Asymmetric synthesis of β-adrenergic blockers through multistep one-pot transformations involving in situ chiral organocatalyst formation

Article abstract of DOI:10.1002/chem.201102931

Two birds one stone: A new atom-economical one-pot approach to enantioselective chiral drug synthesis, involving in situ multistep organocatalyst formation and the application of the reaction for multistep sequential synthesis of β-adrenergic blockers is disclosed (see scheme).

Full text of DOI:10.1002/chem.201102931

DOI: 10.1002/chem.201102931
Asymmetric Synthesis of b-Adrenergic Blockers through Multistep One-Pot
Transformations Involving In Situ Chiral Organocatalyst Formation
Shengwei Wei, Regina Messerer, and Svetlana B. Tsogoeva*[a]
One-pot multistep operations involving organocatalysis
are among the most recent, elegant, economically most at-
tractive and sustainable methods in modern synthetic
chemistry.[1] Whereas successful examples of one-pot orga-
nocatalytic methods[1,2] (and in particular those leading to
chiral drugs or their intermediates[3]) rely on the use of orga-
nocatalysts synthesized in advance (Scheme 1a), to the best
We envisioned that methods employing organocatalysts,
prepared in situ through multistep one-pot transformations
and used for the subsequent enantioselective multistep syn-
thesis, might further contribute to the sustainability of the
one-pot process through the additional reduction of work-
up or purification steps (Scheme 1b). Herein, we disclose
such a facile one-pot multistep synthetic approach applied
directly to chiral drug synthesis.
To realize the idea presented in Scheme 1b, we have
chosen 1,2-amino alcohols as attractive synthetic targets,
which are of significant importance both in the pharmaceuti-
cal context and in nature.[6] Various asymmetric synthesis of
these versatile compounds, for example, by the reduction–
amination of a-tosyloxy and a-halo ketones,[7] the aminohy-
droxylation of olefins,[8] and by different other approaches,[9]
have been reported. The enzymatic and catalytic reduction
of a-amino ketones, as well as resolution of racemic 1,2-
amino alcohols has also been demonstrated.[10]
However, isolation and purification of product intermedi-
ates is almost invariably tedious and difficult with most of
these methods. Moreover, another drawback of some of
these known procedures is the use of metal catalysts,[8,9a–d]
which may be sources of chiral drug contamination as well
as industrial pollution.
Scheme 1. Depiction of the one-pot multistep sequential synthesis of
chiral drugs involving an organocatalytic reaction step: a) using organo-
catalyst prepared in advance; b) by an in situ formed organocatalyst.
According to the suggested new approach (Scheme 1b),
not only the target chiral drug but also the organocatalyst
required for its enantioselective formation can be synthe-
sized in a one-pot procedure without intermediate isolation
or purification steps, therefore leading to the reduction of
costs, saving materials, time and effort. We aimed to realize
this one-pot strategy for the synthesis of selected b-adrener-
gic blockers 1–3 (Scheme 2). The pharmaceuticals nifenalol
(1), pronethalol (2) and dichloroisoproterenol (3) are of
great importance in the therapy of asthma, bronchitis, and
congestive heart failure.[11]
of our knowledge, no one-pot multistep drug synthesis in-
volving an in situ multistep organocatalyst formation has
been reported yet.
Whereas in metal-catalyzed reactions the in situ genera-
tion of chiral metal complexes is a rather common and es-
tablished pathway,[4a–c] amongst the variety of asymmetric or-
ganocatalytic methods the involvement of in situ generated
organocatalysts is still rare (e.g., one step formation of
chiral carbenes from heterocyclic salts,[4d–g] dioxiranes from
ketones,[4h–m] and 1,3,2-oxazaborolidines from amino alco-
hols[4n–q,5]).
The proposed five-step one-pot synthetic outline for the
formation of target 1,2-amino alcohols is given in Scheme 3.
We decided to carry out the in situ synthesis of chiral orga-
[a] Dr. S. Wei, R. Messerer, Prof. Dr. S. B. Tsogoeva
Department of Chemistry and Pharmacy
Chair of Organic Chemistry I, University of Erlangen-Nuremberg
Henkestrasse 42, 91054 Erlangen (Germany)
Fax : (+49)9131-85-26865
E-mail: tsogoeva@chemie.uni-erlangen.de
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201102931.
Scheme 2. b-Adrenergic blockers with antianginal and antiarrhythmic
properties.
