Catalytic one-pot synthesis of 1-substituted 5,6,7,8-tetrahydroindolizine derivatives

Article abstract of DOI:10.1002/ejoc.201201147

The reaction of α,β-unsaturated aldehydes, carbon monoxide, and but-3-en-1-amine in the presence of Ru₃(CO)₁₂ as a precatalyst yields 1-substituted 5,6,7,8-tetrahydroindolizine derivatives for a wide variety of groups R¹ at the C-3 position of the α,β-unsaturated aldehyde.

Full text of DOI:10.1002/ejoc.201201147

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201201147
Catalytic One-Pot Synthesis of 1-Substituted 5,6,7,8-Tetrahydroindolizine
Derivatives
Tobias Biletzki[a] and Wolfgang Imhof*[a]
Keywords: Indolizines / C–H activation / Ruthenium / Homogeneous catalysis / Heterocycles
The reaction of α,β-unsaturated aldehydes, carbon mono-
xide, and but-3-en-1-amine in the presence of Ru3(CO)12 as
a precatalyst yields 1-substituted 5,6,7,8-tetrahydroindolizine
derivatives for a wide variety of groups R1 at the C-3 position
of the α,β-unsaturated aldehyde.
Introduction
The direct catalytic activation of C–H bonds with the
purpose of establishing new C–C bonds is a highly attract-
ive synthetic objective in terms of avoiding synthesis and
isolation of more reactive intermediates as, for example, ha-
logenated compounds. Progress in this field has been thor-
oughly reviewed in recent years.[1]
In earlier works some of us have established a method to
produce heterocyclic products from α,β-unsaturated imines
3. In some recent papers we have developed an approach to
produce mixtures of chiral γ-lactams 4 as the main product
in nonpolar solvents and 2,3-disubstituted pyrroles 5 as by-
products through a 4-component reaction of α,β-unsatu-
rated aldehydes 1, primary amines 2, carbon monoxide, and
ethylene (or terminal alkenes in general) in the presence of
Ru3(CO)12 as precatalyst (Scheme 1).[2]
Scheme 1. 4-Component reaction leading to heterocyclic products
of type 4 and 5, all combinations of R1 = Cp2Fe, Ph, Me, H, 4-F–
C6H4, 2-Fur, 3-Py and R2 = Bn, Ph, 4-Me–C6H4, Cy, tBu, sBu,
nBu, 4-tBu–C6H4 were realized.
The formation of both products obviously proceeds by
the activation of a C–H bond at the β-position relative to
the imine C=N bond. The product ratio is highly dependent
on the relative permittivity of the solvent used. Nonpolar
solvents such as, for example, hydrocarbons lead to the
quantitative and selective formation of racemic lactams 4,
whereas use of more polar solvents or the presence of traces
of water leads to an enhancement of the yield of pyrrole 5
into protein sequences considerably enhances their stability
against proteases, which is an essential prerequisite to use
those proteins as drugs under physiological conditions.[4]
Results and Discussion
In continuation of our previous work, we were interested
(75% in anhydrous MeOH).[2g] γ-Lactams or γ-butyric acid to see whether the reaction of α,β-unsaturated aldehydes
derivatives have been investigated with respect to their po- with molecules exhibiting both the primary amine function
tential as psychotropic drugs, as agents against Alzheimer’s and the alkene function would lead to bicyclic products.
disease, Down’s syndrome, osteoporosis, and cancer, or as There are several products that could possibly be obtained
constituents of peptide mimetics exhibiting γ-turns.[3] It has by this reaction in view of our earlier work (Scheme 2).
also been demonstrated that incorporation of γ-amino acids First of all, it is possible that the reaction also leads to
mixtures consisting of the corresponding derivatives of 4
and 5. Depending on the regioselectivity of the insertion of
the olefin double bond, two isomeric lactams I and II and
pyrroles IV and V may be produced. Nevertheless, 1-aza-
bicyclo[3.2.1]oct-6-en-8-one and 1-azabicyclo[4.2.1]non-7-
en-9-one derivatives I and II, respectively, seem unlikely, be-
cause the amide nitrogen atom would lose its preferred
planar configuration if it were a bridgehead atom as in I
[a] Institute for Integrated Natural Sciences, University of
Koblenz-Landau,
Universitätsstraße 1, 56070 Koblenz, Germany
Fax: +49-261-287-2251
E-mail: imhof@uni-koblenz.de
Homepage: http://www.uni-koblenz-landau.de/koblenz/fb3/ifin/
chemie/ag_organische-chemie
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201201147.
