The study of catalyst free and copper catalyzed reactions of cyanochromenes and sodium azide

Article abstract of DOI:10.1016/j.tet.2012.12.062

Two different roles of sodium azide under two different conditions are described here along with the plausible mechanisms. When sodium azide was treated with cyanochromenes under catalyst free conditions, the azide anion acted as a base. Hence, a base mediated rearrangement of cyanochromenes was resulted in formation of benzofuran derivatives. However, in the presence of catalytic amount of CuI the azide anion acted as a diene to produce chromenotetrazoles.

Full text of DOI:10.1016/j.tet.2012.12.062

Tetrahedron 69 (2013) 1841e1848
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journal homepage: www.elsevier.com/locate/tet
The study of catalyst free and copper catalyzed reactions
of cyanochromenes and sodium azide
Sachin D. Gawande, Mustafa J. Raihan, Manoj R. Zanwar, Veerababurao Kavala,
*
Donala Janreddy, Chun-Wei Kuo, Mei-Ling Chen, Ting-Shen Kuo, Ching-Fa Yao
Department of Chemistry, National Taiwan Normal University, 88, Sec. 4, Tingchow Road, Taipei 116, Taiwan, ROC
a r t i c l e i n f o
a b s t r a c t
Article history:
Two different roles of sodium azide under two different conditions are described here along with the
plausible mechanisms. When sodium azide was treated with cyanochromenes under catalyst free con-
ditions, the azide anion acted as a base. Hence, a base mediated rearrangement of cyanochromenes was
resulted in formation of benzofuran derivatives. However, in the presence of catalytic amount of CuI the
azide anion acted as a diene to produce chromenotetrazoles.
Received 26 September 2012
Received in revised form 9 November 2012
Accepted 25 December 2012
Available online 2 January 2013
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
Cyanobenzofurans
Chromenotetrazoles
Chromenes
Copper iodide
1. Introduction
the chemists are in search of more convenient and easy methods to
synthesize 3-cyanobenzofuran derivatives.
Synthetic, medicinal and pharmaceutical applications of chro-
menes are well explored in a plethora of literature.1e3 In different
occasions, these derivatives are utilized as anti-HIV, antitumour,
antimicrobial, fungicidal, insecticidal agents, photochromic mate-
rials and ingredients of antioxidants.2a Especially, the chromenes
with an electron withdrawing functionality at C-3 are known as
good Michael acceptors.2,3 By applying the same concept, recently,
we have disclosed protocols for the indole and azide addition on 3-
nitrochromenes.3 However, as per the electron withdrawing func-
tionalities at the C-3 of the chromene derivatives are concerned, the
reactivity of the cyanochromenes is less explored than the
nitrochromenes.
Beside chromenes, Benzofuran derivatives are also important
motifs, which are widely occurred in natural products and exhibit
significant biological activities.4,5 Hence, a great effort has been
paid towards the synthesis of these heterocycles. Several attempts
have been made to achieve, particularly, the 3-cyanobenzofurans
due to the huge synthetic utility of the nitrile functionality.5
However, most of the protocols towards the synthesis of 3-
cyanobenzofurans involve multistep reactions, application of
toxic metal catalysts or complex combination of reagents.5 Hence,
Over the last few decades, transition metal catalysts have been
drawing the attention of the chemists due to their wide spectrum of
utilities in numerous chemical transformations.6 These catalytic
activities are mainly originated from the activation of a particular
functionality towards a certain type of reaction via coordination.7
This type of activation promotes the other reagents to react with
that functional group and thus the reaction achieve an added se-
lectivity. As an example, in the present report, we wish to disclose
our finding, which involve the activation of nitrile functionality of
cyanochromenes by a Cu catalyst. This activation promotes the
azide anion to react with the nitrile functionality selectively, as
a 1,3-dipole, to produce chromenotetrazoles. In absence of the
catalyst, the azide anion acts as a base and the cyanochromene
undergo base mediated rearrangement to produce 3-
cyanobenzofuran derivative.
2. Results and discussions
In a recent publication, we reported a protocol for the synthesis
of chromenotriazoles from nitrochromenes (Scheme 1).3a The tri-
azoles were formed via the elimination of the nitro functionality of
the nitrochromenes, when the nitrochromenes were treated with
sodium azide in DMSO under catalyst-free conditions. Prompted by
this success, we planned to use cyanochromenes, in the place of
* Corresponding author. E-mail addresses: cheyaocf@ntnu.edu.tw, cheyaocf@ya-
hoo.com.tw (C.-F. Yao).
0040-4020/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2012.12.062

1842
S.D. Gawande et al. / Tetrahedron 69 (2013) 1841e1848
nitrochromenes, to synthesize triazole derivatives by the elimina-
tion of the cyano functionality. However, on heating cyanochro-
mene (entry 2, Table 1) with sodium azide at 160 ꢀC in DMSO, 2-
methyl-3-cyanobenzofuran derivative was formed. The structure
was verified by 1H NMR, 13C NMR, LRMS, HRMS and single crystal X-
ray analysis data (Fig. 1 and Supplementary data). This result
demonstrates an unprecedented type of transformation, which
leads to the synthesis of 3-cyanobenzofuran derivatives directly
from 3-cyanochromenes.
initial assumption was a thermal cleavage of the pyran ring, fol-
lowed by the 1,2-shift of the cyano functionality and cyclization to
produce intermediate c (Pathway A, Scheme 2). Aromatization of
the intermediate c produces the final product. However, when we
performed a reaction in the absence of sodium azide, no product
was formed and the starting material was recovered. This experi-
mental observation rules out the possibility, which is described in
pathway A. Then we assumed a nucleophilic attack by azide anion
to form intermediate e (Pathway B), which is followed by the ring
opening via the cleavage of CeO bond in SN2 manner to form in-
termediate f. A base mediated E2 reaction on intermediate f, may
produce intermediate b, which would be converted into final
product as described earlier. To clarify this assumption, we
screened the reaction with bases, like DIPA, NaOMe and N,N-
diethylethane-1,2-diamine as they may act as nucleophile as well as
base (Table 1). However, the reactions were either failed (entries 6
and 13) or produced the desired product in poor yields (entry 12).
Scheme 1. Reactivity of sodium azide towards nitro and cyanochromene.
Table 1
Effect of different bases
Entrya,b Base
Temp Solvent Time
(ꢀC)
Yield
(%)c
1
2
3
4
5
6
7
8
NaN3
NaN3
TEA
TEA
DBU
130
160
130
160
160
160
160
160
160
160
rt
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMF
2 h
20 min 76
24 h
1 h
30 min 65
24 h
55 min 30
20 min 67
40 min 56
1 h
d
56
10
15
DIPA
d
DABCO
K2CO3
Na2CO3
Cs2CO3
KOtBu
NaOMe
Scheme 2. Plausible mechanism for the conversion.
