A selective synthesis of 4-bromoisoquinoline and 4-bromoisquinolone

Article abstract of DOI:10.3184/174751913X13744287541024

2-Alkynyl benzyl azides smoothly underwent an electrocyclic reaction catalysed by palladium to selectively afford either 4-bromoisoquinoline and 4-bromoisquinolones under different conditions. 4-Bromoisoquinoline was synthesised in the presence of PdBr

Full text of DOI:10.3184/174751913X13744287541024

556 RESEARCH PAPER
SEPTEMBER, 556–558
JOURNAL OF CHEMICAL RESEARCH 2013
A selective synthesis of 4-bromoisoquinoline and 4-bromoisquinolone
Hong-Ping Zhang*, Hong-Yan Li and Hong-Fang Xiao
Department of Biology and Chemistry Engineering, Shaoyang University, Shaoyang 422000, P. R. China
2-Alkynyl benzyl azides smoothly underwent an electrocyclic reaction catalysed by palladium to selectively afford
either 4-bromoisoquinoline and 4-bromoisquinolones under different conditions. 4-Bromoisoquinoline was synthe-
sised in the presence of PdBr2/CuBr2/LiBr in MeCN, and 4-bromoisoquinolone was selectively produced with
PdBr2/CuBr2/HOAc in CH2ClCH2Cl. A bromine was introduced into the products which makes the methodology more
attractive for organic synthesis.
Keywords: 2-alkynyl benzyl azides, selectivity, cyclisation, isoquinoline, isoquinolone
Isoquinolines1–3 and isoquinolin-1(2H)-ones4–7 are found in
many natural products and synthetic pharmaceuticals and
a number of efficient and selective methods have been
developed for their synthesis.8–13 Although the classical routes
to isoquinolines involve the Bischler-Napieralski14 or Pictet-
Spengler reaction,15 these approaches often have drawbacks,
for example, the use of strong acids and elevated temperature,16
which are not suitable for sensitive substrates. This has led to
the development of new methodology. In recent years, new
transition metal catalysed reactions have been developed for
the synthesis of substituted isoquinolines from phenylacety-
lene substrates.17–24 These reactions have proved to be extremely
efficient in the synthesis of a wide variety of 3,4-substituted
isoquinolines.
There are also a number of approaches to the synthesis of
isoquinolin-1(2H)-ones reported in the literature,25–29 but most
of them suffer from a poor precursor scope and less structural
diversity. Recently, the transition metal catalysed synthesis of
substituted isoquinolin-1(2H)-one derivatives has received
attention, for example, Pd-catalysed multicomponent synthe-
sis of oxazoline and benzoxazole,30 Pd-catalysed synthesis of
4-aminophthalazin-1(2H)-one,31 and synthesis of quinazolino-
[3,2-a]-quinazolines via a palladium-catalysed three-component
reaction.32
annulation of 1a to afford product 4-bromo-3-phenylisoquino-
line (2a). However, unexpectedly 4-bromo-3-phenylisoquino-
lin-1(2H)-one (3a) was isolated (entry 2). During the course of
our investigation, we observed that an identical yield of 3a was
obtained by further decreasing the additive loadings of LiBr to
0.2 equiv, coupled with a decrease in the yield of 2a (entries 3
and 4). Encouraged by these results, we decided to focus on
the development of appropriate protocols for reagent-based
synthesis of 4-bromoisoquinolines and 4-bromoisoquinolin-
1(2H)-one. The effect of additive on the reaction was exam-
ined, employing 5 mol% of PdBr2 and 3 equiv. of CuBr2 in
CH2ClCH2Cl (entries 5–8). After a series of trials, 2 equiv. of
LiBr as additive and MeCN as solvent provided the highest
yield of 4-bromo-3-phenylisoquinoline (2a) (method A), and 2
equiv. of HOAc as additive and a mixture CH2ClCH2Cl/
H2O(50:1) as solvent gave the highest yield of 4-bromo-3-phe-
nylisoquinolin-1(2H)-one (3a) (method B) (entries 1 and 7).
