[3+2] Cycloadditions of α-acyl ketene dithioacetals with propargylamines: Pyrrole synthesis in water

Article abstract of DOI:10.1016/j.tetlet.2013.01.020

A series of 2,3,4-trisubstituted and 1,2,3,4-tetrasubstituted pyrroles were synthesized via [3+2] cycloaddition of α-acyl ketene dithioacetals (or related substrates) with commercially available propargylamines as 1,3-dipoles in water. Most of the reactions can be performed in the absence of an external base. The reaction of secondary amine (N-methylprop-2-yn-1-amine) with α-acyl ketene dithioacetals showed different reaction behaviours depending on the addition or absence of an external base. Copyright

Full text of DOI:10.1016/j.tetlet.2013.01.020

Tetrahedron Letters 54 (2013) 1478–1481
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
[3+2] Cycloadditions of
a-acyl ketene dithioacetals with propargylamines:
pyrrole synthesis in water
⇑
⇑
Chuan-Qing Ren, Chong-Hui Di, Yu-Long Zhao , Jing-Ping Zhang
Department of Chemistry, Northeast Normal University, Changchun 130024, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of 2,3,4-trisubstituted and 1,2,3,4-tetrasubstituted pyrroles were synthesized via [3+2] cycload-
dition of -acyl ketene dithioacetals (or related substrates) with commercially available propargylamines
as 1,3-dipoles in water. Most of the reactions can be performed in the absence of an external base. The
reaction of secondary amine (N-methylprop-2-yn-1-amine) with -acyl ketene dithioacetals showed dif-
ferent reaction behaviours depending on the addition or absence of an external base.
Ó 2013 Elsevier Ltd. All rights reserved.
Received 4 December 2012
Revised 29 December 2012
Accepted 8 January 2013
Available online 16 January 2013
a
a
Keywords:
[3+2] Cycloaddition
Propargylamines
Pyrroles
Synthetic methods
a-Acyl ketene dithioacetals
Increased environmental awareness within the synthetic organ-
construction of substituted pyrroles in aqueous media is important,
in particular, those performed under metal-free conditions.13 Dur-
ing our research on the synthesis of carbocyclic14 and heterocyclic
compounds15–17 by domino reactions, very recently, we developed
a new synthetic strategy for the direct synthesis of 2,3,4-trisubsti-
tuted and 1,2,3,4-tetrasubstituted pyrroles via the [3+2] cycloaddi-
tion strategy in DMF solvent with propargylamines as 1,3-dipoles.17
The concept of green chemistry and our continuing interest in
organic reaction in aqueous media18 prompted us to explore the
feasibility of the [3+2] cycloaddition in water. As a result, it was
found that water is not only an efficient green solvent for the
ic chemistry community has lately pushed green chemistry tech-
nologies to the fore and led to the development of cleaner and
relatively benign chemical processes.1 The use of water as solvent
has attracted increasing interest in recent years.2 Indeed, water is
an environmentally benign, non-flammable liquid over a wide
temperature range and possesses a high heat capacity making it
inherently safe. Furthermore, the insolubility of organic com-
pounds in water causes them to react on its surface and often
exhibits unique reactivity and selectivity that cannot be attained
with conventional organic solvents.2 Consequently, the design of
novel methods to prepare the privileged heterocyclic scaffolds that
combine the synthetic efficiency of domino protocols and the use
of water as the reaction medium constitutes a very important
challenge in green chemistry.3
As one of the most important classes of heterocycles, pyrroles
are not only the key subunits in numerous natural products,4 phar-
maceutical substances5 and organic materials,6 but are also useful
and versatile building blocks in organic synthesis.7 As a conse-
quence, much attention has been paid to the synthesis of pyrrole
derivatives either by classic methods such as the Paal–Knorr,8
Hantzsch,9 Barton–Zard and Van Leusen7b,10 or by transition me-
tal-catalysed cyclization11 and multicomponent reaction.12 How-
ever, most of these methods require the use of organic solvents
which are harmful to the environment. Therefore, the efficient
[3+2] cycloaddition of propargylamines with polarized
a-acyl ke-
tene dithioacetals or vinylogous thiolesters as the dipolarophiles,
but allows a wider choice of products of the reaction. Herein, we
wish to letter a recent development in the use of propargylamines
as 1,3-dipoles for the synthesis of polysubstituted pyrroles in water.
