An efficient intermolecular [Pd]-catalyzed C-C and intramolecular [Cu]-catalyzed C-O bonds formation: Synthesis of functionalized flavans and benzoxepine

Article abstract of DOI:10.1016/j.tetlet.2012.05.050

An efficient three-step strategy for the synthesis of functionalized flavans, starting from readily available 2-bromoiodobenzenes and aryl vinyl alcohols, is presented and successfully extended to benzoxepine. An intermolecular [Pd]-catalyzed C-C and an i

Full text of DOI:10.1016/j.tetlet.2012.05.050

Tetrahedron Letters 53 (2012) 3861–3864
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Tetrahedron Letters
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An efficient intermolecular [Pd]-catalyzed C–C and intramolecular
[Cu]-catalyzed C–O bonds formation: synthesis of functionalized flavans and
benzoxepine
B. Suchand, J. Krishna, B. Venkat Ramulu, D. Dibyendu, A. Gopi Krishna Reddy, L. Mahendar,
⇑
G. Satyanarayana
Department of Chemistry, Indian Institute of Technology (IIT) Hyderabad, Ordnance Factory Estate Campus, Yeddumailaram 502 205, Medak District, AP, India
a r t i c l e i n f o
a b s t r a c t
Article history:
An efficient three-step strategy for the synthesis of functionalized flavans, starting from readily available
2-bromoiodobenzenes and aryl vinyl alcohols, is presented and successfully extended to benzoxepine. An
intermolecular [Pd]-catalyzed C–C and an intramolecular [Cu]-catalyzed C–O bond formations have been
employed as key transformations of the strategy.
Received 27 April 2012
Revised 9 May 2012
Accepted 10 May 2012
Available online 16 May 2012
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Flavans
[Pd]-catalysis
[Cu]-catalysis
Dihydrochalcones
Secondary alcohols
Flavan is a ubiquitous 2-aryl-chroman structural unit present in
a number of flavonoid natural products, which exhibit interesting
biological and pharmacological activities.1 Many members of this
family show interesting biological activity; for example morusyun-
nansin E (1) showed potent inhibitory effects on mushroom tyros-
inase,2 40,6-dichloroflavan (BW683C; 2) inhibits rhinovirus
replication in vitro,3 7-hydroxy-30,40-methylenedioxyflavan (3)
has been traditionally used for the treatment of diabetes, ear and
chest ailments, and some viral infections,4 whereas, 40-hydroxy-
7-methoxy flavan (4) is known as one of the anti-feedant com-
pounds in Lycoris raliata5 (Fig. 1).
assistance of silver salt. It was believed that the size of the substitu-
ent present at the ortho-position of the aromatic ring of the allylic
alcohol is crucial for controlled formation of the allylic alcohol
product, rather producing the expected chalcone product.13b As a
result of consecutive developments for the synthesis of chalocones
and their extensions, herein, we report a new efficient synthetic
route for functionalized flavans (2-aryl and 2-arylmethyl substi-
tuted chromans) and a benzoxepine using two transition metals
OH
Because of their unique structural features and interesting bio-
logical activities, flavans have drawn attention from many synthetic
chemists. Therefore, reasonably a good number of synthetic strate-
gies have been reported for the synthesis of flavan core structure
(various chromans).6–10 Recently, transition metal promoted intra-
molecular [Pd]11 as well as [Cu]12-catalyzed C–O bond forming
reactions between aryl halide and alcohol tether have also been
developed for the synthesis of various chromans. In continuation
of our interest on palladium-catalysis,13 recently we disclosed an
efficient and highly regio- and stereoselective [Pd]-catalyzed b-ary-
lation method for the formation of b-arylallylic alcohols, which is
unexpected under conventional Jeffery’s conditions without the
Cl
OH
O
MeO
HO
O
Cl
OH
4',6-Dichloroflavan (2)
Morusyunnansin E (1)
OH
O
O
MeO
O
O
7-Hydroxy-3',4'-Methylene dioxyflavan (3)
4'-Hydroxy-7-methoxy flavan (4)
⇑
Corresponding author. Fax: +91 40 2301 6032.
E-mail address: gvsatya@iith.ac.in (G. Satyanarayana).
Figure 1. Representative examples of naturally occurring flavonoid natural prod-
ucts with flavan (2-aryl chroman) core structure.
