Highly enantioselective synthesis of α-amino acid derivatives by an NHC-catalyzed intermolecular stetter reaction

Article abstract of DOI:10.1002/anie.201006548

PROTONtype: An NHC-catalyzed (NHC=N-heterocyclic carbene), highly asymmetric intermolecular Stetter reaction allows the atom-economic and efficient formation of valuable α-amino acid derivatives (see scheme); the key step is an intramolecular stereoselective protonation. For the first time, a β-unsubstituted Michael acceptor was employed successfully in this kind of reaction, most importantly, with a broad range of different aldehydes.

Full text of DOI:10.1002/anie.201006548

Communications
DOI: 10.1002/anie.201006548
Asymmetric organocatalysis
Highly Enantioselective Synthesis of a-Amino Acid Derivatives by an
NHC-Catalyzed Intermolecular Stetter Reaction**
Thierry Jousseaume, Nathalie E. Wurz, and Frank Glorius*
Dedicated to Professor Ronald Breslow on the occasion of his 80th birthday
a-Amino acids are one of the most important classes of
compounds in nature and synthetic chemistry, and, conse-
quently, many approaches have been developed for the
synthesis of enantioenriched a-amino acids.[1] Among these
synthetic routes, a particularly versatile and challenging
method to set up the chirality at the a position of a-amino
acids is the formation of a transient enolate through a Michael
addition followed by a stereoselective protonation. Enantio-
selective protonation is important in many biosynthetic
sequences, and the development of powerful catalytic pro-
cesses remains an ongoing challenge.[2,3] Many different
approaches for catalyzed enantioselective protonations have
been realized; however, the combination of a conjugate
addition with the asymmetric protonation of a transiently
formed enolate stands out as being especially efficient and
atom economic.[4–6] In the context of a-amino acid synthesis,
this strategy was first explored by Pracejus et al. in 1977 for
the synthesis of cysteine derivatives with moderate enantio-
selectivities (up to 54% ee) by using methyl 2-phthalimidoa-
crylate as the Michael acceptor and cinchona alkaloids as the
chiral catalysts.[7] More recently, asymmetric approaches
based on the use of organocatalysts[4c] or metal-based
catalysts[6] have shown improved selectivities, but the scope
has remained limited.
the research groups of Enders[11] and Rovis,[12] and very
recently an enzyme-catalyzed variant was reported by Mꢀller
and co-workers[13] (Scheme 1). However, selectivities are not
Scheme 1. Chiral NHC-[11,12] or enzyme-catalyzed[13] intermolecular
asymmetric Stetter reactions. EWG=electron-withdrawing group,
Ar=aromatic group, Het=heteroaromatic group, R=rest.
yet optimal and the substrate scope seems to be very
limited—a general system remains to be found. In addition,
a stereocenter is formed in the b position in each of these
cases. An asymmetric Stetter reaction that builds up only
a stereocenters seems to be even more challenging. The key
to success might be a highly stereoselective intramolecular
proton transfer that relays the stereochemical information
after the initial enantioselective attack of the Breslow
intermediate on the Michael acceptor (Scheme 2). Herein
we report a highly asymmetric intermolecular Stetter reac-
The cyanide or N-heterocyclic carbene (NHC) catalyzed[8]
addition of an aldehyde to a Michael acceptor, the Stetter
reaction,[8,9] is a versatile synthetic transformation. Whereas
the intramolecular asymmetric Stetter reaction has been
investigated extensively by the research groups of Enders,
Rovis, and others,[8,10] the more versatile intermolecular
asymmetric version has proven to be much more challenging.