Initially, the reaction of a,a-diacetyl ketene dithioacetal 1a with
propargylamine 2a was employed as a model reaction in water. It
was found that, in the presence of K2CO3 or DBU (1,8-diazabicy-
clo[5.4.0]undec-7-ene), the desired product 2,3,4-trisubstituted
pyrrole 4a was produced only in moderate to good yields (45–
70%) by treating 1a (0.5 mmol) with 2a (0.5 mmol) in H2O (5 mL)
at 100 °C for 12–20 h (Table 1, entries 1–4). In these cases, the
deacylative product 3a was also obtained and the yield of 3a in-
creased with increasing the amount of base. Interestingly, under
identical conditions but in the absence of an external base, 4a
was produced in higher yield (Table 1, entry 5). To our delight,
the yield of 4a raised to 93% by increasing the amount of 2a to
2.0 equiv without the addition of an external base (Table 1, entry
⇑
Corresponding authors.
E-mail addresses: zhaoyl351@nenu.edu.cn (Y.-L. Zhao), zhangjingping66@ya-
hoo.cn (J.-P. Zhang).
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.01.020

C.-Q. Ren et al. / Tetrahedron Letters 54 (2013) 1478–1481
1479
Table 1
Table 2
Optimization of reaction conditionsa
Synthesis of 2,3,4-trisubstituted pyrroles 4a–oa
R1
O
H2N
O
O
O
O
R1
H2O/100 oC
EWG
EtS
EWG
EtS
2a
R
+
+
conditions
EtS
N
H
N
H
EtS
SEt
SEt
EtS
SEt
H2N
1
2
4a-o
1a
3a
4a
Entry
1
R
EWG
R1
Time
(h)
Product
Yieldb
(%)
Entry
2a (equiv)
Base (equiv)
Solvent
Time (h)
Yieldb (%)
4a 3a 1a
1
2
1a
1b
1c
1d
1e
1f
1g
1h
1i
1j
1k
1l
lm
ln
1a
Me
Et
Me
Ph
MeCO
EtCO
CO2Et
COPh
C6H5NHCO
4-ClC6H4NHCO
2-ClC6H4NHCO
4-MeOC6H4NHCO
2-MeOC6H4NHCO
4-MeC6H4NHCO
2-MeC6H4NHCO
2,4-Me2C6H3NHCO
NO2
H
H
H
H
H
H
H
H
H
H
H
H
H
H
Ph
12
12
12
16
20
20
20
30
30
30
30
30
8
4a
4b
4c
4d
4e
4f
4g
4h
4i
4j
4k
4l
4m
4n
4o
93
90
90
70
90
89
88
86
87
85
82
80
87
71
72
1
2
3
4
5
6
1.0
1.0
1.0
1.0
1.0
2.0
DBU (1.0)
DBU (0.5)
DBU (0.3)
K2CO3 (1.0)
—
H2O
H2O
H2O
H2O
H2O
H2O
12
20
20
12
16
12
45 38 0
60 20 0
70 10 0
65 25 0
80 0 11
93 0 0
3
4c
5
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
6
7
8
9
10
11
12
13
14
15d
—
a
All reactions were carried out in open air.
Isolated yield.
b
6). This result suggested that propargylamine 2a plays a dual role
as both a base and a nucleophile in the above deacylative [3+2]
cycloaddition reaction. In comparison, a-acetyl ketene dithioacetal
CN
MeCO
12
16
a
b
c
Reaction conditions: 1 (0.5 mmol), 2a (1.0 mmol), H2O (5.0 mL), 100 °C, 8–30 h.
Isolated yield.
1.0 equiv K2CO3 was used.
2.0 equiv K2CO3 was used.
3a failed to react with 2a in H2O either in the presence or absence
of K2CO3 (1.0 equiv) with intact substrate 3a recovered, which indi-
cates that 3a would not be involved in the formation of 4a since 3a
(having only one EWG; EWG = electron-withdrawing group) is
much less reactive than 1a (having two EWGs) for SNV reaction.19
Obviously, the above result provides a clean and efficient route
to highly substituted pyrroles from easily available substrates in
water under transition-metal-free conditions in a single operation.
