Synthesis of dihydrofuran-fused perhydrophenanthrenes having a phenolic hydroxyl group as a novel anti-Alzheimer's disease agent

Article abstract of DOI:10.1016/j.bmcl.2011.10.127

As a part of our research program on developing novel anti-Alzheimer's disease medicines, several dihydrofuran-fused perhydrophenanthrenes (DFs) possessing a phenolic hydroxyl group were found to exhibit potent dendritic and axonal regeneration activities. Introduction of a methoxy group into the perhydrophenanthrene skeleton was successfully achieved via a PhI(OAc) ₂-mediated phenolic oxidation of a benzocyclobutene nucleus and subsequent tandem intramolecular electrocyclic reactions based on o-quinodimethane chemistry. We could reveal that a new methoxy derivative having a phenolic hydroxyl group exerted the most significant effects on the dendritic and axonal extensions in the damaged neurons, among DFs examined in this study.

Full text of DOI:10.1016/j.bmcl.2011.10.127

Bioorganic & Medicinal Chemistry Letters 22 (2012) 449–452
Contents lists available at SciVerse ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Synthesis of dihydrofuran-fused perhydrophenanthrenes having a phenolic
hydroxyl group as a novel anti-Alzheimer’s disease agent
Kenji Sugimoto a, Kosuke Tamura a, Naoki Ohta a, Chihiro Tohda b, Naoki Toyooka c,
Hideo Nemoto d, Yuji Matsuya a,
⇑
a Graduate School of Medicine and Pharmaceutical Sciences, University of Toyama, 2630 Sugitani, Toyama 930-0194, Japan
b Institute of Natural Medicine, University of Toyama, 2630 Sugitani, Toyama 930-0194, Japan
c Graduate School of Science and Engineering, University of Toyama, 3190 Gofuku, Toyama 930-8555, Japan
d Lead Chemical Co. Ltd, 77-3 Himata, Toyama 930-0192, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
As a part of our research program on developing novel anti-Alzheimer’s disease medicines, several
dihydrofuran-fused perhydrophenanthrenes (DFs) possessing a phenolic hydroxyl group were found to
exhibit potent dendritic and axonal regeneration activities. Introduction of a methoxy group into the
perhydrophenanthrene skeleton was successfully achieved via a PhI(OAc)2-mediated phenolic oxidation
of a benzocyclobutene nucleus and subsequent tandem intramolecular electrocyclic reactions based on
o-quinodimethane chemistry. We could reveal that a new methoxy derivative having a phenolic hydroxyl
group exerted the most significant effects on the dendritic and axonal extensions in the damaged
neurons, among DFs examined in this study.
Received 27 September 2011
Revised 26 October 2011
Accepted 29 October 2011
Available online 18 November 2011
Keywords:
Perhydrophenanthrenes
Neuronal regeneration activity
Phenolic oxidation
Benzocyclobutenes
o-Quinodimethane
Ó 2011 Elsevier Ltd. All rights reserved.
Alzheimer’s disease, which is caused by the neuronal damages in
the brain,1 is one of the most serious medical problems and requires
a novel drug for the fundamental cure in place of the symptomatic
treatment with cholinesterase inhibitors such as Aricept (Eisai). In
the last decade, several strategies for lowering amyloid b (Ab) have
been studied in basic research and clinical trials, instead of enhanc-
ing cholinergic function, because the Ab targeting is an etiology-
reducing approach for Alzheimer’s disease. However, a decrease in
the level of Ab does not reduce advanced memory disorder.2 If the
brain function can be improved by reactivating and re-establishing
a nervous network, it may be possible to return to a normal memory
even in the state of neuronal degeneration. For the purpose of
exploring novel anti-Alzheimer’s disease medicines with neuro-
regeneration, we recently focused on an Indian folk medicine
Ashwagandha, which was taken to promote learning and memory,3
and finally revealed that withanoside IV and its active metabolite
sominone induced axonal and dendritic regeneration and synaptic
reconstruction.4 Furthermore, we found that denosomin, the
structurally simplified derivative of sominone, demonstrated a
significant axonal extension effect in Ab-damaged neurons.5 In this
context, we turned our attention into dihydrofuran-fused perhydr-
ophenanthrenes (DFs) for further extension of the foregoing
researches, because this series of compounds have been proved to
have high potential as a various drug lead with a variety of bioactiv-
ities. For example, we had previously reported that DF-1, DF-2, and
DF-6 exhibited potent antivirus activities against HVJ6a or various
influenza viruses,6b–d and that DF-2 also significantly induced an
apoptotic tumor cell death under the hyperthermia conditions.6e
These DF derivatives commonly possess a hydrophobic ethereal
substituent on the aromatic ring, such as trifluoromethyl, trialkylsi-
lyl, and benzyl-type appendages (Fig. 1).
O
O
O
O
NC
H
H
NC
H
H
R1
R2
O
O
R
R1 = OH, R2 = H (DF-8)
R = OCF3 (DF-1)
R1 = OMe, R2 = OH (DF-9)
R1 = OH, R2 = OMe (DF-10)
R = OTIPS (DF-2)
R = OH (DF-3)
R = OTf (DF-4)
R = OBn (DF-5)
R = ODFBn (DF-6)
R = Ph (DF-7)
DFBn =
3,5-difluorobenzyl
⇑
Corresponding author. Tel.: +81 76 434 7530; fax: +81 76 434 5047.
E-mail address: matsuya@pha.u-toyama.ac.jp (Y. Matsuya).
Figure 1. Bioactive dihydrofuran-fused compounds (DFs).
0960-894X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2011.10.127