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.05.050

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B. Suchand et al. / Tetrahedron Letters 53 (2012) 3861–3864
[Pd] and [Cu]-catalyzed individual reactions as the key transforma-
tions. During the preparation of our manuscript, a closely related
approach was described by Wang and Franzén12e However, our
strategic approach to the precursors for key cyclization, is different
from Franzén approach and studied extensively with more number
of examples (14 examples) bearing simple to electron rich function-
alities on both aromatic moieties. Most significantly, the present
strategy is amenable for the synthesis of benzoxepine.
Our approach for the synthesis of substituted flavans 9 and 11 is
based on a key intramolecular [Cu]-catalyzed C–O bond formation
between aryl bromide and tethered alcohol moieties of secondary
and tertiary alcohols 8 and 10, respectively. The required precur-
sors (secondary and tertiary alcohols) 8 and 10 can be obtained
from 2-bromoiodbenzenes 5, using a key intermolecular [Pd]-cata-
lyzed C–C bond formation with allylic alcohol coupling partners 6
and followed by reduction and Grignard addition protocol, respec-
tively (Scheme 1).
OH
R3
R4
6a-d
R2
R1
Br
R5
R2
R1
Br
I
Pd(OAc)2 (3 mol%)
O
Et3N (2 equiv)
CH3CN, 80 °C, 24 h
5a-c
7aa-ca
R5
R3
R2
R1
Br
R4
NaBH4 (2 equiv)
MeOH, rt, 30 min.
OH
R5
R3
8aa-ca
R4
Accordingly, treatment of 2-bromoiodobenzenes 5 with cou-
pling partners allylic alcohols 6 in the presence of a catalyst
Pd(OAc)2 (3 mol %) and Et3N (2 equiv) in hot acetonitrile, led to
the dihydrochalcones 7 in very good (64–78%) yields.14,15 Reduc-
tion of dihydrochalcones 7 with NaBH4 in methanol furnished
the corresponding secondary alcohols 8 in near quantitative (97–
99%) yields (Scheme 2, Table 1).
Scheme 2. Synthesis of secondary alcohols 8aa–ca via ketones 7aa–ca using 2-
bromoiodobenzenes 5a–c.
Table 1
Synthesis of secondary alcohols 8aa–ca via ketones 7aa–ca
Entry R1
R2
R3
R4
R5
Yield of 7a
Yield of 8a
With the secondary alcohols 8 in hand, initially the key transi-
tion metal [Pd]-catalyzed C–O bond formation of the alcohol 8ca
was performed under various conditions and the results are sum-
marized in Table 2. Reaction of the alcohol 8ca with Pd(OAc)2
(5 mol %), PPh3 (10 mol %) and Cs2CO3 (2 equiv) in hot DMF failed
to furnish the product 9ca, rather exclusively furnished ketone
9ca0 (entry 1, Table 2). The formation of ketone 9ca0 can be rea-
soned via syn-elimination of b-hydrogen-Pd-species due to the
availability of b-hydrogen and is in good agreement with that of re-
ported by Buchwald et al.11a–c Similarly, other catalytic variants
also found to be inferior to produce the cyclic product 9ca (entries
2 and 4, Table 2). On the other hand, the reaction with the biaryl
ligand L2, which is known to produce cyclic ethers even with sec-
ondary alcohols, produced the product 9ca in good yield 62% (entry
3, Table 2) along with the minor amount of ketone 9ca0 (9%).
Since the [Pd]-catalyzed C–O bond formation was found to be
inferior, we became interested to explore the reaction conditions
using [Cu]-catalyzed C–O bond formations. Gratifyingly, the initial
attempt itself was found to be very efficient in the presence of cat-
alyst CuI (20 mol %)/2,2-bipyridyl (20 mol %), base KOtBu (3 equiv)
in hot DMF (120 °C) for 24 h on secondary alcohol 8ca and resulted
exclusively the cyclized product flavan 9ca, in good yield (68%).
Interestingly, the above conditions proved to be amenable for var-
ious electron releasing substituents as well on aromatic ring bear-
ing bromide and furnished the cyclized flavan products 9 in very
good yields (Table 3).
