A simple synthetic approach for the transformation of (S)-Ugi's amine

Article abstract of DOI:10.1016/j.cclet.2014.06.009

A simple synthetic approach for the transformation of (S)-Ugi's amine, another configuration of (R)-Ugi's amine, one of the most widely used intermediate in the preparation of chiral ferrocene-based ligands, has been developed via esterification using anhydride, alkaline hydrolysis and active manganese dioxide oxidation, and the corresponding ferrocenyl ketone was afforded in good yields.

Full text of DOI:10.1016/j.cclet.2014.06.009

G Model
CCLET 3042 1–4
Chinese Chemical Letters xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Chinese Chemical Letters
journal homepage: www.elsevier.com/locate/cclet
1
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Original article
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A simple synthetic approach for the transformation of (S)-Ugi’s amine
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Q1 Gao-Feng Zha, Wei-Yun Xu, Peng Dai, Xiao-Yan Lai, Wei Liu, Yong-Cun Shen
Department of Pharmacy, The College of Chemical Engineering, Wuhan University of Technology, Wuhan 430070, China
A R T I C L E I N F O
A B S T R A C T
Article history:
A simple synthetic approach for the transformation of (S)-Ugi’s amine, another configuration of (R)-Ugi’s
amine, one of the most widely used intermediate in the preparation of chiral ferrocene-based ligands,
has been developed via esterification using anhydride, alkaline hydrolysis and active manganese dioxide
oxidation, and the corresponding ferrocenyl ketone was afforded in good yields.
ß 2014 Gao-Feng Zha. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights
reserved.
Received 8 April 2014
Received in revised form 27 May 2014
Accepted 5 June 2014
Available online xxx
Keywords:
(S)-Ugi’s amine
Transformation
Esterification
Hydrolysis
Oxidation
6
7
1. Introduction
amine of (R)-2 and (R)-3 [13a,14]. Compared with these methods 30
mentioned above, resolution of its racemic mixture is a better one. 31
8
9
Chiral ferrocene-based ligands have been extensively studied in
asymmetric catalytic reactions and widely used in the industrial
production of chiral compounds [1]. Some of the most efficient
chiral ferrocene ligands, including BoPhoz [2], Josiphos [3],
Taniaphos [4], Walphos [5], PPFA [6], Ferriphos [7], Pigiphos [8],
Trap [9], etc., share a common intermediate (R)-N,N-dimethyl-1-
ferrocenylethyl-amine, namely (R)-Ugi’s amine ((R)-1) (Fig. 1), a
compound first prepared in 1970 [10a]. Presently, a large amount
of (R)-Ugi’s amine is required as an important intermediate.
The classic method to produce optically pure (R)-1 is through
resolution of its racemic mixture (Scheme 1) [10]. Alternatively,
chiral auxiliaries have also been reported to generate a (R)-Ugi’s
amine analog by Togni’s group [11] and Hayashi’s group [12].
Chan’s group reported an efficient approach for chiral ferrocene-
based secondary alcohols via asymmetric hydrogenation of
ferrocenyl ketones, and Ugi’s amine analog could be synthesized
via acylation and amination of chiral ferrocene-based secondary
alcohols [13a]. Li’s group reported a probable approach for
asymmetric transfer hydrogenation of ferrocenyl ketones [13b].
However, very expensive metal catalysts or organic ligands were
used in asymmetric reduction, asymmetric acylation, or stereo-
selective enzymatic acylation to synthesize the precursors of Ugi’s
To decrease the cost of materials, the most economical and eco- 32
benign synthetic pathway to chiral Ugi’s amine is to transform 33
another configuration. It was surprising, however, that few 34
laboratory methods were known for transformation of the (S)- 35
Ugi’s amine to (R)-Ugi’s amine and its precursor. A major progress 36
has been made by Ugi’s group [10], transformation from Ugi’s 37
amine (1) to 1-ferrocenylethanol (3) via two or three steps 38
including methylation and hydrolysis, or including methylation, 39
esterification, and hydrolysis. In other words, the efficient 40
transformation of (S)-Ugi’s amine had not been achieved by Ugi’s 41
group. Additionally, the yield of the process was not very high, and 42
expensive CH3I was used in the methylation. This method, to some 43
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extent, could give us some inspiration in our laboratory study.
