Rh2(OAc)4 and chiral phosphoric acid cocatalyzed highly diastereo- and enantioselective four-component reactions: Facile synthesis of chiral α,β-diamino acid derivatives

Article abstract of DOI:10.1055/s-0033-1341051

A highly diastereo- and enantioselective four-component reaction of a diazo ketone with two molecules of anilines and ethyl glyoxylate was achieved under Rh₂(OAc)₄ and chiral phosphoric acid cocatalyzed conditions. This transformation proceeds through a Mannich-type trapping of the ammonium ylide generated from metal carbene and one molecule of aniline with iminoester derived from another molecule of aniline and ethyl glyoxylate. With this method, a series of chiral α,β-diamino acid derivatives were efficiently constructed in good yields and with good to excellent diastereo- and enantioselectivities. Georg Thieme Verlag Stuttgart New York.

Full text of DOI:10.1055/s-0033-1341051

1348▌
SPECIAL TOPIC
sRpehcia2l(toOpicAc)4 and Chiral Phosphoric Acid Cocatalyzed Highly Diastereo- and
Enantioselective Four-Component Reactions: Facile Synthesis of Chiral
α,β-Diamino Acid Derivatives
Facile Synthesis of Chiral α,β-Diamino Acid Derivatives
Changcheng Jing, Dong Xing, Yu Qian, Wenhao Hu*
Shanghai Engineering Research Center of Molecular Therapeutics and New Drug Development, and Department of Chemistry,
East China Normal University, Shanghai, 200062, P. R. of China
Fax +86(21)62221235; E-mail: whu@chem.ecnu.edu.cn
Received: 02.02.2014; Accepted: 25.02.2014
Multicomponent reactions (MCRs) have been widely rec-
Abstract: A highly diastereo- and enantioselective four-component
ognized as step- and atom-economic ways to construct
reaction of a diazo ketone with two molecules of anilines and ethyl
multiple chemical bonds from three or more starting ma-
glyoxylate was achieved under Rh2(OAc)4 and chiral phosphoric
acid cocatalyzed conditions. This transformation proceeds through
terials in a single operation.3 As part of our continuous in-
a Mannich-type trapping of the ammonium ylide generated from terest in maximizing synthetic efficiency to construct
metal carbene and one molecule of aniline with iminoester derived
from another molecule of aniline and ethyl glyoxylate. With this
method, a series of chiral α,β-diamino acid derivatives were effi-
ylides or zwitterionic intermediates generated from metal
ciently constructed in good yields and with good to excellent diaste-
reo- and enantioselectivities.
structurally diversified molecules, recently we have de-
veloped a novel type of MCR by trapping active onium
carbene4 with electrophiles.5,6 The use of either chiral
metal catalysts or metal/organo cooperative catalysis has
Key words: four-component reaction, metal carbene, chiral Brøn-
further broadened the synthetic potentials of such types of
sted acid, ammonium ylide, chiral α,β-diamino acid
MCRs by providing expedient access to chiral polyfunc-
tional molecules.7,8 Among these progress, enantioselec-
tive trapping of ammonium ylides with electrophiles has
Chiral α,β-diamino acid scaffolds are present widely in
been developed for synthesizing chiral nitrogen-contain-
natural products and biologically active compounds, and
ing molecules. Che and co-workers developed an enan-
have received extensive attention from the synthetic com-
tioselective three-component coupling reaction of
munity (Figure 1).1 As a result, an array of useful methods
diazophosphonates, anilines, and electron-deficient alde-
have been developed to construct α,β-diamino acid deriv-
hydes catalyzed by chiral rhodium catalysts.8a Gong and
atives through carbon–carbon bond formation or carbon–
co-workers reported an enantioselective aldol-type three-
nitrogen bond formation processes.1,2 These methods in-
component reaction of 3-diazooxindoles with anilines and
clude Mannich reaction of nucleophiles with imines,2a–d
glyoxylates by using rhodium/chiral Brønsted acid co-op-
dimerization of glycinates,2e–g amino functionalization of
erative catalysis.8c Our group realized several Mannich-
type enantioselective three-component reaction of diazo-
acetates with anilines or carbamates and imines under rho-
cyclic intermediates,2h–j and amination of α,β-unsaturated
alkenoates or functionalized alkanoates.2k–m Despite these
achievements, however, facile and efficient enantioselec-
dium/chiral Brønsted acid cocatalyzed conditions.9
tive approaches to chiral α,β-diamino acid derivatives
Despites these progresses, however, the construction of
chiral nitrogen-containing molecules based on ylide-trap-
from simple starting materials, are still highly desirable.
O
O
OH
O
O
O
N
O
OH
N
HO
HN
Me3N
OOC
N
OH
N
OH
O
O
H
NH2
N
NH2
OH NH2
O
H
O
L-quisqualic acid
alanosine
β-ODAP
dysibetaine
NH
Me
N
O
O
H2N
N
NH
O
H2N
OH
H
N
N
NH2
O
H
H
O
N
N
NH2
NH2
dapdiamide A
O
O
H
TAN-1057A/B
Figure 1 Natural products and biologically active compounds containing α,β-diamino acid scaffolds
SYNTHESIS 2014, 46, 1348–1354
Advanced online publication: 27.03.2014
0
0
3
9
-
7
8
8
1
1
4
3
7
-
2
1
0
X
DOI: 10.1055/s-0033-1341051; Art ID: SS-2014-C0085-OP
© Georg Thieme Verlag Stuttgart · New York

SPECIAL TOPIC
Facile Synthesis of Chiral α,β-Diamino Acid Derivatives
1349
ping MCRs, especially those using diazo ketones as the
carbene sources to generate ammonium ylides, are rela-
tively rare. As a result, we focused our attention on the
seeking of an enantioselective process starting from diazo
ketones, anilines, and glyoxylates for the efficient synthe-
sis of α,β-diamino acid derivatives.