9
10
11
12
13
14
15
55
dec
On the other hand, bases like DBU, K2CO3, Na2CO3 and Cs2CO3
produced better yields of the desired product. Among the different
bases, K2CO3 produced better yields than the others. This fact tells
more about a base mediated mechanism, which happened via the
abstraction of a proton by a base at high temperature to produce
the unstable anion g. This anion undergoes ring cleavage to pro-
duce the intermediate h. The 1,2-shift of the cyano functionality
was occurred either via pathway D or E to produce the in-
termediate k (allene intermediate). Intermediate k undergoes
cyclization and followed by the aromatization to produce the
desired product. It is not clear whether the reaction was occurred
via pathway D or E. However, in the crude 1H NMR, we did not
observe the formation of product m under air and moisture, we
prefer the pathway D as the most favourable route for the for-
mation of the product d. From our studies with different bases, we
found that a base of moderate strength, like sodium azide, was
more suitable than other strong or weak bases. Hence, we evalu-
ated the scope of our methodology by using sodium azide in
DMSO (entry 2).
160
25 min 45
N,N-Diethylethane-1,2-diamine 160
NaHCO3
NaN3
2 h
2 h
5 h
d
59
40
160
130
a
All reactions were carried out on 0.5 mmol scale.
Base (1.2 equiv) in DMSO at mentioned temp.
Yields refers to isolated and purified compound.
b
c
After determining the best conditions for the conversion, we
utilized different cyanochromene derivatives, to evaluate the scope
of this methodology (Table 2). With unsubstituted cyanochromene,
the reaction produced the expected benzofuran derivative in good
yield (entry 1). A comparable result was observed with 6-methyl-3-
cyanochromene (entry 2). However, the yields of the desired
products were decreased in the case of methoxy substituted cya-
nochromenes (entries 3 and 4). The yield of the product was further
decreased when the dimethoxy substituted cyanochromene
Fig. 1. X-ray crystal structure of 1a (ORTEP diagram).
In this reaction, the cyanide functionality was shifted to the
benzylic position and a six membered pyran ring was rearranged to
a five membered furan ring. As the reaction was occurred only at
high temperature, we first assumed a thermal reaction. Hence, our

S.D. Gawande et al. / Tetrahedron 69 (2013) 1841e1848
Table 2 (continued)
1843
derivative was used as substrate (entry 5). Moreover, the desired
products were obtained in moderate yields from the 6-halo cya-
nochromene derivatives (entries 6, 7). However, the yield of the
desired product was poor in case of 6-iododerivative (entry 8).
Entrya,b
Cyanochromene
Product
Time (min) Yieldc (%)
11
12
20
25
Table 2
Synthesis of benzofuran and benzothiophene derivatives
d
d
de
a
All reactions were carried out on 0.5 mmol scale.
NaN3 (1.2 equiv) in DMSO at 160 ꢀC.
Yields refer to isolated and purified compounds.
Inseparable mixture of compounds.
b
c
d
e
Entrya,b
Cyanochromene
Product
Time (min) Yieldc (%)
NR: reaction was not occurred and only starting material was recovered.
1
20
76
Interestingly, when the benzene ring of the cyanochromene
was replaced with naphthalene moiety (entry 9) the reaction
furnished the corresponding product in good yield. On the other
hand, the reaction of 6-acetyl-2H-chromene-3-carbonitrile in the
present reaction conditions resulted in the formation of an in-
separable mixture of products even at lower temperature (entry
10). The crude 1H NMR did not show a trace amount of the desired
product, although, all the starting materials were consumed. The
present reaction conditions were found to be suitable even for the
preparation of benzothiophene derivative (entry 11). However, in
this case, the desired product was obtained in poor yield. When
the stable aromatic heterocycle, like 2-amino-3-cyanoquinoline
(entry 12), was treated with sodium azide under catalyst free
conditions, no product was observed and the starting material was
recovered.
2
3
4
24
28
25
74
59
54
On the other hand, the result was completely different when the
reaction of sodium azide and cyanochromene (1) was carried out in
the presence of catalytic amount of CuI. In this case, the reaction
was occurred at lower temperature and the resulting product was
chromenotetrazole (1b, Table 3), which was confirmed by 1H NMR,
13C NMR, LRMS, HRMS and single crystal X-ray analysis data (Fig. 2).
5
6
50
30
35
53
Table 3
Synthesis of chromenotetrazoles
7
8
28
40
60
24
Entrya,b
Cyanochromene
Product
Time (h) Yieldc (%)
1
4
89
89
9
35
70
2
3
4.5
10
d
d
dd
5
81
(continued on next page)

1844
S.D. Gawande et al. / Tetrahedron 69 (2013) 1841e1848
Table 3 (continued )
we screened different catalysts to find a better alternative of CuI.
Entrya,b
Cyanochromene
Product
Time (h) Yieldc (%)
However, Cu2O, Cu(OAc)2, TiCl4, AlCl3 and ZnBr2 produced the
desired product in lower yields. Iodine was found to be ineffective
for the conversion (see Supplementary data). Hence, we used DMF
as solvent and CuI as catalyst for evaluating the scopes of this
protocol.
4
5
80
Under the present reaction conditions, several cyanochromenes
underwent cycloaddition to form chromenotetrazoles as shown in
Table 3. The use of unsubstituted and methyl substituted cyano-
chromenes resulted in excellent yields of the product (entries 1 and
2). Moreover, comparable yields were obtained with methoxy
substituted cyanochromenes (entries 3, 4, 11 and 12). The expected
tetrazoles were obtained in excellent yields from cyanochromenes
with electron withdrawing groups (entries 5, 6 and 8). In-
terestingly, the cyanobenzochromene (entry 9) is also produced the
expected tetrazole in excellent yield under the present reaction
conditions. From Table 2, it is evident that the reaction of cyano-
chromene possessing an electron donating group took longer time
compared with unsubstituted cyano chromene and a cyanochro-
mene with an electron withdrawing substituent to form their cor-
responding tetrazoles. It is worthy to note that the present reaction
conditions were utilized for the nitrogen and sulfur analogues of
cyanochromenes like cyanothiochromene (11) and cyanoquinoline
(12). Under these conditions the reaction furnished their corre-
sponding products, i.e., thiochromenotetrazole (11b) and 2-
aminoquinolinotetrazole (12b) in moderate and excellent yields,
respectively.
5
6
4
5
86
85
7
8
5
3
87
88
The most notable point is that under these reaction conditions
the chromene ring remained unaffected and the product formation
was occurred via selective transformation of the cyano function-
ality into tetrazoles. The plausible mechanism is similar to the Zn2þ
catalyzed transformation of nitriles into tetrazole, which was pro-
posed earlier by Sharpless (Scheme 3).8 We assume a similar type of
activation of nitrile functionality by the coordination of Cu(I) with
the nitrogen (intermediate x, Scheme 3) and hence, the azide anion
attacked at the nitrile functionality selectively as outlined in
Scheme 3. To the best of our knowledge, these chromenotetrazole
derivatives are new in the literature.
