Phytic acid: A biogenic organocatalyst for one-pot Biginelli reactions to 3,4-dihydropyrimidin-2(1H)-ones/thiones

Article abstract of DOI:10.1039/c4ra02084g

The natural organocatalyst phytic acid catalyzed one-pot Biginelli reactions by coupling β-ketoesters, aldehydes, and (thio)ureas to afford 3,4-dihydropyrimidin-2(1H)-ones/thiones. This phytic acid catalysis featured good to excellent isolated yields, sol

Full text of DOI:10.1039/c4ra02084g

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Phytic acid: a biogenic organocatalyst for one-pot
Biginelli reactions to 3,4-dihydropyrimidin-2(1H)-
ones/thiones†
Cite this: RSC Adv., 2014, 4, 19710
*
Qiguo Zhang, Xin Wang, Zhenjiang Li, Wenzhuo Wu, Jingjing Liu, Hao Wu, Saide Cui
*
and Kai Guo
Received 10th March 2014
Accepted 7th April 2014
The natural organocatalyst phytic acid catalyzed one-pot Biginelli reactions by coupling b-ketoesters,
aldehydes, and (thio)ureas to afford 3,4-dihydropyrimidin-2(1H)-ones/thiones. This phytic acid catalysis
featured good to excellent isolated yields, solvent-free conditions, a simple workup, environmental
friendliness, and a short reaction time.
DOI: 10.1039/c4ra02084g
www.rsc.org/advances
Based on the original HCl–ethanol solution protocol,3a many
kinds of Lewis acids7 and several kinds of basic catalysts8 have
Introduction
Multicomponent reactions (MCRs)1 are versatile and efficient
for the preparation of complex biologically-active compounds2
from readily-accessible starting materials. The Biginelli reac-
tion,3 one of the most useful MCRs, allows the one-pot forma-
tion of 3,4-dihydropyrimidin-2(1H)-ones/thiones (DHPMs) 4 by
the cyclocondensation of aldehydes 1, b-ketoesters 2, and (thio)
urea 3 (Scheme 1). Interest in Biginelli reactions stems from the
step economy of the synthesis, the diversity of the products, and
the broad spectrum of pharmacological activities of DHPMs
(Fig. 1). For example, Monastrol, which is able to cross cell
membranes, is a potent anticancer drug,4 and adducts SQ 32926
and SWO2 can act as antihypertensive agents.5 In addition,
several alkaloids isolated from marine sources containing
dihydropyrimidine cores exhibit promising biological
properties.6
been utilized in Biginelli reactions. When considering envi-
ronmental effects,9 however, some drawbacks existed in these
methods. For instance, volatile organic solvents, heavy metals,
unusual chemicals, and harsh or sensitive reaction conditions
were employed. Consequently, measures were made to address
these issues, including the use of readily separable and recy-
clable catalysts such as polyvinylsulfonic acid,10 hydrotalcite11
and solid acids,12 and the development of alternative reaction
conditions in ionic liquids,13 by microwave or ultrasonic assis-
tance,14 and in eutectic mixtures.15
New applications for the classical Biginelli reactions are
made possible by the use of the naturally-sourced catalysts
vitamin B1,16 tartaric acid, citric acid,17 bovine serum albumin,18
and even baker's yeast.19 Using carboxylic acids20 and phos-
phoric acids21 as mild Brønsted acidic organocatalysts22 attrac-
ted our attention. We envisioned that the plant-derived,
ubiquitous phosphoric acid, phytic acid (PhyA), could be useful
as a potential catalyst in organic transformations.
PhyA (myo-inositol hexaphosphate; Fig. 2) is a major phos-
phorus reservoir in plants.23 It is widely used as a chelating
agent, food additive, and antioxidant.24 To the best of our
knowledge, no reports on PhyA in catalysis have been pub-
lished. We probed PhyA as an organocatalyst in Biginelli
Scheme 1 The Biginelli reactions.
State Key Laboratory of Materials-Oriented Chemical Engineering, College of
Biotechnology and Pharmaceutical Engineering, Nanjing Tech University, China.
E-mail: zjli@njtech.edu.cn; zjli.njtech@gmail.com; kaiguo@njtech.edu.cn; Fax: +86
25 5813 9935; Tel: +86 25 5813 9935
† Electronic supplementary information (ESI) available: Characterization date
(1H, 13C) for the 3,4-dihydropyrimidin-2(1H)-ones/thiones. See DOI:
10.1039/c4ra02084g
Fig. 1 Pharmacologically active DHPMs.