O
O
N2
O
R
H
COOEt
H2O
XH
+
R
X-H insertion products
ArNH2
1,2-H shift
ORhLn
RhLn
RhLn
To construct optically active α,β-diamino acid deriva-
tives, a rhodium(II) and chiral Brønsted acid cocatalyzed
four-component reaction of a diazo ketone with two mol-
ecules of anilines and ethyl glyoxylate was designed
(Scheme 1). Within this transformation, an ammonium
ylide intermediate I would be generated in situ from a di-
azo ketone and one molecule of aniline in the presence of
a rhodium catalyst, on the other hand, another molecule of
aniline would react with ethyl glyoxylate to afford an im-
inoester, which could be further activated into an iminium
ion II by the assistance of a chiral phosphoric acid. A
Mannich-type trapping of the ammonium ylide I with the
iminium ion II would then occur to afford the desired
four-component product 4. In order to make this designed
transformation to occur smoothly, one of the biggest chal-
lenges need to overcome is the chemoselectivity issue,
such as N–H and O–H insertion reactions, aziridination,10
Mannich-type addition of diazo compounds to imino-
esters,11 hydrolysis of the iminoesters8c,12 as well as the
four-component reaction of diazo ketones, water, anilines,
and glyoxylates; 13 all these are possible competitive side
reaction pathways (Scheme 2). On the other hand, due to
the high reactivity of acceptor carbene derived from diazo
ketones, it would be difficult to achieve high level of en-
antioselective control for this four-component reaction.14
O
O
X = OH, NHAr
XH
R
XH
R
R
RhLn
metal carbene
Mannich-type
trapping
aldol-type
aziridination
trapping
O
O
Ar
O
OH
O
HN
OEt
R
OEt
OEt
R
R
N
X
O
X
O
Ar
Scheme 2 Possible reaction pathways of diazo ketones, anilines, and
ethyl glyoxylate in the presence of Rh2(OAc)4 catalyst
mation of iminoester (Scheme 3). The presence of 4 Å
molecular sieves and racemic phosphoric acid was crucial
in efficiently inhibiting undesired side reactions; as a re-
sult, under such reaction conditions, the formation of any
other obvious by-products was not observed.
R
rac-5a R = H
(S)-5b R = H
(S)-5c R = Ph
(S)-5f R = 9-phenanthryl
(S)-5g R = biphenyl
(S)-5h R = 4-F3CC6H4
O
O
O
(S)-5d R = 3,5-Cl2C6H3 (S)-5i R = 3,5-(CF3)2C6H3
(S)-5e R = 2-naphthyl (S)-5j R = SiPh3
(R)-5j R = SiPh3
P
OH
With these concerns in mind, our initial investigations be-
gan by choosing diazoacetophenone (1a), two molecules
of aniline (2a) (2.2 equiv) and ethyl glyoxylate (3) as the
substrates to react under Rh2(OAc)4-catalyzed conditions.
To our delight, the desired four-component reaction pro-
ceeded smoothly in the presence of 4 Å molecular sieves
at 25 °C with toluene as the solvent, affording 4a in 73%
yield with 83:17 dr. The addition of 5 mol% racemic
phosphoric acid 5a (Figure 2) as a cocatalyst further im-
proved the yield as well as the diastereoselectivity of this
four-component reaction probably via facilitating the for-
R
5
Figure 2 Chiral phosphoric acid cocatalysts 5
With these preliminary results, different chiral phosphoric
acids were screened for enantioselective transformations.
Among the different chiral phosphoric acids being tested,
(S)-5j bearing bulky 3,3′-SiPh3 substituents gave satisfac-
tory result, yielding the desired four-component product
in 84% yield with 86:14 dr and 94% ee (Table 1, entries
O
H
NHAr
RhLn
O
O
R
∗
ArNH2
ArNH2
N2
+
+
+
∗
R
5
H
COOEt
ArHN
COOEt
1
2
2
3
H
4
RO
P
RO
O
Mannich-type
trapping
RhLn
5
OH
RO
P
OR
O
O
O
RhLn
OR
OR
H
N
H
Ar
Ar
H
H
P
H
R
Ar
H
+
N
O
Ar
O
R
HN
N
O
H
COOEt
EtOOC
H
LnRh
I
II
III
Scheme 1 Designed rhodium(II)/chiral PPA cocatalyzed four-component reaction of diazo ketones, anilines, and ethyl glyoxylate for the syn-
thesis of α,β-diamino acids
© Georg Thieme Verlag Stuttgart · New York
Synthesis 2014, 46, 1348–1354

