Combined directed ortho-zincation and palladiumcatalyzed strategies: Synthesis of 4,n-dimethoxysubstituted benzo[b]furans

Article abstract of DOI:10.3762/bjoc.7.146

A new route to regioselectively dialkoxy-functionalized benzo[b]furan derivatives has been developed from 3-halo-2-iodoanisoles bearing an additional methoxy group, which have been accessed through an ortho-zincation/iodination reaction. Two palladiumcata

Full text of DOI:10.3762/bjoc.7.146

Combined directed ortho-zincation and palladium-
catalyzed strategies: Synthesis of 4,n-dimethoxy-
substituted benzo[b]furans
Verónica Guilarte, M. Pilar Castroviejo, Estela Álvarez and Roberto Sanz*
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2011, 7, 1255–1260.
Área de Química Orgánica, Departamento de Química, Facultad de
Ciencias, Universidad de Burgos, Pza. Misael Bañuelos s/n, 09001
Burgos, Spain
doi:10.3762/bjoc.7.146
Received: 29 April 2011
Accepted: 21 July 2011
Email:
Published: 12 September 2011
Roberto Sanz* - rsd@ubu.es
This article is part of the Thematic Series "Directed aromatic
functionalization".
* Corresponding author
Keywords:
Guest Editor: V. Snieckus
benzo[b]furans; o-zincation; palladium; selectivity
© 2011 Guilarte et al; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
A new route to regioselectively dialkoxy-functionalized benzo[b]furan derivatives has been developed from 3-halo-2-iodoanisoles
bearing an additional methoxy group, which have been accessed through an ortho-zincation/iodination reaction. Two palladium-
catalyzed processes, namely a Sonogashira coupling followed by a tandem hydroxylation/cyclization sequence, give rise to new and
interesting dimethoxy-substituted benzo[b]furans.
Introduction
The directed ortho-metallation (DoM) reaction has been widely and halogens), the introduction in the last years of new
used as a powerful and efficient method for regioselective func- organometallic “ate” complexes [8,9], which combine an alkali
tionalization of aromatic compounds and different directing metal with either magnesium, zinc, aluminium, or copper, has
groups have been used to facilitate the deprotonation reaction allowed more selective metallation reactions. The milder reac-
[1-5]. Various strong bases such as alkyl lithiums and their tion conditions required make these deprotonation reactions
derivatives (for instance, TMEDA-activated complexes [6] and tolerant to the coexistence of a wider range of functional
heavier alkali metal tert-butoxide-complexed alkyl lithium groups. In this field, the work of Kondo and Uchiyama is
reagents, known as superbases and introduced by Schlosser [7]), remarkable as they described highly chemo- and regioselective
as well as lithium dialkylamides, have usually been employed to deprotonative zincation [10-12], alumination [13], and
perform deprotonative metallations. Whereas the use of these cupration [14] reactions of some functionalized aromatic and
strong bases has several limitations regarding the presence of heteroaromatic compounds, as well as of meta-functionalized
certain functional groups (mainly carboxylic acid derivatives haloaromatics. In particular, the alkali metal mediated zincation
1255

Beilstein J. Org. Chem. 2011, 7, 1255–1260.
reactions have turned out to be very useful processes and the Pd-catalyzed cross-coupling reactions or halocyclization
structures and reaction pathways of TMP-zincates (TMP = processes. Following our interest in the development of strate-
2,2,6,6-tetramethylpiperidine) have been studied in detail [15- gies for the synthesis of functionalized benzo[b]furan deriva-
19].
