Highly enantio- And diastereoselective mannich reactions of glycine schiff bases with in situ generated N-boc-imines catalyzed by a cinchona alkaloid thiourea

Article abstract of DOI:10.1021/ol902722y

(Figure Presented) Highly ena(Figure Presented)ntlo- and dlastereoselectlve organocatalytlc Mannlch reactions of glycine Schiff bases with N-Boc-protected lmlnes are described. Imlnes were generated In situ from bench-stable a-amldo sulfones. Catalysis mediated by a cinchona alkaloid thiourea provided optically active α,βdlamlno acid derivatives with up to 99% ee and near-perfect dlastereoselection.

Full text of DOI:10.1021/ol902722y

ORGANIC
LETTERS
2010
Vol. 12, No. 4
708-711
Highly Enantio- and Diastereoselective
Mannich Reactions of Glycine Schiff
Bases with in situ Generated
N-Boc-imines Catalyzed by a Cinchona
Alkaloid Thiourea
Haile Zhang, Salahuddin Syed, and Carlos F. Barbas III*
The Skaggs Institute for Chemical Biology and The Departments of Chemistry and
Molecular Biology, The Scripps Research Institute, 10550 North Torrey Pines Road,
La Jolla, California 92037
carlos@scripps.edu
Received November 25, 2009
ABSTRACT
Highly enantio- and diastereoselective organocatalytic Mannich reactions of glycine Schiff bases with N-Boc-protected imines are described.
Imines were generated in situ from bench-stable r-amido sulfones. Catalysis mediated by a cinchona alkaloid thiourea provided optically
active r,ꢀ-diamino acid derivatives with up to 99% ee and near-perfect diastereoselection.
Optically active R,ꢀ-diamino acids have biological signifi-
cance as well as utility in organic synthesis.1 Catalytic
asymmetric synthesis of R,ꢀ-diamino acids through carbon-
carbon bond-forming reactions is challenging because two
vicinal stereocenters must be generated with complete diaste-
reomeric and enantiomeric control. Among various synthetic
approaches, direct catalytic asymmetric Mannich reactions2
are elegant and efficient solutions for the stereocontrolled
assembly of both syn- and anti-R,ꢀ-diamino acid derivatives.
Recently, advances in two complementary synthetic ap-
proaches have been reported: direct Mannich reactions of
glycine Schiff bases with imines3 and nitro-Mannich reac-
tions of nitro compounds with imines facilitated by metal
catalysis and organocatalysis.4 Approaches based on glycine
esters have the obvious synthetic advantage that glycinate
(1) A review on R,ꢀ-diamino acids: (a) Viso, A.; de la Pradilla, R. F.;
Garcia, A.; Flores, A. Chem. ReV. 2005, 105, 3167. For a review on the
synthesis of R,ꢀ-diamino acids by using Mannich reaction strategies, see:
(b) Arrayas, R. G.; Carretero, J. C. Chem. Soc. ReV. 2009, 38, 1940.
(2) For reviews on direct asymmetric Mannich reactions, see: (a)
Verkade, J. M. M.; van Hemert, L. J. C.; Quaedflieg, P. J. L. M.; Rutjes,
F. P. J. T. Chem. Soc. ReV. 2008, 37, 29. (b) Shibasaki, M.; Matsunaga, S.
Chem. Soc. ReV. 2006, 35, 269. (c) Ting, A.; Schaus, S. E. Eur. J. Org.
Chem. 2007, 35, 5797–5815. (d) Marques, M. M. B. Angew. Chem., Int.
Ed. 2006, 45, 348.
(3) Direct Mannich reactions of glycine Schiff bases with imines: (a)
Yan, X. X.; Peng, Q.; Li, Q.; Zhang, K.; Yao, J.; Hou, X. L.; Wu, Y. D.
J. Am. Chem. Soc. 2008, 130, 14362. (b) Shibuguchi, T.; Mihara, H.;
Kuramochi, A.; Ohshima, T.; Shibasaki, M. Chem. Asian J 2007, 2, 794.
