Highly enantioselective direct asymmetric aldol reaction catalyzed by 4,5-methano-L-proline

Article abstract of DOI:10.1002/cjoc.201400732

The 4,5-methano-L-proline was used as chiral organocatalysts in direct asymmetric aldol reactions. Under the optimal conditions, excellent enantioselectivities (up to 99% ee) were obtained with high chemical yields (up to 95%) for a series of aldehydes using only 5 mol% catalyst loading. To show the practicality of the method, the reaction was tested at a large scale. The reaction was complete in 16 h, and the aldol product was obtained in 86% yield and 93% ee.

Full text of DOI:10.1002/cjoc.201400732

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DOI: 10.1002/cjoc.201400732
Highly Enantioselective Direct Asymmetric Aldol Reaction
Catalyzed by 4,5-Methano-L-proline
Yukun Zhang,a Jun Zhu,a Na Yu,a and Han Yu∗,a,b
a School of Chemical and Environmental Engineering, Shanghai Institute of Technology, Shanghai 201418, China
b College of Biological, Chemical Sciences and Engineering, Jiaxing University, Jiaxing, Zhejiang 314001, China
The 4,5-methano-L-proline was used as chiral organocatalysts in direct asymmetric aldol reactions. Under the
optimal conditions, excellent enantioselectivities (up to 99% ee) were obtained with high chemical yields (up to
95%) for a series of aldehydes using only 5 mol% catalyst loading. To show the practicality of the method, the reac-
tion was tested at a large scale. The reaction was complete in 16 h, and the aldol product was obtained in 86% yield
and 93% ee.
Keywords asymmetric organocatalysis, direct asymmetric aldol reaction, 4,5-methano-L-proline
Introduction
chiral organocatalysts in asymmetric Michael addition
of aldehydes to nitroolefins, which afforded excellent
diastereo- and enantioselectivities in high yields for a
series of aldehydes and nitroolefins.[16] We therefore
envisage that they would be good catalysts for asym-
metric catalysis in direct asymmetric aldol reactions of
acetone to various aldehydes. Herein, we report a highly
enantioselective direct asymmetric aldol reactions using
4,5-methano-L-prolines as a chiral organocatalyst.
The stereoselective direct aldol reaction is one of the
most powerful synthetic methods for the synthesis of
β-hydroxycarbonyl structural unit found in many natural
products.[1] Since the publication of a pioneering work
by List and Barbas,[2] the asymmetric aldol reaction
catalyzed by L-proline has been a representative reac-
tion in the field of organocatalysis, after which extraor-
dinary progress has been sought in order to find more
selective and efficient catalytic systems for these direct
aldol reaction.[3,4] A number of catalysts have been re-
ported with a modified pyridine ring moiety of L-proline,
making it effective in asymmetric reactions. For exam-
ple, the groups of Alexakis,[5] Wang,[6] Hayashi,[7]
Zhao,[8] Palomo,[9] Jacobsen,[10] Chen,[11] and Loh,[12]
among many others,[13,14] have developed a series of
efficient proline-like catalysts (molecules containing a
proline moiety) to improve the stereoselectivity in
asymmetric reactions. However, the catalytic systems
retain several shortcomings, such as the low reaction
activity, high catalyst loading requirements and unsatis-
factory results. Thus, the search for highly efficient
proline-like organocatalysts still remains a worthwhile
endeavor.
COOH
COOH
N
H
N
H
1a
1b
Figure 1 The structure of trans-4,5-methano-L-proline (1a) and
cis-4,5-methano-L-proline (1b).
Experimental
General procedure for the aldol reaction
To a mixture of catalyst 1a and benzoic acid in DMF
(1 mL) was added acetone (0.2 mL, 2.5 mmol, 5 equiv.).
Then, the aldehyde (0.5 mmol, 1 equiv.) was added at 0
℃. After TLC analysis indicated complete consumption
of the starting material, the reaction mixture was
quenched with saturated NH4Cl aqueous solution, ex-
tracted with EtOAc, and dried with anhydrous Na2SO4.
The crude product was purified by flash silica gel
chromatography (hexane/EtOAc) to afford pure aldol
products. All aldol products are known compounds, and
their spectroscopic data are identical to those reported in
the literature. The ee values were determined by chiral
The Hanessian group[15] had reported the synthesis
of 4,5-methano-L-prolines and the enzymatic activity of
the corresponding N-(3-mereapto-2-(R)-methyl-propionyl)
analogs as inhibitors of angiotensin converting enzyme
(Figure 1). To the best of our knowledge, 4,5-methano-
proline as a catalysis can surpass the venerable proline
in conjugate additions of nitroalkanes to cyclic enones
in many cases.[15a] Recently, our group have used it as
*
E-mail: hanyu201212@gmail.com
Received October 27, 2014; accepted December 17, 2014; published online XXXX, 2014.
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cjoc.201400732 or from the author.
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1

