Enantioselective Diels–Alder reactions with left-handed G-quadruplex DNA-based catalysts

Article abstract of DOI:10.1016/j.cclet.2020.12.047

Since the discovery of left-handed G-quadruplex (L-G4) structure formed by natural DNA, there has been a growing interest in its potential functions. This study utilised it to catalyse enantioselective Diels-Alder reactions, considering its different opti

Full text of DOI:10.1016/j.cclet.2020.12.047

Chinese Chemical Letters 32 (2021) 1701–1704
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Chinese Chemical Letters
journal homepage: www.elsevier.com/locate/cclet
Communication
Enantioselective Diels–Alder reactions with left-handed G-quadruplex
DNA-based catalysts
*
Kun Chen, Zhiyong He, Wei Xiong, Chun-Jiang Wang, Xiang Zhou
College of Chemistry and Molecular Sciences, Key Laboratory of Biomedical Polymers of Ministry of Education, Wuhan University, Wuhan 430072, China
A R T I C L E I N F O
A B S T R A C T
Article history:
Since the discovery of left-handed G-quadruplex (L-G4) structure formed by natural DNA, there has been
a growing interest in its potential functions. This study utilised it to catalyse enantioselective Diels-Alder
reactions, considering its different optical rotation compared to an ordinary G4. It was determined that
when L-G4 was used with a combination of copper(II) ions, there was a good enantioselectivity (À52% ee)
without further addition of ligands. When further consideration was given by adding G4 ligands, G4 was
further stabilised, even obtaining a better enantioselectivity (up to À80% ee). Moreover, when using
ligands that have regulatory effects on G4, the ee value can be adjusted. In this work, a minimal
left-handed G4 was reported. A follow-up study was also conducted, which recovers that the minimal
left-handed G4 remains its catalytic effect and enantioselectivity, but is not so effective as the former
case. This indicates that a complete G4 structure is relatively conducive to chiral catalysis.
© 2021 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences.
Published by Elsevier B.V. All rights reserved.
Received 4 November 2020
Received in revised form 21 December 2020
Accepted 23 December 2020
Available online 29 December 2020
Keywords:
Diels–Alder reaction
Left-handed G-quadruplex
Asymmetric catalysis
Enantioselectivity
G-quadruplex ligand
Over the past two decades, numerous studies have presented
that DNA not only stores genetic information [1], but also has
greater potential applications. DNA-base hybrid catalysts com-
prised of double-stranded DNA and metal ions is a good example
[2–6]. Due to its chirality and biocompatibility, DNA manifests
excellent advantages in chiral catalysis [7]. Further researches have
discussed that DNA-base hybrid catalysts have certain chemical
effects and enantioselectivity [8]. Through a varied complexation
of metal ions, DNA can participate in numerous catalytic reactions,
such as Diels-Alder (D-A) [9] and Friedel-Crafts [10] reactions.
G-Quadruplex (G4) is different from the conventional double-
strand. It is a secondary structure formed by superimposing
multiple quadruple guanines [11]. Most research objects are
induced by a single-strand DNA. A conventional G4 is generally
classified as either parallel structure or anti-parallel structure.
Based on a classification by means of glycosidic bond angles (GBA),
antiparallel can be divided into structures comprising the same
type of GBA guanine sequences and different types of guanine [12].
G4 has a variety of chain orientations and ring structures [13], and
its internal cavity constitutes a good chiral conversion condition
[14]. In this case, G4 catalysis has attracted the attention of many
researchers. With a more regular and controllable structure, it is
likely that G4 can be used as an applicable chiral template, and
some metal ion cores can be added to enhance its catalytic effect
and enantioselectivity [13,15–17].
Chung et al. has discovered a 28-nt sequence L-G4 (sequence
5'-T(GGT)4TG(TGG)3TGTT-3'), which exhibits a distinct circular
dichroism (CD) profile with a negative peak at ꢀ275 nm and a
positive peak at ꢀ250 nm (Fig. 1), nearly inverted from that of all
right-handed G4 topologies reported to date [18]. Recently, a
second case of mini-L-G4 (sequence 5'-GT(GGT)3G-3') has been
developed, which proves a very similar CD signal as L-G4 [19].
However, recently, studies on the function of L-G4 have not yet
been reported.
