Bifunctional guanidine via an amino amide skeleton for asymmetric michael reactions of ss-ketoesters with nitroolefins: A concise synthesis of bicyclic β- Amino acids

Article abstract of DOI:10.1002/anie.200901337

Two activations are better than one: The chiral bifunctional guanidine 1, which features an amino amide backbone, catalyzes the asymmetric Michael addition of a range of substrates and gives products with excellent stereoselectivities. The method also all

Full text of DOI:10.1002/anie.200901337

Angewandte
Chemie
DOI: 10.1002/anie.200901337
Organocatalysis
Bifunctional Guanidine via an Amino Amide Skeleton for Asymmetric
Michael Reactions of b-Ketoesters with Nitroolefins: A Concise
Synthesis of Bicyclic b-Amino Acids**
Zhipeng Yu, Xiaohua Liu, Lin Zhou, Lili Lin, and Xiaoming Feng*
As a result of the significance of and ever-increasing interest
in guanidine in chemistry and biology, the development of
new synthetic strategies for the efficient construction of such
molecules is an important goal of research carried out in both
academic and industrial laboratories.[1] Chiral guanidine
catalysts share common characteristics, such as high pKa
values and dual hydrogen-bonding modes for the molecular
recognition of b-ketoester anions.[2] Michael addition of cyclic
b-ketoesters to nitroolefins is an efficient synthetic tool for
the construction of nitrogen-containing ketoesters with a
quaternary carbon stereocenter.[3,4] Transformation of the
corresponding adducts, such as reduction to g-amino acids or
oxidation to d-lactones, could yield a variety of useful
Scheme 1. Designation of the bifunctional catalysts evaluated.
Dipp=2,6-diisopropylphenyl, Ad=1-adamantyl, Cy=cyclohexyl.
synthetic intermediates.[4] Over the past few years, chiral
guanidine catalysts have been attractive targets in asymmetric
organocatalysis.[5,6] However, despite the development of
several excellent guanidine catalysts,[2,6] a-amino acids, which
are naturally abundant, have not been widely employed as a
chiral source for bifunctional guanidines,[5d,e,6c] moreover the
development of a facile synthetic method for the production
of guanidines is a challenge of great potential interest.
Therefore, a bifunctional guanidine featuring a chiral amino
amide backbone was designed to promote the asymmetric 1,4-
addition of b-ketoesters to nitroolefins with dual activation in
one molecule (Scheme 1).[3,6]
Rigid cyclic a-amino amides, such as those derived from
l-proline, l-pipecolic acid, or l-ramipril acid, are promising
candidates as the chiral backbone because they can offer a
series of sterically hindered amides simultaneously. Thus, the
practical synthesis of guanidine was achieved by addition of
the lithium amino amide to the carbodiimide to construct the
conjugated trinitrogen carbon plane (Scheme 2).[7] It was
discovered with X-ray diffraction analysis of a single crystal of
1 f[8] that the guanidine formed two H-bonds, one intra-
molecularly and one intermolecularly.
[*] Z. P. Yu, Dr. X. H. Liu, L. Zhou, Dr. L. L. Lin, Prof. Dr. X. M. Feng
Key Laboratory of Green Chemistry & Technology
Ministry of Education, College of Chemistry
Sichuan University, Chengdu 610064 (P.R. China)
Fax: (+86)28-8541-8249
E-mail: xmfeng@scu.edu.cn
[**] We appreciate the National Natural Science Foundation of China
(no. 20732003) and the Ministry of Education (no. 20070610019) for
financial support. We also thank Sichuan University Analytical &
Testing Center for NMR and X-ray diffraction analyses and the State
Key Laboratory of Biotherapy for HRMS analysis.
Scheme 2. Practical synthesis of the amino amide based bifunctional
guanidines[8] and an ORTEP representation of the structure of 1 f.
THF=tetrahydrofuran.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200901337.
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A series of chiral guanidines as organocatalysts were
Under the optimized conditions, various nitroolefin sub-
strates were investigated to afford a wide range of products 8
containing quaternary chiral centers with high ee values (83–
97% ee) and excellent diastereomeric ratios (99:1 d.r. in most
cases; Table 2). It is interesting to note that not only the
synthesized and evaluated in the Michael addition of
b-ketoesters to nitroolefins under mild conditions, a reaction
that affords adducts with a chiral quaternary carbon atom.
With regard to the amino amide backbones, the l-pipecolic
acid derivative 1 f was superior to the l-proline- and
l-ramipril-acid-derived catalysts 1a and 1h; 1 f gave the
