L-valine dipeptide organocatalysts with two amide units for the direct asymmetric aldol reaction in brine

Article abstract of DOI:10.1007/s10562-011-0589-z

A series of valine dipeptide organocatalysts containing a primary amine group and two amide units have been developed and evaluated in the direct asymmetric intermolecular aldol reaction of 4-nitrobenzaldehyde and cyclohexanone. When 2,4-dinitrophenol (DN

Full text of DOI:10.1007/s10562-011-0589-z

Catal Lett (2011) 141:872–876
DOI 10.1007/s10562-011-0589-z
L-Valine Dipeptide Organocatalysts with Two Amide Units
for the Direct Asymmetric Aldol Reaction in Brine
•
Wenbo Huang Hua Tian Hao Xu
•
•
•
Liangyu Zheng Qingwen Liu Suoqin Zhang
•
Received: 21 February 2011 / Accepted: 23 March 2011 / Published online: 7 April 2011
Ó Springer Science+Business Media, LLC 2011
Abstract A series of valine dipeptide organocatalysts
containing a primary amine group and two amide units
have been developed and evaluated in the direct asym-
metric intermolecular aldol reaction of 4-nitrobenzalde-
hyde and cyclohexanone. When 2,4-dinitrophenol (DNP)
was used as an acidic additive, the catalyzed reactions of
various aldehydes and ketones gave the corresponding
aldol products with moderate to high enantioselectivities
(up to 95%) and diastereoselectivities (up to [99/1, anti/
syn) in the presence of 3c in brine.
of attention [9–13]. On the other hand, the development of
stereoselective reactions using water as reaction medium is
another attractive research field [14, 15], mainly due to the
low cost, safety and environmentally benign nature of
water. Therefore, a great deal of emphasis has been
devoted to the development of water-tolerant organocata-
lyst for direct asymmetric aldol reaction, and some groups
have reported organocatalytic direct asymmetric aldol in
water without using any organic co-solvent [16–19].
Most of the reported peptide organocatalysts for the
direct asymmetric aldol reaction are N-terminal prolyl
´
dipeptides [20–29]. Cordova et al. showed that amino acids
Keywords Valine Á Dipeptide Á Aldol reaction Á Brine
[30, 31] and small di- to tetra-peptides [32, 33] with a
primary amino functionality are able to catalyze the
asymmetric intermolecular aldol reaction with high stere-
oselectivity. In addition, the small peptides can catalyze the
aldol reaction in water and aqueous media with high
asymmetric induction [33]. Since these findings, linear
amino acids and their derivatives are used more often as
catalysts for highly enantioselective intermolecular aldol
reactions [34–37]. However, the reports about dipeptides of
N-terminal primary amine catalyzed asymmetric aldol
reaction in aqueous medium are limited [33].
1 Introduction
The asymmetric aldol reaction is one of the most useful
carbon–carbon bond-forming reactions [1–5] and plays an
important role in organic synthesis. Since the pioneering
work by List et al. [6–8] in asymmetric organocatalysis,
development of simple small moleculars as efficient
organocatalysts for this reaction has attracted a great deal
Herein we report a series of N-primary-amine terminal
valine dipeptide organocatalysts which containing a pri-
mary amine group and two amide units for the asymmetric
aldol reaction in brine. These catalysts were synthesized
from N-t-butyloxycarbonyl protected valine and aryl-
amines. The catalyst properties can be readily tuned by
varying the stereo and electronic effects of the terminal
amide to affect the ability of hydrogen bonding formation
between the catalysts and the substrate. Strong electron-
withdrawing nitro group was introduced to enhancing the
amide NH acidity in the hope of improving stereocontrol
[38–41].
W. Huang Á H. Tian Á H. Xu Á Q. Liu Á S. Zhang (&)
College of Chemistry, Jilin University, Changchun 130012,
People’s Republic of China
e-mail: suoqin@jlu.edu.cn
L. Zheng (&)
Key Laboratory for Molecular Enzymology and Engineering of
Ministry of Education, Jilin University, Changchun 130012,
People’s Republic of China
e-mail: lyzheng@jlu.edu.cn
123

Direct Asymmetric Aldol Reaction in Brine
2 Experimental
873
with brine (2 9 15 mL), and dried over anhydrous
Na2SO4. The solvent was evaporated to give compound 2.
