Synthesis of diverse indole libraries on polystyrene resin - Scope and limitations of an organometallic reaction on solid supports

Article abstract of DOI:10.3762/bjoc.8.132

The synthesis of diverse substituted indole structures on solid supports is described. The immobilization of nitrobenzoic acid onto Merrifield resin and the subsequent treatment with alkenyl Grignard reagents delivered indole carboxylates bound to solid s

Full text of DOI:10.3762/bjoc.8.132

Synthesis of diverse indole libraries on
polystyrene resin – Scope and limitations of an
organometallic reaction on solid supports
Kerstin Knepper1, Sylvia Vanderheiden2 and Stefan Bräse*1,2
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2012, 8, 1191–1199.
1Institute of Organic Chemistry, Karlsruhe Institute of Technology
(KIT), Fritz-Haber-Weg 6, D-76131 Karlsruhe, Germany and 2ITG -
Complat, Karlsruhe Institute of Technology (KIT),
Hermann-von-Helmholtz-Platz 1, D-76344
doi:10.3762/bjoc.8.132
Received: 30 March 2012
Accepted: 06 June 2012
Published: 26 July 2012
Eggenstein-Leopoldshafen, Germany
Email:
This article is part of the Thematic Series "Recent developments in
chemical diversity".
Stefan Bräse* - braese@kit.edu
* Corresponding author
Guest Editor: J. A. Porco Jr.
Keywords:
© 2012 Knepper et al; licensee Beilstein-Institut.
License and terms: see end of document.
chemical diversity; cross-coupling reactions; indole; Merrifield resin
Abstract
The synthesis of diverse substituted indole structures on solid supports is described. The immobilization of nitrobenzoic acid onto
Merrifield resin and the subsequent treatment with alkenyl Grignard reagents delivered indole carboxylates bound to solid supports.
In contrast to results in the liquid phase, ortho,ortho-unsubstituted nitroarenes also delivered indole moieties in good yields. Subse-
quent palladium-catalyzed reactions (Suzuki, Heck, Sonogashira, Stille) delivered, after cleavage, the desired molecules in
moderate to good yields over four steps. The scope and limitations are presented.
Introduction
Indoles are heterocyclic structures of unquestionable impor- nitroarenes. Some limitations of this useful reaction have been
tance. It is well recognized that the indole moiety is a privi- reported, e.g., only ortho-substituted nitroarenes gave good
leged structural motif found in numerous natural products, such yields of the indoles. Despite the interest in new indoles [3-5]
as alkaloids or peptides, and various synthetic compounds [1,2]. and in particular for libraries of combinatorial compounds [2,6-
Moreover, a large number of indole-containing compounds 17], there has only been one report for the application of the
show potential as therapeutic agents.
Bartoli reaction [18-23] on solid supports so far [24].
Organometallic reactions on solid supports are always particu-
A large number of synthetic approaches have been published larly challenging [25].
over the past hundred years. Recently, versatile and modular
organometallic reactions have been used to create the indole In the past, indoles were synthesized by using, for example,
core. Among these, the Bartoli reaction is a straightforward Fischer indole synthesis [26-30], Bartoli indole synthesis,
route for the generation of the indole moiety starting from Nenitzescu synthesis [31], Wittig indole synthesis [32],
1191

Beilstein J. Org. Chem. 2012, 8, 1191–1199.
Scheme 1: Diverse synthesis of indoles using Bartoli reactions. aSee [24].
