Direct catalytic enantioselective aza-Diels-Alder reactions

Article abstract of DOI:10.1002/anie.200500811

(Chemical Equation Presented) Proline and its derivatives catalyze the asymmetric one-pot three-component aza-Diels-Alder reaction between unmodified α,β-unsaturated cyclic ketones, aqueous formaldehyde, and aryl amines with excellent chemo- and enantiose

Full text of DOI:10.1002/anie.200500811

Angewandte
Chemie
Asymmetric Catalysis
DOI: 10.1002/anie.200500811
Direct Catalytic Enantioselective Aza-Diels–
Alder Reactions**
tion that yields the corresponding products with excellent
stereoselectivity.
Henrik SundØn, Ismail Ibrahem, Lars Eriksson, and
Armando Córdova*
In an initial experiment, 2-cyclohexen-1-one (1a,
2
mmol), aqueous formaldehyde (1 mmol, 36 vol% aqueous
solution), and p-anisidine (1.1 mmol) were mixed in the
presence of a catalytic amount of (S)-proline (30 mol%).
After vigorously stirring the mixture for 24 h, the reactions
were quenched by extraction and the crude product purified
by column chromatography on silica gel to furnish the desired
aza-Diels–Alder product 2a in 30% yield with excellent
chemoselectivity and 99% ee [Eq. (2)]. Encouraged by this
The synthesis of optically active nitrogen-containing com-
pounds is a very important task in chemistry as they are key
building blocks for the construction of valuable compounds
such as amino acids, aza sugars, and alkaloids. The aza-Diels–
Alder reaction is one of the most powerful CÀC bond-forming
reactions for the preparation of nitrogen-containing com-
[
1,2]
pounds such as piperidines and quinolidine derivatives,
and thus chemists have developed several diastereoselective
[
3,4]
aza-Diels–Alder reactions.
Despite the potential advan-
tages of utilizing asymmetric catalysis, there are only a few
examples of catalytic asymmetric indirect aza-Diels–Alder
reactions between preformed imines and dienes or enolethers.
For example, the research groups of Kobayashi and Jørgensen
have successfully used chiral Lewis acid complexes as
[
5–7]
catalysts for these transformations.
However, there is to
experiment, we investigated different reaction conditions and
the utilization of different proline derivatives as catalysts to
increase the yield of the reaction (Table 1).
our knowledge no report of a direct catalytic enantioselective
aza-Diels–Alder reaction. Organocatalysis is a rapidly grow-
ing research field and has been applied successfully to several
different enantioselective reactions. In particular, amino
acid derivatives have been utilized as catalysts for enantio-
[
8]
Table 1: Amine-catalyzed direct enantioselective aza-Diels–Alder reac-
[
a]
tion.
[
9–13]
selective cycloadditions such as the Diels–Alder reaction.
This research and our interest in applying amino acids as
[
14]
catalysts in asymmetric synthesis
led to us becoming
interested in whether an amino acid derivative would be
able to mediate the classical aza-Diels–Alder reaction
through a catalytic enamine mechanism. Retrosynthetic
analyses suggested that an imine generated in situ would be
able to react with a catalytically generated chiral diene and
[
15]
form an aza-Diels–Alder product [Eq. (1)].
Thus, we
embarked on the quest to develop a one-pot three-component
asymmetric aza-Diels–Alder reaction. Herein, we report the
first direct catalytic enantioselective aza-Diels–Alder reac-
[
b]
[c]
Entry
Cat.
Solvent
t [h]
T [8C]
Yield [%]
ee [%]
1
2
3
4
5
6
7
8
9
3
3
3
3
4
5
3
3
3
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMF
24
24
24
24
48
24
24
24
24
RT
50
50
75
RT
RT
50
50
50
30
52
99
99
99
99
94
99
98
97
n.d.