9
3.5
66
89
10
3
11
12
7
5
84
84
a
All reactions were carried out on 0.5 mmol scale.
NaN3 (1.2 equiv)/CuI (20 mol %) in DMF at 120 ꢀC.
Yields refers to isolated and purified compounds.
b
c
Scheme 3. Plausible mechanism for the formation of chromenotetrazole.
After exploring the scope of this protocol, we attempted to
evaluate the synthetic applicability of the tetrazole derivatives
(Scheme 4). To pursue this goal, we transformed the tetrazole ring
into oxadiazole and pyrazole rings. The chromeno oxadiazole de-
rivatives (1c and 1d) were obtained by treating 1b with benzoyl
chloride and 2-chloroacetyl chloride.
Fig. 2. X-ray crystal structure of 1b (ORTEP diagram).
To prepare the pyrazole (1f) derivative, the chromenotetrazole
derivative (1b) was refluxed with dibromoethane in acetonitrile in
the presence of triethylamine as a base to obtain 1e in good yield.
The compound 1e was heated at 150 ꢀC in xylene to obtain the
desired pyrazole (1f). Next, in order to evaluate further utility of the
With this exciting experimental outcome in hand, we focused
our attention towards exploring the most favourable conditions
for the reaction. On screening with different solvents, we found
that the reaction produced better result in DMF than DMSO. Next,

S.D. Gawande et al. / Tetrahedron 69 (2013) 1841e1848
1845
25
20
15
10
5
Ligand
NaCl
KCl
MnCl2
CaCl2
CdCl2
CoCl2
CrCl3
CuCl2
HgCl2
NiCl2
ZnCl2
0
350
400
450
500
550
600
650
Wavelength (nm)
Fig. 4. Change of fluorescence intensity of 12b with different metal salts. 200
mL so-
lution of the metal ion in DMF was added to 3 mL 0.1 M solution of the ligand
(compound 12b) and the fluorescence enhancement was measured.
Co2þ and Cu2þ also showed fluorescence quenching properties. On
the other hand, Naþ, Kþ, Mn2þ, Ca2þ, Cd2þ, Cr3þ and Hg2þ showed
comparable enhancement of fluorescence. However, with Hg2þ, the
enhancement is greater than Naþ, Kþ, Mn2þ, Ca2þ, Cd2þ and Cr3þ
.
As shown in Fig. 3, compound 12b showed the highest fluorescence
enhancement to Zn2þ ions, which was quite distinct from all the
others. This selectivity of 12b towards Zn2þ is very important as
Zn2þ is the second most abundant transition metal ion in the hu-
man body after iron.10a
Scheme 4. Synthetic utilities of chromenotetrazole.
chromenotetrazoles, compound 1b was treated with morpholine
and formaline in methanol at room temperature and the 3-
methylmorpholine substituted chromenotetrazole (1g) was selec-
tively produced in moderate yield. The structure of the compound
1g was confirmed by single crystal X-ray analysis (Fig. 3). It is
noteworthy that pyrazoles, oxadiazoles and morpholines are very
important due to their synthetic and medicinal utilities as de-
scribed in several literature.9 More studies on the biological activ-
ities of the tetrazole, oxadiazole and pyrazole derivatives are
currently underway in our laboratory.
3. Conclusion
In conclusion, we have demonstrated a novel method for the
synthesis of 3-cyanobenzofuran derivatives from cyanochromenes
under catalyst free conditions. The reaction was occurred via a so-
dium azide mediated rearrangement of the pyran ring into furan
ring and a 1,2-shift of the nitrile functionality. The mechanism was
supported by the experimental outcomes. In the presence of CuI,
a chromenotetrazole was formed due to the selective activation of
the nitrile functionality. In addition, this report demonstrates the
utilities of the tetrazole derivative as potential Zn2þ ion sensor as
well as synthetic intermediate. Studies aimed at exploring more
utilities of benzofurans and chromenotetrazoles in biological and
synthetic fields are currently underway in our laboratory.
4. Experimental section
4.1. General
General remarks: all reactions were carried out under nitrogen
atmosphere and in glassware, which had been oven-dried. Unless
otherwise noted, all reagents were commercially obtained and,
where appropriate, purified prior to use. Analytical thin-layer chro-
matography was performed with E. Merck silica gel 60F glass plates
and flash chromatography by use of E. Merck silica gel 60
(230e400 mesh). MS were measured by JEOL JMS-HX110 spec-
Fig. 3. X-ray crystal structure of 1g (ORTEP diagram).
Further, when we placed the solutions of different tetrazole
derivatives in front of UV lamp, compound 12b displayed fluores-
cent properties. In this context, it is noteworthy to mention that the
fluorescence sensing activities of quinoline derivatives are reported
in the literature.10 Hence, we focused our attention towards ex-
ploring the metal sensing activity of the tetrazole derivative (Fig. 4).
To pursue this goal, we prepared a 0.1 M solution of ligand (12b,
Table 3) in DMF. A 0.1 M solution of the metal ion in DMF was then
added to 3 mL of a 0.1 M solution of the ligand (compound 12b) and
the fluorescence enhancement was measured. With Zn2þ, the
1
trometer. H and 13C NMR spectra were recorded with Bruker Ad-
vance EX 400. Chemical shifts were reported in parts per million (d)
using DMSO-d6 and CDCl3 as internal standard and coupling con-
stants were expressed in hertz. UV study was done on Agilent
HP8453 spectrophotometer and the Fluorescence study was per-
formed on Varian Carry Eclipse Fluorescence spectrophotometer. All
the substrates were prepared by following reported procedures.11
4.1.1. General procedure for the synthesis of 2-methylbenzofuran-3-
carbonitrile (1a). To a stirred solution of cyanochromene (1 mmol)
in DMSO (2 mL) in a 50 mL round bottom flask was added NaN3
(1.1 equiv). The reaction mixture was then heated to 160 ꢀC. The
progress of reaction was monitored by TLC. After the completion of
the reaction, the reaction mixture was quenched with ice water
maximum fluorescence enhancement was observed when 200 mL
of the metal ion solution was added to the ligand solution. Later, we
measured the fluorescence enhancement with other metal ions,
including Naþ, Kþ, Mn2þ, Ca2þ, Cd2þ, Co2þ, Cr3þ, Cu2þ, Hg2þ and
Ni2þ (Fig. 4). With Ni2þ, the fluorescence was quenched abruptly.

1846
S.D. Gawande et al. / Tetrahedron 69 (2013) 1841e1848
(25 mL) and extracted with ethyl acetate. Organic layer was dried
over MgSO4 then filtered and evaporated. The crude compound was
then column purified to yield corresponding pure compound.