Madelung indole synthesis [33], palladium-catalyzed indole graphic steps and the direct use of ortho,ortho-unsubstituted
synthesis [34-40], cycloaddition strategies [41], C-arylation of nitroarenes.
substituted acetonitriles or 1,3-dicarbonyl compounds [42],
Results and Discussion
halocyclization [43,44] and finally, reduction of ortho-fluoro-
nitroarenes [42]. The significant biological properties and the Synthetic plan
distinctive structural features of indole moieties prompted us to Based on our initial findings, we attached a variety of nitroben-
investigate the scope and limitations of this reaction. In this zoic acids onto solid supports. The resin of choice was Merri-
paper, we describe our various strategies towards synthesis of field resin with an initial loading of 0.97 mmol/g. Hence, a
the indole core. A solid-phase Bartoli reaction on solid supports number of substituted polymer-bound nitroarenes 1{a–k} are
is advantageous due to the decreased number of chromato- available (Scheme 1, Figure 1).
Figure 1: Nitroarenes on solid supports. In red: Nitroarenes failed to give indoles. aResin has been reported before, [24].
1192

Beilstein J. Org. Chem. 2012, 8, 1191–1199.
Reaction optimization
indoles was observed. Very common byproducts are anilines
In order to optimize the reaction conditions, the temperature for [45-47]. It should be noted at this point that the anilines are
the Bartoli reaction on solid supports was systematically more prone to cleavage during the Bartoli reaction, and this led
changed. For the reaction of (4-chloro-3-nitrophenyl)- to a high purity (albeit with low yields) of the indole products.
carboxymethyl-polystyrene (1{h}) with 1-methyl-1-propenyl-
magnesiumbromide (2{b}), an optimum between −20 and 0 °C
was determined (Figure 2). In over 90% of the cases, the puri-
ties of the crude material after cleavage were above 80%, and in
a few cases even over 90% (according to GC-MS) [24]. Pure
material was obtained after thin-layer chromatography.
Figure 4: Optimization studies of ester 1{h} with a Grignard reagent
2{d} to give indole 3{h,d} and methyl 3-amino-4-chlorobenzoate
Figure 2: Temperature optimization with Grignard reagent 2{b}.
(aniline): Isolated yields.
A slightly higher temperature was optimal for the reaction of After optimization, we immobilized a number of nitrobenzoic
(3-methyl-4-nitrophenyl)carboxymethyl-polystyrene (1{i}) with acids onto solid supports. The resulting indoles after reaction
the less active vinyl magnesium bromide 2{a} (Figure 3).
with different Grignard reagents and subsequent cleavage are
summarized in Table 1.
In general, most of the nitroarenes were successfully converted
to the indoles. As yields and purities were found to be higher
when −40 °C and 3 equiv of Grignard reagents were used, we
adopted these conditions for our library synthesis. Functional
groups such as halides were tolerated. Two exceptions were the
hydroxycarboxylic acids 1{f},1{j} and the methoxycarboxylic
acids 1{g},1{k}, which all failed in that very complex mixtures
were obtained. Recently, it was reported that methoxyni-
troarene gave somewhat different products in the presence of
Grignard reagent [48].
As reported before, ortho,ortho-unsubstituted arenes such as
1{a} were suitable substrates in contrast to their liquid-phase
counterparts. However, they gave a mixture of products
(Supporting Information File 1).
Figure 3: Temperature optimization with Grignard reagent 2{a}.
Isolated yield.
In addition to this, we also varied the amount of Grignard The next stage is the functionalization by cross-coupling reac-
reagent for the reaction of 1{h} (Figure 4). Interestingly, even tions. In our first communication, we employed Suzuki and
with substoichiometric amounts considerable formation of Heck reactions [24].
1193

Beilstein J. Org. Chem. 2012, 8, 1191–1199.
Table 1: Cleavage of indoles from solid supports.
Yield over 3
steps (%)
Entry
Resin
Grignard reagent
Indole
1
1{a}
CH2=CHMgBr (2{a})
21
4{a,a}
21
14
4{a,a’}
CH2=CHMgBr
2
3
4
5
6
7
8
9
1{b}
1{b}
1{b}
1{b}
1{c}
1{c}
1{c}
1{c}
(2{a})
4{b,a}
CH3CH=CHMgBr
16
18
20
6
(2{b})
{4b,b}
4{b,c}
CH2=C(CH3)MgBr
(2{c})
CH3CH=CCH3MgBr
(2{d})
4{b,d}
4{c,a}
CH2=CHMgBr
(2{a})
CH3CH=CHMgBr
15
(2{b})
{4c,b}
4{c,c}
CH2=C(CH3)MgBr
6
(2{c})
CH3CH=CCH3MgBr
10
(2{d})
4{c,d}
1194

Beilstein J. Org. Chem. 2012, 8, 1191–1199.