44
Chen [15] reported that the Ugi’s amine derivatives could be 45
transformed to related substituted ferrocenylethyl acetates in the 46
solution of anhydride at ambient temperature with high yield. 47
Nicolosi [16] gave an example of alkaline hydrolysis of substituted 48
ferrocenylethyl acetates to related ferrocenyl alcohols, and 49
followed by manganese dioxide oxidation in aprotic solvent to 50
give related ketones. But in the transformation, a large amount of 51
anhydride would be used, and the reaction time was very long.
52
This paper describes our attempt to report the first example of 53
the transformation of (S)-Ugi’s amine via esterification using 54
anhydride, alkaline hydrolysis, and oxidation (Scheme 2). Acet- 55
ylferrocene (4) was obtained which could be changed to (R)-Ugi’s 56
*
Corresponding author.
amine via Ugi’s method (Scheme 1).
57
E-mail address: yongcunshen@163.com (Y.-C. Shen).
http://dx.doi.org/10.1016/j.cclet.2014.06.009
1001-8417/ß 2014 Gao-Feng Zha. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Please cite this article in press as: G.-F. Zha, et al., A simple synthetic approach for the transformation of (S)-Ugi’s amine, Chin. Chem.
Lett. (2014), http://dx.doi.org/10.1016/j.cclet.2014.06.009

G Model
CCLET 3042 1–4
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G.-F. Zha et al. / Chinese Chemical Letters xxx (2014) xxx–xxx
ambient temperature, the reaction mixture was then stirred until
90
TLC (petroleum ether:ethyl acetate, 10:1) analysis indicated the
complete disappearance of (S)-2. The solution was diluted with
water (20 mL), extracted with ethyl acetate (3 Â 20 mL), and dried
over anhydrous Na2SO4, and the solvent was removed in vacuo.
The resulting product was recrystallized from n-hexane (2 mL),
affording 1.05 g (92% yield) (S)-3 as a yellow powder, mp 75–76 8C,
91
92
93
94
95
96
97
[a
]
D
20 +30.0 (c 1.2, benzene), >99% ee. 1H NMR (500 MHz, CDCl3):
d
Fig. 1. Structure of Ugi’s amine (1).
4.56 (m, 1H, J = 4.2, 6.3 Hz), 4.35–4.10 (m, 9H), 1.85 (d, 1H,
J = 4.2 Hz), 1.46 (d, 3H, J = 6.3 Hz). 13C NMR (126 MHz, CDCl3):
98
99
d
OH
O
OAc
94.81, 68.63, 68.36, 67.93, 66.18, 65.59, 23.73. MS (ESI): m/z 214.50
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
(MÀOH+1), 231.04 (M+1). IR (KBr, cmÀ1):
n
3220, 1411, 1308,
NaBH4/MeOH
90%
Ac2O/py
87%
Me2NH/MeOH
94%
Fe
Fe
1237, 1069, 1001, 869, 806.
Fe
General procedure of oxidation of (S)-1-ferrocenylethanol ((S)-
3): (S)-1-ferrocenylethanol (S)-3) (575 mg, 2.5 mmol) was stirred
at room temperature for 4 h with a suspension of active manganese
dioxide (2.17 g, 25 mmol) in toluene (10 mL), The reaction mixture
was then stirred until TLC (petroleum ether:ethyl acetate, 10:1)
analysis indicated the complete disappearance of (S)-3. The
mixture was filtered and the filtrate evaporated. The residue
was dissolved in dichloromethane and washed with water. The
dichloromethane fractions were dried over anhydrous Na2SO4, and
concentrated under reduced pressure. The resulting products were
purified by silica gel flash column chromatography (PE:EtOAc,
10:1) to afford acetylferrocene (4) (542 mg, 95% yield) as a yellow
4
3
Rac-
2
Rac-
NMe2
NMe2
NMe2
(R)-(+)-tartaric acid resolution
+
Fe
Fe
Fe
80-90%
(R)-1
Rac-1
(S)-1
Scheme 1. Ugi’s method to prepare (R)-Ugi’s amine ((R)-1).