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Scheme 2 The iminium mechanism of Biginelli reactions.
non-polar toluene or polar water did not favor interactions
between the two. The solvent-free procedure resulted in a very
good yield (entry 5). Catalyst loading (entries 5 to 9) was optimal
at 10 mol%; increasing PhyA to 15% and 30% did not improve
yields, and 5% catalyst resulted in moderate yield at the desig-
nated reaction time (entry 6).
Fig. 2 The structure of phytic acid and the Brønsted acidic/Lewis basic
pair within one phosphate group.
Next, we expanded the scope of the substrates by using
combinations of methyl/ethyl acetoacetate,
a series of
substituted benazaldehydes and butylaldehyde, urea/thoiurea,
and methyl(thio)urea under solvent-free conditions (Table 2).
Condensations of aldehydes, acetoacetates, and ureas
produced the corresponding DHPMs (4a to 4v) by good to
excellent yields, while aliphatic aldehyde yields were moderate
(4h and 4p). The yields were also good when methylurea was
used instead of urea (4q to 4u). Thioureas, as expected, gave
similar yields to their counterpart ureas (4v to 4d0). Substitu-
tions on the benzene ring of aromatic aldehydes 1 had the
regular effect on the condensations (Table 2). Aromatic alde-
hydes with electron-donating groups afforded higher yields
(e.g. 4b and 4c) than those with electron-withdrawing groups
(e.g. 4e and 4m). Chlorine as activating group by the electron-
donating conjugative effect resulted in yields around 80% (e.g.
4d, 4r, and 4y).
In order to test the limits and expand the scope of PhyA
catalysis in Biginelli-type reactions, we used 1,3-diketones in
place of b-ketoesters. As expected, the yields were good to
excellent (Table 3) with short reaction times.
The mechanism of Biginelli reactions was investigated in the
last eight decades.25 Three hypotheses including (1) iminium
mechanism,25c (2) Knovenagel mechanism,25b and (3) enamine
mechanism25d were widely accepted in Brønsted acid catalysis.
The iminium mechanism has been demonstrated to be kineti-
cally and thermodynamically favorable by ESI(+)-MS(/MS)
characterization and DTF calculations.25d
reactions expecting advantages in: (1) natural abundance, (2)
eco-friendliness, and (3) air, water, and substrate tolerance. We
reported the rst application of PhyA as a catalyst for one-pot,
solvent-free Biginelli condensations to 3,4-dihydropyrimidin-
2(1H)-ones/thiones.
Results and discussion
We initially explored the catalytic effect of PhyA under classical
conditions on the Biginelli reaction of benzaldehyde, urea, and
acetoacetate to produce DHPM 4a. Encouraged by the primary
result (entry 1, Table 1), solvent and solvent-free reactions were
tested. Entries 1 to 4 showed that ethanol was an appropriate
solvent, performing better than toluene, dichloromethane, and
tap water. Since aldehyde and urea are required in the rst step
to produce the intermediate 5 (Scheme 2),25c the extremes of
Table 1 Reaction of benzaldehyde, methyl acetoacetate, and urea
under catalysis of phytic acida
The phosphoric acid catalyst PhyA was able to function as an
ordinary Brønsted acid to catalyze Biginelli reactions via the
iminium mechanism25c (Scheme 2). Phosphoric acid, however,
exhibited mild Brønsted acidity in the –OH group and Brønsted/
Lewis basicity in the P]O group, which is typical for the
phosphoric acid structure (Fig. 2).22 Phosphoric acids acting as
bifunctional catalysts via both their acidic –OH site and basic
P]O moiety had been recently exploited and substantiated by
theory.22a,27 The acidic and basic sites of phosphate showed
cooperative effects, acting as both the nucleophilic and elec-
trophilic substrates in one reaction.22a,27
Catalyst loading
(mol%)
Entry
Solvent
Time (h)
Yield (%)
1
2
3
4
5
6
7
8
9
EtOHb
3.5
3
4.5
2
1/3
1/3
1/3
1/3
1/3
10
10
10
10
10
5
15
20
30
61
46
54
40
81
51
82
80
79
Toluenec
d
CH2Cl2
H2Oe
Nonef
Nonef
Nonef
Nonef
Nonef
PhyA possessed six phosphate groups.26 Each phosphate
group contained two hydroxyl groups and one phosporyl group.
