5 h under an argon atmosphere.
Once cooled to room temperature the CHCl3 and triethylphosphite
were removed by vacuum distillation and the residue was purified
by column chromatography (SiO2, ethyl acetate: acetone, 3:1) with
the desired compound 17, eluting (Rf¼0.35), collected as a colour-
less oil (124 mg, 53% yield) after solvent removal. 1H NMR
Synthesis conducted following a modified literature method.9
A
solution of ethyl 2-acetyl isonicotinate (11) (4.00 g,
20.70 mmol), and iodine (5.25 g, 20.70 mmol), in dry pyridine
(40 mL) was heated to reflux for 3 h under an argon atmosphere.
After this time the reaction mixture was cooled and then stored
at 4 ꢀC overnight whilst remaining under argon. The precipitate
formed in the cooled mixture was filtered by suction filtration,
washed with CHCl3 (1ꢂ10 mL) and diethyl ether (3ꢂ20 mL) then
dried in a vacuum desiccator, giving the desired product 13 as
a beige crystalline solid (5.75 g, 70%). Product used without
(400 MHz, CDCl3):
d
1.28 (t, 6H, J¼7.1 Hz, CH3), 1.34 (t, 3H, J¼7.1 Hz,
further purification. 1H NMR (400 MHz, DMSO-d6):
d
1.35 (t, 3H,
CH3), 3.12 (d, 2H, J¼21.8 Hz, CH2PO(OEt)2), 4.01e3.96 (m, 4H, OCH2),
4.36 (q, 2H, J¼7.1 Hz, OCH2), 7.72 (dt, 1H, J¼8.2 and 2.4 Hz, ArH),
7.77 (dd, 1H, J¼5.0 and 1.6 Hz, ArH), 8.29 (d, 1H, J¼8.2 Hz, ArH), 8.53
(t, 1H, J¼2.4 Hz, ArH), 8.71 (dd, 1H, J¼5.0 and 0.8 Hz, ArH), 8.81 (d,
1H, J¼1.6 Hz, ArH). 13C NMR (100 MHz, CDCl3): 14.3, 16.4, 16.5, 61.8,
62.3, 62.4, 120.3, 120.9, 122.8, 138.1, 138.2, 150.0, 150.1, 154.1, 157.0,
165.2. MS (LR-ESI): Calculated m/z [MH]þ 379.1, Observed m/z
[MH]þ 379.0. Elemental Analysis for C18H23N2O5P: C, 56.99; H, 6.38;
J¼7.1 Hz, CH3), 4.41 (q, 2H, J¼7.1 Hz, OCH2), 6.55 (s, 2H, CH2), 8.24
(d, 1H, J¼4.9 Hz, Py-H), 8.30 (t, 2H, J¼7.0 Hz, Py-H), 8.35 (s, 1H,
Py-H), 8.75 (t, 1H, J¼7.8 Hz, Py-H), 9.03 (d, 2H, J¼5.9 Hz, Py-H),
9.09 (d, 1H, J¼4.9 Hz, Py-H). 13C NMR (100 MHz, DMSO-d6):
14.4, 62.6, 67.1, 120.7, 124.5, 128.2, 139.4, 146.8, 149.7, 151.5, 152.0,
164.2, 191.3. MS (ESI): Calculated m/z [MH]þ 271.1, Observed m/z
[MH]þ 271.0.

C.P. Woodward et al. / Tetrahedron 71 (2015) 5238e5247
5245
N, 7.38. Found (Run 1) C, 56.82; H, 6.30; N, 7.21. (Run 2) C, 56.81; H,
6.43; N, 7.24.
151.3, 154.6, 158.8, 165.1, 167.1. MS (ESI): Calculated m/z [MH]þ
588.2, Observed m/z [MH]þ 588. Selected IR bands (cmꢁ1): 3110 w,
2981,w, 2934 w, 1716 versus, 1611 m, 1591 s, 1549 s, 1469 m, 1366 s,
1235 versus, 1219 s, 1169 m, 1139 s, 1112 s, 1068 m, 1016 versus 990
m, 894 m, 852 m, 795 m, 749 s, 713 s, 681 m. Anal. calcd for
4.17. (E)-Diethyl-6-(3-(furan-2-yl)acryloyl)pyridine-3,4-
dicarboxylate (19)
C
6.92.