(c) Okada, A.; Shibuguchi, T.; Ohshima, T.; Masu, H.; Yamaguchi, K.;
Shibasaki, M. Angew. Chem., Int. Ed. 2005, 44, 4564. (d) Ooi, T.; Kameda,
M.; Fujii, J.; Maruoka, K. Org. Lett. 2004, 6, 2397. (e) Bernardi, L.; Gothelf,
A. S.; Hazell, R. G.; Jorgensen, K. A. J. Org. Chem. 2003, 68, 2583.
10.1021/ol902722y  2010 American Chemical Society
Published on Web 01/19/2010

esters are inexpensive and the commonly used benzophenone
protecting/activating group is readily removed. Surprisingly,
however, only a handful of efficient catalytic asymmetric
reactions based on this strategy have been reported.
enantioselectivities.8 In this approach, however, the diaste-
reoselectivities were not satisfactory, and the use of azi-
doketones on a large scale might not be practical.
It is apparent from both our own studies and the studies
of others that significant improvements in this reaction are
needed. Although organocatalytic phase-transfer reactions
have shown promise, organocatalytic routes not based on
PTC have not been explored. Finally, the protecting group
employed on the imine is critical. N-Carbamoylimines are
known to be sensitive and unstable, but recent advances allow
for the in situ generation of N-carbamoylimines from bench-
stable R-amido sulfones.9,10 Herein, we describe our con-
tributions to the development of practical and efficient
syntheses of the R,ꢀ-diamino derivatives and report the first
examples of bifunctional alkaloid thiourea catalysts for highly
enantio- and diastereoselective organocatalytic direct Man-
nich reactions of a glycine Schiff base with in situ generated
N-carbamoylimines.11
In order to address the stability issue of tert-butoxycar-
bonyl (N-Boc) imines while preserving the advantages of
the N-Boc protecting group, we decided to base our approach
on R-amido-sulfone precursors. As a model reaction we
chose to study the reaction of glycine methyl ester 112 with
the bench-stable R-amido sulfone derived from benzaldehyde
2. We focused our catalyst screening efforts on thiourea
catalysts13,14 and performed the reaction in the presence of
a saturated solution of Na2CO3 to facilitate in situ generation
of the N-Boc-imine of benzaldehyde (Scheme 1).
The use of glycine Schiff bases in asymmetric phase
transfer catalysis (PTC) was pioneered by O’Donnell.5
Recently, there have been advances in asymmetric Lewis
acid and PTC catalyzed Mannich reactions of glycine Schiff
bases. Copper salt catalyzed Mannich reactions of glycine
Schiff bases with toluene-4-sulfonyl (Ts) imines suffer the
drawbacks of an air-sensitive catalyst and the difficulties
inherent with the Ts protecting group that is only removed
under very harsh reaction conditions.3a,e Maruoka has
reported a spiro quaternary ammonium salt based phase-
transfer catalyst that works with a limited number of PMP-
protected glyoxylate imine acceptors to provide Mannich
products with moderate diastereomeric ratios and
enantioselectivity.3d Other quaternary ammonium salts have
been reported by Shibasaki as catalysts of the Mannich
reaction of a glycine Schiff base with N-Boc-imines to
provide R,ꢀ-diamino acids with moderate ee’s.3b,c Finally,
Liu et al. have reported Mannich reactions of preformed
chiral Ni complexes of glycine with R-amido sulfones;6
however, this methodology suffers with respect to efficiency
and practicality. As an alternative to either of these ap-
proaches, we have previously explored organocatalytic direct
asymmetric Mannich reactions7 of azidoketones with imines.
These reactions afforded R,ꢀ-diamino acids with excellent
Gratifyingly, the desired product was obtained with good
enantiomeric excess. The ee was 88% when a thiourea catalyst
prepared from the cinchona alkaloid dihydroquinine (DHQ)14
was used (3a). The reaction, however, did not go to comple-
tion.15 Takemoto’s catalyst 3b gave the best conversion to the
(4) Direct nitro-Mannich reactions of nitro compounds with imines: (a)
Singh, A.; Johnston, J. N. J. Am. Chem. Soc. 2008, 130, 5866. (b) Rueping,
M.; Antonchick, A. P. Org. Lett. 2008, 10, 1731. (c) Chen, Z. H.; Morimoto,
H.; Matsunaga, S.; Shibasaki, M. J. Am. Chem. Soc. 2008, 130, 2170. (d)
Singh, A.; Yoder, R. A.; Shen, B.; Johnston, J. N. J. Am. Chem. Soc. 2007,
129, 3466. (e) Cutting, G. A.; Stainforth, N. E.; John, M. P.; Kociok-Kohn,
G.; Willis, M. C. J. Am. Chem. Soc. 2007, 129, 10632. (f) Knudsen, K. R.;
Risgaard, T.; Nishiwaki, N.; Gothelf, K. V.; Jorgensen, K. A. J. Am. Chem.
