α-Glucosidase Inhibition by Usnic Acid Derivatives

Article abstract of DOI:10.1002/cbdv.202000906

This study investigated a set of new potential antidiabetes agents. Derivatives of usnic acid were designed and synthesized. These analogs and nineteen benzylidene analogs from a previous study were evaluated for enzyme inhibition of α-glucosidase. Analogs synthesized using the Dakin oxidative method displayed stronger activity than the pristine usnic acid (IC₅₀>200 μM). Methyl (2E,3R)-7-acetyl-4,6-dihydroxy-2-(2-methoxy-2-oxoethylidene)-3,5-dimethyl-2,3-dihydro-1-benzofuran-3-carboxylate (6b) and 1,1′-(2,4,6-trihydroxy-5-methyl-1,3-phenylene)di(ethan-1-one) (6e) were more potent than an acarbose positive control (IC₅₀ 93.6±0.49 μM), with IC₅₀ values of 42.6±1.30 and 90.8±0.32 μM, respectively. Most of the compounds synthesized from the benzylidene series displayed promising activity. (9bR)-2,6-Bis[(2E)-3-(2-chlorophenyl)prop-2-enoyl]-3,7,9-trihydroxy-8,9b-dimethyldibenzo[b,d]furan-1(9bH)-one (1c), (9bR)-3,7,9-trihydroxy-8,9b-dimethyl-2,6-bis[(2E)-3-phenylprop-2-enoyl]dibenzo[b,d]furan-1(9bH)-one (1g), (9bR)-2-acetyl-6-[(2E)-3-(2-chlorophenyl)prop-2-enoyl]-3,7,9-trihydroxy-8,9b-dimethyldibenzo[b,d]furan-1(9bH)-one (2d), (9bR)-2-acetyl-6-[(2E)-3-(3-chlorophenyl)prop-2-enoyl]-3,7,9-trihydroxy-8,9b-dimethyldibenzo[b,d]furan-1(9bH)-one (2e), (6bR)-8-acetyl-3-(4-chlorophenyl)-6,9-dihydroxy-5,6b-dimethyl-2,3-dihydro-1H-[1]benzofuro[2,3-f][1]benzopyran-1,7(6bH)-dione (3e), (6bR)-8-acetyl-6,9-dihydroxy-5,6b-dimethyl-3-phenyl-2,3-dihydro-1H-[1]benzofuro[2,3-f][1]benzopyran-1,7(6bH)-dione (3h), (6bR)-3-(2-chlorophenyl)-8-[(2E)-3-(2-chlorophenyl)prop-2-enoyl]-6,9-dihydroxy-5,6b-dimethyl-2,3-dihydro-1H-[1]benzofuro[2,3-f][1]benzopyran-1,7(6bH)-dione (4b), and (9bR)-6-acetyl-3,7,9-trihydroxy-8,9b-dimethyl-2-[(2E)-3-phenylprop-2-enoyl]dibenzo[b,d]furan-1(9bH)-one (5c) were the most potent α-glucosidase enzyme inhibitors, with IC₅₀ values of 7.0±0.24, 15.5±0.49, 7.5±0.92, 10.9±0.56, 1.5±0.62, 15.3±0.54, 19.0±1.00, and 12.3±0.53 μM, respectively.