tives [39], we envisaged that 4,n-dimethoxybenzo[b]furans
could be regioselectively synthesized from 3-halo-2-
On the other hand, benzo[b]furan is a basic skeleton found in a iodoanisoles bearing an additional methoxy group, by
variety of significant natural products [20], and some deriva- combining two palladium-catalyzed processes, that is a
tives are also used as organic materials, due to their optical and selective Sonogashira coupling and a tandem hydroxylation/
electronic properties [21]. Thus, many synthetic efforts have heterocyclization reaction. The required o-dihaloanisole deriva-
been devoted to the synthesis of this type of compound [22,23]. tives could be prepared by a selective ortho-metallation reac-
In particular, several benzo[b]furan derivatives with oxygen- tion and subsequent electrophilic quenching with iodine
bearing substituents, such as hydroxy, or alkoxy, at the benzene (Scheme 1).
moiety are known to be biologically active compounds [24-28]
(Figure 1).
Scheme 1: Retrosynthetic analysis of 4,n-dimethoxy-substituted
benzo[b]furans.
Results and Discussion
As established in our proposed retrosynthetic analysis
Figure 1: Some representative dihydroxybenzofuran derived natural
products.
(Scheme 1), we needed to develop a convenient synthesis of
3-halo-2-iodoanisoles bearing an additional methoxy
group. Taking advantage of the deprotonative ortho-zincation of
Among the various approaches developed for the synthesis of meta-functionalized haloaromatics by using TMP-zincates,
the benzofuran ring system, the cyclodehydration of α-aryloxy described by Uchiyama and co-workers [11,12], we previously
ketones [29], the Claisen rearrangement of an allyl aryl ether developed efficient syntheses of 3-chloro-2-iodoanisole (2a)
followed by Pd-catalyzed intramolecular oxidative cyclization and 3-bromo-2-iodoanisole (2b) from the corresponding
[30], and the tandem Sonogashira coupling/heterocyclization of 3-haloanisoles 1, through their treatment with lithium di-tert-
2-halophenols with terminal alkynes [31], are some of the most butyltetramethylpiperidinozincate, followed by electrophilic
used. However, their application to the synthesis of 4-substi- trapping of the intermediate arylzincate with iodine (Scheme 2)
tuted benzo[b]furans is especially challenging, because the [40].
meta-substituted starting materials tend to cyclize at the less
hindered ortho-position, leading to the formation of 6-substi- Thus, we decided to test this deprotonative metallation on the
tuted heterocycles or a mixture of 6- and 4-substituted ones commercially available dimethoxyhalobenzene derivatives 3
[32].
The DoM strategy, when linked with different cross-coupling
Pd-catalyzed reactions, could provide a superior approach
for the construction of polysubstituted aromatic and heteroaro-
matic compounds [33-38]. In this context, we studied the
o-lithiation of 3-halophenols and the resulting 2,3-difunctional-
ized phenol derivatives were applied to the synthesis of 4-func-
tionalized benzo[b]furans [39], 4- or 7-alkoxyindoles [40], and
Scheme 2: Deprotonative zincation of 3-haloanisoles 1.
7-oxy-substituted benzo[b]thiophenes [41] by employing
1256

Beilstein J. Org. Chem. 2011, 7, 1255–1260.
Table 1: Synthesis of methoxy-substituted 3-halo-2-iodoanisoles 4.
Entry
Starting material
X
OMea
Product
Yield (%)b
1
2
3
4
5
6
3a
3b
3c
3d
3e
3f
Cl
Br
Cl
Br
Cl
Br
ortho
ortho
meta
meta
para
para
4a
4b
4c
4d
4e
4f
70
73
88
75
78
80
aPosition of the additional methoxy group relative to the existing one. bIsolated yield based on the starting material 3.
with the same t-Bu2Zn(TMP)Li, easily prepared by reaction of Having an efficient protocol for the synthesis of dimethoxy-
preformed di-tert-butylzinc with lithium tetramethylpiperidide. haloiodobenzene derivatives 4, and with the retrosynthetic
Under these reaction conditions the zincation reactions took analysis outlined in Scheme 1 in mind, we tackled the selective
place regioselectively and, after treatment with iodine, afforded introduction of an alkynyl moiety at the iodinated position. The
the corresponding 3-halo-2-iodoanisole derivatives 4 in good presence of two different halogen atoms in compounds 4
yields (Table 1). It is interesting to note that substrates 3c and implies that a selective Sonogashira coupling reaction should
3d bearing the two methoxy groups in a meta relationship selec- occur (Table 2). This has been achieved in two different ways.
