New methodology for the synthesis of 3-substituted coumarins via palladium-catalyzed site-selective cross-coupling reactions

Article abstract of DOI:10.1055/s-0030-1259014

A particularly useful, easy, and concise synthesis of diversified 3-aryl coumarin was achieved using Heck coupling reactions between coumarin and aryliodides. The introduction of the aryl moiety occurred regioselective at the 3-position of the heteroaromatic ring. Georg Thieme Verlag Stuttgart - New YorK.

Full text of DOI:10.1055/s-0030-1259014

2918
LETTER
New Methodology for the Synthesis of 3-Substituted Coumarins
via Palladium-Catalyzed Site-Selective Cross-Coupling Reactions
3
-Substituted C
é
oumarins
r
via Pd-C
g
atalyzed Rea
i
ctionso Martins,a Paula S. Branco,b María C. de la Torre,c Miguel A. Sierra,d António Pereira*a
a
Centro de Química, Departamento de Química,, Universidade de Évora, 7000-671 Évora, Portugal
Fax +351(266)745303; E-mail: amlp@uevora.pt
b
REQUIMTE, Departamento de Química, Faculdade de Ciências e Tecnologia, FCT, Universidade Nova de Lisboa,
2829-516 Caparica, Portugal
Instituto de Química Orgánica General, Consejo Superior de Investigaciones Científicas (CSIC), 28006 Madrid, Spain
Departamento de Química Orgánica, Facultad de Química, Universidad Complutense, 28040 Madrid, Spain
c
d
Received 21 July 2010
tions which proceed stepwise and display site selectivity
for the specific introduction of a heteroaryl moiety at the
3-position of the coumarin ring.
Abstract: A particularly useful, easy, and concise synthesis of di-
versified 3-aryl coumarin was achieved using Heck coupling reac-
tions between coumarin and aryliodides. The introduction of the
aryl moiety occurred regioselective at the 3-position of the heteroar-
omatic ring.
When stoichiometric amounts of coumarin (1) and iodo-
benzene (2a) were allowed to react using Pd(OAc)2 as cat-
alyst, the insertion of the phenyl moiety occurred at the 3-
position of the coumarin ring in trace amounts (Table 1,
entry 1).
Key words: coumarin, Heck reaction, palladium, coupling reac-
tion, 3-aryl coumarin
Changing reaction conditions as the catalyst [PdCl2,
Pd2(dba)3], the base (Et3N, NaHCO3), the temperature, the
solvent (CH2Cl2, MeCN), and the presence of a phase-
transfer catalyst (Bu4NBr) revealed that the system cou-
marin (3.0 equiv), PhI (1.0 equiv), Pd(PPh3)4 (10 mol%),
MeCO2Ag (1.1 equiv), DMF, 80 °C, 72 h was the most ef-
ficient to furnish the desired aryl substituted coumarin12
3a (Scheme 1) in 52% yield while 1-bromo-4-iodoben-
zene afforded the cross-coupled product 3b18,19 in 66%
yield. (Table 1, entries 5 and 16). A slight excess of the
coumarin was needed to ensure complete conversion of
the aryl iodide. Competitive byproducts, as 1,1¢-biphenyl
or 4,4¢dibromo-1,1¢-biphenyl, were observed in particular
on those reaction that presents a reduced yield of cou-
marin. These compounds result from competitive reac-
tions between aryl iodide due to the poorer reactivity of
the coumarin ring. Unexpectedly, 3-arylcoumarins 3 were
the only monocoupled products formed. To demonstrate
the generality of this strategy, the method was applied to
other aryl iodides.20 Remarkably, electron-withdrawing as
well as electron-donating substituents on the aryliodide
are suitable partners in this process, affording the corre-
sponding monosubstituted products in fair to good yields
(Table 1, entries 17–21) although, electron-withdrawing
groups gave best results as expected. Indeed, with elec-
tron-donating groups (Table 1, entries 17 and 21) the yield
of the reaction decreased substantial which is related to
the palladium insertion step.