The D-A reaction is a significant carbon-carbon bond formation
reaction in organic synthesis [20,21]. Over the past few decades,
the search for innovative catalytic strategies for controlling the
establishment of new carbon-carbon bonds and stereo-centres has
gained extensive attention [22–24]. Among these strategies,
biomolecules and particularly nucleic acid materials have been
regarded as promising catalysts due to their biocompatibility and
mild reaction conditions [13,15–17,25,26]. In this study, L-G4 is
applied with a special conformation for D-A reaction. Initially,
the D-A model reaction [27] between aza-chalcone (1) and
cyclopentadiene (2) was selected to probe the catalytic perfor-
mance of L-G4. It was found that L-G4 alone could promote D-A
reactions with its distinct laevorotatory properties, while the
enantiomeric excess of the endo isomer of product 3a was À6%
(Table 1, entry 3), which was contrary to previous reports and has
caught researchers’ attention. The enantioselectivity of the D-A
* Corresponding author.
E-mail address: xzhou@whu.edu.cn (X. Zhou).
https://doi.org/10.1016/j.cclet.2020.12.047
1001-8417/© 2021 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences. Published by Elsevier B.V. All rights reserved.

K. Chen, Z. He, W. Xiong et al.
Chinese Chemical Letters 32 (2021) 1701–1704
Table 2
Kinetic parameters for L-G4, Cu2+ and L-G4-Cu2+ a
.
b
c
Entry
Catalyst
kapp (L molÀ1 s-1
)
krel
1
2
3
4
None
(2.26 Æ 0.5) Â 10À3
(8.50 Æ 0.9) Â 10À3
(7.07 Æ 1.0) Â 10À3
(2.69 Æ 0.4) Â 10À2
1.0
3.8
3.1
Cu2+
L-G4
L-G4-Cu2+
11.9
a
D-A reactions of 2 (1 mmol/L) and 1a at fixed concentrations (10, 30, 50, 70 and
mol/L) are carried out without and with catalysts of L-G4 (10
mol/L), Cu2+
mol/L), L-G4-Cu2+ (comprised of 50 mol/L Cu2+, in MOPS
mol/L L-G4 and 50
90
(50
m
m
m
m
m
buffer (20 mmol/L, pH 6.5), 100 mmol/L KCl, 20 mmol/L NaCl), at 298 K.
b
The apparent second-order rate constant (kapp) was determined following the
procedure described in the literature. The following expression was used to
compute kapp:kapp: d[A1a]/dt
Á
(d
Á
(
e1a
-
e3a
)
Á
[1a]0
Á
[2]0)À1 = Vinit /([1a]0
Á
[2]0).
c
Rate acceleration (krel) was computed by the ratio of kappcatalyst/kappuncatalyzed
,
where kappuncatalyzed was the apparent second-order rate constant without
catalyst.
a
Fig. 1. CD spectra of different oligodeoxynucleotides. L-G4: L-G4 (10 mmol/L)
reaction as promoted by L-G4 was a little higher than that of the
uncatalyzed reaction (Table 1, entry 3 vs. entry 1), indicating that
L-G4 may be utilised as an enantioselective catalyst reaction of D-A.
A complex of Cu(NO3)2 and L-G4 was assembled to test the
ability of (L-G4-Cu2+) to catalyse D-A reaction enantioselectivity.
with KCl (100 mmol/L) and NaCl (20 mmol/L) in MOPS buffer (20 mmol/L, pH 6.5);
mini-L-G4: mini-L-G4 (10 mol/L) with KCl (100 mmol/L) and NaCl (20 mmol/L) in
MOPS buffer (20 mmol/L, pH 6.5); L1-L-G4single: L-G4-single (no G4 structure
m
formed, 10
mmol/L), L1 (50 mmol/L), without any buffer, do not need anneal.
L-G4-Cu2+ provides
a significant reaction rate enhancement
Table 1
Diels–Alder reaction catalyzed by L-G4-based catalysts.a
(Table 1, entry 4), and L-G4 (Table 1, entry 3) or Cu(NO3)2 (Table 1,
entry 2) have been observed. An excellent diastereoselectivity
(endo/exo 96:4) and good enantioselectivity (À52% ee) of product
3a has also been observed. These results indicate that L-G4-Cu2+
can be utilised as an effective catalyst to provide stereoselectivity
and increase reaction rate. To further understand L-G4-Cu2+, the
influences of the different cations (Table S1 in Supporting
information) in divalent metal salts are assessed. In all cases,
these complexes in the D-A reaction, lower conversion and
enantioselectivity are obtained (Fig. S9 and Table S1 in Supporting
information). With the presence of G4 ligand, catalytic ability and
enantioselectivity of L-G4-Cu2+ have been further improved
(Table 1, entries 5 and 6). Over the course of this research, the
second case of mini-L-G4 was reported [19]. A study was then
performed on its catalytic performance, eventually determining
that the catalytic effect of mini-L-G4 (Table 1, entries 7-9) was not
as good as that of the first L-G4. It was speculated that the
configuration and enantioselectivity of the absolute product rely
on G4DNA conformation.
