Bronsted acid ionic liquid catalyzed facile synthesis of 3-vinylindoles through direct C3 alkenylation of indoles with simple ketones

Article abstract of DOI:10.1039/c4gc00840e

A direct dehydrative coupling protocol for the synthesis of 3-vinylindoles using easily available indoles and simple ketones as substrates was developed with the aid of a sulfonyl-containing Bronsted acid ionic liquid. The salient features of this protoco

Full text of DOI:10.1039/c4gc00840e

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Brønsted acid ionic liquid catalyzed facile
synthesis of 3-vinylindoles through direct C3
Cite this: DOI: 10.1039/c4gc00840e
alkenylation of indoles with simple ketones†
Received 7th May 2014,
Accepted 21st May 2014
Amir Taheri,a Changhui Liu,a Bingbing Lai,a Cheng Cheng,a Xiaojuan Pana and
DOI: 10.1039/c4gc00840e
Yanlong Gu*a,b
www.rsc.org/greenchem
A direct dehydrative coupling protocol for the synthesis of 3-vinyl- indole that can be prepared from 3-alkylidene oxindoles and
indoles using easily available indoles and simple ketones as sub- TBS-triflate with the aid of Et3N.15 Acid-catalyzed alkenylation
strates was developed with the aid of
a sulfonyl-containing of indoles with an aldehyde or its congeners has been
Brønsted acid ionic liquid. The salient features of this protocol are employed for introducing a carbon–carbon double bond into
high synthetic efficiency, a metal- and solvent-free system, a the indole skeleton.16 Direct alkenylation of indoles with α-oxo
recyclable catalyst, mild conditions and easy product isolation. ketene dithioacetals was also reported by Yu.17 Although
With the ionic liquid catalyst, a hitherto unreported straight- various methods have been reported for the synthesis of
forward method for the construction of the indolo[3,2-b]carbazole 3-vinylindoles, most of these methods often involve the use of
skeleton was also developed using 2-hydroxymethylindole and expensive reagents or metal-based catalysts. Some of them
acetophenone as starting materials.
involve the use of harsh conditions and suffer from the lack of
simplicity. Therefore, the development of simple, convenient,
and environmentally friendly approaches is desirable.
Indoles are found in many naturally occurring compounds.
Because of their unique biological activities, the modification
of the indole structure has proved to be of great interest to
organic chemists.1,2 3-Vinylindoles are a class of unique func-
tionalized indoles,3 which can be utilized in the synthesis of a
number of biologically significant compounds,4 such as indole
alkaloids,5 carbazoles,6 and carbolines.7 Some 3-vinylindole
derivatives can be used as diene equivalents for the synthesis
of polyfunctional indoles.8 3-Vinylindoles have been reported
recently to exhibit interesting biological activities, such as anti-
cancer,9 antiviral,10 and antibacterial activities.11 Vinylindoles
could be synthesized by many methods, and among which,
metal-based catalyst mediated direct oxidative alkenylation of
indoles has been extensively investigated recently.12 A metal-
free alkenylation protocol has also been developed by Jiao and
co-workers.13 Acid–base catalysis has also been frequently used
in the synthesis of 3-vinylindoles. For instance, hydroarylation
of indoles with alkynes has been established using acid cata-
lysts.14 Rassu et al. have introduced a novel 3-alkenyl-2-silyloxy-
Direct dehydrative couplings of indoles with ketones are
clean reactions to synthesize 3-alkenylated indoles as the only
by-product is water. However, these reactions are strictly
limited to the use of highly active ketones, such as β-diketones,
β-ketoesters and phenyl benzyl ketones, as starting materials.18
The use of inexpensive and abundantly available simple
ketones as substrates in this alkenylation reaction is very
attractive from the viewpoint of lowering the synthetic cost and
extending the product diversity. We have recently reported a
novel sulfonyl-containing Brønsted acid ionic liquid (IL) that
can promote various organic reactions under solvent-free con-
ditions (Table 1, 1a).19 The simultaneous existence of sulfonyl
and sulfonic groups ensures an outstanding catalytic activity
of the IL. In continuation of our research on the selective syn-
thesis of indole derivatives,20 we tried in this work to use the
IL as a catalyst for establishing the direct dehydrative coupling
of indoles and simple ketones. It was found that the protocol
for the synthesis of 3-vinylindoles is indeed practicable. The
established method not only opens an avenue to access 3-viny-
lindole derivatives from cheap and easily available substrates,
but also possesses many characteristic features of green
organic synthesis, such as high reaction yields, recyclable cata-
lyst and easy product separation.