As shown in Table 2, the scope of 2-alkynylbenzyl azides 2
for the halopalladation cyclisation reaction was explored under
the standard reaction conditions. In the case of the method
Table 1 Annulation of 2-phenylethynyl benzyl azide (1a) in
presence of PdBr2a
We recently reported the first successful application of the
halopalladation cyclisation of alkynes with azides to the
synthesis of 4-haloisoquinolines and 3-haloindoles.33 As part
of our continuing studies on the efficient preparation of
biologically important heterocycles,34 we describe herein
strategies for the synthesis of 4-bromoisoquinolines and
4-bromoisoquinolin-1(2H)-one via a reagent-based cyclisation
of 2-alkynyl benzyl azides (Scheme 1). To the best of our
knowledge, there are no reports of an efficient synthesis of
4-bromoisoquinolines and 4-bromoisoquinolin-1(2H)-one
using these intermediates.
In our earlier studies with 2-phenylethynyl benzyl azide
(1a), we have devised route for the preparation of bromoiso-
quinolines in the presence of 5 mol% of PdBr2, 3 equiv. of
CuBr2, and 2 equiv. of LiBr as additive in MeCN.33 As shown
in Table 1, exploratory experiments revealed that treatment
of 2-phenylethynyl benzyl azide with PdBr2/CuBr2/LiBr in
CH2ClCH2Cl was unsuccessful for the bromopalladation
Entry
Additive/equiv.
Solvent
Time/h Yield/%b
2a 3a
65 Trace
1
2
3
4
5
6
7
8
LiBr (2)
LiBr (2)
LiBr (1)
LiBr (0.2)
HOAc (1)
HOAc (2)
MeCN
26
26
26
26
22
22
22
22
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
31
20
12
18
25
27
Trace 40
Trace 70
Trace 83
Trace 68
HOAc (2)/H2O (0.1 mL) ClCH2CH2Cl
HOAc (2)/H2O (0.5 mL) ClCH2CH2Cl
a Reaction conditions: 1a (0.3 mmol), PdBr2 (5 mol%), CuBr2
(3 equiv.) and additive in the indicated solvent (5 mL) at 80 °C.
b Isolated yield.
Scheme 1 Synthesis of bromoisoquinoline and bromoisoquinolone.
* Correspondent. E-mail: zhp4901@sina.com

JOURNAL OF CHEMICAL RESEARCH 2013 557
Experimental
A-based cyclisation, substrates 1b, 1c and 1h–j successfully
gave the corresponding bromoisoquinolines. Azides with an
electron-donating group (compounds 1b and 1c) were sub-
jected to method A, and gave 3-substituted 4-bromoisoquino-
lines 2b and 2c in good yields (entries 1 and 2). The aliphatic
alkynyl benzyl azides 1h–j provided good yields of 3-substi-
tuted 4-bromo-1,2-dihydroisoquinoline 4h–j under the same
conditions (entries 7–9). For example, under the reaction con-
dition of 5 mol% of PdBr2, 3 equiv. of CuBr2, and 2 equiv. of
LiBr, a 78% yield of 4-bromo-3-(4-methoxyphenyl)isoquinoli
ne (2b) was obtained, and a 68% yield of 4-bromo-3-octyl-1,2-
dihydroisoquinoline (4h) was obtained (entries 2 and 7).
In the presence of 5 mol% of PdBr2, 3 equiv. of CuBr2, and
2 equiv. of HOAc, palladium-catalysed annulations of 2-
alkynylbenzyl azides (1b–g) provided moderate to good yields
of the corresponding 3-substituted 4-bromoisoquinolin-1(2H)-
one (3b–g). As shown in Table 2, substituents affected the
selectivity of reactions. The results showed that electron-
withdrawing groups in the aromatic ring connecting with
alkyne favored the desired reaction, but an electron-donating
group suppressed it (entries 1–3). For example, the substrate
1d bearing a nitro group gave 81% yield of the target product
3d exclusively in the presence of PdBr2 and CuBr2, whereas
the substrate 1c bearing a methoxyl group provided the desired
product 3c in 42% yield (entries 1 and 3). The 2-heteroaryle-
thynylbenzyl azides (1e) successfully underwent the cyclisa-
tion to afford the dibromo-addition product 3e in moderate
yield (entry 4). Substrates with the aromatic ring bearing a
substituted group 1f and 1g were compatible with annulations,
provided products 3f and 3g in 57% and 59% yield, respec-
tively. (entries 5 and 6). However, when the aliphatic alkynyl
benzyl azides (1h–j) were subjected to method B the expected
products were not obtained (entries 7–9).