(%)
(%)
1
2
3
4
5
6
7
8
9
H
H
H
H
OMe
OMe
OMe
OMe
H
H
H
H
H
H
H
H
H
OMe
OMe
OMe
H
OMe
OMe
H
H
7aa 78
7ab 70
OMe 7ac 69
8aa 99
8ab 98
8ac 98
8ad 97
8ba 97
8bb 99
8bc 98
8bd 98
8ca 97
OCH2– OCH2–
H
H
H
7ad 71
7ba 72
7bb 74
OMe
OMe
OMe
H
OMe
OMe
OMe 7bc 72
OCH2– OCH2–
H
H
7bd 75
7ca 64
OMe OMe
H
a Isolated yields of chromatographically pure products; for compounds 7 and 8 the
first letter refers to the 2-bromoiodobenzenes part 5a–5c whereas the second letter
indicates the aromatic ring coming from the allylic alcohol 6a–6d.
Table 2
Attempts of [Pd]-catalyzed intramolecular C–O bond formation on 8ca
OMe
MeO
MeO
Br
[Pd]/L
OH
9ca'
base
O
OMe
toluene
+
80 °C, 24 h
8ca
OMe
MeO
O
9ca
R3
R4
R2
R1
Br
Entry
Catalyst
(mol %)
Ligand
(mol %)
Base
(equiv)
Yield of
Yield of
OH
R5
9caa (%)
9ca0 (%)
a
R6
n
R2
R1
O
1
2
3
4
Pd(OAc)2 (5)
Pd(OAc)2 (3)
Pd(OAc)2 (3)
PPh3 (10)
L1 (6)
L2 (6)
Cs2CO3 (2)
Cs2CO3 (2)
Cs2CO3 (2)
KOtBu (3)
—
—
62
9
69
65
9
R6=H, 8
R6=alkyl or vinyl, 10
R5
R6
R3
n
Pd2(dba)3 (3)
L3 (3)
17
R6=H,9
R4
R6=alkyl or vinyl, 11
n = 1 or 2
PtBu2
R2
R1
Br
I
P(Cy)2
NMe2
O
PPh2
L1
Isolated yields of chromatographically pure products.
PPh2
5
L2
L3
Scheme 1. Retrosynthetic plan for flavans 9 and 10 starting from 2-bromoiodo-
benzenes 5.
a

B. Suchand et al. / Tetrahedron Letters 53 (2012) 3861–3864
3863
R2
R1
Br
R2
R1
Br
Table 3
Synthesis of flavans 9aa–ca from secondary alcohols 8aa–caa
OH
R6
R6MgX
O
R2
Br
R3
THF, -10 °C to rt
R4
R5
CuI (20 mol%)
2,2-bipyridyl
(20 mol%)
KOtBu (3 equiv)
DMF, 120 °C
24 h
OH
R1
R2
R1
O
R5
R3
R5
R3
R4
R4
8aa-ca R5
R3
7aa
7ac
7ad
7bc
7bd
R1=R2=R4=R5=H, R3=OMe, R6=Me, 10aam (88%)
9aa-ca
R4
1
2
3
4
5
6
10acm
R =R =H, R =R =R =OMe, R =Me,
(97%)
10adm
1
2
3
4
5
6
R =R = R =H, R =R =OCH2-, R =Me,
(93%)
(93%)
R1=OMe, R2=R3=H, R4=R5=OCH2-, R6=CH=CH2, 10bdv (93%)
1
3
4
5
2
6
OMe
10bcm
R =R =R =R =OMe, R =H, R =Me,
O
O
OMe
OMe
Scheme 3. Synthesis of tertiary alcohols 10aam–bdv from dihydrochalcones 7aa–
bd. Isolated yields of chromatographically pure products. Grignard reagents
prepared in anhydrous diethyl ether and added to the substrates (ketones)
dissolved in anhydrous THF. For compounds 10 the first letter refers to the 2-
bromoiodobenzenes part 5a–5c, second letter indicates the aromatic ring coming
from the allylic alcohol 6a–6d, whereas the third letter indicates sixth substituent
that connected to the quaternary center (m for R6 = Me and v for R6 = vinyl).
9aa (68%)
9ab (70%)
OMe
O
O
OMe
OMe
O
O
9ad (71%)
9ac (85%)
Table 4
OMe
Synthesis of flavans 11aam–bdv from tertiary alcohols 10aam–bdva
O
O
OMe
OMe
R3
R4
R2
R1
Br
MeO
MeO
MeO
9ba (71%)
9bb (68%)
CuI (20 mol%)
2,2-bipyridyl
R5
OH
R6
OMe
O
O
R2
R1
O
(20 mol%)
R6
OMe
OMe
O
KOtBu (3 equiv)
O
DMF, 120 °C
R5
R3
24 h
10aam-bdm R4
11aam-bdv
9bd (70%)
MeO
MeO
9bc (70%)
O
OMe
MeO
OMe
OMe
OMe
9ca (68%)
O
Me
a
O
Isolated yields of chromatographically pure products; for compounds 9 the first
letter refers to the 2-bromoiodobenzenes part 5a–5c whereas the second letter
indicates the aromatic ring coming from the allylic alcohol 6a–6d.