tively undergo metalation at the position between the methoxy Considering the steric hindrance of the required position (o,o-
and the halide groups, irrespective of the nature of the halogen disubstituted) we employed a copper- and solvent-free method-
atom (chloro or bromo) (Table 1, entries 3 and 4).
ology for the Sonogashira coupling that uses tetrabutylammo-
Table 2: Synthesis of dimethoxy-substituted o-alkynylhaloarenes 5–7.
Entry
Starting material
X
OMea
R
Product
Methodb
Yield (%)c
1
2
3
4
5
6
4a
4a
4b
4b
4b
4b
Cl
Cl
Br
Br
Br
Br
ortho
ortho
ortho
ortho
ortho
ortho
n-Bu
c-C6H9d
n-C6H13
n-C6H13
Ph
5a
5b
5c
5c
5d
5d
A
A
A
B
A
B
65
79
91
90
83
87
Ph
1257

Beilstein J. Org. Chem. 2011, 7, 1255–1260.
Table 2: Synthesis of dimethoxy-substituted o-alkynylhaloarenes 5–7. (continued)
7
4c
4c
4d
4d
4d
4d
4d
4e
4f
Cl
Cl
Br
Br
Br
Br
Br
Cl
Br
Br
Br
Br
Br
meta
meta
meta
meta
meta
meta
meta
para
para
para
para
para
para
n-Bu
c-C6H9d
n-C5H11
n-C5H11
Ph
6a
6b
6c
6c
6d
6d
6e
7a
7b
7b
7c
7c
7d
A
A
A
B
A
B
A
A
A
B
A
B
A
57
69
63
79
52
80
75
69
55
95
79
92
93
8
9
10
11
12
13
14
15
16
17
18
19
Ph
4-MeC6H4
n-Bu
n-C5H11
n-C5H11
Ph
4f
4f
4f
Ph
4f
3-FC6H4
aPosition of the additional methoxy group referred to the existing one. bMethod A: alkyne (1.5–2 equiv), PdCl2(PPh3)2 (6 mol %), TBAF·3H2O
(3 equiv), 50–60 °C. Method B: alkyne (1.2 equiv), PdCl2(PPh3)2 (3 mol %), CuI (5 mol %), Et2NH (1.5 equiv), DMF, 50 °C. cIsolated yield based on
the starting material 4. d1-Cyclohexenyl.
nium fluoride as base [42] (Table 2, method A). Subsequently, triisopropylbiphenyl) at 100 °C in a 1:1 mixture of H2O:1,4-
we checked that the selective coupling could be carried out dioxane, gave rise to the corresponding benzo[b]furan
under standard Pd–Cu catalysis by controlling the reaction derivatives 8–10 in moderate to good yields (Table 3, method
temperature [43,44] (Table 2, method B).
C). In general, slightly better results were obtained starting from
aryl bromides instead of aryl chlorides. In addition, we also
By either of these procedures, A or B, o-alkynylhaloarenes 5–7 observed better yields for the corresponding benzofuran deriva-
were prepared usually in high yields from the corresponding tives 9 (Table 3, entries 5–9) derived from the starting
dimethoxyhaloiodobenzenes 4 (Table 2). Starting from o-alkynylhalobenzene derivatives 6, bearing the two
chloroiodo derivatives 4a,c and e good yields were obtained for methoxy groups in a 3,5-relationship relative to the halide.
the corresponding coupled products 5–7 by using the Pd/TBAF Moreover, because of the extended reaction times needed for
method A, with no significant side-products (Table 2, entries complete consumption of the starting materials, we
1,2,7,8 and 14). However, under these conditions bromine- developed an alternative protocol under microwave irradiation
containing compounds 4b,d and f afforded variable amounts of that shortens the time required for the coupling to a few
dialkynylation products in some cases, and so the Pd/Cu minutes (method D), and the final products were obtained in
catalyzed procedure B gave rise to better selectivities and yields similar yields to those obtained by the conventional coupling
of the desired alkynes 5–7 (Table 2, entries 4,6,10,12,16 and method C.