With this viable protocol in hand, the scope of the reaction was
then investigated by the reactions of propargylamine 2a with
d
O
O
O
°C
H2O / 100
+
Ar
N
H
H2N
2a
: Ar = Ph, 30 h, 85% yield;
Ar
SEt
5
4p-r
a
-acyl ketene dithioacetals 1 under optimal conditions (Table 1,
4p
4q
entry 6) and the results are summarized in Table 2. It was found
that the [3+2] cycloaddition is not sensitive to the nature of the
EWG of substrates 1 and exhibits good flexibility. All the reactions
of 2a with acyl ketene dithioacetals 1a–n bearing the EWG as acet-
yl, propionyl, ethoxycarbonyl, benzoyl, and N-arylcarbamoyl with
either electron-neutral, electron-deficient or electron-rich group
on the benzene ring, nitro and cyano group, obtained the desired
2,3,4-trisubstituted pyrroles 4a–n in high to excellent yields
(Table 2, entries 1–14). The above results mean that an acetyl,
propionyl, ethoxycarbonyl, benzoyl, N-arylcarbamoyl, nitro or
cyano group can be introduced regiospecifically in the 3-position
of 2,3,4-trisubstituted pyrroles 4 accomplished by chemoselective
deacylation (Table 2, entries 1–14). Notably, under otherwise
identical conditions as above, the [3+2] cycloaddition of 1a with
3-phenylprop-2-yn-1-aminium chloride 2b also proceeded
smoothly to give the desired product 4o in 72% yield in the
presence of 2.0 equiv of K2CO3 (Table 2, entry 15). The successful
synthesis of 4o shows that the deacylative [3+2] cycloaddition is
applicable to the synthesis of a wide variety of 2,3,4-trisubstituted
pyrroles bearing variable substituents at the 4-position.
: Ar = 4-MeO-C6H4,
33 h, 88% yield; 4r: Ar = 4-Cl-C6H4, 30 h, 80% yield
Scheme 1. Synthesis of 2,3,4-trisubstituted pyrroles 4p–r.
above, all selected acetyl ketene dithioacetals 1 with a variety of
electron-withdrawing groups at the -position, including acetyl,
a
ethoxycarbonyl, cyano and N-arylcarbamoyl with either electron-
neutral, electron-deficient or electron-rich group on the benzene
ring (Table 3, entries 1–6), could react with 2c smoothly to give
the desired 1,2,3,4-tetrasubstituted pyrroles 4s–x in high yields.
However, no reaction was detectable for substrate 1o bearing a
phenyl group at the a-position under identical conditions (Table 3,
entry 7). As a comparison, 1,2,3,4-tetrasubstituted pyrroles 4s–x
cannot be obtained from the same reaction by using DMF as the
solvent.17
Interestingly, under identical conditions as above but in the
presence of an external K2CO3 (0.5 equiv), the 1,2,3,4-tetrasubsti-
tuted pyrrole 6o bearing a acetyl group at the C2 of the pyrrole core
could be obtained in 63% yield (Table 4, entry 4). Similarly, the cor-
responding 1,2,3,4-tetrasubstituted pyrroles 6c, 6d, 6n and 6p
were synthesized in good to high yields from reactions of 2c with
1c, 1d, 1n and 1p, respectively (Table 4, entries 1–3 and 5), which
indicated that 1,2,3,4-tetrasubstituted pyrroles 6 could be pre-
pared from reactions of 2c with suitable substrates 1 in water
(5.0 mL) in the presence of K2CO3 (0.5 equiv).
To further extend the scope of the deacylative [3+2] cycloaddi-
tion in water, the reactions of propargylamine 2a with selected 3-
(ethylthio(aryl)methylene)pentane-2,4-diones 5 were then exam-
ined. As a result, 2,3,4-trisubstituted pyrroles 4p–r were prepared
in high yields from the reaction of 2a (1.0 mmol) with the corre-
sponding 5 (0.5 mmol) in H2O at 100 °C for 30–33 h (Scheme 1).
These results offer further enhancement of the efficiency of the
deacylative [3+2] cycloaddition using water as the solvent.