14380
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 14380 – 14384

COMMUNICATION
isolation are not necessarily
well suited for one-pot sequen-
tial transformations. Hence, we
decided to optimize the select-
ed one-pot reaction sequence
directly.
To find out the best suitable
amino acid (among the selected
Scheme 3. Designed one-pot reaction sequence.
nocatalysts using a-amino acids as particularly attractive key
starting compounds, justifying the requirements of ready
availability and low costs.
Table 1. Synthesis of precatalysts: amino alcohols 5a–c.
Enantioselective 1,3,2-oxazaborolidine-catalyzed borane
reduction of prochiral ketones to chiral secondary alcohols
(see step 3, Scheme 3) is one of the most useful reactions in
asymmetric synthesis.[4o,5] Amino alcohols 5a–c are known
as well suitable pre-catalysts for such Corey–Bakshi–Shibata
(CBS)-type reduction of acetophenone.[12] l-Prolinol-derived
1,3,2-oxazaborolidine catalysts prepared from l-proline and
giving the reduction product with moderate enantiomeric
excess (ee) values were also studied.[13] Taking note of these
reports we selected amino acids l-Leu (4a), l-Phe (4b), and
l-Val (4c) with a primary amino functionality as starting
compounds for realization of the proposed one-pot reaction
sequence.
À
Entry
Amino
acid
BH3
X
Solvent
t
Yield
[%][a]
[h]
1
2
3
4
5
6
7
8
9
4a
4a
4a
4b
4b
4b
4c
4c
4c
4c
THF
THF
DMS
THF
THF
DMS
THF
DMS
DMS
THF
THF
THF
toluene
THF
THF
toluene
THF
toluene
THF
17
3
3
17
3
3
17
3
9
56
51
79
61
65
88
<78
<58
47
10
toluene
20
39
The first step is the amino alcohol formation through re-
duction of selected amino acids. Reductions of amino acids
have been reported using lithium aluminium hydride,[14a]
sodium borohydride,[14a,b] H2 with Rh/Pt oxide (Nishimura
catalyst),[14c] and BH3-DMS (borane dimethyl sulfide com-
plex) activated by BF3-OEt2 (boron trifluoride etherate).[14d]
Since the first two methods are accompanied by the forma-
tion of trace impurities and partial racemization of the prod-
uct, and Rh/Pt oxide catalyzed reaction is not suitable for
the designed metal-free one-pot sequence, we selected the
reduction employing borane for optimization of our first
step.
After preliminary studies (see Table S1 of the Supporting
Information) we found that the use of 1.5 equiv of borane
reagent (relative to amino acid) and the reflux conditions
were important to obtain good yields in this reduction reac-
tion. It was further found that, whereas BH3-DMS in tolu-
ene was the more appropriate system for the reduction of l-
Leu and l-Phe (entries 3 and 6, Table 1), the combination of
BH3-THF in THF was much better suited for the reduction
of l-Val (entry 7, Table 1). Notably, no racemization of
amino alcohols 5a–c under the selected reaction conditions
was observed (see Table S2 in the Supporting Information).
At the outset, improved conditions to those previously re-
ported[14d] for the reduction of amino acids with borane were
developed (entries 3, 6, and 7, Table 1) and applied in step 1
of our one-pot reaction.
[a] Yields of isolated products. DMS=dimethyl sulfide.
ones 4a–c) for the designed sequential transformation and
to reach an efficient one-pot protocol for the synthesis of
chiral drugs 1–3, the three-step one-pot reaction sequence
was next investigated under different conditions (Table 2).
The in situ generation of chiral 1,3,2-oxazaborolidines 6a–c
and their subsequent application (without isolation) as cata-
lysts for the reduction of acetophenone 7a with BH3-DMS
at room temperature was selected as a model one-pot se-
quence. Variation of reducing agent, solvent and reaction
time in step 1 and of reaction temperature in step 2 allowed
us to make successful optimization of the one-pot process
over three steps.
À
It was expected that B OMe derivatives might perform
better than the B-unsubstituted oxazaborolidines.[13,15] Nota-
bly, preliminary studies (see Table S3 in the Supporting In-
formation) demonstrated that the use of 0.6 equiv of B-
AHCTUNGRTEG(NNUN OMe)3 (relative to the in situ generated amino alcohols
5a–c, taking into consideration their theoretically possible
yield from Table 1) was crucial to obtain good yield and
enantioselectivity in the reduction of acetophenone. There-
À
fore, B OMe-derived oxazaborolidines 6a–c were prepared
in situ by addition of B
A
ring for 1 h at the desired temperature (RT or 608C). Then,
BH3-DMS (1.2 equiv relative to 7a) was added dropwise at
room temperature within 1 h. Subsequently, acetophenone
was slowly added over a 1 h period. A syringe pump was
used for the addition of both reducing agent and reactant
7a.