3H); 13C NMR (100 MHz, CDCl3)
d 164.9, 152.4, 134.4, 126.9, 126.3,
119.5,113.7,111.2, 91.2, 21.5,14.1; LRMS (EI) (m/z) (relative intensity)
171 (100) [M]þ, 156 (30), 115 (55); HRMS calcd for C11H9NO [M]þ:
171.0684, found 171.0683.
4.1.2. General procedure for the synthesis of chromene tetrazole by
using NaN3/CuI (1b). To a stirred solution of cyanochromene
(0.5 mmol) in DMF (2 mL) in a 50 mL round bottom flask was added
NaN3 (1.1 equiv) and CuI (20 mol %). The reaction mixture was then
heated to 120 ꢀC. The progress of reaction was monitored by TLC.
After the completion of the reaction, the reaction mixture was
quenched with ice water (25 mL) and acidified (pH¼3.5) by aq HCl
(4 M). The precipitate was filtered and washed with ice water
(25 mL) to obtain the pure product.
4.2.3. 6-Methoxy-2-methylbenzofuran-3-carbonitrile (3a). Yellow
solid; mp: 158e160 ꢀC; 1H NMR (400 MHz, CDCl3)
d
(ppm) 7.45 (d,
J¼8.56 Hz,1H), 6.99 (d, J¼2.0 Hz,1H), 6.95 (dd, J¼2.16, 11.44 Hz, 1H),
3.85 (s, 3H), 2.61 (s, 3H); 13C NMR (100 MHz, CDCl3)
163.9, 159.0,
d
154.9, 119.7, 119.2, 113.7, 113.2, 96.5, 91.2, 55.9, 13.9; LRMS (EI) (m/z)
(relative intensity) 187 (97) [M]þ, 172 (100); HRMS calcd for
C11H9NO2 [M]þ: 187.0633, found 187.0636.
4.2.4. 7-Methoxy-2-methylbenzofuran-3-carbonitrile (4a). Yellow
4.1.3. Synthesis of 2-(2H-chromen-3-yl)-5-phenyl-1,3,4-oxadiazole
(1c). A mixture of 1b (1 mmol) and benzoyl chloride (1.1 mmol)
in toluene (10 mL) in a 50 mL round bottom flask was refluxed
overnight. The reaction was monitored by TLC. After the completion
of the reaction, the reaction mixture was cooled to room temper-
ature and the solvent was evaporated under reduced pressure. The
crude compound was then purified by column chromatography to
afford yellow solid.
solid; mp: 151e153 ꢀC; 1H NMR (400 MHz, CDCl3)
d
(ppm)
7.25e7.29 (m, 1H), 7.20 (d, J¼8 Hz, 1H), 6.87 (d, J¼7.92 Hz, 1H), 4.02
(s, 3H), 2.68 (s, 3H); 13C NMR (100 MHz, CDCl3)
164.9, 145.4, 143.2,
d
127.8, 125.5, 113.5, 111.8, 107.8, 92.0, 56.4, 14.1; LRMS (EI) (m/z)
(relative intensity) 187 (100) [M]þ, 144 (15); HRMS calcd for
C11H9NO2 [M]þ: 187.0629, found 187.0633.
4.2.5. 5,6-Dimethoxy-2-methylbenzofuran-3-carbonitrile (5a). Off
Compound 1d was prepared by following a similar procedure
and by using chloroacetylchloride (1.1 mmol) in place of benzoyl
chloride.
white solid; mp: 171e173 ꢀC; 1H NMR (400 MHz, CDCl3)
d
(ppm)
7.02 (s, 2H), 3.95 (s, 3H), 3.93 (s, 3H), 2.62 (s, 3H); 13C NMR
(100 MHz, CDCl3) 163.5, 149.1, 148.6, 148.1, 118.2, 113.9, 100.9, 95.8,
d
91.4, 56.7, 56.6, 14.1; LRMS (EI) (m/z) (relative intensity) 217 (100)
[M]þ, 202 (40); HRMS calcd for C12H11NO3 [M]þ: 217.0739, found
217.0741.
4.1.4. Synthesis of 5-(2H-chromen-3-yl)-1-vinyl-1H-tetrazole (1e). To
a stirred solution of dibromoethane (2 mmol) and 1b (1 mmol) in
5 mL acetonitrile in a 25 mL round bottom flask was added solution
of Et3N (4 mmol) in acetonitrile (3 mL) dropwise. The reaction
mixture was then refluxed and the progress of the reaction was
monitored by TLC. After the completion of the reaction, the reaction
mixture was cooled to room temperature and acetonitrile was re-
moved under reduced pressure. The residue was dissolved in
dichloromethane (30 mL) and washed with H2O (25 mL). The or-
ganic solution was dried over MgSO4 and the solvent was evapo-
rated. The product was purified by column chromatography.
4.2.6. 5-Chloro-2-methylbenzofuran-3-carbonitrile (6a). White solid;
mp: 166e168 ꢀC; 1H NMR (400 MHz, CDCl3)
d
(ppm) 7.61 (d,
J¼1.96 Hz, 1H), 7.41 (d, J¼8.64 Hz, 1H), 7.28e7.34 (m, 1H), 2.67 (s,
3H); 13C NMR (100 MHz, CDCl3)
165.9, 152.3, 130.5, 127.5, 126.1,
d
119.4, 112.7, 91.4, 14.1; LRMS (EI) (m/z) (relative intensity) 191 (100)
[M]þ, 165 (5); HRMS calcd for C10H6ONCl [M]þ: 191.0138, found
191.0133.
4.2.7. 5-Bromo-2-methylbenzofuran-3-carbonitrile
(7a). White
(ppm) 7.76 (d,
4.1.5. Synthesis of 5-(2H-chromen-3-yl)-1H-pyrazole (1f). A stirred
solution of compound 1e (1 mmol) in xylene (15 mL) was heated at
140 ꢀC and the progress of the reaction was monitored by TLC. After
the completion of the reaction, the solvent was evaporated under
reduced pressure. The product was purified by column chroma-
tography to yield yellow solid.
solid; mp: 169e171 ꢀC; 1H NMR (400 MHz, CDCl3)
d
J¼1.96 Hz, 1H), 7.47 (dd, J¼1.80, 8.72 Hz, 1H), 7.37 (d, J¼8.76 Hz, 1H)
2.68 (s, 3H); 13C NMR (100 MHz, CDCl3)
d
166.2, 152.8, 128.9, 128.0,
122.6, 117.9, 113.2, 112.8, 91.4, 14.1; LRMS (EI) (m/z) (relative in-
tensity) 235 (100) [M]þ, 156 (20); HRMS calcd for C10H6NOBr [M]þ:
234.9633, found 234.9634.