Zhang et al.
COMMUNICATION
carried out with 2－10 mol% of benzoic acid (Table 1,
Entries 6, 14, and 15). Then, we tested a lower loading
of catalyst 1a in DMF. To our delight, the decreased
catalyst loading (from 30 to 10 mol%) could still main-
tain the product yield and enantioselectivity (Table 1,
Entries 4, 9 and 10). When further decreasing the
amount of catalyst 1a to 5 mol%, up to 85% yield and
95% enantioselectivity were obtained by extending the
reaction time. Even with 2 mol% of catalyst 1a, similar
results were still obtained in this reaction (Table 1, En-
try 12). Thus, the most efficient catalytic system in-
volved 5 mol% of catalyst 1a and 2 mol% of benzoic
acid in DMF.
Influence of solvents on the reaction was further ex-
plored and the aldol reaction was successful in many
organic solvents. The best enantioselectivity (95% ee)
was achieved in DMF (Table 2, Entry 1). Although a
slight difference in enantioselectivity was observed in
THF and CHCl3 (Table 2, Entries 2 and 6), the reaction
required longer reaction time. However, the enanti-
omeric excess decreased further when water was used as
a solvent (Table 2, Entry 4). When decreasing the reac-
tion temperature to −20 ℃, product 3a was obtained
with 99% ee. It was found that the reaction temperature
has an effect on improving the enantioselectivity of the
aldol product.
HPLC analysis.
Results and Discussion
We first applied 1a and 1b in the direct aldol reac-
tion between acetone and p-nitrobenzaldehyde as the
model reaction in DMF to determine the effect of dif-
ferent configurations of the catalyst used in the reaction.
The results showed that high yields and excellent enan-
tioselectivities could be obtained in the presence of
catalyst 1a, whereas a poorer yield and a moderate ee
value were obtained with catalyst 1b (Table 1, Entries 1
and 2). Next, the effect of additive of this reaction was
investigated. All of the acid additives dramatically im-
proved the yield and enantioselectivity in the reaction
(Table 1, Entries 4－8). The best results were obtained
with benzoic acid as the additive. The addition of
trifluoroacetic acid also gave similar results with ben-
zoic acid (Table 1, Entries 4 and 5). It could be con-
cluded that the combination of organocatalyst 1a and
benzoic acid is crucial for the reactivity of this catalytic
system. For further details, the loading of the additive
was also studied. The reaction afforded a good ee value
but only a moderate yield when a larger amount of addi-
tive was used (20 mol%; Table 1, Entry 13). However, a
much higher yield was obtained when this reaction was
Table 1 Influence of catalysts and additives for aldol reaction
between acetone 2 and 4-nitrobenzaldehyde 3aa
Table 2 Influence of solvents and temperature for aldol reaction
between acetone 2 and 4-nitrobenzaldehyde 3a catalyzed by 1aa
Catalyst
PhCOOH
DMF, 0 oC
OH
O
O
CHO
O
OH
O
1a (5 mol%)
PhCOOH (2 mol%)
+
O2N
+
CHO
DMF, T
O2N
O2N
Entry Catalyst/mol% Time/h Additive/mol% Yieldb/%eec /%
O2N
1
1a (30)
1b (30)
Proline
1a (30)
1a (30)
1a (30)
1a (30)
1a (30)
1a (20)
1a (10)
1a (5)
14
14
14
6
－
－
80
78
68
90
89
85
86
75
91
89
85
83
70
84
82
89
73
76
98
95
90
90
92
96
95
95
92
91
92
95
Entry Solvent
Temp./℃ Time/h Yieldb/% eec/%
2
1
2
3
4
5
6
7
8
DMF
THF
0
0
9
11
11
14
82
82
75
73
47
56
80
85
95
91
72
45
67
90
58
99
3
－
4
PhCOOH (10)
TFA (10)
MeCN
Water
Acetone
Chloroform
Toluene
DMF
0
0
5
6
0
9.5
6
10 CH3COOH (10)
TsOH (10)
11 TfOH (10)
0
0
−20
12
13
12
7
9
8
a
9
8
7
9
PhCOOH (10)
The reaction was carried out with acetone (2.5 mmol) and
10
11
12
13
14
PhCOOH (10)
PhCOOH (10)
4-nitrobenzaldehyde (0.5 mmol) in solvent at suitable tempera-
ture according to the general procedure. b Isolated yield. c Deter-
mined by chiral HPLC analysis by using a Chiralpak AS-H col-
umn, and the configuration was assigned as R by comparison
with the literature data.
1a (2)
12 PhCOOH (10)
1a (5)
9
9
9
PhCOOH (20)
PhCOOH (5)
PhCOOH (2)
1a (5)
15
a
1a (5)
To increase the scope of the methodology, the aldol
reaction was extended to several aromatic and aliphatic
aldehydes and the results are summarized in Table 3.
The reaction of acetone with aromatic aldehydes bear-
ing electron-withdrawing group led to excellent enanti-
oselectivities (97%－99%) and high yields (85%－92%)
due to the strong electrophilicity of the substrates (Table
The reaction was carried out with acetone (2.5 mmol) and
4-nitrobenzaldehyde (0.5 mmol) at 0 ℃ in DMF (1 mL) ac-
b
c
cording to the general procedure. Isolated yield. Determined
by chiral HPLC analysis by using a Chiralpak AS-H column, and
the configuration was assigned as R by comparison with the lit-
erature data.
2
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© 2015 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Chin. J. Chem. 2015, XX, 1—4