N-t-butyloxycarbonyl-L-valine and DCC (1.0 equiv) were
dissolved in 20 mL dichloromethane. Then the solution was
cooled to 0 °C and stirred for 0.5 h. After that, compound 2
was added and the reaction mixture was warmed to room
temperature and stirred for another 5 h. After filtration and
the removal of solvent under reduced pressure, the residue
was purified through chromatography on silica gel column
[eluent, V(chloroform)/V(ethyl acetate) = 2:1] to afford the
N-t-butyloxycarbonyl protected L-valine dipeptide.
2.1 Materials and Instruments
Arylamines and N-t-butyloxycarbonyl protected valine
were purchased from commercial suppliers. 1H NMR
spectra were recorded on a 300 MHz FT-NMR Mercury-
300B (Varian) spectrometer with tetramethylsilane as an
internal reference. Solvent for NMR is CDCl3. 13C NMR
were recorded on 75 MHz. Elemental analysis (C, H, and
N) were performed on a Perkin-Elmer 2400 CHN Ele-
mental Analyzer. The enantiomeric excess was determined
by chiral HPLC with Daicel Chiralpak AD-H (4.6 mm 9
250 mm), OD-H (4.6 mm 9 250 mm) and AS-H (4.6 mm 9
250 mm) column. Diastereoselectivities were determined
N-t-butyloxycarbonyl protected L-valine dipeptide was
dissolved in 10 mL dichloromethane, to which 2 mL TFA
was added. The solution was stirred for 5 h at room
temperature. Then the reaction was quenched with con-
centrated ammonia solution and extracted with dichloro-
methane (3 9 10 mL). The organic layer was washed with
brine (2 9 10 mL), and dried over anhydrous Na2SO4.
After the removal of the solvent under reduced pressure,
the residue was purified through flash column chromatog-
raphy on silica gel (eluent, V(chloroform)/V(metha-
nol) = 5:1) to afford compound 3.
1
by H NMR analysis of the aldol product.
2.2 Catalyst Preparation
Organocatalyst 3a–3d was prepared from N-t-butyloxy-
carbonyl protected L-valine and arylamines as shown in
Scheme 1.
N-t-butyloxycarbonyl-L-valine (10 mmol) and aryl-
amine (10 mmol) were dissolved in 30 mL dry pyridine
[42]. Then the clear yellowish solution was cooled to
-15 °C and phosphorus oxychloride (1.0 mL, 11 mmol)
was added dropwise with vigorous stirring over 15 min.
The reaction mixture was stirred until the reaction was
completed (monitored by TLC). Quenched with crushed
ice/water, and extracted with ethyl acetate (30 mL 9 4).
Combined organic layer was washed with saturated
NaHCO3 solution and brine, dried over anhydrous Na2SO4.
After the removal of the solvent under reduced pressure,
the residue was purified through flash column chromatog-
raphy on silica gel [eluent, V(chloroform)/V(ethyl ace-
tate) = 5:1] to afford compound 1.
2.2.1 Catalyst 3a
White solid, 1H NMR (300 MHz, CDCl3), d: 1.85 (d,
J = 6.9 Hz, 3H), 1.00–1.05 (m, 9H), 1.61 (br, 2H),
2.22–2.39 (m, 2H), 3.33 (d, J = 2.4 Hz, 1H), 4.43 (t,
J = 8.2 Hz, 1H), 7.07 (t, J = 7.3 Hz, 1H), 7.25–7.30 (m,
2H), 7.53 (d, J = 7.8 Hz, 2H), 8.14 (d, J = 8.1 Hz, 1H),
8.84 (br, 1H); 13C NMR (75 MHz, CDCl3), d: 15.64, 18.09,
18.82, 19.18, 30.43, 30.77, 58.38, 59.70, 119.41, 123.37,
128.09, 137.64, 169.82, 174.52; (EI) m/z 291 (rel intensity)
(M? ? 1); Analysis calculated for C16H25N3O2: C, 65.95;
H, 8.65; N, 14.42. Found: C, 66.12; H, 8.74; N, 14.31.
