Design and synthesis of dual active neovibsanin derivatives based on a chemical structure merging method

Article abstract of DOI:10.1016/j.bmcl.2020.127497

A hybrid compound consisting of neovibsanin and trans-banglene was designed according to a structure merging method and synthesized via a sequence of key steps including a Diels–Alder cycloaddition, stereoselective alkynylation, and intramolecular oxa-Mic

Full text of DOI:10.1016/j.bmcl.2020.127497

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Design and synthesis of dual active neovibsanin derivatives based on a chemi‐
cal structure merging method
Tsuyoshi Yanagimoto, Suguru Kishimoto, Yusuke Kasai, Nobuaki Matsui,
Miwa Kubo, Hirofumi Yamamoto, Yoshiyasu Fukuyama, Hiroshi Imagawa
PII:
S0960-894X(20)30608-9
DOI:
Reference:
https://doi.org/10.1016/j.bmcl.2020.127497
BMCL 127497
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Bioorganic & Medicinal Chemistry Letters
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11 August 2020
Please cite this article as: Yanagimoto, T., Kishimoto, S., Kasai, Y., Matsui, N., Kubo, M., Yamamoto, H.,
Fukuyama, Y., Imagawa, H., Design and synthesis of dual active neovibsanin derivatives based on a chemical
structure merging method, Bioorganic & Medicinal Chemistry Letters (2020), doi: https://doi.org/10.1016/j.bmcl.
2020.127497
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Graphical Abstract
Design and synthesis of dual active neovibsanin
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derivatives based on a chemical structure merging method
Tsuyoshi Yanagimoto, Suguru Kishimoto, Yusuke Kasai, Nobuaki Matsui, Miwa Kubo,
Hirofumi Yamamoto, Yoshiyasu Fukuyama, Hiroshi Imagawa

Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com
Design and synthesis of dual active neovibsanin derivatives based on a chemical
structure merging method
Tsuyoshi Yanagimotoa, Suguru Kishimotoa, Yusuke Kasaia, Nobuaki Matsuic, Miwa Kubob,
Hirofumi Yamamotoa, Yoshiyasu Fukuyamab, Hiroshi Imagawa*a
a
Chemistry of functional molecule, Faculty of Pharmaceutical Sciences, Tokushima Bunri University, 180 Nishihamabouji Yamashiro-cyo,Tokushima, 770-
8514, Japan
b
Chemistry of natural products, Faculty of Pharmaceutical Sciences, Tokushima Bunri University, 180 Nishihamabouji Yamashiro-cyo,Tokushima, 770-
8514, Japan
c
Pharmacology, Department of Pharmacy, Faculty of Pharmacy, Gifu University of Medical Science, 4-3-3 Nijigaoka, Kani, Gifu, 509-0293, Japan
ARTICLE INFO
ABSTRACT
Article history:
Received
Revised
Accepted
Available online
A hybrid compound consisting of neovibsanin and trans-banglene was designed
according to a structure merging method and synthesized via a sequence of key steps
including
a
Diels–Alder cycloaddition, stereoselective alkynylation, and
intramolecular oxa-Michael addition reaction. The biological activity of the
synthetized acetal compound and its hemiacetal analogue was investigated in PC12
cells. These studies revealed that the designed hybrid compounds displayed
neuritogenic activity. Furthermore, a relatively strong neurite outgrowth promoting
activity was observed in the presence of NGF. These results suggest that the designed
hybrid compound exhibited a dual activity.
Keywords:
Neovibsanin
phenylbutadiene dimer
neurite-outgrowth promoting activity
neuritogenic activity
structure merging method
2009 Elsevier Ltd. All rights reserved.
Neovibsanins are polyfunctionalized diterpenoid compounds
that were originally isolated in 1996 from the shrub Viburnum
awabuki by Fukuyama et al.1 These natural products such as
Neovibsanin A and B (1 and 2, Figure 1), were found to display
neurite outgrowth promoting activity in PC12 cells in the
presence of a small amount of nerve growth factor (NGF),
suggesting that neovibsanin-type compounds might be promising
candidates for the development of novel therapeutic agents for
Alzheimer’s disease. In 2009, we reported the first total synthesis
of (±)-2 based on two key steps: (1) an intramolecular Diels–
Alder reaction that was accelerated by 1,3-dimethyl-2-
imidazolidinone (DMI), and (2) subsequent oxa-Michael
addition–lactonization reaction.2 By using a similar synthetic
strategy, several neovibsanin analogues were synthesized and
further evaluated for their biological activity. These studies
revealed that the neurite outgrowth activity of structurally
simplified neovibsanin derivative 3, which contained the tricyclic
core of 2, was approximately the same as that of natural (+)-2.3
On the other hand, Fukuyama and Akagi reported that
phenylbutadiene dimer 4 (trans-banglene), which was isolated
from Indonesian Zingiberaceae plants, exhibited neurotrophic
activity such as neuritogenesis and promotion of neurite
outgrowth.4 Interestingly, both 3 and 4 contain a cyclohexene
ring moiety. Such structural similarity between neovibsanins and
banglene inspired us to explore a novel molecular design, and
specifically the creation of dual active compounds by structurally
merging two compounds that exhibit different mechanisms of
action. Here, we report the design and synthesis of a novel hybrid
neovibsanin–banglene compound along with its dual biological
activities.
Figure 1. Structures of Neovibsanins (1-3), and trans-Banglene (4).
Figure 2 shows the structure of target molecular 5, which was
designed by merging a structural part shared by 3 and 4 that