Soc. 2001, 123, 5843.
(8) Chowdari, N. S.; Ahmad, M.; Albertshofer, K.; Tanaka, F.; Barbas,
C. F. Org. Lett. 2006, 8, 2839.
(5) For reviews and selected examples concerning catalysis with glycine
Schiff base substrates, see: (a) O’Donnell, M. J.; Bennett, W. D.; Wu, S. D.
J. Am. Chem. Soc. 1989, 111, 2353. (b) O’Donnell, M. J. Acc. Chem. Res.
2004, 37, 506. (c) O’Donnell, M. J. Aldrichim. Acta 2001, 34, 3. (d) Lygo,
B.; Andrews, B. I. Acc. Chem. Res. 2004, 37, 518.
(9) For a comprehensive review of use of R-amido sulfones as stable
precursors of N-acylimino derivatives, see: Petrini, M. Chem. ReV. 2005,
105, 3949
.
(10) For examples of R-amido sulfones as imine precursors in Mannich-
type reactions, see: (a) Song, J.; Shih, H. W.; Deng, L. Org. Lett. 2007, 9,
603. (b) Palomo, C.; Oiarbide, M.; Laso, A.; Lopez, R. J. Am. Chem. Soc.
2005, 127, 17622. (c) Lou, S.; Dai, P.; Schaus, S. E. J. Org. Chem. 2007,
72, 9998. (d) Fini, F.; Sgarzani, V.; Pettersen, D.; Herrera, R. P.; Bernardi,
(6) Wang, J.; Shi, T.; Deng, G. H.; Jiang, H. L.; Liu, H. J. Org. Chem.
2008, 73, 8563.
(7) For select studies concerning organocatalytic direct asymmetric
Mannich reactions reported by this laboratory, see: (a) Zhang, H.; Mitsumori,
S.; Utsumi, N.; Imai, M.; Garcia-Delgado, N.; Mifsud, M.; Albertshofer,
K.; Cheong, P. H.-Y.; Houk, K. N.; Tanaka, F.; Barbas, C. F., III. J. Am.
Chem. Soc. 2008, 130, 875. (b) Utsumi, N.; Kitagaki, S.; Barbas, C. F., III.
Org. Lett. 2008, 10, 3405. (c) Utsumi, N.; Zhang, H.; Tanaka, F.; Barbas,
C. F., III. Angew. Chem., Int. Ed. 2007, 46, 1878. (d) Ramasastry, S. S. V.;
Zhang, H.; Tanaka, F.; Barbas, C. F., III. J. Am. Chem. Soc. 2007, 129,
288. (e) Zhang, H.; Mifsud, M.; Tanaka, F.; Barbas, C. F., III. J. Am. Chem.
Soc. 2006, 128, 9630. (f) Mitsumori, S.; Zhang, H.; Cheong, P. H.-Y.; Houk,
K. N.; Tanaka, F.; Barbas, C. F., III. J. Am. Chem. Soc. 2006, 128, 1040.
(g) Cheong, P. H.-Y.; Zhang, H.; Thayumanavan, R.; Tanaka, F.; Houk,
K. N.; Barbas, C. F., III. Org. Lett. 2006, 8, 811. (h) Notz, W.; Watanabe,
S.-i.; Chowdari, N. S.; Zhong, G.; Betancort, J. M.; Tanaka, F.; Barbas,
C. F., III. AdV. Synth. Catal. 2004, 346, 1131. (i) Notz, W.; Tanaka, F.;
Barbas, C. F., III. Acc. Chem. Res. 2004, 37, 580. (j) Chowdari, N. S.;
Suri, J. T.; Barbas, C. F., III. Org. Lett. 2004, 6, 2507. (k) Notz, W.; Tanaka,
F.; Watanabe, S.; Chowdari, N. S.; Turner, J. M.; Thayumanavan, R.;
Barbas, C. F. J. Org. Chem. 2003, 68, 9624. (l) Chowdari, N. S.; Ramachary,
D. B.; Barbas, C. F., III. Synlett 2003, 1906. (m) Watanabe, S.; Cordova,
A.; Tanaka, F.; Barbas, C. F. Org. Lett. 2002, 4, 4519. (n) Cordova, A.;
Watanabe, S.-i.; Tanaka, F.; Notz, W.; Barbas, C. F., III. J. Am. Chem.
