Three-component tandem reaction involving acid chlorides, terminal alkynes, and ethyl 2-amino-1H-indole-3-carboxylates: Synthesis of highly diversified pyrimido[1,2-a]indoles via sequential Sonogashira and [3+3] cyclocondensation reactions

Article abstract of DOI:10.1016/j.tetlet.2011.06.021

A simple, highly efficient three-component reaction involving acid chlorides, terminal alkynes, and ethyl 2-amino-1H-indole-3-carboxylates, for the synthesis of highly diversified pyrimido[1,2-a]indoles has been described. The salient feature of the react

Full text of DOI:10.1016/j.tetlet.2011.06.021

Tetrahedron Letters 52 (2011) 4288–4291
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Three-component tandem reaction involving acid chlorides, terminal alkynes,
and ethyl 2-amino-1H-indole-3-carboxylates: synthesis of highly diversified
pyrimido[1,2-a]indoles via sequential Sonogashira and [3+3]
cyclocondensation reactions q
Sahaj Gupta a, Sudhir K. Sharma a, Anil K. Mandadapu a, Harsh M. Gauniyal b, Bijoy Kundu a,
⇑
a Medicinal and Process Chemistry Division, Central Drug Research Institute, CSIR, Lucknow 226001, India
b Sophisticated Analytical Instrument Facility, Central Drug Research Institute, CSIR, Lucknow 226001, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A simple, highly efficient three-component reaction involving acid chlorides, terminal alkynes, and ethyl
2-amino-1H-indole-3-carboxylates, for the synthesis of highly diversified pyrimido[1,2-a]indoles has
been described. The salient feature of the reaction involves sequential Sonogashira and [3+3] cyclocon-
densation reactions.
Received 10 May 2011
Revised 3 June 2011
Accepted 4 June 2011
Available online 13 June 2011
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Pyrimido[1,2-a]indoles
Alkynes
Ethyl 2-amino-1H-indole-3-carboxylate
Multicomponent reaction
Sonogashira coupling
In the past decade, one-pot multicomponent formats involving
condensation of three or more reactants either in tandem or in sin-
gle step have become an integral part of drug discovery program.1
This in turn has laid challenges for the design of new multicompo-
nent formats by combining organic reactions in one-pot in a man-
ner that will lead to the formation of several bonds without the
isolation of the intermediate(s).
In continuation of our studies aiming at the efficient construc-
tion of indole-based polyheterocycles involving minimal multi-
step2/multicomponent3 reaction formats, we embarked with a
search for a one-pot three component reaction involving ethyl
2-amino-1H-indole-3-carboxylate as one of the reactants. In multi-
component formats although alkynes continue to remain as one of
the key components,4 functionalized indoles, in general, despite
being a privileged5 template with proven medicinal pedigree have
remained relatively under explored.6 Interestingly, reaction of
alkynyl derivatives having two electrophilic centers with suitably
functionalized indoles having nucleophilic centers is likely to pro-
vide numerous options for the design and synthesis of unlimited
combinations of novel indole-based annulated polycyclic skeletons
with diverse physical and chemical properties. In the literature
although there are reports dealing with reactions involving indoles
and alkynyl derivatives for the synthesis of indole-based polyhet-
erocycles,7 such reactions in multicomponent format are scarce.3
The latter format due to its ability to allow multi-bond forma-
tion in a single step is efficient, cost effective, and less wasteful
and leads to diversity oriented synthesis in one-pot. For our stud-
ies, we envisaged that the ethyl 2-amino-1H-indole-3-carboxylate
owing to its ability to act as a bifunctional nucleophile (NaH and
NbH2) may undergo [3+3] cyclocondensation with bifunctional
3-carbon building block alkynones to furnish a pyrimidine ring
annulated to the indole. Since alkynones can be generated from
acid chloride and terminal alkyne, a three-component strategy
combining the two organic reactions involving alkynone formation
and cyclocondensation in one-pot may provide a convenient route
to indole-based polyheterocycles. For our studies, the C-3 position
in the indole moiety, which is also known to act as a nucleophile,
has been deliberately blocked with –COOC2H5 group in order to
prevent the formation of regioisomers. We recently reported
involvement of position C-3 of the indole ring as a nucleophile dur-
ing the synthesis of a
-carbolines in a multicomponent format.3b In
this Letter we report a three component tandem reaction involving
acid chlorides (1), terminal alkynes (2), and ethyl 2-amino-1H-in-
dole-3-carboxylates (3) leading to the synthesis of pyrimido[1,2-
a]-indoles (Scheme 1). A careful survey of the literature did not
q
CDRI Communication No. 8073.