aKey Laboratory for Large-Format Battery Materials and System, Ministry of
Education, School of Chemistry and Chemical Engineering, Huazhong University of
Science and Technology (HUST), 1037 Luoyu Road, Hongshan District, Wuhan,
430074, P.R. China. E-mail: klgyl@hust.edu.cn; Fax: +86-27-87 54 45 32
bState Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute
Initially, a direct dehydrative coupling of 2-methylindole 2a
and acetophenone 3a was investigated. The reaction was per-
formed at 60 °C under solvent-free conditions. Brønsted acids,
such as toluenesulfonic acid (TsOH) and trifluoromethane-
of Chemical Physics, Lanzhou, 730000, P.R. China
†Electronic supplementary information (ESI) available. CCDC 1000775 and
1000776. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c4gc00840e
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Table 1 Dehydrative alkenylation of 2a with 3aa
ation, 1a was neutralized with an aqueous solution of sodium
hydroxide (1.0 N) at the end of the reaction. No precipitate was
observed after adding an AgNO3 (aqu.) to the system, indicat-
ing that sulphur oxides were not formed in our system. This
manifested that IL 1a is quite robust. In addition, under the
optimal conditions, the reactions scaled up to multigram
quantities provided uniform results (entry 5), indicating the
practical usefulness of this method.
We probed then the scope of the reaction with respect to
both the indole and ketone components. Acetophenones with
different substituents smoothly reacted with 2-methylindole,
producing 2-methyl-3-(α-arylvinyl)indoles in moderate to excel-
lent yields (Fig. 1). Both electron-rich and electron-poor aceto-
phenones readily participated in the reaction. Even those
containing substituents in the ortho-position of the acetyl
group, such as 2-bromoacetophenone and 2-fluoroaceto-
phenone, can be used as well, producing the desired products
in high yields. Some bulky ketones, such as 1-tetralone and
2-acetonaphthone, also reacted with 2-substituted indoles
smoothly. Structures of the obtained products, 4n and 4o, have
been unambiguously confirmed by X-ray structural analysis.22
It should be noted that preparation of these indole derivatives
through conventional methods is not easy, which either
involves the use of expensive reagents, such as alkynes23 and
benzoylindoles,24 or is plagued by a low yield resulting from
Entry
Catalyst
Yield (%)
1
2
3
4
TsOH
TfOH
FeCl3
Sc(OTf)3
1a
1b
1a
37 (20,b 73c)
45 (36,b 84c)
29 (19)b
19 (15)b
95 (91)g
42 (73)c
70
5
6
7d
8e
9f
1a
1a
45
95
a 2a: 2.5 mmol, 3a: 2.5 mmol, catalyst: 0.08 mmol. b DCE as the solvent
(1.0 ml). c The reaction time is 1 hour. d 40 °C. e 1a: 0.01 mol%. f 1a was
reused in the fifth run. g The reaction was performed on the
10.0 mmol scale.
sulfonic acid (TfOH), are moderately active for this reaction,
and after 15 minutes of reaction, the expected product 4a was multi-step or insufficient reaction.25 The present system pro-
obtained in 37% and 45% yields, respectively (Table 1, entries
1 and 2). An appreciable yield decrease was observed when the
reactions were performed in an organic solvent, dichloro-
ethane (DCE). An increase of the reaction time is effective for
improving the reaction yields. After 1 hour, the yield with
TsOH and TfOH reached 73% and 84%, respectively. We also
screened many other catalysts. Lewis acids, such as FeCl3 and
Sc(OTf)3, are less active as compared with the examined
Brønsted acids (entries 3 and 4). To our great delight, Brønsted
acid IL 1a showed an excellent performance in the model reac-
tion, and 95% of yield could be obtained within 15 minutes
(entry 5). The reaction with Forbes’s IL, 1b, only provided 4a in
42% yield (entry 6). Although a good yield (72%) was obtained
after 1 hour of reaction with 1b, due to the fact that the
polarities of 4a and 2a were nearly the same this gave rise to a
difficulty in the isolation of the desired product. These results
demonstrated clearly that IL 1a is indeed an efficient catalyst
for the direct dehydrative coupling of 2a and 3a. Further inves-
tigation revealed that the reaction was also affected by the
temperature and catalyst amount (entries 7 and 8), and the
optimal conditions are 60 °C and 3 mol% of 1a catalyst.