Full text of DOI:10.1002/cbdv.202000906

DOI: 10.1002/cbdv.202000906
FULL PAPER
α-Glucosidase Inhibition by Usnic Acid Derivatives
Huy Truong Nguyen,a Asshaima Paramita Devi,b, c Tran-Van-Anh Nguyen,a Warinthorn Chavasiri,b
Duc-Dung Pham,d Jirapast Sichaem,e Ngoc-Hong Nguyen,f Bui-Linh-Chi Huynh,g
Van-Kieu Nguyen,*h, i and Thuc Huy Duong*d
a
Faculty of Pharmacy, Ton Duc Thang University, Ho Chi Minh City 700000, Vietnam
b
Center of Excellence in Natural Products Chemistry, Department of Chemistry, Faculty of Science,
Chulalongkorn University, Pathumwan, Bangkok 10330, Thailand
Program in Biotechnology, Faculty of Science, Chulalongkorn University, Pathumwan 10330, Thailand
c
d
Department of Chemistry, Ho Chi Minh City University of Education, 280 An Duong Vuong Street, District 5
Ho Chi Minh City 748342, Vietnam, e-mail: huydt@hcmue.edu.vn
Research Unit in Natural Products Chemistry and Bioactivities, Faculty of Science and Technology,
e
Thammasat University Lampang Campus, Lampang 52190, Thailand
CirTech Institute, Ho Chi Minh City University of Technology (HUTECH), Ho Chi Minh City 700000, Vietnam
Department of Nature, Dong Nai University, Dong Nai Province 810000, Vietnam
Institute of Fundamental and Applied Sciences, Duy Tan University, Ho Chi Minh City 700000, Vietnam
f
g
h
i
Faculty of Natural Sciences, Duy Tan University, Da Nang 550000, Vietnam,
e-mail: nguyenvankieu2@duytan.edu.vn
This study investigated a set of new potential antidiabetes agents. Derivatives of usnic acid were designed
and synthesized. These analogs and nineteen benzylidene analogs from a previous study were evaluated for
enzyme inhibition of α-glucosidase. Analogs synthesized using the Dakin oxidative method displayed stronger
activity than the pristine usnic acid (IC50 >200 μM). Methyl (2E,3R)-7-acetyl-4,6-dihydroxy-2-(2-methoxy-2-
oxoethylidene)-3,5-dimethyl-2,3-dihydro-1-benzofuran-3-carboxylate (6b) and 1,1’-(2,4,6-trihydroxy-5-methyl-1,3-
phenylene)di(ethan-1-one) (6e) were more potent than an acarbose positive control (IC50 93.6�0.49 μM), with
IC50 values of 42.6�1.30 and 90.8�0.32 μM, respectively. Most of the compounds synthesized from the
benzylidene series displayed promising activity. (9bR)-2,6-Bis[(2E)-3-(2-chlorophenyl)prop-2-enoyl]-3,7,9-trihy-
droxy-8,9b-dimethyldibenzo[b,d]furan-1(9bH)-one (1c), (9bR)-3,7,9-trihydroxy-8,9b-dimethyl-2,6-bis[(2E)-3-phenyl-
prop-2-enoyl]dibenzo[b,d]furan-1(9bH)-one (1g), (9bR)-2-acetyl-6-[(2E)-3-(2-chlorophenyl)prop-2-enoyl]-3,7,9-tri-
hydroxy-8,9b-dimethyldibenzo[b,d]furan-1(9bH)-one (2d), (9bR)-2-acetyl-6-[(2E)-3-(3-chlorophenyl)prop-2-enoyl]-
3,7,9-trihydroxy-8,9b-dimethyldibenzo[b,d]furan-1(9bH)-one (2e), (6bR)-8-acetyl-3-(4-chlorophenyl)-6,9-dihydroxy-
5,6b-dimethyl-2,3-dihydro-1H-[1]benzofuro[2,3-f][1]benzopyran-1,7(6bH)-dione (3e), (6bR)-8-acetyl-6,9-dihydroxy-
5,6b-dimethyl-3-phenyl-2,3-dihydro-1H-[1]benzofuro[2,3-f][1]benzopyran-1,7(6bH)-dione (3h), (6bR)-3-(2-chloro-
phenyl)-8-[(2E)-3-(2-chlorophenyl)prop-2-enoyl]-6,9-dihydroxy-5,6b-dimethyl-2,3-dihydro-1H-[1]benzofuro[2,3-
f][1]benzopyran-1,7(6bH)-dione (4b), and (9bR)-6-acetyl-3,7,9-trihydroxy-8,9b-dimethyl-2-[(2E)-3-phenylprop-2-
enoyl]dibenzo[b,d]furan-1(9bH)-one (5c) were the most potent α-glucosidase enzyme inhibitors, with IC50 values
of 7.0�0.24, 15.5�0.49, 7.5�0.92, 10.9�0.56, 1.5�0.62, 15.3�0.54, 19.0�1.00, and 12.3�0.53 μM,
respectively.
Keywords: usnic acid, benzylidene derivative, Dakin oxidation, α-glucosidase inhibition.
