Selective copper(II) acetate and potassium iodide catalyzed oxidation of aminals to dihydroquinazoline and quinazolinone alkaloids

Article abstract of DOI:10.3762/bjoc.9.135

Copper(II) acetate/acetic acid/O₂ and potassium iodide/tert-butylhydroperoxide systems are shown to affect the selective oxidation of ring-fused aminals to dihydroquinazolines and quinazolinones, respectively. These methods enable the facile preparation of a number of quinazoline alkaloid natural products and their analogues.

Full text of DOI:10.3762/bjoc.9.135

Selective copper(II) acetate and potassium iodide
catalyzed oxidation of aminals to dihydroquinazoline
and quinazolinone alkaloids
Matthew T. Richers, Chenfei Zhao and Daniel Seidel*
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2013, 9, 1194–1201.
Department of Chemistry and Chemical Biology, Rutgers, The State
University of New Jersey, Piscataway, New Jersey 08854, USA
doi:10.3762/bjoc.9.135
Received: 27 March 2013
Accepted: 28 May 2013
Published: 20 June 2013
Email:
Daniel Seidel* - seidel@rutchem.rutgers.edu
* Corresponding author
This article is part of the Thematic Series "Transition-metal and
organocatalysis in natural product synthesis".
Keywords:
aminal; copper; oxygen; tert-butylhydroperoxide; quinazoline alkaloid
Guest Editors: D. Y.-K. Chen and D. Ma
© 2013 Richers et al; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
Copper(II) acetate/acetic acid/O2 and potassium iodide/tert-butylhydroperoxide systems are shown to affect the selective oxidation
of ring-fused aminals to dihydroquinazolines and quinazolinones, respectively. These methods enable the facile preparation of a
number of quinazoline alkaloid natural products and their analogues.
Introduction
Quinazoline alkaloids are a class of naturally occurring com- common method involves the condensation of an ortho-
pounds with a range of medicinal properties and have been indi- aminobenzoic ester with a lactam promoted by phosphoryl
cated for use as bronchodilators, vasodilators, anti-inflamma- chloride, known as the Niementowski reaction [3,27-30]
tory agents and acetylcholinesterase inhibitors [1-5]. Many of (Figure 2). The availability, or lack thereof, of the corres-
the plants these products have been isolated from, such as ponding lactam can determine the length and efficiency of the
Adhatoda vasica, Peganum harmala and Evodia rutaecarpa, route. Access to the sometimes more biologically active dihy-
have been used in folk medicine for centuries [6-9]. Since the droquinazolines, such as deoxyvasicine (2), from quinazoli-
original isolation of vasicine (1, Figure 1) in 1888 [10], the bio- nones requires a subsequent reduction of the amide. In 2008,
logical properties of this class of alkaloids have been exten- our group reported the syntheses of deoxyvasicinone (4) and
sively studied.
rutaecarpine (6) by the potassium permanganate promoted
oxidation of aminals, which in turn were obtained from the con-
A number of synthetic strategies have been employed to gain densation of ortho-aminobenzaldehydes and simple secondary
access to quinazoline alkaloids [5,11-26]. Perhaps the most amines [31,32]. A number of these aminal precursors were
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Beilstein J. Org. Chem. 2013, 9, 1194–1201.
The conversion of the aminals formed from the condensation of
aminobenzaldehydes and secondary amines to the corres-
ponding dihydroquinazoline and quinazolinone structures under
mild and catalytic conditions would be preferable to using harsh
oxidants and strong bases. Han et al. have recently shown the
ability of copper salts, in conjunction with oxygen, to catalyze
oxidations of 2-substituted tetrahydroquinazoline aminals to
quinazolines [35] (Figure 2). In addition, Reddy and co-workers
have developed a catalytic system in which 2,3-substituted
tetrahydroquinazoline aminals are converted to quinazolinones
using tert-butylhydroperoxide (TBHP) and catalytic potassium
iodide [36,37]. While these examples deal with the oxidation of
bicyclic aminals, we were interested in developing methods to
create dihydroquinazoline and quinazolinone alkaloids from
ring-fused aminals. Here we present catalytic methods for the
synthesis of both these compound classes from aminals using
Figure 1: Examples of naturally occurring quinazoline alkaloids.
prepared in generally good to excellent yields with the scope Cu(OAc)2/O2/AcOH and KI/TBHP systems, respectively.
encompassing various cyclic amines and substituents on the
Results and Discussion
aminobenzaldehyde aryl ring. Since then, we have demon-
Copper-catalyzed oxidations of aminals to
dihydroquinazolines
strated that the reaction can be run on a multigram scale [33]
and have shown that dihydroquinazolines vasicine (1) and
deoxyvasicine (2) can be synthesized from their corresponding
aminals by using an iodine-promoted oxidation [34]. While
resulting in good yields, these oxidations have the drawback of
requiring large amounts of a strong oxidant for the perman-
ganate oxidation and the necessity of stoichiometric n-butyl-
lithium for the iodine reaction.