In the last two years, the first moderately to highly enantio-
selective intermolecular Stetter reactions were reported by
[*] Dr. T. Jousseaume, N. E. Wurz, Prof. Dr. F. Glorius
Universitꢀt Mꢁnster, Organisch-Chemisches Institut
Corrensstrasse 40, 48149 Mꢁnster (Germany)
Fax: (+49)251-833-3202
E-mail: glorius@uni-muenster.de
Homepage: http://www.uni-muenster.de/Chemie.oc/glorius/
index.html
[**] We thank the Deutsche Telekom Stiftung (N.E.W.) for generous
support, Isabel Piel for the preparation of catalyst 3 and for helpful
comments, and Karin Gottschalk for technical assistance. The
research of F.G. has been supported by the Alfried Krupp Prize for
Young University Teachers of the Alfried Krupp von Bohlen und
Halbach Foundation. NHC=N-heterocyclic carbene.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201006548.
Scheme 2. Proposed reaction pathway for an intermolecular enantiose-
lective Stetter reaction.
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tion, starting from two rather simple starting materials
(aldehydes and N-acylamido acrylate) and resulting in the
formation of valuable highly enantioenriched a-amino acid
derivatives [Eq. (1)].
we observed that the use of two equivalents of the Michael
acceptor had a positive effect on conversion (Table 1,
entries 6 and 7). Switching to stronger bases such as
KHMDS and KOtBu restored the enantioselectivity to
95%, and the conversion increased substantially (67% and
65%; Table 1, entries 8 and 9, respectively).[16] We surmise
that these bases are strong enough to be fully protonated by
the triazolium salt, so that free base, which might racemize the
product, is no longer present in the reaction mixture. Low-
ering the reaction temperature to 08C led to full conversion
and high levels of enantioinduction after only 4 h (Table 1,
entry 10). Clearly, racemization is not a problem under these
mild reaction conditions, even with a prolonged reaction time
(Table 1, entry 11).
Two equivalents of 2 were essential for full conversion
under these conditions (Table 1, entry 12). Nevertheless, the
catalyst and base loadings could be further reduced without
affecting the yield or enantioselectivity; this led to the
optimized reaction conditions (Table 1, entry 13; highlighted
in bold). The reaction has also been applied successfully on a
gram scale (5 mmol) under these optimized conditions to
yield enantiomerically enriched a-amino acid derivative 4a in
excellent yield and stereoinduction (Table 1, entry 13). How-
ever, lowering the amount of catalyst significantly below
10 mol% resulted in an incomplete reaction (not shown).
Furthermore, we tried other Michael acceptors in the
reaction to evaluate their reactivity. Other amino protecting
groups such as tert-butoxycarbonyl (Boc) or phthalimido
failed to provide the desired product. In addition, the N-H
group of the amide seems to be crucial for the reactivity, since
the tertiary N-methylated variant of Michael acceptor 2 did
not react.[17] Similarly, the b-substituted acrylate (Z)-methyl-
acetamido cinnamate (MAC) also did not react under our
standard conditions.
In the course of our research on NHC organocatalysis[14]
we decided to explore an enantioselective Stetter reaction in
which a stereoselective protonation was the key step by using
aromatic or aliphatic aldehydes and a dehydroamino ester as
the Michael acceptor. Our study commenced with the
observation that the reaction between aldehyde 1a and
Michael acceptor
2 was catalyzed by chiral NHC 3
(Table 1).[15] The Stetter product 4a was obtained with an
excellent enantioselectivity of 97% but with a poor yield of
only 10% (Table 1, entry 1). An extensive screening of the
reaction conditions revealed several crucial parameters. We
first showed that the use of TBD as the base and toluene as a
solvent gave an improved yield compared to dioxane or THF
(Table 1, entries 2–4); however, the enantioselectivity
decreased to 84%. Similar results were obtained with DBU
or K2CO3 as the base (Table 1, entries 5 and 6). Additionally,
Table 1: Optimization of the reaction conditions.[a]
Thereafter, we studied the scope and the generality of this
reaction. First, we screened aromatic aldehydes bearing an
electron-withdrawing group (Scheme 3). Methyloxycarbonyl
(4b), trifluoromethyl (4c), and cyano (4d) groups were
compatible with the reaction conditions. In all cases, the
reaction led to the desired products in good yields and with
excellent ee values over 90%. It is noteworthy that halides
such as bromide in position 4 or 3 (products 4e and 4 f,
respectively) or chloride (product 4g) were also tolerated,
and the corresponding products could undergo further
functionalization by cross-coupling reactions for the con-
struction of more elaborate molecules. Challenging aldehydes
with a substituent in the ortho position were then evaluated.
