Catalytic asymmetric reductive animation of α-branched ketones

Article abstract of DOI:10.1002/anie.201001715

Chemical equcation Presented Branching out: An organocatalytic reductive amination of -αbranched ketones using dynamic kinetic resolution is reported. The e/s-2-substituted cyclohexyl amines were obtained in high diastereoselectivity and enantioselectivit

Full text of DOI:10.1002/anie.201001715

Communications
DOI: 10.1002/anie.201001715
Organocatalysis
Catalytic Asymmetric Reductive Amination of a-Branched Ketones**
Vijay N. Wakchaure, Jian Zhou, Sebastian Hoffmann, and Benjamin List*
The reductive amination of carbonyl compounds is a powerful
and versatile method for the creation of carbon–nitrogen
bonds. However, despite its importance in both, medicinal
chemistry and process chemistry, only a few asymmetric
versions exist.[1] Recently, we have developed a chiral acid-
catalyzed reductive amination of ketones using Hantzsch
esters as a hydride source and 3,3’-bis(2,4,6-triisopropyl-
phenyl)-1,1’-binaphthyl-2,2’-diyl hydrogen phosphate (TRIP)
as a Brønsted acid organocatalyst.[2–4] The groups of Rue-
ping[3a] and MacMillan[3b] have both independently developed
alternative variants and additional applications of this meth-
odology appeared subsequently.[3,5] However, asymmetric
reductive amination reactions of racemic ketones by dynamic
kinetic resolution (DKR) have not yet been reported.
The 2-substituted cyclohexylamine pharmacophore is a
privileged and frequently occurring motif in drug design and,
in addition to the ACE-inhibitor perindorpil, there are
hundreds of reported pharmacologically active compounds
that incorporate this subunit (Scheme 1).[6,7] We hypothesized
that reductive amination of the corresponding a-branched
cyclohexanones should provide a powerful approach towards
such enantiopure cyclohexylamines using dynamic kinetic
Scheme 1. The direct reductive amination of racemic ketones by
resolution. Herein, we show that this concept is indeed
possible and report, to the best of our knowledge, the first
example of the catalytic asymmetric reductive amination of
racemic ketones using DKR.
Despite the enormous advances in the asymmetric
reduction of configurationally labile carbonyl compounds by
DKR,[8] the analogous reductive amination reactions are
underexplored. In 2005, Lassaletta and co-workers reported
the first example of a transition-metal-catalyzed asymmetric
transfer-hydrogenation of a-branched ketimines.[9] Most
dynamic kinetic resolution leads to biologically active amines.
Accordingly, treating cyclohexanones 1 with para-anisi-
dine (2, PMP-NH2, 1.1 equiv), Hantzsch ester 3 (1.4 equiv),
and only 1 mol% of TRIP readily provided the desired
products 4. Remarkably, with this combination of commer-
cially available reagents and catalyst, a broad array of ketones
1 could be converted into their corresponding cyclohexyl-
amines (4) in good yields and diastereoselectivities, and high
enantioselectivities.
Although the previously developed unsymmetrical
Hantzsch ester 5 afforded slightly higher enantioselectivity
for the reaction of 2-methylcyclohexanone (rac-1a; Table 1,
entry 1 vs 2), for practical reasons we used its inexpensive
analogue 3 throughout our studies. The substrate scope of the
reaction is summarized in Table 1. An important feature of
our process is its tolerance of a variety of different substitu-
ents whilst maintaining excellent enantioselectivity. Simple
alkyl-substituted substrates are particularly reactive, requir-
ing only a very low amount of catalyst (1 mol%; Table 1,
entries 1–5). With sterically more-demanding substrates
(Table 1, entries 6 and 7), as well as with aromatic substrates
(Table 1, entries 8–10), slightly higher catalyst loadings were
used. Whilst diastereoselectivities varied from reasonably
good to excellent, the enantioselectivities were generally very
high. The relative and absolute configuration of product 4a
was determined to be 1R,2S by deprotection using H5IO6 to
give known cis-2-methyl cyclohexyl amine (see the Support-