Supporting information for this article is available on the
WWW under https://doi.org/10.1002/cbdv.202000906
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Chem. Biodiversity 2021, 18, e2000906
Results and Discussion
Chemistry
Introduction
Diabetes poses a potential risk of death if cardiovas-
cular complications arise.[1,2] A combination of ramipril We successfully synthesized 11 usnic acid analogs
and vitamin E, which maintains blood glucose levels from (+)-usnic acid isolated from Usnea baileyi. These
and prevents glucose oxidation, has been reported to include six previously-unreported compounds (1g, 4c,
be a successful therapy.[3] However, novel antidiabetic 5c, 5d, 6a, and 6b). The benzylidene derivatives (1a–
agents from natural or synthetic sources are required. 5d) were obtained using the method reported in our
Usnic acid is known to be an antibacterial,[4] antiviral, earlier article.[15]
antiprotozoal, antiproliferative, anti-inflammatory, anti-
The identification of compounds 1g, 4c, 5c, and 5d
α-glucosidase,[5] and analgesic,[6,7] but its use has been was readily established on the basis of their H, 13C-
limited by its low solubility in organic solvents or NMR, and HR-ESI mass spectra as well as by reference
water.[8,9] Although usnic acid derivatives including to the literature.[14,15] The 1H-NMR spectrum of 1g
triazole, imine, enamine, pyrazole, and chalcone have displayed three singlets of signals at δH 14.01, 11.18,
been reported as antivirals,[10–13] and cytotoxins and 6.11 representing 8-OH, 10-OH, and H-4, respec-
against breast cancer cells,[14] the α-glucosidase inhib- tively, ten aromatic protons [δH 7.71 (dd, J=5.5, 2.0,
itory have been not reported yet. In our previous 2H), 7.66 (dd, J=7.5, 4.0, 2H), 7.46–7.47 (m, 3H), and
study, 25 benzylidene analogs were reported to be 7.45–7.46 (m, 3H)], and two trans double bond [δH
potential anticancer agents.[15] In this study, we 8.29 (d, J=15.5, 1H), 8.07 (d, J=15.5, 1H), 7.95 (d, J=
describe the synthesis of new benzylidene derivatives 15.0, 1H), 7.89 (d, J=15.0, 1H)] indicated two aldoliza-
1
of usnic acid using Dakin oxidation.
tions occurred at C-15 and C-18. NMR comparison of
1g and other similar derivatives 1a–1e[14–16] indicated
they shared the same skeleton (Figure 1). Furthermore,
NMR data of 4c was highly similar to those of 4b,
Figure 1. Chemical structures of 1a–1e, 1g, 2a–2f, 2i, 3a–3f, 3h, 4b–4c, 5b–5d, and 6a–6e.
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Chem. Biodiversity 2021, 18, e2000906
except for the absence of chlorine atoms in phenyl
Five compounds, 6a–6e, were synthesized from 1
groups. Moreover, compounds 5c and 5d were using optimized Dakin oxidation with hydrogen
identified as an aldolized product occurred at C-18 peroxide and K2CO3 as catalyst. The synthesis was
when comparing their NMR and HR-ESI mass data with carried out at room temperature. The synthesis routes
those of the previously reported compound 5b.
are shown in Scheme 1. Compound 6a was identified
from its 1D and 2D NMR spectra. The 1H-NMR
Scheme 1. General synthesis route towards analogs 1g, 2i, 3h, 4c, 5c–5d, and 6a–6e.
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Chem. Biodiversity 2021, 18, e2000906
spectrum showed a hydrogen-bonded hydroxy che- 166.2). Compounds 6a and 6b were therefore found
lated signal at δH 13.52, an olefin proton at δH 5.82, to have novel scaffolds. The structures of 6c–6e were
1
one methoxy group at δH 3.57, and three singlet analyzed from their H- and 13C-NMR spectra and by
methyl groups at δH 2.63, 1.95, and 1.74 ppm. The 13C- reference to the literature.[17,18]
NMR spectrum contained 16 carbon signals. From the
The mechanism is believed to involve initial
HSQC and HMBC spectra, signals at δH 13.52, 5.82, oxidation of 1 at ketone C-14 of usnic acid, providing a
1.95, and 7.9 ppm represented 6-OH, H-2, H3-14, and hydroxy group at C-2. This intermediate is further
H3-11, respectively. The presence of a benzofuran oxidized to yield the corresponding triketone Ia.[19,20]
moiety (rings A and B) was further supported by The oxidative cleavage of Ia at C-1 and C-3 provides
comparing the NMR results with those of usnic acid.[16] diacid Ib,[21] which is then methylated to form 6a and
The absence of both 10-OH and H3-15 of usnic acid 6b. The diacid Ib then initiates decarboxylation,
suggested that an oxidation reaction occurred at C-1 forming 6c and 6d. Finally, the C=C oxidative cleavage
and C-4. This was supported by HMBCs between H3-11 of the furan ring B of 6d produces 6e (Scheme 2).[22,23]
or H3-13 (δH 3.57) and C-12 (δC 169.0), and between H-
4 and C-1 (δC 174.7). Compound 6b was identified as a
methyl ester of 6a, and this was confirmed by HMBCs
between the methoxy protons at δH 3.73 and C-1 (δC
Scheme 2. Proposed mechanism of Dakin reaction of 1 to form 6a–6e.