With 2-fluorobenzaldehyde, we obtained the desired product
4h with an exceptional enantioselectivity (99% ee). More
sterically hindered aldehydes such as 2-methylbenzaldehyde
or 2-chlorobenzaldehyde failed to participate in the reaction
(not shown). Notably, heteroaromatic aldehydes such as
furfuraldehyde were also successful in yielding the expected
product 4i with high stereocontrol (98% ee). The highly
reactive 2-naphthalenecarboxaldehyde can also participate in
the reaction to give the desired product in excellent yield and
selectivity. As often reported, electron-rich aromatic alde-
hydes are known to be less reactive in NHC-catalyzed
processes. For example, attempts to carry out the reaction
Entry
Base
Solvent
T
[8C]
t
[h]
Yield
[%][b]
ee
[%]
1[c]
KHMDS
TBD
TBD
TBD
DBU
K2CO3
K2CO3
KHMDS
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
dioxane
dioxane
THF
25
25
25
25
25
25
25
25
25
0
24
24
24
24
24
24
24
24
24
4
(10)
22
17
41
49
23
31
67
65
97
2[d]
3[d]
4[d]
5[d]
6[d]
7
n.d.
n.d.
84
85
n.d.
90
95
95
95
95
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
8[e]
9[e]
10[e]
11[e]
12[e,f]
13[g]
99 (98)
99
72
0
0
0
65
24
3
n.d.
95
95 (93)
[a] General reaction conditions: 1a (0.2 mmol, 1.0 equiv), 2 (0.4 mmol,
2.0 equiv), NHC·HCl 3 (10 mol%), base (10 mol%), 0.67 mL solvent
(0.3m). [b] Yield determined by 1H NMR spectroscopy using 1,3,5-
trimethoxybenzene as an internal standard. Yields of isolated products
are given in parentheses. [c] 1a (0.4 mmol, 2.0 equiv), 2 (0.2 mmol,
1.0 equiv). [d] 2 (0.22 mmol, 1.1 equiv). [e] NHC·HCl 3 (15 mol%). [f] 2
(0.3 mmol, 1.5 equiv). [g] Base (8 mol%). On a 5 mmol scale, 95% yield
and 94% ee were obtained. n.d.=not determined. DBU=1,8-
diazabicyclo[5.4.0]undec-7-ene, KHMDS=potassium hexamethyldisil-
azide, TBD=1,5,7-Triazabicyclo[4.4.0]dec-5-ene.
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enine 3-hydroxylase inhibitor FCE28833 6g.[19]
Simple deprotection under standard conditions gave
the hydrochloride salt 6g quantitatively and without
apparent racemization [Eq. (2)].[17]
The mechanism and the asymmetric induction of
this transformation can be rationalized as follows.
First, the reaction between the free carbene derived
from 3 and the aldehyde 1 leads to the formation of a
nucleophilic enamine, the Breslow intermediate A
(Scheme 4).[20] It can be assumed on the basis of
computational calculations performed by Dudding
and Houk[21] that the shown E isomer of A is more
favorable than the Z isomer (not shown). The benzyl
group in intermediate A successfully shields the top
face of the Breslow intermediate. Thus, the Michael
acceptor 2 approaches from the bottom face in an anti
fashion, most likely supported by a hydrogen bond
between the enol hydrogen atom and the carbonyl
oxygen atom of the Michael acceptor (B). In this
process, enolate C bearing a new but transient
stereocenter is formed highly stereoselectively. The
configuration is relayed to the a position by a
stereoselective protonation of the transiently
formed enolate. Finally, the NHC is released, thereby
destroying the initially generated stereocenter and
forming the final product 4. However, C might not be
a local energy minimum of this reaction and a
concerted conjugate addition/protonation mecha-
nism[22,23] cannot be ruled out.