2.2.2 Catalyst 3b
Compound 1 was dissolved in 8 mL dichloromethane, to
which 2 mL TFA was added. The solution was stirred for
4 h at room temperature. Then the reaction was quenched
with concentrated ammonia solution and extracted with
chloroform (3 9 15 mL). The organic layer was washed
Yellow solid, 1H NMR (300 MHz, CDCl3), d: 0.87 (d,
J = 6.9 Hz 3H), 0.97 (d, J = 6.9 Hz 3H), 1.03–1.07 (m,
6H), 1.58 (br, 2H), 2.39–2.45 (m, 1H), 2.50–2.62 (m, 1H),
3.52 (d, J = 6.9 Hz 1H), 4.52–4.56 (m, 1H), 7.16–7.22
Scheme 1 Preparation of
organocatalysts
R
NH2
R
O
O
POCl3
pyridine, -15 oC
TFA
CH2Cl2
+
N
H
OH
NHBoc
NHBoc
1
R
O
O
H
N
3a
1) Boc-L-Valine, DCC
: R = H
3b
: R = 2-NO2
N
N
H
2) TFA, CH2Cl2
3c: R = 3-NO2
3d: R = 4-NO2
R
H
NH2
O
NH2
2
3a-3d
123

874
W. Huang et al.
Table 1 Aldol reaction of cyclohexanone and 4-nitrobenzaldehyde under various conditions
O
O
OH
O
cat
H
+
O2N
O2N
4
5
6
Entry
Solvent
Additive
Catalyst
Time (h)
Yield (%)a
Anti/synb
ee for anti (%)c
1
CH2Cl2
CHCl3
Toluene
DMF
None
None
None
None
None
None
None
None
3a
3a
3a
3a
3a
3a
3a
3a
3a
3a
3a
3a
3a
3a
3c
3d
75
77
92
72
52
42
48
68
14
12
9
65
73
58
25
77
79
76
67
81
73
75
65
82
79
84
83
91:9
94:6
93:7
83:17
94:6
95:5
97:3
94:6
97:3
95:5
94:6
92:8
99:1
94:6
98:2
99:1
42
55
53
38
80
81
82
81
86
83
83
82
88
92
95
93
2
3
4
5
Water
Brine
Brine
Brine
Brine
Brine
Brine
Brine
Brine
Brine
Brine
Brine
6
7d
8e
9
PhCOOH
10
11
12
13
14
15
16
3,5-Dinitrobenzoic acid
2-Nitrobenzoic acid
TFA
DNP
DNP
DNP
DNP
7.5
33
29
26
24
Conditions: aldehyde (0.25 mmol), cyclohexanone (2.5 mmol), catalyst (10 mol%), 1.0 mL brine, rt
a
Isolated yield
b
c
d
e
1
Determined by H NMR
Determined by chiral HPLC
Reaction temperature 5 °C
5 mol % of catalyst used
2.2.4 Catalyst 3d
(m,1H), 7.63–7.69 (m, 1H), 8.21–8.24 (m, 1H), 8.30 (d,
J = 8.7 Hz, 1H), 8.87–8.90 (m, 1H), 11.01 (br, 1H); 13C
NMR (75 MHz, CDCl3), d: 15.89, 17.10, 19.58, 19.62,
29.53, 30.35, 59.18, 59.87, 121.63, 123.31, 125.66, 134.38,
136.02, 170.74, 175.28; (EI) m/z 336 (rel intensity)
(M? ? 1); Analysis calculated for C16H24N4O4: C, 57.13;
H, 7.19; N, 16.66. Found: C, 57.28; H, 7.33; N, 16.52.
White solid, 1H NMR (300 MHz, CDCl3), d: 0.86 (d,
J = 6.9 Hz, 3H), 1.03–1.07 (m, 9H), 1.49 (br, 2H),
2.23–2.46 (m, 2H), 3.38 (d, J = 3.3 Hz, 1H), 4.41 (t,
J = 8.2 Hz,1H), 7.67 (d, J = 9.0 Hz, 2H), 8.10 (d,
J = 9.3 Hz, 2H), 8.26 (d, J = 8.1 Hz, 1H), 9.78 (br, 1H);
13C NMR (75 MHz, CDCl3), d: 16.04, 18.47, 19.28, 19.63,
30.86, 30.91, 59.22, 60.09, 118.96, 124.53, 142.94, 144.15,
170.89, 175.60; (EI) m/z 336 (rel intensity) (M? ? 1);