31H29N3O9; C, 63.37; H, 4.97; N, 7.15. Found C, 62.34; H, 5.06; N,
Synthesis conducted following a modified literature method.12
A mixture of diethyl 6-acetylpyridine-3,4-dicarboxylate (7)
(5.00 g, 18.85 mmol) and 2-furaldehyde (5.43 g, 56.55 mmol) was
added to 10 g of basic alumina and the resulting mixture was
ground together in a mortar and pestle for 20 min then left to stand
for 48 h with occasional grinding of the solid mixture. The mixture
then transferred onto a sinter and the organics were extracted by
continual washing with CHCl3 until the washings ran clear. The
combined organic were taken to dryness by heating under vacuum.
The solid was then placed under high vacuum to remove the excess
2-furaldehyde to give the desired product as crystalline yellow solid
4.20. Tetraethyl-40-(thien-2-yl)-[2,20:60,200-terpyridine]-
4,400,5,500-tetracarboxylate (22)
Synthesis conducted following the same procedure as for 21
using 20 as the chalcone to give the desired product as a beige solid
(860 mg, 47% yield); mp: 133 ꢀC; 1H NMR (400 MHz, CDCl3):
d 1.43
(t, 6H, J¼7.2 Hz, CH3ꢂ2), 1.46 (t, J¼7.2 Hz, 6H, CH3ꢂ2), 4.45 (q, 4H,
J¼7.2 Hz, OCH2ꢂ2), 4.50 (q, 4H, J¼7.2 Hz, OCH2ꢂ2), 7.20 (dd, 1H,
J¼5.0 and 3.4 Hz, Thienyl-H), 7.50 (dd,1H, J¼5.0 and 1.2 Hz, Thienyl-
H), 7.80 (dd, 1H, J¼3.4 and 1.2 Hz, Thienyl-H), 8.74 (d, 2H, J¼0.78 Hz,
ArH), 8.80 (s, 2H, ArH), 9.20 (d, 2H, J¼0.7 Hz, ArH). 13C NMR
(100 MHz, CDCl3): 14.2, 14.4, 62.3, 62.6, 119.1, 119.4, 124.7, 126.5,
128.0, 128.7, 141.3, 142.6, 144.1, 150.6, 154.9, 158.9, 165.2, 167.2. Se-
lected IR bands (cmꢁ1): 3116 w, 2982 w, 1729 s, 1711 s, 1584 s, 1547
m, 1462 m, 1441 m, 1396 m, 1367 m, 1270 s, 1266 s, 1226 s, 1172 m,
1124 s, 1089 s, 1066 s, 1009 s, 889 m, 851 m, 835 m, 794 m, 773 m,
712 s, 682 m, 625 w. MS (ESI): Calculated m/z 604.17 [MH]þ, Ob-
served m/z 604.2. Anal. calcd for C31H29N3O8S; C, 61.68; H, 4.84; N,
6.96. Found C, 61.20; H, 4.79; N, 6.64.
(6.02 g, 93% yield). 1H NMR (400 MHz, CDCl3):
d
1.39 (t, 6H, J¼7.1 Hz,
CH3ꢂ2), 4.42 (q, 4H, J¼7.1 Hz, OCH2ꢂ2), 6.51 (dd, 1H, J¼3.4 and
1.8 Hz, Furan-H), 6.79 (d, 1H, J¼3.4 Hz, Furan-H), 7.55 (d, 1H,
J¼1.8 Hz, Furan-H), 7.71 (d, 1H, J¼15.7 Hz, Alkene-H), 8.06 (d, 1H,
J¼15.7 Hz, Alkene-H), 8.32 (d, 1H, J¼0.7 Hz, ArH), 9.10 (d, 1H,
J¼0.7 Hz, ArH). 13C NMR (100 MHz, CDCl3):
d 14.2, 62.5, 62.7, 113.0,
117.3, 118.2, 121.2, 128.1, 131.8, 141.6, 145.7, 149.9, 152.1, 156.5, 165.1,
165.9,187.8. Selected IR bands (cmꢁ1): 3147 w, 2984 w,1731 m,1715
s, 1660 m, 1598 m, 1580 s, 1548 s, 1473 w, 1443 w, 1391 w, 1382 w,
1372 w, 1361 w, 1287 s, 1264 s, 1235 s, 1205 s, 1182 s, 1144 s, 1110 m,
1082 s, 1075 s, 1030 m, 974 s, 775 s, 749 s, 683 m, 673 m, 592 m. MS
(ESI): Calculated m/z [MH]þ 344.1, Observed m/z [MH]þ 344. Anal.
calcd for C18H17NO6; C, 62.97; H, 4.99; N, 4.08. Found C, 62.54; H,
5.30; N, 4.04.
