NHC-initiated cascade, metal-free synthesis of quinoxaline derivatives under solvent-free conditions

Article abstract of DOI:10.1007/s11164-013-1108-1

2,3-Diaryl quinoxaline derivatives have been efficiently synthesized by cascade, metal-free, and solvent-free reaction of aryl aldehydes with aryl 1,2-diamines initiated by N-heterocyclic carbene. Compared with reported methods, this procedure has the advantages of easy work-up, reduced energy consumption, and elimination of solvent waste, hazards, and toxicity. A group of potentially bioactive compounds has been prepared for biomedical screening.

Full text of DOI:10.1007/s11164-013-1108-1

Res Chem Intermed
DOI 10.1007/s11164-013-1108-1
NHC-initiated cascade, metal-free synthesis
of quinoxaline derivatives under solvent-free conditions
•
Lijiu Gao Rui Liu Chenxia Yu Changsheng Yao
•
•
•
•
Tuanjie Li Zhaoxin Xiao
Received: 20 December 2012 / Accepted: 7 February 2013
Ó Springer Science+Business Media Dordrecht 2013
Abstract 2,3-Diaryl quinoxaline derivatives have been efficiently synthesized by
cascade, metal-free, and solvent-free reaction of aryl aldehydes with aryl 1,2-dia-
mines initiated by N-heterocyclic carbene. Compared with reported methods, this
procedure has the advantages of easy work-up, reduced energy consumption, and
elimination of solvent waste, hazards, and toxicity. A group of potentially bioactive
compounds has been prepared for biomedical screening.
Keywords Cascade reaction Á Quinoxalines Á N-Heterocyclic carbene (NHC) Á
Solvent-free synthesis
Introduction
Heterocycles are used in the design and discovery of new physiologically active
compounds [1, 2]. Functionalized quinoxalines are an important class of N-con-
taining heterocycles with a wide range of biological activity, including antiviral,
antibacterial, anthelmintic, kinase inhibitory, anti-inflammatory, and anticancer
activity [3–9]. Quinoxaline derivatives have also been used in dyes, semiconductors,
and ligands in coordination chemistry [10–13]. Much progress has been made in the
synthesis of quinoxaline derivatives. Reactions using 1,2-diketones and their
analogs with catalysts such as Bi(0), HClO4/SiO2, transition metals, b-cyclodextrin
L. Gao Á R. Liu Á C. Yu Á T. Li
School of Chemistry and Engineering, Jiangsu Key Laboratory of Green Synthetic Chemistry
for Functional Materials, Jiangsu Normal University, Xuzhou, Jiangsu 221116,
People’s Republic of China
C. Yao (&) Á Z. Xiao
School of Chemistry and Chemical Engineering, Key Laboratory of Biotechnology on Medical
Plant, Jiangsu Normal University, Xuzhou, Jiangsu 221116, People’s Republic of China
e-mail: csyao@jsnu.edu.cn
123

L. Gao et al.
(b-CD), TMSCl, and ion-exchanged molybdophosphoric acid have been reported
[14–24]. In addition, improved methods, for example 1,4-addition of 1,2-diamines
to diazenylbutenes, condensations catalyzed by Ce(IV) ammonium nitrate, cellulose
sulfuric acid, and Zn-[(L)-proline], and cyclization of a-arylimino oximes, have
recently been discovered and used successfully [25–33]. Despite their importance in
synthetic chemistry, these methods are limited by energy consumption, inaccessible
starting materials, tedious work-up procedures, use of volatile organic solvents,
difficult recovery of catalyst, and use of toxic transition-metal catalysts. Hence,
developing new means of synthesis of quinoxaline derivatives is still a vital
challenge in organic chemistry. N-Heterocyclic carbene (NHC)-catalyzed umpolung
reactions, which have become a quickly growing topic of research in the past
decade, are unusual means of synthesis of quinoxalines [34–48]. Recently, a large
number of NHC-catalyzed inter and intramolecular benzoin condensation reactions
have been used to construct a variety of functionalized organic compounds [49–53].
Because of inherently simple experimental procedures, high bond-forming
efficiency, and great potential for generation of variety, cascade reactions have
aroused much attention from the synthetic community and have been used to
prepare compound libraries of functional molecules for screening [54–65]. To date,
many successful applications of NHC-triggered cascade reactions have been
reported [34–38, 63–65].
