A Highly Enantioselective Mannich-Type Reaction of Glycine Schiff Base Catalyzed by a Cinchoninium Salt

Article abstract of DOI:10.1002/cjoc.201400453

A chiral phase-transfer catalyst, derived from the combination of cinchona alkaloid backbone and BINOL skeleton, enabled a Mannich reaction of glycine Schiff base with N-Boc-imines to generate α,β-diamino acid derivatives in excellent yields (up to 99%) a

Full text of DOI:10.1002/cjoc.201400453

COMMUNICATION
DOI: 10.1002/cjoc.201400453
A Highly Enantioselective Mannich-Type Reaction of Glycine
Schiff Base Catalyzed by a Cinchoninium Salt†
Zhonglin Tao, Arafate Adele, Xiang Wu, and Liuzhu Gong*
Hefei National Laboratory for Physical Sciences at the Microscale and Department of Chemistry,
University of Science and Technology of China, Hefei, Anhui 230026, China
A chiral phase-transfer catalyst, derived from the combination of cinchona alkaloid backbone and BINOL
skeleton, enabled a Mannich reaction of glycine Schiff base with N-Boc-imines to generate α,β-diamino acid de-
rivatives in excellent yields (up to 99%) and with high diastereo- and enantioselectivities (up to＞20∶1 dr, 96%
ee).
Keywords phase-transfer catalysis, Mannich reaction, α,β-diamino acid, asymmetric catalysis, cinchoninium salt
Introduction
as an efficient phase transfer catalyst capable of render-
ing a Mannich-like reaction of azlactones with sulfonyl
imines to give amino acid precursors with excellent
enantioselectivity, but with modest diastereomeric ratios
(Scheme 1, Eq. 2).[4d] A highly enantioselective Man-
nich reaction of azlactones with aliphatic imines was
established by using a bis(betaine) catalysts (IV), which
were derived from BINOL and quinidine, delivering
α,β-diamino acid derivatives with excellent enantiose-
lectivities but with only moderate diastereoselectivities
(Scheme 1, Eq. 2).[4e] Despite of many highly stereose-
lective approaches available to access α,β-diamino acid
derivatives,[6,7] a practical and straight-forward route
still remains desirable and needs to be disclosed. Herein,
we will present a highly diastereo- and enantioselective
direct Mannich reaction of glycine Schiff base with
N-Boc-imines under the phase transfer catalysis of
chiral quininium salts[8-10] prepared from cinchona al-
kaloids and binaphthols (up to＞20/1 dr and 96% ee).
Initially, a library of chiral ammonium salts 1－4
were readily prepared from substitution reactions of
cinchona alkaloid derivatives with structurally different
aryl bromides in CHCl3/EtOH at 40 ℃ by following
known procedure reported previously.[11] Among them,
chiral catalysts 1 and 2 were prepared from the reaction
of quinine with benzyl bromide and 1-(bromomethyl)-
naphthalene, respectively. On the other hand, the corre-
sponding newly designed ammonium salts 3 featured by
the installation of an additional axial chiral element
were accessed by the substitution reactions of quinine
with 3-(bromomethyl)-bis-O-protected (R)-BINOL de-
rivatives,[4e] as described in Supporting Information.
Phase-transfer catalysis (PTC) has been a versatile
strategy in organic synthesis, featured by several
advantages including simple operations and mild
reaction conditions. PTC has also been exploited in in-
dustry for practical preparation of chiral molecules.[1]
Recently, the use of chiral ammonium in asymmetric
phase-transfer catalysis has attracted intensive attention,
leading to notable accomplishments, enabling various
highly enantioselective reactions to occur readily under
mild conditions.[2] Among these transformations, the
direct Mannich reaction[3] of a glycine Schiff bases or
α-amino acid derivatives with imines turns out to be a
straightforward protocol to stereoselectively access
α,β-diamino acid derivatives[4] that have been found
frequently in natural products, peptides and pharmaceu-
tical compounds.[5]
In 2004, Maruoka and co-workers reported a Man-
nich-type addition of glycine Schiff base to α-imino
esters using a chiral N-spiro quaternary ammonium
bromide I as catalyst, delivering the protected 3-amino-
aspartates with moderate diastereomeric ratios and en-
antioselectivities (Scheme 1, Eq. 1).[4a] Later, Ohshima,
Shibasaki and co-workers described a tartrate-derived
diammonium salt (II)-catalyzed asymmetric Mannich
reaction of a glycine Schiff base with N-Boc-imines
with excellent diastereoselectivity, but the enantioselec-
tivity obtained was not satisfactory (Scheme 1, Eq.
1).[4b,4c]
More recently, Ooi and co-workers designed a chiral
aminophosphonium pivalate III, which was able to act
*
E-mail: gonglz@ustc.edu.cn; Fax: 0086-0551-3606266
Received July 5, 2014; accepted September 13, 2014; published online September 29, 2014.
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cjoc.201400453 or from the author.
Dedicated to Professor Chengye Yuan and Professor Li-Xin Dai on the occasion of their 90th birthdays.
†
Chin. J. Chem. 2014, 32, 969—973
© 2014 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
969