Copper-catalyzed oxidation reactions have received a great deal
of interest in recent years [38-44]. Han’s copper-catalyzed
method for the synthesis of aminals to quinazolines results in
high yields [35], but the process is not applicable to mono-
oxidation as dihydroquinazolines are not isolated as products in
these reactions. We set out to develop a method for the syn-
Figure 2: Different approaches to the synthesis of quinazoline alkaloid structures.
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Beilstein J. Org. Chem. 2013, 9, 1194–1201.
thesis of dihydroquinazolines that would prevent further amidine moiety apparently activates the molecule for oxidation
oxidation at the benzylic position. A factor complicating this at the benzylic position; we have observed that samples of
effort was that dihydroquinazolines like deoxyvasicine (2) are aminal 7 can remain stable in the freezer for years, whereas 2
known to auto-oxidize to their quinazolinone counterparts by begins to convert to 4 within a day when exposed to atmos-
exposure to air [3,45-47]. We initiated our efforts by exposing pheric oxygen. Considering this, we reasoned that addition of a
aminal 7 to stoichiometric amounts of CuCl2 in acetonitrile weak acid to protonate the relatively basic amidine moiety of 2
under a nitrogen atmosphere, which led to the formation of 2 in might deactivate the benzylic position toward oxidation while
81% yield (Table 1, entry 1). To improve the efficiency of the not interfering with the initial aminal oxidation. Indeed, using
process, catalytic conditions were subsequently evaluated. 1.1 equivalents of acetic acid as an additive with catalytic
When aminal 7 was heated under reflux in an oxygen atmos- Cu(OAc)2 in acetonitrile led to the formation of 2 in 53% yield
phere and in the presence of 20 mol % of CuCl2, 2 was only without formation of 4 and 8 (Table 1, entry 4). A simple
observed in trace amounts; deoxyvasicinone (4) and peroxide 8 change of the solvent from acetonitrile to methanol drastically
were also formed as products. Switching the catalyst to improved the yield of 2 to 81% (Table 1, entry 5). A number of
Cu(OAc)2 led to a 15% yield of the desired product 2, but the different copper salts, solvents and acids were then evaluated,
process was still unselective.
but none of the changes led to a further improvement in yield. It
appears that under certain conditions catalyst deactivation via
It appears that the first oxidation occurs exclusively at the copper oxide formation decreased the catalyst turnover and
aminal site to form deoxyvasicine (2). The presence of the consequently product yields.
Table 1: Optimization of conditions for deoxyvasicine (2) formation.a
Entry Solvent (0.2 M) Catalyst (mol %)
Acid (equiv)
Temp. (°C) Time (h) Yield of 2 (%) Yield of 4 (%) Yield of 8 (%)
1b
2
MeCN
MeCN
MeCN
CuCl2∙2H2O (100)
CuCl2∙2H2O (20)
–
–
–
rt
6
2
3
81
–
–
81
81
trace
15
14
17
10
3
Cu(OAc)2∙H2O
trace
(20)
4
5
6
7
8
MeCN
MeOH
MeOH
AcOH
DMF
Cu(OAc)2∙H2O
(20)
AcOH (1.1)
81
65
65
80
80
3
53
81
33
18c
17
–
–
Cu(OAc)2∙H2O
(20)
AcOH (1.1)
4
–
–
Cu(OAc)2∙H2O
(20)
–
4
6
24
–
Cu(OAc)2∙H2O
(20)
–
24
4
–
Cu(OAc)2∙H2O
AcOH (1.1)
20
–
(20)
9
MeOH
MeOH
EtOHd
Cu(2-EH)2 (20)
CuBr (20)
2-EHA (1.1)
AcOH (1.1)
AcOH (1.1)
65
65
78
12
8
71
72
73
–
–
–
–
10
11
–
Cu(OAc)2∙H2O
1.5
trace
(20)
12
13
14
MeOH
MeOH
MeOH
Cu(OAc)2∙H2O
(10)
AcOH (1.1)
AcOH (1.1)
AcOH (1.1)
65
40
65
18
24
24
67
–
trace
trace
trace
Cu(OAc)2∙H2O
(20)
61
trace
Cu(acac)2 (10)
68c
trace
aReactions were performed on a 0.25 mmol scale. Cu(2-EH)2 = copper(II) 2-ethylhexanoate. 2-EHA = 2-ethylhexanoic acid. bNitrogen atmosphere.
cThe reaction was incomplete. d95% Solution.
