Lipase-Initiated Tandem Biginelli Reactions via in situ-Formed Acetaldehydes in One Pot: Discovery of Single-Ring Deep Blue Luminogens

Article abstract of DOI:10.1002/adsc.201700599

A facile approach for the synthesis of 3,4-dihydropyrimidin-2(1H)-ones was developed by a tandem multi-component reaction (MCR) in one pot. This approach involves two steps, lipase-catalyzed in situ generation of acetaldehyde and the Biginelli reaction in

Full text of DOI:10.1002/adsc.201700599

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Title: Lipase-Initiated Tandem Biginelli Reactions via In Situ-Formed
Acetaldehydes in One Pot: Discovery of Single-Ring Deep Blue
Luminogens
Authors: Xiao-Qi Yu, Wei Zhang, Na Wang, Zeng-Jie Yang, Yan-Rong
Li, Yuan Yu, and Xue-Mei Pu
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.201700599
Link to VoR: http://dx.doi.org/10.1002/adsc.201700599

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10.1002/adsc.201700599
Advanced Synthesis & Catalysis
FULL PAPER
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Lipase-Initiated Tandem Biginelli Reactions via In Situ-Formed
Acetaldehydes in One Pot: Discovery of Single-Ring Deep Blue
Luminogens
Wei Zhang, Na Wang,* Zeng-Jie Yang, Yan-Rong Li, Yuan Yu, Xue-Mei Pu, and
Xiao-Qi Yu*
Key Laboratory of Green Chemistry and Technology, Ministry of Education, College of Chemistry, Sichuan University,
Chengdu 610064, P.R. China.
Fax: (+86)-28-8541-5886; e-mail: wnchem@scu.edu.cn, xqyu@scu.edu.cn
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.((Please
delete if not appropriate))
Abstract:
A facile approach for synthesis of 3,4- structurally diverse products (yields up to 98% under the
dihydropyrimidin-2(1H)-ones was developed by a tandem optimized conditions in this paper) and provided an
multi-component reaction (MCR) in one pot. This approach exceptional synthesis tool for the discovery of the minimal
involves two steps, lipase-catalyzed in situ generation of deep-blue luminogen in the solid state, namely, a single
acetaldehyde and the Biginelli reaction in turns. Several ring.
control experiments were performed using acetaldehydes
directly to explore the possible mechanism of this procedure.
Moreover, owing to the distinct modularity and highly
efficient features of the MCR, it assembles libraries of
Keywords: enzyme catalysis; Candida antarctica Lipase
B; tandem reaction; one-pot synthesis; fluorescence
structures with evident advantages, such as simplified
workup procedures, high overall yields and versatile
product libraries. As one of the most useful MCRs,
the Biginelli reaction enables straightforward access
Introduction
Enzyme catalysts, by allowing the development of
green and efficient synthetic reactions, are
increasingly becoming the best complements to the
traditional chemocatalysts.[1] Among the enzymes,
lipases (EC 3.1.1.3) normally catalyze hydrolytic
cleavage of ester bonds in the aqueous media but also
transesterification in the organic media.[2] The
inherent advantages of lipases such as good stability,
simple processing requirements, and mild reaction
conditions greatly expand their application.[3] Of the
lipases from different organisms, Candida antarctica
lipase B (CALB) is a robust enzyme, especially in
immobilized form (Novozyme 435), and can be
readily expressed industrially in large quantities.
Currently, it serves as one of the most frequently used
lipases in both academic and industrial laboratories.[4]
to
privileged
3,4-dihydropyrimidin-2(1H)-ones
(DHPMs) and relevant heterocyclic scaffolds.[6]
DHPM derivatives are well-known a class of
biologically active molecules including calcium
channel blockers, antitumor activity, HIV inhibitor,
and interaction with sequence-selective DNA or
bovine serum albumin, etc.[7] Up to now, the Biginelli
reactions for synthesis of DHPMs have been reported
mainly involving catalyst variations on the following
types: Lewis acids, ionic liquids, Brønsted acids,
biocatalysts, and organocatalysts.[6,8] However, these
existing species suffer with certain drawbacks in
terms of complicated procedures for preparations of
catalysts, tedious reaction conditions, unsatisfactory
yields, and environment pollution. In aspects of basic
skeleton, almost all the DHPMs were obtained by
using aromatic aldehydes as the substrates instead of
acetaldehydes, because acetaldehydes are difficult to
handle with low boiling points and tendency to
polymerize.[6,8] Therefore, the in situ generation of
acetaldehydes from the corresponding substrates
under mild conditions is an ideal method for further
reactions. Several groups (including ours) have
developed some cascade strategies like this through
elegant enzyme catalysis recently.[9]
Additionally,
numerous
publications
have
demonstrated that CALB could exhibit promiscuity
through catalyzing the generation of various carbon-