CAUTION: Many low-molecular-weight azides are explosive. In
this lab, no problems have been encountered, but great caution
should be exercised when heating compounds of this type, espe-
cially neat. The reactions described here were run on only a few
grams; an increase in the scale of these reactions will decrease the
efficiency of heat dissipation and explosions may result.
NMR spectra were obtained with an INOVA-400 (Varian) and AMX-
400 (Bruker) spectrometer, operating at 400 MHz (1H NMR) and
100 MHz (13C NMR). TMS was used an internal standard and CDCl3
was used as the solvent. Mass spectrometric analysis was performed
on a GC-MS instrument (Shimadzu GCMS-QP2010 plus). Melting
points were recorded on an Electrothermal type 9100 melting point
apparatus. All melting points are uncorrected.
Synthesis of 3-substituted 4-bromoisoquinolines (2) and 3-substituted
4-bromo-1,2-dihydroisoquinoline (4); typical procedure
A mixture of the 2-alkynyl benzyl azide 1 (0.3 mmol), PdBr2 (5 mol
%), CuBr2 (3 equiv.), LiBr (2 equiv.) and CH3CN (5 mL) was stirred
at the indicated reaction temperature until consumption of the starting
material was complete as judged by TLC and GC analysis. Then the
mixture was filtered and evaporated, the residue was purified by flash
column chromatography to afford 2 and 4 (hexane/ethyl acetate).
Synthesis of 3-substituted 4-bromoisoquinoline 1(2H)-ones (3);
typical experimental procedure
A mixture of the 2-alkynyl benzyl azides 1 (0.3 mmol), PdBr2 (5 mol %),
CuBr2 (3 equiv.), HOAc (2 equiv.), H2O (0.1 mL) and ClCH2CH2Cl
(5 mL) was stirred at the indicated reaction temperature until con-
sumption of the starting material was complete as judged by TLC and
GC analysis. Then the mixture was filtered and evaporated, the
residue was purified by flash column chromatography to afford 3
(hexane/ethyl acetate).
4-Bromo-3-phenylisoquinoline (2a):35,36 Colourless oil; 1H NMR
(500 MHz, CDCl3) δ 9.23 (s, 1H), 8.33 (d, J = 8.5 Hz, 1H), 8.01 (d,
J = 8.0 Hz, 1H), 7.84 (t, J = 8.0 Hz, 1H), 7.73 (d, J = 8.5 Hz, 2H), 7.68
(t, J = 8.0 Hz, 1H), 7.50 (t, J = 7.8 Hz, 2H), 7.45 (t, J = 8.0 Hz, 1H);
13C NMR (125 MHz, CDCl3) δ 152.3, 151.1, 140.7, 136.0, 131.9,
129.9, 128.6, 128.3, 127.9, 127.7, 127.0, 118.3; LRMS (EI, 70 eV)
m/z (%): 285 (M++2, 23), 283 (M+, 25), 204 (-Br, 100).
In summary, an efficient selective synthesis method has
been developed for the synthesis of 4-bromoisoquinolines and
4-bromoisoquinolin-1(2H)-ones. In the presence of PdBr2,
CuBr2, and additive, a variety of 2-alkynyl benzyl azides
smoothly underwent the halopalladation cyclisation to afford
the corresponding 4-bromoisoquinolines and 4-bromoiso-
quinolin-1(2H)-ones in moderate to good yields. Most impor-
tantly, these isoquinolines and isoquinolones with a halo group
would be useful introduction of various functional groups,
which should be significant for drug discovery.
4-Bromo-3-o-tolylisoquinoline (2b): Orange solid, m.p. 110.2–
1
111.6 °C (uncorrected); H NMR (500 MHz, CDCl3) δ 9.24 (s, 1H),
8.30 (d, J = 8.5 Hz, 1H), 8.03 (d, J = 8.5 Hz, 1H), 7.85 (t, J = 8.3 Hz,
1H), 7.70 (t, J = 8.0 Hz, 1H), 7.38–7.30 (m, 4H), 2.16 (s, 3H); 13C
NMR (125 MHz, CDCl3) δ 153.3, 151.0, 140.7, 135.9, 135.6, 131.8,
130.1, 129.0, 128.6, 128.4, 127.9, 127.8, 126.6, 125.6, 119.5, 19.5;
LRMS (EI, 70 eV) m/z (%): 299 (M++2, 8), 297 (M+, 8), 218 (-Br,
100); HRMS (EI) for C16H1279BrN (M+): calcd 297.0153, found
297.0150.