1350
C. Jing et al.
SPECIAL TOPIC
O
Table 1 Catalyst Screening and Optimization of Reaction Condi-
tions for the Four-Component Reaction of 1a, 2a (2.2 equiv), and 3a
Rh2(OAc)4
(2 mol%)
H
H
O
O
NHPh
Ph
PhHN
N2 PhNH
+
+
2
O
toluene
4 Å MS, 25 °C
Rh2(OAc)4
(2 mol%)
5
Ph
H
COOEt
3
2a
H
COOEt
1a
O
O
NHPh
(2.2 equiv)
Ph
PhHN
+
anti-4a
N2
+
PhNH2
2a
Ph
H
COOEt
solvent
4 Å MS
without co-catalyst
73% yield, 83:17 dr
77% yield, 90:10 dr
COOEt
(2.2 equiv)
1a
3
H
with 5 mol% of rac-5a
anti-4a
Scheme 3 Initial results of the four-component reaction of diazoace-
tophenone (1a), two molecules of aniline (2a), and ethyl glyoxylate
(3)
Entry
5
Solvent
Temp Yield drc
(°C)
ee
(mol%)
(%)b (anti/syn) (%)d
1
2
(S)-5b (5)
(S)-5c (5)
(S)-5d (5)
(S)-5e (5)
(S)-5f (5)
(S)-5g (5)
(S)-5h (5)
(S)-5i (5)
(S)-5j (5)
(S)-5j (2)
(S)-5j (5)
(S)-5j (5)
(S)-5j (5)
(S)-5j (5)
(R)-5j (5)
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
DCE
25
25
25
25
25
25
25
25
25
25
25
25
40
0
78
76
78
79
81
80
83
86
84
76
84
78
86
83
84
91:9
19
65
73
67
89
54
33
89
94
71
91
89
95
90
–92
1–9). When the amount of chiral phosphoric acid (S)-5j
was decreased from 5 mol% to 2 mol%, the enantioselec-
tivity was decreased to 71% (entry 10). The screening of
different solvents and temperatures revealed that the use
of toluene as the solvent at 40 °C was the most optimal
(entries 11–14). The use of 5 mol% of (R)-5j as the cocat-
alyst gave (2S,3R)-anti-4a in 84% yield with 84:16 dr and
92% ee (entry 15).
88:12
82:18
85:15
83:17
84:16
85:15
81:19
86:14
90:10
67:33
70:30
87:13
88:12
84:16
3
4
5
6
7
With the optimized reaction conditions in hand, the sub-
strate scope of this Rh2(OAc)4/chiral phosphoric acid co-
catalyzed four-component reaction was investigated.
When anilines bearing different substituents were applied,
the desired four-component reactions proceeded smooth-
ly, providing the desired products in high level of diaste-
reo- and enantioselective control (Table 2, entries 1–6).
The highest diastereomeric ratio was achieved when 3,4-
difluoroaniline (2f) was used as the substrate, affording
the desired product 4f in 72% yield with 95:5 dr and 91%
ee (entry 6). On the other hand, different diazo ketones
were also tested. The results indicated that the existence of
either electron-donating or electron-withdrawing substit-
uents on the aromatic ring of diazo ketone 1 could all af-
ford the desired products in good yield with high dr and ee
(entries 7–11). When styryl-substituted diazo ketone 1f
was used, the desired product was obtained in 50% yield
with 92:8 dr, albeit with 67% ee (entry 12). The absolute
configurations of the four-component products were ten-
tatively established as 2R,3S by comparing the circular di-
chroism spectrum of anti-4h with a known compound.13,15
8
9
10
11
12
13
14
15
CH2Cl2
toluene
toluene
toluene
40
a The reactions were carried out on a 0.10 mmol scale, 1a/2a/3a =
1.2:2.2:1.0. Diazoacetophenone 1a (0.12 mmol) in 0.5 mL of solvent
was added to a mixture of 2a (0.22 mmol), 3 (0.10 mmol), 5 (5.0
mol%), Rh2(OAc)4 (2.0 mol%), and 4 Å MS (100 mg) in 0.5 mL of
solvent within 1 h via a syringe pump.
b Isolated yield of 4a.
c The dr for 4a was determined by chiral HPLC with an IA column.
d The ee for anti-4a was determined by chiral HPLC with an IA col-
umn.
In order to rationalize the high diastereo- and enantio-
selectivity observed in this Rh2(OAc)4/chiral phosphoric
O
O
O
P
R
O
H
O
O
O
R
C2
H
P
R
N
OH
Ar
EtOOC
R
H
H
R
N
ORhLn
H
TSIII
Ar
(S)-5j, R = SiPh3
Scheme 4 Proposed preferential conformation for transition state
Synthesis 2014, 46, 1348–1354
© Georg Thieme Verlag Stuttgart · New York

SPECIAL TOPIC
Facile Synthesis of Chiral α,β-Diamino Acid Derivatives
1351
Table 2 Enantioselective Four-Component Mannich-Type Reactions of Diazo Ketones 1, Anilines 2 (2.2 equiv) and Ethyl Glyoxylate (3)a
Rh2(OAc)4
(2 mol%)
(S)-5j
(5 mol%)
SiPh3
O
H
O
O
NHAr
O
O
O
R
P
ArNH2
2
+
+
N2
OH
R
H
COOEt
toluene
4 Å MS, 40 °C
ArHN
COOEt
(2.2 equiv)
1
3
H
SiPh3
anti-4
(S)-5j
Entry
1
Ar
Ph
4
Yield (%)b
drc (anti/syn)
ee (%)d
1
2
1a (R = Ph)
1a (R = Ph)
1a (R = Ph)
1a (R = Ph)
1a (R = Ph)
1a (R = Ph)
1b (R = 4-ClC6H4)
4a
4b
4c
4d
4e
4f
86
82
86
71
77
72
69
65
62
62
74
50
87:13
91:9
95
92
94
97
88
91
94
92
94
93
93
67
4-BrC6H4
4-ClC6H4
3
90:10
85:15
92:8
4
3,4-Cl2C6H3
4-FC6H4
5
6
3,4-F2C6H3
4-ClC6H4
4-ClC6H4
4-ClC6H4
4-BrC6H4
4-BrC6H4
4-BrC6H4
95:5
7
4g
4h
4i
87:13
87:13
91:9
8e
9
1c (R = 4-MeOC6H4)
1d (R = 3-BrC6H4)
1c (R = 4-MeOC6H4)
1e (R = β-naphthyl)
1f (R = trans-styryl)
10e
11
12
4j
89:11
88:12
92:8
4k
4l
a The reactions were carried out on a 0.20 mmol scale, 1/2/3 = 1.2:2.2:1.0. Diazo compound 1 (0.24 mmol) in 1.0 mL of toluene was added to
a mixture of 2 (0.44 mmol), 3 (0.20 mmol), (S)-5j (5.0 mol%), Rh2(OAc)4 (2.0 mol%), and 4 Å MS (200 mg) in 1.0 mL of toluene at 40 °C
within 1 h via a syringe pump.
