Asymmetric mannich reaction of fluorinated ketoesters with a tryptophan-derived bifunctional thiourea catalyst

Full text of DOI:10.1002/anie.200903635

Communications
DOI: 10.1002/anie.200903635
Asymmetric Catalysis
Asymmetric Mannich Reaction of Fluorinated Ketoesters with a
Tryptophan-Derived Bifunctional Thiourea Catalyst**
Xiao Han, Jacek Kwiatkowski, Feng Xue, Kuo-Wei Huang,* and Yixin Lu*
In memory of George Just
Small organic molecules capable of hydrogen-bonding inter-
actions with substrates have found widespread application in
asymmetric catalysis.[1] In particular, thiourea-based organic
molecules have become the most prominent hydrogen-bond-
donor catalysts in a wide variety of organic reactions. In this
context, bifunctional[2] organic molecules containing a tertiary
amino functionality and a thiourea moiety are remarkably
useful organic catalysts.[3] Despite their tremendous utility,
Scheme 1. Thiourea catalyst based on a primary amino acid (trypto-
phan).
these bifunctional catalysts are derived from a very limited
range of chiral structural scaffolds, including cyclohexane-1,2-
diamine, 1,1’-binaphthyl-2,2’-diamine, and cinchona alkaloids.
The development of readily accessible novel bifunctional
catalysts of this nature would be highly desirable. As part of
our research program towards the development of practical
organocatalysts based on primary amino acids,[4] we were
intrigued by the possibility of designing novel tertiary amine–
thiourea catalysts on the basis of simple amino acids. The
facile conversion of natural amino acids into 1,2-diamines and
the availability of structurally diverse side chains make this
method very attractive. To investigate the validity of this
approach, we selected l-tryptophan as the chiral precursor.
We reasoned that the indole moiety would be capable of
engaging in aromatic and hydrogen-bonding interactions with
substrates, and these effects may result in efficient chiral
induction (Scheme 1).
metal catalysis have been reported;[6] however, organocata-
lytic approaches for the creation of fluorinated quaternary
centers are rather limited.[7] Recently, organocatalytic syn-
thetic methods with fluorinated substrates have become an
alternative and viable option for accessing chiral fluorinated
molecules. In such approaches, racemic fluorinated nucleo-
À
philes are used as substrates. A C C bond is formed rather
À
than a C F bond, and full advantage is taken of the high
electronegativity and small molecular radius of the fluorine
atom. We and others[8] have used fluorinated substrates in this
way in organocatalytic Michael and alkylation reactions for
the construction of fluorinated chiral molecules.
To assess the utility of tryptophan-based bifunctional
catalysts, we chose to focus on the direct asymmetric Mannich
reaction of a-fluorinated b-ketoesters, as such reactions yield
structurally demanding and biologically important a-fluoro-
b-amino acids. Organocatalytic asymmetric Mannich reac-
tions of b-ketoesters and malonates were reported recently by
the research groups of Schaus, Deng, and Dixon, all of whom
employed organic catalysts derived from cinchona alkaloids.[9]
Herein, we report that tryptophan-based bifunctional thio-
urea derivatives promote the asymmetric Mannich reaction of
fluorinated substrates to afford highly optically enriched
fluorine-containing molecules containing adjacent quaternary
and tertiary stereocenters.
Fluorinated molecules are of high importance in the
pharmaceutical industry, and their asymmetric preparation
has drawn great attention.[5] The catalytic construction of
fluorinated quaternary carbon stereocenters is a formidable
synthetic challenge. A number of excellent methods based on
[*] X. Han, J. Kwiatkowski, Prof. Dr. F. Xue, Prof. Dr. K.-W. Huang,
Prof. Dr. Y. Lu
Department of Chemistry, National University of Singapore
3 Science Drive 3, Singapore 117543 (Singapore)
Fax: (+65)6779-1691
E-mail: hkw@nus.edu.sg
chmlyx@nus.edu.sg
We selected the Mannich reaction of a-fluoro-b-ketoester
1a with N-Boc imine 2a[10] as a model reaction and examined
the catalytic effects of various bifunctional catalysts (Table 1).
Quinidine-derived thioureas and a quinidine-derived sulfo-
namide[11] gave disappointing results (Table 1, entries 1–3).