Soc. 2002, 124, 1866. (o) Cordova, A.; Notz, W.; Zhong, G.; Betancort,
J. M.; Barbas, C. F., III. J. Am. Chem. Soc. 2002, 124, 1842. (p) Notz, W.;
Sakthivel, S.; Bui, T.; Barbas, C. F., III. Tetrahedron Lett. 2001, 42, 199.
L.; Ricci, A. Angew. Chem., Int. Ed. 2005, 44, 7975
.
(11) For thiourea catalyzed 1,3-dipolar cycloaddition of azomethine
ylides of glycine Schiff bases with nitroalkenes, see: Xue, M. X.; Zhang,
X. M.; Gong, L. Z. Synlett 2008, 691.
(12) When glycine tert-butyl ester was employed in the Mannich reaction
with in situ generated N-carbamoylimines in the presence of thiourea catalyst
and base under the same conditions, little of the desired product was ob-
tained.
(13) Reviews on organocatalysts, including urea and thiourea catalysts:
(a) Connon, S. J. Synlett 2009, 354. (b) Miyabe, H.; Takemoto, Y. Bull.
Chem. Soc. Jpn. 2008, 81, 785. (c) Taylor, M. S.; Jacobsen, E. N. Angew.
Chem., Int. Ed. 2006, 45, 1520. (d) Doyle, A. G.; Jacobsen, E. N. Chem.
ReV. 2007, 107, 5713
.
(14) For pioneering work on cinchona alkaloid-derived thiourea catalysts,
see: (a) McCooey, S. H.; Connon, S. J. Angew. Chem., Int. Ed. 2005, 44,
6367. (b) Vakulya, B.; Varga, S.; Csa´mpai, A.; Soo´s, T. Org. Lett. 2005, 7,
1967. (c) Wang, Y.-Q.; Song, J.; Hong, R.; Deng, L. J. Am. Chem. Soc.
2006, 128, 8156. (d) Wang, J.; Li, H.; Zu, L. Z.; Xie, H. X.; Duan, W. H.;
Wang, W. J. Am. Chem. Soc. 2006, 128, 12652. (e) Lou, S.; Taoka, B. M.;
Ting, A.; Schaus, S. E. J. Am. Chem. Soc. 2006, 127, 11256. (f) Bernardi,
L.; Fini, F.; Herrera, R. P.; Ricci, A.; Sagarzabu, V. Tetrahedron 2006, 62,
375. (g) Li, B.; Jiang, L.; Liu, M.; Chen, Y.; Ding, L.; Wu, Y. Synlett
2005, 4, 603. (h) Ye, J.; Dixon, D. J.; Hynes, P. Chem. Commun. 2005, 35,
4481.
Org. Lett., Vol. 12, No. 4, 2010
709

Scheme 1
.
Model Reaction and Catalysts Studieda
Table 1. Solvent Screen: Mannich Reaction of Glycine Schiff
base with in situ Generated N-Boc-phenyliminea
entry
solvent
CHCl3
CH2Cl2
toluene
convnb (%)
drc
eed (%)
1
2
3
4
5
6
7
8
50
45
59
77
63
63
14
48
>99:1
>99:1
>99:1
>99:1
>99:1
>99:1
91
92
96
99
96
92
PhCF3
m-xylene
PhCF3+ CHCl3
THF
cyclohexane
>99:1
95
a Typical reaction conditions: To a mixture of R-amido sulfone (0.15
mmol, 1.5 equiv) and glycine Schiff Base (0.1 mmol, 1.0 equiv) in the
indicated solvent (0.5 mL) was added catalyst (0.01 mmol, 0.01 equiv, 10
mol % to the glycine Schiff base) followed by aqueous base (0.25 mL).