450
K. Sugimoto et al. / Bioorg. Med. Chem. Lett. 22 (2012) 449–452
R1
R2
CN
On the other hand, new DF congeners possessing a phenolic
CN
2 steps
R1
R2
hydroxyl group (a hydrophilic substituent) have been found to
exhibit a potent anti-Alzheimer effect as a dendritic and axonal
regenerator in this research. We will report herein the efficient
synthesis and biological assessment of novel phenol-type DF
derivatives, utilizing hypervalent iodine oxidation7 and original
o-quinodimethane chemistry of benzocyclobutenes.8,9
At first, we surveyed the known DF derivatives (DF-1 to DF-7),
which have been synthesized previously,6a,b,d on the dendritic
extension activity in Ab-damaged neurons (Fig. 2). The compounds
OTHP
R1 = H, R2 = OMe (1a)
R1 = OMe, R2 = H (1b)
R1 = H, R2 = OH (2a)
R1 = OH, R2 = H (2b)
a
a
CN
MeO CN
O
MeO
O
OTHP
OTHP
DF-1 to DF-7 (1
4 days treatment with Ab, and dendrite lengths were quantified
after further 4 days. In the cells treated with 10 M Ab followed
lM) were administered to rat cortical neurons after
MeO
OMe
b
3a
3b
l
by vehicle (0.1% DMSO instead of DFs), the length of the dendrite
was obviously shorter than that in control cells (without Ab treat-
ment). However, DF-3, which was equipped with a phenolic hydro-
xyl group, was revealed to exhibit the significant dendrite
re-extension activity, while the other DFs did not show any activi-
ties as shown in Figure 2. This behavior markedly contrasts with the
previous biological profile of DFs requiring a hydrophobic substitu-
ent on the aromatic ring.6
b
CN
CN
HO
MeO
HO
OTHP
OTHP
MeO
4a
4b
Scheme 2. Hypervalent iodine oxidation of benzocyclobutenes 2a and 2b. Reagents
and conditions: (a) PhI(OAc)2 (2.2 equiv), MeOH, rt, 51% (3a); (b) Zn, THF/H2O, rt,
85% (4a) and 13% (4b, 2 steps).
These results prompted us to investigate the other DF derivatives
bearing a phenolic hydroxyl group, and we prepared 10-hydroxy-
perhydrophenathrene derivative DF-8,10 regioisomer of DF-3, by
6b
simple deprotection of formerly reported DF-20 as shown in
Scheme 1.
For the purpose of getting further information concerning effects
of oxygen functionalities on the bioactivity, we examined the
syntheses of novel mono-protected catechol derivatives (DF-9 and
DF-10). Since the DF tetracyclic framework containing an enol ether
substructure is rather susceptible to harsh reaction conditions, an
oxidative introduction of the oxygen functions on advanced
perhydrophenanthrenes is out of our choice. In addition, a chemose-
lective functionalization of the two hydroxyl groups on a catechol
moiety in the early stage of the synthesis seemed to be difficult.
Consequently, we planned to set up them from the corresponding
CN
CN
R1
R2
R1
R2
a, b
OTHP
OH
R1 = OMe, R2 = OH (4a)
R1 = OH, R2 = OMe (4b)
R1 = OMe, R2 = OTIPS (5a)
R1 = OTIPS, R2 = OMe (5b)
OH
CN
R1
R2
c, d
e, f
O
R1 = OMe, R2 = OTIPS (6a)
R1 = OTIPS, R2 = OMe (6b)
O
O
NC
CN
O
O
R1
R2
g
O
O
R1
R1 = OMe, R2 = OTIPS (7a)
R1 = OTIPS, R2 = OMe (7b)
R2
8a,b
O
O
NC
H
H
h
R1
R2
DF-9
DF-10
Figure 2. Effect of DF-1 to DF-7 on the dendritic extension in Ab-damaged neurons.
O
O
O
R1 = OMe, R2 = OTIPS (9a)
R1 = OTIPS, R2 = OMe (9b)
NC
H
H
a
TIPSO
DF-8
Scheme 3. Synthesis of DF-9 and DF-10. Reagents and conditions: (a) TIPSOTf,
NEt3, CH2Cl2, rt; (b) PPTS, EtOH, reflux, 77% (5a, 2 steps) and 71% (5b, 2 steps); (c)
SO3ꢀPy, NEt3, DMSO, rt; (d) 3-lithiofuran, Et2O, ꢁ78 °C to 0 °C, 30% (6a, 2 steps) and
38% (6b, 2 steps); (e) SO3ꢀPy, NEt3, DMSO, rt; (f) ethylene glycol, TsOH, benzene,
reflux, 62% (7a, 2 steps) and 66% (7b, 2 steps); (g) o-dichlorobenzene, reflux, 65%
(9a) and 65% (9b); h) TBAF, THF, rt, 84% (DF-9) and quant. (DF-10).
O
DF-2'
Scheme 1. Synthesis of DF-8. Reagents and conditions: (a) TBAF, THF, rt, 95%.