Table 2 Effect of 2-alkynylbenzyl azides on the reaction selectivitya
Entry
1d
Time/h
Method A
Method B
Yield/%c,d
Ratiob (2/3)
Yield/%c,d
Ratiob (2/3)
1
2
3
4
5
6
7
8
9
R2 = H, R1 = o-MeC6H4 (1b)
R2 = H, R1 = p-MeOC6H4 (1c)
R2 = H, R1= p-NO2C6H4 (1d)
R2 = H, R1 = 2-thiophene (1e)
R2 = 3-MeO, R1 = C6H5 (1f)
R2 = 3, 4-dimethoxy, R1 = C6H5 (1g)
R2 = H, R1 = n-C8H17 (1h)
20
20
34
22
24
24
22
22
24
88/12
90/10
72 (2b)
78 (2c)
Trace
Trace
Trace
Trace
68 (4h)
62 (4i)
60 (4j)
45/55
39/61
100/0
100/0
100/0
100/0
0/0
38 (3b)
42 (3c)
81 (3d)
76 (3e)E
57 (3f)
59 (3g)
Trace
0/0
0/0
0/0
0/0
100/0 (4/3)
100/0 (4/3)
100/0 (4/3)
R2 = H, R1 = n-C5H11 (1i)
0/0
Trace
R2 = H, R1 = 1-cyclohexenyl (1j)
0/0
Trace
a Reaction conditions of method A: 1 (0.3 mmol), PdBr2 (5 mol%), CuBr2 (3 equiv.) and LiBr (2 equiv.) in MeCN (5 mL) 80 °C. Reaction
conditions of method B: 1 (0.3 mmol), PdBr2 (5 mol%) and HOAc (2 equiv.) in ClCH2CH2Cl (5 mL) and H2O (0.1 mL) at 80 °C.
c
1
b Calculated by using LC/MS. Isolated yield. d All of the new compounds were characterised by H NMR and 13C NMR spectra as
given in the Electronic Supplementary Information. e 3e is 4-bromo-3-(5-bromothiophen-2-yl)isoquinolin-1(2H)-one.

558 JOURNAL OF CHEMICAL RESEARCH 2013
4-Bromo-3-(4-methoxyphenyl)isoquinoline (2c): Orange solid, m.p.
30.7, 27.2, 22.4, 14.0; LRMS (EI, 70 eV) m/z (%): 281 (M++2, 2), 279
(M+, 2), 223 (57), 221 (59), 199 (16), 198 (100), 197 (15); HRMS (EI)
for C14H18BrN (M+): calcd 279.0623, found 279.0625.
1
109.8–110.9 °C (uncorrected); H NMR (500 MHz, CDCl3) δ 9.21
(s, 1H), 8.32 (d, J = 9.0 Hz, 1H), 7.98 (d, J = 3.0 Hz, 1H), 7.82 (t,
J = 8.3 Hz, 1H), 7.72 (d, J = 8.5 Hz, 2H), 7.66 (t, J = 8.0 Hz, 1H), 7.03
(d, J = 9.5 Hz, 2H), 3.88 (s, 3H); 13C NMR (125 MHz, CDCl3) δ 159.7,
152.0, 151.0, 136.1, 133.1, 131.8, 131.4, 128.4, 127.7, 127.6, 126.9,
118.0, 113.4, 55.3; LRMS (EI, 70 eV) m/z (%): 315 (M++2, 45), 313
(M+, 47), 234 (-Br, 62), 219 (41); HRMS (EI) for C16H1279BrNO (M+):
calcd 313.0102, found 313.0100.
4-Bromo-3-cyclohexenyl-1,2-dihydroisoquinoline (4j): Pale yellow
oil; 1H NMR (500 MHz, CDCl3) δ 7.44–7.36 (m, 3H), 7.28–7.26 (m,
1H), 6.06 (t, J = 7.5 Hz, 1H), 4.48–4.39 (m, 2H), 2.29–2.26 (m, 2H),
2.20–2.16 (m, 2H), 1.79–1.74 (m, 2H), 1.68–1.65 (m, 2H); 13C NMR
(125 MHz, CDCl3) δ 139.8, 137.1, 133.0, 131.8, 131.2, 129.3, 128.8,
1127.9, 124.6, 112.6, 52.0, 26.4, 25.2, 22.3, 21.6; LRMS (EI, 70 eV)
m/z (%): 291 (M++2, 26), 289 (M+, 26), 260 (-Br, 11), 259 (29), 258
(10); HRMS (EI) for C15H1679BrN (M+): calcd 289.0466, found
289.0462.