Coumarins, a common motif in a variety of naturally oc-
curring compounds, have attracted intense interest in re-
cent years due to their diverse pharmacological properties,
namely as anticoagulant, antimicrobial, antibacterial, an-
ticancer, anti-HIV, and antioxidant.1 Moreover, these se-
ries of compounds has outstanding optical properties as
they constitute the largest class of laser dyes for the ‘blue–
green’ region2 and are widely used as emission layers in
organic light-emitting diodes (OLED),3 optical brighten-
ers,4 nonlinear optical chromophores,5 fluorescent whit-
eners,6 as well as fluorescent labels and probes for
physiological measurement.7 Recently, coumarins have
found wide applications in the labeling8 and caging.9 An-
other feature of the coumarin derivatives is that photo-
physical and spectroscopic properties can be readily
modified by the introduction of substituents in the cou-
marin ring, giving themselves more flexibility to fit well
into various applications.10 In particular, it seems that the
presence of a heteroaryl moiety at the 3-position of the
coumarinic system induces specific activities.11,12 Al-
though there are classical methods13 for the synthesis of a
wide variety of substituted coumarins, these methods of-
ten require the use of strong acids and high temperatures.
Much effort has been paid to the development of synthetic
methods requiring the use of transition-metal catalyst.14,15
However, most of the methods rely on the construction of
the coumarin skeleton starting from halogenated sub-
strates such as iodophenols and iodoarenes.14,16 Methods
relying on the derivatization of the coumarin ring are rare
and limited to formerly substituted coumarins.17 Here we
proposed to use palladium-catalyzed cross-coupling reac-
Other catalyst were also tried as Pd(OCOCF3)2, Pd2(dba)3,
and PdCl2 (results not shown) but the results were disap-
pointing. The introduction of aryl substituents at the 3-po-
sition of the coumarin scaffold extended the p-system of
the parent chromophore which was observed by a ba-
tochromic shift of the absorbance maximum from 312 nm
to around 325 nm The complete structural elucidation of
3 was accomplished using two-dimensional NMR spec-
SYNLETT 2010, No. 19, pp 2918–2922
x
x
.x
x
.2
0
1
0
Advanced online publication: 03.11.2010
DOI: 10.1055/s-0030-1259014; Art ID: G23010ST
© Georg Thieme Verlag Stuttgart · New York

LETTER
3-Substituted Coumarins via Pd-Catalyzed Reactions
2919
R
I
Pd(PPh3)4 (0.1 equiv)
MeCO2Ag (1.1 equiv)
+
R
O
O
DMF, 80 °C, 72 h
O
O
1.0 equiv
3.0 equiv
1
2a R = H
2e R = CHO
3a R = H 3e R = CHO
2b R = Br 2f R = NO2
2c R = Et 2g R = OMe
2d R = I
3b R = Br
3c R = Et
3d R = I
3f R = NO2
3g R = OMe
Scheme 1 Reagents and conditions for the synthesis of 3-phenyl coumarins
troscopy as the reported citation on literature for some of the benzo[b]furan radical cation.18 Regioselectivity is one
the coumarins described here lack strong spectroscopic of the major problems of Heck reactions and is related
evidence. Characteristic of these structures is the absence only to the coordination–insertion pathway.23 The coordi-
of the pair of doublets at d = 7.7 and 6.4 ppm of the H-4 nation–insertion step of the Heck reaction can follow two
and H-3 coumarin protons, respectively, and the appear- reaction pathways: one involves coordination of the olefin
ance of a high deshielding singlet around d = 7.8–8.0 ppm via dissociation of a neutral ligand and another involves
for H-4 as a result of the absence of the coupling with the coordination of the olefin via dissociation of the anionic
adjacent proton. Also, we observed the principal mode of ligand.