Because the 1a catalyst is not soluble in non-polar organic sol-
vents, the formed product could be easily isolated by extraction
with ethyl acetate. The recycled 1a could be reused in the next
run after 30 minutes of drying at 100 °C under vacuum
(20 mmHg). Reuse experiments manifested that 1a could be
reused at least 5 times without a significant loss of its activity
in the model reaction (entry 9). If decomposition of IL 1a
occurs during the reaction, it will produce probably some
vided a cost-effective and environmentally benign method to
access these compounds. Cyclobutyl phenyl ketone is also a
Fig. 1 Substrate scope of 1a-catalyzed C3 alkenylation of indoles
acidic species, such as SO2 and SO3.21 Out of this consider- with ketones.
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viable reagent for alkenylating 2-methylindole without dama-
ging the cyclobutyl group. The skeleton of 4l has shown to be a
potential antimitotic and antitumor agent.26 The results
obtained with aliphatic ketones are as competent as in the
case of aryl alkyl ketones. Various ketones, such as isopropyl
methyl ketone, cyclohexanone, 2-methylcyclohexanone and
2-methylcyclopentanone, could be successfully applied in this
reaction. Interestingly, the dehydrative coupling selectively
occurred in favour of forming a densely substituted double
bond when a substituent group existed in the α-position of the
ketocarbonyl group. Various indoles, such as 2-phenylindole,
1-methyl-2-phenylindole, 1-ethyl-2-phenylindole, 1,2-dimethyl-
indole, 2-methyl-6-fluoroindole and 2,5-dimethylindole, could
all be successfully used in this reaction. Particularly, 5-bromo-
indole can also be alkenylated with cyclic ketones, such as
cyclohexanone. This result overcomes the difficulty of using
non-C2-substituted indoles in the alkenylation reaction.27
The reaction might proceed according to the mechanism
depicted in Fig. 2. The initial event of the reaction is the for-
mation of a carbocation intermediate (I). The desired product
4a could be generated by the following H+ elimination.28
However, trapping of this intermediate with 2a is also possible,
which results in the formation of 5a. Interestingly, this reac-
tion might be reversible as 2-methylindole is a good leaving
group.26,29 However, no 5a was observed during the reaction.
To verify this hypothesis, 5b was synthesized and then treated
with the 1a catalyst (Scheme 1). As we expected, 4aa was
obtained in 92% yield. This result led us to draw a conclusion
that the reversible carbon–carbon bond formation reaction is
indeed able to maximize the selectivity towards 4a.
Scheme 2 C3 arylation of 2a with 3b catalyzed by 1a.
mediate (II);31 (ii) formation of a 2,6-dione (III) through retro-
Claisen cleavage;32 (iii) intramolecular cyclization of (III) and
(iv) C3-vinylation of 2a. An easiness of the last step contributed
probably the main power that enabled the condensation reac-
tion to be possible.
1,1-Diarylethylene derivatives have recently been used as
π-nucleophiles.33 DFT calculation revealed that the electron
density in the double bond of 4a is indeed non-uniform, and a
distribution in favour of the CH2 terminal is foreseeable.21
Because the reactions of π-nucleophiles are often associated
with the use of acid catalysts, we therefore envisioned that it
might be possible to establish some one-pot step-wise reac-
tions by means of adding a suitable electrophile to the reac-
tion system. As shown in Scheme 3, this idea was proved to be
feasible indeed, and both 7a and 9a are able to act as electro-
philes to react with the generated 4a.