The mixture was stirred at room temperature. b The conversion was
determined by HNMR of the crude product. c The dr was determined by
HPLC. d The ee was determined by chiral-phase HPLC analysis.
tions, the ee and the reactivity of the catalyst decreased.
Significantly, each solvent system provided products with
near perfect diastereoselection.
In order to further optimize the reaction, other parameters
were varied (Table 2). When the reaction was both initiated
a Typical reaction conditions: To a mixture of R-amido sulfone (0.15
mmol, 1.5 equiv) and glycine Schiff base (0.1 mmol, 1.0 equiv) in CHCl3
(0.5 mL) were added catalyst (0.01 mmol, 0.01 equiv, 10 mol % to the
glycine Schiff base) and aqueous base (0.25 mL). The mixture was stirred
at room temperature The conversion was determined by H NMR of the
crude product. c The dr was determined by HPLC. d The ee was determined
by chiral-phase HPLC analysis.
Table 2. Optimization of Conditions: Mannich Reaction of
Glycine Schiff Base with in situ Generated N-Boc-phenyliminea
b
1
temp
time
(h)
convn
(%)b
ee
desired product; however, the ee was significantly lower than
that provided by 3a. Attempts to optimize this cyclohexane-
derived catalyst through the preparation of derivatives 3c-f
failed.16 Neither DHQ 3g nor bisthiourea 3h alone could
efficiently catalyze this reaction, demonstrating that the bifunc-
tional nature of 3a is key. Presumably, the tertiary amine of
this catalyst activates the glycine Schiff base as a nucleophile
while the thiourea portion activates the N-Boc-imine as an
electrophilic acceptor through H-bonding to the nitrogen of the
imine. Having failed to find a catalyst more promising than 3a,
a solvent screen was performed.
entry
base
(°C)
(%)c
1
2
Na2CO3 (sat aq)
Na2CO3 (sat aq)
Na2CO3 (satd aq)
Na2CO3 (satd aq)
4 to rt 36, then 26
77
77
81
80
58
82
66
69
77
100
62
62
99
97
96
99
96
92
92
rt
rt
26
60
3
4
4 to rt 14, then 48
5
Na2CO3 (S, 10 equiv) rt
24
24
24
26
60
24
26
60
6
Na2CO3(satd aq)
45
7
Na2CO3 (S, 10 equiv) 45
8
K2CO3 (satd aq)
K2CO3 (satd aq)
K2CO3 (S, 10 equiv)
CS2CO3 (satd aq)
CS2CO3 (satd aq)
rt
rt
rt
rt
rt
9
10
11
12
45
a Typical reaction conditions: To a mixture of R-amido sulfone (0.15
mmol, 1.5 equiv) and glycine Schiff base (0.1 mmol, 1.0 equiv) in PhCF3
(0.5 mL) was added catalyst (0.01 mmol, 0.01 equiv, 10 mol % to the
glycine Schiff base) followed by the addition of the base (0.25 mL of
aqueous base or 10 equiv of solid base, indicated by S). The mixture was
stirred at room temperature. b The conversion was determined by HNMR
of the crude product. c The ee was determined by chiral-phase HPLC
analysis.
The initial solvent screen was performed at 4 °C. At this
temperature the reaction was very slow. We then evaluated
various solvents in reactions initiated at 4 °C and then
continued at room temperature for 26 h (Table 1). Under
these conditions, toluene provided a product with higher ee
than CH2Cl2 or CHCl3. Among the solvents screened, PhCF3
gave the best results with respect to both ee and conversion.
Attempts to further optimize the reaction by the combining
CHCl3 and PhCF3 were not successful; under these condi-
and run at room temperature a small decrease in ee was noted
compared to the reaction initiated at 4 °C (entry 1 versus
710
Org. Lett., Vol. 12, No. 4, 2010

entry 2). Increasing the reaction time only slightly increased
the reaction conversion (entry 2 versus entry 3). Use of solid
Na2CO3 (no water) decreased the conversion to the desired
product (entry 5). Increasing the reaction temperature
improved the reaction conversion but decreased the ee (entry
2 versus entry 6, entry 5 versus entry 7). Stronger aqueous
bases K2CO3 or Cs2CO3 did not increase the conversion to
the desired product (entries 8, 9, 11, and 12). Complete
conversion was observed when solid K2CO3 (no water) was
used; however, the ee was decreased (entry 10).
o-tolyl imines had reduced reaction conversions (entries 3)
compared to substrates with electron-withdrawing groups
(entries 4-6). The reaction was also readily scaled without
significant changes in yield or enantioselectivity (entry 2).