Eur. J. Org. Chem. 2012, 6513–6516
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T. Biletzki, W. Imhof
SHORT COMMUNICATION
and II. Moreover, also 2,3-dihydro-1H-pyrrolizines IV are
If but-3-ene-1-amine is used as the reactant, the exclusive
less reasonable than 5,6,7,8-tetrahydroindolizines V, with formation of 1-substituted 5,6,7,8-tetrahydroindolizine de-
regard to our investigations concerning the n/iso selectivity rivatives 7 is observed for a wide variety of groups R1 at
of the reaction of 1-alkenes.[2e] These assumptions were sup- the α,β-unsaturated aldehyde (Scheme 3).
ported by DFT calculations for I–IV (R1 = Ph, cf. Support-
ing Information). These calculations show that the forma-
tion of II or V is energetically preferred over that of iso-
meric I or IV, respectively. Nevertheless, the effect is signifi-
cantly less pronounced when IV and V are compared. In
addition, the results also demonstrate that amide nitrogen
atoms in I and II are indeed in a pyramidal bonding situa-
Scheme 3. 3-Component reaction leading to 5,6,7,8-tetra-
tion that leads to a destabilization of the lactams compared
hydroindolizines (R1 = Cp2Fe, Ph, Me, H, 4-Cl–C6H4, 4-Me2N–
C6H4, 2-Fur, 3-Py).
to the pyrroles. So, from DFT calculations it can be con-
cluded that the formation of indolizine derivatives V should
be most reasonable if the reaction products really are small
molecules. It should, of course, be kept in mind that poly-
meric material like III and VI may also be produced.
Eight α,β-unsaturated aldehydes were treated with but-3-
en-1-amine and carbon monoxide. In the presence of
Ru3(CO)12 as the precatalyst and toluene as the solvent, the
reaction was carried out in a 50 cm3 autoclave at 140 °C in
16 h. Workup was easily achieved by removing the solvent
under reduced pressure, and the oily residue was used to
1
get an impression of the outcome of the product 7 by H
NMR spectroscopy. The product was finally separated by
1
column chromatography and characterized by IR, H, and
13C NMR spectroscopy as well as by mass spectrometry
and high-resolution mass spectrometry. Table 1 shows the
α,β-unsaturated aldehydes used and the yields of the corre-
sponding products.
Table 1. Aldehydes used and isolated yields of 7a–h.
R1 (Aldehyde)
Yield [%]
Cp2Fe (3-ferrocenylpropenal)
Ph (trans-cinnamaldehyde)
Me (trans-crotonic aldehyde)
H (acrolein)
7a: 54
7b: 49
7c: traces
–
Scheme 2. Potential products of the catalytic reaction of α,β-unsat-
urated aldehydes with but-3-en-1-amine and CO.
4-Cl–C6H4 (trans-4-chlorocinnam- 7e: 61
Alkaloids containing isolated pyrrole rings occur as sec-
ondary metabolites in a variety of species, including plants,
invertebrates, fungi, and bacteria.[5] Especially prolific are
those natural products in which the pyrrole ring is part of
a fused bicyclic or polycyclic system; in particular, the
number of alkaloids containing indole, carbazole, or β-carb-
oline substructures is enormous.[6] The tetrahydroindoliz-
aldehyde)
4-Me2N–C6H4 (trans-4-dimethyl-
aminocinnamaldehyde)
2-Fur [3-(2-furyl)acrolein]
just the imine was observed
7g: 43
7h: 46
3-Py [trans-3-(3-pyridyl)acrolein]
From the entries in Table 1 it can be concluded that most
ine fragment of compounds V is a structural unit of many combinations for the catalytic transformation of but-3-en-
natural and unnatural compounds possessing diverse bio- 1-amine and unsaturated aldehydes to produce tetra-
logical activities.[7] Naturally occurring 5,6,7,8-tetra- hydroindolizine derivatives 7 work very well. Acrolein is the
hydroindolizines, in which the pyrrole ring remains intact, only substrate examined in this study from which neither
have until recently been found only as part of more complex the intermediate imine nor one of the expected products of
cyclic arrays, for example, in the anticancer alkaloid (–)- type I–VI was detectable by NMR spectroscopy or mass
rhazinilam and the ant alkaloid myrmicarin 217.[8] The first spectrometry. One reason might be that the intermediate
simple bicyclic 5,6,7,8-tetrahydroindolizine alkaloid was re- imine is too reactive and polymerizes without the interim
1
ported in 1997 as a metabolite of Polygonatum sibiricum.[9] formation of heterocycles. The resulting H NMR spectra
Known methods for the preparation of tetrahydroindolizine of the crude reaction mixture showed signals in the ali-
derivatives are tedious, and the overall yield of the target phatic and olefinic region of the spectrum that might be
products is poor.[10] Therefore, development of new pro- indicative of the formation of oligomeric or polymeric
cedures for the synthesis of these compounds is an impor- products. Nevertheless, it was not possible to obtain any
tant challenge in medicinal and organic chemistry. Herein pure material from the reaction with acrolein. For trans-4-
we describe an easy way to prepare tetrahydroindolizine de- dimethylaminocinnamaldehyde, the resulting 1H NMR
rivatives by a three-component reaction in reasonable spectra of the crude reaction mixture showed the typical
yields.