Finally, a highly reactive indole, 2-hydroxymethylindole 2b,
was used as the substrate in the title reaction in conjunction
with 3a as an alkenylation reagent. Unexpectedly, in the pres-
ence of 1a, a 5,6,11,12-tetrahydro-6-methyl-6-phenylindolo-
[3,2-b]carbazole 11a was obtained in 88% yield (Scheme 4).
Literature survey stated that the analogous polyheterocycles
show some unique biological activities.34 A previous method
A bifunctional reagent, 3b, was also utilized in this system,
which has two active sites including the ketocarbonyl and the
acetal. It was found unexpectedly that 6a was obtained in 90%
yield (Scheme 2). Remarkably, 1 mol% of 1a is sufficient to
promote this reaction toward completion. Previous systems for
accomplishing the C3-arylation of indoles involve the use of
either toxic catalysts or a time-consuming procedure.30 This
reaction might proceed through the following pathway: (i) con-
densation of two molecules of 3b, which generated an inter-
Fig. 2 Proposed mechanism.
Scheme 1 Synthesis of 4aa from 5b catalysed by IL 1a.
Scheme 3 One-pot step-wise reactions.
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Notes and references
1 For selected recent reviews, see: (a) J. M. Finefield,
J. C. Frisvad, D. H. Sherman and R. M. Williams, J. Nat.
Prod., 2012, 75, 812–833; (b) M. Shiri, Chem. Rev., 2012,
112, 3508–3549; (c) M. Shiri, M. A. Zolfigol, H. G. Kruger
and Z. Tanbakouchian, Chem. Rev., 2010, 110, 2250–2293;
(d) G. R. Humphrey and J. T. Kuethe, Chem. Rev., 2006, 106,
2875 See some examples: (e) P. Sang, Z. Chen, J. Zou and
Y. Zhang, Green Chem., 2013, 15, 2096–2100; (f) G.-P. Fan,
Z. Liu and G.-W. Wang, Green Chem., 2013, 15, 1659–1664;
(g) J. Engel-Andreasen, B. Shimpukade and T. Ulven, Green
Chem., 2013, 15, 336–340.
2 For reviews on synthesis of indoles, see: (a) G. W. Gribble,
in Comprehensive Heterocyclic Chemistry II, ed. A. R.
Katritzky, C. W. Rees and E. F. V. Scriven, Pergamon,
Oxford, UK, 1996, vol. 2, p. 207; (b) M. Bandini and
A. Eichholzer, Angew. Chem., Int. Ed., 2009, 48, 9608–9645;
(c) J. A. Joule, Indole and its Derivatives, in Science of Syn-
thesis (Houben-Weyl Methods of Molecular Transformations),
ed. E. J. Thomas, Thieme, Stuttgart, 2000, vol. 10, ch. 10.13.
3 For selected recent reviews, see: (a) M. Platon, R. Amardeil,
L. Djakovith and J. Hierso, Chem. Soc. Rev., 2012, 41,
3929–3968; (b) R. Vicente, Org. Biomol. Chem., 2011, 9,
6469–6480; (c) S. Cacchi and G. Fabrizi, Chem. Rev., 2011,
111, PR215–PR283.
Scheme 4 Synthesis of indolo[3,2-b]carbazole 11a.
to access the skeleton of this indolo[3,2-b]carbazole involves
the use of 2,3′-diindolylmethane as a critical precursor, which
is very expensive and difficult to prepare. The reaction might
proceed through the following pathway: (i) self-condensation
of two molecules of 2-hydroxymethylindole that generated the
intermediate (V);35 (ii) dehydroxymethylation of (V) to form
2,3′-diindolylmethane 12a; and (iii) electrophilic alkylation of
3a with 12a forms the final product 11a. Although dehydroxy-
methylation of indole derivatives has rarely been reported,
removal of a hydroxymethyl group from an aromatic system is
often used in organic synthesis.36 Furthermore, treating
2,3′-diindolylmethane 12a with acetophenone in the presence
of 1a produced 11a in nearly quantitative yield, which
supported properly the proposed mechanism.
4 (a) W. Liu, H. J. Lim and T. V. RajanBabu, J. Am. Chem.