It is known that combining separately optimized reactions
(with isolation in each step) into a one-pot sequence does
not guarantee the best outcome.[2] Moreover, the conditions
found to be the best for individual reactions carried out with
Chem. Eur. J. 2011, 17, 14380 – 14384
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
14381

S. B. Tsogoeva et al.
Table 2. One-pot three-step sequential synthesis of chiral alcohols
through in situ organocatalyst generation. Optimization of reaction con-
ditions of steps 1 and 2 in a one-pot reaction.
ticular, the “additive” BY3 and reaction temperature in step
2 as well as the reducing agent BH3-X (BH3-DMS vs. BH3-
THF) in step 3 have been varied (Table 3).
Our studies revealed that the use of BACTHNUTRGENUG(N OMe)3 as an “addi-
tive” at 608C for in situ catalyst synthesis (step 2, Table 3),
and of BH3-DMS as a reducing agent in step 3 had a pro-
Table 4. Scope of the reaction.
Step 1
Amino
acid
Step 2
T
[8C]
Entry
X
Solvent
t
Yield
[%][a]
ee
[h]
[%][b,c]
1
2
3
4
5
6
7
8
4a
4a
4a
4b
4b
4b
4c
4c
DMS
DMS
THF
DMS
DMS
THF
THF
DMS
toluene
toluene
THF
toluene
toluene
THF
3
3
3
3
3
3
17
3
RT
60
60
RT
60
60
>99
>99
>99
>99
>99
>99
>99
>99
24
21
59
16
17
62
80
33
Entry
Product
Yield
[%][a]
ee
[%][b,c]
THF
Toluene
RT
60
1
2
8a
8b
>99
>99
87
71
[a] Yields of isolated products. [b] Determined by chiral phase HPLC
analysis and compared to authentic racemic material. [c] The absolute
configuration at the stereocenter was established by comparison with lit-
erature data (see Table S4 in the Supporting Information).
The in situ generated catalysts 6a–c were applied at ꢀ20
mol% loading relative to 7a (theoretically calculated,
taking into consideration the expected yields for 5a–c given
in Table 1).
The conditions found in entry 7 of Table 2 (with l-Val as
a starting amino acid, BH3-THF as a reducing agent and
THF as a solvent) giving the product 8a in over 99% yield
and 80% ee using the in situ formed catalyst 6c were select-
ed for further studies.
3
4
5
6
7
8c
8d
8e
8 f
8g
98
>99
>99
>99
>99
72
77
81
90
94
Next, the conditions of the in situ synthesis of CBS-type
catalyst 6c (step 2, Table 3) and the subsequent reduction of
ketone 7a (step 3, Table 3) were further optimized. In par-
Table 3. Optimization of reaction conditions of steps 2 and 3.
8
8h
>99
91
9[d]
8i
8j
>99
>99
88
92
10[d]
Step 2
BY3
Step 3
X
nd[e]
Entry
T
[8C]
Yield
[%][a]
ee
11[d]
8k
>99
[a]2D0 =À30.7
(c=1.0, CHCl3)
[%][b]
1
2
3
4
5
6
B
G
RT
RT
RT
RT
60
DMS
THF
DMS
DMS
DMS
DMS
>99
>99
>99
>99
>99
>99
80
31
80
77
78
87
B
ACHTUNGTRENNUNG
[a] Yields of isolated products. [b] Determined by chiral phase HPLC
analysis and compared with authentic racemic material. [c] The absolute
configuration at the stereocenter was established by comparison of the
sign of specific rotations with the literature values (see Table S4 in the
Supporting Information). [d] d-Val was used for the in situ synthesis of
the catalyst. [e] Not determined because no appropriate conditions for
chiral phase HPLC analysis were found for 8k.
BH3-DMS
BH3-THF
BH3-THF
BN
60
[a] Yields of isolated products. [b] Determined by chiral phase HPLC
analysis and compared with authentic racemic material.
14382
www.chemeurj.org
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 14380 – 14384

Asymmetric Synthesis of b-Adrenergic Blockers
COMMUNICATION
nounced effect on the resultant enantioselectivity of the re-
action product 8a (entry 6 vs. entries 1–5, Table 3).
cation steps not only for the synthesis of target compounds,
but also those required for the synthesis of external organo-
catalyst, leading to the further reduction of costs, materials
and labor input and making the one-pot synthetic strategy
even more efficient, atom economical and environmentally
friendly.