ing Information).[11]
ˇ
´
recently, Kocovsky and co-workers reported the synthesis of
b-amino acids using a catalytic trichlorosilane reduction of
enamines, suggesting a DKR mechanism.[10] However, the
direct reductive amination of racemic a-branched ketones
have remained unknown; we have now systematically devel-
oped a powerful and general procedure for this transforma-
tion (Table 1).
[*] Dr. V. N. Wakchaure, Dr. J. Zhou, Dr. S. Hoffmann, Prof. Dr. B. List
Max-Planck-Institut fꢀr Kohlenforschung
Kaiser Wilhelm-Platz 1, 45470 Mꢀlheim an der Ruhr (Germany)
Fax: (+49)208-306-2982
E-mail: list@mpi-muelheim.mpg.de
[**] Generous support by the Max Planck Society, the DFG (SPP 1179,
Organocatalysis), and the Fonds der Chemischen Industrie, is
gratefully acknowledged. We also thank Wacker and Sanofi-Aventis
for support.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201001715.
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Table 1: Reductive amination of a-substituted ketones.[a]
excellent selectivity [Eq. (3)]. As expected, when only
1 equivalent of Hantzsch ester was used, 2-phenylcyclohex-
anone 1g was obtained with complete conversion and poor
enantioselectivity.
Entry
R
Product
Yield [%]
d.r.[b]
5:1
e.r.[c]
1
Me
Me
Et
4a
4a
4b
4c
4d
82
76
87
91
80
93:7
95:5
94:6
95:5
95:5
2[d]
3
5:1
6:1
5:1
8:1
4
5
nPr
6[e]
4e
63
>99:1
98:2
As a further illustration of the synthetic utility of this
transformation, a short synthesis of lactam 10 was developed
[Eq. (4)]. This compound is the key intermediate in the
synthesis of Coversyl (perindopril), a long-acting ACE
inhibitor.[6] Reductive amination of ketone 1k, followed by
an in situ base-mediated cyclization, afforded lactam 9 in
92% yield and a 5:1 cis/trans ratio. The desired isomer cis-9
was isolated in 78% yield with a 97:3 e.r. Oxidative removal
of the PMP group then provided known lactam 10 in 72%
yield. The conversion of lactam 10 into perindopril has
already been established in the patent literature.[6]
7[e]
8[f]
Bn
Ph
4 f
4g
92
63
6:1
16:1
96:4
94:6
9[e]
4h
73
6:1
94:6
10[e,g]
4i
81
17:1
96:4
[a] 0.5 mmol scale. [b] Determined by GC or NMR analysis. [c] The e.r. of
the cis product determined by HPLC analysis on a chiral stationary phase.
[d] Dimethyl 2-isopropyl-6-methyl-1,4-dihydropyridine-3,5-dicarboxylate
(5) was used. [e] 5 mol% catalyst. [f] 10 mol% catalyst. [g] 3 (3 equiv-
alents) was used.
Our reaction is not limited to alkyl- or aryl-substituted
cyclohexanones, even chlorine is tolerated in the a position.
Upon subjecting 2-chlorocyclohexanone (1j) to our standard
reaction conditions, 2-chlorocyclohexyl amine 4j was
obtained in excellent yields and stereoselectivities [Eq. (1)].
We also studied other ring sizes and found that racemic 2-
butylcyclopentanone (6) furnished the corresponding amine 7
in good yield, but somewhat lower stereoselectivities
[Eq. (2)]. Surprisingly, the corresponding cycloheptanone
did not undergo reductive amination under these conditions.
Remarkably though, by employing 2.4 equivalents of the
Hantzsch ester, even a,b-unsaturated, a-branched ketone 8
could be converted into product 4g in reasonable yields and
In conclusion, we have developed a catalytic asymmetric
reductive amination of a-branched ketones using dynamic
kinetic resolution. Our new reaction provides an efficient
diastereoselective and enantioselective synthesis of valuable
cis-2-substituted cyclohexylamines, as illustrated with a syn-
thesis of a key pharmaceutical intermediate.
Currently, substituted cyclohexanones are ideal substrates
for our reaction, and both aromatic and aliphatic substituents
give products with highly stereoselectivity. We expect to
further expand the substrate scope in ongoing studies in our
laboratory.
Received: March 22, 2010
Published online: May 20, 2010
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Communications
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Keywords: asymmetric catalysis · chiral amines · ketones ·
organocatalysis · reductive amination
.
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