[
d]
82
45
31
61
35
10
[
*] H. SundØn, I. Ibrahem, Prof. Dr. A. Córdova
Department of Organic Chemistry
The Arrhenius Laboratory
Stockholm University
106 91 Stockholm (Sweden)
NMP
toluene
Fax: (+46)815-4908
<5
E-mail: acordova1a@netscape.net
E-mail: acordova@organ.su.se
[
a] Experimental conditions: A mixture of 1a (2 mmol, 2 equiv), p-
anisidine (1.1 mmol), aqueous formaldehyde (1 mmol), and catalyst
30 mol%) was stirred at the temperature and conditions displayed for
0–37 h. The crude product 2a obtained after aqueous workup was
Prof. Dr. L. Eriksson
(
2
Department of Structural Chemistry
The Arrhenius Laboratory
Stockholm University
purified by column chromatography. [b] Yield of the isolated pure
products after column chromatography on silica gel. [c] Determined by
chiral-phase HPLC analyses. [d] Yield of the corresponding alcohol (1:1
106 91 Stockholm (Sweden)
[**] We gratefully acknowledge the Swedish National Research council
cis/trans) obtained by in situ reduction of 2a with excess NaBH after
4
and the Wenner-Gren Foundation for financial support.
column chromatography on silica gel.
Angew. Chem. Int. Ed. 2005, 44, 4877 –4880
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4877

Communications
We found that the organocatalytic aza-Diels–Alder reac-
rated cyclohexenones and heptenones were excellent sub-
strates for the direct catalytic enantioselective aza-Diels–
Alder reactions with amino acids as the catalysts, and the
corresponding bicyclic amines 2a–2c protected with p-
methoxyphenyl (PMP) groups were isolated in high yield
with high ee values (up to > 99% ee). For example, azabicy-
clooctanone 2b was isolated in 72% yield with > 99% ee.
Thus, protected azabicycles can be assembled asymmetrically
in one chemical manipulation from simple inexpensive read-
ily available starting materials. Proline was the most efficient
organic catalyst when unsaturated ketone 1c was utilized as
the donor and furnished the corresponding bicycle 2c in high
yield with 98% ee. Furthermore, the amino acid catalyzed
aza-Diels–Alder reactions were operationally simple and
performed in wet solvents. The proline-catalyzed one-pot
three-component reaction with 3-substituted cyclohexanone
1d only furnished the a,b-unsaturated Mannich adduct 2d in
40% yield and 94% ee and not the aza-Diels–Alder product.
The reaction with 2-cyclopentenone (1e) only furnished trace
amounts of the desired aza-Diels–Alder adduct 2e. Proline-
catalyzed reactions between trans-4-phenyl-3-buten-2-one,
formaldehyde, and p-anisidine did not provide any product
under our reaction conditions. The PMP group of the aza-
Diels–Alder adduct 2b was removed with cerium ammonium
tion was most efficient in DMSO and that the yield of 2a
could be increased from 30 to 52% by performing the
reaction at 508C without affecting the stereoselectivity of the
[
16]
reaction. Furthermore, in situ reduction of 2a with excess
NaBH furnished the corresponding bicyclic alcohol product
4
in 82% yield and 99% ee. We also investigated the novel
direct enantioselective aza-Diels–Alder reaction with amine
[
17,18]
catalysts 4 and 5.
Both catalysts 4 and 5 were able to
catalyze the direct three-component reaction with excellent
regio- and enantioselectivity to furnish the corresponding aza-
Diels–Alder adduct 2a in 31 and 61% yields and 94 and
9
9% ee, respectively. Hence, of all the amino acid derivatives
tested proline (3) and tetrazole 5 were the most efficient
catalysts for the aza-Diels–Alder reaction. The amino acid
derivatives also catalyzed the reaction in N-methylpyrrolidine
(
NMP) and DMF with high enantioselectivity.
We next investigated the one-pot three-component aza-
Diels–Alder reaction for a set of different cyclic a,b-
unsaturated ketones (Table 2). We found that a,b-unsatu-
Table 2: Proline-catalyzed direct three-component enantioselective aza-
[
a]
Diels–Alder reaction.
nitrate (CAN) followed by treatment with (Boc) O
2
Table 3: Proline-catalyzed direct three-component enantioselective aza-
Diels–Alder reaction with different anilines.