18).
Conclusion
According to our retrosynthetic analysis (Scheme 1), the final We have developed a synthetic route to access regioselectively
step to achieve the benzofuran derivatives should be the functionalized 4,n-dimethoxybenzo[b]furans through combined
incorporation of the hydroxy group followed by in situ directed ortho-metallation (DoM)/palladium-catalyzed reac-
heterocyclization. In recent years, the direct hydroxylation of tions. The deprotonative ortho-zincation of meta-functionalized
aryl halides has been developed by several groups by using haloanisoles bearing an additional methoxy group, followed by
palladium- or copper-catalyzed protocols. Whereas the reac- electrophilic quenching with iodine allows the regioselective
tions under copper catalysis work well for aryl iodides [45-48], and straightforward synthesis of highly functionalized
the palladium-catalyzed hydroxylation also takes place dihalodimethoxybenzene derivatives. A subsequent selective
with aryl bromides and chlorides [49-51]. Thus, in our case we Sonogashira coupling with terminal alkynes, followed by direct
employed the Pd-catalyzed Buchwald protocol [49] in an hydroxylation with KOH of the resulting o-haloaryl alkyne and
attempt to synthesise the desired dimethoxybenzo[b]furan in situ heterocyclization, afforded the benzo[b]furan derivatives.
derivatives. Thus, reaction of the previously prepared o-haloaryl In addition, we have developed a new procedure for the
alkynes 5–7 with KOH, in the presence of catalytic amounts of Pd-catalyzed hydroxylation reaction that allows the coupling to
Pd2(dba)3 and t-BuXPhos (2-di-tert-butylphosphino-2',4',6'- take place within in a few minutes.
1258

Beilstein J. Org. Chem. 2011, 7, 1255–1260.
Table 3: Synthesis of dimethoxy-substituted benzo[b]furans 8–10.
Entry
Starting material
X
R
Product
OMea
Methodb
Yield (%)
1
2
5a
5b
5c
5d
6a
6b
6c
6d
6e
7a
7b
7c
7d
Cl
Cl
Br
Br
Cl
Cl
Br
Br
Br
Cl
Br
Br
Br
n-Bu
c-C6H9c
n-C6H13
Ph
8a
8b
4,5-(MeO)2
4,5-(MeO)2
4,5-(MeO)2
4,5-(MeO)2
4,6-(MeO)2
4,6-(MeO)2
4,6-(MeO)2
4,6-(MeO)2
4,6-(MeO)2
4,7-(MeO)2
4,7-(MeO)2
4,7-(MeO)2
4,7-(MeO)2
D
C
D
C
C
C
C
C
C
C
C
D
D
55
57
55
50
62
73
70
81
75
60
64
71
65
3
8c
4
8d
5
n-Bu
9a
6
c-C6H9c
n-C5H11
Ph
9b
7
9c
8
9d
9
4-MeC6H4
n-Bu
9e
10
11
12
13
10a
10b
10c
10d
n-C5H11
Ph
3-FC6H4
aPosition of the methoxy groups referred to benzo[b]furan moiety. bMethod C: conventional heating (100 °C, overnight). Method D: MW (50 W,
150 °C, 12 min). c1-Cyclohexenyl.