carbon or carbon-heteroatom bonds.[5] In spite of
continuous enthusiasm dedicated to this field, at the
present stage, integrating a CALB-catalyzed process
into a tandem reaction in one pot becomes fascinating
and remains a considerable challenge.
Multi-component reactions (MCRs) have attracted
sustained attention because they represent a powerful
tool for the construction of complex molecular
1
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Advanced Synthesis & Catalysis
Table 1. The influence of different solvents and Reactant
A on the CALB-initiated tandem reaction.[a]
Herein, following the approach and continuous
research towards enzyme catalysis, we wish to
disclose a facile tandem MCR initiated by a lipase
(CALB) for one-pot synthesis of DHPMs in the
presence of water. This tandem procedure apparently
consists of two steps: CALB-catalyzed generation of
acetaldehydes from vinyl acetate and followed by the
Biginelli reaction of in situ-formed acetaldehyde,
urea and β-dicarbonyl compound. Some proof-of-
principle experiments were designed using
acetaldehydes directly to gain insights into the
possible mechanism and the role of CALB, especially
to the second-step reaction. More importantly, due to
the modular feature of the MCR and high efficiency
of this tandem procedure, it could easily assemble
diverse product libraries by simply altering the
starting materials and gave the yields from moderate
to excellent. These structurally diverse molecules
provided us an opportunity to screen their optical
properties resulting in discovery of single-ring deep
blue fluorogens in the solid state.
Entry Solvent
Reactant A Yield [%] [b]
1
2
3
4
THF
MeCN
n-Hexane
DMSO
i-PrOH
i-PrOH
i-PrOH
i-PrOH
<5
<5
<5
--
5
6
7
8
9
10
11
1,4-dioxane i-PrOH
<5
<5
--
<5
48
54
38
<5
CH2Cl2
Toluene
i-PrOH
H2O
i-PrOH
i-PrOH
i-PrOH
EtOH
H2O
H2O
H2O
i-PrOH
t-BuOH
H2O
Results and Discussion
12
[a]
Conditions: urea (0.2 mmol), ethyl acetoacetate (0.4
Multi-component reactions associated with tandem
processes are networks of diversified reactions. Thus,
finding the suitable parameters for these reaction
systems are likely to be more difficult than for the
stepwise processes. On the basis of our previous
research, the condition optimization of the MCR was
performed for the synthesis of 1a using CALB as a
biocatalyst to initiate the tandem reaction. Reaction
medium has been recognized an important role in
affecting the enzymatic reaction.[10] Accordingly, we
began our research by exploring the effects of various
organic solvents and Reactant A on the model
reaction. It is reasonable that vinyl acetate reacts with
mmol), vinyl acetate (0.2 mL), and CALB (10 mg) in
different solvents (1.0 mL) and Reactant A (0.2 mL) at
50 °C for 3 d. [b] Determined by HPLC.
From the structure of 1a, it contains an ester group
which introduced through the β-dicarbonyl
component. A question should be considered that
whether CALB could catalyze transesterification
between the ester group and alcohol to afford other
types of products? Thus we performed the reaction
using different alcohols and the results were listed
below. It is found that the corresponding products via
transesterification gave trace or no yield (entry 2 and
3, Table S1). Notably, when using ethanol as a
substrate, the yield of the target product 1a actually
showed a decreased (entry 1, Table S1). Due to those
results, we chose i-PrOH as a substrate and did not
consider the effect of transesterification during the
subsequent conditional optimization.
Although higher yields were obtained in the water-
alcohol media, we were puzzled with the results in
the pure organic solvents. In fact, CALB could
catalyze transesterification between vinyl acetate and
isopropanol to generate acetaldehyde in the absence
of water, and some relevant literatures have been
reported. Hence, we rationally considered that the
failure to obtain the target product in the organic
solvents may be ascribed to the second-step reaction
(namely, Biginelli reaction). To demonstrate this, we
directly utilized acetaldehyde as a substrate to
incubate with urea and ethyl acetoacetate in the
different solvents. Before the experiments, the
amount of acetaldehyde was firstly calculated
according to the amount of vinyl acetate in the
tandem process (Scheme S1). When vinyl acetate
(0.20 mL) was completely converted, the generation
Reactant
A
to form vinyl alcohol through
transesterification in the first step, and then
tautomerism of vinyl alcohol produces acetaldehyde.
In this way, we merely limit the Reactant A to
several alcohols. The results are summarized in Table
1 and clearly indicated that the solvent played an
important role in realizing the tandem reaction. No
reaction or only the trace amount of the desired
product was generated when the reactants were