possess neurite outgrowth promoting and neuritogenic activity,
respectively. It can be expected that if the tricyclic moiety of 3,
which is the core element of neovibsanins, and that
corresponding to the phenylbutadiene dimer can interact with
each receptor, an enhanced neurite outgrowth activity might
result upon activation of two different pathways for signal
transduction, namely dual activation. The retrosynthesis of
compound 5 is shown in Figure 3. The key intermediate enone 8
can be obtained by Diels–Alder cycloaddition of acrolein with
diene 10, followed by Horner–Wadsworth–Emmons olefination
and Morita–Baylis–Hillman reaction. The tricyclic moiety in 5
may be constructed from 8 by applying a similar route as that
previously reported for the total synthesis of 2.2
Scheme 1. Synthesis of diene 10.
The Diels–Alder reaction between 10 and acrolein was carried
out under several conditions. As shown in Scheme 2, it was
found that the reaction proceeded smoothly in the presence of
hydroquinone by using acrolein as solvent, thus affording Diels–
Alder adduct 9 in 79% yield as a mixture of endo and exo
isomers.7 Subsequently, the endo product was isomerized to its
thermodynamically stable trans form upon treatment with
imidazole at room temperature. Next, purified trans-aldehyde 9
exo was first reacted with a Horner–Wadsworth–Emmons
reagent, which was in turn prepared from dimethyl (3,4-
dimethoxybenzyl)phosphonate and n-BuLi, followed by
treatment with methanol to afford olefination product 15 in 61%
yield.8,9 Alcohol 15 was further oxidized using a catalytic amount
of TEMPO in the presence of iodobenzene diacetate to give
compound 16 which was isolated in 95% yield.10 The
introduction of a hydroxylmethyl moiety at the cyclohexenone
skeleton of 16 was achieved by Morita–Baylis–Hillman reaction
using DMAP and formaldehyde, leading to the desired key
intermediate 8 in 73% yield.11
Figure 2. Structural design of hybrid compound 5.
Figure 3. Retrosynthetic analysis of hybrid compound 5.
Therefore, the synthesis started with the preparation of diene
10, as shown in Scheme 1. Reduction of methyl (E)-3-(3,4-
dimethoxyphenyl)acrylate (11) with DIBAL, followed by the
oxidation of the resulting allyl alcohol 12 in the presence of
TEMPO afforded aldehyde 13 in 97% yield over 2 steps.5
Compound 13 was then treated with acetone in the presence of
10% aq. NaOH to provide aldol condensation product 14 in 81%
yield. Baeyer–Villiger oxidation of 14 upon reaction with oxone
in DMF furnished diene 10 in 80% yield.6
Scheme 2. Synthesis of enone 8 from 10 via Diels–Alder reaction.
As illustrated in Scheme 3, after protection of the hydroxy group
of 8 with TBS, the resulting compound 17 underwent a selective
alkynylation upon axial attack by methyl lithiopropiolate to give
18 and 18 in 92% yield ( 1:9.5). After purification, 18
was isolated as the major product and subjected to the reduction
of the triple bond in the presence of Red-Al resulting in the
formation of  -unsaturated ester 19 in 89% yield.12 The newly
generated stereochemistry of 18 after the alkynylation step was