b Isolated yield of 4.
c The dr for 4 was determined by chiral HPLC with an IA column.
d The ee for anti-4 was determined by chiral HPLC with an IA column.
e The dr for 4 was determined by 1H NMR spectroscopy of the crude reaction mixture.
acid cocatalyzed four-component reaction, a plausible transformation proceeds through a Mannich-type trapping
preferential conformation for transition state was pro- of the ammonium ylide by the phosphoric acid activated
posed (Scheme 4). Iminoester that was protonated by iminoester. With this co-operative catalytic system, a se-
phosphoric acid would accept a nucleophilic attack from ries of chiral α,β-diamino acid derivatives were efficiently
ammonium ylide intermediate to afford anti-products as produced from simple starting materials in good yields
the major product through preferential conformation for and with good to excellent diastereo- and enantioselectiv-
transition state III (TSIII). A weak hydrogen bond be- ities.
tween the Lewis basic phosphoryl oxygen atom and the
acidic NH proton in the ammonium yilde intermediate
All NMR spectra were recorded on a Bruker spectrometer at 500
MHz (1H NMR) and 125 MHz (13C NMR), a Bruker spectrometer
at 400 MHz (1H NMR) and 100 MHz (13C NMR) or JNM-EX at 400
MHz (1H NMR) and 100 MHz (13C NMR). Chemical shifts (δ val-
ues) were reported in ppm downfield from internal TMS. HRMS
(ESI) mass spectra were recorded on a Bruker micrOTOF II instru-
ment. HPLC analysis was performed on Waters-Breeze (2487 Dual
Absorbance Detector and 1525 Binary HPLC Pump) and Shimadzu
(SPD-20AV UV-VIS Detector and LC-20AT Liquid Chromato-
graph Pump). Chiralpak IA was purchased from Daicel Chemical
Industries, LTD. CD spectra were recorded on a JASCO J-810 spec-
tropolarimeter.
was believed to be the key interaction to define the stereo-
chemical outcome of this transformation.16
In summary, we have developed a highly diastereo- and
enantioselective four-component reaction of a diazo ke-
tone with two equivalents of anilines and ethyl glyoxylate
in the presence of Rh2(OAc)4 and chiral phosphoric acid
catalysts. Rh2(OAc)4 plays a role to catalyze the diazo de-
composition reaction of the diazo ketone and the aniline to
generate an ammonium ylide intermediate. In the mean-
time, reaction of an additional equivalent of aniline with
ethyl glyoxylate promoted by the chiral phosphoric acid
allowed to form an iminium ion of iminoester. The desired
The racemic standards used in HPLC studies were prepared accord-
ing to the general procedure by using a racemic BINOL-derivatized
phosphoric acid catalyst.
© Georg Thieme Verlag Stuttgart · New York
Synthesis 2014, 46, 1348–1354

1352
C. Jing et al.
SPECIAL TOPIC
Four-Component Mannich-Type Reactions of Diazo Ketones 1,
Anilines 2, and Ethyl Glyoxylate (3); General Procedure
A mixture of Rh2(OAc)4 (1.77 mg, 2 mol%), chiral phosphoric acid
(S)-5 (5 mol%), aniline 2 (0.44 mmol, 2.2 equiv), ethyl glyoxylate
3 (40.8 mg, 0.20 mmol, 1.0 equiv), and 4 Å MS (200 mg) in solvent
(1.5 mL) was heated to 40 °C, then the diazo compound 1 (0.24
mmol, 1.2 equiv) in solvent (1.5 mL) was added over 1 h via a sy-
ringe pump. The reaction mixture was purified by flash chromatog-
raphy on silica gel to give the corresponding four-component
reaction product.
13C NMR (100 MHz, CDCl3): δ = 197.55, 170.54, 145.15, 144.87,
135.32, 134.13, 129.45, 129.26, 129.00, 128.42, 124.19, 124.02,
115.62, 115.56, 61.81, 60.33, 58.67, 13.93;
HRMS (ESI): m/z [M + Na]+ calcd for C24H22Cl2N2O3 + Na:
479.0900; found: 479.0930.