Corresponding author. Tel.: +91 522 2612411/18x4383; fax: +91 522 2623405.
E-mail addresses: bijoy_kundu@yahoo.com, b_kundu@cdri.res.in (B. Kundu).
⇑
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.06.021

S. Gupta et al. / Tetrahedron Letters 52 (2011) 4288–4291
4289
O
O
O
R3
OEt
NH2
O
R3
OEt
(i)
O
+
Cl
+
(ii)
N
N
N
R2
R1
N
H
N
R2
R1
1a-d
2a-d
3a-d
4a-r
3a; R3 = H
1a; R1 = 4-CH3C6H4
1b; R1 = 4-CH3OC6H4
2a; R2 = C6H5
2b; R2 = 4-CH3C6H4
3b; R3 = 5-CH3
3c; R3= 6-CF3
3d; R3 = 6-F
1c; R1 = 3,4-di-CH3OC6H3 2c; R2 = 4-C(CH3)3C6H4
= HMBC
= nOe
1d; R1 =C6H5
2d; R2 =C6H9
Figure 1. Important HMBC and nOe correlations for 4a.
Scheme 1. Multicomponent tandem reaction involving 1, 2, and 3. Reagents and
optimal conditions: (i) PdCl2(PPh3)2/CuI, TEA, CH3CN, 1 h, rt; (ii) Cs2CO3, CH3CN, 6 h,
reflux.
manner that will allow both alkynone formation and [3+3] cyclo-
condensation in tandem in one-pot. For optimization studies, we
monitored the progress of reaction for both the steps in situ involv-
ing Sonogashira coupling as well as for [3+3] cyclocondensation by
tlc/HPLC and the results have been summarized in Table 1.
We initiated our investigation by treating 1a and 2a in the pres-
ence of Pd/C and Et3N in THF for 1 h followed by addition of 3a in
the presence of Et3N. Under these conditions although alkynone
formation was complete, its conversion to 4a was not observed
even after changing the base to Na2CO3/DABCO and the solvent
from THF to CH3CN (entries 1–5). This led us to replace Pd/C with
a conventional catalyst PdCl2(PPh3)2/CuI, generally used for Sono-
gashira couplings.10 Yet again, although we observed in situ forma-
tion of alkynone, addition of 3a in the presence of Et3N as base
failed (entries 6–8) to trigger the formation of 4a. However, after
screening a series of bases and solvent, the use of PdCl2(PPh3)2/
CuI, Et3N, in the first step followed by addition of 3a and Cs2CO3
under reflux for 6 h (entry 9) was found to furnish 4a in 73% iso-
lated yield. Next, the isolated yield of 4a was further enhanced to
81% when the concentration of Cs2CO3 was increased from 1.0 to
1.5 equiv (entry 10).11
reveal any report dealing with the synthesis of pyrimido[1,2-a]-in-
doles using one-pot multicomponent strategy, except for a report
involving one-pot synthesis under harsh reaction conditions and
with limited diversity.8
In the first instance, we decided to scrutinize the one-pot reac-
tion conditions that would facilitate both Sonogashira coupling for
the generation of alkynones from acid chlorides and alkynes as
well as [3+3] cyclocondensation with ethyl 2-amino-1H-indole-3-
carboxylate 3a in a sequential format. In our initial experiments
we synthesized 1-(4-methylphenyl)-3-phenylprop-2-yn-1-one via
Sonogashira coupling by treating p-toluoyl chloride (1a) with
phenylacetylene (2a) in the presence of Pd/C and Et3N in THF using