Scheme 3. Substrate scope of the enantioselective Stetter reaction. General
reaction conditions: 1 (0.5 mmol), 2 (1.0 mmol), NHC·HCl 3 (10 mol%),
KOtBu (8 mol%) in toluene (1.8 mL) at 08C for 3–24 h. [a] Using NHC·HCl 3
(20 mol%) and KOtBu (16 mol%).
In conclusion, we have developed an NHC-
catalyzed enantioselective Stetter reaction with a
highly stereoselective proton transfer as the key step
starting from 4-methoxybenzaldehyde have only led to a low
conversion (10% yield). Gratefully, however, the use
of 20 mol% 3 in the reaction with 4-methylbenzalde-
hyde provided 4l in moderate yield but excellent
enantioselectivity. In addition, 4-phenylbenzaldehyde
and ferrocenecarboxaldehyde also furnished the
desired products 4k and 4m in moderate yield but in
high enantioselectivity. Benzaldehyde was also suit-
able and led to the desired amino ester derivative 4n in
good conversion and high enantioselectivity. More-
over, an aliphatic aldehyde such as dodecanal is also a
good substrate, thus demonstrating the value of this
new synthetic method to make enantioenriched a-
amino acid derivatives. In all cases, the absolute
configuration of the major enantiomer can be assigned
as S by comparison with optical rotations described
previously.[17,18] To show the versatility of the products
Scheme 4. Proposed stepwise mechanism for the enantioselective Stetter reac-
4 we converted product 4g into the selective kynur- tion.
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that leads to amino acid derivatives. This reaction is attractive
for a number of reasons: rather simple and general starting
materials are highly stereoselectively coupled under mild
À
reaction conditions. Two valuable steps, C C bond formation
between the Breslow intermediate and the Michael acceptor
and an asymmetric protonation, are efficiently combined.
Further studies on the mechanism of this transformation and
the development of related reactions should be worthwhile.
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Experimental Section
General procedure: Dry KOtBu (4.5 mg, 0.04 mmol) and the
triazolium salt 3 (18.5 mg, 0.05 mmol) were added to a flame-dried
screw-capped test tube equipped with a magnetic stir bar in a glove
box. The mixture was dissolved in toluene (1.8 mL) under argon
outside the glovebox. The resulting reaction mixture was stirred at
258C for 30 min and then cooled to 08C. After 10 min, 4-chloroben-
zaldehyde (1a; 70.3 mg, 0.5 mmol) and the Michael acceptor 2
(143.2 mg, 1.0 mmol) were added successively. After 3 h (TLC
monitoring showed completion of the reaction), the mixture was
diluted with EtOAc (2 mL) and filtered through a pad of silica gel and
eluted with EtOAc (10 mL). Evaporation of the solvent followed by
purification by flash chromatography (40% EtOAc in pentane then
60%) afforded the corresponding amino ester 4a as a white solid
(131.6 mg, 93% yield, 95% ee).
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Received: October 18, 2010
Revised: November 19, 2010
Published online: January 10, 2011
Keywords: amino acids · asymmetric catalysis ·
.
N-heterocyclic carbene · organocatalysis · protonation
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[17] See the Supporting Information for details.
[18] Methyl ester 4n was converted into the corresponding ethyl
ester 5n without epimerization. The optical rotation data for this
ethyl ester was compared with literature data for the same
compound with known absolute configuration.[17]
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[22] For NHC-catalyzed hydroacylation reactions of unactivated
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