58
2. Experimental
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64
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68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
All reactions were carried out in reaction tubes with magnetic
stirring and no special precautions were taken to exclude air from
the reaction vessels. NMR spectra were recorded with a Bruker
Avance HD III 500 NMR spectrometer. Chemical shifts were
reported in parts per million (ppm) downfield from TMS with the
solvent resonance as the internal standard. Coupling constants (J)
are reported in Hz. ESI-MS was recorded on a Thermo-LTQ XL mass
spectrometer. Optical rotation data was obtained at 25 8C. Melting
points were determined on a WRS-2A (Shanghai Jingke) instru-
ment and uncorrected. All other reagents were purchased from
commercial sources and used without further purification.
solid, mp 86–87 8C. 1H NMR (500 MHz, CDCl3):
d 4.82–4.76 (t, 2H,
J = 1.9 Hz, H2 and H5 of subst. Cp ring), 4.55–4.50 (t, 2H, J = 1.9 Hz,
H3 and H4 of subst. Cp ring), 4.22 (s, 5H, unsubst. Cp ring), 2.42 (s,
3H, CH3). 13C NMR (126 MHz, CDCl3):
d 202.03 (C55O), 79.32 (C1 of
subst. Cp ring), 72.33 (C3 and C4 of subst. Cp ring), 69.87 (C of
unsubst. Cp ring), 69.62 (C2 and C5 of subst. Cp ring), 27.42(CH3).
MS (ESI): m/z 229.58 (M+1), 251.41 (M+Na). IR (KBr, cmÀ1):
n 3116,
3096, 3075, 1660, 1457, 1281.
The spectra of 1H NMR, 13C NMR, MS, IR of (S)-2, (S)-3 and 4 are
available in Supporting information.
General procedure of esterification of (S)-Ugi’s amine ((S)-1): a
solution of (S)-1 (2.57 g, 10 mmol) in acetic anhydride (1.88 mL,
20 mmol) and dichloromethane (5 mL) were stirred for 1 h at room
temperature. The reaction mixture was then stirred until TLC
(petroleum ether:ethyl acetate, 10:1) analysis indicated the
complete disappearance of the starting material. The solvents
were removed under reduced pressure. The residue was dissolved
in ethyl acetate (20 mL), washed with the solution of 10% NaHCO3,
washed with water, and dried over anhydrous Na2SO4. Thereafter,
the solvents were removed in vacuo to offer a crude product, which
3. Results and discussion
125
As described above, the nucleophilic displacement of (S)-1 with
acetic anhydride took a very long time and required a large amount
of anhydride, therefore, improvements that cut down the amount
of acetic anhydride and avoid the long reaction time should be
investigated. We tried to add solvents into the reaction system to
form a dilute solution. A solvent study showed that dichloro-
methane as solvent provided the best result (Table 1, entry 3).
Further studies showed that there was no increase in yield when
increasing the quantity of acetic anhydride. The best result was
obtained when a solution of (S)-1 (10 mmol) in acetic anhydride
(20 mmol) and dichloromethane (5 mL) reacted at room tempera-
ture for 1 h. Determination of rotation value showed that no
racemization occurred during the process of esterification of (S)-1.
With (S)-1-ferrocenylethyl acetate ((S)-2) in hand, we further
studied the chemical hydrolysis of (S)-2. It was found that solvent
was important to the hydrolysis yield, and ethanol was the best
solvent. Temperature could influence the speed of reaction but had
no obvious influence on the yield (Table 2). The rational condition
126
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132
133
134
135
136
137
138
139
140
141
142
143
was recrystallized from n-hexane (3 mL), affording (S)-2 2.58 g
20
(95% yield) as an orange solid, mp 66–68 8C, [
ethanol). 1H NMR (500 MHz, CDCl3):
(s, 1H), 4.23 (s, 1H), 4.21 (s, 7H), 2.06 (s, 3H), 1.58 (d, 3H, J = 6.5 Hz).