On the basis of the above experimental results together with re-
lated letters,17,20 the possible mechanisms for the formation of 4
and 6 (Tables 2–4 and Scheme 1) are proposed in Scheme 2 (with
Furthermore, to evaluate the efficiency of secondary propargyl-
amine in the [3+2] cycloaddition, the reactions of
a-acyl ketene
dithioacetals 1 with N-methylprop-2-yn-1-amine 2c were exam-
a-acetyl ketene dithioacetals 1 as an example). In the case of
ined. It was found that, under otherwise identical conditions as
the reaction of 1 and 2 in the absence of an external base, the

1480
C.-Q. Ren et al. / Tetrahedron Letters 54 (2013) 1478–1481
R1
Table 3
R1
Synthesis of 1,2,3,4-tetrasubstituted pyrroles 4s–x from 1 and 2ca
O
O
O
EWG
EtS
EWG
EtS
EWG
EtS
amine
O
2
EWG
H2O/100 o
C
EWG
SEt
R
SEt
+
N
N
EtS
N
N
H
2c
R
A
R
EtS
1
B
1
4s-x
5-exo-dig
cyclization
c
K2CO3
O
2
Entry
1
R
EWG
Time (h)
Product
Yieldb (%)
R1
O
O
GWE
N
1
2
1a
1c
1e
1f
Me
Me
Me
Me
Me
Me
C6H5
MeCO
CO2Et
16
16
30
30
30
16
30
4s
4t
85
83
80
82
75
73
0
5-exo-dig
cyclization
EWG
EtS
EWG
3c
4c
5c
6
C6H5NHCO
4-ClC6H4NHCO
4-MeOC6H4NHCO
CN
4u
4v
4w
4x
4y
EtS
EtS
N
N
EtS
EtS
1h
ln
1o
R
Me
Me
D
E
C
7d
CO2Et
- COMe
aroma-
tization
-EtSH
a
Reaction conditions: 1 (0.5 mmol), 2c (1.0 mmol), H2O (5.0 mL), 100 °C, 16–
30 h.
R1
b
EWG
MeOC
EWG
O
EWG
EtS
Isolated yield.
5.0 equiv of 2c was used.
With recovery of 96% of 1o.
- EtSH
aromatization
c
d
N
Me
6
N
N
R
EtS
Me
F
4
Table 4
Synthesis of 1,2,3,4-tetrasubstituted pyrroles 6a
Scheme 2. Proposed mechanisms for formation of 4 and 6.
O
EWG
EWG
SEt
K2CO3 (0.5 equiv)
H2O/100 oC
R
O
ethylthio group at the C2-position were obtained. Whereas poly-
substituted pyrroles bearing an acetyl group at the C2-position
were obtained when an external base (K2CO3) was added. These re-
sults showed the crucial role of water in organic reactions. Further
studies are in progress.
+
N
N
H
2c
EtS
1
6
Entry
1
R
EWG
Time (h)
Product
Yieldb (%)
1
2
3
4
5
1c
1d
ln
1o
1p
Me
C6H5
Me
C6H5
Me
CO2Et
COC6H5
CN
CO2Et
COC6H5
12
20
10
12
20
6c
6d
6n
6o
6p
69
62
70
63
60
Acknowledgment
Financial support from the National Natural Sciences
Foundation of China (21172032 and 20972026) is gratefully
acknowledged.
a
Reaction conditions: 1 (0.5 mmol), 2c (1.0 mmol), K2CO3 (0.25 mmol), H2O
(5.0 mL), 100 °C, 10–16 h.
b
Isolated yield.
Supplementary data
N,S-acetal intermediate A could be formed via the acetyl enhanced
addition–elimination (SNV) of propargylamines 2 to enones 1.14b,19
Subsequently, in the presence of amine 2 as a base, intermediate A
would undergo a sequential tautomerization and 5-exo-dig cycliza-
tion to give intermediate C. Finally, the 2,3,4-trisubstituted or
1,2,3,4-tetrasubstituted pyrroles 4 are produced via deacetylation
and subsequent isomerization (Scheme 2). However, under identi-
cal conditions but in the presence of an external base (K2CO3), the
reaction of 1 with secondary amine 2c (Table 4) seems to favour
the formation of intermediate D from the intermolecular Michael
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.tetlet.2013.
01.020.
References and notes
1. (a) Anastas, P.; Eghbali, N. Chem. Soc. Rev. 2010, 39, 301; (b) Ahluwalia, V. K.;
Varma, R. S. Green Solvents for Organic Synthesis; Alpha Science, Oxford, U.K.,
2009.