Ethyl (2R,3S)-2,3-Bis(3,4-dichlorophenylamino)-4-oxo-4-phen-
ylbutanoate (4d)
Yield: 74.73 mg (71%); 85:15 dr (anti/syn); 97% ee (anti) deter-
mined by HPLC [Daicel Chiralpak IA, flow rate 1.0 mL/min,
hexane–i-PrOH–EtOH (80:10:10), 254 nm; tR (minor) = 9.7 min, tR (ma-
jor) = 13.6 min].
1H NMR (400 MHz, CDCl3): δ = 7.89–7.87 (m, 2 H), 7.62–7.58 (m,
1 H), 7.50–7.46 (m, 2 H), 7.17 (d, J = 8.8 Hz, 1 H), 7.12 (d, J = 8.6
Hz, 1 H), 6. 78 (d, J = 2.7 Hz, 1 H), 6.61 (d, J = 2.7 Hz, 1 H), 6.53
(dd, J = 8.8, 2.7 Hz, 1 H), 6.38 (dd, J = 8.8, 2.7 Hz, 1 H), 5.37 (dd,
J = 9.8, 3.9 Hz, 1 H), 4.76 (d, J = 10.0 Hz, 1 H), 4.60 (d, J = 10.2
Hz, 1 H), 4.43 (dd, J = 10.2, 4.1 Hz, 1 H), 4.02–3.96 (m, 2 H), 1.10
(t, J = 7.2 Hz, 3 H).
13C NMR (125 MHz, CDCl3): δ = 196.90, 169.87, 145.79, 145.69,
135.14, 134.36, 133.28, 133.08, 131.04, 130.90, 129.14, 128.39,
122.31, 122.11, 115.57, 115.49, 114.03, 114.01, 62.10, 59.99,
58.40, 13.94.
Ethyl (2R,3S)-4-Oxo-4-phenyl-2,3-bis(phenylamino)butanoate
(4a); Typical Procedure
A mixture of Rh2(OAc)4 (1.77 mg, 2 mol%), chiral phosphoric acid
(S)-5j (8.65 mg, 5 mol%), aniline (2a; 40.98 mg, 0.44 mmol, 2.2
equiv), ethyl glyoxylate (3; 40.8 mg, 0.20 mmol, 1.0 equiv), and 4
Å MS (200 mg) in toluene (1.0 mL) was heated to 40 °C. Then, the
diazo compound 1a (35.08 mg, 0.24 mmol, 1.2 equiv) in toluene
(1.0 mL) was added over 1 h via a syringe pump. The reaction mix-
ture was purified by flash chromatography on silica gel (eluent: PE–
EtOAc, 40:1 to 10:1) to give 4a; yield: 66.82 mg (86%); 87:13 dr
(anti/syn); 95% ee (anti) determined by HPLC [Daicel Chiralpak
IA, flow rate 1.0 mL/min, hexane–i-PrOH (95:5); 254 nm; tR (mi-
nor) = 22.4 min, tR (major) = 27.5 min].
1H NMR (400 MHz, CDCl3): δ = 7.98 (d, J = 7.4 Hz, 2 H), 7.61 (t,
J = 7.3 Hz, 1 H), 7.49 (t, J = 7.6 Hz, 2 H), 7.21–7.12 (m, 4 H), 6.82–
6.74 (m, 4 H), 6.61 (d, J = 7.8 Hz, 2 H), 5.54 (dd, J = 9.8, 3.5 Hz, 1
H), 4.78 (d, J = 10.0 Hz, 1 H), 4.68–4.59 (m, 2 H), 4.04–4.01 (m, 2
H), 1.13 (t, J = 7.1 Hz, 3 H).
HRMS (ESI): m/z [M + Na]+ calcd for C24H20Cl4N2O3 + Na:
547.0120; found: 547.0101.
Ethyl (2R,3S)-2,3-Bis(4-fluorophenylamino)-4-oxo-4-phenylbu-
tanoate (4e)
Yield: 65.37 mg (77%); 92:8 dr (anti/syn); 88% ee (anti) deter-
mined by HPLC [Daicel Chiralpak IA, flow rate 1.0 mL/min,
hexane–i-PrOH–EtOH (90:5:5), 254 nm; tR (minor) = 19.9 min, tR
(major) = 25.6 min].
1H NMR (400 MHz, CDCl3): δ = 7.97 (d, J = 7.1 Hz, 2 H), 7.65–
7.61 (m, 1 H), 7.51 (t, J = 7.7 Hz, 2 H), 6.93–6.84 (m, 4 H), 6.77–
6.72 (m, 2 H), 6.57–6.54 (m, 2 H), 5.43 (dd, J = 10.5, 4.2 Hz, 1 H),
4.68 (d, J = 10.5 Hz, 1 H), 4.57 (d, J = 10.8 Hz, 1 H), 4.48 (dd, J =
10.8, 4.2 Hz, 1 H), 4.06–3.98 (m, 2 H), 1.13 (t, J = 7.2 Hz, 3 H).
13C NMR (100 MHz, CDCl3): δ = 198.08, 170.98, 146.73, 146.53,
135.67, 133.87, 129.56, 129.37, 128.92, 128.50, 119.40, 119.24,
114.59, 114.52, 61.59, 60.55, 58.83, 13.97.
HRMS (ESI): m/z [M + Na]+ calcd for C24H24N2O3 + Na: 411.1679;
found: 411.1674.