13C NMR (126 MHz, CDCl3):
170.46, 88.03, 68.82, 68.35, 67.98,
a
]
+28.5 (c 1.5,
D
d
5.85 (q, 1H, J = 6.5 Hz), 4.28
d
66.02, 21.39, 20.04. MS (ESI): m/z 273.1 (M+1), 213.76
(MÀCH3CO2H+1), 295.86 (M+Na); IR (KBr, cmÀ1):
n 1723, 1235.
General procedure of hydrolysis of (S)-1-ferrocenylethyl ace-
tate ((S)-2): a solution of (S)-2 (1.36 g, 5 mmol) in ethanol (10 mL)
and 2 mol/L NaOH (7 mL) were allowed to stand for 5 min at
OAc
NMe2
OH
O
Ac2O/CH2Cl2
2 moL/L NaOH(aq)/EtOH
active MnO2/toluene
Fe
Fe
r.t.
Fe
Fe
r.t.
r.t.
95%
95%
92%
1
(S)-
(S)-2
3
(S)-
4
Scheme 2. Synthesis of acetyl ferrocene (4) from (S)-Ugi’s amine ((S)-1).
Please cite this article in press as: G.-F. Zha, et al., A simple synthetic approach for the transformation of (S)-Ugi’s amine, Chin. Chem.
Lett. (2014), http://dx.doi.org/10.1016/j.cclet.2014.06.009

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G.-F. Zha et al. / Chinese Chemical Letters xxx (2014) xxx–xxx
3
Table 1
Table 3
Esterification of (S)-Ugi’s amine ((S)-1) under various conditionsa.
Oxidation of (S)-1-ferrocenyl alcohols ((S)-3) under various conditionsa.
.
.
Entry
Oxidant
Solvent
Time (h)
Yield(%)b
20c
Entry
Ac2O (mmol)
Solvent
Time (h)
Yield (%)b
[a]
D
c
1
2
3d
4e
5f
6g
7h
8
MnO2
Toluene
Toluene
MeCN
48
4
–
95
35
–
Active MnO2
DMSO/N2H4.H2O
DMSO/Ac2O
KMnO4
1
2
50
15
20
25
30
20
20
20
20
20
No
2
3
80
90
95
95
95
79
87
78
–
28.4
28.4
28.5
28.5
28.4
28.5
28.0
28.3
–
3
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
MeCN
Acetone
Toluene
MeOH
EtOAc
Et3N
48
48
24
48
4
3
1
AcOH/H2O
i-PrOH
CH2Cl2
CH2Cl2
Et2O
–
4
1
Al(O-i-Pr)3/acetone
PCC
–
5
1
–
6
2
Active MnO2
Active MnO2
Active MnO2
Active MnO2
94
93
95
90
7
3
9
4
8
3
10
11
Benzene
MeCN
4
9
>24
>24
4
10
–
–
a
a
Reaction conditions: the oxidation was carried out by allowing a mixture of (S)-
3 (2.5 mmol), active manganese dioxide (25 mmol) and solvent (10 mL) to stand at
room temperature.
Reaction coditions: (S)-Ugi’s amine ((S)-1) (10 mmol) was allowed to react in
acetic anhydride and solvent (5 mL) at room temperature.
b
Isolated yield.
b
c
Isolated yield.
1.5, ethanol.
c
Normal commercial manganese dioxide.
d
The reaction was according to literature procedures [18].
Table 2
e
DMSO/Ac2O/Et3N was used instead of active manganese dioxide, reflux or not.
a
Hydrolysis of (S)-1-ferrocenylethyl acetate ((S)-2) under various conditions
.
f
Acidic potassium permanganate solution was used instead of active manganese
dioxide.
g
OAc
OH
The Oppenauer oxidation.
h
PCC: pyridinium chlorochromate. The reaction was according to literature
procedures [19].