We have recently focused our research interests on NHC-catalyzed or NHC-
induced cascade synthesis [66–68]. Our group has reported a new cascade strategy
for synthesis of 2,3-diarylated indoles via metal-free reaction of aryl aldehydes and
aryl amines initiated by NHC under solvent-free conditions [66]. During the course
of further studies on the design and efficient synthesis of potentially biologically
active heterocyclic compounds, we found that NHC-initiated reaction of aryl
aldehydes and aryl 1,2-diamines could give the functionalized quinoxalines in good
yields under solvent-free and metal-free conditions (Scheme 1) [69]. Herein, we
report our recent results.
Results and discussion
Because optimized conditions have been established for synthesis of benzoin from
benzaldehydes [66], we commenced a detailed investigation of conditions for the
cyclization reaction of 1,2-bis(4-bromophenyl)-2-hydroxyethanone with benzene-
1,2-diamine (Table 1). For optimization of the reaction conditions, different
reaction temperatures and Brønsted acids were investigated. Although several
R
NH2
NH2
H
O
N
I
R
N
N
Ar
Ar
Ar
Ar
O
S
(10 mol %)
A
3
ArCHO
DBU (75 mol %)
Solvent-free
OH
TsOH(250 mol%)
Solvent-free
4
2
1
Scheme 1 NHC-initiated cascade, solvent-free synthesis of quinoxalines
123

Synthesis of quinoxaline derivatives
Table 1 Optimization of
Entry
Acid (mol%)
Temp. (°C)
Time (h)
Yield (%)
conditions for reaction of
1,2-bis(4-bromophenyl)-2-
hydroxyethanone and benzene-
1,2-diamine
1
SA (200)
H2SO4 (200)
H2SO4 (200)
H2SO4 (240)
TsOH (200)
TsOH (220)
TsOH (240)
TsOH (240)
TsOH (250)
TsOH (250)
TsOH (250)
TsOH (250)
TsOH (280)
TsOH (280)
120
120
120
120
120
120
120
130
120
120
130
150
120
130
6
23
35
36
26
39
44
51
61
57
55
76
69
65
56
2
5
3
6
4
5
5
5
6
4.5
5
7
8
3.5
3.5
5
9
10
11
12
13
14
3.5
4
4
3.5
Brønsted acids, for example SA (sulfonic acid), and H2SO4 (18.4 mol/L) were
effective in the reaction, p-toluenesulfonic acid (TsOH) was the optimum acid. To
obtain the best yield in this transformation, we then optimized the reaction
temperature and the amount of TsOH. It was found that the reaction gave the
expected products in good yields in the presence of 250 mol% TsOH at 130 °C.
The generality of this reaction was investigated on the basis of these optimized
reaction conditions. The results are summarized in Table 2. Aryl 1,2-diamines
bearing the electron-donating methyl group afforded high yields of 2,3-diphenyl-
quinoxaline derivatives. Next, we investigated the range of aryl aldehydes suitable
for synthesis of quinoxalines. It is apparent that this procedure can be successfully
applied to aryl aldehydes bearing electron-withdrawing groups, for example
halogens. However, electron-rich aryl aldehydes, heteroaromatic aldehydes, and
fatty aldehydes did not participate in this cascade reaction.
Although the detailed mechanism of this reaction must still be fully elucidated,
formation of 2,3-diphenylquinoxaline could possibly be explained by the reaction
sequence depicted in Scheme 2. With an efficient pathway to a-hydroxyethanone
derivatives from aryl aldehydes in the presence of NHC, subsequent reaction of the
a-hydroxyethanone derivatives with aryl 1,2-diamines catalyzed by acid gives the
corresponding quinoxaline derivatives. The specific route involving 5 possibly
proceeds via protonation, addition, and dehydration to form intermediate 7;
substitution and cyclization subsequently gives intermediate 9. Oxidation of 9 in air
then affords the target molecule.