the literature procedure.9 The resulting alkynone obtained in 94%
isolated yield was then treated with ethyl 2-amino-1H-indole-3-
carboxylate (3a) in the presence of Na2CO3/CH3CN at 90 °C for
16 h to furnish pyrimido[1,2-a]-indole (4a) as a single regioisomer
in 78% isolated yield via [3+3] cyclocondensation. Unfortunately,
when both the conditions were combined in multicomponent tan-
dem format in one-pot, the strategy failed to yield the desired
product. This prompted us to optimize the reaction conditions by
applying different palladium catalysts, bases, and solvents in a
The regioselectivity during [3+3] cyclocondensation reaction of
the alkynone with ethyl 2-amino-1H-indole-3-carboxylate 3a was
established on the basis of 2D NMR and nOe studies. The important
Table 1
Optimization of multicomponent tandem reaction conditions for the one-pot conversion of 1a, 2a, and 3a to 4a via in situ formation of alkynone
Entry
Tandem reaction conditions in one-pot
(i) Sonogashira coupling at rt
Reaction time for both steps
%Yields of alkynones#/products (4a)$
(ii) [3+3] cyclocondensation at reflux
1
(i) Pd/C, TEA, THF
(ii) TEA in THF
(i) Pd/C, TEA, THF
(ii) Na2CO3, THF
(i) Pd/C, TEA, CH3CN
(ii) Na2CO3, CH3CN
1 h/24 h
1 h/24 h
1 h/24 h
18 h/–
94/0
93/0
90/0
0/–
2
3
4
(i) Pd/C, DABCO, CH3CN
(ii) NA*
5
(i) Pd/C, Na2CO3, THF
(ii) NA*
18 h/–
0/–
6
(i) PdCl2 (PPh3)2/CuI, TEA, THF
(ii) TEA, THF
(i) PdCl2 (PPh3)2/CuI, TEA, CH3CN
(ii) TEA, CH3CN
(i) PdCl2 (PPh3)2/CuI, TEA, THF
(ii) Na2CO3, CH3OH
(i) PdCl2 (PPh3)2/CuI, TEA, CH3CN
(ii) Cs2CO3 (1.0 equiv), CH3CN
(i) PdCl2 (PPh3)2/CuI, TEA, CH3CN
(ii) Cs2CO3 (1.5 equiv), CH3CN
(i) PdCl2 (PPh3)2/CuI, TEA, THF
(ii) Cs2CO3 (1.0 equiv), THF
(i) PdCl2 (PPh3)2/CuI, Cs2CO3, CH3CN
(ii) NA*
1 h/24 h
1 h/24 h
1 h/24 h
1 h/6 h
1 h/6 h
1 h/16 h
18 h/–
96/0
94/0
94/0
98/73
97/81
97/0
0/–
7
8
9
10
11
12
#
$
Yield of alkynone is based on HPLC.
Isolated yields.
*
NA, not applicable.

4290
S. Gupta et al. / Tetrahedron Letters 52 (2011) 4288–4291
Table 2
Synthesis of pyrimido[1,2-a]-indoles from acid chlorides, terminal alkynes, and 2-amino-1H-indole-3-carboxylates, in three-component tandem format
Entry
1
2
3
Product
Yield#(%)
Entry
1
2
3
Product
Yield#(%)
o
O
O
O
2a
O
O
Cl
F3C
1
3a
81
10
1a
2a
3c
83
N
N
N
N
N
4a
4j
1a
O
o
O
O
Cl
F3C
N
N
o
N
4b
4k
1b
2
3
4
2a
3a
3a
3a
81
85
68
11
12
13
1d
1b
1c
2c
2b
2d
3c
3c
3c
77
83
63
O
O
O
O
O
O
2b
F3C
1b
1a
N
N
N
O
N
N
4c
4l
O
O
O
O
2d
F3C
N
N
N
4m
4d
O
O
O
O
O
O
F
N
4n
N
N
N
4e
5
6
1a
1a
1b
2a
3b
3b
86
84
14
15
1b
1d
2c
2a
3d
3d
87
81
O
O
O
O
O
2c
F
N
N
4f
N
N
4o
O
O
O
O
F
N
N
N
N
4g
4p
7
8
2b
2a
3b
3b
78
76
16
17
1a
1b
2c
3d
3d
79
82
O
o
O
O
o
o
O
O
Cl
F
F
1c
2b
N
N
N
N
4h
N
N
4q
O
O
O
o
O
O
O
O
Cl
1d
N
N
4i
4r
9
2d
3b
61
18
1c
2b
3d
80
O
O
#
Isolated yields.
and selected HMBC and nOe correlations required for the confir-
mation of the regioselectivity of 4a are shown in Figure 1.