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Evaluation of the α-Glucosidase Inhibition
Molecular Docking Computation
The usnic acid analogs from Dakin oxidation (6a–6e) AutoDock 4 and Autodock Vina are among of the
and the 25 benzylidene analogs (1–5) were evaluated most popular free packages to roughly and rapidly
for α-glucosidase inhibition. The results are shown in estimate the binding affinity and binding pose of a
Table 1. In most of the derivatives, the IC50 values ligand to a protein.[24] The successful-docking rate of
against α-glucosidase enzyme were below those of the AutoDock 4 package was up to 77% with higher
both the original (1, IC50 >200 μM) and the control accuracy and precision to Autodock Vina in overall
(acarbose, IC50 93.6�0.49). Compound 3e was the according to our previous benchmarkof its perform-
most active (IC50 1.5�0.62 μM). We next investigated ance over 800 protein-ligand complexes.[25] Bhatia
the relationship between biological activity and chem- et al. (2019) evaluated the binding affinities of various
ical structure. The bioactivity of the benzylidene isolated compound’s structures on alpha-glucosidase
analogs was attributed to electronic effects and protein by AutoDock 4, which gave consistent results
changes in the structure of the scaffolds. The presence with their in vitro discovery.[26] Therefore, in this
of chlorine substituents in groups I–III significantly project, AutoDock 4.2 was employed to evaluate the
increased the level of activity. The rank-order of binding affinity of the synthesized ligands to α-
potency reflected the electronic effect of the R- glucosidase. The results of free binding energy
substituent: chlorinated > nonsubstituent > electron- obtained by IC50 experiments and docking were
donating. Within group I, compound 1c showed high- shown in Table S1.
er activity than 1a and 1b, while 1a was more active
than 1b.
The ligand site interactions were analyzed using
MOE 2015.104. The ligands 6a, 2c, and 5c had the
Similar results were found for groups II–V. In group highest, average, and lowest docking affinities toward
III, the lack of conjugation in the D-ring made all 4 J5T, respectively. The ligand 3e has the best
compounds relatively active. Derivatives 3e (IC50 1.5� inhibitory activity (IC50 value).
0.62 μM) and 3h (IC50 15.3�0.54 μM) contained 4-Cl
According to previous literatures, the catalytic
and 4-H groups and exhibited the highest activity. residues in α-glucosidase I protein (GluI) include Asp
Compounds 1c, 2d, and 3e had the lowest IC50 values, 568 and Glu 771. These residues catalyze the hydrol-
and therefore have potential as antidiabetes agents.
ysis reaction of trisaccharide based on the acid-base
The Dakin oxidative analogs (6a–6e) showed lower mechanism.[27,28] In detail, the three non-reducing
enzyme inhibition of α-glucosidase, reflecting the role terminal glucose occupied À 1, +1, +2 subsites in GIu
played by the ring C of the usnic acid.
active site and two residues Asp 568, Glu 771 would
cleave the bond between glycan 1 and glycan 2 in À 1,
Table 1. α-Glucosidase activity of usnic acid derivatives.
Compounds
α-Glucosidase, IC50 (μM)[a]
Compounds
α-Glucosidase, IC50 (μM)[a]
1a
1b
1c
1d
1e
1g
2a
2b
2c
2d
2e
2f
53.6�0.84
84.8�0.59
7.0�0.24
3d
3e
3f
3h
4b
5b
5c
5d
6a
6b
6c
6d
6e
Usnic acid
Acarbose
34.5�0.88
1.5�0.62
72.9�1.21
15.3�0.54
19.0�1.00
100.1�1.11
12.3�0.53
55.3�0.79
>200
45.9�0.14
68.7�1.21
15.5�0.49
62.8�0.72
25.2�1.31
62.1�0.22
7.5�0.92
42.6�1.30
10.9�0.56
86.7�1.52
28.1�1.13
43.9�0.20
96.0�1.10
72.2�1.21
>200
>200
2i
90.8�0.32
3a
3b
3c
>200
93.6�0.49
[a]
IC50 values presented as mean�SD (n=3).