References
1. Hartung, C. G.; Snieckus, V. The Directed ortho Metalation Reaction –
Supporting Information
A Point of Departure for New Synthetic Aromatic Chemistry. In Modern
Experimental procedures and spectroscopic data for all new
Arene Chemistry; Astruc, D., Ed.; Wiley-VCH: New York, 2002;
compounds. Copies of 1H NMR and 13C NMR spectra for
pp 330–367. doi:10.1002/3527601767.ch10
new compounds.
2. Clayden, J. Regioselective Synthesis of Organolithiums by
Deprotonation. In Organolithiums: Selectivity for Synthesis;
Tetrahedron Organic Chemistry Series, Vol. 23; Pergamon: Oxford,
2002; pp 9–109.
Supporting Information File 1
Experimental and analytical data.
3. Whisler, M. C.; MacNeil, S.; Snieckus, V.; Beak, P.
Angew. Chem., Int. Ed. 2004, 43, 2206–2225.
doi:10.1002/anie.200300590
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-7-146-S1.pdf]
4. Anctil, E. J.-G.; Snieckus, V. In Metal-Catalyzed Cross-Coupling
Reactions, 2nd ed.; de Meijere, A.; Diederich, F., Eds.; Wiley-VCH:
Weinheim, 2004; pp 761–813.
Supporting Information File 2
NMR spectra.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-7-146-S2.pdf]
5. Macklin, T.; Snieckus, V. In Handbook of C–H Transformations;
Dyker, G., Ed.; Wiley-VCH: New York, 2005; pp 106–119.
6. Eberhardt, G. G.; Butte, W. A. J. Org. Chem. 1964, 29, 2928–2932.
doi:10.1021/jo01033a029
7. Schlosser, M.; Choi, J. H.; Takagishi, S. Tetrahedron 1990, 46,
5633–5648. doi:10.1016/S0040-4020(01)87763-2
8. Mulvey, R. E.; Mongin, F.; Uchiyama, M.; Kondo, Y.
Angew. Chem., Int. Ed. 2007, 46, 3802–3824.
doi:10.1002/anie.200604369
Acknowledgements
We acknowledge MICINN (CTQ2010-15358) and Junta de
Castilla y León (BU021A09 and GR-172) for financial support.
We are also grateful to MEC (FPU predoctoral fellowship to
V.G.).
9. Mulvey, R. E. Acc. Chem. Res. 2009, 42, 743–755.
doi:10.1021/ar800254y
1259

Beilstein J. Org. Chem. 2011, 7, 1255–1260.
10.Kondo, Y.; Shilai, M.; Uchiyama, M.; Sakamoto, T. J. Am. Chem. Soc.
1999, 121, 3539–3540. doi:10.1021/ja984263t
35.Nerdinger, S.; Kendall, C.; Cai, X.; Marchart, R.; Riebel, P.;
Johnson, M. R.; Yin, C.-F.; Hénaff, N.; Eltis, L. D.; Snieckus, V.
J. Org. Chem. 2007, 72, 5960–5967. doi:10.1021/jo062543i
36.MacNeil, S. L.; Gray, M.; Gusev, D. G.; Briggs, L. E.; Snieckus, V.
J. Org. Chem. 2008, 73, 9710–9719. doi:10.1021/jo801856n
37.James, C. A.; Snieckus, V. J. Org. Chem. 2009, 74, 4080–4093.
doi:10.1021/jo9001454
11.Uchiyama, M.; Miyoshi, T.; Kajihara, Y.; Sakamoto, T.; Otani, Y.;
Ohwada, T.; Kondo, Y. J. Am. Chem. Soc. 2002, 124, 8514–8515.
doi:10.1021/ja0202199
12.Uchiyama, M.; Kobayashi, Y.; Furuyama, T.; Nakamura, S.;
Kajihara, Y.; Miyoshi, T.; Sakamoto, T.; Kondo, Y.; Morokuma, K.