4.21. 40-(Furan-2-yl)-[2,20:60,200-terpyridine]-4,400,5,500-tetra-
carboxylic acid (23)
Terpyridine 21 (2.00 g, 3.40 mmol) and lithium hydroxide
monohydrate (1.43 g, 34.04 mmol) were combined in 80 mL of THF/
water (1:1 ratio) and the resulting mixture was heated to reflux
whilst stirring for 6 h. The reaction mixture was cooled to room
temperature and THF was removed by rotary evaporation. The so-
lution was adjusted to pH 2 with 1 M H2SO4 resulting the formation
of a yellow solid. The solid was collected by filtration, washed with
cold water (3ꢂ20 mL) and dried under high vacuum to give the
desired product as a yellow solid (1.56 g, 96% yield); mp: >250 ꢀC
4.18. (E)-Diethyl-6-(3-(thien-2-yl)acryloyl)pyridine-3,4-
dicarboxylate (20)
Synthesis conducted following the same procedure as for 19
using
56.55 mmol) to give the desired product as yellow solid (1.25 g, 93%
yield). 1H NMR (400 MHz, CDCl3):
7 (5.00 g, 18.85 mmol) and 2-thenaldehyde (6.34 g,
d
1.40 (t, 6H, J¼7.1 Hz, CH3ꢂ2),
4.44 (q, 4H, J¼7.1 Hz, OCH2ꢂ2), 7.11 (dd, 1H, J¼5.0 and 3.5 Hz,
Thienyl-H), 7.45 (d, 1H, J¼3.5 Hz, Thienyl-H), 7.48 (d, 1H, J¼5.0 Hz,
Thienyl-H), 8.01 (d, 1H, J¼15.8 Hz, Alkene-H), 8.11 (d, 1H, J¼15.8 Hz,
Alkene-H), 8.35 (s, 1H, ArH), 9.13 (s, 1H, ArH).
(decomp.); 1H NMR (400 MHz, DMSO-d6):
d 6.74e6.78 (m,1H, ArH),
7.54e7.60 (m, 1H, ArH), 7.99 (s, 1H, ArH), 8.71 (s, 2H, ArH), 8.79 (s,
2H, ArH), 9.12 (s, 2H, ArH). 13C NMR (100 MHz, CDCl3): 112.9, 115.8,
118.6, 139.5, 143.0, 145.1, 150.1, 150.2, 154.5, 157.3, 166.3, 167.7. Se-
lected IR bands (cmꢁ1): 3364 br m, 3073 br m, 2641 br w, 2508 br
m, 1980 br w, 1721 s, 1585 s, 1531 s, 1435 m, 1402 m, 1365 m, 1307 s,
1262 s, 1144 m, 1019 m, 988 m, 917 s, 896 s, 838 m, 777 s, 746 s, 625
m, 561 m. Accurate Mass (ESI): C23H9N3O7 (dianhydride form) e
Calculated m/z [MH]þ 439.0440, Observed m/z [MH]þ 439.0437.
4.19. Tetraethyl-40-(furan-2-yl)-[2,20:60,200-terpyridine]-
4,400,5,500-tetracarboxylate (21)
Synthesis conducted following a modified literature method.9
A mixture of pyridinium salt (8) (4.50 g, 9.57 mmol), (E)-diethyl
6-(3-(furan-2-yl)acryloyl)pyridine-3,4-dicarboxylate (19) (13.28 g,
9.57 mmol) and ammonium acetate (7.28 g, 95.67 mmol) dissolved
in absolute ethanol (80 mL) was heated to reflux for 3 h under ar-
gon. The ethanol was then removed by rotary evaporation, water
(100 mL) was added and the crude mixture was extracted with
ethyl acetate (4ꢂ75 mL). The combined organic fractions were
dried over MgSO4, filtered and solvent removed in vacuo to give
a crystalline dark brown solid. The solid was dissolved in a minimal
amount of hot ethanol then stored at 4 ꢀC for 3 h. The solid was
filtered and washed with cold ethanol (4ꢂ20 mL) to give the de-
sired product as a beige/yellow solid (3.67 g, 65% yield); mp: 124 ꢀC;
4.22. 40-(Thiophen-2-yl)-[2,20:60,2ʺ-terpyridine]-4.4ʺ,5,5ʺ-tet-
racarboxylic acid (24)
Terpyridine 22 (400 mg, 0.66 mmol) and lithium hydroxide
monohydrate (278 mg, 6.63 mmol) were combined in 30 mL of
THF/water (1:1 ratio) and the resulting mixture was heated to
reflux whilst stirring for 6 h. The reaction mixture was cooled to
room temperature and THF was removed by rotary evaporation.