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Beilstein J. Org. Chem. 2013, 9, 1194–1201.
Using the optimized reaction conditions, a range of different idine and piperidine revealed that the pyrrolidine-containing
aminals were selectively oxidized to the corresponding dihydro- aminal adopts a bent structure, whereas the piperidine aminal
quinazolines (Table 2). In general, these products were obtained appears relatively strain-free [34]), this finding likely relates to
in moderate to good yields. Product 10, containing a piperidine the reduced propensity of six-membered rings to engage in
ring, required a higher reaction temperature and resulted in a reactions that form exocyclic double bonds. The isolation of
lower yield than the corresponding pyrrolidine and azepane azepinoquinazoline 12 in 73% yield was gratifying but some-
products (2 and 12, respectively). While differences in con- what unexpected since Decker reported that samples of the
formation may in part account for the observed differences in compound completely oxidized to quinazolinone 23 when
reactivity (X-ray crystal structures of aminals containing pyrrol- exposed to air for 24 h [3]. This demonstrates the need for
Table 2: Scope of the copper-catalyzed conversion of aminals to dihydroquinazolines.a
Entry
1
Starting material
Product
Time (h)
Yield (%)
86
7
2
7
9
2b,c
3c
8
7
57
73
10
12
11
4
24
72
82
47
18
14
16
13
15
5
8
6
4
17
19
18
20
7b,d
72
aReactions run on a 1 mmol scale. bEtOH used as solvent. c0.5 mmol scale. dReaction incomplete.
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Beilstein J. Org. Chem. 2013, 9, 1194–1201.
acetic acid to protonate the amidine, preventing further The Cu/TEMPO/DABCO catalyst system employed by Han et
oxidation. While product 16 was obtained in good yields from al. [35] for the oxidation of aminals to quinazolines provided an
tetrahydroisoquinoline-aminal 15, rutaecarpane-derived prod- increased yield of 50% (Table 3, entry 9). The best yields were
uct 18 was formed in only 47% yield, apparently due to uniden- obtained by using the conditions developed by Reddy and
tified side-reactions. The reaction leading to the synthesis of the co-workers [36], namely the combined use of catalytic amounts
dibromo- analogue of deoxyvasicine 20, even under elevated of potassium iodide (20 mol %) and excess TBHP (5 equiv),
temperature and extended reaction time, still did not reach followed by the addition of piperidine. In this instance, deoxy-
completion after 3 days. The attenuated reactivity of aminal 19 vasicinone was isolated in 80% yield (Table 3, entry 12). In the
is most likely the result of the decreased electron density on the course of this reaction, the TBHP adduct 22 is formed as an
anilinic nitrogen.
intermediate that is subsequently converted to the quinazoli-
none upon addition of piperidine. A slight modification of
Reddy’s conditions, in which piperidine was added directly to
the solution after 36 hours instead of the removal of solvent
KI-catalyzed oxidations of aminals to quina-
zolinones
Different conditions for the direct catalytic oxidation of aminals from the intermediate peroxide beforehand, resulted in iden-
to quinazolinones were also explored. The use of Cu(OAc)2 and tical yields.