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Chem. Biodiversity 2021, 18, e2000906
+1 subsites.[29] Therefore, blocking these residues values of 7.0, 7.5, and 1.5 μM, respectively. Compound
would result in the cancellation of the catalytic activity 3e exhibited the strongest inhibition and therefore
of the hydrolysis reaction of a trisaccharide. This might be considered as a leading candidate for the
phenomenon has been structurally observed in other treatment and delay of complications from diabetes.
glycoside hydrolases inhibited by monosaccharide
analogs.[30]
In this study, the lowest binding affinities between Experimental Section
protein-ligand complexes and molecule 5c and its
analogs (5b and 5d) which have two H-bonds donors
General Information
with Glu 771, Leu 563, and one H-bond acceptor with (+)-Usnic acid was isolated from the dichloromethane
Trp 710 (Table S2).
fraction of Usnea baileyi (Stirt.) Zalbr. according to the
The second rank of the binding affinity group was previously reported procedure.[16] Solvents were dis-
1a–1g compounds. They are all high molecular mass tilled before use. All reagents were purchased from
substrates which neither block Asp 568 nor Glu 771 Sigma-Aldrich. All reactions were monitored by silica
because in this group, Arg 428 and Asn 448 are two gel 60 F254 TLC plates (Merck). Column chromatog-
main residues that are responsible for forming H-bond raphy was performed on silica gel 60–120 mesh. The
acceptor. Therefore, the predominant binding affinity IR spectra was measured on IS50 ATR Thermo
of this group could be explained by many favorable Scientific, Switzerland, and FT-IR/NIR Spectrometer-
1
hydrophilic interactions of their hydroxy, ether, or Frontier/PerkinElmer, USA, instruments. The H- and
carbonyl groups with polar residues Trp 391, Asp 392, 13C-NMR spectra were measured on a Bruker Avance
Arg 428, Gly 566, Asp 568, Trp 710, and Glu 771 in the spectrometers (400 MHz and 500 MHz for 1H-NMR,
active site of GluI protein. Next, the groups of 2a–2i, 100 MHz, and 125 MHz for 13C-NMR). Proton chemical
3a–3h, and 4b reserve one H-bond donor for Leu 563, shifts were referenced to the solvent residual signal of
while being primary H-bonds acceptors for Asn 453 CD3COCD3 at δH 2.05, and of CDCl3 at δH 7.26. The 13C-
and Arg 428 residues. Hence, Asp 568 blocking was NMR spectra were referenced to the central peak of
not a high priority option for these compounds CD3COCD3 at δC 206.26/ 29.84, and of CDCl3 at δC
because their H-bond donors with this residue were 77.16. Mass spectra were obtained on the Agilent ESI-
observed only with three molecules (2b, 2c, and 2f).
The final group, 6a–6e showed a good agreement
between their molecular size and inhibitor capacity. It
could be explained that their center benzofuran
QTOF instrument.
Synthesis of the Compounds
scaffold and short side chain or small substituent General procedure for preparing the title compounds 1g,
groups were unable to cover a large active site in α- 2i, 3h, 4c, and 5c–5d
glucosidase I protein. In brief, the inhibitor capacity of
all the studied compounds showed good consistency
Compounds (1g, 2i, 3h, 4c, and 5c) and (1a, 2a, 3a,
with literature in which G2lu 771 residue is considered and 5d) were prepared according to our previously
as the main target for blocking the glycoside hydrol- reported procedure[15] as the benzylidene analogs of
ysis reaction.
usnic acid with benzaldehyde and 4-meth-
ylbenzaldehyde, respectively. The IR, 1H-NMR, 13C-
NMR, and HR-MS data of compounds 1g, 2i, 3h, 4c,
and 5d are described in Supporting Information.