The application of this facile and straightforward method
to the synthesis of important chiral drugs, b-adrenergic
blockers 1–3, has been successfully demonstrated. Further
efforts to evaluate the scope of the related processes are un-
derway.
Motivated by this result, we next examined the scope of
the developed one-pot process for different prochiral ke-
tones using the optimized reaction conditions. The results
are summarized in Table 4. To our delight, all the reactions
investigated can be effected in excellent yields (98–99%)
and with good to high enantioselectivities (71–94%). Gener-
ally, higher enantioselectivities were obtained using a-halo
ketones (Table 4, entries 6–10).
Consequently, the anticipated one-pot sequential transfor-
mations leading to selected chiral drugs 1–3 was successfully
attempted (for optimization of epoxidation and aminolysis
reaction steps; see Tables S5 and S6 in the Supporting Infor-
mation). Whereas l-Val gave the S-configured products (see
Table S7 of the Supporting Information), application of d-
Val as a starting amino acid gratifyingly gave the desired R-
configured products 1–3 with good yields and excellent
enantioselectivities (96–99% ee, entries 1–3, Table 5).
Whereas the individual reactions selected for realization
of our one-pot synthesis (e.g., reduction, epoxide formation
and aminolysis) are already known,[16] the combination of
these transformations with an in situ organocatalyst genera-
tion in a one-pot sequential multistep process leading to
chiral drugs is unprecedented.
Acknowledgements
The authors gratefully acknowledge financial support from the Deutsche
Forschungsgemeinschaft (DFG) through the priority programme SPP
1179 “Organocatalysis” and the BMBF (DLR, Project MDA 08/010).
Keywords: amino alcohols · asymmetric synthesis · drug
synthesis · one-pot reactions · organocatalysis · reduction
[1] a) K. C. Nicolaou, T. Montagnon, S. A. Snyder, Chem. Commun.
2003, 551; b) L. F. Tietze, G. Brasche, K. Gericke, Domino Reactions
in Organic Synthesis Wiley-VCH, Weinheim, 2006; c) K. C. Nico-
laou, D. J. Edmonds, P. G. Bulger, Angew. Chem. 2006, 118, 7292;
Angew. Chem. Int. Ed. 2006, 45, 7134; d) D. Enders, C. Grondal,
M. R. M. Hꢁttl, Angew. Chem. 2007, 119, 1590; Angew. Chem. Int.
Ed. 2007, 46, 1570; e) C. Vaxelaire, P. Winter, M. Christmann,
Angew. Chem. 2011, 123, 3685; Angew. Chem. Int. Ed. 2011, 50,
3605; f) Ł. Albrecht, H. Jiang, K. A. Jørgensen, Angew. Chem. 2011,
123, 8642; Angew. Chem. Int. Ed. 2011, 50, 8492 and references
therein.
Table 5. One-pot sequential synthesis of b-blockers 1–3 by in situ orga-
nocatalyst generation.
[2] For selected examples, see: a) Y. Huang, A. M. Walji, C. H. Larsen,
D. W. C. MacMillan, J. Am. Chem. Soc. 2005, 127, 15051; b) D.
Enders, M. R. M. Hꢁttl, J. Runsink, G. Raabe, B. Wendt, Angew.
Chem. 2007, 119, 471; Angew. Chem. Int. Ed. 2007, 46, 467; c) G.-L.
Zhao, R. Rios, J. Vesely, L. Eriksson, A. Cꢂrdova, Angew. Chem.
2008, 120, 8596; Angew. Chem. Int. Ed. 2008, 47, 8468; d) S. Belot,
K. A. Vogt, C. Besnard, N. Krause, A. Alexakis, Angew. Chem.
2009, 121, 9085; Angew. Chem. Int. Ed. 2009, 48, 8923; e) H. Jiang,
N. Holub, M. W. Paix¼o, C. Tiberi, A. Falcicchio, K. A. Jørgensen,
Chem. Eur. J. 2009, 15, 9638; f) C. Chandler, P. Galzerano, A. Mi-
chrowska, B. List, Angew. Chem. 2009, 121, 2012; Angew. Chem. Int.
Ed. 2009, 48, 1978; g) C. Grondal, M. Jeanty, D. Enders, Nat. Chem.