desired product 8 in 95:5 d.r. with 87% ee (Table 1, entry 6
Table 2: Substrate scope for the asymmetric Michael reaction.[a]
Table 1: Optimization of the reaction conditions.[a]
Entry R4
x
Prod. Yield
[%][b]
syn/
ee
[mol%]
anti[c]
[%][c]
1
2
3
4
5
6
Ph
2
3
3
2
2
2
8a 98 (79)[d]
8b 98
8c 90
8d 99
8e 82
99:1 95 (>99)[d]
89:11 96
99:1 95
96:3 92
>99:1 95
98:2 94
2-MeC6H4
3-MeC6H4
4-MeC6H4
3-MeOC6H4
4-MeOC6H4
Enrty
Cat.
x
R3
Solvent
Yield
[%][b]
syn/
ee
[mol%]
anti[c]
[%][c]
1
2
3
4
5
6
7
8
1a
1b
1c
1d
1e
1 f
1g
1h
1i
1 f
1g
1 f
1 f
1 f
1 f
1 f
1 f
1 f
1 f
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
2
2
Et
Et
Et
Et
Et
Et
Et
Et
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
74
60
95
99
99
98
98
84
91
97
80
96
94
67
81
58
92
96
98
78:22
87:13
88:12
92:8
91:9
95:5
95:5
92:8
89:11
96:4
98:2
97:3
98:2
97:3
98:2
94:6
98:2
98:2
99:1
7
8 f 80
53
60
68
61
87
86
40
50
90
92
90
93
93
89
73
91
94
95
7
8
5
5
8g 75[e,f]
>99:1 93
>99:1 95
8h 76[e,f]
9
2-ClC6H4
3-ClC6H4
4-ClC6H4
2,4-Cl2C6H3
2,6-Cl2C6H3
4-FC6H4
2
2
2
5
5
2
2
4
2
2
2
2
8i 99
8j 82
8k 95
>99:1 95
>99:1 95
>99:1 95
99:1 96
88:12 83
99:1 97
9
Et
10
11
12
13
14
15
16
17
18
19
20
10
11
12
13
14
15
16
17
18[d]
19[d,e]
tBu
tBu
Cy
Ad
tBu
tBu
tBu
tBu
tBu
tBu
8l 99[f,g]
8m 54[f]
8n 93
8o 99 (80)[d]
8p 94
8q 80
THF
Et2O
PhMe
CH2Cl2
EtOAc
EtOAc
EtOAc
4-BrC6H4
99:1 95 (>99)[d,h]
99:1 92
>99:1 91
85:15 96
99:1 96
4-NO2C6H4
1-naphthyl
2-naphthyl
4-PhC6H4
3-PhO-4-FC6H3
8r 99
8s 80
8t 93
>99:1 96
[a] Unless otherwise noted, the reaction was carried out with 0.15 mmol
b-ketoester and 0.2 mmol nitroolefin in solvent (1 mL) at 08C for 20 h.
[b] Yield of isolated product. [c] Determined by chiral HPLC. [d] In 0.5 mL
of solvent. [e] Reaction was carried out at À158C for 32 h.
21
2
8u 99 (75)[d] >99:1 93 (>99)[d]
22
2
8v 83
99:1 93
23
24
25
2-furyl
2-thienyl
c-hexyl
2
2
5
8w 99
8x 99
8y 70[f]
99:1 90
>99:1 93
99:1 92
versus entries 1 and 8). Further examinations were focused on
the sterically hindered amide subunit. The results suggested
that the amide subunit in the guanidine had a significant
impact on the enantioselectivity of the reaction, and the 2,6-
diisopropylphenyl amide was the best one (Table 1, entry 6
versus entries 2–5 and 9). The replacement of the Cy group by
an isopropyl group in the guanidine moiety provided an
equivalent outcome (Table 1, entry 7 versus entry 6); how-
ever, catalyst 1g is hard to synthesize. Thus, we moved on to
an evaluation of the b-ketoester by varying the R3 substituent
and found that the tBu group was suitable to improve both the
d.r. and ee values (Table 1, entry 10 versus entries 12 and 13).
Finally, substantial improvement was realized by solvent
screening. The reaction was completed at À158C in EtOAc in
the presence of 2 mol% of 1 f to furnish 8a quantitatively
with a 99:1 syn/anti ratio and a 95% ee value for the major
product (Table 1, entry 19 versus entries 14–18).
[a] Unless otherwise noted, the reaction was carried out with 0.15 mmol
b-ketoester and 0.2 mmol nitroolefin in EtOAc (0.5 mL) at À158C for
32 h. [b] Yield of isolated product. [c] Determined by 1H NMR spectros-
copy and chiral HPLC. Results are in accordance with literature data.[3b]
[d] Data in parentheses were obtained after a single recrystallization.
[e] In 1.0 mL EtOAc/THF (1:1). [f] Reaction was carried out for 48 h.
[g] In 1.0 mL of EtOAc. [h] The absolute configuration was determined to
be (2R,6S) by X-ray diffraction analysis.[14]
monosubstituted aryl substrates but also condensed-ring and
a,b-unsaturated nitroolefins had no obvious effects on the
enantioselectivities and reactivities (Table 2, entries 1–6, 9–
11, and 14–22). However, the disubstituted aryl substrates
slightly influenced the reactivities; this was caused by the
electronic or steric properties of the substrates (Table 2,
entries 7, 8, 12, 13, and 20). In particular, despite catalyst
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loadings of 5 mol%, electron-donating disubstituted aromatic
substrates suffered lower reactivities (Table 2, entries 7 and
8). It is noteworthy that excellent ee and d.r. values have been
achieved in the asymmetric Michael addition of heteroar-
omatic and aliphatic nitroolefins (up to 93% ee; Table 2,
entries 23–25) because the corresponding adducts have great
potential in natural product synthesis.
On account of the high efficiency in the guanidine
organocatalysis approach and the synthetic potential of the
Michael adducts, the reaction was carried out on a 7 mmol