Ethyl (2R,3S)-2,3-Bis(4-bromophenylamino)-4-oxo-4-phenyl-
butanoate (4b)
Yield: 89.59 mg (82%); 91:9 dr (anti/syn); 92% ee (anti) deter-
mined by HPLC [Daicel Chiralpak IA, flow rate 1.0 mL/min, hex-
ane–EtOH (80:20), 254 nm; tR (minor) = 12.0 min, tR (major) = 15.0
min].
13C NMR (100 MHz, CDCl3): δ = 197.96, 170.88, 158.05, 155.68,
142.90, 142.71, 142.68, 135.47, 133.99, 128.94, 128.40, 115.92,
115.88, 115.85, 61.62, 61.26, 59.48, 13.92.
1H NMR (500 MHz, CDCl3): δ = 7.95 (d, J = 7.4 Hz, 2 H), 7.63 (t,
J = 7.6 Hz, 1 H), 7.51 (t, J = 7.7 Hz, 2 H), 7.28 (d, J = 8.7 Hz, 2 H),
7.23 (d, J = 8.7 Hz, 2 H), 6.65 (d, J = 8.7 Hz, 2 H), 6.47 (d, J = 8.7
Hz, 2 H), 5.47 (dd, J = 10.1, 4.1 Hz, 1 H), 4.78 (d, J = 10.1 Hz, 1
H), 4.65 (d, J = 10.4 Hz, 1 H), 4.51 (dd, J = 10.4, 4.1 Hz, 1 H), 4.03–
3.99 (m, 2 H), 1.13 (t, J = 7.1 Hz, 3 H).
13C NMR (100 MHz, CDCl3): δ = 197.44, 170.44, 145.57, 145.29,
135.29, 134.11, 132.31, 132.10, 128.98, 128.39, 116.00, 115.95,
111.24, 111.09, 61.80, 60.13, 58.49, 13.92.
HRMS (ESI): m/z [M + Na]+ calcd for C24H22F2N2O3 + Na:
447.1491; found: 447.1489.
Ethyl (2R,3S)-2,3-Bis(3,4-difluorophenylamino)-4-oxo-4-phen-
ylbutanoate (4f)
Yield: 66.30 mg (72%); 95:5 dr (anti/syn); 91% ee (anti) deter-
mined by HPLC [Daicel Chiralpak IA, flow rate 1.0 mL/min,
hexane–i-PrOH–EtOH (90:5:5), 254 nm; tR (minor) = 16.4 min, tR
(major) = 23.0 min].
1H NMR (400 MHz, CDCl3): δ = 7.96 (d, J = 7.3 Hz, 2 H), 7.66 (t,
J = 7.5 Hz, 1 H), 7.53 (t, d, J = 7.8 Hz, 2 H), 7.03–6.91 (m, 2 H),
6.63–6.58 (m, 1 H), 6.47–6.38 (m, 2 H), 6.30–6.28 (m, 1 H), 5.39
(dd, J = 10.2, 4.1 Hz, 1 H), 4.74 (d, J = 10.2 Hz, 1 H), 4.61 (d, J =
10.5 Hz, 1 H), 4.44 (dd, J =10.5, 4.1 Hz, 1 H), 4.08–4.00 (m, 2 H),
1.15 (t, J = 7.1 Hz, 3 H).
13C NMR (100 MHz, CDCl3): δ = 197.32, 170.34, 152.01, 149.56,
143.38, 135.21, 134.28, 129.08, 128.38, 118.01, 117.81, 117.62,
109.96, 109.93, 109.79, 103.70, 103.61, 103.49, 103.41, 61.93,
60.71, 59.01, 13.91.
HRMS (ESI): m/z [M + Na]+ calcd for C24H22Br2N2O3 + Na:
566.9889; found: 566.9923.
Ethyl (2R,3S)-2,3-Bis(4-chlorophenylamino)-4-oxo-4-phenyl-
butanoate (4c)
Yield: 78.66 mg (86%); 90:10 dr (anti/syn); 94% ee (anti) deter-
mined by HPLC [Daicel Chiralpak IA, flow rate 1.0 mL/min,
hexane–i-PrOH (80:20), 254 nm; tR (minor) = 15.5 min, tR (ma-
jor) = 19.8 min].
1H NMR (400 MHz, CDCl3): δ = 7.96 (d, J = 7.3 Hz, 2 H), 7.64 (t,
J = 7.6 Hz, 1 H), 7.52 (t, J = 7.6 Hz, 2 H), 7.17–7.14 (m, 2 H), 7.12–
7.08 (m, 2 H), 6.73–6.70 (m, 2 H), 6.54–6.51 (m, 2 H), 5.47 (dd, J =
10.0, 3.9 Hz, 1 H), 4.76 (d, J = 10.2 Hz, 1 H), 4.64 (d, J = 10.5 Hz,
1 H), 4.51 (dd, J = 10.5, 4.2 Hz, 1 H), 4.07–3.99 (m, 2 H), 1.14 (t,
J = 7.1 Hz, 3 H).
HRMS (ESI): m/z [M + Na]+ calcd for C24H20F4N2O3 + Na:
483.1302; found: 483.1282.