Me
11aam (70%)
11acm (82%)
O
MeO
O
OMe
OMe
After successful accomplishment of flavans 9aa–ca, we turned
our attention to determine the scope and limitation of the method.
Hence, [Cu]-catalysis of tertiary alcohols 10 was also investigated.
The required tertiary alcohols 10 were synthesized by the addition
of alkyl/alkenyl Grignard reagents to dihydrochalcones 7, in very
good yields (Scheme 3).
O
O
Me
Me
MeO
11adm (85%)
11bcm (67%)
O
In general, the results were fairly comparable to those observed
for secondary alcohols 8aa–ca, and furnished the products
11aam–bdm possessing simple as well as electron rich aromatic
functionality on either of aromatic rings, in good yields (Table 4).
In addition to the NMR spectroscopic confirmation, the struc-
ture of one flavan was unambiguously further confirmed by single
crystal X-ray diffraction analysis on 9ac (Fig. 2).
Finally to the check the scope and applicability of the method,
we explored the synthesis of 2-aryl-2,3,4,5-tetrahydro-1-benzoxe-
pine (seven-membered cyclic ether). Interestingly, compounds fea-
turing bezoxepines core are also found to be pharmaceutically
important as they exhibit interesting biological activities. The req-
uisite coupling partner, homoallylic alcohol 12a was synthesized
by using Barbier reaction under sonochemical acceleration. Unlike
the case of allylic alcohols, the Jeffery–Heck coupling on homoal-
lylic alcohol 12a with 2-bromoidobenzene 5a, resulted in ketone
13aa in moderate yield (51%) along with a mixture of other
unidentified products. Finally, [Cu]-catalyzed intramolecular C–O
O
O
CH2
11bdv (65%)
MeO
a Isolated yields of chromatographically pure products; For compounds 11 the first
letter refers to the 2-bromoiodobenzenes part 5a–5c, second letter indicates the
aromatic ring coming from the allylic alcohol 6a–6d, whereas the third letter
indicates sixth substituent that connected to the quaternary center (m for R6 = Me
and v for R6 = vinyl).
bond formation on the secondary alcohol 14aa, proved to be ame-
nable to the standard conditions and gave the cyclic ether 15aa in
very good yield 78% (Scheme 4).
In summary, we have developed an efficient three-step strategy
for the synthesis of functionalized flavans and a benzoxepine,

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B. Suchand et al. / Tetrahedron Letters 53 (2012) 3861–3864
References and notes
1. (a) Abdel-Razik, A. F.; Nassar, M. I.; El-Khrisy, E. D. A.; Dawidar, A. A. M.; Mabry,
T. J. Fitoterapia 2005, 76, 762; (b) Morikawa, T.; Xu, F. M.; Matsuda, H.;
Yoshikawa, M. Chem. Pharm. Bull. 2006, 54, 1530; (c) Li, L. J.; Zhang, Y.; Zhang,
P.; Pi, H. F.; Ruan, H. L.; Wu, J. Z. J. Asian Nat. Prod. Res. 2011, 13, 367; (d)
Saengchantara, S. T.; Wallace, T. W. Nat. Prod. Rep. 1986, 3, 465; (e) Veitch, N. C.;
Grayer, R. J. Nat. Prod. Rep. 2011, 28, 1626.
2. Hu, X.; Wu, J.-W.; Wang, M.; Yu, Mei-Hua; Zhao, Q.-S.; Wang, H.-Y.; Hou, A.-J. J.
Nat. Prod. 2012, 75, 82.
3. Bauer, D. J.; Selway, J. W. T.; Batchelor, J. F.; Tisdale, M.; Caldwell, I. C.; Young, D.
A. Nature (London) 1981, 292, 369.
4. Ghosal, S.; Singn, S. K.; Srivastava, R. S. Phytochemistry 1985, 24, 151.
5. Numata, A.; Takemura, T.; Ohbayashi, H.; Katsuno, T.; Yamamoto, K.; Sato, K.;
Kobayashi, S. Chem. Pharm. Bull. 1983, 31, 2146.