Table 1: Cleavage of indoles from solid supports. (continued)
CH2=CHMgBr
10
11
1{d}
1{d}
13
(2{a})
4{d,a}
CH3CH=CHMgBr
12
(2{b})
4{d,b}
4{d,c}
CH2=C(CH3)MgBr
12
13
1{d}
1{d}
7
(2{c})
CH3CH=CCH3MgBr
13
(2{d})
4{d,d}
4{e,a}
CH2=CHMgBr
14
15
1{e}
1{e}
14
14
(2{a})
CH3CH=CHMgBr
(2{b})
4{e,b}
CH3CH=CCH3MgBr
16
17
1{e}
1{h}
15
(2{d})
4{e,d}
4{h,d}
CH3CH=CCH3MgBr
25a
(2{d})
CH2=CHMgBr
18
1{i}
54a
(2{a})
4{i,a}
aSee [25].
1195

Beilstein J. Org. Chem. 2012, 8, 1191–1199.
Scheme 2: Stille reaction on solid supports.
In addition to these reactions, Stille reactions have been used to It should be noted at this point that the basic cleavable linker
expand the utility of the Bartoli solid-phase reaction. The reac- prevents the addition of water usually observed under cleavage
tion with tributyl(vinyl)tin proceeded smoothly and gave the with acids. The details are given in Scheme 4. It should also be
vinylindoles in moderate yields but good purity, in four steps noted that alkynyl-substituted indole carboxylic acids are only
(Scheme 2).
reported scarcely [49].
We also performed a couple of Suzuki reactions on solid Conclusion
supports. The details are given in Scheme 3. Again, these reac- In conclusion, we presented an extension of our Bartoli indole
tions proceeded smoothly.
syntheses with application of a diverse set of vinyl Grignard
reagents and cross-coupling reactions. Although the overall
We also performed Sonogashira–Hagihara reactions on solid yield is only moderate, due to the facile solid-phase approach, a
supports. This reaction gave rise to alkynyl-substituted indoles. small library of highly substituted indoles is readily available.
Scheme 3: Suzuki reaction on solid supports.
1196

Beilstein J. Org. Chem. 2012, 8, 1191–1199.
Scheme 4: Sonogashira–Hagihara reaction on solid supports.
Experimental
light at 254 nm and/or dipped into a solution of molybdato
Instrumentation and reagents: 1H NMR spectra were phosphate (5% phosphor molybdic acid in ethanol, dipping
recorded on Bruker DP 300 (300 MHz), Bruker DP 400 solution) or potassium permanganate (0.45 g of potassium
(400 MHz). Chemical shifts are expressed in parts per million permanganate and 2.35 g of sodium carbonate in 90 mL of
(δ/ppm) downfield from tetramethylsilane (TMS) and are refer- water) and heated with a heat gun. Solvent mixtures are under-
enced to chloroform (7.26 ppm), dimethylsulfoxide (2.50 ppm) stood as volume/volume. Solid materials were powdered.
or methanol (3.31 ppm) as internal standard. All coupling Solvents, reagents and chemicals were purchased from Aldrich,
constants are absolute values and J values are expressed in Fluka, ABCR, Acros, Merck and Lancaster. Solvents, reagents
Hertz (Hz). The description of signals include: s = singlet, bs = and chemicals were used as purchased unless stated otherwise.