4.1.6. Synthesis of 4-((5-(2H-chromen-3-yl)-1H-tetrazol-1-yl)methyl)
morpholine (1g). To an ice cold solution of 1a (1 mmol) in methanol
(2 mL) was added formalin (1.6 equiv) and stirred at room temper-
ature for 15 min. Then morpholine (1 equiv) was added dropwise.
The progress of the reaction was monitored by TLC. After the com-
pletion of the reaction, the solvent was evaporated under reduced
pressure. The compound was purified by column chromatography to
yield off white solid.
4.2.8. 5-Iodo-2-methylbenzofuran-3-carbonitrile (8a). Yellow solid;
mp: 158e160 ꢀC; 1H NMR (400 MHz, CDCl3)
d
(ppm) 7.98 (d,
J¼1.56 Hz, 1H), 7.67 (dd, J¼1.64, 8.60 Hz, 1H), 7.29 (d, J¼2.0 Hz, 1H),
2.69 (s, 3H); 13C NMR (100 MHz, CDCl3)
165.9, 153.4, 134.6, 128.6,
d
113.6, 112.8, 111.9, 90.9, 88.1, 14.1; LRMS (EI) (m/z) (relative in-
tensity) 283 (100) [M]þ, 156 (20); HRMS calcd for C10H7INO [M]þ:
282.9494, found 282.9494.
4.2.9. 2-Methylnaphtho[1,2-b]furan-3-carbonitrile (9a). Off white
4.2. Spectral data
solid; mp: 163e165 ꢀC; 1H NMR (400 MHz, CDCl3)
d (ppm) 8.61 (d,
J¼8.28 Hz, 1H), 7.97 (s, J¼8.20 Hz, 1H), 7.78 (d, J¼9.0 Hz, 1H), 7.67 (t,
4.2.1. 2-Methylbenzofuran-3-carbonitrile (1a). White solid; mp:
J¼7.72 Hz, 1H), 7.61 (d, J¼9.04 Hz, 1H), 7.67 (t, J¼7.32 Hz, 1H) 2.75 (s,
148e150 ꢀC; 1H NMR (400 MHz, CDCl3)
d
(ppm) 7.61e7.63 (m, 1H),
3H); 13C NMR (100 MHz, CDCl3)
d 164.3, 151.6, 131.1, 129.1, 127.5,
126.9, 126.8, 125.8, 122.7, 120.2, 115.0,111.9, 91.5, 14.0; LRMS (EI) (m/
z) (relative intensity) 207 (100) [M]þ HRMS calcd for C14H9NO [M]þ:
207.0684, found 207.0683.
7.47e7.49 (m, 1H), 7.35e7.37 (m, 2H), 2.67 (s, 3H); 13C NMR
(100 MHz, CDCl3)
d 165.0, 153.9, 126.2, 125.8, 124.6, 119.7, 113.6,
111.7, 91.6, 14.1; LRMS (EI) (m/z) (relative intensity) 157 (100) [M]þ;
HRMS calcd for C10H7NO [M]þ: 157.0528, found 157.0532.
4.2.10. 2-Methylbenzo[b]thiophene-3-carbonitrile (11a). White solid;
4.2.2. 2,5-Dimethylbenzofuran-3-carbonitrile (2a). White solid;
mp: 138e140 ꢀC; 1H NMR (400 MHz, CDCl3)
d
(ppm) 7.85 (d,
J¼7.92 Hz, 1H), 7.78 (d, J¼8.0 Hz, 1H), 7.46e7.50 (m, 1H), 7.38e7.42
(m, 1H), 2.79 (s, 3H); 13C NMR (100 MHz, CDCl3)
154.0, 138.1, 137.7,
mp: 137e139 ꢀC; 1H NMR (400 MHz, CDCl3)
d
(ppm) 7.39 (s, 1H),
7.35 (d, J¼8.44 Hz, 1H), 7.15 (d, J¼8.44 Hz, 1H), 2.64 (s, 3H), 2.46 (s,
d

S.D. Gawande et al. / Tetrahedron 69 (2013) 1841e1848
1847
126.0, 125.7, 122.5, 122.1, 114.4, 105.7, 15.9; LRMS (EI) (m/z) (relative
intensity) 173 (100) [M]þ, 172 (85); HRMS calcd for C10H7NS [M]þ:
173.0299, found 173.0298.
131.5, 131.2, 128.6, 124.4, 120.8, 118.7, 115.9, 65.3, 26.5; LRMS (EI) (m/
z) (relative intensity) 242 (12) [M]þ, 241 (100), 185.2 (16); HRMS
calcd for C12H10N4O2 [M]þ: 242.0804, Found 242.0760.
4.2.11. 5-(2H-Chromen-3-yl)-1H-tetrazole (1b). White solid; mp:
4.2.19. 5-(2H-Thiochromen-3-yl)-1H-tetrazole (11b). Yellow solid;
208e210 ꢀC; 1H NMR (400 MHz, DMSO-d6)
d
(ppm) 7.44 (s, 1H),
7.22e7.28 (m, 2H), 6.96 (t, J¼7.4 Hz, 1H), 6.87 (d, J¼8.0 Hz, 1H), 5.19
(s, 2H); 13C NMR (100 MHz, DMSO-d6)
153.7, 153.0, 131.0, 128.2,
mp: 191e193 ꢀC; 1H NMR (400 MHz, DMSO-d6)
d
(ppm) 7.52 (s, 1H),
7.31e7.37 (m, 2H), 7.20e7.27 (m, 2H), 4.02 (s, 2H); 13C NMR
d
(100 MHz, DMSO-d6) d 156.1, 131.7, 131.4, 129.6, 129.3, 129.2, 126.8,
126.1, 121.9, 120.9, 116.9, 115.7, 64.3; LRMS (EI) (m/z) (relative in-
tensity) 201 (100) [Mþ1]þ, 172 (15); HRMS calcd for C10H8N4O
[Mþ1]þ: 201.0776, found 201.0780.
126.0, 118.7, 24.8; LRMS (EI) (m/z) (relative intensity) 216 (55) [M]þ,
188 (68),172 (100); HRMS calcd for C10H8N4S [M]þ: 216.0470, found
216.0472.