Based on the bifunctional behavior of phosphoric acids22,27 and
on the basis of the iminium mechanism,25c we proposed that
a
All the yields are isolated yield, and the molar ratio of benzaldehyde,
b
methyl acetoacetate, and urea is 1 : 1.2 : 1.5. Reaction at 78 ꢀC
c
f
d
e
Reaction at 110 ꢀC. Reaction at 60 ꢀC. Reaction at 100 ꢀC.
Reaction at 100 ꢀC.
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Table 2 Phatic acid catalysis in Biginelli reactions to 3,4-dihydropyrimidin-2-(1H)-ones/thionesa
Product
R1
R2
R3
X
Time (h)
Yields (%)
Yields (%) lit.b
Mp (ꢀC)
4a
4b
4c
4d
4e
4f
4g
4h
4i
4j
4k
4l
4m
4n
4o
4p
4q
4r
Me
Me
Me
Me
Me
Me
Me
Me
Et
Et
Et
Et
Et
Et
Et
Et
Me
Me
Me
Et
Et
Me
Et
–C6H5
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
–CH3
–CH3
–CH3
–CH3
–CH3
H
H
H
H
–CH3
–CH3
–CH3
–CH3
–CH3
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
S
0.3
2
4
4
6
4.5
4
10
3
3
3
3.5
7
4.5
6
11
0.5
2
81
86
89
81
66
53
68
57
89
86
81
86
66
76
64
56
96
81
60
94
85
85
94
80
78
87
85
77
85
83
—
—
—
—
—
—
—
—
213.4–215.4
251.0–252.5
199.9–200.3
209.6–210.0
246.1–246.7
249.1–250.7
284.6–258.4
168.1–168.6
209.9–212.0
238.7–239.2
207.9–209.0
216.7–217.0
215.5–216.1
199.9–201.0
233.5–234.1
175.6–178.2
196.8–197.0
142.6–143.4
209.4–209.8
179.8–181.1
139.2–139.7
226.4–227.5
204.6–207.7
152.7–154.4
192.0–194.0
163.7–164.6
157.2–158.2
144.2–151.5
150.3–151.3
87.9–90.0
4-(OH)–C6H4
4-(OCH3)–C6H4
4-(Cl)–C6H4
4-(NO2)–C6H4
3-(Cl)–C6H4
3-(NO2)–C6H4
n-Bu
–C6H5
70,8b 85 (ref. 12b)
4-(OH)–C6H4
4-(OCH3)C6H4
4-(Cl)–C6H4
4-(NO2)–C6H4
3-(Cl)–C6H4
3-(NO2)–C6H4
n-Bu
55,8b 78 (ref. 12b)
58,8b 86 (ref. 12b)
85 (ref. 12b)
91 (ref. 12b)
—
78 (ref. 12b)
—
—
—
–C6H5
4-(Cl)–C6H4
3-(NO2)–C6H4
–C6H5
4-(OCH3)–C6H4
–C6H5
4s
4t
8
—
—
—
—
0.6
4.5
0.6
0.6
3
3
3.5
3
4u
4v
4w
4x
4y
4z
4a0
4b0
4c0
4d0
–C6H5
S
S
S
S
S
S
S
S
50,8b 67 (ref. 12b)
Et
Et
Me
Me
Me
Et
4-(OCH3)–C6H4
4-(Cl)–C6H4
–C6H5
4-(OCH3)–C6H4
3-(Cl)–C6H4
–C6H5
87 (ref. 31)
89 (ref. 31)
—
—
—
—
—
3
1.5
2.5
Et
4-(OCH3)–C6H4
a
Reaction at 100 ꢀC; all the yields are isolated ones; the molar ratio of b-ketoesters, aldehydes, and (thio)ureas is 1.2 : 1 : 1.5, the catalyst loading is
10 mol%. b Some yields of DHPMs under solvent-free condition from the literature.
Table 3 Reaction of aldehydes, 1,3-diketone compounds, and ureaa
of water from 10. The transition state 12 might then be stabi-
lized by the HB of PhyA. Finally, cyclization to 8 and the elim-
ination of another water molecule yielded the target product 4.