broad singlet, d = doublet, bd = broad doublet, t = triplet, dd = Merrifield resin (1–2% cross-linked, 0.97 mmol/g, 200–400
doublet of doublets, dt doublet of triplets, m = multiplet. The mesh), was purchased from CalBiochem-NovaBiochem.
spectra were analyzed according to first order. 13C NMR
spectra were recorded on Bruker DP 300 (75 MHz) and Bruker General washing procedure for resins: After reaction the
DP 400 (100 MHz). Chemical shifts are expressed in parts per resins are subsequently washed according to the following
million (ppm, δ) downfield from tetramethylsilane (TMS) and procedure: (MeOH, THF, n-pentane, CH2Cl2) three times,
are referenced to CDCl3 (77.4 ppm), DMSO-d6 (39.52 ppm) or (MeOH, DMF, n-pentane, THF) once, (n-pentane, CH2Cl2,
methanol-d4 (49.00) as internal standard. Perkin Elmer FT-IR n-pentane) twice.
1750: IR spectra of soluble substances were recorded in
distilled dichloromethane; IR spectra of resins were recorded in GP 1 - Immobilization of benzoic acids on Merrifield resin:
KBr on Bruker IFS88 IR. EI-MS (electron impact mass spec- In a three-necked round-bottom flask equipped with a mechan-
trometry): Kratos MS 50 (70 eV) and Thermo Quest Finnigan ical stirrer, 5 equiv of cesium carbonate are suspended in DMF
MAT 95 XL (70 eV). The molecular fragments are quoted as (mL/mmol) and are stirred for 30 min at 50 °C. Next, 5 equiv of
the relation between mass and charge (m/z), the intensities as a benzoic acid are added and the mixture is stirred for another
percentage value relative to the intensity of the base signal 30 min. Afterwards, one equiv of Merrifield resin is added and
(100%). The abbreviation [M]+ refers to the molecular ion. the suspension is stirred for 24 h at 50 °C. After being cooled
Elemental analysis: Elementar Vario EL. Routine monitoring of down to room temperature the resin is filtered off and first
reactions was performed by using silica-gel-coated glass plates washed with water, then treated according to the general
(Merck, silica gel 60, F254), which were analyzed under UV washing procedure, and dried in high vacuum. The loading of
1197

Beilstein J. Org. Chem. 2012, 8, 1191–1199.
the resin was calculated according to the nitrogen values of the in high vacuum. The loading of the resulting resin was calcu-
elemental analysis.
lated as if complete conversion had taken place.
GP 2 - Bartoli-indole synthesis: Under an argon atmosphere, GP 6 - General cleavage protocol: To one equiv of the resin in
one equiv of the resin is suspended in dry THF (0.1 mmol/mL), dry THF (0.1 mmol/mL), a solution of 30% sodium methoxide
cooled down to −40 °C and three equiv of the Grignard reagent in MeOH (2.00 mL/mmol of resin) is added and the mixture is
are added, while the color of the mixture changes to orange-red. agitated for three hours at room temperature. The resin is
The reaction mixture is allowed to warm to 0 °C, and then a filtered off, and the filtrate is hydrolyzed with water and subse-
saturated aqueous solution of ammonium chloride is added. The quently extracted with ethyl acetate three times. After being
resin is filtered off, and first washed with water, then according dried over magnesium sulfate, the solvent is removed under
to the general washing procedure, and dried in high vacuum. reduced pressure and the residue is purified by column chroma-
The loading of the resulting resin was calculated as if complete tography by using cyclohexane/ethyl acetate, 3:1 as eluent.
conversion had taken place.
Supporting Information
GP 3 - Suzuki reaction: Under an argon atmosphere, one equiv
of the respective 7-bromo-1H-indole-6-carboxymethyl-poly-
Supporting Information File 1
styrene is suspended in DMF (0.1 mmol/mL) together with
0.10 equiv of tetrakis(triphenylphosphine)palladium and two
equiv of boronic acid. An aqueous solution of sodium carbonate
(2.5 equiv, 1.5 M) is added and the mixture agitated for two
days at 80 °C. After cooling down to room temperature, 10 mL
of a 25% aqueous solution of ammonium acetate are added, the
resin is filtered off, washed according to the general washing
procedure, and dried in high vacuum. The loading of the
resulting resin was calculated as if complete conversion had
taken place.