Analysis calculated for C16H24N4O4: C, 57.13; H, 7.19; N,
16.66. Found: C, 57.18; H, 7.26; N, 16.51.
2.2.3 Catalyst 3c
White solid, 1H NMR (300 MHz, CDCl3), d: 0.89 (d,
J = 6.9 Hz, 3H), 1.04–1.08 (m, 9H), 1.51 (br, 2H),
2.19–2.30 (m, 1H), 2.37–2.43 (m, 1H), 3.48 (d,
J = 3.3 Hz, 1H), 4.595 (t, J = 8.2 Hz, 1H), 7.36 (t,
J = 8.2 Hz, 1H), 7.85 (d, J = 7.8 Hz, 1H), 8.01 (d,
J = 8.1 Hz, 1H), 8.35, (m, 2H), 9.98 (br, 1H); 13C NMR
(75 MHz, CDCl3), d: 16.07, 18.55, 19.22, 19.58, 30.95,
31.31, 58.91, 60.11, 114.22, 118.09, 125.11, 129.37,
139.46, 148.11, 170.69, 175.47; (EI) m/z 336 (rel intensity)
(M? ? 1); Analysis calculated for C16H24N4O4: C, 57.13;
H, 7.19; N, 16.66. Found: C, 57.26; H, 7.30; N, 16.48.
2.3 General Procedure for the Catalytic Direct
Asymmetric Aldol Reaction
All solvents were refined according to standard methods.
Aldehyde, ketone (freshly distilled), catalyst and 1.0 mL
solvent were added to a 5 mL glass reactor. The reaction
mixture was stirred until all the aldehyde has been con-
sumed (monitored by TLC). Quenched with saturated
123

Direct Asymmetric Aldol Reaction in Brine
875
ammonium chloride solution (5 mL), and extracted with
ethyl acetate (10 mL 9 3). Combined organic layer was
washed with saturated NaHCO3 solution and dried over
anhydrous Na2SO4. After the removal of the solvent under
reduced pressure, the residue was purified through flash
column chromatography on silica gel [eluent, V(ethyl
acetate)/V(petroleum ether) = 1:3] to afford pure aldol
product.
stereocontrol, and the steric factors also influenced the
enantioselectivity.
To further explore the scope of the reaction, the direct
asymmetric aldol reactions of various aldehydes and
cyclohexanone in the presence of 3c were examined as
shown in Table 2. The benzaldehydes substituted by strong
electron-withdrawing nitro and canyo groups were con-
verted to the corresponding aldol products in good yields,
with moderate to high enantioselectivities (86–95% ee)
and high diastereoselectivities (dr up to [99:1). Enanti-
oselectivity was influenced by the electron-withdrawing
ability of the substituent on the benzene ring. The aldol
reactions of cyclohexanone and benzaldehydes which
bearing chloro and fluoro substituents on the benzene ring
(entrys 7–10) were very sluggish, giving the corresponding
aldol product in moderate yields with moderate enanti-
oselectivities (68–82%) and high diastereoselectivities (up
to 99:1). Under the same condition, reaction of less reactive
benzaldehyde with cyclohexanone afforded trace product.
The direct asymmetric aldol reactions between various
ketones and aldehydes were also examined as shown in
Table 3. The aldol reactions of cyclopentanone with
4-nitrobenzaldehyde and 3-nitrobenzaldehyde gave the
corresponding aldol product in high yield but with low
enantioselectivity and diastereoselectivity. The reaction of
3 Results and Discussion
As shown in Scheme 1, dipeptide organocatalysts 3a–3d
was prepared from Boc-L-valine and arylamines in four
steps. The catalytic activity of these new catalysts was then
evaluated in the direct asymmetric intermolecular aldol
reaction of 4-nitrobenzaldehyde and cyclohexanone.