signal corresponding to the CH function of an imine. So
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1-Substituted 5,6,7,8-Tetrahydroindolizine Derivatives
2922 w (CH2), 2852 w (CH2), 1659 m (C=C), 1411 w (CH2), 1260
m (CH2), 1103 s (CH2), 1011 vs (CH2), 801 vs (CH2) cm–1.
we can assume that the formation of the imine took place
but no further reaction towards one of the bicyclic products
occurred. For the remaining six α,β-unsaturated aldehydes,
a successful catalytic transformation could be achieved al-
though the reaction of trans-crotonic aldehyde gave very
low yields. Compound 7c was therefore detectable from the
1-Phenyl-5,6,7,8-tetrahydroindolizine (7b): Yield: 97 mg (49.17%);
eluent: light petroleum/CH2Cl2 (70:30). 1H NMR (200 MHz,
CDCl3): δ = 1.78–2.04 (m, 4 H, CH2), 2.95 (t, J = 6.2 Hz, 2 H,
CH2), 3.99 (t, J = 6.2 Hz, 2 H, CH2), 6.37 (d, J = 2.8 Hz, 1 H,
crude reaction mixture by NMR spectroscopy and mass =CH), 6.59 (d, J = 2.8 Hz, 1 H, =CH), 7.12–7.45 (m, 5 H, CHPh
)
ppm. 13C NMR (50 MHz, CDCl3): δ = 21.40 (CH2), 23.39 (CH2),
23.70 (CH2), 45.70 (CH2), 107.39 (=CH), 119.14 (=CH), 119.55
(C), 124.66 (CHPh), 126.00 (C), 127.08 (CHPh), 128.26 (CHPh),
137.06 (CPh) ppm. MS (DEI): m/z (%) = 197 (100) [M]+, 169 (18)
[M – C2H4]+, 141 (3) [M – C4H8]+, 77 (3) [C6H5]+. HRMS: calcd.
for C14H15N 197.12045; found 197.11954; Δ = 0.91 mmu. IR
spectrometry, but the yield was too low to obtain any pure
material after chromatographic workup. The isolated yields
of compounds 7a, 7b, 7e, 7g, and 7h are at about 50%.
Most probably, the rest of the reaction mixture may mainly
consist of the suggested polymer VI, as NMR spectra of
the crude reaction mixtures also showed additional signals
in the region that is indicative of methylene units.
(298 K): ν = 2959 w (CH ), 2926 w (CH ), 2856 w (CH ), 1672 m
˜
2
2
2
(C=C), 1600 m (CHPh), 1503 m (CHPh), 1447 m (CH2), 1259 m
(CH2), 1074 m (CH2), 1015 m (CH2), 799 m (CH2), 762 s (CHPh),
695 vs (CHPh) cm–1.