Conclusions
In conclusion, compounds synthesized from Dakin
(9bR)-3,7,9-Trihydroxy-8,9b-dimethyl-6-[(2E)-3-
oxidation of usnic acid and benzylidene analogs from (3-methylphenyl)prop-2-enoyl]-2-[(2E)-3-(4-meth-
a previous study were evaluated for α-glucosidase ylphenyl)prop-2-enoyl]dibenzo[b,d]furan-1(9bH)-
inhibition. The results suggest that benzylidene or one (1a). Isolated yield 4.5%. Light yellow powder.[15]
benzofuran analogs of usnic acid have increased
activity against α-glucosidase. The benzylidene ana-
(9bR)-3,7,9-Trihydroxy-8,9b-dimethyl-2,6-bis
logs in this study also displayed higher α-glucosidase [(2E)-3-phenylprop-2-enoyl]dibenzo[b,d]furan-
inhibition than the benzofuran analogs. The chloro- 1(9bH)-one (1g). Isolated yield 4.4%. Light yellow
benzylidene analogs (1c, 2d, and 3e) have potential as powder.
α-glucosidase inhibitors, given their respective IC50
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(9bR)-2-Acetyl-3,7,9-trihydroxy-8,9b-dimethyl-6-
[(2E)-3-(4-methylphenyl)prop-2-enoyl]dibenzo[b,d]
(2E)-[(3R)-7-Acetyl-4,6-dihydroxy-3-(methoxy-
carbonyl)-3,5-dimethyl-1-benzofuran-2(3H)-
furan-1(9bH)-one (2a). Isolated yield 16.5%. Light ylidene]acetic acid (6a). Isolated yield 31%. Light
yellow powder.[15]
brown powder.
(9bR)-2-Acetyl-3,7,9-trihydroxy-8,9b-dimethyl-6-
[(2E)-3-phenylprop-2-enoyl]dibenzo[b,d]furan-
Methyl
(2E,3R)-7-acetyl-4,6-dihydroxy-2-(2-
methoxy-2-oxoethylidene)-3,5-dimethyl-2,3-dihy-
1(9bH)-one (2i). Isolated yield 18.0%. Light yellow dro-1-benzofuran-3-carboxylate (6b). Isolated yield
powder.
3.0%. Light brown powder.
(6bR)-8-Acetyl-6,9-dihydroxy-5,6b-dimethyl-3-
(4-methylphenyl)-2,3-dihydro-1H-[1]benzofuro[2,3-
(7-Acetyl-4,6-dihydroxy-3,5-dimethyl-1-benzo-
furan-2-yl)acetic Acid (6c). Isolated yield 18.0%.
f][1]benzopyran-1,7(6bH)-dione (3a). Isolated yield White powder.
18.2%. Light yellow powder.[15]
Methyl (7-acetyl-4,6-dihydroxy-3,5-dimethyl-1-
benzofuran-2-yl)acetate (6d). Isolated yield 4.5%.
White powder.
(6bR)-8-Acetyl-6,9-dihydroxy-5,6b-dimethyl-3-
phenyl-2,3-dihydro-1H-[1]benzofuro[2,3-f][1]-
benzopyran-1,7(6bH)-dione (3h). Isolated yield
24.4%. Light yellow powder.
1,1’-(2,4,6-Trihydroxy-5-methyl-1,3-phenylene)
di(ethan-1-one) (6e). Isolated yield 6.0%. White
powder.
(6bR)-6,9-Dihydroxy-5,6b-dimethyl-3-phenyl-8-
[(2E)-3-phenylprop-2-enoyl]-2,3-dihydro-1H-[1]-
benzofuro[2,3-f][1]benzopyran-1,7(6bH)-dione (4c).
Isolated yield 3.4%. Light yellow powder.
α-Glucosidase Inhibition Assay
α-Glucosidase inhibitory activity was determined using
(9bR)-6-Acetyl-3,7,9-trihydroxy-8,9b-dimethyl-2- the method of Nguyen et al.[31] with slight modifica-
[(2E)-3-phenylprop-2-enoyl]dibenzo[b,d]furan-
tions. Enzymatic activity was quantified by measuring
1(9bH)-one (5c). Isolated yield 4.6%. Light yellow absorbance at 405 nm (ALLSHENG microplate reader
powder.
AMR-100). All samples were tested in triplicate at
different concentrations to obtain the IC50 value of
(9bR)-6-Acetyl-3,7,9-trihydroxy-8,9b-dimethyl-2- each compound. The mean values and standard
[(2E)-3-(4-methylphenyl)prop-2-enoyl]dibenzo[b,d]
furan-1(9bH)-one (5d). Isolated yield 3.8%. Light
yellow powder.
deviations were also determined.