The existence of HB could be detected by various methods
such as IR spectroscopy,28 electron spectroscopy,29 and NMR.30
In order to investigate the proposed mechanism, NMR studies
were carried out. Here, our main aim was to evaluate the
Product R2
1,3-Diketone Time (h) Yield (%) Mp (ꢀC)
possibility of the rst step interactions shown in 9. The results
of NMR detection were shown in Fig. 3 and 4. The chemical shi
14a
14b
–C6H5
4-(OH)–C6H4
2/3
1
65
89
242.3–243.0
261.4–261.6
1
of urea protons in the H NMR spectrum of urea in D2O was
4.70 ppm. When PhyA was added, the urea proton peak widened
shied slightly towards low eld (PhyA/urea ¼ 0.4/1, 4.76 ppm;
PhyA/urea ¼ 0.6/1, 4.78 ppm). This phenomenon may indicate
that the P]O of PhyA affected the protons of –NH–, possibly by
the HB between PhyA and urea.30 Next, the effect of PhyA on
benzaldehyde was examined. The chemical shi of the –CHO
proton was not very obvious because the solubility of PhyA in
CDCl3 was poor. But when PhyA was added, the width of the
–CHO proton peak increased, which might be caused by HB
between the –OH of PhyA and the oxygen of –CHO (Fig. 4). These
phenomena suggested that the three proposed interactions in 9
are possible. Moreover, these results demonstrate the H-bond
interactions between the substrates and catalyst in the
proposed bifunctional activation mode. Based on the
a
Reaction at 100 ꢀC; all the yields are isolated ones; the molar ratio of
1,3-diketone, aldehydes, and urea is 1.2 : 1 : 1.5.
PhyA could catalyze Biginelli reactions through a bifunctional
activation mode. The plausible steps were shown in Scheme 3.
Initially, interactions as in 9 were formed through hydrogen
bonding (HB) in which the acidic –OH of PhyA paired with the
carbonyl of aldehyde 1, and the –NH of urea 3 paired with the
basic P]O.22a,27 The urea nitrogen subsequently attacked
carbonyl, forming the intermediate of type 10.22b,27 The inter-
mediate state 11 might be formed aer releasing one molecule
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bifunctional catalytic behavior of phosphoric acid in the acti-
vation of carbonyl27 and the results of the NMR investigation,
the bifunctional activation mode for Biginelli reactions under
PhyA catalysis may be validated.
Conclusions
In summary, we demonstrated the use of PhyA as a natural
phosphoric acid organocatalyst in solvent-free Biginelli reac-
tions. Compared with other catalysts in solvent-free con-
ditions,8b,12b,31 PhyA provided wider applicability with similar
yields. A plausible Brønsted/Lewis bifunctional activation mode
for the condensations using the acidic –OH site and the basic
P]O site in PhyA was proposed. The preparation featured good
to excellent isolated yields, solvent-free conditions, a simple
workup, environmental friendliness, and a short reaction time.
Experimental
1
Melting points were determined on a Koer hot stage. H and
13C NMR spectra were recorded in DMSO or CDCl3 solutions.
The 1H NMR spectral measurements were performed at 300
MHz and 13C NMR were at 75 MHz.
Scheme 3 A plausible mechanism for the Biginelli reaction under
phytic acid catalysis in bifunctional activation mode.
General procedure for the synthesis of DHPMs
Aldehydes (3 mmol), b-ketoesters or 1,3-diketones (3.6 mmol),
ureas or thioureas (4.5 mmol), PhyA (0.3 mmol, 10 mol%) in
solvent-free conditions were heated at 100 ꢀC under stirring for
the time shown in Table 3. Aer completion of the reaction was
indicated by TLC, the mixture was cooled to room temperature
and ethanol (10 mL) was added (the unreacted substrates and
PhyA will dissolve in ethanol and the crystallized product will
separate). The resultant solid was separated from the mixture
by ltration, rinsed with ethanol (3 ꢁ 10 mL) and water (3 ꢁ
10 mL), and dried in vacuum at 40 ꢀC to afford the desired
product as a crystalline solid.
Acknowledgements
Financial support from the National High Technology Research
and Development Program of China (2011AA02A202), the
Research Fund for the Doctoral Program of Higher Education of
China (20123221110009), and the Project Funded by the Priority
Academic Program Development of Jiangsu Higher Education
Institutions is acknowledged.
Fig. 3 Chemical shifts of urea protons in the 1H NMR spectrum
observed with different urea to PhyA ratios in D2O.
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