2. (a) Simon, M.-O.; Li, C.-J. Chem. Soc. Rev. 2012, 41, 1415; (b) Butler, R. N.; Coyne,
A. G. Chem. Rev. 2010, 110, 6302; (c) Chanda, A.; Fokin, V. V. Chem. Rev. 2009,
109, 725; (d) Li, C.-J.; Chen, L. Chem. Soc. Rev. 2006, 35, 68; (e) Li, C.-J. Chem. Rev.
2005, 105, 3095; (f) Li, C.-J.; Chan, T.-H. Comprehensive Organic Reactions in
Aqueous Media; Wiley: New York, 2007; (g) Lindström, U. M. Chem. Rev. 2002,
102, 2751; (h) Lubineau, A.; Augé, J.; Queneau, Y. Synthesis 1994, 741.
3. For selected recent examples, see: (a) Sarrafi, Y.; Sadatshahabi, M.;
Alimohammadi, K.; Tajbakhsh, M. Green Chem. 2011, 13, 2851; (b) Moseley, J.
D.; Kappe, C. O. Green Chem. 2011, 13, 794; (c) Ke, F.; Qu, Y.; Jiang, Z.; Li, Z.; Wu,
D.; Zhou, X. Org. Lett. 2011, 13, 454; (d) De, S.; Dutta, S.; Saha, B. Green Chem.
2011, 13, 2859; (e) Carvalho, L. C. R.; Fernandes, E.; Marques, M. M. B. Chem.-
Eur. J. 2011, 17, 12544; (f) Jung, E. J.; Park, B. H.; Lee, Y. R. Green Chem. 2010, 12,
2003; (g) Ma, N.; Jiang, B.; Zhang, G.; Tu, S.-J.; Wever, W.; Li, G. Green Chem.
2010, 12, 1357; (h) Ding, Q.; Cao, B.; Liu, X.; Zong, Z.; Peng, Y.-Y. Green Chem.
2010, 12, 1607; (i) Zhou, Y.; Zhai, Y.; Li, J.; Ye, D.; Jiang, H.; Liu, H. Green Chem.
2010, 12, 1397; (j) Kumaravel, K.; Vasuki, G. Green Chem. 2009, 11, 1945.
4. For reviews, see: (a) Fan, H.; Peng, J.; Hamann, M. T.; Hu, J.-F. Chem. Rev. 2008,
108, 264; (b) Fürstner, A. Angew. Chem., Int. Ed. 2003, 42, 3582; (c) Agarwal, S.;
Cämmerer, S.; Filali, S.; Fröhner, W.; Knöll, J.; Krahl, M. P.; Reddy, K. R.; Knölker,
H.-J. Curr. Org. Chem. 2005, 9, 1601; (d) Hoffmann, H.; Lindel, T. Synthesis 2003,
1753; (e) Walsh, C. T.; Garneau-Tsodikova, S.; Howard-Jones, A. R. Nat. Prod.
Rep. 2006, 23, 517.
addition of N-methylprop-2-yn-1-amine 2c to
a-acetyl ketene
dithioacetals 1. Then the methylene pyrrolidine intermediate E,
generated via an intramolecular 5-exo-dig cyclization of intermedi-
ate D, undergoes a sequential ethylthio elimination and 1,2-acetyl
migration to generate intermediate F, which is followed by elimi-
nation of the second ethylthio group and subsequent aromatiza-
tion leading to 1,2,3,4-tetrasubstituted pyrroles 6 (Scheme 2).17,20c
In conclusion, a series of 2,3,4-trisubstituted and 1,2,3,4-tetra-
substituted pyrroles were synthesized in a single-step in good to
excellent yields via the [3+2] cycloaddition of a-acyl ketene dithio-
acetals (or related substrates) with commercially available propar-
gylamines as 1,3-dipoles by using water as the solvent. The
reaction of secondary amine (N-methylprop-2-yn-1-amine) with
a-acyl ketene dithioacetals showed different reaction behaviours
depending on the addition or absence of an external base. In the
absence of an external base, polysubstituted pyrroles bearing an

C.-Q. Ren et al. / Tetrahedron Letters 54 (2013) 1478–1481
1481
5. (a) Bellina, F.; Rossi, R. Tetrahedron 2006, 62, 7213; (b) Huffman, J. W.; Padgett,
L. W. Curr. Med. Chem. 2005, 12, 1395; (c) Huffman, J. W. Curr. Med. Chem. 1999,
6, 705.
