Comparative analysis of stilbene and benzofuran neolignan derivatives as acetylcholinesterase inhibitors with neuroprotective and anti-inflammatory activities

Article abstract of DOI:10.1016/j.bmcl.2019.07.026

Stilbenes and benzofuran neolignans are important groups of plant phenolics therefore they play a significant role in plants and human health. The objective of this study was to investigate the structure-activity relationships of naturally occurring stilbene and benzofuran neolignan derivatives as acetylcholinesterase inhibitors. A series of these compounds were prepared and assessed for their inhibition on acetylcholinesterase activity. δ-Viniferin, pterostilbene trans-dehydrodimer, pallidol, grossamide, and boehmenan exerted acetylcholinesterase inhibitory potential. The several oligomeric compounds protected against cell damage resulting from t-BHP exposure and inhibited lipopolysaccharide/interferon-gamma (LPS/IFNγ)-induced NO production in vitro. Our findings highlight the great potential of pterostilbene trans-dehydrodimer, pallidol, and boehmenan as multifunctional nutraceuticals for management of neurodegenerative diseases.

Full text of DOI:10.1016/j.bmcl.2019.07.026

Bioorganic & Medicinal Chemistry Letters xxx (xxxx) xxx–xxx
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Comparative analysis of stilbene and benzofuran neolignan derivatives as
acetylcholinesterase inhibitors with neuroprotective and anti-inflammatory
activities
Mina Nagumo , Masayuki Ninomiya , Natsuko Oshima , Tomohiro Itoh , Kaori Tanakac,d,
a
a
a
b
e
a,⁎
Atsuyoshi Nishina , Mamoru Koketsu
a
b
Department of Chemistry and Biomolecular Science, Faculty of Engineering, Gifu University, 1-1 Yanagido, Gifu 501-1193, Japan
Laboratory for Molecular Chemistry of Aquatic Materials, Department of Life Sciences, Graduate School of Bioresources, Mie University, 1577 Kurimamachiya, Tsu, Mie
14-8507, Japan
5
c
Division of Anaerobe Research, Life Science Research Center, Gifu University, 1-1 Yanagido, Gifu 501-1194, Japan
United Graduate School of Drug Discovery and Medicinal Information Sciences, Gifu University, 1-1 Yanagido, Gifu 501-1194, Japan
College of Science and Technology, Nihon University, Chiyoda, Tokyo 101-0062, Japan
d
e
A R T I C L E I N F O
A B S T R A C T
Keywords:
Stilbenes and benzofuran neolignans are important groups of plant phenolics therefore they play a significant
role in plants and human health. The objective of this study was to investigate the structure-activity relationships
of naturally occurring stilbene and benzofuran neolignan derivatives as acetylcholinesterase inhibitors. A series
of these compounds were prepared and assessed for their inhibition on acetylcholinesterase activity. δ-Viniferin,
pterostilbene trans-dehydrodimer, pallidol, grossamide, and boehmenan exerted acetylcholinesterase inhibitory
potential. The several oligomeric compounds protected against cell damage resulting from t-BHP exposure and
inhibited lipopolysaccharide/interferon-gamma (LPS/IFNγ)-induced NO production in vitro. Our findings high-
light the great potential of pterostilbene trans-dehydrodimer, pallidol, and boehmenan as multifunctional nu-
traceuticals for management of neurodegenerative diseases.
Stilbene dimer
Benzofuran neolignan
Acetylcholinesterase inhibitor
Neuroprotective activity
Anti-inflammatory activity
Introduction
Benzofuran neolignans form a subgroup of naturally occurring di-
meric phenylpropanoids present in plants. They vary widely in struc-
Stilbenes constitute a vast group of polyphenols that occur naturally
ture due to the various types of substitutions in their basic rings, which
can influence their biological effects. These compounds were also re-
1
in plant origin. Their carbon skeleton which is known as 1,2-diphe-
8
nylethylene (C
6
–C
–C
2 6
), consists of a central ethylene moiety with
ported as antibacterial, anticancer, and anti-inflammatory agents.
benzene rings on each end of the double bond. From the viewpoint of
Structurally interesting benzofuran neolignan derivatives, grossamide
both constitutive and inducible defense mechanisms, stilbenes are of
and boehmenan were isolated from bell pepper (Capsicum annuum var.
2
9,10
primary importance in plants. The best-known stilbene, resveratrol
grossum) and kenaf (Hibiscus cannabinus), respectively.
was firstly isolated from the roots of white hellebore (Veratrum grand-
In the nervous system, acetylcholinesterase (AChE) is a key enzyme
that terminates nerve impulses by splitting the neurotransmitter acet-
3
iflorum). It has attracted extensive scientific attention over the years
1
1
due to its health benefits related with its cardiovascular, chemopre-
ylcholine into choline and acetate. Most promising drugs for symp-
tomatic treatment of AD are AChE inhibitors. This study focuses on the
structure-activity relationships of stilbene and benzofuran neolignan
derivatives as AChE inhibitors. Moreover, these compounds were
evaluated for their neuroprotective and anti-inflammatory activities.
In the plant kingdom, biosynthetic dimerization modes of stilbenes
typically occur through the 8-10′ coupling, 8-8′ coupling, or 3-8′ cou-
pling.1 Subsequently, highly reactive p-quinone methides can undergo
4
,5
ventive, antiobesity, antidiabetic, and neuroprotective properties.
Another interesting stilbene is pterostilbene found in Pterocarpus and
6
Vitis spp. Besides, different hydroxylation patterns of stilbenes bring a
wide variety of polymerization and oligomeric construction. Stilbene
oligomers were found to possess wide biological activities, and some of
them displayed the efficacy in the prevention of Alzheimer's and Par-
kinson's diseases (AD and PD).7
⁎
Corresponding author.
E-mail address: koketsu@gifu-u.ac.jp (M. Koketsu).
https://doi.org/10.1016/j.bmcl.2019.07.026
Received 8 May 2019; Received in revised form 12 July 2019; Accepted 16 July 2019
0960-894X/ © 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Mina Nagumo, et al., Bioorganic & Medicinal Chemistry Letters, https://doi.org/10.1016/j.bmcl.2019.07.026