Entry
1
Ketone
Product
t
T
Yield ee
[
a]
[
b]
[c]
[h] [8C] [%] [%]
[
d]
1a
2a 24 50 82
99
2
3
48 50 72
17 RT 70
>99
>99
Entry
Ketone
Ar
Product
T
Yield ee
[b]
[c]
[
8C] [%]
[%]
1
1
b
c
2b
1
2
3
1b PMP
2b RT 70
>99
>96
98
[
[
e]
f]
4
5
6
24 50 90
24 RT 75
24 RT 20
98
98
96
2c
[f]
1a
1b
Ph
Ph
2 f RT 54
2g RT 69
7
8
1d
1e
2d 24 RT 40
94
[
g]
2e 24 RT 10
n.d.
4
5
1b
1b
4-BrC H
2h RT 20
>99
>99
6
4
[a] Experimental conditions: A mixture of 1 (2 mmol, 2 equiv), aniline
(
(
1.1 mmol), aqueous formaldehyde (1 mmol), and (S)-proline
30 mol%) was stirred at the temperature and conditions displayed for
4-ClC H
2i RT 32
6
4
2
0–72 h. The crude product 2 obtained after aqueous workup was
purified by column chromatography. [b] Yield of the isolated pure
products after column chromatography on silica gel. [c] Determined by
chiral-phase HPLC analyses. [d] Yield of the corresponding alcohol (1:1
[a] Experimental conditions: A mixture of 1 (2 mmol, 2 equiv), aniline
(1.1 mmol), aqueous formaldehyde (1 mmol), and (S)-proline
(30 mol%) was stirred at the temperature and conditions displayed for
20–72 h. The crude product 2 obtained after aqueous work-up was
purified by column chromatography. [b] Yield of the isolated pure
products after column chromatography on silica gel. [c] Determined by
chiral-phase HPLC analyses.
cis/trans) obtained by in situ reduction of 2a with excess NaBH after
4
column chromatography on silica gel. [e] Combined yield of 2c and the
minor amount of the retro-Michael adduct, which was formed upon
column chromatography. [f] Reaction performed with catalyst 5. [g] Not
determined.
4
878 www.angewandte.org
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2005, 44, 4877 –4880

Angewandte
Chemie
(
Boc = tert-butoxycarbonyl) to furnish the
desired Boc-protected azabicycle.
We next investigated the effect of the
amine component on the reaction catalyzed
by an amino acid (Table 3). The reaction
advanced with excellent chemo-, regio-, and
stereoselectivity to yield the corresponding
bicycles 2b and 2 f–2i with up to > 99% ee.
In particular, the hetero-Diels–Alder reac-
tions with anilines having an electron-donat-
ing substituent at the para position furnished
the corresponding aza-Diels–Alder products
with excellent stereocontrol. The yields of the
products derived from the aza-Diels–Alder
reactions with p-chloro- and p-bromoanilines
were moderate in comparison to the reactions
with aniline and p-anisidine.
The stereochemical outcome of the reac-
tion was determined by X-ray structure
analysis of 2b (Figure 1), which revealed
that bicycle (1R,4S)-2b was assembled asym-
metrically when amino acid derivatives 3, 4,
and 5 were used as catalysts.
Scheme 1. The plausible reaction pathway and transition state.
proline-catalyzed reaction with the substituted ketone 1d
failed to ring-close and provided the a,b-unsaturated Man-
[
16]
nich base 2d with excellent enantioselectivity. Attempts to
perform 6-endo-trig cyclizations of the unsaturated Mannich
bases failed under our reaction conditions.
The proline derivatives also catalyzed the direct enantio-
selective aza-Diels–Alder reaction with preformed imines.
For example, proline catalyzed the aza-Diels–Alder reaction
between ketone 1b and ethyl N-PMP-a-imino glyoxylate in
wet DMSO, and the corresponding synthetically valuable
bicyclic amino acid derivative 2j was isolated exclusively as
the exo adduct in moderate yield with 96% ee [Eq. (3)].
Figure 1. Structure of aza-Diels–Alder product 2b (ORTEP picture).
On the basis of the absolute stereochemistry of the aza-
Diels–Alder adducts and the isolation of Mannich adduct
[
19]
2
d,
account for the stereochemical outcome of the reaction
Scheme 1). The first step is that the proline-derived catalyst
we propose the following reaction mechanism to
(
forms a chiral enamine with the a,b-unsaturated ketone 1.