Regardless of electronic and structural differences in the
imines, all products were prepared as virtually single
diastereomers.17
To confirm the stereochemistry of the Mannich adducts,
imidazolidinone 14 was synthesized from 4 as shown in
Scheme 2. The relative and absolute configuration of 14 was
We then studied the scope of this Mannich reaction using
a variety of aromatic and heteroaromatic imines generated
in situ. As summarized in Table 3, all of the reactions
Scheme 2. Efficient Synthesis of cis-Imidazolidinone 14
Table 3. Reaction Scope: Mannich Reaction of Glycine Schiff
Base with in situ Generated N-Boc-alkyliminesa
then determined by comparison with data reported in the
literature and the configuration of all other products were
assigned by analogy with this product.18
entry
R
product convnb (%) yieldc (%) eed (%)
In summary, we have developed a highly efficient and
practical strategy for the synthesis of chiral R,ꢀ-diamino acid
derivatives with remarkable levels of diastereo- and enan-
tiocontrol (up to >99% ee, dr > 99:1). We employed direct
organocatalytic Mannich reactions of a glycine Schiff base
with N-Boc-imines generated in situ. This is the first
characterization of highly enantioselective direct Mannich
reactions of a glycine Schiff base promoted by a DHQ-
derived thiourea catalyst and not based on PTC. We believe
that the tertiary amine thiourea catalyst activates the glycine
Schiff base as a nucleophile through general base catalysis
while simultanously activating the imine as an electrophile
through H-bonding to the nitrogen of the imine. Further
investigations of the scope and application of this strategy
are in progress.
1
2e
3
4
5
6
7
8
9
C6H5
C6H5
4
4
5
6
7
8
9
10
11
12
80
81
68
100
100
100
82
97
100
93
73
76
62
86
87
90
74
80
98
89
99
99
98
4-MeO-C6H5
4-NO2-C6H5
4-Cl-C6H5
4-Br-C6H5
2-furyl
R-naphthyl
4-CF3-C6H5
4-Me-C6H5
94
>99
>99
>99
98
>97
>95
10
a Typical reaction conditions: To a mixture of R-amido sulfone (0.6
mmol, 2.0 equiv or 0.45 mmol, 1.5 equiv) and glycine Schiff base (0.3
mmol, 1.0 equiv) in solvent (1.5 mL) was added catalyst (0.03 mmol, 0.03
equiv, 10 mol % to the glycine Schiff base) followed by the addition of the
base. The mixture was stirred at 4 °C for 14 h and then incubated at room
temperature for 28-48 h. b The conversion was determined by 1H NMR of
the crude product. c Isolated yield. d The ee was determined by chiral-phase
HPLC analysis. e R-Amido sulfone (3 mmol, 1.5 equiv) and glycine Schiff
base (2 mmol, 1.0 equiv).
Acknowledgment. This research was supported by the
Skaggs Institute for Chemical Biology.
Supporting Information Available: Experimental pro-
cedures and spectral data for all compounds. This material
is available free of charge via the Internet at http://pubs.acs.org.
afforded product with excellent ee and dr. The electronic
character of substrate dramatically affected the rate of the
Mannich transformation. Electron-rich substrates such as
OL902722Y
(15) The glycine Schiff base is quite stable in basic aqueous conditions.
The reaction conversion was judged by the ratio of desired product to the
remaining glycine Schiff base. H NMR was used to monitor Boc-imine
1
(17) Diastereomers were resolved by HPLC. See the Supporting
consumption and decomposition to benzaldehyde.
Information.
(16) By 1H NMR of the crude reaction mixture, N-Boc-phenylimine was
formed and little desired product was observed.
(18) Lee, S.-H.; Yoon, J.; Chung, S.-H.; Lee, Y.-S. Tetrahedron 2001,
57, 2139.
Org. Lett., Vol. 12, No. 4, 2010
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