On the other hand, the tryptophan-based thiourea derivatives
Trp-1–Trp-3 were found to be good catalysts. They afforded
the Mannich product 3a in quantitative yield and with good
diastereoselectivity and enantioselectivity (Table 1, entries 4–
6). Under optimized reaction conditions, the fluorinated
product containing adjacent quaternary and tertiary stereo-
Prof. Dr. Y. Lu
Medicinal Chemistry Program, Life Sciences Institute
National University of Singapore (Singapore)
J. Kwiatkowski, Prof. Dr. Y. Lu
NUS Graduate School for Integrative Sciences and
Engineering (Singapore)
[**] We thank the National University of Singapore and the Ministry of
Education (MOE) of Singapore (R-143-000-362-112) for generous
financial support.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200903635.
7604
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Table 1: Screening of bifunctional catalysts for the asymmetric Mannich
reaction of 1a and 2a.[a]
Table 2: Scope of the Mannich reaction catalyzed by tryptophan-based
catalyst Trp-1.[a]
Entry
R/R’
t [h] Yield
d.r.
ee
Entry
Catalyst
T
[8C]
Solvent
Conversion
[%]
d.r.
ee
[%][b]
[%][c]
[%][d]
[%][b]
[%][c]
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15[e]
16[e]
17[e]
18[f]
19[g]
20[g]
p-MeC6H4/p-MeC6H4 (3b)
p-FC6H4/p-MeC6H4 (3c)
p-ClC6H4/p-MeC6H4 (3d)
p-BrC6H4/p-MeC6H4 (3e)
p-NO2C6H4/p-MeC6H4 (3 f)
thiophenyl/p-MeC6H4 (3g)
2-naphthyl/p-MeC6H4 (3h)
Ph/p-BrC6H4 (3i)
72
48
24
20
20
24
24
20
20
20
20
36
20
20
24
36
24
24
20
20
92
96
95
92
93
95
93
92
96
95
92
95
92
92
90
93
95
70
92
93
12:1
9:1
8:1
7:1
8:1
4:1
5:1
8:1
7:1
10:1
8:1
11:1
6:1
8:1
1:1
1:1
>19:1
4:1
97
96
95
95
92
95
96
96
95
97
95
92
97
99
90/93
96/97
84
1
2
3
4
5
6
7
8
9
QD-1
QD-2
QD-3
Trp-1
Trp-2
Trp-3
Trp-1
Trp-1
Trp-1
Trp-1
Trp-1
RT
RT
RT
RT
RT
RT
RT
RT
toluene
toluene
toluene
toluene
toluene
toluene
CH2Cl2
THF
>99
>99
<20
>99
>99
>99
>99
>99
>99
>99
>99
4:1
3:1
–
À43
À4
–
5:1
4:1
4:1
4:1
3:1
5:1
8:1
9:1
88
59
67
80
15
53
93
97
p-ClC6H4/p-BrC6H4 (3j)
Ph/p-FC6H4 (3k)
RT
CH3CN
toluene
toluene
10[d]
11[e]
À20
p-BrC6H4/p-FC6H4 (3l)
Ph/o-MeC6H4 (3m)
Ph/p-CF3C6H4 (3n)
Ph/m-NO2C6H4 (3o)
Me/p-MeC6H4 (3p)
iPr/p-FC6H4 (3q)
tBu/p-MeC6H4 (3r)
Ph/c-C6H11 (3s)
Ph/c-C6H11 (3t)
À50
[a] Reactions were carried out with 1a (0.05 mmol), 2a (0.075 mmol),
and the catalyst (0.005 mmol) in the solvent (1 mL) at the specified
temperature. The conversion was estimated by H NMR spectroscopic
analysis of the crude product. [b] The diastereomeric ratio was
determined by 1H NMR spectroscopic analysis of the crude product.
[c] The ee value of the major diastereomer is given, as determined by
HPLC analysis on a chiral stationary phase. [d] The reaction time was
24 h. [e] The reaction time was 48 h. Boc=tert-butoxycarbonyl, THF=
tetrahydrofuran, Ts=p-toluenesulfonyl.