Experimental details.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-8-132-S1.pdf]
Acknowledgements
We thank the Deutsche Forschungsgemeinschaft (DFG BR
1750) for financial support. We gratefully acknowledge the
valuable experimental assistance of Dr. Frank-Peter Köhler (see
also Dissertation, Universität Bonn).
References
GP 4 - Sonogashira–Hagihara reaction: Under an argon
atmosphere, one equiv of the respective 7-bromo-1H-indole-6-
carboxymethyl-polystyrene is suspended in DMF (0.1 mmol/
mL) together with 10.0 mol % bis(triphenylphosphine)palla-
dium(II) chloride, 15.0 mol % copper(I) iodide and one equiv of
triphenylphosphine. Then, two equiv of triethylamine and
2.5 equiv of 4-ethynylanisole are added and the mixture is
agitated for two days at 80 °C. After cooling down to room
temperature, 10 mL of a 25% aqueous solution of ammonium
acetate are added, the resin is filtered off, washed according to
the general washing procedure, and dried in high vacuum. The
loading of the resulting resin was calculated as if complete
conversion had taken place.
1. Palmisano, G.; Penoni, A.; Sisti, M.; Tibiletti, F.; Tollari, S.;
Nicholas, K. M. Curr. Org. Chem. 2010, 14, 2409–2441.
doi:10.2174/138527210793358277
2. Patil, S. A.; Patil, R.; Miller, D. D. Curr. Med. Chem. 2009, 16,
2531–2565. doi:10.2174/092986709788682010
3. Fiandanese, V.; Bottalico, D.; Marchese, G.; Punzi, A. Tetrahedron
2008, 64, 53–60. doi:10.1016/j.tet.2007.10.100
4. Fiandanese, V.; Bottalico, D.; Marchese, G.; Punzi, A. Tetrahedron
2008, 64, 7301–7360. doi:10.1016/j.tet.2008.05.059
5. Stoll, A. H.; Knochel, P. Org. Lett. 2008, 10, 113–116.
doi:10.1021/ol7025872
6. Gil, C.; Bräse, S. J. Comb. Chem. 2009, 11, 175–197.
doi:10.1021/cc800102t
7. Ziegert, R. E.; Knepper, K.; Bräse, S. In Targets in Heterocyclic
Systems – Chemistry and Properties; Attanasi, O. A.; Spinelli, D., Eds.;
2005; Vol. 9, pp 230–253.
GP 5 - Stille reaction: Under an argon atmosphere, one equiv
of the respective 7-bromo-1H-indole-6-carboxymethyl-poly-
styrene is suspended in DMF (0.1 mmol/mL) together with
10.0 mol % bis(triphenylphosphine)palladium(II) chloride,
15.0 equiv of lithium chloride and one equiv of triphenylphos-
phine. Then, three equiv of tributyl(vinyl)tin are added and the
mixture is agitated for two days at 80 °C. After being cooled
down to room temperature, 10 mL of a 25% aqueous solution of
ammonium acetate are added, and the resin is filtered off,
washed according to the general washing procedure, and dried
8. Knepper, K.; Ziegert, R. E.; Bräse, S. PharmaChem 2003, 2, 4–7.
9. Bräse, S.; Gil, C.; Knepper, K. Bioorg. Med. Chem. 2002, 10,
2415–2437. doi:10.1016/S0968-0896(02)00025-1
10.Main, C. A.; Petersson, H. M.; Rahman, S. S.; Hartley, R. C.