Ethyl (2R,3S)-4-(4-Chlorophenyl)-2,3-bis(4-chlorophenylami-
no)-4-oxobutanoate (4g)
Yield: 67.80 mg (69%); 87:13 dr (anti/syn); 94% ee (anti) deter-
mined by HPLC [Daicel Chiralpak IA, flow rate 1.0 mL/min,
Synthesis 2014, 46, 1348–1354
© Georg Thieme Verlag Stuttgart · New York

SPECIAL TOPIC
Facile Synthesis of Chiral α,β-Diamino Acid Derivatives
1353
hexane–i-PrOH–EtOH (90:5:5), 254 nm; tR (minor) = 28.4 min, tR
(major) = 41.0 min].
HRMS (ESI): m/z [M + Na]+ calcd for C25H24Br2N2O4 + Na:
596.9995; found: 596.9985.
1H NMR (500 MHz, CDCl3): δ = 7.90 (d, J = 8.5 Hz, 2 H), 7.48 (d,
J = 8.5 Hz, 2 H), 7.17 (d, J = 8.7 Hz, 2 H), 7.10 (d, J = 8.7 Hz, 2 H),
6.70 (d, J = 8.7 Hz, 2 H), 6.51 (d, J = 8.7 Hz, 2 H), 5.42 (dd, J =
10.4, 4.2 Hz, 1 H), 4.71 (d, J = 10.4 Hz, 1 H), 4.63 (d, J = 10.5 Hz,
1 H), 4.48 (d, J = 10.5, 4.2 Hz, 1 H), 4.08–4.03 (m, 2 H), 1.15 (t, J =
7.1 Hz, 3 H).
Ethyl (2R,3S)-2,3-Bis(4-bromophenylamino)-4-(naphthalen-2-
yl)-4-oxobutanoate (4k)
Yield: 88.25 mg (74%); 88:12 dr (anti/syn); 93% ee (anti) deter-
mined by HPLC [Daicel Chiralpak IA, flow rate 1.0 mL/min,
hexane–i-PrOH–EtOH (80:10:10), 254 nm; tR (minor) = 23.1 min,
tR (major) = 27.4 min].
13C NMR (125 MHz, CDCl3): δ = 196.49, 170.61, 145.19, 144.61,
140.68, 133.58, 129.86, 129.53, 129.31, 129.26, 124.40, 124.16,
115.62, 115.58, 61.88, 60.37, 58.64, 13.96.
HRMS (ESI): m/z [M + Na]+ calcd for C24H21Cl3N2O3 + Na:
513.0510; found: 513.0505.
1H NMR (400 MHz, CDCl3): δ = 8.49 (s, 1 H), 7.99–7.89 (m, 4 H),
7.67–7.58 (m, 2 H), 7.31–7.23 (m, 4 H), 6.70 (d, J = 8.0 Hz, 2 H),
6.50 (d, J = 8.0 Hz, 2 H), 5.62 (dd, J = 12.0, 4.0 Hz, 1 H), 4.85 (d,
J = 8.0 Hz, 1 H), 4.69 (d, J = 12.0 Hz, 1 H), 4.59 (dd, J = 8.0, 4.0
Hz, 1 H), 4.05–3.96 (m, 2 H), 1.11 (t, J = 6.0 Hz, 3 H).
13C NMR (100 MHz, CDCl3): δ = 197.34, 170.49, 145.57, 145.46,
136.01, 132.66, 132.41, 132.38, 132.20, 130.20, 129.65, 129.21,
129.09, 127.93, 127.30, 123.86, 116.15, 116.04, 111.34, 111.19,
61.86, 60.33, 58.79, 13.96.
HRMS (ESI): m/z [M + Na]+ calcd for C28H24Br2N2O3 + Na:
617.0046; found: 617.0072.
Ethyl (2R,3S)-2,3-Bis(4-chlorophenylamino)-4-(4-methoxyphe-
nyl)-4-oxobutanoate (4h)
Yield: 63.36 mg (65%); 87:13 dr (anti/syn); 92% ee (anti) deter-
mined by HPLC [Daicel Chiralpak IA, flow rate 1.0 mL/min,
hexane–i-PrOH–EtOH (80:10:10), 254 nm; tR (minor) = 28.6 min,
tR (major) = 38.6 min].
1H NMR (500 MHz, CDCl3): δ = 7.95 (d, J = 8.8 Hz, 2 H), 7.14 (d,
J = 8.7 Hz, 2 H), 7.08 (d, J = 8.7 Hz, 2 H), 6.96 (d, J = 8.8 Hz, 2 H),
6.69 (d, J = 8.7 Hz, 2 H), 6.51 (d, J = 8.8 Hz, 2 H), 5.43 (dd, J =
10.3, 4.2 Hz, 1 H), 4.76 (d, J = 10.3 Hz, 1 H), 4.65 (d, J = 10.6 Hz,
1 H), 4.49 (dd, J = 10.6, 4.2 Hz, 1 H), 4.04–4.00 (m, 2 H), 3.89 (s,
3 H), 1.12 (t, J = 7.1 Hz, 3 H).
Ethyl (2R,3S,5E)-2,3-Bis(4-bromophenylamino)-4-oxo-6-phen-
ylhex-5-enoate (4l)
Yield: 57.23 mg (50%); 92:8 dr (anti/syn); 67% ee (anti) deter-
mined by HPLC [Daicel Chiralpak IA, flow rate 1.0 mL/min,
hexane–i-PrOH (90:10), 254 nm; tR (minor) = 18.0 min, tR (ma-
jor) = 19.4 min].
13C NMR (125 MHz, CDCl3): δ = 195.69, 170.74, 164.33, 145.25,
144.89, 130.87, 129.40, 129.18, 128.08, 123.98, 123.84, 115.54,
115.49, 114.17, 61.70, 59.60, 58.62, 55.57, 13.93.