Conclusion
We report a cascade reaction initiated by NHC for direct and efficient synthesis of
functionalized 2,3-aryl quinoxaline derivatives. The ready availability of the starting
123

L. Gao et al.
Yield (%)
Table 2 Solvent-free synthesis
of 4
Entry
Ar
R
Product
1
4-FC6H4
3-FC6H4
4-BrC6H4
4-ClC6H4
C6H4
H
4a
4b
4c
4d
4e
4f
78
2
H
67
3
H
76
4
H
78
5
H
74
6
3-ClC6H4
4-FC6H4
4-BrC6H4
4-ClC6H4
3-ClC6H4
C6H4
H
69
7
CH3
CH3
CH3
CH3
CH3
CH3
CH3
4g
4h
4i
80
8
79
9
70
10
11
12
13
4j
73
4k
4l
75
3-CH3OC6H4
2,3-Cl2C6H4
Trace
Trace
4m
H
O
H2N
H2N
R
N
Ar
Ar
O
S
Ar
Ar
HO
HO
3
H
ArCHO
OH
1
Ar
2
5
Ar
Ar
OH
NH
Ar
N
R
Ar
Ar
N
-
H
R
2O
H
OH
H2N
OH
H2N
OH2
H2N
R
7
8
6
Ar
Ar
Ar
Ar
R
N
R
N
O
-
H
2O
N
N
H
4
9
Scheme 2 Proposed mechanistic pathway of the cascade reaction
materials, atom economy of the reaction, and useful structure of the products makes
this strategy quite useful. Investigation of further applications of NHC for synthesis
of other heterocycles of pharmaceutical importance is in progress in our laboratory.
Experimental
Common reagents and materials were purchased from commercial sources and
purified by recrystallization or distillation. Infrared (IR) spectra were recorded on a
123

Synthesis of quinoxaline derivatives
Tensor 27 spectrophotometer in KBr pellets and are reported as absorption
1
frequency (cm-1). NMR spectra were recorded on a Bruker DPX 400; data for H
are reported as follows: chemical shift (ppm), multiplicity (s, singlet; d, doublet; t,
triplet; q, quartet; m, multiplet; and br, broad), coupling constant (Hz) and
integration. HRMS (ESI) was performed with a microTOF-QII HRMS/MS
instrument (Bruker).
General procedure for synthesis of 2,3-diaryl quinoxalines (4a–4k)
Aryl aldehyde (1.0 mmol), precatalyst A (0.029 g, 10 mol%), and DBU (0.114 g,
75 mol%) were triturated together in an agate mortar for 45 min at 55 °C. TsOH
(0.237 g, 250 mol%) and aryl amine (0.5 mmol) were then added and the mixture
was kept at 130 °C. Upon completion of the reaction, monitored by TLC, the
mixture was cooled to room temperature and purified by column chromatography
(silica gel, ethyl acetate–petroleum ether, 1:28, v/v) to afford the desired pure
product.
2,3-Bis(4-fluorophenyl)quinoxaline (4a)
White solid; M.P: 130.1–132.0 °C (132–134 °C) [70]; IR (potassium bromide) (v,
1
cm-1): 3,055, 1,584, 1,540, 1,487, 842; H NMR (400 MHz, CDCl3): d 7.04–7.08
(m, 4H, ArH), 7.49–7.53 (m, 4H, ArH), 7.77–7.81 (m, 2H, ArH), 8.14–8.18 (m, 2H,
ArH); HRMS (ESI) m/z: Calcd for [M ? H]? C20H13F2N2: 319.1047 found:
319.1053.
2,3-Bis(3-fluorophenyl)quinoxaline (4b)
White solid; M.P: 134.3 –136.7 °C; IR (potassium bromide) (v, cm-1): 3,063,
1
1,521, 1,487, 796, 712; H NMR (400 MHz, CDCl3): d 7.07–7.12 (m, 2H, ArH),
7.23–7.26 (m, 2H, ArH), 7.28–7.34 (m, 4H, ArH), 7.80–7.83 (m, 2H, ArH),
8.17–8.20 (m, 2H, ArH); HRMS (ESI) m/z: Calcd for [M ? H]? C20H13F2N2:
319.1047 found: 319.1044.
2,3-Bis(4-bromophenyl)quinoxaline (4c)
White solid; M.P: 191.6 –192.7 °C (190–191 °C) [71]; IR (potassium bromide) (v,
1
cm-1): 3,060, 1,595, 1,581, 1,460, 831; H NMR (400 MHz, CDCl3): d 7.39–7.42
(m, 4H, ArH), 7.50–7.52 (m, 4H, ArH), 7.78–7.81 (m, 2H, ArH), 8.14–8.17 (m, 2H,
ArH); HRMS (ESI) m/z: Calcd for [M ? H]? C20H13Br2N2: 440.9425 found:
440.9429. 13C NMR (100 MHz, DMSO-d6): d 151.9, 141.3, 137.7, 131.7, 131.4,
130.4, 129.2, 123.7.