Next, the in situ generated imine attacks the si face of the
chiral diene via transition state I, and an activated iminium
salt is formed. The secondary amine of the chiral iminium salt
performs a subsequent selective 6-endo-trig cyclization to
furnish the corresponding chiral azabicycle. Next, the amino
acid derivative is released and the desired aza-Diels–Alder
adduct 2 is obtained by hydrolysis and the catalytic cycle can
be repeated. Thus, the reaction proceeds through a tandem
one-pot three-component Mannich/Michael reaction path-
way. The stepwise mechanism was further supported by the
fact that in the aza-Diels–Alder reactions with p-chloroani-
line and p-bromoaniline, minor amounts of the corresponding
a,b-unsaturated Mannich bases were formed in addition to
products 2h and 2i. This result is in accordance with the lower
nucleophilicity of the secondary amine intermediate in the
Michael step as compared to p-anisidine. Furthermore, the
In summary, we have reported the first one-pot three-
component direct catalytic enantioselective aza-Diels–Alder
reaction. The reaction is catalyzed by proline and its
derivatives with excellent chemo-, regio-, and stereoselectiv-
ity. For example, the amino acid catalyzed asymmetric aza-
Diels–Alder reactions between aqueous formaldehyde, a,b-
unsaturated cyclic ketones, and aromatic amines furnished
the desired azabicyclic ketones with up to > 99% ee. The
reactions are operationally simple, performed in wet solvents,
and environmentally friendly. Moreover, the reaction can be
applied for the synthesis of protected azabicyclic amino acids
with excellent exo and enantioselectivity. Further elaboration
of this transformation and its synthetic application is ongoing.
Angew. Chem. Int. Ed. 2005, 44, 4877 –4880
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 4879

Communications
5
573; c) R. O. Duthaler, Angew. Chem. 2003, 115, 1005; Angew.
Experimental Section
Chem. Int. Ed. 2003, 42, 975; d) P. I. Dalko, L. Moisan, Angew.
Chem. 2004, 116, 5248; Angew. Chem. Int. Ed. 2004, 43, 5138.
9] For organocatalytic Diels–Alder reactions, see a) A. B.
Northrup, D. W. C. MacMillan, J. Am. Chem. Soc. 2002, 124,
Typical experimental procedure (Table 2, entry 2): Ketone 1b
(
(
2 mmol) was added to a vial containing aqueous formaldehyde
1 mmol, 36% aqueous solution), p-anisidine (1.1 mmol), and a
[
catalytic amount of (S)-proline (30 mol%) in DMSO (4 mL). After
vigorously stirring the mixture for 24 h at 508C, the reaction was
quenched by purifying the reaction mixture by column chromatog-
raphy on silica gel (EtOAc/pentane 1:5) to afford 2b in 72% yield as a
slightly yellow solid. The ee value of 2b was > 99% as determined by
2
458; b) K. A. Ahrendt, C. J. Borths, D. W. C. MacMillan, J. Am.
Chem. Soc. 2000, 122, 4243; c) D. B. Ramachary, N. S. Chowdari,
C. F. Barbas III, Angew. Chem. 2003, 115, 4365; Angew. Chem.
Int. Ed. 2003, 42, 4233.
1
[10] For reverse-electron-demand Diels–Alder reaction, see a) K.
Juhl, K. A. Jørgensen, Angew. Chem. 2003, 115, 1536; Angew.
Chem. Int. Ed. 2003, 42, 1498.
11] For hetero-Diels–Alder reactions, see a) Y. Huang, K. Unni,
A. N. Thadani, V. H. Rawal, Nature 2003, 424, 146; b) K. A.
Unni, N. Tanaka, H. Yamamoto, V. H. Rawal, J. Am. Chem. Soc.
HPLC analysis on a chiral stationary phase. H NMR (400 MHz,
CDCl ): d = 1.08 (s, 3H), 1.10 (s, 3H), 1.77 (d, J = 2.98 Hz, 2H), 2.47
3
(dd, J = 18.7, 3.4 Hz 1H), 2.62 (m, 1H), 2.68 (dd, J = 18.9, 2.3 Hz 1H),
[
3
2
3
2
.48, (d, J = 2.5 Hz, 2H), 3.75 (m, 1H), 3.76 (s, 3H), 6.61–6.63 (m,
H), 6.84–6.86 ppm (m, 2H); C NMR (100 MHz, CDCl ): d = 28.8,
0.2, 36.1, 38.9, 41.3, 46.0, 47.9, 56.1, 58.5, 112.1, 115.54, 141.1, 151.4,
1
3
3
2005, 127, 1336.
