Lipase-catalysed decarboxylative aldol reaction and decarboxylative Knoevenagel reaction

Article abstract of DOI:10.1039/b914653a

Acrylic resin immobilized Candida antarctica lipase B (CAL-B) is able to catalyse decarboxylative aldol reaction and decarboxylative Knoevenagel reaction with good to excellent yields.

Full text of DOI:10.1039/b914653a

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Lipase-catalysed decarboxylative aldol reaction and decarboxylative
Knoevenagel reaction†
Xing-Wen Feng, Chao Li, Na Wang,* Kun Li, Wei-Wei Zhang, Zao Wang and Xiao-Qi Yu*
Received 21st July 2009, Accepted 1st September 2009
First published as an Advance Article on the web 14th September 2009
DOI: 10.1039/b914653a
Acrylic resin immobilized Candida antarctica lipase B
(CAL-B) is able to catalyse decarboxylative aldol reaction
and decarboxylative Knoevenagel reaction with good to
excellent yields.
In continuation of our work with enzymatic synthetic
methodologies, we report herein two environmentally-benign
decarboxylative addition protocols. CAL-B was found to be able
to catalyse decarboxylative aldol reactions and decarboxylative
Knoevenagel reactions. Good substrate scopes were obtained,
as well as yields (Scheme 2).
Biocatalysis is an efficient and green tool for modern organic
synthesis due to its high selectivity and mild conditions.1
Recently, a new frontier in biocatalysis is catalytic promiscuity,
which is mainly exploring the catalytic activities of enzymes on
the unnatural substrates or alternative reactions.2 Biocatalytic
promiscuity provides new tools for organic synthesis, and thus
expands largely the application of enzymes. To the best of
our knowledge, some promiscuous biocatalytic basic reactions
such as aldol3a–b and Mannich condensations3c and Michael3d–e
and Markovnikov additions3f have been reported. However,
more complex and useful reaction systems are relatively scarce.
Hilvert and coworkers reported that oxaloacetate went through
decarboxylation and then an aldol reaction with aldehydes in the
presence of macrophomate synthase.4 Ohta et al. demonstrated
a decarboxylase catalysed intramolecular aldol reaction after a
decarboxylation process first.5 Nevertheless, limited industrial
outputs of these enzymes restrict their large-scale application.
Utilizing the promiscuity of widely-used hydrolases to accom-
plish some novel processes becomes interesting and challenging.
As one of the important carbon–carbon bond formation
reactions in organic synthesis, the decarboxylative aldol reaction
provides a good protocol for regioselective aldol reactions
(Scheme 1). Though lots of efforts have been contributed,
the most common approaches require strong bases6 or metal
catalysts.7 Thus, the application of enzymes will effectively
circumvent the harsh reaction conditions.
Scheme 2 (I) CAL-B catalysed decarboxylative aldol reaction. (II)
CAL-B catalysed decarboxylative Knoevengel reaction
Initially, we focused on demonstrating the specific catalytic
effect of the CAL-B in the decarboxylative aldol reaction
by performing some control experiments. The reaction of
4-nitrobenzaldehyde (1a) with ethyl acetoacetate (2a) in the
absence of CAL-B led to no product being detected, even after
two weeks. When the reactants were incubated with denatured
CAL-B or bovine serum albumin (B.S.A.), it was equal to the
background reaction, suggesting that the tertiary structure of the
enzyme was necessary and the effect of the polymeric support
was excluded (for details, see ESI).†
For optimization of the decarboxylative aldol reaction con-
ditions, the reaction of 1a and 2a was chosen as a model.
The results are summarized in Table 1. To our delight, a very
interesting result was obtained using an amine as additive. The
best results were obtained in up to 96% yield with 10 mol%
1,4,7,10-tetraazacyclododecane (cyclen) added in acetonitrile
after 20 h. No product was detected only using cylen in the
absence of CAL-B, it showed that cyclen may cooperate with
CAL-B to promote the reaction. Though CAL-B showed high
activity in this decarboxylative aldol reaction, the product 3aa
exhibited little enantioselectivity, which was similar to other
promiscuous CAL-B catalyzed reactions.3a,3d
Scheme 1 (1) General aldol reaction. (2) Decarboxylative aldol reac-
tion, a good protocol for regioselective aldol reaction.
Subsequently, some substituted aromatic aldehydes and
b-ketoesters were tested under the optimized conditions, the
results are shown in Table 2. Though only aldehydes with
strong electron-withdrawing group can take part in the reaction
(Table 2, entry 1–3), b-ketoesters have an extensive scope
(Table 2, entry 4–11), and good to excellent yields were obtained.
Key Laboratory of Green Chemistry and Technology, Ministry of
Education, College of Chemistry, Sichuan University, Chengdu, 610064,
P. R. China. E-mail: xqyu@tfol.com; Fax: (+86)288-541-5886
† Electronic supplementary information (ESI) available: Experiment
details, characterization and spectra. See DOI: 10.1039/b914653a