HRMS (ESI): m/z [M + Na]+calcd for C25H24Cl2N2O4 + Na:
509.1005; found: 509.0992.
1H NMR (500 MHz, CDCl3): δ = 7.73 (d, J = 15.9 Hz, 1 H), 7.54–
7.53 (m, 2 H), 7.41–7.39 (m, 3 H), 7.32–7.30 (m, 2 H), 7.24–7.22
(m, 3 H), 6.98 (d, J = 15.9 Hz, 1 H), 6.63 (d, J = 8.8 Hz, 1 H), 6.56–
6.55 (m, 1 H), 6.51 (d, J = 8.8 Hz, 1 H), 4.86 (dd, J = 9.9, 3.8 Hz, 1
H), 4.71 (d, J = 9.9 Hz, 1 H), 4.56 (d, J = 10.0 Hz, 1 H), 4.50–4.49
(m, 1 H), 4.20–4.15 (m, 2 H), 1.22 (t, J = 7.1 Hz, 3 H).
Ethyl (2R,3S)-4-(3-Bromophenyl)-2,3-bis(4-chlorophenylami-
no)-4-oxobutanoate (4i)
Yield: 66.50 mg (62%); 91:9 dr (anti/syn); 94% ee (anti) deter-
mined by HPLC [Daicel Chiralpak IA, flow rate 1.0 mL/min,
hexane–i-PrOH–EtOH (92:6:2), 254 nm; tR (minor) = 23.9 min, tR
(major) = 33.5 min].
13C NMR (100 MHz, CDCl3): δ = 196.75, 170.85, 145.83, 145.27,
145.19, 133.90, 132.37, 132.11, 131.95, 131.24, 129.04, 128.68,
121.85, 116.65, 115.91, 115.68, 111.10, 111.07, 63.25, 61.97,
58.23, 14.06.
HRMS (ESI): m/z [M + Na]+ calcd for C26H24Br2N2O3 + Na:
593.0046; found: 593.0045.
1H NMR (500 MHz, CDCl3): δ = 8.06–7.91 (m, 1 H), 7.87–7.81 (m,
1 H), 7.75–7.59 (m, 1 H), 7.46–7.38 (m, 1 H), 7.16–7.10 (m, 4 H),
6.69 (d, J = 7.5 Hz, 2 H), 6.52 (d, J = 7.9 Hz, 2 H), 5.40–5.39 (m, 1
H), 4.72 (d, J = 10.0 Hz, 1 H), 4.61 (d, J = 10.0 Hz, 1 H), 4.51–4.49
(m, 1 H), 4.07–4.05 (m, 2 H), 1.17 (t, J = 6.6 Hz, 3 H).
Acknowledgment
We are grateful for the financial support from NSF of China
(21125209 and 21332003), the MOST of China (2011CB808600),
STCSM (12JC1403800), specialized Research Fund for the Docto-
ral Program of Higher Education (20100076110005 and
20130076120031), Shanghai Postdoctoral Scientific Program
(13R21412800), and the Scholarship Award for Excellent Doctoral
Student granted by East China Normal University (XRZZ2013021).
We also thank Prof. X. F. Mei for the CD analysis.
13C NMR (125 MHz, CDCl3): δ = 196.52, 196.44, 170.45, 170.42,
145.07, 145.05, 144.63, 136.88, 135.35, 133.98, 131.42, 130.55,
130.31, 129.52, 129.31, 128.51, 126.90, 126.45, 124.41, 124.18,
123.29, 115.65, 61.97, 60.64, 58.70, 13.99.
HRMS (ESI): m/z [M + Na]+ calcd for C24H21BrCl2N2O3 + Na:
557.0005; found: 556.9993.
Ethyl (2R,3S)-2,3-Bis(4-bromophenylamino)-4-(4-methoxyphe-
nyl)-4-oxobutanoate (4j)
Yield: 71.46 mg (62%); 89:11 dr (anti/syn); 93% ee (anti) deter-
mined by HPLC [Daicel Chiralpak IA, flow rate 1.0 mL/min,
hexane–i-PrOH–EtOH (240:15:50), 254 nm; tR (minor) = 26.1 min,
tR (major) = 32.9 min].
1H NMR (500 MHz, CDCl3): δ = 7.95 (d, J = 8.3 Hz, 2 H), 7.28–
7.21 (m, 4 H), 6.96 (d, J = 8.5 Hz, 2 H), 6.64 (d, J = 8.2 Hz, 2 H),
6.47 (d, J = 8.3 Hz, 2 H), 5.44–5.42 (m, 1 H), 4.77 (d, J = 9.4 Hz, 1
H), 4.66 (d, J = 10.0 Hz, 1 H), 4.49 (d, J = 10.2 Hz, 1 H), 4.04–4.01
(m, 2 H), 3.88 (s, 3 H), 1.12 (t, J = 6.9 Hz, 3 H).
13C NMR (125 MHz, CDCl3): δ = 195.62, 170.66, 164.35, 145.70,
145.32, 132.29, 132.07, 130.87, 128.09, 115.97, 115.92, 114.18,
111.09, 110.97, 61.71, 59.44, 58.51, 55.57, 13.93.
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synthesis.SnoIufproi
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Synthesis 2014, 46, 1348–1354
© Georg Thieme Verlag Stuttgart · New York