The solution was adjusted to pH 2 with 1 M H2SO4 resulting the
formation of a yellow solid. The solid was collected by filtration,
washed with cold water (3ꢂ10 mL) and dried under high vacuum to
give the desired product as a yellow solid (316 mg, 97% yield); mp:
1H NMR (400 MHz, CDCl3):
d
1.39e1.43 (m, 12H, CH3ꢂ4), 4.41 (q,
4H, J¼7.2 Hz, OCH2ꢂ2), 4.47 (q, 4H, J¼7.2 Hz, OCH2ꢂ2), 6.53 (dd,1H,
J¼3.4 and 1.5 Hz, Furan-H), 7.08 (d, 1H, J¼3.4 Hz, Furan-H), 7.57 (d,
1H, J¼1.5 Hz, Furan-H), 8.68 (d, 2H, J¼0.6 Hz, ArH), 8.73 (s, 2H, ArH),
9.14 (d, 2H, J¼0.6 Hz, ArH). 13C NMR (100 MHz, CDCl3): 14.2, 14.3,
62.1, 62.5, 110.0, 112.4, 116.9, 119.2, 124.6, 139.9, 142.4, 144.3, 150.5,
>225 ꢀC (decomp.); 1H NMR (400 MHz, DMSO-d6):
and 3.8 Hz, 1H, ArH), 7.85 (dd, J¼5.1 and 1.1 Hz, 1H, ArH), 8.05 (dd,
J¼3.8 and 1.1 Hz, 1H, ArH), 8.75 (m, 4H, ArH), 8.79 (s, 2H, ArH), 9.16
(s, 2H, ArH). 13C NMR (100 MHz, CDCl3): 117.7, 118.7, 126.0, 127.5,
d
7.30 (dd, J¼5.1

5246
C.P. Woodward et al. / Tetrahedron 71 (2015) 5238e5247
129.2, 129.4, 139.9, 143.1, 143.4, 150.1, 154.7, 157.3, 166.4, 167.8. Ac-
curate Mass (ESI): C23H13N3O8S (dianhydride form)dCalculated m/
z 455.0207 [MH]þ, Observed 455.0208 m/z [MH]þ.
and the mixture was placed at 4 ꢀC for 4 h. The mixture was then
filtered and washed with cold MeOH then dried in a vacuum des-
iccator, giving the desired product as an orange solid (2.31 g, 41%);
mp: 153 ꢀC (mp: 156.8e158.9 ꢀC);15 1H NMR (400 MHz, CDCl3):
4.23. 4,4ʺ,5,5ʺ-Tetrakis(ethoxycarbonyl)-[2,20:60,2ʺ-terpyr-
d 6.46e6.56 (2H, m, Fur-H), 6.80, (2H, d, Fur-H), 7.31 (2H, d, Fur-H),
idine]-40-carboxylic acid (25)
7.58 (2H, d, CH), 7.63 (2H, d, CH). LR-MS 243.0 m/z [MH]þ.