1
85
96
81
3:1
3:1
Ph/nBu (3u)
[a] Reactions were performed with the ketoester (0.05 mmol), the imine
(0.075 mmol), and Trp-1 (0.005 mmol) in toluene (1 mL). [b] Yield of the
isolated product. [c] The diastereomeric ratio was determined by
1H NMR spectroscopic analysis of the crude product. [d] Single ee values
are for the major diastereomer. The ee values were determined by HPLC
analysis on a chiral phase. [e] The benzyl ester was employed instead of
the ethyl ester. [f] The reaction was carried out at room temperature with
100 mol% of Trp-1. [g] The N-tosylimine was used.
the best reported for this type of reaction. In their study, high
catalyst loading (100 mol%) was required for good enantio-
selectivity, and the yields were only moderate. By employing
the N-Boc derivative of an alkyl imine and increasing the
catalyst loading of Trp-1, we were able to obtain the desired
Mannich product in good yield and with high enantioselec-
tivity (Table 2, entry 18). Gratifyingly, when N-tosylcyclohex-
ylmethanimine was used, the Mannich reaction proceeded
very efficiently in the presence of Trp-1 (10 mol%) to afford
the desired fluorinated Mannich product with 96% ee
(Table 2, entry 19).
centers was obtained with d.r. 9:1 and 97% ee (Table 1,
entry 11).
Next, we investigated the scope of the reaction. Consis-
tently good diastereoselectivity and high enantioselectivity
were observed with a wide range of aromatic a-fluorinated
b-ketoesters (Table 2, entries 1–14). In the case of aliphatic
ketoester substrates, although the reactions proceeded with
little diastereoselectivity, the diastereomers were formed with
excellent enantioselectivity (Table 2, entries 15 and 16).
However, the use of the tert-butyl ketoester resulted in high
diastereoselectivity and good enantioselectivity (Table 2,
entry 17).
Tryptophan-based Trp-1 is a versatile catalyst for the
Mannich reaction of a variety of 1,3-dicarbonyl substrates.
The reactions are not limited to fluorinated substrates;
nonfluorinated and chlorinated ketoesters also proved to be
suitable substrates. Excellent enantioselectivity was also
observed when malonates were used (Scheme 2).
We carried out density functional theory calculations to
elucidate the stereochemical outcome of this novel Mannich
reaction.[12] Our preliminary efforts were focused on the
identification of the structure of the pre-transition-state
complex. Complex IMa (for the formation of 3a) was located
The Mannich reaction of alkyl imines is a daunting
synthetic challenge. The results of Deng and co-workers[9b] are
as the most plausible intermediate. With a C C bond distance
À
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Figure 1. Intermediate IMa formed from 1a, 2a, and Trp-1. Hydrogen-
bond distances are given in ꢀ (non-hydrogen-bonded hydrogen atoms
were omitted for clarity).
reaction mechanism, the preparation of diverse bifunctional
catalysts based on primary amines, and applications of these
catalysts to various organic reactions are currently ongoing in
our laboratories.
Scheme 2. Mannich reaction of other ketoesters and malonates.
Bn=benzyl.
of 3.582 ꢀ, it is ready to undergo the
bond-forming step (Figure 1). The dieth-
ylamino group of Trp-1 could first
deprotonate 1a to yield an ammonium
group. The indole group was found to
assist the thiourea moiety in binding the
resulting ketoenolate through an addi-
À
tional N H···O hydrogen-bonding inter-
action. The ammonium group could
later direct and bind the incoming
imine to bring it into proximity with
the ketoenolate in a locked conforma-
tion.
Given the importance of b-amino
acids and b-lactams[13] in biological sci-
ences and medicinal chemistry, the syn-
Scheme 3. Preparation of the a-fluorinated b-amino acid 6 and b-lactam 8. DCC=N,N’-
dicyclohexylcarbodiimide, DMAP=4-dimethylaminopyridine, TFA=trifluoroacetic acid.
thesis of a-fluorinated analogues of these compounds is of
great interest. As an illustration of the synthetic applicability
of our methodology, the Mannich product 3a was converted
into a-fluoro-b-amino acid 6,
a-fluoro-b-lactam 8, and a-fluoro-b-lactone 9 in good yields
(Scheme 3). We were also able to obtain the X-ray crystal
structure of lactam 8 (Figure 2).[14]
In summary, we have introduced a novel tryptophan-
based bifunctional thiourea catalyst that was remarkably
effective in promoting the asymmetric Mannich reaction of a-
fluoro-b-ketoesters. The resulting compounds with fluori-
nated quaternary and tertiary stereocenters can be converted
readily into a-fluoro-b-amino acids and a-fluoro-b-lactams.
Preliminary computational studies suggest that the indole
moiety of the catalyst plays a crucial role in substrate binding.
We have shown that tertiary amine–thiourea bifunctional
catalysts can be derived readily from natural amino acids; this
strategy may lead to the discovery of various novel multi-
functional organic catalysts. Further investigations into the
Figure 2. ORTEP structure of lactam 8.
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[14] CCDC 745901 (8) 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.
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