whereas R2 in terminal alkynes includes both aromatic and ali-
phatic moieties. The resulting reactants 1a–d, 2a–d, and 3a–d were
subjected to multicomponent reaction under the optimized reac-
tion condition to furnish 18 compounds 4a–r in 61–87% isolated
yield (Table 2). In general reactions were not sensitive to the elec-
tronic properties of the substituent on the phenyl ring of the indole
or in acid chlorides. Similarly the use of substituent on the phenyl
ring of terminal alkynes had no effect on isolated yields while
replacement of aromatic ring with aliphatic group reduced the
The scope and limitation of the optimized reaction condition
was examined by studying the effect of different substituents on
the acid chlorides (R1), terminal alkynes (R2), and indoles (R3). In
the indole derivatives, the phenyl ring has been substituted by
introducing electron-donating CH3 group at position 5 and electron
with-drawing groups (F, CF3) at position 6. The R1 in the acid chlo-
rides has been predominantly restricted to aromatic moieties

S. Gupta et al. / Tetrahedron Letters 52 (2011) 4288–4291
4291
Leardini, R.; Nanni, D.; Zanardi, G. Tetrahedron 1993, 49, 10157–10174; (g)
Appendino, G.; Cicione, L.; Minassi, A. Tetrahedron Lett. 2009, 50, 5559–5561.
5. (a) Horton, D. A.; Bourne, G. T.; Smythe, M. L. Chem. Rev. 2003, 103, 893–930;
(b) Bennasar, M. L.; Vidal, B.; Bosch, J. J. Org. Chem. 1997, 62, 3597–3609; (c)
Zhang, H. C.; Ye, H.; Moretto, A. F.; Brumeld, K. K.; Maryano, B. E. Org. Lett. 2000,
2, 89–92; (d) Marugan, J. J.; Manthey, C.; Anaclerio, B.; Lafrance, L.; Lu, T.;
Markotan, T.; Leonard, K. A.; Crysler, C.; Eisennagel, S.; Dasgupta, M.; Tomczuk,
B. J. Med. Chem. 2005, 48, 926–934; (e) Faul, M. M.; Winneroski, L. L.; Krumrich,
C. A. J. Org. Chem. 1998, 63, 6053–6058.
yield of pyrimido[1,2-a]-indoles to 61%. Among acid chlorides ali-
phatic moiety failed to initiate alkynone formation.
In summary, we have developed a simple, efficient, and highly
diverse multicomponent tandem reaction for the synthesis of
pyrimido[1,2-a]-indoles in good to excellent yields. The salient fea-
ture of the protocol involves Sonogashira coupling and [3+3] cyclo-
condensation in sequential format in one-pot. Studies to extend
multicomponent format to other functionalized indoles and alky-
nyl derivatives are in progress in our laboratory and results will
be published elsewhere.
6. For review see: Sapi, J.; Laronze, J. Y. Arkivoc 2004, 208–222 and references
cited therein.
7. (a) Zhang, H.; Larock, R. C. Org. Lett. 2001, 3, 3083–3086; (b) Zhang, H.; Larock,
R. C. J. Org. Chem. 2002, 67, 9318–9330; (c) Verma, A. K.; Kesharwani, T.; Singh,
J.; Tandon, V.; Larock, R. C. Angew. Chem., Int. Ed. 2009, 48, 3168–3210; (d) Shi,
Z.; Cui, Y.; Jiao, N. Org. Lett. 2010, 12, 2908–2911; (e) Chernyak, N.; Tilly, D.; Li,
Z.; Gevorgyan, V. Chem. Commun. 2010, 150–152; (f) Shi, Z.; Zhang, B.; Cui, Y.;
Jiao, N. Angew. Chem., Int. Ed. 2010, 49, 4036–4041; (g) Lu, Y.; Du, X.; Jia, X.; Liu,
Y. Adv. Synth. Catal. 2009, 351, 1517–1522; (h) Renrong, L.; Junlinag, Z. Adv.
Synth. Catal. 2011, 353, 36–40; (i) Shi, Z.; Shengtao, D.; Cui, Y.; Jiao, N. Angew.
Chem., Int. Ed. 2009, 48, 7895–7898; (j) Todd, K. H.; Tomislav, R. J. Am. Chem.
Soc. 2010, 132, 10565–10569.
Acknowledgment
S.G., S.K.S., and A.K.M. are thankful to CSIR, New Delhi, India for
fellowships.
8. Gaster, L. M.; Wyman, P. A. U.S. 5998409 (A1), Appl. No. 08/459,934; 1999.
9. Likhar, P. R.; Subhas, M. S.; Roy, M.; Roy, S.; Kantam, M. L. Helv. Chim. Acta 2008,
91, 259–264.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2011.06.021.