fragmentation of 3-substituted coumarins which involves
the elimination of carbon monoxide and the formation of
served when silver salts were used as bases (and as se-
Taking into consideration that the best results were ob-
Table 1 Yields and Reaction Conditions for the Synthesis of 3-Arylcoumarins
I
Entry
Catalyst (equiv)
Base (equiv)
Solvent
Ph3P
Temp (°C) Time (h) Yield (%)
O
O
R
1
1
2
1.0
2a
2a
2a
2a
2a
2b
2b
2b
2b
2b
2b
2b
2b
2b
2b
2b
2c
2d
2e
2f
Pd(OAc)2 (0.1)
Pd(OAc)2 (0.1)
Pd(OAc)2 (0.1)
Pd(PPh3)4 (0.1)
Pd(PPh3)4 (0.1)
Pd(OAc)2 (0.1)
Pd(OAc)2 (0.1)
Pd(OAc)2 (0.1)
Pd(OAc)2 (0.1)
Pd(OAc)2 (0.1)
Pd(OAc)2 (0.02)
Pd(OAc)2 (0.02)
Pd(OAc)2 (0.02)
Pd(OAc)2 (0.1)
Pd(OAc)2 (0.1)
Pd(PPh3)4 (0.1)
Pd(PPh3)4 (0.1)
Pd(PPh3)4 (0.1)
Pd(PPh3)4 (0.1)
Pd(PPh3)4 (0.1)
Pd(PPh3)4 (0.1)
NaOAc (2.0)
NaOAc (2.0)
AgOAc (1.1)
NaOAc (4.0)
AgOAc (1.1)
AgOAc (1.1)
NaOAc (1.1)
AgOAc (1.1)
AgOAc (1.1)
AgOAc (1.1)
AgOAc (1.1)
AgOAc (1.1)
AgOAc (1.1)
AgOAc (1.1)
NaOAc (1.1)
AgOAc (1.1)
AgOAc (1.1)
AgOAc (1.1)
AgOAc (1.1)
AgOAc (1.1)
AgOAc (1.1)
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
0.4
0.4
0.4
–
60
100
100
100
80
24
52
24
24
72
24
24
48
24
24
24
48
43
24
24
72
72
72
72
72
72
trace
32
3.0
3.0
3.0
3.0
1.1
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3
10
4
35
5
–
52
6
0.4
0.4
0.4
0.4
2.0
0.08
0.08
0.08
–
100
100
100
80
trace
trace
15
7
8
9
21
10
11
12
13
14
15
16
17
18
19
20
21
100
100
80
trace
35
13
75
33
100
100
80
trace
trace
66
–
–
–
80
28
–
80
5419,21
74
–
80
–
80
8122
12
2g
–
80
Synlett 2010, No. 19, 2918–2922 © Thieme Stuttgart · New York

2920
S. Martins et al.
LETTER
thyl trans-cinnamate derivatives 4a and 4b we observed
the exclusive formation of the 3-phenyl monocoupled
products 5a24 and 5b25 in 58% and 11% yield, respective-
ly. The difference in the yield is probably related to stereo
constraint due to the ortho-substitution. The spectroscopic
data for 5a agree with those from literature.24 On the other
hand, the reaction with the Z-derivative 6, a mimetic
structure of the coumarin ring produces a mixture of 3-
and 2-phenyl monocoupled products 7 and 8 in the pro-
portion of 1:3, respectively (obtained by 1H NMR integra-
tion).25,26 The spectroscopic data26 reported for compound
8 do not match our result. Indeed the expected downfield
signal for H-3 (present in our case at d = 7.95 ppm) is ab-
sent on the report spectra. Our spectra is in harmony with
that reported for the E-isomer of compound 8 that presents
the H-3 signal at d = 8.16 ppm.27 The reported data for
compounds 728 and 5b25 does not include NMR assign-
ment which prevent us from a comparison with published
data. Even so, the presence of a singlet near d = 6.2 and
6.4 ppm, respectively, for the H-2 proton is characteristic
for these types of compounds. In view of these results the
stereoelectronic properties of the alkene/ester group in the
coumarin ring have a definitive role on the coordination–
insertion step of the group at C-3 position. Additional
studies at the level of DFT (density functional theory) are
currently under way.
