Multiple-organocatalyst-promoted cascade reaction: A fast and efficient entry into fully substituted piperidines

Article abstract of DOI:10.1002/chem.201000059

(Figure presented) Chemical Equation presented Multiple reactions in a single pot: The development of a catalytic asymmetric approach to fully substituted piperidines, which previously remained elusive, is achieved by the use of a multicatalyst-promoted cascade reaction (see scheme). The one-pot reaction cascade is easy to perform, highly stereoselective and broad in scope, and the reaction products are high-value intermediates for multiple synthetic transformations.

Full text of DOI:10.1002/chem.201000059

DOI: 10.1002/chem.201000059
Multiple-Organocatalyst-Promoted Cascade Reaction: A Fast and Efficient
Entry into Fully Substituted Piperidines
Yao Wang,[a] De-Feng Yu,[a] Yao-Zong Liu,[a] Hao Wei,[a] Yong-Chun Luo,[a]
Darren J. Dixon,[b] and Peng-Fei Xu*[a]
Piperidines have been used widely in the construction of
natural products and pharmaceutical compounds.[1] Recently,
the synthesis of polysubstituted piperidines has received
much attention. For example, Hayashi et al. reported an
enantioselective, formal aza-[3+3] cycloaddition reaction
for their construction[2] and Chen et al. introduced two aza-
Diels–Alder reactions for the formation of piperidines with
high enantioenrichment.[3] New, non-asymmetric approaches
to the formation of polysubstituted piperidines continue to
be developed within the synthetic community.[4] Notably,
one important class of polysubstituted piperidines, which are
characterized by 3-nitro and 2,4-diaryl substituent groups, is
bioactive.[5] Unfortunately, the synthetic route to these bio-
active molecules requires resolution procedures to obtain
enantioenriched products.[5] Current methods for the synthe-
sis of fully substituted, optically active piperidines generally
employ lengthy routes. Despite the potential utility of these
valuable compounds, to the best of our knowledge, an enan-
tioselective, one-pot catalytic cascade approach to optically
active, fully substituted piperidines remains an elusive prob-
lem; this is, presumably, due to the challenge of constructing
five contiguous stereocenters around the six-membered-ring
heterocycle. In this context, the development of a simple-to-
perform and single-operation catalysis cascade to form fully
substituted piperidines that contain the core structure of
some anti-cancer compounds would be a timely endeavour.
During the last few years, the development of asymmetric
organocatalytic domino/cascade reactions has been hotly
pursued.[6] Most of these elegant approaches involve two-
step cascades and are based on covalent, or hydrogen-bond-
ing catalysis.[6] However, many useful reactions cannot be ef-
fectively carried out by the use of a single organocatalyst.
Thus, the combination of two organocatalysts has been em-
ployed in one-pot reactions with the controlled, sequential
addition of reagents and/or catalysts throughout the course
of the reaction.[7] The major challenge in the development
of single-operation, catalysed cascade-reactions involving
multiple organocatalysts is due to the incompatibility of re-
actants, intermediates, and catalyst-activated species with re-
spect to the desired product outcome. Recently, a few such
reactions have been successfully used in the synthesis of im-
portant chiral building blocks.[8] To make further advances
in the field, our aim was 1) to develop a new, singly-operat-
ed, multiple-organocatalyst-promoted cascade reaction and
2) to combine covalent bond[9] and bifunctional-base/Brønst-
ed acid catalysis[10] in a triple cascade reaction for the direct,
asymmetric synthesis of fully substituted piperidines.
Herein, we report our efforts towards meeting these chal-
lenges.
Our proposed cascade process is to couple three compo-
nents, which comprise an aldehyde 1, a nitroalkene 2, and
an imine 3, to give product 4 in the presence of two com-
mercially available, or readily prepared, organocatalysts, 5
and 6.[11] The sequential transformation would involve three
steps (Scheme 1). Initial activation of aldehyde 1, by catalyst
5 (enamine activation), would facilitate selective addition to
the hydrogen-bond-activated nitroalkene 2 in a Michael-
type reaction.[12] In situ hydrolysis would liberate nitroal-
kane 2a, which can then able to participate in the relay cat-
alysis cycle. Catalyst 6 would promote the nitro-Mannich re-
action of intermediate 2a with imine 3 to generate the per-
substituted N-Tos-protected aminoaldehyde 3a (Tos=p-tol-
uenesulfonyl), which can then undergo cyclization to form
the final hemiaminal 4. If successful, this sequence would