Highly Enantioselective Direct Asymmetric Aldol Reaction Catalyzed by 4,5-Methano-L-proline
2, Entries 2－9). The effect of steric hindrance on the
reaction outcome has been tested. When an electron-
withdrawing group was introduced at the ortho-position
of the phenyl ring, products remain in high enantiose-
lectivities (97%－98%) (Table 2, Entries 2, 4 and 6).
Weak electrophilic aldehydes, such as 4-methylbenzal-
dhyde, required a longer reaction time to give the cor-
responding aldol products in moderate yield (Table 2,
Entry 10). The benzaldehyde and 2-naphthaldehyde re-
acted with acetone to generate the aldol product with
extremely high enantioselectivities (Table 2, Entries 1
and 12; ee up to 95% and 98%, respectively). Neverthe-
less, as for less-reactive aliphatic aldehydes such as
isobutyraldehyde, only a trace amount of the aldol ad-
duct could be obtained in this system (Table 2, Entry
11). This may be due to the fact that the formation of
imidazolidinethiones between isobutyraldehyde and the
organocatalyst occurs more quickly in this reaction sys-
tem. To expand the range of the substrates, compounds
containing heterocycles such as furyl rings were also
employed in this reaction, which proceeded with excel-
lent enantioselectivities (up to 80% ee, Entry 13).
mmol) was allowed to react with benzaldehyde (1.27
mL, 12.5 mmol) by using the catalyst 1a (32 mg, 0.25
mmol) with benzoic acid as an additive in DMF (12.5
mL) at 0 ℃. The reaction was complete in 16 h, and the
aldol product was obtained in 86% yield and 93% ee
(Table 2, Entry 14).
Conclusions
In summary, we have successfully applied the
4,5-methano-L-prolines in direct asymmetric aldol reac-
tions. As a result, they showed excellent enantioselec-
tivity (99% ee) together with high yields in the catalytic
reactions. The methodology could also be expanded to a
series of aldehydes with excellent asymmetric catalytic
results using 1a as a chiral organocatalyst. Encouraged
by its excellent performance, a broader application is
under investigation in our laboratory.
Acknowledgement
Financial support by the NSFC (No. 21402065) and
the start-up fund of Shanghai Institute of Technology is
gratefully acknowledged.
To show the practicality of the method, the reaction
was tested at a large scale. Acetone (3.69 mL, 50.3
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