Tetrahedron 2007, 64, 901–914. doi:10.1016/j.tet.2007.08.119
11.Mentel, M.; Schmidt, A. M.; Gorray, M.; Eilbracht, P.; Breinbauer, R.
Angew. Chem., Int. Ed. 2009, 48, 5841–5844.
doi:10.1002/anie.200901643
12.Worlikar, S. A.; Neuenswander, B.; Lushington, G. H.; Larock, R. C.
J. Comb. Chem. 2009, 11, 875–879. doi:10.1021/cc900057n
1198

Beilstein J. Org. Chem. 2012, 8, 1191–1199.
13.Nören-Müller, A.; Wilk, W.; Saxena, K.; Schwalbe, H.; Kaiser, M.;
Waldmann, H. Angew. Chem., Int. Ed. 2008, 47, 5973–5977.
doi:10.1002/anie.200801566
39.Liao, Y.; Reitman, M.; Zhang, Y.; Fathi, R.; Yang, Z. Org. Lett. 2002, 4,
2607–2609. doi:10.1021/ol020111y
40.Liao, Y.; Fathi, R.; Yang, Z. Org. Lett. 2003, 5, 909–912.
doi:10.1021/ol030009g
14.Kasahara, T.; Kondo, Y. Heterocycles 2006, 67, 95–100.
doi:10.3987/COM-05-S(T)24
41.Nicolaou, K. C.; Roecker, A. J.; Pfefferkorn, J. A.; Cao, G.-Q.
J. Am. Chem. Soc. 2000, 122, 2966–2967. doi:10.1021/ja994373f
42.Stephensen, H.; Zaragoza, F. Tetrahedron Lett. 1999, 40, 5799–5802.
doi:10.1016/S0040-4039(99)01117-X
15.Ruhland, T.; Pedersen, H.; Andersen, K. Synthesis 2003, 2236–2240.
doi:10.1055/s-2003-42086
16.Wu, Z.; Ede, N. J. Org. Lett. 2003, 5, 2935–2938.
doi:10.1021/ol035153g
43.Barluenga, J.; Trincado, M.; Rubio, E.; González, J. M.
Angew. Chem., Int. Ed. 2003, 42, 2406–2409.
17.Wang, W.; Herdtweck, E.; Dömling, A. Chem. Commun. 2010, 46,
770–772. doi:10.1039/b917660h
doi:10.1002/anie.200351303
18.Bartoli, G.; Palmieri, G.; Bosco, M.; Dalpozzo, R. Tetrahedron Lett.
1989, 30, 2129–2132. doi:10.1016/S0040-4039(01)93730-X
(Original discovery).
44.Yao, T.; Yue, D.; Larock, R. C. J. Comb. Chem. 2005, 7, 809–812.
doi:10.1021/cc050062r
45.Vlachou, M.; Tsotinis, A.; Kelland, L. R.; Thurston, D. E. Heterocycles
2002, 57, 129–133. doi:10.3987/COM-01-9373
19.Brown, N.; Luo, D.; Decapo, J. A.; Buszek, K. R. Tetrahedron Lett.
2009, 50, 7113–7115. doi:10.1016/j.tetlet.2009.09.083
20.Silva, L. F., Jr.; Craveiro, M. V. Org. Lett. 2008, 10, 5417–5420.
doi:10.1021/ol8023105
46.Vlachou, M.; Tsotinis, A.; Kelland, L. R.; Thurston, D. E.