4-Bromo-3-phenylisoquinolin-1(2H)-one (3a):37 White solid, m.p.
158.4–158.6 °C (uncorrected); 1H NMR (500 MHz, CDCl3) δ 7.91 (s,
1H), 7.83 (d, J = 7.5 Hz, 1H), 7.54–7.48 (m, 5H), 7.42 (t, J = 7.5 Hz,
1H), 7.27 (t, J = 7.5 Hz, 1H), 6.64 (d, J = 8.0 Hz, 1H); 13C NMR (125
MHz, CDCl3) δ 167.1, 137.5, 135.2, 133.5, 132.2, 131.1, 130.2,130.1,
129.3, 129.1, 123.9, 122.8, 103.6; LRMS (EI, 70 eV) m/z (%): 301
(M++2, 43.8), 299 (M+, 49.1), 220 (-Br, 100.0).
Electronic Supplementary Information
1H NMR and 13C NMR spectra of the new compounds have
been deposited in the ESI available through stl.publisher.
ingentaconnect.com/content/stl/jcr/supp-data.
4-Bromo-3-o-tolylisoquinolin-1(2H)-one (3b): White solid, m.p.
172.4–172.6 °C; 1H NMR (400 MHz, CDCl3) δ 7.86 (s, 1H), 7.83 (d,
J = 7.6 Hz, 1H), 7.44–7.40 (m, 2H), 7.36–7.30 (m, 3H), 7.27–7.24 (m,
2H), 6.27 (d, J = 8.0 Hz, 1H), 2.30 (s, 3H); 13C NMR (100 MHz,
CDCl3) δ 167.2, 137.8, 136.5, 135.2, 133.9, 132.6, 131.0, 130,4,
130.2, 129.3, 128.5, 126.9, 123.9, 122.4, 102.8; LRMS (EI, 70 eV)
m/z (%): 315 (M++2, 13.1), 313 (M+, 15.8), 234 (-Br, 37.3), 216 (21.5).
HRMS (EI) for C16H1279BrNO (M+): calcd 313.0102, found 313.0100.
4-Bromo-3-(4-methoxyphenyl)isoquinolin-1(2H)-one (3c): White
The authors thank the Scientific and Technological Innovative
team of Shaoyang University (2012) for financial support.
Received 2 May 2013; accepted 24 June 2013
Paper 1301924 doi: 10.3184/174751913X13744287541024
Published online: 6 September 2013
1
solid, m.p. 168.5–168.9 °C; H NMR (400 MHz, CDCl3) δ 7.80 (s,
1H), 7.83 (d, J = 7.6 Hz, 1H), 7.45–7.41 (m, 3H), 7.30 (t, J = 7.4 Hz,
1H), 6.70 (d, J = 8.4 Hz, 2H), 6.74 (d, J = 8.0 Hz, 1H), 3.90 (s, 3H);
13C NMR (100 MHz, CDCl3) δ 167.1, 135.3, 134.3, 133.3, 132.2,
131.7, 131.1, 129.7, 129.2, 123.8, 122.8, 114.6, 104.3, 55.5; LRMS
(EI, 70 eV) m/z (%): 331 (M++2, 46.9), 329 (M+, 40.9), 250 (-Br, 100).
HRMS (EI) for C16H1279BrNO2 (M+): calcd 329.0051, found 329.0050.
4-Bromo-3-(4-nitrophenyl)isoquinolin-1(2H)-one (3d): Brown
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34 H.-P. Zhang, X.-H. Yang, P. Peng and J.-H. Li, Synthesis, 2011, 1219.
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Chem. Int. Ed., 2007, 46, 4764.