methanol, while appropriate for furnishing deoxyvasicine (2)
from aminal 7, did not result in satisfactory yields of deoxyvasi- Using the optimized conditions, a range of different quinazoli-
cinone (4, Table 1). Attempts to use other copper(I) or nones were synthesized (Table 4). In general, yields were
copper(II) salts and solvents under oxygen without the addition moderate to good for substrates with varying ring sizes. In this
of acid to promote the full oxidation of aminal 21 to deoxyvasi- manner the natural products deoxyvasicine (4), mackinazoli-
cinone (4) were met with disappointment, with yields of 4 for none (5) and rutaecarpine (6) were prepared, in addition to the
these conditions reaching a maximum of around 40% (Table 3). azepinoquinazolone 23, which has been demonstrated to be a
In most cases, peroxide 8 was observed as a major side product. more effective antitussive agent than codeine [48]. Dibromo-
Table 3: Optimization of conditions for deoxyvasicinone (4) formation.a
Entry
Solvent (0.2 M)
Catalyst (mol %)
Oxidant (equiv) Additive (equiv) Temp. (°C)
Time (h) Yield of 4 (%)
1
2
3
4
5
6
7
8
9
DMSO
DMSO
DMSO
DMSO
MeCN
DMF
CuBr (20)
CuBr (20)
CuBr (20)
CuBr (20)
CuBr(10)
O2
O2
O2
O2
O2
O2
O2
O2
O2
–
100
100
100
60
2
21
25
22
28
43
42
29
19
50
DBU (0.4)
17
3
DBU (2)
–
–
–
–
–
3
80
24
24
3
CuBr(10)
80
DMSO
MeCN
MeCN
CuI (20)
60
CuCl2·2H2O (20)
CuCl (10)
50
5
DABCO (0.1),
TEMPO (0.05)
80
12
10
DMSO
CuCl (10)
O2
DABCO (0.1),
TEMPO (0.05)
100
3
38
11
12
PhMe
EtOH
CuBr (20)
KI (20)
TBHP (5)
TBHP (5)
piperidineb (5)
piperidineb (5)
rt
rt
0.5
36
61
80
aReactions run on a 0.25 mmol scale. DBU = 1,8-Diazabicyclo[5.4.0]undec-7-ene. DABCO = 1,4-Diazabicyclo[2.2.2]octane. TEMPO = 2,2,6,6-
Tetramethylpiperidine-1-oxy radical. bPiperidine was added at the end of the reaction and the reaction mixture was heated at 50 °C for 1 h.
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Beilstein J. Org. Chem. 2013, 9, 1194–1201.
Table 4: Scope of KI-catalyzed conversion of aminals to quinazolinones.a
Entry
1
Starting material
Product
Yield [%]
84
7
9
4
5
2b
3b
59
69
11
23
4
5
60
58
15
17
24
6
6
7
88
50
19
26
25
27
aReactions run on a 1 mmol scale. b0.5 mmol scale.
deoxyvasicinone analogue 25 was obtained in relatively high Interestingly, when quaternary aminal 28 was subjected to
yield (88%) whereas the corresponding analogue of mackina- oxidative conditions in an attempt to prepare compound 29,
zolinone (27) was obtained in only 50% yield.
deoxyvasicinone (4) was obtained as the major product in a
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Beilstein J. Org. Chem. 2013, 9, 1194–1201.
process that involved demethylation (Scheme 1, reaction 1). Acknowledgements
The demethylation of aminals has been previously reported in Financial support from the NIH–NIGMS (grant R01GM-
cases where the product achieves aromaticity [49-51], which is 101389-01) is gratefully acknowledged.
presumably the driving force for this transformation. Aminal
References
1. D’yakonov, A. L.; Telezhenetskaya, M. V. Chem. Nat. Compd. 1997,
30, which contains two tertiary amines and is readily obtain-
able by an acid-promoted hydride shift process [52-54], was
33, 221. doi:10.1007/BF02234869
also exposed to oxidative conditions (Scheme 1, reaction 2). We
2. Sheu, J.-R. Cardiovasc. Drug Rev. 1999, 17, 237.
had hypothesized that quinazolinone 32 might be formed in this
doi:10.1111/j.1527-3466.1999.tb00017.x
reaction by the debenzylation of an intermediate iminium ion.
3. Decker, M. Eur. J. Med. Chem. 2005, 40, 305.
However, the major product from this reaction was identified to
doi:10.1016/j.ejmech.2004.12.003
be 31, the apparent product of iminium hydrolysis.
4. Rachana, S.; Ujata, B.; Mamta, P.; Priyanka, K. M.; Sonam, S.
Indo Global J. Pharm. Sci. 2011, 1, 85.
5. Nepali, K.; Sharma, S.; Ojha, R.; Dhar, K. L. Med. Chem. Res. 2013,
22, 1. doi:10.1007/s00044-012-0002-5
Conclusion
We have demonstrated that quinazoline alkaloids and their
analogues can be synthesized from aminals by using Cu(OAc)2/
O2/AcOH and KI/TBHP catalyst systems. The use of acetic acid
in addition to oxygen and catalytic copper(II) salts was deter-
mined to prevent overoxidation of dihydroquinazolines,
allowing access to these structures under mild conditions. A
number of natural products and their analogues were obtainable
by these methods, which should facilitate the preparation of
novel materials for biological studies.