4.2.12. 5-(6-Methyl-2H-chromen-3-yl)-1H-tetrazole (2b). Off white
4.2.20. 3-(1H-Tetrazol-5-yl)quinolin-2-amine (12b). Green solid;
solid; mp: 235e237 ꢀC; 1H NMR (400 MHz, DMSO-d6)
d
(ppm) 7.39
(s, 1H), 7.03e7.06 (m, 2H), 6.77 (d, J¼8.0 Hz, 1H), 5.14 (s, 2H), 2.22 (s,
3H); 13C NMR (100 MHz, DMSO-d6)
153.0,151.6,131.4,130.8,128.4,
mp: 239e241 ꢀC; 1H NMR (400 MHz, DMSO-d6)
d (ppm) 8.65 (s,
1H), 7.74 (d, J¼8.0 Hz, 1H), 7.65 (t, J¼7.4 Hz, 1H), 7.51 (d, J¼8.5 Hz,
d
1H), 7.26 (t, J¼7.6 Hz, 1H), 6.92 (br s, 2H); 13C NMR (100 MHz,
126.3, 120.8, 116.9, 115.5, 64.3, 20.0; LRMS (EI) (m/z) (relative in-
tensity) 214 (55) [M]þ, 185 (100), 170 (38); HRMS calcd for
C11H10N4O [M]þ: 214.0855, found 214.0848.
DMSO-d6) d 155.9,149.2,145.5,133.0,128.7,125.6,123.0,121.1,116.6,
94.7; LRMS (EI) (m/z) (relative intensity) 212 (100) [M]þ, 184. HRMS
Calcd for C10H8N6 [M]þ: 212.0810, found 212.0804.
4.2.13. 5-(7-Methoxy-2H-chromen-3-yl)-1H-tetrazole
white solid; mp: 235e237 ꢀC; 1H NMR (400 MHz, DMSO-d6)
(ppm) 7.40 (s, 1H), 7.20 (d, J¼8.4 Hz, 1H), 6.56 (dd, J¼2.4, 8.4 Hz,
1H), 6.49 (s, 1H), 5.16 (s, 2H), 3.75 (s, 3H); 13C NMR (100 MHz,
(3b). Off
4.2.21. 5-(6,8-Dimethoxy-2H-chromen-3-yl)-1H-tetrazole
(13b). Light brown solid; mp: 206e208 ꢀC; 1H NMR (400 MHz,
d
DMSO-d6)
d
(ppm) 7.43 (s, 1H), 7.03 (d, J¼8.5 Hz, 1H), 6.69 (d,
J¼8.6 Hz, 1H), 5.18 (d, J¼1.1 Hz, 2H), 3.81 (s, 3H), 3.72 (s, 3H); 13C
DMSO-d6)
d
161.8, 155.2, 152.9, 129.2, 126.2, 114.1, 113.4, 108.1, 101.5,
NMR (100 MHz, DMSO-d6) d 154.8, 152.9, 147.1, 136.6, 126.5, 123.2,
64.4, 55.4; LRMS (EI) (m/z) (relative intensity) 230 (67) [M]þ, 201
(100), 186 (40); HRMS calcd for C11H10N4O2 [M]þ: 230.0804, found
230.0798.
115.6, 114.2, 105.7, 64.3, 60.3, 55.9; LRMS (EI) (m/z) (relative in-
tensity) 260 (77) [M]þ, 232 (100); HRMS calcd for C12H12N4O3 [M]þ:
260.0909, found 260.0911.
4.2.14. 5-(8-Methoxy-2H-chromen-3-yl)-1H-tetrazole
yellow solid; mp: 203e205 ꢀC; 1H NMR (400 MHz, DMSO-d6)
(ppm) 7.43 (s, 1H), 6.99 (d, J¼6.9 Hz, 1H), 6.88e6.93 (m, 2H), 5.17
(s, 2H), 3.77 (s, 3H); 13C NMR (100 MHz, DMSO-d6)
152.9, 147.6,
(4b). Light
4.2.22. 5-(6-Bromo-8-methoxy-2H-chromen-3-yl)-1H-tetrazole
(14b). Yellow solid; mp: 196e198 ꢀC; 1H NMR (400 MHz, DMSO-d6)
d
d
(ppm) 7.33 (s, 1H), 7.12 (s, 2H), 5.19 (d, J¼1.1 Hz, 2H), 3.80 (s, 3H);
d
13C NMR (100 MHz, DMSO-d6)
d
154.3, 148.5, 141.8, 123.7, 122.7,
142.7,126.3, 121.7, 121.6, 120.0, 116.9, 114.5, 64.2, 55.7; LRMS (EI) (m/
z) (relative intensity) 230 (100) [M]þ; HRMS calcd for C11H10N4O2
[M]þ: 230.0804, found 230.0802.
121.7, 120.3, 116.3, 112.6, 64.8, 56.1; LRMS (EI) (m/z) (relative in-
tensity) 308 (62) [M]þ, 281 (100); HRMS calcd for C11H9N4O2Br
[M]þ: 307.9909, found 307.9902.
4.2.15. 5-(6-Chloro-2H-chromen-3-yl)-1H-tetrazole (6b). Off white
4.2.23. 2-(2H-Chromen-3-yl)-5-phenyl-1,3,4-oxadiazole (1c). White
solid; mp: 237e239 ꢀC; 1H NMR (400 MHz, DMSO-d6)
d
(ppm) 7.34
solid; mp: 145e147 ꢀC; 1H NMR (400 MHz, CDCl3)
d (ppm) 8.12 (d,
(d, J¼2.5 Hz, 1H), 7.24 (s, 1H), 7.17 (dd, J¼8.5, 2.5 Hz, 1H), 6.86 (d,
J¼7.9 Hz, 2H), 7.51e7.58 (m, 3H), 7.38 (s,1H), 7.18e7.27 (m, 2H), 6.97
J¼8.6 Hz, 1H), 5.22 (d, J¼0.7 Hz, 2H); 13C NMR (100 MHz, DMSO-d6)
(t, J¼7.5 Hz, 1H), 6.91 (d, J¼8.0 Hz, 1H), 5.29 (s, 2H); 13C NMR
d
155.8, 152.1, 128.8, 126.6, 125.2, 123.9, 122.9, 119.9, 117.1, 65.5;
(100 MHz, CDCl3) d 164.5, 162.0, 154.9, 132.1, 131.7, 129.3, 128.5,
LRMS (EI) (m/z) (relative intensity) 234 (100) [M]þ, HRMS calcd for
127.9, 127.3, 123.9, 122.2, 121.3, 116.5, 116.2, 64.3; LRMS (EI) (m/z)
(relative intensity) 276 (100) [M]þ; HRMS calcd for C17H12N2O2
[M]þ: 276.0899, found 276.0899.
C10H7 N4OCl [M]þ: 234.0304, found 234.0308.