14.0 ppm; HPLC (Daicel Chiralpak AD, hexanes/iPrOH 99:1, flow
À1
[12] For nitroso-Diels–Alder reactions, see a) Y. Yamamoto, N.
Momiyama, H. Yamamoto, J. Am. Chem. Soc. 2004, 126, 5962;
b) Y. Hayashi, J. Yamaguchi, K. Hibino, T. Sumiya, T. Urushima,
M. Shoji, D. Hashizume, H. Koshino, Adv. Synth. Catal. 2004,
rate 1.2 mLmin , l = 254 nm): major isomer: t = 24.94 min; minor
R
isomer: t = 27.31 min; [a] = À71.8 (c = 1.7, CHCl ); MALDI-TOF
R
D
3
+
MS: 256.1689; C H NO [M+H] : calcd 261.1683.
1
6
22
2
3
46, 1435; c) H. SundØn, N. Dahlin, I. Ibrahem, H. Adolfsson, A.
Received: March 4, 2005
Published online: June 23, 2005
Córdova, Tetrahedron Lett. 2005, 46, 3385.
[
13] For 1,3-dipolar cycloadditions, see a) W. S. Jen, J. J. M. Wiener,
D. W. C. MacMillan, J. Am. Chem. Soc. 2000, 122, 9874; b) S.
Karlsson, H. Högberg, Tetrahedron: Asymmetry 2002, 13, 923;
for [4+3]cycloadditions, see c) M. Harmata, S. K. Ghosh, X.
Hong, S. Wacharasindhu, P. Kirchhoefer, J. Am. Chem. Soc.
Keywords: asymmetric catalysis · bicyclic amino acids ·
.
cycloaddition · enantioselectivity · ketones
2003, 125, 2058.
[
14] a) J. Casas, M. Engqvist, I. Ibrahem, B. Kaynak, A. Córdova,
[
1] For reviews, see a) K. A. Jørgensen, Angew. Chem. 2000, 112,
702; Angew. Chem. Int. Ed. 2000, 39, 3558; b) D. L. Boger, S. M.
Angew. Chem. 2005, 117, 1367; Angew. Chem. Int. Ed. 2005, 44,
1
Casas, J. Am. Chem. Soc. 2004, 126, 8914; c) H. SundØn, M.
Engqvist, J. Casas, I. Ibrahem, A. Córdova, Angew. Chem. 2004,
1
Acc. Chem. Res. 2004, 37, 102; e) A. Bøgevig, H, SundØn, A.
Córdova, Angew. Chem. 2004, 116, 1129; Angew. Chem. Int. Ed.
2
Johansson, F. Himo, Chem. Eur. J. 2004, 10, 3673; g) A. Córdova,
Chem. Eur. J. 2004, 10, 1987; h) A. Córdova, Synlett 2003, 1651,
and references therein.
3
343; b) A. Córdova, H. SundØn, M. Engqvist, I. Ibrahem, J.
Weinreb, Hetero Diels–Alder Methodology in Organic-Syntehsis,
Academic Press, San Diego. 1987, chap. 2; c) H. Waldmann,
Synlett 1995, 133; d) L. F. Tietze, G. Kettschau, Top. Curr. Chem.
16, 6694; Angew. Chem. Int. Ed. 2004, 43, 6532; d) A. Córdova,
1997, 190, 1; e) S. M. Weinreb, Top. Curr. Chem. 1997, 190, 131;
f) S. Kobayashi, H. Ishitani, Chem. Rev. 1999, 99, 1069.
2] For examples of synthesis of complex compounds by an aza-
Diels–Alder reaction, see a) R. T. Bailey, R. S. Garigipati, J. A.
Morton, S. M. Weinreb, J. Am. Chem. Soc. 1984, 106, 3240; b) R.
Lock, H. Waldmann, Liebgs. Ann. Chem. 1994, 511; c) A. B.
Holmes, A. Kee, T. Ladduwahetty, D. F. Smith, J. Chem. Soc.
Chem. Commun. 1990, 1412.