Tao et al.
COMMUNICATION
Scheme 1 Chiral organocatalysts in Mannich-type reaction of a glycine Schiff base derivatives with imines
Boc or PMP
H
N
Boc or PMP
Cat. I or II
CO2t-Bu
Ph
N
CO2t-Bu
N
(1)
R
+
N
Ph
Ph
R
Ph
Ar
Br
2BF4
Ph
Me
N
4-MeC6H4
N
O
O
4-MeC6H4
4-MeC6H4
N
Ar
4-MeC6H4
Me
II
Ph
Ar = 3,4,5-(F)3C6H2
I
Maruoka, 2004
Ohshima & Shibasaki, 2007
Ar2O2S
O
NH
O
SO2Ar2
R1
Cat. III or IV
N
R2
(2)
*
*
+
O
R1
O
N
R2
H
N
Ar1
Ar1
OMe
N
OMe
OH
OH
.
N
OCOPiv PivCOOH
N
Me
O O
H
Me
H
H
N
H
Ph
N
N
N
P
Ph
Ph
N
H
H
Ph
III
Ooi , 2008
IV
Gong, 2012
The ammonium salts 4 represent the derivatives of 3 by
alkylation of the OH at C-9 of the quinine moiety and
have been prepared by following procedures similar to
those for the preparation of 3 (See Supporting Informa-
tion).
butoxycarbonylamino)-2-[(diphenylmethylene)amino]-
3-(4-methoxyphenyl) propaonate (7a).
Results and Discussion
The chiral ammonium salts 1－4 were evaluated for
the Mannich reaction of tert-butyl glycinate 5 and
N-Boc imine 6a. The reaction was conducted in toluene
at 0 ℃ and in the presence of Cs2CO3 (2 equiv.),
wherein anhydrous Na2SO4 was added to remove resid-
ual water and thereby to avoid the hydrolysis of imines.
As shown in Table 1, although all the catalysts were
able to accelerate the reaction, basically favouring the
generation of syn-diamine adduct 7a, the stereoselectiv-
ity was greatly dependent on the structure of ammonium
salts. Either 1 or 2 enabled the reaction to give 7a in a
high yield and with high diastereomeric ratio, but a very
low enantioselectivity was obtained (entries 1 and 2).
Interestingly, much higher enantioselectivities were ob-
served upon using the ammonium salts 3 incorporated
with an additional axial chirality as the catalysts, sug-
gesting that the additional chirality really exerted great
impact on the stereochemical control (entries 3－5). In
particular, promising stereochemical outcomes of 67%
ee and 8/1 dr were obtained in the presence of 3c (entry
5). In sharp contrast, the catalysts 4a－4c with the
Experimental
Representative procedure for asymmetric Mannich
reaction
To a mixture of N-(diphenylmethylene) glycine
tert-butyl ester (5) (29.5 mg, 0.10 mmol), (2R,4R,8S)-1-
(((R)-2,2'-bis(anthracen-9-ylmethoxy)-1,1'-binaphthyl-
3-yl)methyl)-2-((S)-hydroxy(6-methoxyquinolin-4-yl)-
methyl)-8-vinyl-1-azoniabicyclo[2.2.2]octane bromide
(3c) (10.8 mg, 0.01 mmol), Cs2CO3 (65.2 mg, 0.20
mmol) and Na2SO4 (100 mg, 0.70 mmol) in toluene (0.7
mL) was added a solution of (E)-tert-butyl 4-methoxy-
benzylidenecarbamate (6a) (28.2 mg, 0.12 mmol) in
toluene (0.3 mL) at −20 ℃. After 48 h, the reaction was
quenched with water. The aqueous layer was extracted
with EtOAc (10 mL×3). The combined organic layers
were dried over anhydrous magnesium sulfate and then
concentrated under vacuum. The residue was purified
by flash column chromatography on silica gel
[V(hexane)/V(ether)＝10/1] to give tert-butyl 3-(tert-
970
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© 2014 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Chin. J. Chem. 2014, 32, 969—973