To validate the catalytic activity of L-G4-Cu2+, the initial rate
(Vinit) of the D–A reaction catalysed by L-G4, Cu2+ and L-G4-Cu2+
(Fig. S2 in Supporting information) was measured. At a fixed
concentration of 1a, VinitL-G4-Cu2+ was greater than VinitCu2+ and VinitL-
G4 (Fig. S2), signifying that L-G4 and Cu2+ assemble into a whole L-
G4-Cu2+ complex. Such little difference between Vinit L-G4 and Vinit
un-catalysing that the catalytic function of L-G4 was weak. The
apparent second-order rate constant (kapp) was then estimated
from the initial rate of the D–A reaction [3,27]. Cu2+ and L-G4
increased kapp by 3.1 times and 3.8 times, respectively (Table 2,
entries 2 and 3), while L-G4-Cu2+ caused a rate acceleration of up to
11.9 times (Table 2, entry 4). Compared with Cu2+ as a catalyst only,
L-G4-Cu2+ has a moderate 4-fold acceleration in the D–A reaction
rate, which was probably because of the coordination of L-G4 with
Cu2+ and the substrate. When L-G4, Cu2+, and 1a were mixed in a
certain proportion, the color of the solution changed obviously,
from nearly colorless to obvious yellow.
In this case, a research on the combination of L-G4, Cu2+ and 1a
was conducted. First, using a job plot (Fig. S5 in Supporting
information), two sets of separate substrates 1a and L-G4 CD were
made; measuring the signal value of 280 nm. It was found that the
two were not combined. Another group entails 2.5% equiv. Cu2+
being added; in this case, the signal has changed. The calculated
difference was linear, implying that the addition of Cu2+ was
aThe reaction details see the Experimental Section in Supporting information. All
data are averaged over two experiments.
bDetermined for the crude product by HPLC analysis on a chiral stationary phase
(Scheme S1 in Supporting information); results are reproducible within Æ 5%.
cDetermined for the endo isomer by HPLC analysis on a chiral stationary phase;
results are reproducible within Æ 5%.
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K. Chen, Z. He, W. Xiong et al.
Chinese Chemical Letters 32 (2021) 1701–1704
Table 3
Whilst optical terms have depicted that the structure of L-G4
and mini-L-G4 are similar, there are remarkable differences in
catalytic effects (Table 1), which further signify that a complete G4
structure highlights the significance of enantioselectivity. At the
same time, to achieve a better catalytic efficiency, the G4 ligand
[28], was used through the screening of ligands L1 [29] and L2 [30]
(Supporting information) to stabilise G4 and to coordinate with
Cu2+ and 1a, so as to obtain better results (Table 1). Through G4
ligands, one of them can speculate that 1a and Cu2+ may interact
with L-G4. For the mini-L-G4, the same strategy was used to
implement the above experiment, in which it was observed that
the catalytic effect has become better, but yet not as good as L-G4.
Taken together, this work proposes an approach based on the
currently known asymmetric catalysis of left-handed G4.
Using the preliminary catalytic results, the substrate specificity
of L-G4-Cu2+ has been assessed with various substituted aza-
chalcones (1a–d) (Table 3). L-G4-Cu2+, L1 and L2 are the active
catalysts for all tested substrates in the D-A reaction. Likewise, the
ee values of 3c and 3d have changed from negative to positive
(Table 1, entries 3 and 4). It was speculated that there was a
coordination with the methoxy group and the nitro group to the
ring part of L-G4. Nevertheless, after adding a G4 ligand, the
catalytic effect can be significantly enhanced. Related experiments
on mini-L-G4 have also been conducted. Unfortunately, the results
are unsatisfactory (Table S2 in Supporting information). The
coordination structure of the metal ion or metal complex with the
host macromolecule in the metalloenzyme was highly vital for
catalytic performance, especially in an enantioselective reaction.