2010, 2, 167; h) B. Westermann, M. Ayaz, S. S. van Berkel, Angew.
Chem. 2010, 122, 858; Angew. Chem. Int. Ed. 2010, 49, 846; i) N. T.
Jui, E. C. Y. Lee, D. W. C. MacMillan, J. Am. Chem. Soc. 2010, 132,
10015; j) Ł. Albrecht, H. Jiang, G. Dickmeiss, B. Gschwend, S. G.
Hansen, K. A. Jørgensen, J. Am. Chem. Soc. 2010, 132, 9188;
k) K. L. Jensen, P. T. Franke, C. Arrꢂniz, S. Kobbelgaard, K. A. Jør-
gensen, Chem. Eur. J. 2010, 16, 1750; l) T. Urushima, D. Sakamoto,
H. Ishikawa, Y. Hayashi, Org. Lett. 2010, 12, 4588; m) K. Weiß, S.-
W. Wei, S. B. Tsogoeva, Org. Biomol. Chem. 2011, 9, 3457.
Entry
Product
Yield
[%][a]
ee
[%][b]
1
2
1
2
82
78
96
98
3
3
80
99
[a] Yields of isolated products. [b] Determined by chiral phase HPLC
analysis and compared to authentic racemic material.
ˇ
[3] a) D. Enders, V. Terteryan, J. Palecek, Synthesis 2008, 14, 2278;
In conclusion, we have demonstrated here a new one-pot
approach to enantioselective synthesis, involving in situ or-
ganocatalyst formation (from commercially available and in-
expensive starting materials, for example, amino acids) and
the application of the reaction for further multistep sequen-
tial synthesis. This obviates the need of work-up and purifi-
b) H. Ishikawa, T. Suzuki, Y. Hayashi, Angew. Chem. 2009, 121,
1330; Angew. Chem. Int. Ed. 2009, 48, 1304; c) H. Ishikawa, T.
Suzuki, H. Orita, T. Uchimaru, Y. Hayashi, Chem. Eur. J. 2010, 16,
12616; d) H. Ishikawa, M. Honma, Y. Hayashi, Angew. Chem. 2011,
123, 2876; Angew. Chem. Int. Ed. 2011, 50, 2824; e) G. Dickmeiss,
K. L. Jensen, D. Worgull, P. T. Franke, K. A. Jørgensen, Angew.
Chem. 2011, 123, 1618; Angew. Chem. Int. Ed. 2011, 50, 1580.
Chem. Eur. J. 2011, 17, 14380 – 14384
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
14383

S. B. Tsogoeva et al.
[4] For selected examples, see: a) C. Bolm, Coord. Chem. Rev. 2003,
237, 245; b) C. Bolm, J. Legros, J. Le Paih, L. Zani, Chem. Rev.
2004, 104, 6217; c) N. Grimster, L. Zhang, V. V. Fokin, J. Am. Chem.
Soc. 2010, 132, 2510; d) D. Enders, T. Balensiefer, Acc. Chem. Res.
2004, 37, 534; e) V. Cꢃsar, S. Bellemin-Laponnaz, L. H. Gade,
Chem. Soc. Rev. 2004, 33, 619; f) S. S. Sohn, E. L. Rosen, J. W. Bode,
J. Am. Chem. Soc. 2004, 126, 14370; g) A. T. Biju, N. Kuhl, F. Glo-
rius, Acc. Chem. Res. 2011, DOI: 10.1021/ar2000716; h) W. Adam,
R. Curci, J. Edwards, Acc. Chem. Res. 1989, 22, 205; i) Z.-X. Wang,
Y. Tu, M. Frohn, J.-R. Zhang, Y. Shi, J. Am. Chem. Soc. 1997, 119,
11224; j) Y. Shi, Acc. Chem. Res. 2004, 37, 488; k) D. Goeddel, L.
Shu, Y. Yuan, O. A. Wang, B. Wang, Y. Shi, J. Org. Chem. 2006, 71,
1715; l) C. P. Burke, Y. Shi, J. Org. Chem. 2007, 72, 4093; m) C. P.
Burke, L. Shu, Y. Shi, J. Org. Chem. 2007, 72, 6320; n) M. Masui, T.
Shioiri, Synlett 1997, 273; o) E. J. Corey, C. J. Helal, Angew. Chem.