A Highly Enantioselective Mannich-Type Reaction of Glycine Schiff Base
Scheme 2 Synthesis of catalyst library
Table 1 Evaluation of chiral catalysts and optimization of con-
ditionsa
R1
R1
Boc
N
Br
Ar
H
N
H
N
Br
Ar
OR2
CHCl3/EtOH
CO2t-Bu
N
Ph
OR2
+
(V:V = 10:1)
N
N
40 o
C
Ph
MeO
6a
5
NHBoc
CO2t-Bu
Cat. (10 mol%)
Cs2CO3 (2.0 equiv.)
OMe
H
N
Br
Ph
H
N
N
Ph
Br
MeO
Na2SO4, 48 h
Ar =
Ar
Ph
OH
OH
7a
N
N
Entry Cat. Temp./℃ Solvent Yieldb/%
drc
eed/%
9
2
1
1
2
0
0
PhCH3
PhCH3
PhCH3
PhCH3
PhCH3
PhCH3
PhCH3
PhCH3
CH2Cl2
Et2O
83
94
73
66
84
79
65
82
92
99
87
69
70
79
87
10∶1
13∶1
5∶1
1
OMe
N
OMe
0
2
H
N
Br
Ar
H
N
Br
Ar
3
0
64
55
67
16
13
32
16
0
3a
3b
3c
4a
4b
4c
3c
3c
3c
3c
3c
3c
3c'
OR
OH
4
0
6∶1
N
5
0
8∶1
4
3
6
0
14∶1
14∶1
11∶1
19∶1
10∶1
13∶1
15∶1
14∶1
13∶1
13∶1
Ar =
Ar =
7
0
OR
OR
OMOM
OMOM
8
0
9
0
10
11
12
13
14
15
0
3a R = MOM
3b R = Bn
3c R = 9-Anthrylmethyl
4a R = Me
4b R = Bn
4c R = 9-Anthrylmethyl
0
THF
8
0
PhF
55
60
86
78
0
PhCl
hydroxyl of the quinine moiety being protected by alkyl
groups offered much diminished enantioselectivities,
but gave slightly improved diastereoselectivity (entries
6－8). These results clearly indicated that the hydroxyl
group in ammonium salts may coordinate with the
enolate generated from the protonation of substrate 5
with base, by the hydrogen-bonding interaction, to im-
prove the enantioselectivity.[10e,12] In the presence of the
optimal catalyst 3c, a variety of solvents were examined
and found that toluene appeared to be the most suitable
media (entries 5, 9－13). Significantly, conducting the
reaction at –20 ℃ led to a considerable improvement
in the diastereomeric ratio (13/1) and enantioselectivity
(86% ee) (entry 5 vs. 14). To clarify the role played by
the axial chirality in the catalysis, the (S)-binaphthyl
skeleton was adopted in the synthesis of 3c', an epimer
of catalyst 3c. Under the otherwise identical conditions,
similar yield and diastereoslectivity (87% yield, 13∶1
dr) of the product was obtained, but the enantioselectiv-
ity was considerably decreased to 78% ee (entry 15).
These results implied that the (S)-axial chirality might
mismatch with quinine in the control of enantioselectiv-
ity (entry 15 vs. 14).
PhCH3
−20
−20e
PhCH3
a Unless indicated otherwise, reactions of 5 (0.10 mmol), 6a (0.12
mmol), catalyst (0.01 mmol), Cs2CO3 (0.20 mmol) and Na2SO4
(100 mg) were carried out in a solvent (1 mL) for 48 h. b Isolated
yield. Determined by H NMR analysis. Determined by HPLC
analysis. Catalyst 3c' was used in the reaction. The (S)-BINOL
c
1
d
e
skeleton was used in the synthesis of the epimer 3c of the catalyst
3c.
able to smoothly undergo the Mannich reaction to give
rise to α,β-diamino acid derivatives in high yields while
both diastereo- and enantioselectivities have an apparent
correlation with the aryl substituent of aldimines (en-
tries 1－13). Obviously, the presence of electronically
withdrawing or neutral substituents at ortho or para-
position of the benzene ring basically provided higher
enantioselectivity than those bearing electron-donating
substituents (entries 1－2 and 4－6 vs. 7). In contrast,
the introduction of either an electronically rich or defi-
cient meta-substituent at the benzene ring of aldimines
was nicely tolerated to give excellent levels of enanti-
oselectivity (entries 3 and 8－9). Moreover, the disub-
stituted aldimine also underwent the Mannich reaction
in a perfect yield and with 90% ee (entry 10). Addition-
Having established the optimal conditions, we then
investigated reaction generality for imine substrates
(Table 2). A wide scope of aromatic aldimines were
Chin. J. Chem. 2014, 32, 969—973
© 2014 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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971