Soc., 2012, 134, 5496–5499; (b) T. P. Pathak, J. G. Osiak,
R. M. Vaden, B. E. Welm and M. S. Sigman, Tetrahedron,
2012, 68, 5203–5208; (c) G. Bergonzini, L. Gramigna,
A. Mazzanti, M. Fochi, L. Bernardi and A. Ricci, Chem.
Commun., 2010, 46, 327–329; (d) C. Zhang, L.-X. Zhang,
Y. Qiu, B. Xu, Y. Zong and Q.-X. Guo, RSC Adv., 2014, 4,
6916–6919; (e) M. Terada, K. Moriya, K. Kanomata and
K. Sorimachi, Angew. Chem., Int. Ed., 2011, 50,
12586–12590; (f) J. McNulty and D. McLeod, Synlett, 2011,
717–721; (g) C. Liu, W. Zhang, L.-X. Dai and S.-L. You,
Chem. – Asian J., 2014, DOI: 10.1002/asia.201402071.
5 (a) M. Jida, O.-M. Soueidan, B. Deprez, G. Laconde and
R. Deprez-Poulain, Green Chem., 2012, 14, 909–911;
(b) A. Kumar, M. K. Gupta and M. Kumar, Green Chem., 2012,
14, 290–295; (c) C. C. Silveira, S. R. Mendes, M. A. Villetti,
D. F. Back and T. S. Kaufman, Green Chem., 2012, 14, 2912–2921.
6 (a) D. Shu, G. N. Winston-McPherson, W. Song and
W. Tang, Org. Lett., 2013, 15, 4162–4165; (b) P.-L.
T. Boudreault, S. Wakim, M. L. Tang, Y. Tao, Z. Bao and
M. Leclerc, J. Mater. Chem., 2009, 19, 2921–2928.
Conclusions
Using a sulfonyl-containing Brønsted IL as the catalyst, we
have successfully synthesized various 3-vinylindoles through
direct dehydrative alkenylation of indoles with inexpensive and
abundantly available simple ketones. Compared with the con-
ventional methods for the synthesis of 3-vinylindoles, this
method showed many advantages including high synthetic
efficiency, cost-effective reaction, recyclable catalyst and easy
product isolation. Particularly, using 2-hydroxymethylindole as
the substrate, a hitherto unreported straightforward method
for the synthesis of the indolo[3,2-b]carbazole derivative was
established. In addition, an unexpected method for accom-
plishing C3-arylation of 2-methylindole was also developed
using acetylacetaldehyde dimethyl acetal as a non-aromatic
arylation reagent. All these results demonstrated clearly that
the Brønsted IL is indeed an invaluable powerful catalyst for
the derivatization of indoles.
The authors thank the National Natural Science Foundation
of China for financial support (21173089 and 21373093). The
authors are also grateful to the Analytical and Testing Centre
of HUST. The Chutian Scholar Program of the Hubei Provincial
Government and the Cooperative Innovation Center of Hubei
Province are also acknowledged. This work was also supported
by the Fundamental Research Funds for the Central Univer-
sities of China (2014ZZGH019).
7 (a) A. W. Schmidt, K. R. Reddy and H.-J. Knölker, Chem.
Rev., 2012, 112, 3193–3328; (b) G. S. Singh and Z. Y. Desta,
Chem. Rev., 2012, 112, 6104–6155; (c) B. Tan, G. Hernández-
Torres and C. F. Barbas III, J. Am. Chem. Soc., 2011, 133,
12354–12357; (d) T. Lemster, U. Pindur, G. Lenglet,
S. Depauw, C. Dassi and M.-H. David-Cordonnier,
Eur. J. Med. Chem., 2009, 44, 3235–3252.
8 (a) C. Gioia, A. Hauville, L. Bernardi, F. Fini and A. Ricci,
Angew. Chem., Int. Ed., 2008, 47, 9236–9239; (b) Y. Liu,
Green Chem.
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M. Nappi, E. C. Escudero-Adán and P. Melchiorre, Org.
Lett., 2012, 14, 1310–1313.