J = 7.5 Hz, 1H), 7.89 (s, 1H), 7.84 (d, J = 7.5 Hz, 1H), 7.49 (t, J =
7.5 Hz, 1H), 7.42 (t, J = 8.0 Hz, 1H), 7.09 (d, J = 3.5 Hz, 1H), 7.01 (t,
J = 5.75 Hz, 1H); 13C NMR (125 MHz, CDCl3) δ 166.9, 140.6, 136.6,
134.7, 132.7, 130.9, 130.7, 130.5, 130.3, 130.0, 124.1, 123.0, 116.3;
LRMS (EI, 70 eV) m/z (%): 387 (M++4, 8.2), 385 (M++2, 15.5), 383
(M+, 10.2), 306 (-Br, 11.6), 304 (-Br, 11.3), 225 (-2Br, 41.3). HRMS
(EI) for C13H779Br2NOS (M+): calcd 382.8615, found 382.8613.
4-Bromo-7-methoxy-3-phenylisoquinolin-1(2H)-one (3f): White
1
solid, m.p. 168.4–168.8 °C; H NMR (500 MHz, CDCl3) δ 7.90 (s,
1H), 7.58–7.51 (m, 5H), 7.33 (t, J = 2.0 Hz, 1H), 6.86 (d, J = 10.0 Hz,
1H), 6.70 (d, J = 9.0 Hz, 1H), 3.86 (s, 3H); 13C NMR (125 MHz,
CDCl3) δ 166.9, 160.8, 136.0, 132.4, 131.3, 130.2, 129.9, 129.1,
127.9, 123.9, 120.4, 111.7, 106.4, 55.7; LRMS (EI, 70 eV) m/z (%):
331 (M++2, 11.8), 329 (M+, 11.1), 250 (-Br, 8.7). HRMS (EI) for
C16H1279BrNO2 (M+): calcd 329.0051, found 329.0050.
4-Bromo-6,7-dimethoxy-3-phenylisoquinolin-1(2H)-one (3g): White
1
solid, m.p. 171.4–171.9 °C; H NMR (400 MHz, CDCl3) δ 7.73 (s,
1H), 7.55–7.45 (m, 5H), 7.23 (s, 1H), 6.01 (s, 1H), 3.91 (s, 3H), 3.75
(s, 3H); 13C NMR (100 MHz, CDCl3) δ 167.5, 152.5, 150.7, 137.7,
133.7, 130.4, 129.7, 129.2, 129.1, 124.0,104.9, 101.7, 77.3, 56.3,
55.6; LRMS (EI, 70eV) m/z (%): 361 (M++2, 14.8), 359 (M+, 13.1),
280 (-Br, 9.9). HRMS (EI) for C17H1479BrNO3 (M+): calcd 359.0157,
found 359.0153.
1
4-Bromo-3-octyl-1,2-dihydroisoquinoline (4h): Colourless oil; H
NMR (500 MHz, CDCl3) δ 7.41 (t, J = 8.75 Hz, 1H), 7.38–7.35 (m,
2H), 7.22 (t, J = 4.5 Hz, 1H), 4.42–4.34 (m, 2H), 2.90–2.82 (m, 2H),
1.72–1.68 (m, 2H), 1.45–1.31 (m, 10H), 0.90 (t, J = 7.0 Hz, 3H); 13
C
NMR (125 MHz, CDCl3) δ 140.1, 133.0, 129.4, 129.3, 128.7, 128.6,
126.5, 113.6, 52.0, 40.3, 31.8, 29.3, 29.2, 28.6, 27.5, 22.6, 14.1;
LRMS (EI, 70 eV) m/z (%): 321 (M++2, 2), 319 (M+, 2), 237 (2), 236
(11), 235 (3), 223 (97), 220 (100); HRMS (EI) for C17H2479BrN (M+):
calcd 321.1092, found 321.1089.
1
4-Bromo-3-pentyl-1,2-dihydroisoquinoline (4i): Colourless oil; H
NMR (500 MHz, CDCl3) δ 7.43–7.34 (m, 3H), 7.22 (t, J = 4.5 Hz,
1H), 4.43–4.34 (m, 2H), 2.91–2.79 (m, 2H), 1.75–1.69 (m, 2H), 1.45–
1.39 (m, 4H), 0.96 (t, J = 7.0 Hz, 3H); 13C NMR (125 MHz, CDCl3)
δ 140.1, 133.0, 129.4, 129.3, 128.8, 128.6, 126.5, 113.6, 52.0, 40.3,
36 X.Yu and J. Wu, J. Comb. Chem., 2009, 11, 895.
37 C.O. Usifoh, J. Heterocycl. Chem., 2001, 38, 597.

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