Docking Study
The α-glucosidase protein (PDB ID: 4 J5T) structure
was obtained from the Protein Data Bank database
(PDB), while the ligand structures were drawn and
General Procedure for Preparing the Title Compounds
6a–6e
(+)-Usnic acid (1.0 g, 1.9 mmol) and K2CO3 (1.6 g, then converted to three-dimensional bond-line struc-
11.6 mmol) were dissolved in MeOH (200 mL). 890 μL tures by https://chemicalize.com/, developed by
of H2O2 30% (8.7 mmol) was added while stirring at ChemAxon.
room temperature for 10 h. The reaction was
quenched by adding acidic solution, 1 M HCl, followed
AutodockTools version 1.5.6 was utilized to set up
by extraction with AcOEt/H2O (1:1; v/v) 3 times and parameters for the alpha-glucosidase protein and the
evaporation under vacuum. The purification using synthesized compounds, which were reported into
chromatography was proceeded using CH2Cl2/EtOH/ PDBQT files. The atom models of receptor and ligands
H2O (8:0.2:0.01) to obtain the desired derivatives 6a– were contained the polar hydrogen atoms and
6e. The IR, 1H-NMR, 13C-NMR, and HR-MS data of evaluated the charges via Gasteiger-Marsili method.[32]
compounds 6a–6e are described in Supporting In-
formation.
Molecular docking of the protein-ligand complexes
was then simulated by AutoDock version 4.2.[33]
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Chem. Biodiversity 2021, 18, e2000906
patients with type 2 diabetes prescribed oral antidiabetes
Herein, the center of the α-glucosidase protein
binding site was referred from the docking results of
the complex of taxifolin and α-glucosidase by the grid
center, and the grid box was 2.4×2.4×2.4 nm in
size.[34] The number of genetic algorithms (GA) run
was 250 with the selection of the number of
generations and population size being 27000 and 300,
respectively. The maximum number of evals was
selected as long option, which corresponds to
25,000,000. The lowest estimated free energy of a
distinct conformational cluster that has the largest
series of repetitive runs was regarded as the represen-
tative conformation of the complexes. The RMSD
analysis was also taken into account using an RMSD-
tolerance of 2.0 A.
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Acknowledgment
The authors would like to thank Dr. Son-Tung Ngo,
Head of Theoretical and Computational Biophysics
Laboratory, Ton Duc Thang University, Vietnam, for his
precious advice regarding molecular docking.
Author Contribution Statement
Van-Kieu Nguyen and Thuc Huy Duong conceived and
designed the experiments. Huy Truong Nguyen, Van-
Kieu Nguyen and Thuc Huy Duong performed the
isolation work. Thuc Huy Duong, Jirapast Sichaem,
Van-Kieu Nguyen analyzed the data. Huy Truong
Nguyen and Tran-Van-Anh Nguyen designed and
performed the DOCKING calculations. Asshaima Para-
[12] A. A. Shtro, V. V. Zarubaev, O. A. Luzina, D. N. Sokolov, O. I.
Kiselev, N. F. Salakhutdinov, ‘Novel derivatives of usnic acid
effectively inhibiting reproduction of influenza A virus’,
Bioorg. Med. Chem. 2014, 22, 6826–6836.
[13] N. R. Vanga, A. Kota, R. Sistla, M. Uppuluri, ‘Synthesis and
anti-inflammatory activity of novel triazole hybrids of
(+)-usnic acid, the major dibenzofuran metabolite of the
lichen Usnea longissima’, Mol. Diversity 2017, 21, 273–282.
mita Devi and Warinthorn Chavasiri performed the [14] H. Y. Ebrahim, M. R. Akl, H. E. Elsayed, R. A. Hill, K. A.
El Sayed, ‘Usnic acid benzylidene analogs as potent
mechanistic target of rapamycin inhibitors for the control
of breast malignancies’, J. Nat. Prod. 2017, 80, 932–952.
biological activity. Van-Kieu Nguyen, Huy Truong
Nguyen and Thuc Huy Duong wrote the manuscript.
All the authors reviewed and validated the present
manuscript prior to its being submitted.
[15] V.-K. Nguyen, J. Sichaem, H.-H. Nguyen, X. H. Nguyen, T.-T.-
L. Huynh, T.-P. Nguyen, N. Niamnont, D.-H. Mac, D.-D.
Pham, W. Chavasiri, K.-P.-P. Nguyen, T.-H. Duong, ‘Synthesis
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Received November 4, 2020
Accepted February 4, 2021
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