9 (a) M. W. Robinson, J. H. Overmeyer, A. M. Young,
348, 331–338; (b) S. Santra, A. Majee and A. Hajra, Tetra-
hedron Lett., 2011, 52, 3825–3827; (c) P. Jaisankar and
P. C. Srinivasan, Synth. Commun., 2005, 35, 923–927.
P. W. Erhardt and W. A. Maltese, J. Med. Chem., 2012, 55, 19 T. Amir, X. Pan, C. Liu and Y. Gu, ChemSusChem, 2014,
1940–1956; (b) E. Dolušić, P. Larrieu, L. Moineaux, DOI: 10.1002/cssc.201402220.
V. Stroobant, L. Pilotte, D. Colau, L. Pochet, B. V. Eynde, 20 (a) M. Li and Y. Gu, Adv. Synth. Catal., 2012, 354, 2484–2494;
B. Masereel, J. Wouters and R. Frédérick, J. Med. Chem.,
2011, 54, 5320–5334.
(b) D. Jiang, X. Pan, M. Li and Y. Gu, ACS Comb. Sci., 2014,
16, 287–292; (c) M. Li, A. Taheri, M. Liu, S. Sun and Y. Gu,
Adv. Synth. Catal., 2014, 356, 537–556; (d) M. Li, B. Zhang
and Y. Gu, Green Chem., 2012, 14, 2421–2428; (e) X. Pan,
M. Li and Y. Gu, Chem. – Asian J., 2014, 9, 268–274.
10 (a) C. Steuer, C. Gege, W. Fischl, K. H. Heinonen,
R. Bartenschlager and C. D. Klein, Bioorg. Med. Chem.,
2011, 19, 4067–4074; (b) R. S. Kusurkar, S. K. Goswami and
S. M. Vyas, Tetrahedron Lett., 2003, 44, 4761–4763.
11 P. Venkatesan and S. J. Sumathi, Heterocycl. Chem., 2010,
47, 81–84.
21 (a) G. Chatel, R. Pflieger, E. Naffrechoux, S. I. Nikitenko,
J. Suptil, C. Goux-Henry, N. Kardos, B. Andrioletti and
M. Draye, ACS Sust. Chem. Eng., 2013, 1, 137–143;
(b) A.-O. Diallo, A. B. Morgan, A. Len and G. Marlair, Energy
Environ. Sci., 2013, 6, 699–710.
12 See some recent examples: (a) W.-L. Chen, Y.-R. Gao,
S. Mao, Y.-L. Zhang, Y.-F. Wang and Y.-Q. Wang, Org. Lett.,
2012, 14, 5920–5923; (b) S. R. Kandukuri, J. A. Schiffner 22 See the ESI.†
and M. Oestreich, Angew. Chem., Int. Ed., 2012, 51, 23 G. Bhaskar, C. Saikumar and P. T. Perumal, Tetrahedron
1265–1269; (c) H. Yu and Z. Yu, Angew. Chem., Int. Ed.,
2009, 48, 2929–2933; (d) A. García-Rubia, R. G. Arrayás and 24 W. E. Noland, C. L. Etienne and N. P. Lanzatella, J. Hetero-
J. C. Carretero, Angew. Chem., Int. Ed., 2009, 48, 6511–6515; cycl. Chem., 2011, 48, 381–388.
(e) S. Mochida, K. Hirano, T. Satoh and M. Miura, J. Org. 25 X. Zhao, Z. Yu, T. Xu, P. Wu and H. Yu, Org. Lett., 2007, 9,
Chem., 2011, 76, 3024–3033; (f) L. Ackermann, L. Wang, 5263–5266.
R. Wolfram and A. V. Lygin, Org. Lett., 2012, 14, 728–731; 26 Z. Liu, L. Liu, Y. Han, Z. Li and J. Jiang, Faming Zhuanli
(g) B. Li, J. Ma, N. Wang, H. Feng, S. Xu and B. Wang, Org. Shenqing, CN 102786394 A 20121121, 2012.