M. Nagumo, et al.
Bioorganic & Medicinal Chemistry Letters xxx (xxxx) xxx–xxx
Scheme 1. Dimerization of resveratrol (1a) and pterostilbene (1b).
Friedel-Crafts reactions, nucleophilic trappings, or tautomerizations, to
produce a variety of stilbene dimers. δ-Viniferin is a 3-8′ coupled re-
sveratrol dehydrodimer found in wines and grape (Vitis vinifera) cell
50% inhibition. The AchE inhibitory effects of these compounds were
comparable with that of galantamine. Their IC50 values were calculated
as 4.3 ± 0.4 μM for 2a, 11.7 ± 0.7 μM for 2b, and 10.5 ± 1.1 μM for
3a. Contrastingly, resveratrol (1a) and pterostilbene (1b) showed re-
latively weak effects. Overall, dimeric stilbenes denoted a marked
tendency to possess higher inhibition than monomeric counterparts.
Benzofuran neolignans were biosynthesized via oxidative coupling
1
2,13
cultures.
Pallidol is an 8-8′ coupled resveratrol dimer isolated from
12,14
plants of the Vitaceae family.
Previously, a regio- and stereo-
selective biomimetic synthetic method of oligostilbenoids from resver-
1
5
atrol analogues was reported. The starting materials, resveratrol (1a)
was commercially available and pterostilbene (1b) was prepared from
8
of two molecules of phenylpropanoid. The benzofuran neolignan de-
1
6
p-hydroxybenzaldehyde. The dimerization of 1 employing 1.5 equiv.
of silver(I) acetate (AgOAc) in MeOH at 50 °C for 1 h furnished the
desired 3-8′ coupled dehydrodimers (δ-viniferin: 2a, pterostilbene
trans-dehydrodimer: 2b) in 20% and 41% yields (Scheme 1). When 0.9
rivatives, grossamide is produced by dimerization of feruloyl tyramines
and boehmenan is formed by polymerization of four phenylpropanoid
units. Their synthetic sequences are outlined in Schemes 2 and 3. Acidic
methyl esterification of ferulic acid (4) easily gave the product 5 in 84%
yield. The dihydrobenzofuran skeleton in 6 was formed in 30% yield by
equiv. of potassium ferricyanide (K
sence of K CO in MeOH/H
All the spectral data were consistent with the previous reports.
3
[Fe(CN) ]) was used in the pre-
6
1
5,18,19
2
3
2
O, pallidol (3a) was obtained in 11% yield.
silver(I) oxide (Ag
2
O)-mediated dimerization.
This compound 6
1
2–17
was evaluated as a racemic mixture of the trans-diastereoisomers, as
evidenced by the coupling constant (J = 7.3 Hz) between adjacent two
protons in the dihydrobenzofuran ring, which is lower for the trans
The inhibitory effects of stilbene monomers (1) and their dimers (2
and 3) were assessed against AChE with galantamine as a positive
control using the Ellman’s method, in which the enzymatic activity is
measured using acetylcholine iodide (AChEI) as a substrate and 5,5′-
dithiobis(2-nitrobenzoic acid) (DTNB) as a chromogenic reagent. As
shown in Fig. 1, even at a concentration of 12.5 μM, δ-viniferin (2a),
pterostilbene trans-dehydrodimer (2b), and pallidol (3a) exhibited over
1
0,20–22
isomer (J = 6.5–7.3 Hz) than for the cis isomer (J = 8.2–8.4 Hz).
The hydroxy group of 6 was protected in 92% yield by chloromethyl
methyl ether (MOMCl) in the presence of N,N-diisopropylethylamine
(DIPEA) and N,N-dimethyl-4-aminopyridine (DMAP, catalytic amount).
For conjugation of tyramines, the MOM-protected compound 7 was
Fig. 1. Acetylcholinesterase inhibitory effects of monomeric and dimeric stilbenes. Values (means ± SEMs, n = 3) are expressed as % inhibition of AChE alone
treated without any samples).
(
2