Figure 2. X-ray crystal structure of 9ac. Thermal ellipsoids are drawn at 50%
6. For recent examples of racemic chromans, see: (a) Chiba, K.; Hirano, T.; Kitano,
Y.; Tada, M. Chem. Commun. 1999, 691; (b) Barluenga, J.; Trincado, M.; Eduardo
Rubio, E.; González, J. M. J. Am. Chem. Soc. 2004, 126, 3416; (c) Yamamoto, Y.;
Itonaga, K. Org. Lett. 2009, 11, 717; (d) Dang, T. T.; Boeck, F.; Hintermann, L. J.
Org. Chem. 2011, 76, 9353; (e) Hamamoto, H.; Hata, K.; Nambu, H.; Shiozaki, Y.;
Tohma, H.; Kita, Y. Tetrahedron Lett. 2004, 45, 2293; (f) Sugimoto, H.;
Nakamura, S.; Ohwada, T. Adv. Synth. Catal. 2007, 349, 669.
7. For recent examples of enantioselective synthesis of chromans, see: (a) Trost, B.
M.; Shen, H. C.; Dong, L.; Surivet, J.-P.; Sylvain, C. J. Am. Chem. Soc. 2004, 126,
11966; (b) Couladouros, E. A.; Moutsos, V. I.; Maria Lampropoulou, M.; Little, J.
L.; Hyatt, J. A. J. Org. Chem. 2007, 72, 6735; (c) Mull, E. S.; Van Zandt, M.;
Golebiowski, A.; Beckett, R. P.; Sharma, P. K.; Schroeter, H. Tetrahedron Lett.
2012, 53, 1501.
8. For a recent review on C–O bond formations, see: (a) Shen, H. C. Tetrahedron
2009, 65, 3931; (b) Pitsinos, E. N.; Vidali, V. P.; Couladouros, E. A. Eur. J. Org.
Chem. 2011, 1207; (c) Evano, G.; Blanchard, N.; Toumi, M. Chem. Rev. 2008, 108,
3054; (d) Schlummer, B.; Scholz, U. Adv. Synth. Catal. 2004, 346, 1599.
9. For highlights on C–C, C–N, and C–O bond formations, see: (a) Monnier, F.;
Taillefer, M. Angew. Chem. 2008, 120, 3140. Angew. Chem., Int. Ed. 2008, 47,
3096.
10. For recent examples of flvans (2-aryl chromans), see: (a) Oyama, K.; Kondo, T. J.
Org. Chem. 2004, 69, 5240; (b) Li, Y.; Yang, J.; Li, W. Z.; Hou, L.; Xue, J.; Li, Y. J.
Nat. Prod. 2001, 64, 214; (c) Li, K.; Vanka, K.; Thompson, W. H.; Tunge, J. A. Org.
Lett. 2006, 8, 4711; (d) Hodgetts, K. J. Tetrahedron 2005, 61, 6860; (e) Zhang, L.;
Zhang, W.; Ma, E.; Wu, L.; Bao, K.; Wang, X.; Wang, Y.; Song, H. Arch. Pharm.
Chem. Life Sci. 2007, 340, 650; (f) Mazimba, O.; Masesane, I. B.; Majinda, R. R.
Tetrahedron Lett. 2011, 52, 6716; (g) Nay, B.; Collet, M.; Lebon, M.; Chèze, C.;
Vercauteren, J. Tetrahedron Lett. 2002, 43, 2675; (h) Radomkit, S.; Sarnpitak, P.;
Tummatorn, J.; Batsomboon, P.; Ruchirawat, S.; Ploypradith, P. Tetrahedron
2011, 67, 3904.
probability level.
Br
5a
I
OHC
OMe
Zn/
Br
OMe
HO
Pd(OAc)2 (3 mol%)
))))), THF
rt, 4 h, 76%
Et3N (2 equiv)
CH3CN, 80 °C, 24 h, 51%
12a
Br
Br
CuI (20 mol%)
2,2-bipyridyl
NaBH4 (2 equiv)
MeOH, rt, 30 min.
97%
(20 mol%)
OH KOtBu (3 equiv)
O
DMF, 120 °C
24 h, 78%
14aa
13aa
OMe
OMe
O
OMe
15aa
Scheme 4. Synthesis of benzoxepine 15aa starting from meta-anisaldehyde.