2,3-Bis(4-chlorophenyl)quinoxaline (4d)
White solid; M.P: 169.0 –171.0 °C (174.9 °C) [72]; IR (potassium bromide) (v,
1
cm-1): 3,062, 1,557, 1,449, 839, 801; H NMR (400 MHz, CDCl3): d 7.35 (d,
123

L. Gao et al.
J = 8.8 Hz, 4H, ArH), 7.47 (d, J = 8.4 Hz,4H, ArH), 7.80 (dd, J1 = 3.2 Hz,
J2 = 6.4 Hz, 2H, ArH), 8.16 (dd, J1 = 3.2 Hz, J2 = 6.4 Hz, 2H, ArH); HRMS
(ESI) m/z: Calcd for [M ? H]? C20H13Cl2N2: 351.0456 found: 351.0440.
2,3-Diphenylquinoxaline (4e)
White solid; M.P: 113.4–115.0 °C (120–122 °C) [73]; IR (potassium bromide) (v,
1
cm-1): 3,055, 1,575, 1,556, 1,446, 766; H NMR (400 MHz, CDCl3): d 7.31–7.36
(m, 6H, ArH), 7.52 (dd, J1 = 1.2 Hz, J2 = 7.2 Hz, 4H, ArH), 7.75–7.79 (m, 2H,
ArH), 8.16–8.20 (m, 2H, ArH); HRMS (ESI) m/z: Calcd for [M ? H]? C20H15N2:
283.1235 found: 283.1231.
2,3-Bis(3-chlorophenyl)quinoxaline (4f)
White solid; M.P: 176.0–178.0 °C; IR (potassium bromide) (v, cm-1): 3,063, 1,557,
1
1,487, 1,447, 732, 694; H NMR (400 MHz, CDCl3): d 7.23–7.28 (m, 4H, ArH),
7.36–7.39 (m, 2H, ArH), 7.67 (d, J = 1.6 Hz, 2H, ArH), 7.81–7.84 (m, 2H, ArH),
8.19 (dd, J1 = 3.2 Hz, J2 = 6.4 Hz, 2H, ArH); HRMS (ESI) m/z: Calcd for
[M ? H]? C20H13Cl2N2: 351.0456 found: 351.0465.
2,3-Bis(4-fluorophenyl)-6-methylquinoxaline (4g)
White solid; M.P: 152.2 –155.5 °C (163–165 °C) [70]; IR (potassium bromide) (v,
cm-1): 3,058, 1,509, 1,489, 827; 1H NMR (400 MHz, CDCl3): d 2.62 (s, 3H, CH3),
7.02–7.07 (m, 4H, ArH), 7.47–7.51 (m, 4H, ArH) 7.62 (dd, J1 = 2.0 Hz,
J2 = 8.8 Hz, 1H, ArH), 7.93 (s,1H, ArH),8.04 (d, J = 8.4 Hz, 1H, ArH); HRMS
(ESI) m/z: Calcd for [M ? H]? C21H15F2N2: 333.1203 found: 333.1218.
2,3-Bis(4-bromophenyl)-6-methylquinoxaline (4h)
White solid; M.P: 181.4 –183.2 °C (183–184 °C) [74]; IR (potassium bromide) (v,
1
cm-1): 3,029, 1,528, 1,486, 832, 722; H NMR (400 MHz, CDCl3): d 2.62 (s, 3H,
CH3), 7.38 (d, J = 8.0 Hz, 4H, ArH), 7.50 (d, J = 7.6 Hz, 4H, ArH), 7.62 (d,
J = 8.4 Hz 1H, ArH), 7.92 (s, 1H, ArH), 8.04 (d, J = 7.6 Hz, 1H, ArH); HRMS
(ESI) m/z: Calcd for [M ? H]? C21H15N2Br2: 454.9585 found: 454.9591.
2,3-Bis(4-chlorophenyl)-6-methylquinoxaline (4i)
White solid; M.P: 179.2 –181.5 °C (178–180 °C) [75]; IR (potassium bromide) (v,
1
cm-1): 1,541, 1,456, 834, 730; H NMR (400 MHz, CDCl3): d 2.62 (s, 3H, CH3),
7.34 (d, J = 8.4 Hz, 4H, ArH), 7.45 (d, J = 8.4 Hz, 4H, ArH), 7.62 (dd,
J1 = 1.6 Hz, J2 = 8.4 Hz, 1H, ArH), 7.93 (s,1H, ArH),8.04 (d, J = 8.4 Hz, 1H,
ArH); HRMS (ESI) m/z: Calcd for [M ? H]? C21H15Cl2N2: 365.0612 found:
365.0593.