10. Karpov, A. S.; Muller, T. J. J. Synthesis 2003, 18, 2815–2826.
11. To a degassed solution of Pd(PPh3)2Cl2 (0.02 equiv), CuI (0.04 equiv), TEA
(1.25 equiv), in CH3CN (5.0 mL) was added benzoyl chloride (1.0 equiv, 1a–d)
and terminal alkyne (1.0 equiv, 2a–d) under N2 atmosphere and the reaction
was stirred at rt for 1 h. After formation of alkynone as monitored on tlc, ethyl
2-amino-1H-indole-3-carboxylate (1.0 equiv, 3a–d) and Cs2CO3 (1.5 equiv)
were sequentially added to the reaction mixture and was refluxed for 6 h. The
crude reaction mixture was filtered through a bed of celite, washed, and
extracted with ethyl acetate (3 Â 10 mL). The combined organic extracts were
washed with brine, dried over anhydrous sodium sulfate, and evaporated
under reduced pressure in vacuo. The crude reaction mixture was purified by
column chromatography on 60–120 mesh silica using EtOAc/hexane (1:19) as
eluent to afford 4a–r.
References and notes
1. (a) Biggs-Houck, J. E.; Younai, A.; Shaw, J. T. Curr. Opin. Chem. Biol. 2010, 14,
371–382; (b) Toure, B. B.; Hall, D. G. Chem. Rev. 2009, 109, 4439–4486; (c) Lu, J.;
Gong, X.; Yang, H.; Fu, H. Chem. Commun. 2010, 45, 4172–4174; (d) Zhu, Y.;
Zhai, C.; Yue, Y.; Yang, L.; Hu, W. Chem. Commun. 2009, 11, 1362–1364; (e)
Ganem, B. Acc. Chem. Res. 2009, 42, 463–472; (f) Zhu, J.; Bienayme, H.
Multicomponent Reactions; Wiley-VCH: Weinheim, 2005; (g) Bienayme, H.;
Hulme, C.; Oddon, G.; Schmitt, P. Chem. Eur. J. 2000, 6, 3321–3329; (h) Domling,
A.; Ugi, I. Angew. Chem., Int. Ed. 2000, 39, 3168–3210; (i) Zhu, J. Eur. J. Org. Chem.
2003, 7, 1133–1144; (j) Ramon, D. J.; Yus, M. Angew. Chem., Int. Ed. 2005, 44,
1602–1634; (k) Ulaczyk-Lesanko, A.; Hall, D. G. Curr. Opin. Chem. Biol. 2005, 9,
266–276; (l) Willy, B.; Muller, J. J. T. Arkivoc 2008, 195–208; (m) Weber, L. Drug
Discovery Today 2002, 7, 143–147.
2. (a) Sharma, S. K.; Sharma, S.; Agarwal, P. K.; Kundu, B. Eur. J. Org. Chem. 2009, 9,
1309–1312; (b) Saha, B.; Sharma, S.; Sawant, D.; Kundu, B. Tetrahedron Lett.
2007, 48, 1379–1383; (c) Saha, B.; Kumar, R.; Maulik, P. R.; Kundu, B.
Tetrahedron Lett. 2006, 47, 2765–2769; (d) Sawant, D.; Kumar, R.; Maulik, P.
R.; Kundu, B. Org. Lett. 2006, 8, 1525–1528; (e) Saifuddin, M.; Agarwal, P. K.;
Sharma, S. K.; Mandadapu, A. K.; Gupta, S.; Harit, V. K. Eur. J. Org. Chem. 2010,
26, 5108–5117; (f) Agarwal, P. K.; Sawant, D.; Sharma, S.; Kundu, B. Eur. J. Org.
Chem. 2009, 2, 292–303; (g) Sharma, S.; Kundu, B. Tetrahedron Lett. 2008, 49,
7062–7065; (h) Saha, B.; Sharma, S.; Sawant, D.; Kundu, B. Tetrahedron Lett.
2007, 48, 1379–1383.
Ethyl 2-(4-methylphenyl)-4-phenylpyrimido[1,2-a]indole-10-carboxylate (4a).