Results were summarized in Table 1. Initially, various
solvents were screened with 3a at room temperature. It was
found that brine [43–46] was the more suitable reaction
medium (entry 6). Higher stereoselectivity and diastere-
oselectivity were obtained in brine compared to organic
solvent (entry 1–5). Decreasing the reaction temperature
did not increase the enantioselectivity as indicated in entry
7. Influence of catalyst loading was also examined.
Reducing the amount of catalyst (entry 8) from 10 to
5 mol% did not cause the drop of enantioselectivity but led
to a prolonged reaction time. Considering the effect of acid
additives on increasing the yield and enantioselectivity [29,
46], a series of acidic additives were tested in the same
reaction. As expected, the acidic additives increased the
enantioselectivity and abbreviated the reaction time. The
increase of the enantioselectivities caused by strong acidic
additives such as 2-nitrobenzoic acid and TFA was founded
to be slightly (entry 9–12). When DNP (2,4-dinitrophenol)
was added as an additive, both the higher enantioselectivity
and diastereoselectivity were obtained. Hence DNP was
chose as the additive for this reaction. The influence of the
ratio of ketone and aldehyde on the reaction was also
determined. Increasing the ratio of ketone and aldehyde
from 10:1 to 20:1 did not cause the improved enantiose-
lectivity. Decreasing the ratio of ketone and aldehyde from
10:1 to 5:1 caused the drop in enantioselectivity (from 88
to 84% ee). Next, the catalytic activity of catalysts 3b–3c
was evaluated under the optimized condition. As can be
seen from the results summarized in Table 1, catalysts 3b,
3c and 3d which bearing an electron-withdrawing nitro
group on the benzene ring gave the desired product in high
enantioselectivity and diastereoselectivity, 3c gave the best
result (ee up to 95%, dr up to 98:2). Catalyst 1a which
having no electron-withdrawing group on the benzene ring
gave lower enantioselectivity and diastereoselectivity. The
resluts indicated that NH acidity plays a pivotal role in
Table 2 Aldol reaction of various aldehydes and cyclohexanone in
the presence of 3c
O
OH
O
O
10mol% 3c
brine, rt
H
R
R
+
5
4
6
Entry
R
Product Time Yield Anti/synb ee for
(h)
(%)a
anti (%)c
1
4-NO2 6a
3-NO2 6b
2-NO2 6c
26
29
38
43
72
72
96
96
96
96
72
84
98:2
99:1
[99:1
98:2
96:4
[99:1
92:8
98:2
99:1
99:1
–
95
93
94
87
86
87
72
68
82
83
–
2
79
3
76
4
4-CN
3-CN
2-CN
3-Cl
2-Cl
3-F
6d
6e
6f
77
5
73
6
69
7
6g
6h
6i
45
8
43
9
51
10
11
2-F
6j
48
H
6k
Trace
Conditions: aldehyde (0.25 mmol), cyclohexanone (2.5 mmol), cat-
alyst (10 mol%), 2,4-dinitrophenol (10 mol%), 1.0 mL brine, rt
a
Isolated yield
1
Determined by H NMR
b
c
Determined by chiral HPLC
123

876
W. Huang et al.
9. List B (2004) Acc Chem Res 37:548
Table 3 Aldol reaction of aldehydes and various ketones in the
presence of 3c
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O
OH
O
O
¨
12. Groger H, Wilken J (2001) Angew Chem Int Ed 40:529
H
10mol% 3c
R1 brine, rt
R1
R2
R
+
R
13. Carrea G, Ottolina G, Lazcano A, Pirontic V, Colonna S (2007)
Tetrahedron:Asymmetr 18:1265
14. Lindstrom UM (2002) Chem Rev 102:2751
15. Li C-J (2005) Chem Rev 105:3095
16. Mlynarski J, Paradowska J (2008) Chem Soc Rev 37:1502
17. Pan C-F, Wang Z-Y (2008) Coord Chem Rev 252:736
18. Gruttadauria M, Giacalone F, Noto R (2009) Adv Synth Catal
351:33
19. Mase N, Barbas CF III (2010) Org Biomol Chem 8:4043
20. Krattiger P, Kovasy R, Revell JD, Ivan S, Wennemers H (2005)
Org Lett 7:1101
21. Shi L-X, Sun Q, Ge Z-M, Zhu Y-Q, Cheng T-M, Li R-T (2004)
Synlett 2215
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R2
6
Entry
R
R1
R2 Product Time Yield anti/ ee
(h)
(%)a synb (%)c
1
2
3
4-NO2 –(CH2)3–
3-NO2 –(CH2)3–
4-NO2 CH3–
6l
34
45
32
92
89
63
22:78 89
37:66 86
29
6m
6n
H
Conditions: aldehyde (0.25 mmol), ketone (2.5 mmol), catalyst
(10 mol%), 2,4-dinitrophenol (10 mol%), 1.0 mL brine, rt
a
Isolated yield
1
Determined by H NMR
b
c
Determined by chiral HPLC
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uct in moderate yield with low enantioselectivity.
4 Conclusions
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16:2487
In summary, a series of valine dipeptide organocatalysts
containing a primary amine group and two amide units
have been developed and applied to the asymmetric direct
aldol reactions in brine with the addition of DNP as
cocatalyst. Aldol products were obtained in mederate
yields with moderate to high enantioselectivities (up to
95%) and diastereoselectivities (up to [99:1).
29. Chen F-B, Huang S, Zhang H, Liu F-Y, Peng Y-G (2008) Tet-
rahedron 65:9585
´
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Acknowledgements We are grateful for the financial support from
the National Natural Science Foundation of China (no. 20802025) and
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