11. For [Pd]-catalyzed intramolecular C–O bond formations, see: (a) Palucki, M.;
Wolfe, J. P.; Buchwald, S. L. J. Am. Chem. Soc. 1996, 118, 10333; (b) Torraca, K. E.;
Kuwabe, S. I.; Buchwald, S. L. J. Am. Chem. Soc. 2000, 122, 12907; (c) Kuwabe, S.;
Torraca, K. E.; Buchwald, S. L. J. Am. Chem. Soc. 2001, 123, 12202; (d) Shelby, Q.;
Kataoka, N.; Mann, G.; Hartwig, J. J. Am. Chem. Soc. 2000, 122, 10718; (e)
Kataoka, N.; Shelby, Q.; Stambuli, J. P.; Hartwig, J. F. J. Org. Chem. 2002, 67,
5553; (f) Neogi, A.; Majhi, T. P.; Achari, B.; Chattopadhyay, P. Eur. J. Org. Chem.
2008, 2, 330; (g) Xu, B.; Xue, J.; Zhu, J.; Li, Y. Chem. Lett. 2008, 37, 202.
12. For [Cu]-catalyzed intramolecular C–O bond formations, see: (a) Niu, J. J.; Guo,
P. R.; Kang, J. T.; Li, Z. G.; Xu, J. W.; Hu, S. J. J. Org. Chem. 2009, 74, 5075; (b)
Adams, H.; Gilmore, N. J.; Jones, S.; Muldowney, M. P.; von Reuss, S. H.; Vemula,
R. Org. Lett. 2008, 10, 1457; (c) Zhao, J.; Zhao, Y. F.; Fu, H. Angew. Chem., Int. Ed.
2011, 50, 3769; (d) Fang, Y.; Li, C. J. Org. Chem. 2006, 71, 6427; (e) Wang, Y.;
Franzén, R. Synlett 2012, 925.
employing an intermolecular [Pd]-catalyzed C–C and intramolecu-
lar [Cu]-catalyzed C–O bond formations as the key steps. The strat-
egy is efficient and amenable for the synthesis of a number of
analogs. Further investigations on the application of the current
strategy for other benzoxepine analogs and for the total synthesis
of flavonoid natural products are under progress.
Acknowledgments
13. (a) Reddy, A. G. K.; Krishna, J.; Satyanarayana, G. Synlett 2011, 1756–1760; (b)
Krishna, J.; Reddy, A. G. K.; Mahendar, L.; Ramulu, B. V. Synlett 2012, 375; (c)
Mahendar, L.; Krishna, J.; Reddy, A. G. K.; Ramulu, B. V. Org. Lett. 2012, 14, 628.
14. For some relevant examples of [Pd]-catalyzed coupling of iodobenzenes with
aryl vinyl alcohols, see: Briot, A.; Baehr, C.; Brouillard, R.; Wagner, A.;
Mioskowski, C. J. Org. Chem. 2004, 69, 1374.
15. For some relevant examples of [Pd]-catalyzed coupling of 2-
bromoiodobenzenes with various simple alkenols, see: (a) Tietze, L. F.; Kahle,
K.; Raschke, T. Chem. Eur. J. 2002, 8, 401; (b) Bruyére, D.; Bouyssi, D.; Balme, G.
Tetrahedron 2004, 60, 4007; (c) Satyanarayana, G.; Maier, M. E. Tetrahedron
2008, 64, 356; (d) Satyanarayana, G.; Maier, M. E. J. Org. Chem. 2008, 73, 5410;
(e) Satyanarayana, G.; Maier, M. E. Org. Lett. 2008, 12, 2316; (f) Satyanarayana,
G.; Maichle-Mössmer, C.; Maier, M. E. Chem. Commun. 2009, 1571.
Financial support by the Department of Science and Technology
[(DST), CHE/2010-11/006/DST/GSN], New Delhi is gratefully
acknowledged. We thank Dr. S.J.Gharpure and Prof. Dr. Martin E.
Maier for their valuable suggestions. J.K., B.V.R., A.G.K. and L.M.
thanks CSIR, New Delhi, for the award of research fellowship.
Supplementary data
Supplementary data associated with this article can be found, in
the onlineversion, at http://dx.doi.org/10.1016/j.tetlet.2012.05.050.