Eur. J. Pharm. Sci. 2002, 17, 139–143.
doi:10.1016/S0928-0987(02)00163-X
21.Mudadu, M. S.; Singh, A.; Thummel, R. P. J. Org. Chem. 2006, 71,
7611–7617. doi:10.1021/jo061011z
47.Miller, T. A.; Vachaspati, P. R.; Labroli, M. A.; Thompson, C. D.;
Bulman, A. L.; Macdonald, T. L. Bioorg. Med. Chem. Lett. 1998, 8,
1065–1070. doi:10.1016/S0960-894X(98)00176-0
48.Egris, R.; Villacampa, M.; Menéndez, J. C. Chem.–Eur. J. 2009, 15,
10930–10939. doi:10.1002/chem.200901322
22.Pirrung, M. C.; Wedel, M.; Zhao, Y. Synlett 2002, 143–145.
doi:10.1055/s-2002-19346
23.Dalpozzo, R.; Bartoli, G. Curr. Org. Chem. 2005, 9, 163–178.
doi:10.2174/1385272053369204
49.Chang, K.-J.; Kang, B.-N.; Lee, M.-H.; Jeong, K.-S. J. Am. Chem. Soc.
2005, 127, 12214–12215. doi:10.1021/ja0547984
(For an example and references cited therein).
(Review).
24.Knepper, K.; Bräse, S. Org. Lett. 2003, 5, 2829–2832.
doi:10.1021/ol034851y
25.Ljungdahl, N.; Bromfield, K.; Kann, N. Top. Curr. Chem. 2007, 278,
89–134. doi:10.1007/128_2007_125
License and Terms
26.Hutchins, S. M.; Chapman, K. T. Tetrahedron Lett. 1996, 37,
4869–4872. doi:10.1016/0040-4039(96)00997-5
27.Cheng, Y.; Chapman, K. T. Tetrahedron Lett. 1997, 38, 1497–1500.
doi:10.1016/S0040-4039(97)00139-1
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.
28.Tanaka, H.; Ohno, H.; Kawamura, K.; Ohtake, A.; Nagase, H.;
Takahashi, T. Org. Lett. 2003, 5, 1159–1162. doi:10.1021/ol020230d
29.Rosenbaum, C.; Baumhof, P.; Mazitschek, A.; Müller, O.; Giannis, A.;
Waldmann, H. Angew. Chem., Int. Ed. 2004, 43, 224–228.
doi:10.1002/anie.200352582
The license is subject to the Beilstein Journal of Organic
Chemistry terms and conditions:
30.Rosenbaum, C.; Katzka, C.; Marzinzik, A.; Waldmann, H.
Chem. Commun. 2003, 1822–1823. doi:10.1039/b305497g
31.Ketcha, D. M.; Wilson, L. J.; Portlock, D. E. Tetrahedron Lett. 2000, 41,
6253–6257. doi:10.1016/S0040-4039(00)00697-3
32.Hughes, I. Tetrahedron Lett. 1996, 37, 7595–7598.
doi:10.1016/0040-4039(96)01669-3
(http://www.beilstein-journals.org/bjoc)
The definitive version of this article is the electronic one
which can be found at:
33.Wacker, D. A.; Kasireddy, P. Tetrahedron Lett. 2002, 43, 5189–5191.
doi:10.1016/S0040-4039(02)01025-0
doi:10.3762/bjoc.8.132
34.Collini, M. D.; Ellingboe, J. W. Tetrahedron Lett. 1997, 38, 7963–7966.
doi:10.1016/S0040-4039(97)10140-X
35.Fagnola, M. C.; Candiani, I.; Visentin, G.; Cabri, W.; Zarini, F.;
Mongelli, N.; Bedeschi, A. Tetrahedron Lett. 1997, 38, 2307–2310.
doi:10.1016/S0040-4039(97)00300-6
36.Fancelli, D.; Fagnola, M. C.; Severino, D.; Bedeschi, A.
Tetrahedron Lett. 1997, 38, 2311–2314.
doi:10.1016/S0040-4039(97)00301-8
37.Smith, A. L.; Stevenson, G. I.; Swain, C. J.; Castro, J. L.
Tetrahedron Lett. 1998, 39, 8317–8320.
doi:10.1016/S0040-4039(98)01824-3
38.Wang, Y.; Huang, T.-N. Tetrahedron Lett. 1998, 39, 9605–9608.
doi:10.1016/S0040-4039(98)02337-5
1199