L
L
I
L
L
– I
+ I
L
L
Pd
Pd
Pd
O
Ar
Ar
Ar
O
L
L
Pd
O
Ar
Ar
Ar
O
– H
O
O
O
O
Scheme 2 Proposed mechanism for the synthesis of 3-aryl couma-
rins
questring agents for the halide anions), the last pathway
mentioned above should be involved (Scheme 2). When
the reaction proceeds via dissociation of the counterion as
shown, electronic factors predominate. In fact, the coordi-
nation of the p-system with the cationic complex deter-
mines an increase of the polarization, and selective
migration of the aryl moiety onto the carbon with the
highest charge density is observed. With cyclic systems as
is the case of the coumarin molecule, a syn-b-hydride
elimination–dissociation process cannot be involved be-
cause there is no syn-b-hydrogen and rotation cannot oc-
cur. The efficiency of the process is related to the
stabilized carbocation formed after dissociation of the cat-
alytically active species of 14-electron complex L2P(0). In
view of our results we investigated the cross-coupling of
methyl cinnamate derivatives and iodobenzene when the
aforementioned reaction conditions were applied
(Scheme 3). When the reaction was carried out with me-
The success of this method opens a new way to 3-substi-
tuted coumarins. Armed with this encouraging strategy,
we briefly explored this synthetic methodology to the syn-
thesis of diversified coumarins. For example a tandem
Heck reaction of iodine derivative 3d with methyl acry-
late, in the same conditions, provide the corresponding
Pd(PPh3)4 (0.1 equiv)
MeCO2Ag (1.1 equiv)
I
CO2Me
+
CO2Me
DMF, 80 °C, 72 h
R
only product
R
1.0 equiv
3.0 equiv
5a R = H
4a R = H
4b R = OMe
5b R = OMe
Pd(PPh3)4 (0.1 equiv)
MeCO2Ag (1.1 equiv)
I
+
+
CO2Me
OMe
DMF, 80 °C, 72 h
CO2Me
OMe
CO2Me
OMe
1.0 equiv
3.0 equiv
7
8
6
1:3
Scheme 3 Reaction of methyl cinnamate derivatives and iodobenzene
I
CO2Me
Pd(PPh3)4 (0.1 equiv)
MeCO2Ag (1.1 equiv)
+
CO2Me
DMF, 80 °C, 72 h
96%
O
O
O
O
3.0 equiv
1.0 equiv
9
3d
Scheme 4 Synthesis of (E)-methyl-3-[4-(coumarin-3-yl)phenyl]acrylate (9)
Synlett 2010, No. 19, 2918–2922 © Thieme Stuttgart · New York

LETTER
3-Substituted Coumarins via Pd-Catalyzed Reactions
2921
Joo, T.; Kim, J. S. J. Am. Chem. Soc. 2009, 131, 2008.
(E)-methyl-3-[4-(coumarin-3-yl)phenyl]acrylate
9
in
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96% yield (Scheme 4). This promising results display the
potential of this strategy for numerous synthetic applica-
tions to diversified coumarins.
In summary, this new protocol considerably facilitates the
concise synthesis of diversified 3-arylated coumarins by a
Heck coupling reaction that proceeds essentially at C-3
rather than at C-4 position on the coumarin ring. We also
have demonstrated that sequential Heck reactions will en-
large the scope of this work with the synthesis of new and
promising systems. To the best of our knowledge these
types of reactions relying on the derivatization of the cou-
marin ring have not been widely published.
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett.
Acknowledgment
We thank Fundação para a Ciência e Tecnologia (Lisbon, Portugal),
PRAXIS program, for partial financial support FCOMP-01-0124-
FEDER-007448.
(12) Kabeya, L. M.; de Marchi, A. A.; Kanashiro, A.; Lopes,
N. P.; da Silva, C.; Pupo, M. T.; Lucisano-Valima, Y. M.
Bioorg. Med. Chem. 2007, 15, 1516.
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Under a nitrogen atmosphere, a mixture of coumarin (146
mg, 1.0 mmol, 3.0 equiv), aryl iodide (1.0 equiv), Pd(PPh3)4
(10 mol%), and MeCO2Ag (1.1 equiv) in DMF (2.0 mL) was
stirred at 80 °C for a period of 72 h. The reaction mixture
was diluted with EtOAc and washed with H2O. The organic
layer was dried (Na2SO4), filtered, and concentrated under
vacuum. The residue was purified by flash column chroma-
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3-(4-Bromophenyl)coumarin (3b)
1H NMR (400 MHz, CDCl3): d = 7.31 (1 H, dd, J = 7.5, 7.4
Synlett 2010, No. 19, 2918–2922 © Thieme Stuttgart · New York