Conclusions
1-para-Chlorophenyl-5,6,7,8-tetrahydroindolizine (7e): Yield: 142 mg
(61.28%); eluent: light petroleum/CH2Cl2 (70:30). 1H NMR
(400 MHz, CDCl3): δ = 1.80–1.86 (m, 2 H, CH2), 1.93–1.99 (m, 2
H, CH2), 2.89 (t, J = 6.4 Hz, 2 H, CH2), 3.97 (t, J = 6.4 Hz, 2 H,
CH2), 6.31 (d, J = 2.8 Hz, 1 H, =CH), 6.57 (d, J = 2.8 Hz, 1 H,
=CH), 7.26–7.33 (m, 4 H, CHPh) ppm. 13C NMR (50 MHz,
CDCl3): δ = 21.31 (CH2), 23.31 (CH2), 23.69 (CH2), 45.73 (CH2),
107.24 (=CH), 118.42 (C), 119.39 (=CH), 128.01 (C), 128.16
(CHPh), 128.38 (CHPh), 130.24 (CPh), 135.53 (CCl) ppm. MS (DEI):
m/z (%) = 231 (100) [M]+, 203 (12) [M – C2H4]+, 120 (10) [M –
C6H4Cl]+, 111 (7) [C6H4Cl]+. HRMS: calcd. for C14H14NCl
The reaction of α,β-unsaturated aldehydes, carbon
monoxide, and but-3-en-1-amine in the presence of
Ru3(CO)12 as a precatalyst yields 1-substituted 5,6,7,8-tetra-
hydroindolizine derivatives for a wide variety of groups R1
at the C-3 position of the α,β-unsaturated aldehyde. In ac-
cordance with theoretical calculations, no γ-lactam deriva-
tives were produced. The concomitant presence of the pri-
mary amine and alkene functional groups in but-3-en-1-
amine therefore leads to an enhanced chemoselectivity of
the ruthenium-catalyzed multicomponent reaction. The ob-
served tetrahydroindolizines are easily separated from ad-
ditionally formed polymeric material by column
chromatography.
231.08148; found 231.08076; Δ = 0.72 mmu. IR (298 K): ν = 2963
˜
w (CH2), 2922 w (CH2), 2852 w (CH2), 1674 w (C=C), 1489 w
(CH2), 1259 m (CH2), 1088 s (CH2), 1011 vs (CH2), 793 vs (CCl)
cm–1.
1-Furan-2-yl-5,6,7,8-tetrahydroindolizine (7g): Yield:
80 mg
(42.73%); eluent: light petroleum/CH2Cl2 (70:30). 1H NMR
(200 MHz, CDCl3): δ = 1.86–1.96 (m, 4 H, CH2), 2.93 (t, J =
6.0 Hz, 2 H, CH2), 3.94 (t, J = 6.0 Hz, 2 H, CH2), 6.11 (d, J =
3.2 Hz, 1 H, =CH), 6.40 (d, J = 3.2 Hz, 1 H, =CH), 6.40 (dd, J =
1.6, J = 3.2 Hz, 1 H, =CH), 6.51 (d, J = 3.2 Hz, 1 H, =CH), 7.34
(d, J = 1.6 Hz, 1 H, =CH) ppm. 13C NMR (100 MHz, CDCl3): δ
= 20.96 (CH2), 23.25 (CH2), 23.36 (CH2), 45.54 (CH2), 101.71
(=CH), 105.46 (=CH), 110.83 (C), 110.83 (=CH), 119.08 (=CH),
126.03 (C), 139.36 (=CH), 152.23 (C) ppm. MS (DEI): m/z (%) =
187 (100) [M]+, 159 (12) [M – C2H4]+, 130 (23) [M – C4H9]+.
HRMS: calcd. for C12H13NO 187.09971; found 187.09962; Δ =
Experimental Section
Experimental Procedure: In a typical reaction, a 50 cm3 autoclave
was charged with an α,β-unsaturated aldehyde (1 mmol), but-3-en-
1-amine (71 mg, 1 mmol), Ru3(CO)12 (19 mg, 0.03 mmol), and an-
hydrous, freshly distilled toluene (4 mL). The autoclave was then
pressurized with carbon monoxide (20 bar) and heated up to
140 °C for a reaction time of 16 h. After the reaction mixture had
cooled to room temperature, it was transferred to a Schlenk tube,
and the solvent was removed in vacuo. Preparative separation of 7
was achieved by column chromatography (10ϫ2 cm, silica gel).
Using a mixture of light petroleum (b.p. 40–60 °C) and CH2Cl2,
depending on the reaction mixture, led to the elution of 7. The
exact ratios of solvents for the elution of all derivatives of 7 are
specified below together with the physical data of each compound.