incubated in the different pure organic solvents
(entries 1-8, Table 1). On the contrary, we found that
water could markedly promote the reaction.
Therefore, followed condition parameters were
evaluated in the “wet” alcohol media. The tandem
process provided a better result in the wet i-PrOH,
giving the product 1a in 54% yield (entry 9, Table 1).
An attempt to carry out the reaction in the aqueous
medium led to the formation of trace product (entry
10, Table 1). Excess i-PrOH and vinyl acetate were
utilized to produce acetaldehyde constantly due to
acetaldehyde being a volatile liquid with a low
boiling point. Actually, the mixture of water, i-PrOH
and vinyl acetate was the true medium in this one-pot
system.
2
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Entry VAcOH [mL] Yield [%] [b]
amount of acetaldehyde was ~0.122 mL. Considering
the loss of volatilization, the actual volume of
acetaldehyde we took was 0.150 mL.
1
2
3
4
5
6
0
<5
<5
12
18
29
36
42
0.002
0.004
0.006
0.008
0.100
0.125
The experimental data are summarized in Table 2.
It is obvious that the reactions barely proceed in all
involved solvent systems. Especially, only trace
product was detected in the water-alcohol mixture
solvent (entry 2, Table 2), which is very close to the
conditions of entry 10 in Table 1. Such a confusing
but intriguing result promoted us to think what
species make the second-step reaction proceed
smoothly in the tandem process. Looking back at the
first-step reaction, when there is water, hydrolysis of
vinyl acetate by CALB could also generate
acetaldehyde but acetic acid as a by-product.
Therefore, we took into account that whether acetic
acid has an effect on the Biginelli reaction. To prove
the hypothesis, we calculated the amount of acetic
acid at first when vinyl acetate (0.20 mL) was totally
hydrolyzed (Scheme S2).
7
[a]
Conditions: urea (0.2 mmol), ethyl acetoacetate (0.4
mmol), acetaldehyde (0.15 mL), i-ProOH (0.2 mL), acetic
acid (0-0.125 mL) and CALB (10 mg) in deionized water
(1.0 mL) at 50 °C for 3 d. [b] Determined by HPLC.
These results prompted us to further explore the
influence of acetic acid on the second-step reaction in
the other solvent systems. Keeping 0.125 mL of
acetic acid in each reaction, we detected the yields of
1a in the different solvents and the results are
summarized in Table 4. It is clear that almost all of
the reactions respectively give improved yields than
those in Table 2 except in the pure water. By
comparing the three sets of the experimental data
(Table 2-4), we temporarily think that the second-step
reaction may be related to acetic acid. However, we
considered that if the role of acetic acid is a catalyst,
in that way, the reaction has less relevant to its
amount. But in truth, this judgment is inconsistent
with the results of Table 3.
Table 2. The influence of different solvents on the
Biginelli reaction.[a]
Entry Solvent A Solvent B Yield [%] [b]
Table 4. The influence of acetic acid on the Biginelli
1
2
3
4
5
6
H2O
H2O
H2O
i-PrOH
EtOH
Toluene
MeCN
H2O
i-PrOH
EtOH
i-PrOH
EtOH
i-PrOH
i-PrOH
<5
<5
<5
<5
<5
--
reaction in different solvents.[a]
7
<5
[a]
Conditions: urea (0.2 mmol), ethyl acetoacetate (0.4
Entry Solvent A Solvent B Yield [%] [b]
mmol), acetaldehyde (0.15 mL), and CALB (10 mg) in
different solvents (Solvent A: 1.0 mL, Solvent B: 0.2 mL)
at 50 °C for 3 d. [b] Determined by HPLC.
1
2
3
4
5
6
H2O
H2O
i-PrOH
EtOH
MeCN
Toluene
H2O
<5
42
35
49
25
10
i-PrOH
i-PrOH
EtOH
i-PrOH
i-PrOH
Subsequently, we added different amount of acetic
acid in the reaction system (based on the entry 2,
Table 2) to examine its effect on the yield. The
volume of acetic acid changes from 0 mL to the
largest (0.125 mL). As shown in Table 3, the yields
were significantly affected by the amount of acetic
acid, and the highest yield was reached when the
reaction system contained 0.125 mL of acetic acid.
Besides, we found that the yields displayed a gradual
increasing trend along with enhancing acetic acid.
[a]
Conditions: urea (0.2 mmol), ethyl acetoacetate (0.4
mmol), acetaldehyde (0.15 mL), acetic acid (0.125 mL)
and CALB (10 mg) in different solvents (Solvent A: 1.0
mL, Solvent B: 0.2 mL) at 50 °C for 3 d. [b] Determined by
HPLC.
To better understand the role of acetic acid, we
continued to perform some control experiments on
the foundation. At first, we investigated the influence
of the amount of acetic acid on the Biginelli reaction
when CALB was absent (Table 5). The yields
dropped slightly along with increasing the volume of
acetic acid. It means that acetic acid could slightly
inhibit the reaction. Additionally, in comparison with
the reactions which contained CALB (Table 3), these
cases gave higher yields under the corresponding
Table 3. The influence of acetic acid on the Biginelli
reaction.[a]
3
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