2922
S. Martins et al.
LETTER
Hz, H-6), 7.37 (1 H, d, J = 8.5 Hz, H-8), 7.53–7.61 (6 H, m,
J = 8.8 Hz, H-2¢, H-6¢), 7.95 (1 H, s, H-4), 8.33 (2 H, d,
H-5, H-7, H-2¢, H-3¢, H-5¢, H-6¢), 7.82 (1 H, s, H-4). 13
C
J = 8.8 Hz, H-3¢, H-5¢). 13C NMR (100 MHz, CDCl3): d =
116.7 (C-8), 119.1 (C-4a), 123.7 (C-3¢, C-5¢), 124.9 (C-6),
126.1 (C-3), 128.4 (C-5), 129.5 (C-2¢, C-6¢), 132.6 (C-7),
141.0 (C-1¢), 141.6 (C-4), 147.8 (C-4¢), 153.9 (C-8a), 159.8
(C-2). MS (EI+): m/z (%) = 267 [M]+(100), 239 [M – CO]+
(17.4). HRMS (EI+): m/z calcd for C15H9NO4 [M]+:
267.0532; found: 267.0533.
NMR (100 MHz, CDCl3): d = 116.6 (C-8), 119.5 (C-4a),
123.2 (C-4¢), 124.6 (C-6), 127.2 (C-3), 128.0 (C-5), 130.1
(C-2¢, C-6¢), 131.7 (C-7, C-3¢, C-5¢), 133.6 (C-1¢), 139.9
(C-4), 153.6 (C-8a), 160.3 (C-2). MS (EI+): m/z (%) = 299
[M79Br – H]+(100), 273 [M81Br – CO]+(71). HRMS (EI+):
m/z calcd for C15H9BrO2 [M]+: 299.9786; found: 299.9790.
3-(4-Ethylphenyl)coumarin (3c)
3-(4-Methoxyphenyl)coumarin (3g)
1H NMR (400 MHz, CDCl3): d = 1.27 (3 H, t, J = 7.6 Hz,
CH2CH3), 2.70 (2 H, q, J = 7.6 Hz, CH2CH3), 7.29 (2 H, d,
J = 8.2 Hz, H-3¢, H-5¢), 7.30 (1 H, dd, J = 7.7, 7.7 Hz, H-6),
7.37 (1 H,d, J = 8.0 Hz, H-8), 7.52 (1 H, dd, J = 8.0, 7.7 Hz,
H-7), 7.54 (1 H, d, J = 7.7 Hz, H-5), 7.63 (2 H, d, J = 8.2 Hz,
H-2¢, H-6¢), 7.80 (1 H, s, H-4). 13C NMR (100 MHz, CDCl3):
d = 15.5 (CH2CH3), 28.7 (CH2CH3), 116.4 (C-8), 119.8 (C-
4a), 124.4 (C-6), 127.8 (C-5), 128.0 (C-3¢, C-5¢), 128.5 (C-3,
C-2¢, C-6¢), 131.2 (C-7), 132.0 (C-1¢), 139.2 (C-4), 145.2 (C-
4¢), 153.4 (C-8a), 160.7 (C-2). MS (EI+): m/z (%) = 250 [M]+
(99.5), 235 [M – CH3]+(100), 222 [M – CO]+ (17.87), 207
[M – CO2 + H]+(77). HRMS (EI+): m/z calcd for C17H14O2
[M]+: 250.0994; found: 250.0993.
1H NMR [400 MHz, CO(CD3)2]: d = 3.84 (3 H, s, OCH3),
7.01 (2 H, d, J = 8.0 Hz, H-3¢, H-5¢), 7.36 (2 H, m, H-6, H-
8), 7.59 (1 H, dd, J = 7.8, 7.5 Hz, H-7), 7.75 (3 H, m, H-5,
H-2¢, H-6¢), 8.07 (1 H, s, H-4). 13C NMR [100 MHz,
CO(CD3)2]: d = 55.6 (OCH3), 114.5 (C-3¢, C-5¢), 116.7 (C-
8), 120.9 (C-4a), 126.0 (C-6, C-3), 129.1 (C-5, C-1¢), 130.7
(C-2¢, C-6¢), 131.9 (C-7), 139.4 (C-4), 154.0 (C-8a), 160.5
(C-4¢), 161.1 (C-2). MS (EI+): m/z (%) = 252 [M]+(100), 209
[M – CO + CH3]+ (53.2). HRMS (EI+): m/z calcd for
C16H12O3 [M]+: 252.0786; found: 252.0785.