6. Jia, S.; Hu, C. Molecules 2010, 15, 1873.
doi:10.3390/molecules15031873
7. Dhankhar, S.; Kaur, R.; Ruhil, S.; Balhara, M.; Dhankhar, S.;
Chhillar, A. K. Afr. J. Plant Sci. 2011, 5, 620.
8. Corrêa, G. M.; Alcântara, A. F. de C. Rev. Bras. Farmacogn. 2012, 22,
220. doi:10.1590/S0102-695X2011005000196
9. Asgarpanah, J.; Ramezanloo, F. Afr. J. Pharm. Pharmacol. 2012, 6,
1573.
10.Hooper, D. Pharm. J. Trans. 1888, 18, 841.
11.Bergman, J.; Bergman, S. Heterocycles 1981, 16, 347.
doi:10.3987/R-1981-03-0347
12.Mhaske, S. B.; Argade, N. P. J. Org. Chem. 2001, 66, 9038.
doi:10.1021/jo010727l
Supporting Information
13.Yadav, J. S.; Reddy, B. V. S. Tetrahedron Lett. 2002, 43, 1905.
doi:10.1016/S0040-4039(02)00135-1
Supporting Information File 1
14.Mhaske, S. B.; Argade, N. P. Tetrahedron 2006, 62, 9787.
doi:10.1016/j.tet.2006.07.098
Experimental details, characterization data and 1H and
13C NMR spectra for all new compounds.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-9-135-S1.pdf]
15.Bowman, W. R.; Elsegood, M. R. J.; Stein, T.; Weaver, G. W.
Org. Biomol. Chem. 2007, 5, 103. doi:10.1039/b614075k
Scheme 1: Oxidation of other aminal systems.
1200

Beilstein J. Org. Chem. 2013, 9, 1194–1201.
16.Zhou, J.; Fu, L.; Lv, M.; Liu, J.; Pei, D.; Ding, K. Synthesis 2008, 3974.
doi:10.1055/s-0028-1083245
46.Johns, S. R.; Lamberton, J. A.; Suares, H. Aust. J. Chem. 1985, 38,
1007. doi:10.1071/CH9851007
17.Kraus, G. A.; Guo, H. J. Org. Chem. 2009, 74, 5337.
doi:10.1021/jo900718g
47.Nakagawa, Y.; Stevens, R. V. J. Org. Chem. 1988, 53, 1873.
doi:10.1021/jo00244a007
18.Giri, R.; Lam, J. K.; Yu, J.-Q. J. Am. Chem. Soc. 2010, 132, 686.
doi:10.1021/ja9077705
48.Nepali, K.; Bande, M. S.; Sapra, S.; Garg, A.; Kumar, S.; Sharma, P.;
Goyal, R.; Satti, N. K.; Suri, O. P.; Dhar, K. L. Med. Chem. Res. 2012,
21, 1271. doi:10.1007/s00044-011-9641-1
19.Kshirsagar, U. A.; Puranik, V. G.; Argade, N. P. J. Org. Chem. 2010,
75, 2702. doi:10.1021/jo100400z
49.Elderfield, R. C.; Kreysa, F. J.; Dunn, J. H.; Humphreys, D. D.
J. Am. Chem. Soc. 1947, 69, 186. doi:10.1021/ja01193a506
50.Yamazaki, C. J. Org. Chem. 1981, 46, 3956. doi:10.1021/jo00333a005
51.Schulze, K.; Richter, C.; Ludwig, R. Tetrahedron Lett. 1989, 30, 2369.
doi:10.1016/S0040-4039(01)80401-9
20.Dabiri, M.; Salehi, P.; Bahramnejad, M.; Alizadeh, M. Monatsh. Chem.
2010, 141, 877. doi:10.1007/s00706-010-0341-1
21.Zeng, F.; Alper, H. Org. Lett. 2010, 12, 3642. doi:10.1021/ol101428v
22.Kshirsagar, U. A.; Argade, N. P. Org. Lett. 2010, 12, 3716.
doi:10.1021/ol101597p
52.Zhang, C.; Murarka, S.; Seidel, D. J. Org. Chem. 2009, 74, 419.
doi:10.1021/jo802325x
23.Tseng, M.-C.; Cheng, H.-T.; Shen, M.-J.; Chu, Y.-H. Org. Lett. 2011,
13, 4434. doi:10.1021/ol201793v
53.Mori, K.; Ohshima, Y.; Ehara, K.; Akiyama, T. Chem. Lett. 2009, 38,
524. doi:10.1246/cl.2009.524
24.Granger, B. A.; Kaneda, K.; Martin, S. F. Org. Lett. 2011, 13, 4542.