Yield = 0.238 g (81%); orange solid; mp 190–192 °C; Rf = 0.50 (1:4, EtOAc/
hexane); IR (KBr) m ;
max 3036, 2975, 1757, 1668, 1595, 1481, 1218, 1102 cmÀ1 1H
NMR (400 MHz, CDCl3) d = 8.52 (1H, d, J = 8.2 Hz, ArH), 8.20 (2H, d, J = 8.2 Hz,
ArH), 7.67À7.60 (3H, m, ArH), 7.55 (2H, d, J = 7.0 Hz, ArH), 7.37 (1H, t, J = 7.6 Hz,
ArH), 7.26 (2H, d, J = 8.0 Hz, ArH), 7.08 (1H, s, ArH), 6.94À6.90 (1H, m, ArH), 6.52
(1H, d, J = 8.6 Hz, ArH), 4.57 (2H, q, J = 7.1 Hz, OCH2), 2.40 (3H, s, CH3), 1.58 (3H,
t, J = 7.1 Hz, CH3); 13C NMR (100 MHz, CDCl3) d = 165.3, 157.9, 148.6, 147.0,
141.7, 134.0, 133.8, 130.8, 130.5, 129.7, 129.4, 128.4, 128.0, 127.6, 125.1, 121.9,
121.2, 114.9, 105.4, 95.1, 59.6, 21.5, 14.8 ppm; Exact Mass: 406.17; HR-MS (ESI)
found 407.1760 [M+H]+.
Ethyl 4-cyclohex-1-en-1-yl-2-(3,4-dimethoxyphenyl)-7-(trifluoromethyl )pyri
mido[1,2-a]indole-10-carboxylate (4m). Yield = 0.217 g (63%); yellow solid; mp
222À224 °C; Rf = 0.38 (2:3, EtOAc/Hexane); IR (KBr) mmax 3074, 2939, 2846,
1678, 1596, 1422, 1226, 1106 cmÀ1 1H NMR (300 MHz, CDCl3) d = 8.61 (1H, d,
;
J = 8.6 Hz, ArH), 8.27 (1H, s, ArH), 8.05 (1H, d, J = 1.3 Hz, ArH), 7.82–7.79 (1H, m,
ArH), 7.70 (1H, d, J = 8.5 Hz, ArH), 7.07 (1H, s, ArH), 6.98 (1H, d, J = 8.5 Hz, ArH),
6.35 (1H, s, @CH), 4.55 (2H, q, J = 7.1 Hz, OCH2), 4.08 (3H, s, OCH3), 3.99 (3H, s,
OCH3), 2.53–1.77 (8H, s, 4 Â CH2), 1.56 (3H, t, J = 7.1 Hz, CH3); 13C NMR (75 MHz,
CDCl3) d = 165.0, 159.4, 152.4, 150.9, 149.6, 148.1, 133.0 (d, J = 96.0 Hz), 129.6,
126.7 (d, J = 84.0 Hz), 123.4 (d, J = 24.0 Hz), 122.9, 122.3, 121.6, 112.5, (d,
J = 18.0 Hz), 110.9, 110.4, 104.4, 94.9, 59.9, 56.2, 27.2, 25.4, 21.9, 21.4, 14.9 ppm;
Exact Mass: 524.19; HR-MS (ESI) found 525.3635 [M+H]+.
3. (a) Sharma, S. K.; Mandadapu, A. K.; Saifuddin, M.; Gupta, S.; Agarwal, P. K.;
Mandwal, A. K.; Gauniyal, H. M.; Kundu, B. Tetrahedron Lett. 2010, 51, 6022–
6024; (b) Sharma, S. K.; Gupta, S.; Saifuddin, M.; Mandadapu, A. K.; Agarwal, P.
K.; Gauniyal, H. M.; Kundu, B. Tetrahedron Lett. 2011, 52, 65–68.
4. (a) Wei, C.; Zhang, L.; Li, C. J. Synlett 2004, 1472–1483; (b) Guchhait, S. K.;
Jadeja, K.; Madaan, C. Tetrahedron Lett. 2009, 50, 6861–6865; (c) Kikuchi, S.;
Iwai, M.; Fukuzawa, S. Synlett 2007, 2639–2642; (d) Baudelle, R.; Melnyk, P.;
Deprez, B.; Tartar, A. Tetrahedron 1998, 54, 4125–4140; (e) Cao, K.; Zhang, F.; Tu,
Y. Q.; Zhuo, X. T.; Fan, C. A. Chem. Eur. J. 2009, 15, 6332–6334; (f) Bortolotti, B.;