1998, 110, 2092; Angew. Chem. Int. Ed. 1998, 37, 1986; p) V. Santhi,
J. M. Rao, Tetrahedron: Asymmetry 2000, 11, 3553; q) Z. Dalicsek,
F. Pollreisz, ꢄ. Gçmçry, T. Soꢂs, Org. Lett. 2005, 7, 3243.
[9] a) S. Miyano, L. D.-L. Lu, S. M. Viti, K. B. Sharpless, J. Org. Chem.
1985, 50, 4350; b) A. Baeza, C. Najeri, J. M. Sansano, J. M. Saa,
Chem. Eur. J. 2005, 11, 3849; c) A. K. Yadav, M. Manju, Indian J.
Chem. Sect. B 2006, 45, 2770; d) Y. Xiong, F. Wang, X. Huang, Y.
Wen, X. Feng, Chem. Eur. J. 2007, 13, 829; e) T. X. Mꢃtro, D. G.
Pardo, J. Cossy, J. Org. Chem. 2007, 72, 6556.
[10] a) F. D. Klingler, Acc. Chem. Res. 2007, 40, 1367; b) J. C. Moore,
D. J. Pollard, B. Kosjek, P. N. Devin, Acc. Chem. Res. 2007, 40, 1412.
[11] A. Klleman, J. Engel, B. Kutscher, D. Reichert, Pharmaceutical Sub-
stances: Syntheses, Patents, Applications, 3rd ed., Thieme Medical
Publishers, New York, 1999.
[12] M. Hoogenraad, G. M. Klaus, N. Elders, S. M. Hooijschuur, B.
McKay, A. A. Smith, E. W. P. Damen, Tetrahedron: Asymmetry
2004, 15, 519.
[13] a) T. Mehler, W. Behnen, J. Wilken, J. Martens, Tetrahedron: Asym-
metry 1994, 5, 185; b) J. Xu, T. Wei, S.-S. Lin, Q. Zhang, Helvetica
Chimica Acta 2005, 88, 180.
[14] Selected examples for the reduction of amino acids with LiAlH4 and
NaBH4, see: a) G. S. Poindexter, A. I. Meyers, Tetrahedron Lett.
1977, 40, 3527; b) M. J. McKennon, A. I. Meyers, K. Drauz, M.
Schwarm, J. Org. Chem. 1993, 58, 3568; for reduction with H2 and
Rh/Pt oxide, see: c) M. Studer, H. Blaser, S. Burkhardt, Adv. Synth.
Catal. 2001, 343, 802; for reduction with BH3-DMS/BF3-OEt2, see:
d) V. Guerlavais, P. J. Carroll, M. M. Joulliꢃ, Tetrahedron: Asymme-
try 2002, 13, 675.
[5] For selected reviews, see: a) S. Wallbaum, J. Martens, Tetrahedron:
Asymmetry 1992, 3, 1475; b) V. K. Singh, Synthesis 1992, 605; c) L.
Deloux, M. Srebnik, Chem. Rev. 1993, 93, 763.
[6] a) R. T. Brittain, D. Jack, A. C. Ritchie, Adv. Drug Res. 1970, 197;
b) D. J. Ager, I. Prakash, D. R. Schaad, Chem. Rev. 1996, 96, 835–
875; c) S. C. Bergmeier, Tetrahedron 2000, 56, 2561 and references
therein.
[7] a) B. T. Cho, S. K. Kang, W. K. Yang, Bull. Korean Chem. Soc. 2002,
23, 1328; b) J. Duquette, M. Zhang, L. Zhu, R. S. Reeves, Org. Pro-
[15] N. J. Gilmore, S. Jones, M. P. Muldowney, Org. Lett. 2004, 6, 2805.
[16] a) C. P. Baird, P. C. Taylor, J. Chem. Soc. Perkin Trans. 1 1998, 3399;
b) C. Barbieri, L. Bossi, P. DꢅArrigo, G. P. Fantoni, S. Servi, J. Mol.
Catal. B 2001, 11, 415; c) T. Hamada, T. Torii, K. Izawa, R. Noyori,
T. Ikariya, Org. Lett. 2002, 4, 4373.
´
cess Res. Dev. 2003, 7, 285; c) M. Zaidlewicz, M. Krzeminski, K.
Ła˛czkowski, Encyclopedia of Reagents for Organic Synthesis J.
Wiley, 2007, first update, DOI:10.1002/047084289X.rn00701.
[8] J. A. Bodkin, M. D. McLeod, J. Chem. Soc. Perkin Trans. 1 2002,
2733 and references therein.
Received: September 18, 2011
Published online: November 23, 2011
14384
www.chemeurj.org
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 14380 – 14384