2 mol/L NaOH(aq)/Solvent
Fe
Fe
(S)-3
(S)-2
material [17], it has been found essential to precipitate it in the 159
presence of alkali or to treat it with alkali after precipitation and 160
before drying. The latter process is also critical, and both under- 161
and over-drying can profoundly reduce the activity of manganese 162
dioxide. The active material is a hydrated oxide and a chocolate- 163
brown color. The solvent effect was also investigated. Different 164
polar solvents such as toluene, benzene, dichloromethane, and 165
.
Entry
Temp (8C)
Solvent
Time (min)
Yield (%)b
ee
(%)c
1
2
r.t.
40
70
r.t.
r.t.
r.t.
r.t.
r.t.
EtOH
EtOH
EtOH
MeOH
Acetone
THF
5
92
92
91
78
60
46
38
75
>99
>99
>99
>99
>99
>99
>99
>99
4
3
3
ether, were suitable for this oxidation.
166
4
30
5
60
6
120
4. Conclusion
167
7
8d
MeCN
EtOH
120
overnight
In summary, we successfully developed a simple synthetic 168
approach for the transformation of S-Ugi’ s amine via esterification 169
using anhydride, alkaline hydrolysis, and oxidation using active 170
manganese dioxide. It compares favorably with and represents a 171
valid alternative to the existing methods. The important features of 172
our method are: mild reaction conditions, simple work-up, 173
inexpensive reaction agents, and high total yield (up to 83%). 174
Additionally, this research can also provide a method for the 175
transformation of other representative amines. Further efforts will 176
be directed at synthesizing large amounts of bisphosphine ligands 177
a
Reaction coditions: (S)-1-ferrocenylethyl acetate ((S)-2) (5 mmol) was allowed
to react in 2 mol/L aqueous sodium hydroxide solution (7 mL) and solvent (10 mL).
b
Isolated yield.
c
Determined by chiral HPLC analysis (Chiral OJ-H, n-hexane/i-propanol = 97:3,
0.5 mL/min, tR of (S)-isomer = 23.3, tR of (R)-isomer = 24.7).
d
The reaction was carried out in 50% aqueous ethanol (25 mL) without 2 mol/L
NaOH (aq).
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
was a solution of (S)-2 (5 mmol) in ethanol (10 mL) and 2 mol/L
aqueous sodium hydroxide solution (7 mL). The reaction was
allowed to stand for 5 min at ambient temperature. Determination
of ee value showed that no racemization occurred during the
process of alkaline hydrolysis of (S)-2 (see HPLC of Supporting
information)
To accomplish the transformation of (S)-Ugi’s amine ((S)-1),
oxidation of (S)-1-ferrocenethanol ((S)-3) is the key step. We have
tested several oxidants to achieve this transformation (Table 3,
entries 1–7). It was found that active manganese dioxide was the
best oxidant. After optimizing the reaction conditions, a nearly
complete oxidation within 4 h at room temperature was achieved
with an activated form of manganese dioxide. Normal commercial
precipitated manganese dioxide commonly has a much lower
activity and is often inactive. In the preparation of the active
based on ferrocenyl used in the asymmetric catalytic reaction.
178
Acknowledgments
179
We are grateful for financial support from the National Natural Q2180
Science Foundation of China (No. 20972123) and Wuhan Science 181
and Technology Bureau.
182
183
Appendix A. Supplementary data
Supplementary data associated with this article can be found, 184
in the online version, at http://dx.doi.org/10.1016/j.cclet. 185
2014.06.009.
186
Please cite this article in press as: G.-F. Zha, et al., A simple synthetic approach for the transformation of (S)-Ugi’s amine, Chin. Chem.
Lett. (2014), http://dx.doi.org/10.1016/j.cclet.2014.06.009

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[10] (a) D. Marquarding, H. Klusacek, G. Gokel, P. Hoffman, I. Ugi, Stereoselective
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Please cite this article in press as: G.-F. Zha, et al., A simple synthetic approach for the transformation of (S)-Ugi’s amine, Chin. Chem.
Lett. (2014), http://dx.doi.org/10.1016/j.cclet.2014.06.009