Tetraethyl-40-(furan-2-yl)-[2,20:60,2ʺ-terpyridine]-4.4ʺ,5,5ʺ-tet-
racarboxylate (21) (4.00 g, 6.81 mmol) was added to a 2:1 mixture
of BuOH/water (90 mL) then potassium permanganate (10.76 g,
4.26. (Tetraethyl-40,4ʺ-di(furan-2-yl)-[2,20:60,2ʺ:6ʺ,2000-qua-
terpyridine]-4,4000,5.5000-tetracarboxylate (28)
t
68.08 mmol) was added in portions over a 10 min period. The
resulting mixture was left to stir at 70 ꢀC overnight. The reaction
mixture was treated with a saturated solution of sodium meta-
bisulphite until all residual purple coloration had been removed
then the mixture was filtered through a Celite pad to remove the
spent MnO2 and was washed with water (2ꢂ50 mL) and warm
acetone (2ꢂ50 mL). The solution was concentrated to half of the
original volume then pH adjusted to 2 using 1 M H2SO4 and the
resulting precipitate was collected by suction filtration, giving the
desired compound 25 as a white solid (2.70 g, 70% yield); mp:
A mixture of pyridinium salt 8 (2.00 g (45% purity), 1.91 mmol),
(1E,5E)-1,6-difurylhexa-1,5-diene-3,4-dione (231.8 mg, 0.96 mmol)
and ammonium acetate (1.48 g, 19.14 mmol) dissolved in dry eth-
anol (10 mL) was heated to reflux for 3 h under argon. After being
left to standing at room temperature overnight the mixture was
filtered and washed with ethanol to give a gray solid. The collected
solid was then dissolved in CHCl3 and triturated with heptane, with
the resultant solid collected by suction filtration then dried in
a vacuum desiccator, giving the desired product 28 as a brown solid
(220 mg, 31%); mp: 211 ꢀC (melt-decomp.); 1H NMR (400 MHz,
323 ꢀC (melt-decomp.); 1H NMR (400 MHz, DMSO-d6):
d
1.35 (m,
CDCl3): d 1.36e1.54 (12H, m, CH3), 4.40e4.60 (8H, m, CH2),
12H, CH3ꢂ4), 4.35e4.43 (m, 8H, OCH2ꢂ4), 8.76 (s, 2H, ArH), 8.92 (s,
2H, ArH), 9.14 (s, 2H, ArH). 13C NMR (100 MHz, CDCl3): 13.7, 13.9,
62.0, 62.2, 118.8, 121.6, 125.2, 141.4, 150.1, 154.4, 157.1, 164.5, 165.5,
165.6. Accurate Mass (ESI): C28H28N3O10dCalculated m/z for [MH]þ
566.1769, Observed m/z [MH]þ 566.1764. Selected IR bands (cmꢁ1):
3148 w, 2986 w, 2938 w, 2905 w, 1723 versus, 1591 s, 1549 s, 1466
m, 1444 w, 1386 w, 1367 s, 1266 versus, 1229 s, 1204 s, 1173 m, 1128
s, 1116 versus, 1088 m, 908 m, 882 s, 773 s, 690 versus, 677 m. Anal.
calcd for C28H27N3O10: C, 59.47; H, 4.81; N, 7.43. Found C, 58.47; H,
4.65; N, 7.07.
6.54e6.64 (2H, m, Fur-H), 7.17 (2H, d, Fur-H), 7.66 (2H, s, Fur-H),
8.87 (6H, t, Py-H), 9.09 (2H, s, Py-H). 13C NMR (100 MHz, CDCl3):
d
14.2, 14.3, 62.2, 62.5, 109.7, 112.4, 116.4, 116.5, 119.5, 124.9, 139.8,
142.0, 144.2, 150.4, 151.8, 154.3, 156.1, 159.2, 165.4, 167.0. MS (ESI):
Calculated m/z for C40H34N4O10 [M]þ 730.2, observed m/z [M]þ
730.1.
4.27. 40,4ʺ-Di(furan-2-yl)-[2,20:60,2ʺ:6ʺ,2000-quaterpyridine]-
4.4000,5.5000-tetracarboxylic acid (29)
Tetraethyl 40,4ʺ-di(furan-2-yl)-[2,20:60,2ʺ:6ʺ,2000-quaterpyridine]-
4.4000,5.5000-tetracarboxylate (115 mg, 0.27 mmol) and lithium hy-
droxide monohydrate (225 mg, 5.38 mmol) were combined in 10 mL
of THF/water (1:1 ratio) and the resulting mixture was heated to
reflux whilst stirring for 6 h. The reaction mixture was cooled to
room temperature and THF was removed by rotary evaporation. The
solution was adjusted to pH 2 with 1 M H2SO4 resulting the forma-
tion of a gel. Additional amounts of water were added (20 mL) and
the resulting mixture was stirred with a plastic spatula (not nickel
metal) to break up the gel. The mixture was heated to 60e70 ꢀC on
a rotary evaporator then hot filtered to remove the precipitated solid.