scale in the presence of 1 f (1.8 mol%) with the cinnamonic
substrate 7u and gave the desired product in 99% yield and
with 99:1 d.r. and 93% ee (Table 2, entry21; Scheme 3). The
NH proton of the amide in 1 f showed a strong deshielding
effect, with a broad peak shape at d = 11.43 ppm due to the
characteristic strong intramolecular hydrogen bonding,[12]
À
which implies that the N H moiety of the amide in catalyst
1 f might act as a Brønsted acid.[13] Based on the X-ray
diffraction analysis of both the guanidine and the adducts, a
preliminary mechanism for this direct nitro-Michael reaction
of cyclic b-ketoesters has been proposed to illustrate the dual-
activation mode. As depicted in Figure 1, the intramolecular
Figure 1. The dual-activation mode of guanidine 1 f (TS) and the
ORTEP representation of product 8o (RC9,SC7) from the X-ray analy-
sis.[14]
H-bond of catalyst 1 f was released and transformed to
activate the two substrates simultaneously. The most favor-
able transition state (TS) shows the guanidine unit to be a
Brønsted base, on which strong zwitterionic hydrogen bonds
with the Michael donor can be built,[5,6f,i] while the NH moiety
of the amide acts as a Brønsted acid to activate the Michael
acceptor.[2d,6] This plausible TS leads to mostly syn products,
in accordance with the substrate generality.
Scheme 3. Large-scale synthesis of 8u and the ramipril analogues.
Boc=tert-butoxycarbonyl.
In conclusion, we have presented an example of the
introduction of amino amides into the guanidine framework
to create organocatalysts for the asymmetric Michael addition
of b-ketoesters to nitroolefins. Catalyst 1 f demonstrated high
stereoselectivities (up to > 99:1 d.r. and 97% ee) and yields
(up to 99%) for a wide range of substrates. The reaction could
be easily scaled up under mild conditions to facilitate a
concise synthesis of a bicyclic b-amino acid. The comparative
experiments and X-ray diffraction analysis of the catalyst
structures revealed both the guanidine group and the NH
proton of the amide are important for the dual-activation
mode.
optically pure (99% ee after a single recrystallization) prod-
uct 8u was successfully converted exclusively into the
corresponding aza-bicyclocarboxylate 9 in good yield by
zinc-mediated reduction[9] followed by an intramolecular aza-
cyclization without any loss of stereoselectivity.[10] Further
reductive addition of the imine with NaCNBH3 under weak-
acid conditions afforded a ramipril analogue, amino acid ester
10, which was then N-protected by using (Boc)2O. The N-Boc-
b-ramipril-type amino acid ester 11, featuring a chiral func-
tional group with an adjacent quaternary carbon stereocenter,
possesses great potential in pharmaceutical synthesis.
To gain insight into the dual-activation mode, comparative
À
experiments were carried out with the N Me derivative of the
amide catalyst.[11] Under optimal conditions, it gave 54%
yield, a syn/anti ratio of 93:7, and 71% ee for the major
product. These results indicate that the NH proton of the
amide moiety is vital for the high activity and stereoselectiv-
ity. Direct evidence was observed by NMR spectroscopy
analyses and deduced from experimental observations. The
Experimental Section
b-Ketoester 6b (27.6 mg, 0.15 mmol) was added to a stirred solution
of nitroolefin (1.33 equiv, 0.2 mmol) and guanidine 1 f (2 mol%,
1.5 mg, 0.005 mmol) in EtOAc (0.50 mL, analytical-reagent grade) at
À158C. After being stirred for 32 h, the reaction mixture was
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5197

Communications
concentrated in vacuum. The residue was purified by column
chromatography on silica gel to afford the desired product.
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Received: March 10, 2009
Published online: June 5, 2009
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Keywords: amino acids · asymmetric synthesis · guanidines ·
Michael addition · organocatalysis
.
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[10] The stereoselectivity of product 9 was determined to be cis/trans
> 99:1, with 99% ee. For detailed information, see the Support-
ing Information.
À
[11] The catalysis was carried out by using the N Me derivative of 1 f
À
for comparison with the N H catalyst 1 f. For detailed informa-
tion, see the Supporting Information.
[12] For detailed information, see the Supporting Information.
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[14] CCDC 722586 (8o) 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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