4.2.16. 5-(6-Bromo-2H-chromen-3-yl)-1H-tetrazole (7b). Off white
solid; mp: 229e231 ꢀC; 1H NMR (400 MHz, DMSO-d6)
d
(ppm) 7.40
4.2.24. 2-(Chloromethyl)-5-(2H-chromen-3-yl)-1,3,4-oxadiazole
(d, J¼2.4 Hz, 1H), 7.22 (dd, J¼8.9, 2.5 Hz, 1H), 7.10 (br s, 2H), 6.76 (d,
(1d). White solid; mp: 150e151 ꢀC; 1H NMR (400 MHz, CDCl3)
J¼8.5 Hz, 1H), 5.21 (d, J¼1.6 Hz, 2H); 13C NMR (100 MHz, DMSO-d6)
d
(ppm) 7.33 (s, 1H), 7.24 (t, J¼8.0 Hz, 1H), 7.16 (d, J¼7.4 Hz, 1H), 6.95
d
157.4, 152.3, 130.5, 128.8, 125.8, 125.3, 117.2, 116.2, 112.7, 66.0;
(t, J¼7.4 Hz, 1H), 6.88 (d, J¼8.1 Hz, 1H), 5.20 (s, 2H), 4.73 (s, 2H); 13C
LRMS (EI) (m/z) (relative intensity) 278 (46) [M]þ, 251 (100); HRMS
calcd for C10H7N4OBr [MþH]þ: 278.9803, found 278.9804.
NMR (100 MHz, CDCl3) d 163.3, 161.9, 154.9, 132.0, 129.3, 128.7,
122.3, 120.9, 116.5, 115.4, 63.9, 33.1; LRMS (EI) (m/z) (relative in-
tensity) 248 (100) [M]þ; HRMS calcd for C12H9N2O2Cl [M]þ:
248.0353, found 248.0359.
4.2.17. 5-(2H-Benzo[h]chromen-3-yl)-1H-tetrazole (9b). Yellow solid;
mp: 230e232 ꢀC; 1H NMR (400 MHz, DMSO-d6)
d (ppm) 8.12 (d,
J¼8.44 Hz,1H), 7.94 (s,1H), 7.85 (d, J¼8.04 Hz, 1H), 7.78 (d, J¼8.84 Hz,
4.2.25. 5-(2H-Chromen-3-yl)-1-vinyl-1H-tetrazole (1e). Yellow solid;
1H), 7.52e7.57 (m, 1H), 7.39 (t, J¼7.28 Hz, 1H), 7.16 (d, J¼8.8 Hz, 1H),
mp: 159e161 ꢀC; 1H NMR (400 MHz, CDCl3)
d (ppm) 7.58 (s,1H), 7.53
5.33 (s, 2H); 13C NMR (100 MHz, DMSO-d6)
d
157.2,151.6,129.6,129.3,
(dd, J¼15.6, 8.7 Hz, 1H), 7.17e7.24 (m, 2H), 6.95 (m, 1H), 6.89 (d,
129.2, 128.9, 128.5, 127.0, 123.9, 121.8, 121.4, 117.3, 115.5, 65.6; LRMS
(EI) (m/z) (relative intensity) 250 (50.5) [M]þ, 221 (100); HRMS calcd
for C14H10N4O [M]þ: 250.0855, found 250.0852.
J¼8.0 Hz,1H), 6.22 (dd, J¼15.6,1.4 Hz,1H), 5.39 (dd, J¼8.7,1.4 Hz,1H),
5.29 (d, J¼1.2 Hz, 2H); 13C NMR (100 MHz, CDCl3)
d 162.1, 154.4,
130.6, 129.6, 127.9, 126.1, 121.8, 121.5, 118.9, 115.9, 108.4, 64.9; LRMS
(EI) (m/z) (relative intensity) 226 (100) [M]þ, 200 (60); HRMS calcd
for C12H10N4O [M]þ: 226.0855, found 226.0850.
4.2.18. 1-(3-(1H-Tetrazol-5-yl)-2H-chromen-6-yl)ethanone
Yellow solid; mp: 228e230 ꢀC; 1H NMR (400 MHz, DMSO-d6)
(ppm) 7.82e7.87 (m, 2H), 7.46 (s, 1H), 6.96 (d, J¼8.3 Hz, 1H), 5.31 (s,
2H), 2.51 (s, 3H); 13C NMR (100 MHz, DMSO-d6)
196.5, 157.6, 153.8,
(10b).
d
4.2.26. 5-(2H-Chromen-3-yl)-1H-pyrazole (1f). Yellow solid; mp:
d
144e146 ꢀC; 1H NMR (400 MHz, CDCl3)
d (ppm) 12.90 (br s, 1H),

1848
S.D. Gawande et al. / Tetrahedron 69 (2013) 1841e1848
Monrad, O.; Bekker-Jensen, A.; Nielsen, C. W.; Frydenvang, K.; Jensen, A. A.; Bunch, L.
J. Med. Chem. 2010, 53, 7180e7191; (e) Kureshy, R. I.; Khan, N. H.; Abdi, S. H. R.; Patel,
S. T.; Iyer, P. K.; Jasra, R. V. Tetrahedron Lett. 2002, 43, 2665e2668; (f) Kureshy, R. I.;
Khan, N. H.; Abdi, S. H. R.; Patel, S. T.; Jasra, R. V. Tetrahedron: Asymmetry 2001, 12,
433e437.
2. (a) Reddy, B. V. S.; Divya, B.; Swain, M.; Rao, T. P.; Yadav, J. S.; Vardhan, M. V. P.
S. V. Bioorg. Med. Chem. Lett. 2012, 22, 1995e1999; (b) Bera, K.; Sarkar, S.;
Biswas, S.; Maiti, S.; Jana, U. J. Org. Chem. 2011, 76, 3539e3544; (c) Conti, C.;
Desideri, N. Bioorg. Med. Chem. 2010, 18, 6480e6488.
7.78 (s, 1H), 7.08e7.14 (m, 2H), 6.98 (s, 1H), 6.90 (t, J¼7.2 Hz, 1H),
6.81 (d, J¼7.8 Hz, 1H), 6.66 (s, 1H), 5.15 (s, 2H); 13C NMR (100 MHz,
CDCl3)
d 153.1, 147.5, 129.8, 128.6, 126.7, 122.8, 121.5, 117.9, 115.2,
102.9, 101.5, 65.3; LRMS (EI) (m/z) (relative intensity) 198 (100)
[M]þ, 197 (62), 169 (20); HRMS calcd for C12H10N2O [M]þ: 198.0793,
found 198.0795.
3. (a) Habib, P. M.; Raju, B. R.; Kavala, V.; Kuo, C.-W.; Yao, C.-F. Tetrahedron 2009,
65, 5799e5804; (b) Habib, P. M.; Kavala, V.; Raju, B. R.; Kuo, C.-W.; Yao, C.-F. Eur.
J. Org. Chem. 2009, 4503e4514.
4. (a) Melzig, L.; Rauhut, C. B.; Naredi-Rainer, N.; Knochel, P. Chem.dEur. J. 2011, 17,
5362e5372; (b) Huang, X.-C.; Liu, Y.-L.; Liang, Y.; Pi, S.-F.; Wang, F.; Li, J.-H. Org.
Lett. 2008, 10, 1525e1528.
5. (a) Okamoto, K.; Watanabe, M.; Murai, M.; Hatano, R.; Ohe, K. Chem. Commun.