confirmed by the presence of a NOE correlation between H-5 and
H-11 in 19 (Figure 4). The deprotection of the TBS group with
TBAF induced an oxa-Michael reaction, however the major
product of this step was  -unsaturated -lactone alcohol 20,
which was formed via retro-Michael reaction from the desired
tricyclic lactone 6. Nonetheless, 20 could be smoothly converted
to lactone 6 upon treatment with sodium methoxide in MeOH.13
The stereochemistry of the newly generated stereocenter in 6 was
confirmed by NOE experiment (Figure 4). Finally, methylation
of 6 with the Tebbe’s reagent, and subsequent treatment with
MeOH provided 5 as a diastereomeric mixture in 86% yield over
two steps (Scheme 3).14
Scheme 4. Preparation of hemiacetal 5OH from acetal 5.
Figure 5 shows the results of the neuritogenic activity assay
conducted in the presence of 5OH. A clear neuritogenesis was
observed in the absence of NGF as compared to the control (B, C
in Figure 5). By quantitatively evaluating the length of the
protrusions, it was found that 5OH undoubtedly showed
neuritogenic activity, although lower than that of 4 (Figure 6).
Based on these results, 5OH was demonstrated to be the first
neuritogenic active compound having a neovibsanin skeleton.
A
B
C
Figure 5. Results of the neuritogenic activity assay; A: 30 M of 4 as
positive control, B: 30 M of 5OH, and C: control (0.1% DMSO).
Scheme 3. Synthesis of 5 from enone 8.
Figure 4. Confirmation of the stereochemistry of compounds 6, 19, and 20.
An assay for determining the neuritogenic activity of 5 was
performed by using PC12 cells,15 However, 5 crystalized in the
culture medium for PC12 cells due to its insolubility in water.
Therefore, we decided to prepare 5OH, which could be expected to
exhibit an increased solubility in water, upon hydrolysis of the
acetal moiety of 5 to a hydroxy group.16 The hemiacetal 5OH was
obtained as a mixture of keto form by acid treatment of 5
(Scheme 4).
Figure 6. Quantitative evaluation of the neuritogenic activity in the presence
of 3, 4 and 5OH. Data are expressed as the mean ± SE (n = 100).**P <0.01
versus control (0.1% DMSO); Dunnett’s t-test.

1.
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Kawazu, K. Tetrahedron Lett. 1996, 37, 6767–6770. (b)
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Next, the neurite outgrowth promoting activity of hybrid
compound 5OH was quantitatively analyzed using PC12 cells in
the presence of NGF (Figure 7), and it was found that 5OH
exhibited a higher activity than 3. Although, it is not clear
whether this activity originates from the phenylbutadiene dimer
or neovibsanin moiety in 5OH, or both of them, it was much
higher than that estimated from the neuritogenic activity owing to
the phenylbutadiene dimer structure in 5OH. Therefore, it can be
assumed that the main contribution to the activity is due to the
2.
3.
4.
neovibsanin part in 5OH
for 3 and 4 have not been elucidated yet, therefore it is difficult at
this time to fully prove the dual activation mechanism of 5OH
.
17,18 However, the mechanisms of actions
.
5.
6.
Currently, in order to elucidate the detailed mechanism of this
activity expression, studies on the receptors of 3 and 4 are
ongoing by using photoaffinity label derivatives.
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13. During this reaction, oxa-Michael adduct 6 was obtained as a
white precipitate from the reaction solution.
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15. PC12 cells were cultured in a 24 well plate at a density of 2×103
cells/cm3 in DMEM + 10% HS, 5% FBS, 100 IU/mL penicillin
and 100 g/mL streptomycin at 37 C under a humidified
atmosphere of 95% air and 5% CO2 for 24 h, and the culture
medium was changed to DMEM + 2% HS, 1% FBS, 100 IU/mL
penicillin, 100 g/mL streptomycin. Then samples in DMSO at 30
M final concentration was added. After incubation with samples
Figure 7. Quantitative evaluation of neurite outgrowth promoting activities in
the presence of 3, 4, 5OH. Each sample contained NGF 10 ng/mL, Data are
expressed as the mean ± SE (n = 100).* P <0.05; **P <0.01 versus NGF;
Dunnett’s t-test..
for
4
days, the cultures were fixed with 4%
paraformaldehyde/PBS, and stained with methylene blue. And the
cell morphology was observed under a phase-contrast microscope.
16. A 3 mol of 5 was suspended in 100 L DMSO, while 5OH
completely dissolved under the same conditions.
17. We presume that there is a parallel relationship between the
neuritogenic activity of compound 4 and the neurite outgrowth
promoting activity.
18. The activity of 4 (30 M) in the presence of 3 (30 M) were tested
for both neuritogenic and neurite outgrowth promoting activity.
Under these conditions, these activities showed an additive trend.
Acknowledgments
This research project was financially supported
by Nagai
memorial research scholarship from the pharmaceutical society
of Japan, the Sasakawa scientific research grant from the Japan
science society (2018-3008). We thank Dr. Masami Tanaka and
Dr. Yasuko Okamoto (Center for Instrumental Analysis,
Tokushima Bunri University) for 600 MHz NMR spectroscopy
and mass spectrometry measurements.
A. Supplementary Data
Supplementary data associated with this article can be found, in the online
version, a
References