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Table 1 Reaction conditions optimization for decarboxylative aldol
Table 2 Decarboxylative aldol reaction of aromatic aldehydes and
b-ketoestersa
reactiona
Entry
1
1
2
3
Yield (%)b
Entry
Solvent
Additive
t (h)
Yield (%)b
1a p-NO2
2a 3aa 96
2a 3ba 81c
2a 3ca 87c
2b 3ab 97
2c 3ac 95
2d 3ad 94
2e 3ae 91
1
2
3
4
5
6
7
CHCl3
—
—
—
—
72
72
72
72
72
30
20
25
28
53
74
Toluene
t-BuOMe
THF
CH3CN
CH3CN
CH3CN
2
3
4
5
6
7
1b o-NO2
—
83
Et3N
87
Cyclend
96 (0)c
1c m-NO2
a Reaction conditions: 1a (0.1 mmol), 2b (0.12 mmol), CAL-B 10 mg,
additive 10% mol, solvent 0.5 mL, 30 ◦C. b Isolated yields. c The value
given in parenthesis refers to yield without CAL-B. d Cyclen = 1,4,7,10-
tetraazacyclododecane.
1a
1a
1a
1a
Substrate 2a and 2b indicated that group R3 had no obvious
effect on the reaction (Table 2, entries 1 and 4). They had
the same product, and both of them obtained excellent yields.
When the methylene of b-ketoesters was substituted by ethyl,
the reaction didn’t take place (Table 2, entry 8), whereas high
yield was observed for substrate 2g due to reduction of the steric
hindrance (Table 2, entry 9), indicating that the reaction system
was sensitive to steric effects.
Interestingly, when
a primary amine such as aniline,
p-toluidine and benzylamine was added to the decarboxylative
aldol reaction, a, b-unsaturated ketones were obtained as the
final products due to the occurrence of the decarboxylative
Knoevenagel reaction. After some optimization, we found a
good result using water as cosolvent (Table S1, ESI).† The
best results were obtained in 90% yield with 5% (v/v) water
added in acetonitrile. We next investigated a series of substituted
aromatic aldehydes and b-ketoesters, and this time a wide range
of aromatic aldehydes were able to take part in the reaction
(Table 3, entry 1–9). A Schiff base was formed firstly as a result
of adding aniline. As the reactivity of the Schiff base is higher
than aldehyde, so good substrate scope was obtained. The yields
of aldehydes with electron-withdrawing groups were higher than
those with electron-donating groups. Also, no product was
observed for 2f due to steric effects. In contrast, diethyl malonate
and ethyl cyanoacetate were also tested, which are often used
in Knoevenagel reactions, but no corresponding products were
formed. It suggested that the structure of the b-ketoester was
necessary for CAL-B catalysed decarboxylative Knoevenagel
reactions. As an effective immobilized hydrolase, CAL-B is
frequently applied in the pharmaceutical industry. It was easy
to recycle from the reaction system and makes full use of its
catalytic function. When CAL-B was reused for five times in the
decarboxylative Knoevenagel reaction, the yield of product only
decreased slightly (Fig S1).†
8
9
1a
1a
2f
—
N.R.
2g 3ag 95
10
11
1a
1a
2h 3ah 87c
2i
3ai
85c
a Reaction conditions: 1 (0.1 mmol), 2 (0.12 mmol), cyclen (0.01 mmol),
CAL-B 10 mg, CH3CN 0.5 mL, 30 ◦C, 20 h. b Isolated yields. c 30 h.
N.R. means no reaction.
between 1 and 2, then a sequential process of hydrolysis
and decarboxylation was carried out (Scheme 3). Firstly, 5
was generated through an aldol condensation after the enol
form of substrate 2 was stabilized by the Asp-His dyad and
oxyanion hole in the active site. Then the Asp-His-Ser triad
catalysed the hydrolysis of 5 to get acid 6. Subsequently, carbon
dioxide was released from 6 after an electron transfer process
under the influence of the Asp-His dyad and oxyanion hole.
Then the final product 3 was formed. For decarboxylative
Knoevenagel reactions, a Schiff base was formed firstly, then
To realize the reaction mechanism, we first performed the
experiment using acetone instead of ethyl acetoacetate, no
product was detected. It proved indirectly that decarboxylation
was not occurring before addition; then a tentative reaction
mechanism was proposed. CAL-B catalysed aldol condensation
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Scheme 3 Proposed mechanism of decarboxylative aldol reaction
Table 3 Decarboxylative Knoevenagel of aromatic aldehydes and
b-ketoestersa
In conclusion, here we report an unprecedented CAL-B
catalysed decarboxylative aldol reaction and decarboxylative
Knoevenagel reaction. It provides a novel case of biocatalytic
promiscuity and might be a potential synthetic method for
organic chemistry.
Acknowledgements
Entry
1
2
4
t (h)
Yield (%)b
This work was financially supported by the National Science
Foundation of China (Nos. 20725206, 20732004 and 20702034)
and the Specialized Research Fund for the Doctoral Program
of Higher Education. We also thank the Sichuan University
Analytical and Testing Centre for NMR analysis.
1
2
3
4
5
6
7
8
1a p-NO2
1b o-NO2
1c m-NO2
1d p-CN
1e H
2a
2a
2a
2a
2a
2a
2a
2a
2a
2b
2c
2d
2e
2f
4aa
4ba
4ca
4da
4ea
4fa
4ga
4ha
4ia
4ab
4ac
4ad
4ae
—
12
24
24
24
24
24
24
24
24
12
24
24
24
24
12
24
90
85
88
89
62
81
72
58
56
91
88
86
82
N.R.
85
84
1f p-CH3
1g p-CH3O
1h o-CH3O
1i m-CH3O
1a
Notes and references
9
10
11
12
13
14
15
16
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