123

Synthesis of quinoxaline derivatives
2,3-Bis(3-chlorophenyl)-6-methylquinoxaline (4j)
White solid; M.P: 106.2–107.1 °C; IR (potassium bromide) (v, cm-1): 3,059, 1,595,
1
1,542, 800, 680; H NMR (400 MHz, CDCl3): d 2.63 (s, 3H, CH3), 7.21–7.26 (m,
4H, ArH), 7.35–7.37 (m, 2H, ArH), 7.63–7.65 (m, 3H, ArH), 7.95 (s,1H, ArH), 8.06
(d, J = 8.8 Hz, 1H, ArH); HRMS (ESI) m/z: Calcd for [M ? H]? C21H15Cl2N2:
365.0612 found: 365.0607.
6-Methyl-2,3-diphenylquinoxaline (4k)
White solid; M.P: 110.0–111.2 °C (112–114 °C)[73]; IR (potassium bromide) (v,
1
cm-1): 3,055, 1,538, 1,504, 808, 701; H NMR (400 MHz, CDCl3): d 2.62 (s, 3H,
CH3), 7.32–7.36 (m, 6H, ArH), 7.49–7.51 (m, 4H, ArH), 7.61 (dd, J1 = 1.6 Hz,
J2 = 8.4 Hz, 1H, ArH), 7.95 (s,1H, ArH), 8.06 (d, J = 8.4 Hz, 1H, ArH); HRMS
(ESI) m/z: Calcd for [M ? H]? C21H17N2: 297.1392 found: 297.1397.
Acknowledgments We are grateful for financial support by NSFC (no. 21242014), a project funded by
the Priority Academic Program Development of Jiangsu Higher Education Institutions, and the Major
Basic Research Project of the Natural Science Foundation of the Jiangsu Higher Education Institutions
(09KJA430003). This project was also sponsored by the Qing Lan Project (10qlg005).
References
1. L. Wang, K.W. Woods, Q. Li, K.J. Barr, R.W. McCroskey, S.M. Hannick, L. Gherke, R.B. Credo,
Y.H. Hui, K. Marsh, R. Warner, J.Y. Lee, N. Zielinski-Mozng, D. Frost, S.H. Rosenberg, H.L. Sham,
J. Med. Chem. 45, 1697 (2002)
2. E.A. Couladouros, A.T. Strongilos, Angew. Chem. Int. Ed. 41, 3677 (2002)
3. A. Gomtsyan, E.K. Bayburt, R.G. Schmidt, G.Z. Zheng, R.J. Perner, S. Didomenico, J.R. Koenig, S.
Turner, T. Jinkerson, I. Drizin, S.M. Hannick, B.S. Macri, H.A. McDonald, P. Honore, C.T. Wismer,
K.C. Marsh, J. Wetter, K.D. Stewart, T. Oie, M.F. Jarvis, C.S. Surowy, C.R. Faltynek, C.H. Lee, J.