M. Nagumo, et al.
Bioorganic & Medicinal Chemistry Letters xxx (xxxx) xxx–xxx
Scheme 2. Synthesis of grossamide (9).
Scheme 3. Synthesis of boehmenan (13).
Fig. 2. Acetylcholinesterase inhibitory effects of benzofuran neolignan derivatives. Values (means ± SEMs, n = 3) are expressed as % inhibition of AChE alone
treated without any samples).
(
converted into the free carboxylate under basic conditions. Without
further purification by silica gel column chromatography, two mole-
cules of tyramine were inserted using 1-(3-dimethylaminopropyl)-3-
ethylcarbodiimide (EDCI), 1-hydroxybenzotriazole (HOBt), and DIPEA
Pd(OH)
2
/C catalytic hydrogenation, a double bond of compound 6 was
reduced in 89% yield. Methyl ester groups of 10 were also reduced to
primary alcohols in 73% yield by LiAlH (8.0 equiv.). Esterification of
4
11 with two molecules of MOM-protected ferulic acid using EDCI,
HOBt, and DMAP in DCM for 7 days allowed access to 12 in 74% yield.
The final step was deprotection of three MOM groups which was ef-
2
3
in DMF for 40 h, giving 8 in 28% yield. Unfortunately, acidic de-
protection (TFA/DCM, conc. HCl/THF, or AcCl/MeOH) of the MOM
group in 8 was failed. Under mild conditions with CBr
4
and PPh
3
in
fected in the CBr
4
/PPh system in DCE, affording the targeted boeh-
3
2
4
DCE, 8 was steadily deprotected to grossamide (9) in 70% yield. Upon
menan (13) in 60% yield. In addition, compounds 14 and 15 depicted
3