The solid was collected by filtration, washed with cold water
(3ꢂ20 mL, pH 2) and dried to give 29 as a brown solid (95 mg, 95%);
4.24. [2,20:60,2ʺ-Terpyridine]-4,40,4ʺ,5,5ʺ-pentacarboxylic acid
(26)
4,4ʺ,5,5ʺ-Tetrakis(ethoxycarbonyl)-[2,20:60,2ʺ-terpyridine]-40-
carboxylic acid (25) (5.5 g, 9.73 mmol) and lithium hydroxide
monohydrate (4.08 g, 97.25 mmol) were combined in 160 mL of
THF/water (1:1 ratio) and the resulting mixture was heated to
reflux whilst stirring for 6 h. The reaction mixture was cooled to
room temperature and THF was removed by rotary evaporation.
The solution was adjusted to pH 2 with 1 M H2SO4 resulting the
formation of a gel. Additional amounts of water were added
(100 mL) and the resulting mixture was stirred and heated to
70e80 ꢀC on a rotary evaporator for 20 min then immediately hot
filtered collect the precipitated solid. The solid was collected by
filtration, washed with cold water (3ꢂ20 mL) and dried under high
vacuum to give 26 as a white solid (2.66 mg, 60% yield); mp: 325 ꢀC
mp: >240 ꢀC (decomp.); 1H NMR (400 MHz, CDCl3):
d 6.81 (2H, q,
Fur-H), 7.60 (2H, d, Fur-H), 8.03 (2H, s, Fur-H), 8.82 (2H, d, Py-H) 8.87
(2H, s, Py-H) 8.89 (2H, d, Py-H), 9.2 (2H, s, Py-H). 13C NMR (100 MHz,
DMSO-d6): d 110.9, 112.8, 115.4, 115.5, 118.5, 125.7, 139.3, 143.0, 145.3,
(melt-decomp.); 1H NMR (400 MHz, DMSO-d6):
d
8.75 (s, 2H, ArH),
149.9, 150.5, 154.2, 155.4, 157.4, 166.4, 167.8. Accurate Mass (ESI):
8.96 (s, 2H, ArH), 9.14 (s, 2H, ArH). 13C NMR (100 MHz, CDCl3):
118.67, 121.5, 126.2, 141.3, 143.2, 150.2, 155.0, 156.9, 165.7, 166.3,
167.7. Accurate Mass (ESI): C20H12N3O10 ‒ Calculated m/z [MH]þ
454.0517, Observed m/z [MH]þ 454.0516. Selected IR bands (cmꢁ1):
2965 br m, 2662 br w, 2526 br m, 1700 versus, 1593 s, 1553 s, 1483
w, 1420 s, 1374 m, 1340 m, 1279 m, 1260 s, 1180 m, 1128 m, 1092 s,
961 m, 926 m, 908 s, 773, 804 m, 777 m, 725 m, 688 m, 660 s. Anal.
calcd for C20H13N3O11 (ligand þ 1H2O): C, 50.97; H, 2.78; N, 8.92.
Found C, 51.59; H, 2.96; N, 8.57.
C
32H18N4NaO10 e Calculated m/z [MþNa]þ 641.0915, Observed m/z
[MþNa]þ 641.0918.
5. Crystallographic data
Crystallographic data (excluding structure factors) for the
structures in this paper have been deposited with the Cambridge
Crystallographic Data Centre as supplementary publication no.
CCDC 1056206. 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).
4.25. (1E,5E)-1,6-Difurylhexa-1,5-diene-3,4-dione (27)
To a solution of furfural (8.93 g, 92.93 mmol) and 2,3-
butanedione (2.00 g, 23.23 mmol) in MeOH (25 mL), piperidine
(395 mg, 4.65 mmol) and glacial acetic acid (279.02 mg, 2.65 mmol)
were added sequentially. The mixture was then sto reflux for 6.5 h.
Once cooled to room temperature the MeOH was removed in vacuo
Acknowledgements
The Authors would like to acknowledge the Australian Growth
Partnership (AGP) for funding support under a collaborative
agreement between CSIRO and Dyesol Australia.

C.P. Woodward et al. / Tetrahedron 71 (2015) 5238e5247
5247
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