doi:10.1021/ol201739u
54.He, Y.-P.; Du, Y.-L.; Luo, S.-W.; Gong, L.-Z. Tetrahedron Lett. 2011,
52, 7064. doi:10.1016/j.tetlet.2011.10.062
25.Fang, J.; Zhou, J. Org. Biomol. Chem. 2012, 10, 2389.
doi:10.1039/c2ob07178a
26.Murai, K.; Komatsu, H.; Nagao, R.; Fujioka, H. Org. Lett. 2012, 14, 772.
doi:10.1021/ol203313n
27.von Niementowski, S. J. Prakt. Chem. 1895, 51, 564.
doi:10.1002/prac.18950510150
License and Terms
28.Pachter, I.; Suld, G. J. Org. Chem. 1960, 25, 1680.
doi:10.1021/jo01079a615
This is an Open Access article under the terms of the
Creative Commons Attribution License
29.D’yakonov, A. L.; Telezhenetskaya, M. V.; Yunusov, L. Yu.
Chem. Nat. Compd. 1986, 22, 435. doi:10.1007/BF00579819
30.Lee, E. S.; Park, J.-G.; Jahng, Y. Tetrahedron Lett. 2003, 44, 1883.
doi:10.1016/S0040-4039(03)00080-7
(http://creativecommons.org/licenses/by/2.0), which
permits unrestricted use, distribution, and reproduction in
any medium, provided the original work is properly cited.
31.Zhang, C.; De, C. K.; Mal, R.; Seidel, D. J. Am. Chem. Soc. 2008, 130,
416. doi:10.1021/ja077473r
The license is subject to the Beilstein Journal of Organic
Chemistry terms and conditions:
32.Dieckmann, A.; Richers, M. T.; Platonova, A. Yu.; Zhang, C.;
Seidel, D.; Houk, K. N. J. Org. Chem. 2013, 78, 4132.
doi:10.1021/jo400483h
(http://www.beilstein-journals.org/bjoc)
33.Zhang, C.; De, C. K.; Seidel, D. Org. Synth. 2012, 89, 274.
34.Richers, M. T.; Deb, I.; Platonova, A. Y.; Zhang, C.; Seidel, D.
Synthesis 2013, 45, 1730–1748. doi:10.1055/s-0033-1338852
35.Han, B.; Yang, X.-L.; Wang, C.; Bai, Y.-W.; Pan, T.-C.; Chen, X.;
Yu, W. J. Org. Chem. 2012, 77, 1136. doi:10.1021/jo2020399
36.Kumar, R. A.; Maheswari, C. U.; Ghantasala, S.; Jyothi, C.;
Reddy, K. R. Adv. Synth. Catal. 2011, 353, 401.
doi:10.1002/adsc.201000580
The definitive version of this article is the electronic one
which can be found at:
doi:10.3762/bjoc.9.135
37.Kumar, R. A.; Saidulu, G.; Prasad, K. R.; Kumar, G. S.; Sridhar, B.;
Reddy, K. R. Adv. Synth. Catal. 2012, 354, 2985.
doi:10.1002/adsc.201200679
38.Li, C.-J. Acc. Chem. Res. 2009, 42, 335. doi:10.1021/ar800164n
39.Punniyamurthy, T.; Rout, L. Coord. Chem. Rev. 2008, 252, 134.
doi:10.1016/j.ccr.2007.04.003
40.Wendlandt, A. E.; Suess, A. M.; Stahl, S. S. Angew. Chem., Int. Ed.
2011, 50, 11062. doi:10.1002/anie.201103945
41.Hirano, K.; Miura, M. Chem. Commun. 2012, 48, 10704.
doi:10.1039/c2cc34659a
42.Zhang, C.; Tang, C.; Jiao, N. Chem. Soc. Rev. 2012, 41, 3464.
doi:10.1039/c2cs15323h
43.Jones, K. M.; Klussmann, M. Synlett 2012, 159.
doi:10.1055/s-0031-1290117
44.Ratnikov, M. O.; Doyle, M. P. J. Am. Chem. Soc. 2013, 135, 1549.
doi:10.1021/ja3113559
45.Mehta, D. R.; Naravane, J. S.; Desai, R. M. J. Org. Chem. 1963, 28,
445. doi:10.1021/jo01037a041
1201