To explore the location of Cu2+ in an L-G4-Cu2+ complex, the
method of proton nuclear magnetic resonance (1H NMR) was used
for investigation. This result suggests that, in the G4 loop ring, it
can coordinate with Cu2+ (Fig. 2). After adding 1a, a remarkable
result emerges, and a new hydrogen bond may be formed.
Specifically, it was believed that L-G4-Cu2+-1a can reduce the
LUMO of 1a, and the kinetic instability of ligand substitution may
exist in a carbon-forming system within a rapid equilibrium
between the electrons [31]. Significantly, this analysis determines
the attractive prospect that L-G4-Cu2+ may function as enantio-
selective catalysts.
Diels–Alder reaction catalyzed by L-G4-based catalysts.a
Entry
Ligand
Substrate
Conversion (%)b
endo/exob
ee (%)c
1
2
3
4
5
6
7
8
none
none
none
none
L1
L1
L1
L1
L2
1a
1b
1c
1d
1a
1b
1c
1d
1a
1b
1c
1d
99
93
98
99
99
97
99
99
99
98
99
99
96:4
97:3
90:10
92:8
98:2
98:2
96:4
96:4
97:3
98:2
94:6
94:6
À52
À58
46
55
À80
À91
78
67
9
À73
À87
65
10
11
12
L2
L2
L2
56
a
Refer to the Experimental Section in Supporting information for reaction
details. All data are taken as the means of two experiments.
b
Determined by the HPLC spectroscopy of the crude product; results are
reproducible within Æ 10%.
c
Determined for the endo isomer by HPLC analysis on a chiral stationary phase;
results are reproducible within Æ 5%.
To summarise, it was found that the various enantioselective
D-A reactions can be attained using a catalyst based on L-G4.
Furthermore, the enantioselectivity of the reaction can be
increased by adding a G4 ligand (L1 and L2). By comparing the
unsatisfactory effects of mini-L-G4, it was determined that a
complete G4 structure was a necessary condition to ensure smooth
catalysis. This special kind of L-G4 may have other applications in
chemical synthesis and biology. More in-depth research on this
type of G4 will be conducted in due course.
Fig. 2. 1H NMR of L-G4 in different conditions. 1: L-G4 (200
(100 mmol/L) and NaCl (20 mmol/L); 2: L-G4 (200 mol/L) and 1a (200
with KCl (100 mmol/L) and NaCl (20 mmol/L); 3: L-G4 (200
mol/L) and Cu2+
(200 mol/L) with KCl (100 mmol/L) and NaCl (20 mmol/L). 4: L-G4 (200 mol/L),
Cu2+ (2 mmol/L) and 1a (9 mmol/L) with KCl (100 mmol/L) and NaCl (20 mmol/L); 5:
L-G4 (200 mol/L) and 1a (200 mol/L) with KCl (100 mmol/L)
mol/L), Cu2+ (200
and NaCl (20 mmol/L).
m
mol/L) with KCl
mol/L)
m
m
m
m
m
m
m
m
Declaration of competing interest
The authors report no declarations of interest.
Acknowledgments
combined. The UV spectrum was measured after annealing L-G4
and Cu2+ (Fig. S6 in Supporting information), taking the signal
value of 260 nm, and calculating the molar ratio as approximately
L-G4:Cu2+ = 1:12. In the case of fixed L-G4:Cu2+ = 1:12, the
combination of L-G4 and 1a (Fig. S8 in Supporting information)
was explored; a rough combination of L-G4:Cu2+:1a = 1:12:45 was
then obtained. To rule out that 2 can be combined with L-G4 and
Cu2+, another experiment (Fig. S7 in Supporting information) was
performed, which eventually finds that 2 was not combined. At the
same time, it can be deduced from the UV–vis method that the
addition of Cu2+ promotes a better binding than other ions (Fig. S9
in Supporting information), which was consistent with the
catalytic results (Table S1).
This work was supported by the National Natural Science
Foundation of China (Nos. 21432008, 91753201, and 21721005). We
thank the large-scale instrument and equipment sharing founda-
tion of Wuhan University.
Appendix A. Supplementary data
Supplementary material related to this article can be
found, in the online version, at doi:https://doi.org/10.1016/j.
cclet.2020.12.047.
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K. Chen, Z. He, W. Xiong et al.
Chinese Chemical Letters 32 (2021) 1701–1704
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