Med. Chem. 48, 744 (2005)
4. R. Sarges, H.R. Howard, R.G. Browne, L.A. Label, P.A. Seymour, B.K. Koe, J. Med. Chem. 33, 2240
(1990)
5. L.E. Seitz, W.J. Suling, R.C. Reynolds, J. Med. Chem. 45, 5604 (2002)
6. A. Jaso, B. Zarranz, I. Aldana, A. Monge, J. Med. Chem. 48, 2019 (2005)
7. W. He, M.R. Myers, B. Hanney, A.P. Spada, G. Bilder, H. Galzcinski, D. Amin, S. Needle, K. Page,
Z. Jayyosi, M.H. Perrone, Bioorg. Med. Chem. Lett. 13, 3097 (2003)
8. M.M. Ali, M.M.F. Ismail, M.S.A. El-Gaby, M.A. Zahran, Y.A. Ammar, Molecules 5, 864 (2000)
9. G. Sakata, K. Makino, Y. Kurasawa, Heterocycles 27, 2481 (1988)
10. E.D. Brock, D.M. Lewis, T.I. Yousaf, H.H. Harper, WO patent No. 9951688, 1999
11. S. Dailey, J.W. Feast, R.J. Peace, I.C. Sage, S. Till, E.L. Wood, J. Mater. Chem. 11, 2238 (2001)
12. S. Saravanakumar, M.K. Kindermann, J. Heinicke, M. Koeckerling, Chem. Commun. 14(6), 640
(2006)
13. M.S. Kulkarni, A.S. Kumbhar, H. Mohan, B.S.M. Rao, Dalton Trans. 6185 (2009)
14. S. Antoniotti, E. Dunach, Tetrahedron Lett. 43, 3971 (2002)
15. B. Das, K. Venkateswarlu, K. Suneel, A. Majhi, Tetrahedron Lett. 48, 5371 (2007)
16. (a) S. A. Raw, C. D. Wilfred, R. J. K. Taylor, Chem. Commun. 2286 (2003)
17. S.A. Raw, C.D. Wilfred, R.J.K. Taylor, Org. Biomol. Chem. 2, 788 (2004)
18. R.S. Robinson, R.J.K. Taylor, Synlett 1003 (2005)
19. S.Y. Kim, K.H. Park, Y.K. Chung, Chem. Commun. 1321 (2005)
20. C.S. Cho, S.G. Oh, J. Mol. Catal. A Chem. 276, 205 (2007)
21. F. Pan, T.M. Chen, J.J. Cao, J.P. Zou, W. Zhang, Tetrahedron Lett. 53, 2508 (2012)
123

L. Gao et al.
22. B. Madhav, S. Narayana Murthy, V. Prakash Reddy, K. Rama Rao, Y.V.D. Nageswar, Tetrahedron
Lett. 50, 6025 (2009)
23. J.P. Wan, S.F. Gan, J.M. Wu, Y.J. Pan, Green Chem. 11, 1633 (2009)
24. K.T. Venkateswara Rao, P.S. Sai Prasad, N. Lingaiah, J. Mol. Catal. A Chem. 312, 65 (2009)
25. D. Aparicio, O.A. Attanasi, P. Filippone, R. Ignacio, S. Lillini, F. Mantellini, F. Palacios, J.M.
Santos, J. Org. Chem. 71, 5897 (2006)
26. J.J. Cai, J.P. Zou, X.Q. Pan, W. Zhang, Tetrahedron Lett. 49, 7386 (2008)
27. S.V. More, M.N.V. Sastry, C.F. Yao, Green Chem. 8, 91 (2006)
28. M.M. Heravi, S. Taheri, K. Bakhtiari, H.A. Oskooie, Catal. Commun. 8, 211 (2007)
29. B.C. Raju, N.D. Theja, J.A. Kumar, Synth. Commun. 39, 175 (2009)
30. A. Shaabani, A.H. Rezayan, M. Behnam, M. Heidary, C. R. Chimie. 12, 1249 (2009)
31. M.M. Heravi, M.H. Tehrani, K. Bakhtiari, H.A. Oskooie, Catal. Commun. 8, 1341 (2007)
32. N.P. Xekoukoulotakis, C.P. Hadjiantoniou-Maroulis, A.J. Maroulis, Tetrahedron Lett. 41, 10299 (2000)
33. H.R. Darabi, S. Mohandessi, K. Aghapoor, F. Mohsenzadeh, Catal. Commun. 8, 389 (2007)
34. D. Enders, T. Balensiefer, Acc. Chem. Res. 37, 534 (2004)
35. K. Zeitler, Angew. Chem. Int. Ed. 44, 7506 (2005)
36. D. Enders, O. Niemeier, A. Henseler, Chem. Rev. 107, 5606 (2007)
37. N. Marion, S. Diez-Gonzalez, S.P. Nolan, Angew. Chem. Int. Ed. 46, 2988 (2007)