0.09 mmu. IR (298 K): ν = 2922 m (CH ), 2856 w (CH ), 1644 m
˜
2
2
(C=C), 1504 m (CH2), 1322 s (CH2), 1259 s (CH2), 1092 s (CH2),
1015 vs (CH2), 796 vs (CH2), 722 vs (C=C) cm–1.
1-Pyrid-3-yl-5,6,7,8-tetrahydroindolizine
(7h):
Yield:
92 mg
(46.40%); eluent: light petroleum/CH2Cl2 (50:50). 1H NMR
1-Ferrocenyl-5,6,7,8-tetrahydroindolizine (7a): Yield: 164 mg
(200 MHz, CDCl3): δ = 1.85–2.01 (m, 4 H, CH2), 2.90 (t, J =
(53.74%); eluent: light petroleum/CH2Cl2 (70:30). 1H NMR 6.0 Hz, 2 H, CH2), 3.96 (t, J = 6.0 Hz, 2 H, CH2), 6.34 (d, J =
(200 MHz, CDCl3): δ = 1.84–1.95 (m, 4 H, CH2), 2.85 (t, J = 3.0 Hz, 1 H, =CH), 6.58 (d, J = 3.0 Hz, 1 H, =CH), 7.18–7.24 (m,
6.0 Hz, 2 H, CH2), 3.92 (t, J = 6.0 Hz, 2 H, CH2), 4.07 (s, 5 H, 1 H, CHPy), 7.65 (dt, J = 1.8, J = 7.8 Hz, 1 H, CHPy), 8.34 (m, 1
FeCp), 4.15 (pt, J = 1.6 Hz, 2 H, FeCpR), 4.38 (pt, J = 1.6 Hz, 2
H, FeCpR), 6.25 (d, J = 2.6 Hz, 1 H, =CH), 6.47 (d, J = 2.6 Hz,
H, CHPy), 8.66 (m, 1 H, CHPy) ppm. 13C NMR (150 MHz, CDCl3):
δ = 21.17 (CH2), 23.18 (CH2), 23.55 (CH2), 45.67 (CH2), 107.10
1 H, =CH) ppm. 13C NMR (150 MHz, CDCl3): δ = 21.38 (CH2), (=CH), 115.91 (C), 119.69 (=CH), 123.19 (C), 126.68 (CHPy),
23.26 (CH2), 23.55 (CH2), 45.57 (CH2), 66.51 (FeCpR), 66.99 132.78 (CPy), 133.68 (CHPy), 145.66 (CHPy), 148.13 (CHPy) ppm.
(FeCpR), 68.96 (FeCp), 83.35 (CpR), 107.67 (=CH), 115.11 (C),
118.37 (=CH), 125.42 (C) ppm. MS (DEI): m/z (%) = 305 (100)
[M]+, 212 (99) [M – C2H4 – C5H5]+, 184 (38) [M – C5H5Fe]+, 121
MS (DEI): m/z (%) = 198 (82) [M]+, 170 (14) [M – C2H4]+, 157 (6)
[M – C3H5]+, 120 (6) [M – C5H4N]+, 41 (100) [C3H5]+, 28 (58)
[C2H4]+. HRMS: calcd. for C13H14N2 198.11570; found 198.11515;
(64) [FeC H ]+, 56 (18) [C H ]+. IR (298 K): ν = 2956 w (CH ), Δ = 0.55 mmu. IR (298 K): ν = 2926 m (CH ), 2856 w (CH ), 1693
˜
˜
5
5
4
8
2
2
2
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T. Biletzki, W. Imhof
SHORT COMMUNICATION
[3] a) D. F. Horrobin, Eur. Pat. Appl. 37175, 1981 [Chem. Abstr.
1981, 96, 24802]; b) S. R. Norrby, Drug Saf. 1996, 15, 87; c)
M. Armat, M. Canto, N. Llor, C. Escolano, E. Molins, E. Espi-
nosa, J. Bosch, J. Org. Chem. 2002, 67, 5343; d) L. A. Thomp-
son, U. S. Pat. Appl. 20020010172, 2002 [Chem. Abstr. 2002,
136, 118401]; e) K. J. Herzog, R. Jacobs, J. Kang, T. R. Simp-
son, J. M. Woods, PCT Int. Appl. WO 2004080983, 2004
[Chem. Abstr. 2004, 141, 296296]; f) T. R. Elworthy, E. R. Brill,
S.-S. Chiou, F. Chu, J. R. Harris, J. Med. Chem. 2004, 47, 6124;
g) X. Xu, X. Che, S. Gao, J. Wu, X. Bai, Synlett 2005, 1865.