constitute an atom economical,[13] selective,[14] and environ-
[a] Y. Wang, D.-F. Yu, Y.-Z. Liu, H. Wei, Y.-C. Luo, Prof. Dr. P.-F. Xu
State Key Laboratory of Applied Organic Chemistry
College of Chemistry and Chemical Engineering
Lanzhou University, Lanzhou 730000 (P. R. China)
Fax : (+86)931-8915557
E-mail: xupf@lzu.edu.cn
[b] Prof. Dr. D. J. Dixon
Department of Chemistry, Chemistry Research Laboratory
University of Oxford, Mansfield Road
Oxford, OX1 3TA (UK)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201000059.
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COMMUNICATION
Table 1. Optimization of the cascade reaction.[a]
Scheme 1. Proposed reaction mechanism.
mentally friendly synthesis of fully substituted piperidines.
Furthermore, it would be accomplished with readily avail-
able organocatalysts, from low cost, simple starting materi-
als, and would exploit a rare nitro-Mannich reaction involv-
ing a hindered long-chain nitroalkane.[15]
To probe the feasibility of the proposed cascade process,
preliminary experiments were performed using propionalde-
hyde, nitrostyrene and N-Tos-benzaldimine in the presence
of catalysts 5a and 6a in dry toluene at 128C. Pleasingly, the
cascade process proceeded smoothly to afford the desired
product 4c in 43% yield and >99% ee as, initially, a 1:1
mixture of a- and b-diastereomers that slowly equilibrated
to a 2.5:1 mixture when left in [D6]DMSO at room tempera-
ture for 18 h (Table 1, entry 1). This result led us to investi-
gate the cascade reaction with various catalyst combinations
and solvent conditions in order to improve the reaction effi-
ciency (Table 1). The use of quinine 6d and achiral com-
pounds 6e–h as catalysts resulted in lower yields (Table 1,
entries 6–10). The best results were obtained using a combi-
nation of 5a (15 mol%) and 6b (15 mol%) in dry toluene
at 128C. The product was then obtained in moderate yield
and excellent stereoselectivity; it should be noted that under
these conditions, aside from the diastereomers arising from
the hemiaminal stereocenter, no other diastereomeric prod-
ucts were detectable in the crude reaction mixture (Table 1,
entry 4; 56% yield, >99:1 d.r., >99% ee).
Entry
Catalysts
Solvent
Yield[b] [%]
ee[c] [%]
1
2
3
4
5
6
7
8
9
5a + 6a
5b + 6a
5c + 6a
5a + 6b
5a + 6c
5a + 6d
5a + 6e
5a + 6 f
5a + 6g
5a + 6h
5a + 6b
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
CH2Cl2
43
15
8
56
32
12
29
<10
<10
<10
48
>99
>99
>99
>99
>99
>99
>99
n.d.
n.d.
n.d.
>99
10
11
[a] All the reactions were performed by using aldehyde 1 (0.4 mmol), ni-
troalkene 2 (0.4 mmol), imine 3 (0.2 mmol) and each organocatalyst
(0.03 mmol) in dry solvent (0.4 mL) at 128C. [b] Yield of the isolated
product. [c] Determined by chiral phase HPLC analysis.
Table 2. Survey of the reaction scope.[a]
With optimal reaction conditions established, the scope of
this triple cascade reaction was investigated. The fact that
the residues R1–R3 of precursors 1–3, respectively, can be
varied (see Table 2) demonstrates the flexibility of our ap-
proach and allows the generation of a diverse range of reac-
tion products. The moderate-to-good reaction yields and ex-
cellent enantioselectivities were independent of the elec-
tronic and structural characteristics of the substituents (elec-
tron-withdrawing, -donating, neutral, and heterocyclic).
Likewise the substitution pattern (ortho-, meta-, and para-)
of the substituents on aromatic rings had little-to-no adverse
effect on the reaction efficiency and selectivity. The enantio-
meric excesses of hemiaminal products 4k and 4l could not
be determined directly by using chiral phase HPLC analysis.
Accordingly, 4k and 4l were oxidized to their corresponding
lactams and subsequently analysed to reveal that they were
formed in excellent enantioselectivity (>99% ee). Unfortu-
nately, alkyl substrates did not produce good results under
these catalytic conditions. However, we were pleased that,
Entry R1
R2
R3
4
Yield[b] [%] ee[c] [%]
1
2
nC5H11 Ph
Et
Ph
Ph
Ph
Ph
4a 51
4b 65
4c 56
4d 48
4e 52
4 f 65
4g 61
4h 51
>99
>99
>99
98
>99
>99
>99
>99
>99
>99
>99
>99
Ph
Ph
Ph
3
4
5
6
7
8
9
10
11[e]
12[e]
Me
Bn[d]
Me
Me
Me
Me
Me
Me
Me
Me
4-CH3C6H4 Ph
2-furyl
3-ClC6H4
Ph
Ph
Ph
Ph
Ph
3-FC6H4
4-CH3C6H4 4i 63
4-CNC6H4
Ph
2-furyl
4j 47
4k 54
4l 71
2-BrC6H4
Ph
[a] Unless otherwise noted, all the reactions were performed by using al-
dehyde 1 (0.4 mmol), nitroalkene 2 (0.4 mmol), imine 3 (0.2 mmol), and
each organocatalyst (0.03 mmol) in dry toluene (0.4 mL) at 128C.
[b] Yield of the isolated product. [c] Determined by chiral phase HPLC
analysis. [d] Bn=benzyl. [e] ee was determined by first oxidising to the
corresponding lactam.
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3923