Lett., 2012, 14, 736–739; (h) N. P. Grimster, C. Gauntlett, 27 Q. Yang, L. Wang, T. Guo and Z. Yu, J. Org. Chem., 2012, 77,
C. R. A. Godfrey and M. J. Gaunt, Angew. Chem., Int. Ed., 8355–8361.
2005, 44, 3125–3129; (i) B. Gong, J. Shi, X. Wang, Y. Yan, 28 W. E. Noland and M. R. Venkiteswaran, J. Org. Chem., 1960,
Q. Li, Y. Meng, H. E. Xu and W. Yi, Adv. Synth. Catal., 2014, 26, 4263–4269.
356, 137–143; ( j) Z.-L. Yan, W.-L. Chen, Y.-R. Gao, S. Mao, 29 H. Li, J. Yang, Y. Liu and Y. Li, J. Org. Chem., 2009, 74,
Y.-L. Zhang and Y.-Q. Wang, Adv. Synth. Catal., 2014, 356, 6797–6801.
1085–1092; (k) L. Yang, G. Zhang and H. Huang, Adv. 30 (a) F. Bellina, F. Benelli and R. Rossi, J. Org. Chem., 2008,
Lett., 2010, 51, 3141–3145.
Synth. Catal., 2014, 356, 1509–1515.
13 S.-K. Xiang, B. Zhang, L.-H. Zhang, Y. Cui and N. Jiao,
Chem. Commun., 2011, 47, 8097–8099.
14 See some examples: (a) L. L. Suarez and M. F. Greaney,
Chem. Commun., 2011, 47, 7992–7994; (b) T. Tsuchimoto,
73, 5529–5535; (b) Y. Chen, S. Guo, K. Li, J. Qu, H. Yuan,
Q. Hua and B. Chen, Adv. Synth. Catal., 2013, 355, 711–715.
31 (a) S. Maeda, Y. Obora and Y. Ishii, Eur. J. Org. Chem., 2009,
4067–4072; (b) W. Liu, S. Wang, H. Zhan and M. Li, Synlett,
2014, 25, 1478–1481.
H. Matsubayashi, M. Kaneko, Y. Nagase, T. Miyamura and 32 S. Biswas, S. Maiti and U. Jana, Eur. J. Org. Chem., 2010,
E. Shirakawa, J. Am. Chem. Soc., 2008, 130, 15823–15835; 2861–2866.
(c) T. Tsuchimoto and M. Kanbara, Org. Lett., 2011, 13, 33 (a) L. Cui, Y. Zhu, S. Luo and J. Cheng, Chem. – Eur. J.,
912–915.
2013, 19, 9481–9484; (b) B. Qian, G. Zhang, Y. Ding and
H. Huang, Chem. Commun., 2013, 49, 9839–9841; (c) D. Liu,
C. Liu, H. Lia and A. Lei, Chem. Commun., 2014, 50,
3623–3626.
15 (a) G. Rassu, V. Zambrano, R. Tanca, A. Sartori,
L. Battistini, F. Zanardi, C. Curti and G. Casiraghi,
Eur. J. Org. Chem., 2012, 466–470; (b) B. Ranieri, A. Sartori,
C. Curti, L. Battistini, G. Rassu, G. Pelosi, G. Casiraghi and 34 J. Tholander and J. Bergman, Tetrahedron, 1999, 55,
F. Zanardi, Org. Lett., 2014, 16, 932–935. 6243–6260.
16 (a) W. Q. Wang and T. Ikemoto, Tetrahedron Lett., 2005, 46, 35 L. Jong, F. Jiang, G. Li and K. Mortelmans, U.S. Pat. Appl.
3875–3878; (b) G. Fridkin, N. Boutard and W. D. Lubell,
J. Org. Chem., 2009, 74, 5603–5606.
17 H. Yu and Z. Yu, Angew. Chem., Int. Ed., 2009, 48,
2929–2933.
18 See some examples: (a) A. Arcadi, M. Alfonsi, G. Bianchi,
G. D’Annicalle and F. Marinelli, Adv. Synth. Catal., 2006,
Publ., 20100069355, 2010.
36 (a) S. S. Dhareshwar and V. J. Stella, J. Pharm. Sci., 2009, 98,
1804–1812; (b) L. Peng, M. Ma, X. Zhang, S. Zhang and
J. Wang, Tetrahedron Lett., 2006, 47, 8175–8178;
(c) A. R. Katritzky and K. Akutagawa, J. Org. Chem., 1989,
54, 2949–2952.
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