J. Am. Chem. Soc. 2008, 130, 472–480. doi:10.1021/ja071268u
13.Naka, H.; Uchiyama, M.; Matsumoto, Y.; Wheatley, A. E. H.;
McPartlin, M.; Morey, J. V.; Kondo, Y. J. Am. Chem. Soc. 2007, 129,
1921–1930. doi:10.1021/ja064601n
38.James, C. A.; Coelho, A. L.; Gevaert, M.; Forgione, P.; Snieckus, V.
J. Org. Chem. 2009, 74, 4094–4103. doi:10.1021/jo900146d
39.Sanz, R.; Castroviejo, M. P.; Fernández, Y.; Fañanás, F. J.
J. Org. Chem. 2005, 70, 6548–6551. doi:10.1021/jo0508402
40.Sanz, R.; Castroviejo, M. P.; Guilarte, V.; Pérez, A.; Fañanás, F. J.
J. Org. Chem. 2007, 72, 5113–5118. doi:10.1021/jo070643y
41.Sanz, R.; Guilarte, V.; Hernando, E.; Sanjuán, A. M. J. Org. Chem.
2010, 75, 7443–7446. doi:10.1021/jo101436f
14.Usui, S.; Hashimoto, Y.; Morey, J. V.; Wheatley, A. E. H.; Uchiyama, M.
J. Am. Chem. Soc. 2007, 129, 15102–15103. doi:10.1021/ja074669i
15.Clegg, W.; Dale, S. H.; Drummond, A. M.; Hevia, E.; Honeyman, G. W.;
Mulvey, R. E. J. Am. Chem. Soc. 2006, 128, 7434–7435.
doi:10.1021/ja061898g
42.Liang, Y.; Xie, Y.-X.; Li, J.-H. J. Org. Chem. 2006, 71, 2535–2537.
doi:10.1021/jo051882t
16.Uchiyama, M.; Matsumoto, Y.; Nobuto, D.; Furuyama, T.;
Yamaguchi, K.; Morokuma, K. J. Am. Chem. Soc. 2006, 128,
8748–8750. doi:10.1021/ja060489h
43.Sanz, R.; Guilarte, V.; García, N. Org. Biomol. Chem. 2010, 8,
3860–3864. doi:10.1039/c004360e
17.Uchiyama, M.; Matsumoto, Y.; Usui, S.; Hashimoto, Y.; Morokuma, K.
Angew. Chem., Int. Ed. 2007, 46, 926–929.
44.Guilarte, V.; Castroviejo, M. P.; García-García, P.;
Fernández-Rodríguez, M. A.; Sanz, R. J. Org. Chem. 2011, 76,
3416–3437. doi:10.1021/jo200406f
doi:10.1002/anie.200602664
18.Kondo, Y.; Morey, J. V.; Morgan, J. C.; Naka, H.; Nobuto, D.;
Raithby, P. R.; Uchiyama, M.; Wheatley, A. E. H. J. Am. Chem. Soc.
2007, 129, 12734–12738. doi:10.1021/ja072118m
19.Hevia, E.; Kennedy, A. R.; Klett, J.; McCall, M. D. Chem. Commun.
2009, 3240–3242. doi:10.1039/b903592c
45.Tlili, A.; Xia, N.; Monnier, F.; Taillefer, M. Angew. Chem., Int. Ed. 2009,
48, 8725–8728. doi:10.1002/anie.200903639
46.Zhao, D.; Wu, N.; Zhang, S.; Xi, P.; Su, X.; Lan, J.; You, J.
Angew. Chem., Int. Ed. 2009, 48, 8729–8732.
doi:10.1002/anie.200903923
20.De Luca, L.; Nieddu, G.; Porcheddu, A.; Giacomelli, G.
Curr. Med. Chem. 2009, 16, 1–20. doi:10.2174/092986709787002826
21.Hwu, J. R.; Chuang, K.-S.; Chuang, S. H.; Tsay, S.-C. Org. Lett. 2005,
7, 1545–1548. doi:10.1021/ol050196d
47.Jing, L.; Wei, J.; Zhou, L.; Huang, Z.; Li, Z.; Zhou, X. Chem. Commun.