2012, 3127e3129; (b) Ding, S.; Jiao, N. J. Am. Chem. Soc. 2011, 133, 12374e12377;
(c) Swamy, N. K.; Yazici, A.; Pyne, S. G. J. Org. Chem. 2010, 75, 3412e3419; (d)
Liang, Z.; Hou, W.; Du, Y.; Zhang, Y.; Pan, Y.; Mao, D.; Zhao, K. Org. Lett. 2009, 11,
4978e4981; (e) Okitsu, T.; Nakazawa, D.; Taniguchi, R.; Wada, A. Org. Lett. 2008,
10, 497e4970; (f) RamaRao, V. V. V. N. S.; Reddy, G. V.; Maitraie, D.; Ravikanth,
S.; Yadla, R.; Narsaiah, B.; Rao, P. S. Tetrahedron 2004, 60, 12231e12237; (g)
Yang, Z.; Liu, H. B.; Lee, C. M.; Chang, H. M.; Wong, H. N. C. J. Org. Chem. 1992, 57,
7246e7257.
4.2.27. 4-((5-(2H-Chromen-3-yl)-1H-tetrazol-1-yl)methyl)morpho-
line (1g). White solid; mp: 132e134 ꢀC; 1H NMR (400 MHz, DMSO-
d6)
d (ppm) 7.51 (s, 1H), 7.34 (d, 1H) 7.15e7.22 (m, 1H), 6.94 (t,
J¼7.4 Hz, 1H), 6.88 (d, J¼8.1 Hz, 1H), 5.60 (s, 2H), 5.22 (d, J¼0.96 Hz,
2H), 3.58 (t, J¼4.8 Hz, 4H), 2.57 (t, J¼4.7 Hz, 4H); 13C NMR (100 MHz,
DMSO-d6) d 153.6, 130.6, 128.1, 124.6, 124.5, 121.8, 121.3, 119.1, 115.6,
73.8, 65.9, 64.4, 49.2; LRMS (EI) (m/z) (relative intensity) 299 (100)
[M]þ; HRMS calcd for C15H17N5O2 [M]þ: 299.1382, found 299.1380.
Acknowledgements
This work was supported by the National Science Council of the
Republic of China (NSC-100-2113-M-003-008-MY3) and National
Taiwan Normal University (99T3030-2, 99-D and 100-D-006) and
Instrumentation Center at National Taiwan Normal University is
gratefully acknowledged for providing spectral data. The authors
are grateful to Professor Dr. Chen-Hsiung Hung of Academia Sinica
for providing access to UV and Fluorescence instruments for
obtaining the data presented in this paper.
6. (a) Yamamoto, Y. Chem. Rev. 2012, 112, 4736e4769; (b) Chelucci, G. Chem. Rev.
2012, 112, 1344e1466; (c) Gladysz, J. A. Chem. Rev. 2011, 111, 1167e1169; (d)
Corma, A.; García, H. Chem. Rev. 2002, 102, 3837e3892.
7. For recent reports please see: (a) Ueda, S.; Buchwald, S. L. Angew. Chem., Int. Ed.
2012, 51, 1e5; (b) Imae, K.; Konno, T.; Ogata, K.; Fukuzawa, S.-i. Org. Lett. 2012,
14, 4410e4413; (c) Narumi, T.; Kobayakawa, T.; Aikawa, H.; Seike, S.; Tamamura,
H. Org. Lett. 2012, 14, 4490e4493; (d) Senadi, G. C.; Hu, W.-P.; Hsiao, J.-S.;
Vandavasi, J. K.; Chen, C.-Y.; Wang, J.-J. Org. Lett. 2012, 14, 4478e4481.
8. (a) Popova, E. A.; Trifonov, R. E.; Ostrovskii, V. A. Arkivoc 2012, i, 45e65; (b)
Demko, Z. P.; Sharpless, K. B. J. Org. Chem. 2001, 66, 7945e7950.
9. (a) Zheng, Y.; Batsanov, A. S.; Jankus, V.; Dias, F. B.; Bryce, M. R.; Monkman, A. P.
J. Org. Chem. 2011, 76, 8300e8310; (b) Katritzky, A. R.; El-Gendy, B. E. D. M.;
Draghici, B.; Hall, C. D.; Steel, P. J. J. Org. Chem. 2010, 75, 6468e6476; (c) Rusinov,
G. L.; Ishmetova, R. I.; Chupakhin, O. N. Russ. J. Org. Chem. 1997, 33, 524e531; (d)
Binda, M.; Dziklinska, A.; Hachiam, A. F. H.; Plenkiewicz, J. Pol. J. Chem. 1992, 66,
1257e1261.
10. (a) Zhou, X.; Li, P.; Shi, Z.; Tang, X.; Chen, C.; Liu, W. Inorg. Chem. 2012, 51,
9226e9231; (b) Pramanik, A.; Das, G. Tetrahedron 2009, 65, 2196e2200; (c) Ito,
H.; Matsuoka, M.; Ueda, Y.; Takuma, M.; Kuda, Y.; Iguchi, K. Tetrahedron 2009,
65, 4235e4238; (d) Moody, C. J.; Rees, C. W.; Young, R. G. J. Chem. Soc., Perkin
Trans. 1 1991, 329e333.
11. (a) Sun, F.; Zhang, G.; Zhang, D.; Xue, L.; Jiang, H. Org. Lett. 2011, 13, 6378e6381;
(b) Lee, J. H.; Lee, H.; Seo, S.; Jaworski, J.; Seo, M. L.; Kang, S.; Lee, J. Y.; Jung, J. H.
New J. Chem. 2011, 35, 1054e1059; (c) Popova, E. A.; Trifonov, R. E.; Ostrovskii,
V. A. Arkivoc 2011, i, 552e572; (d) Rajesha; Bhojya Naik, H. S.; Harish Kumar, H.
N.; Hosamani, K. M.; Mahadevan, K. M. Arkivoc 2009, ii, 11e19.
Supplementary data
Supplementary data related to this article can be found online at
http://dx.doi.org/10.1016/j.tet.2012.12.062.
References and notes
ꢀ
1. (a) Graham, T. J. A.; Doyle, A. G. Org. Lett. 2012, 14, 1616e1619; (b) Fernandaz-Bach-
ꢀ
iller, M. I.; Perez, C.; Monjas, L.; Rademann, J.; Rodríguez-Franco, M. I. J. Med. Chem.
2012, 55, 1303e1317; (c) Das, S. G.; Srinivasan, B.; Hermanson, D. L.; Bleeker, N. P.;
Doshi, J. M.; Tang, R.; Beck, W. T.; Xing, C. J. Med. Chem. 2011, 54, 5937e5948; (d)
Erichsen, M. N.; Huynh, T. H. V.; Abrahamsen, B.; Bastlund, J. F.; Bundgaard, C.;