(E)-Methyl 3-[4-(coumarin-3-yl)phenyl]acrylate (9)
1H NMR (400 MHz, CDCl3): d = 3.82 (3 H, s, OCH3), 6.49
(1 H, d, J = 16.0 Hz, CHCHCO), 7.32 (1 H, dd, J = 7.6, 7.6
Hz, H-6), 7.38 (1 H, d, J = 8.3 Hz, H-8), 7.55 (1 H, dd,
J = 8.3, 7.6 Hz, H-7), 7.56 (1 H, d, J = 7.6 Hz, H-5), 7.60 (2
H, d, J = 8.1 Hz, H-3¢, H-5¢), 7.72 (1 H, d, J = 16.0 Hz,
CHCHCO), 7.76 (2 H, d, J = 8.4 Hz, H-2¢, H-6¢), 7.87 (1 H,
s, H-4). 13C NMR (100 MHz, CDCl3): d = 51.8 (OCH3),
116.5 (C-8), 118.6 (CHCHCO), 119.5 (C-4a), 124.6 (C-6),
127.5 (C-3), 128.0 (C-5), 128.1 (C-3¢, C-5¢), 129.0 (C-2¢, C-
6¢), 131.8 (C-7), 134.8 (C-4¢), 136.5 (C-1¢), 140.1 (C-4),
144.0 (CHCHCO), 153.6 (C-8a), 160.3 (C-2), 167.3
(CHCHCO). MS (EI+): m/z (%) = 306 [M]+ (63.15). HRMS
(EI+): m/z calcd for C19H14O4 [M]+: 306.0892; found:
306.0896.
3-(4-Iodophenyl)coumarin (3d)
1H NMR (400 MHz, CDCl3): d = 7.31 (1 H, dd, J = 7.1, 7.1
Hz, H-6), 7.37 (1 H, d, J = 8.2 Hz, H-8), 7.46 (2 H, d, J = 7.0
Hz, H-2¢, H-6¢), 7.53–7.56 (2 H, m, H-5, H-7), 7.79 (2 H, d,
J = 7.0 Hz, H-3¢, H-5¢), 7.82 (1 H, s, H-4). 13C NMR (100
MHz, CDCl3): d = 95.0 (C-4¢), 116.5 (C-8), 119.5 (C-4a),
124.6 (C-6), 127.3 (C-3), 128.0 (C-5), 130.2 (C-2¢, C-6¢),
131.7 (C-7), 134.2 (C-1¢), 137.6 (C-3¢, C-5¢), 139.9 (C-4),
153.6 (C-8a), 160.2 (C-2). MS (EI+): m/z (%) = 347
[M]+(61), 57 [C2HO2](100). HRMS (EI+): m/z calcd for
C15H9IO2 [M]+: 347.9647; found: 347.9645.
4-(Coumarin-3-yl)benzaldehyde (3e)
1H NMR [400 MHz, CO(CD3)2]: d = 7.40 (2 H, m, H-6, H-
8), 7.67 (1 H, dd, J = 7.2, 6.8 Hz, H-7), 7.80 (1 H, d, J = 6.8
Hz, H-5), 8.02 (4 H, br s, H-2¢, H-3¢, H-5¢, H-6¢), 8.31 (1 H,
s, H-4), 10.10 (1 H, s, CHO). 13C NMR [100 MHz,
CO(CD3)2]: d = 116.9 (C-8), 120.5 (C-4a), 125.5 (C-6),
127.5 (C-3), 129.7 (C-5), 130.1 (C-2¢, C-6¢), 130.2 (C-3¢,
C-5¢), 133.0 (C-7), 137.4 (C-4¢), 141.8 (C-1¢), 142.4 (C-4),
154.8 (C-8a), 160.2 (C-2), 192.5 (CHO). MS (EI+): m/z (%)
249 [M – H]+ (73.71), 221 [M – CO]+ (14.6), 220 [M –
CHO]+ (17.9), 57 [C2HO2](100). HRMS (EI+): m/z calcd for
C16H10O3 [M]+ 250.0630; found: 250.0632.
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1H NMR (400 MHz, CDCl3): d = 7.36 (1 H, dd, J = 7.6, 6.8
Hz, H-6), 7.41 (1 H, d, J = 8.2 Hz, H-8), 7.60 (1 H, d, J = 7.6
Hz, H-5), 7.61 (1 H, dd, J = 8.2, 6.8 Hz, H-7), 7.92 (2 H, d,
Synlett 2010, No. 19, 2918–2922 © Thieme Stuttgart · New York