Tao et al.
COMMUNICATION
ally, the protocol was also amenable to 2-thiophenyl,
2-furyl and 1-naphthyl imines, leading to the generation
of desired products in excellent yields ranging from
93% to 99% and with high enantioselectivity of up to
94% ee (entries 11－13). However, the Mannich reac-
tion of aliphatic imines with tert-butyl glycinate 5 was
unable to give satisfactory enantioselectivities.[13] The
relative configurations and absolute configuration of the
same products 7 were assigned by comparing the data
with those in the literatures by 1H NMR, HPLC and op-
tical rotation,[4c,4e] and other products were assigned by
analogy.
chiral phase transfer catalysts.
Acknowledgement
We are grateful for financial support from the Na-
tional Natural Science Foundation of China (No.
21232007).
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NHBoc
Cat. 3c (10 mol%)
Cs2CO3 (2 equiv.)
CO2t-Bu
Boc
Ph
N
CO2t-Bu
Ar
N
+
N
Ph
Na2SO4, Toluene
-20 oC, 48 h
Ph
Ar
Ph
5
6
7
Entry
1
Ar
6b (C6H5)
Yield b/%
drc
eed/%
93
96
92
94
94
92
88
94
93
90
92
94
90
99
99
99
99
99
90
99
99
64
＞20∶1
19∶1
2
6c (2-FC6H4)
6d (3-FC6H4)
6e (4-FC6H4)
6f (4-ClC6H4)
6g (4-BrC6H4)
6h (4-MeC6H4)
6i (3-MeC6H4)
6j (3-MeOC6H4)
3
19∶1
4
20∶1
5
＞20∶1
＞20∶1
15∶1
6
7
8
17∶1
9
＞20∶1
13∶1
10
11
12
13
6k (3,4-Cl2C6H3)
6l (2-thiophenyl)
6m (2-furyl)
99
99
93
99
10∶1
14∶1
6n (1-naphthyl)
6∶1
a Unless indicated otherwise, reactions of 5 (0.10 mmol), 6 (0.12
mmol), catalyst 3c (0.01 mmol), Cs2CO3 (0.20 mmol) and
Na2SO4 (100 mg) were carried out in toluene (1 mL) for 48 h.
b Isolated yield. c Determined by H NMR analysis. d Determined
by HPLC analysis.
Conclusions
In conclusion, we have developed a family of qui-
nine-based ammonium salts featured by the incorpora-
tion of an additional axial chirality, capable of effi-
ciently promoting the Mannich-type reaction of a gly-
cine Schiff base with N-Boc imines. In particular, the
ammonium salt derived from (R)-BINOL and quinine
was found to be the most efficient catalyst, rendering
the Mannich reaction to give α,β-diamino ester deriva-
tives in up to 99% yield and with high diastereo- and
enantioselectivities (up to＞20∶1 dr and 96% ee).
More importantly, the finding that the axial chirality
plays a crucial role in the control of stereoselectivity
might provide a possible clue for the design of new
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Chin. J. Chem. 2014, 32, 969—973

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[13] Only 50% ee was observed for the tert-butyl (cyclohexylmethyl-
ene)carbamate.
(Zhao, X.)
Chin. J. Chem. 2014, 32, 969—973
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