2010, 46, 4767–4769. doi:10.1039/c0cc00434k
48.Yang, D.; Fu, H. Chem.–Eur. J. 2010, 16, 2366–2370.
doi:10.1002/chem.200903468
22.Hou, X.-L.; Yang, Z.; Yeung, K.-S.; Wong, H. N. C.
Prog. Heterocycl. Chem. 2009, 21, 179–223.
49.Anderson, K. W.; Ikawa, T.; Tundel, R. E.; Buchwald, S. L.
J. Am. Chem. Soc. 2006, 128, 10694–10695. doi:10.1021/ja0639719
50.Schulz, T.; Torborg, C.; Schäffner, B.; Huang, J.; Zapf, A.; Kadyrov, R.;
Börner, A.; Beller, M. Angew. Chem., Int. Ed. 2009, 48, 918–921.
doi:10.1002/anie.200804898
doi:10.1016/S0959-6380(09)70034-0
23.Yeung, K.-S.; Yang, Z.; Peng, X.-S.; Hou, X.-L.
Prog. Heterocycl. Chem. 2011, 22, 181–216.
doi:10.1016/S0959-6380(11)22007-5
51.Sergeev, A. G.; Schulz, T.; Torborg, C.; Spannenberg, A.;
Neumann, H.; Beller, M. Angew. Chem., Int. Ed. 2009, 48, 7595–7599.
doi:10.1002/anie.200902148
24.Ober, A. G.; Fronczek, F. R.; Fischer, N. H. J. Nat. Prod. 1985, 48,
242–248.
25.Parodi, F. J.; Fischer, N. H.; Flores, H. E. J. Nat. Prod. 1988, 51,
594–595.
26.Wang, L.-Q.; Zhao, Y.-X.; Hu, J. M.; Jia, A.-Q.; Zhou, J.
Helv. Chim. Acta 2008, 91, 159–164.
License and Terms
27.Stevenson, P. C.; Veitch, N. C. Phytochemistry 1998, 48, 947–951.
28.Joshi, A. S.; Li, X.-C.; Nimrod, A. C.; ElSholy, H. N.; Walker, L. A.;
Clark, A. M. Planta Med. 2001, 67, 186–188.
This is an Open Access article under the terms of the
Creative Commons Attribution License
(http://creativecommons.org/licenses/by/2.0), which
permits unrestricted use, distribution, and reproduction in
any medium, provided the original work is properly cited.
29.Kim, I.; Lee, S.-H.; Lee, S. Tetrahedron Lett. 2008, 49, 6579–6584.
doi:10.1016/j.tetlet.2008.09.034
30.Youn, S. W.; Eom, J. I. Org. Lett. 2005, 7, 3355–3358.
doi:10.1021/ol051264z
31.Heravi, M. M.; Sadjadi, S. Tetrahedron 2009, 65, 7761–7775.
doi:10.1016/j.tet.2009.06.028
The license is subject to the Beilstein Journal of Organic
Chemistry terms and conditions:
32.Tsuchikama, K.; Hashimoto, Y.-k.; Endo, K.; Shibata, T.
Adv. Synth. Catal. 2009, 351, 2850–2854.
(http://www.beilstein-journals.org/bjoc)
33.Alessi, M.; Larkin, A. L.; Ogilvie, K. A.; Green, L. A.; Lai, S.; López, S.;
Snieckus, V. J. Org. Chem. 2007, 72, 1588–1594.
doi:10.1021/jo0620359
The definitive version of this article is the electronic one
which can be found at:
34.Blanchet, J.; Macklin, T.; Ang, P.; Metallinos, C.; Snieckus, V.
J. Org. Chem. 2007, 72, 3199–3206. doi:10.1021/jo062385v
doi:10.3762/bjoc.7.146
1260