38. J.L. Moore, T. Rovis, Top. Curr. Chem. 291, 77 (2010)
39. L. He, T.Y. Jian, S. Ye, J. Org. Chem. 72, 7466 (2007)
40. J.H. Song, Z.L. Tan, J.T. Reeves, N.K. Yee, C.H. Senanayake, Org. Lett. 9, 1013 (2007)
41. H. Takikawa, K. Suzuki, Org. Lett. 9, 2713 (2007)
42. P.C. Chiang, J. Kaeobamrung, J.W. Bode, J. Am. Chem. Soc. 129, 3520 (2007)
43. W. Jeong, J.L. Hedrick, R.M. Waymouth, J. Am. Chem. Soc. 129, 8414 (2007)
44. E.M. Phillips, M. Wadamoto, A. Chan, K.A. Scheidt, Angew. Chem. Int. Ed. 46, 3107 (2007)
45. J.W. Bode, S.S. Sohn, J. Am. Chem. Soc. 129, 13798 (2007)
46. E.M. Phillips, T.E. Reynolds, K.A. Scheidt, J. Am. Chem. Soc. 130, 2416 (2008)
47. Y.R. Zhang, L. He, X. Wu, P.L. Shao, S. Ye, Org. Lett. 10, 277 (2008)
48. J. Seayad, P.K. Patra, Y.G. Zhang, J.Y. Ying, Org. Lett. 10, 953 (2008)
49. T. Ema, Y. Oue, K. Akihara, Y. Miyazaki, T. Sakai, Org. Lett. 11, 4866 (2009)
50. S.P. Lathrop, T. Rovis, J. Am. Chem. Soc. 131, 13628 (2009)
51. D. Enders, A. Henseler, Adv. Synth. Catal. 351, 1749 (2009)
52. D. Enders, A. Grossmann, J. Fronert, G. Raabe, Chem. Commun. 46, 6282 (2010)
53. Y. Shimakawa, T. Morikawa, S. Sakaguchi, Tetrahedron Lett. 51, 1786 (2010)
54. A. Shaabani, E. Soleimani, A. Sarvary, A.H. Rezayan, Bioorg. Med. Chem. Lett. 18, 3968 (2008)
´
55. K. Bedjeguelal, H. Bienayme, A. Dumoulin, S. Poigny, P. Schmitt, E. Tam, Bioorg. Med. Chem.
Lett. 16, 3998 (2006)
´
56. B.B. Toure, D.G. Hall, Chem. Rev. 109, 4439 (2009)
57. A. Doemling, Chem. Rev. 106, 17 (2006)
58. J.D. Sunderhaus, S.F. Martin, Chem. Eur. J. 15, 1300 (2009)
59. V. Estevez, M. Villacampa, J.C. Menendez, Chem. Soc. Rev. 39, 4402 (2010)
60. G.H. Harris, A.E. Graham, Tetrahedron Lett. 51, 6890 (2010)
61. F.Z. Peng, Z.H. Shao, Curr. Org. Chem. 15, 4144 (2011)
62. L.Q. Lu, J.R. Chen, W.J. Xiao, Acc. Chem. Res. 45, 1278 (2012)
63. A.R. Bharadwaj, K.A. Scheidt, Org. Lett. 6, 2465 (2004)
64. L.R. Wen, C. Liu, M. Li, L.J. Wang, J. Org. Chem. 75, 7605 (2010)
65. M. Li, H. Cao, Y. Wang, X.L. Lv, L.R. Wen, Org. Lett. 14, 3470 (2012)
66. C.S. Yao, D.L. Wang, J. Lu, B.B. Qin, H.H. Zhang, T.J. Li, C.X. Yu, Tetrahedron Lett. 52, 6162
(2011)
67. C.X. Yu, J. Lu, T.J. Li, D.L. Wang, B.B. Qin, H.H. Zhang, Synlett 2420 (2011)
68. C.S. Yao, D.L. Wang, J. Lu, T.J. Li, W.H. Jiao, C.X. Yu, Chem. Eur. J. 18, 1914 (2012)
69. Y. Lin, X.L. Lei, Q. Yang, J.J. Yuan, Q.P. Ding, J.S. Xu, Y.Y. Peng, Synthesis 44, 2699 (2012)
70. A. Hasaninejad, M. Shekouhy, A. Zare, M.A. Zolfigol, Synth. Commun. 39, 569 (2009)
71. H.Y. Lue, S.H. Yang, J. Deng, Z.H. Zhang, Aust. J. Chem. 63, 1290 (2010)
72. J.F. Zhou, G.X. Gong, S.J. Zhi, X.L. Duan, Synth. Commun. 39, 3743 (2009)
73. J. Li, D.N. Jiang, J.X. Chen, M.C. Liu, J.C. Ding, H.Y. Wu, J. Heterocycl. Chem. 48, 403 (2011)
74. J.T. Hou, Y.H. Liu, Z.H. Zhang, J. Heterocycl. Chem. 47, 703 (2010)
75. K. Niknam, M.A. Zolfigol, Z. Tavakoli, Z. Heydari, J. Chin. Chem. Soc. 55, 1373 (2008)
123