6. For reviews, see: (a) Guernion, N. J. L.; Hayes, W. Curr. Org. Chem. 2004, 8, 637;
(b) Novák, P.; Müller, K.; Santhanam, K. S. V.; Haas, O. Chem. Rev. 1997, 97, 207;
(c) Curran, D.; Grimshaw, J.; Perera, S. D. Chem. Soc. Rev. 1991, 20, 391.
7. (a) Jolicoeur, B.; Chapman, E. E.; Thompson, A.; Lubell, W. D. Tetrahedron 2006,
62, 11531; (b) Ono, N. Heterocycles 2008, 75, 243.
8. (a) Pall, C. Ber. Dtsch. Chem. Ges. 1885, 18, 367; (b) Banik, B. K.; Samajdar, S.;
Banik, I. J. Org. Chem. 2004, 69, 213; (c) Minetto, G.; Raveglia, L. F.; Sega, A.;
Taddei, M. Eur. J. Org. Chem. 2005, 24, 5277.
9. (a) Hantzsch, A. Ber. Dtsch. Chem. Ges. 1890, 23, 1474; (b) Palacios, F.; Aparicio,
D.; De los Santos, J. M.; Vicario, J. Tetrahedron 2001, 57, 1961.
R.; Scheidt, K. A. Org. Lett. 2004, 6, 2465; (g) Cadierno, V.; Gimeno, J.; Nebra, N.
Chem. Eur. J. 2007, 13, 9973.
13. (a) Rueping, M.; Parra, A. Org. Lett. 2010, 12, 5281 (from (E)-
bromonitrostyrenes and enaminones); (b) Khalili, B.; Jajarmi, P.; Eftekhari-
Sis, B.; Hashemi, M. M. J. Org. Chem. 2008, 73, 2090 (three-component reaction
of b-dicarbonyl compounds with arylglyoxals in the presence of ammonium
acetate).
14. For selected recent letters, see: (a) Han, X.-D.; Zhao, Y.-L.; Meng, J.; Ren, C.-Q.;
Liu, Q. J. Org. Chem. 2012, 77, 5173; (b) Zhao, Y.-L.; Yang, S.-C.; Di, C.-H.; Han, X.-
D.; Liu, Q. Chem. Commun. 2010, 46, 7614; (c) Zhao, Y.-L.; Chen, L.; Yang, S.-C.;
Tian, C.; Liu, Q. J. Org. Chem. 2009, 74, 5622; (d) Bi, X.; Dong, D.; Liu, Q.; Pan, W.;
Zhao, L.; Li, B. J. Am. Chem. Soc. 2005, 127, 4578.
15. For selected recent letters, see: (a) Li, Y.; Xu, X.; Tan, J.; Xia, C.; Zhang, D.; Liu, Q.
J. Am. Chem. Soc. 2011, 133, 1775; (b) Tan, J.; Xu, X.; Zhang, L.; Li, Y.; Liu, Q.
Angew. Chem., Int. Ed. 2009, 48, 2868; (c) Wang, H.; Zhao, Y.-L.; Ren, C.-Q.;
Diallo, A.; Liu, Q. Chem. Commun. 2011, 47, 1236; (d) Meng, J.; Zhao, Y.-L.; Ren,
C.-Q.; Li, Y.; Li, Z.; Liu, Q. Chem. Eur. J. 2009, 15, 1830; (e) Zhao, Y.-L.; Li, D.-Z.;
Han, X.-D.; Chen, L.; Liu, Q. Adv. Synth. Catal. 2008, 350, 1537; (f) Zhao, Y.-L.;
Zhang, W.; Wang, S.; Liu, Q. J. Org. Chem. 2007, 72, 4985; (g) Chen, L.; Zhao, Y.-
L.; Liu, Q.; Chen, C.; Piao, C.-R. J. Org. Chem. 2007, 72, 9259.
16. Jon Grenning, A. J.; Tunge, A. Angew. Chem., Int. Ed. 2011, 50, 1688; (b) Yuan, H.;
Wang, M.; Liu, Y.; Wang, L.; Liu, J.; Liu, Q. Chem. Eur. J. 2010, 16, 13450.