[4] J. Frackenpohl, P. I. Arvidsson, J. V. Schreiber, D. Seebach,
ChemBioChem 2001, 2, 445.
s (C=C), 1592 w (CHPh), 1411 m (CH2), 1326 m (CH2), 1159 m
(CH2), 1026 m (CH2), 807 s (CHPh), 711 vs (CHPh) cm–1.
Supporting Information (see footnote on the first page of this arti-
cle): Theoretical calculations, calculated molecular structures, 1H
and 13C NMR spectra.
Acknowledgments
The authors gratefully thank the Deutsche Bundesstiftung Umwelt
(DBU) for financial support and the Ohio Supercomputing Center,
Columbus, OH, USA for the provision of computing time.
[5] a) A. Gossauer, Prog. Chem. Org. Nat. Prod. 2003, 86, 1; b) A.
Dinsmore, K. Mandy, J. P. Michael, Org. Biomol. Chem. 2006,
4, 1032.
[6] a) I. W. Southon, J. Buckingham, Dictionary of Alkaloids,
Chapman & Hall, London, 1989; b) J. P. Michael, Nat. Prod.
Rep. 2005, 22, 603 and references cited therein.
[7] a) Q. Zhang, G. Tu, Y. Zhao, T. Cheng, Tetrahedron 2002, 58,
6798; b) J. Szawkalo, A. Zawaddzka, K. Wojtasiewicz, A. Len-
iewski, J. Drabowicz, Z. Czaronocki, Tetrahedron: Asymmetry
2005, 16, 3619; c) D. Pla, A. Marchal, C. A. Olsen, F. Albericio,
M. Alvarezi, J. Org. Chem. 2005, 70, 8231; d) A. M. Magerra-
mov, I. A. Aliev, S. F. Farzaliev, N. D. Sadykhova, I. M. Akh-
medov, Russ. J. Org. Chem. 2010, 46, 1101.
[8] F. Schröder, S. Franke, W. Francke, H. Baumann, M. Kaib,
J. M. Pasteels, D. Daloze, Tetrahedron 1996, 52, 13539.
[9] L.-R. Sun, S.-X. Wang, X. Li, Chin. J. Med. Chem. 1997, 7,
129 [Chem. Abstr. 1999, 130, 2168].
[1] a) G. Dyker, Angew. Chem. 1999, 111, 1808; Angew. Chem. Int.
Ed. 1999, 38, 1698; b) Y. Guari, S. Sabo-Etienne, B. Chaudret,
Eur. J. Inorg. Chem. 1999, 1047; c) K. Kakiuchi, S. Murai, Top.
Organomet. Chem. 1999, 3, 47; d) V. Ritleng, C. Sirlin, M.
Pfeffer, Chem. Rev. 2002, 102, 1731; e) F. Kakiuchi, Top. Or-
ganomet. Chem. 2007, 11, 1; f) N. Chatani, Chem. Rec. 2008,
8, 201; g) F. Kakiuchi, T. Kochi, Synthesis 2008, 3013; h) L.
Ackermann, Chem. Commun. 2010, 46, 4866.
[2] a) D. Berger, W. Imhof, Chem. Commun. 1999, 1457; b) D.
Berger, W. Imhof, Tetrahedron 2000, 56, 2015; c) D. Berger, A.
Göbel, W. Imhof, J. Mol. Catal. A 2001, 165, 37; d) W. Imhof,
D. Berger, M. Kötteritzsch, M. Rost, B. Schönecker, Adv.
Synth. Catal. 2001, 343, 795; e) D. Dönnecke, W. Imhof, Tetra-
hedron 2003, 59, 8499; f) W. Imhof, A. Göbel, J. Organomet.
Chem. 2005, 690, 1092; g) G. Gillies, D. Dönnecke, W. Imhof,
Monatsh. Chem. 2007, 138, 683; h) T. Biletzki, W. Imhof, Syn-
thesis 2011, 24, 3979.
[10] a) F. J. Sardina, H. Rapoport, Chem. Rev. 1996, 96, 1825; b)
C. W. Jefford, Curr. Org. Chem. 2000, 4, 205.
Received: August 28, 2012
Published Online: October 11, 2012
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Eur. J. Org. Chem. 2012, 6513–6516