M. Nagumo, et al.
Bioorganic & Medicinal Chemistry Letters xxx (xxxx) xxx–xxx
Fig. 3. Neuroprotective effects of compounds
at a concentration of 10 μM against t-BHP-in-
duced cytotoxicity in PC12 cells. Cells were
pretreated with samples for 1 h, and were
subsequently exposed to t-BHP (200 μM) for
3
h. Values (means ± SEMs, n = 3) are ex-
pressed as % of control (pretreated without any
*
**
samples). p < 0.05 and p < 0.01, respec-
tively (vs. cells treated with t-BHP alone).
in Fig. 2 were prepared from dimeric compound 6 through catalytic
pteroresveratrol (1b), pterostilbene trans-dehydrodimer (2b), grossa-
mide (9), and boehmenan (13) turned out to be excellent inhibitors of
NO production. These compounds are worthy of further investigation
on LPS-stimulated microglia.
hydrogenation and LiAlH reduction.
4
To gain insights into the structure-activity relationships, we
screened for the AChE inhibitory effects of synthetic benzofuran neo-
lignan derivatives (6, 9, and 13–15). Results shown in Fig. 2 indicated
that grossamide (9) and boehmenan (13) dose-dependently behaved as
the AChE inhibitors showing IC50 values of 39.3 ± 1.2 μM and
Polyphenols with their ability to attenuate the symptoms present
3
3–38
therapeutic options in neurodegenerative diseases.
In particular,
3
9,40
flavonoids are continuously in the limelight.
Research on quercetin
2
9.8 ± 0.9 μM. On the other hand, compounds 6, 14, and 15 were
has been expanded in animal models, and the dietary supplementation
has shown its ability to enhance memory and improve cognitive func-
found to be weak inhibitors of AChE. Interestingly, the addition of two
molecules of tyramine or ferulic acid significantly enhanced their AChE
inhibition efficacy.
4
1–43
tions.
In the present study, we have synthesized naturally occur-
ring stilbene and benzofuran neolignan derivatives, and evaluated for
their inhibitory effects on the enzymatic activity of AChE. Our results
suggest that oligomerization of plant phenolics favors AChE inhibition.
The attractive multifunctional properties of pterostilbene trans-dehy-
drodimer (2b), pallidol (3a), and boehmenan (13) could help to delay
the progression of neurodegeneration and to mitigate the associated
symptoms. Further studies are currently underway and will be reported
in the near future.
Oxidative stress appears as a major part of pathophysiologic process
2
5
in neurodegenerative diseases. Next, we examined the possible pro-
tective effects of compounds (1–3, 6, 9, and 13–15) against tert-butyl
hydroperoxide (t-BHP)-induced neurotoxicity on rat pheochromocy-
toma PC12 cells, by preincubating the cells in the presence or absence
2
6
of the compounds. The viability of PC12 cells was measured by CCK-8
assay and results are presented in Fig. 3. A large decrease in cell via-
bility (27.3%) was observed upon treatment of the cells with 200 μM of
t-BHP. Pretreatment with resveratrol (1a), δ-viniferin (2a), pallidol
Acknowledgment
(
3a), pterostilbene trans-dehydrodimer (2b), and boehmenan (13) at a
concentration of 10 μM powerfully prevented against cell death elicited
by t-BHP with cell viability of 57.5%, 60.2%, 92.2%, 75.5%, and 72.7%,
respectively. Notably, 2b, 3a, and 13 were more effective than quer-
cetin (cell viability: 65.1%), which served as a positive control. In
contrast to these compounds, grossamide (9) was slightly cytotoxic in
this concentration.
The authors are grateful to Mr. Atsushi Shimozuma for his fruitful
assistance.
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://
doi.org/10.1016/j.bmcl.2019.07.026.
Inflammation is associated with the pathogenesis of neurodegen-
2
7
erative diseases. Nitric oxide (NO) produced by activated microglia is
2
8
an important contributor to neuronal damage. Huang T-C and co-
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