3] For examples of diastereoselective aza-Diels–Alder reactions,
see a) J. Barluenga, F. Aznar, C. ValdØz, A. Martín, S. García-
Granada, E. Martín, J. Am. Chem. Soc. 1993, 115, 4403; b) A. S.
Timen, A. Ficher, P. Somfai, Chem. Commun. 2003, 1150; c) H.
Waldmann, M. Braun, M. Dräger, Angew. Chem. 1990, 102,
[
[
004, 43, 1109; f) A. Córdova, H. SundØn, A. Bøgevig, M.
[
15] This strategy has been used successfully in a direct catalytic one-
pot three-component Mannich reaction, see a) B. List, J. Am.
Chem. Soc. 2000, 122, 9336; b) B. List, P. Porjaliev, W. T. Biller,
H. J. Martin, J. Am. Chem. Soc. 2002, 124, 827; c) I. Ibrahem, J.
Casas, A. Córdova, Angew. Chem. 2004, 116, 6690; Angew.
Chem. Int. Ed. 2004, 43, 6528, and references therein.
16] The aza-Diels–Alder products decomposed and underwent a
retro-Michael reaction upon chromatography on silica gel, which
decreased the yield.
17] For the first use of catalyst 4, see a) A. J. A. Cobb, D. M. Shaw,
S. V. Ley, Synlett 2004, 558; b) H. Torii, M. Nakadai, K. Ishihara,
S. Saito, H. Yamamoto, Angew. Chem. 2004, 116, 2017; Angew.
Chem. Int. Ed. 2004, 43, 1983; c) A. Hartikaa, P. I. Arvidsson,
Tetrahedron: Asymmetry 2004, 15, 1831.
[
[
1445; Angew. Chem. Int. Ed. Engl. 1990, 29, 1468; d) S. D.
Larsen, P. A. Grieco, J. Am. Chem. Soc. 1985, 107, 1768.
4] Stoichiometric amounts of chiral boron complexes have also
been used to mediate enantiosalective aza-Diels–Alder reac-
tions, see a) K. Hattori, H. Yamamoto, J. Org. Chem. 1992, 57,
[
[
[
3264; b) K. Ishihara, M. Miyata, K. Hattori, T. Tada, H.
Yamamoto, J. Am. Chem. Soc. 1994, 116, 10520.
5] a) H. Ishitani, S. Kobayashi, Tetrahedron Lett. 1996, 37, 7357;
b) S. Kobayashi, S. Komiyama, H. Ishitani, Angew. Chem. 1998,
[
18] Arylsulfonylcarboxamides have been used in aldol reactions, see
a) A. Berkessel, B. Koch, J. Lex, Adv. Synth. Catal. 2004, 346,
1
10, 1026; Angew. Chem. Int. Ed. 1998, 37, 979; c) S. Kobayashi,
K. Kusakabe, S. Komiyama, H. Ishitani, J. Org. Chem. 1999, 64,
220.
1
141; for the use of catalyst 5, see reference [12c] and b) A. J. A.
Cobb, D. M. Shaw, D. A. Longbottom, J. B. Gold, S. V. Ley, Org.
Biomol. Chem. 2005, 3, 84.
4
6] a) S. Yao, M. Johannsen, R. G. Hazell, K. A. Jørgensen, Angew.
Chem. 1998, 110, 3318; Angew. Chem. Int. Ed. 1998, 37, 3121;
b) S. Yao, S. Saaby, R. G. Hazell, K. A. Jørgensen, Chem. Eur. J.
[
19] CCDC-265109 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge from
the Cambridge Crystallographic Data Centre via www.ccdc.
cam.ac.uk/data_request/cif.
2000, 6, 2435.
[
7] see also: a) N. S. Josephsohn, M. L. Snapper, A. H. Hoveyda, J.
Am. Chem. Soc. 2003, 125, 4018; b) O. G. Mancheæo, R. G.
Arrayµs, J. C. Carretero, J. Am. Chem. Soc. 2004, 126, 456.
8] a) P. I. Dalko, L. Moisan, Angew. Chem. 2001, 113, 3840; Angew.
Chem. Int. Ed. 2001, 40, 3726; b) B. List, Tetrahedron 2002, 58,
[
4
880 www.angewandte.org
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2005, 44, 4877 –4880