17. Zhao, Y.-L.; Di, C.-H.; Liu, S.-D.; Meng, J.; Liu, Q. Adv. Synth. Catal. 2012, 354,
3545.
10. For reviews, see: Van Leusen, D.; Van Leusen, A. M. Org. React. 2001, 57, 417.
11. (a) Jiang, Y.; Chan, W. C.; Park, C.-M. J. Am. Chem. Soc. 2012, 134, 4104; (b) Trost,
B. M.; Lumb, J.-P.; Azzarelli, J. M. J. Am. Chem. Soc. 2011, 133, 740; (c) Rakshit, S.;
Patureau, F. W.; Glorius, F. J. Am. Chem. Soc. 2010, 132, 9585; (d) Mizuno, A.;
Kusama, H.; Iwasawa, N. Angew. Chem., Int. Ed. 2009, 48, 8318; (e) Larionov, O.
ˇ
V.; de Meijere, A. Angew. Chem., Int. Ed. 2005, 44, 5664; (f) Kim, J. T.; Kelin, A. V.;
Gevorgyan, V. Angew. Chem., Int. Ed. 2003, 42, 98; (g) Kamijo, S.; Kanazawa, C.;
Yamamoto, Y. J. Am. Chem. Soc. 2005, 127, 9260; (h) Dhawan, R.; Arndtsen, B. A.
J. Am. Chem. Soc. 2004, 126, 468; (i) Tejedor, D.; González-Cruz, D.; García-
Tellado, F.; Marrero-Tellado, J. J.; Rodríguez, M. L. J. Am. Chem. Soc. 2004, 126,
8390; (j) Barber, D. M.; Sanganee, H.; Dixon, D. J. Chem. Commun. 2011, 47,
4379; (k) Kramer, S.; Madsen, J. L. H.; Rottländer, M.; Skrydstrup, T. Org. Lett.
2010, 12, 2758; (l) Ackermann, L.; Sandmann, R.; Kaspar, T. Org. Lett. 2009, 11,
2031; (m) Merkul, E.; Boersch, C.; Frank, W.; Müller, T. J. J. Org. Lett. 2009, 11,
2269; (n) Wang, Y.-F.; Toh, K. K.; Chiba, S.; Narasaka, K. Org. Lett. 2008, 10,
5019.
18. (a) Ouyang, Y.; Dong, D.; Yu, H.; Liang, Y.; Liu, Q. Adv. Synth. Catal. 2006, 348,
206; (b) Dong, D.; Ouyang, Y.; Yu, H.; Liu, Q.; Liu, J.; Wang, M.; Zhu, J. J. Org.
Chem. 2005, 70, 4535.
19. For reviews, see: (a) Dieter, R. K. Tetrahedron 1986, 42, 3029; (b) Junjappa, H.;
Ila, H.; Asokan, C. V. Tetrahedron 1990, 46, 5423.
12. (a) Balme, G. Angew. Chem., Int. Ed. 2004, 43, 6238; (b) Estévez, V.; Villacampa,
M.; Menéndez, J. C. Chem. Soc. Rev. 2010, 39, 4402; (c) Frolova, L. V.; Evdokimov,
N. M.; Hayden, K.; Malik, I.; Rogelj, S.; Kornienko, A.; Magedov, I. V. Org. Lett.
2011, 13, 1118; (d) Maiti, S.; Biswas, S.; Jana, U. J. Org. Chem. 2010, 75, 1674; (e)
St. Cyr, D. J.; Martin, N.; Arndtsen, B. A. Org. Lett. 2007, 9, 449; (f) Bharadwaj, A.
20. (a) Xin, X.; Wang, D.; Li, X.; Wan, B. Angew. Chem., Int. Ed. 2012, 51, 1693; (b)
Cacchi, S.; Fabrizi, G.; Filisti, E. Org. Lett. 2008, 10, 2629; (c) Mo, D.-L.; Ding, C.-
H.; Dai, L.-X.; Hou, X.-L. Chem. Asian J. 2011, 6, 3200; (d) Misra, N. C.; Panda, K.;
Ila, H.; Junjappa, H. J. Org. Chem. 2007, 72, 1246.