P.-F. Xu et al.
in the vast majority of cases (11 out of 12 examples in
Table 2), complete enantiocontrol (ee>99%) was achieved.
The absolute, and relative, configuration of the cascade-
product hemiaminal 4k was established by single-crystal X-
ray crystallography and analysis of NMR data (see Support-
ing Information for details).[16]
excellent enantiocontrol, atom economy, and the products
are valuable for numerous synthetic applications. The possi-
ble anticancer activity of the synthesized piperidines, the
catalytic activity of 8, and the mechanistic course of the re-
action are currently under investigation and will be reported
in due course.
To illustrate the value of this triple cascade reaction, we
devised several useful transformations of product 4c
(Scheme 2). Partial reduction of piperidine 4c with nickel
Experimental Section
General procedure for the triple cascade synthesis of piperidines: Freshly
distilled aldehyde 1 (0.4 mmol) and imine 3 (0.2 mmol) were added to a
solution of nitroalkene 2 (0.4 mmol) and organocatalysts 5a (0.03 mmol,
11.0 mg, 15 mol%) and 6b (0.03 mmol, 12.0 mg, 15 mol%) in dry toluene
(0.4 mL) at 128C. After stirring for 28–54 h, the reaction was completed.
A portion of the desired product, 4, sometimes precipitated and Et2O
(0.5 mL) was added to the reaction mixture. Any precipitated product
was then isolated by filtration, the filtrate concentrated and the residue
purified by column chromatography to afford the remaining portion of
the desired product, 4. In cases in which no precipitation occurred, the
reaction mixture was directly purified by flash column chromatography
to afford the products.
Scheme 2. Synthetic transformations of hemiaminal 4c. a) NiCl2·6H2O,
NaBH4, MeOH, 70%; b) NaBH4, EtOH; c) Fe, AcOH, 32% over two
steps; d) Ac2O, pyridine, CH2Cl2, 94%; e) NiCl2·6H2O, NaBH4, MeOH,
56%; f) CF3COOH, CH2Cl2, 73%
Acknowledgements
We are grateful for the funding of the National Basic Research Program
of China (No. 2010CB833200), the Foundation of NSFC (20772051,
20972058), and the “111” program from MOE of P. R. China. We would
like to thank Syngenta for awarding a PhD studentship to Y.W.
boride afforded the corresponding nitrone 7, the structure
of which was unambiguously established by single-crystal X-
ray diffraction[16] (see Supporting Information for details), in
good yield. Nitrones are reliable intermediates with a high
value in a plethora of synthetic applications[17] and our ap-
proach provides rapid access to them. An two-step reduc-
tion of nitrone 7 then provided a route for the preparation
of substituted pyrrolidine 8. Pyrrolidines are important
building blocks with attractive chemical and biological prop-
erties and this short route provides many opportunities. Im-
portantly, these substituted pyrrolidines could be potential
organocatalysts.[9c] Typical routes to single-enantiomer pyr-
rolidine organocatalysts rely on the manipulation of chiral-
pool starting materials. After optimization, our route could
provide a method for the discovery of new and effective pyr-
rolidine organocatalysts. In a further demonstration of the
synthetic utility of the cascade products, the reduction of pi-
peridine 4c to the corresponding 3-amino derivative 9 was
also achieved. Elimination of MeOH from 9 to form the val-
uable tetrahydropyridine derivative 10 was also facile. 3-
Amino piperidine alkaloids are found extensively in nature
and possess a wide range of biological activities;[1] thus, our
route provides rapid access to a potentially diverse set of
such compounds.
Keywords: asymmetric catalysis
·
domino reactions
·
enantioselectivity · organocatalysis · piperidines
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Fully Substituted Piperidines
COMMUNICATION
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Received: January 26, 2010
Published online: March 5, 2010
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