K. Sugimoto et al. / Bioorg. Med. Chem. Lett. 22 (2012) 449–452
451
monohydroxy benzocyclobutenes via the phenolic oxidation
(Scheme 2).7 The benzocyclobutene derivative 2a, which was pre-
pared in the established manner from 1a,6a was once oxidized by
2 equiv of PhI(OAc)2 in the presence of MeOH to afford the desired
methoxyquinone derivative 3a with satisfactory yield. Successive
reductive rearomatization smoothly proceeded to give the requisite
methoxybenzocyclobutene 4a within 85% yield. In the same man-
ner, the regioisomeric product4b couldbe obtained from2b,6b albeit
rather in low yield because of the instability of the intermediate 3b,
which could not be isolated.
With these methoxybenzocyclobutenes in hand, we then trans-
ferred these compounds into the corresponding perhydrophenanth-
renes by means of o-quinodimethane chemistry (Scheme 3). After
protection of the phenolic hydroxyl group on 4a as TIPS ether, acidic
hydrolysis of THP ether gave the alcohol 5a. Parikh-Döring oxidation
and subsequent nucleophilic addition of lithiofuran to the resulting
aldehyde furnished the alcohol 6a. The compound 7a as a precursor
of the o-quinodimethane 8a was prepared via oxidation followed by
acetalization of the resultant ketone. By the thermal treatment of 7a
in refluxing o-dichlorobenzene, intramolecular cycloaddition via
o-quinodimethane intermediate proceeded uneventfully to furnish
the desired perhydrophenanthrene 9a as a single diastereomer
through an exclusive endo transition state 8a. The stereochemistry
of 9a was determined as an all-cis configuration by the comparison
of the 1H NMR spectra of 9a with that of another perhydrophenanth-
rene derivative whose structure was unambiguously confirmed by
the X-ray crystallographic analysis.6a Finally, desilylation of 9a by
the treatment with TBAF quantitatively accomplished DF-9.11 In
accordance with the preparation of DF-9, regioisomer DF-1012 was
also obtained satisfactorily.
Having prepared several phenolic DFs, we further estimated the
efficacy of them on dendrite and axon extensions in Ab-damaged
neurons (Figure 3). Sominone, denosomin, DF-3, DF-8 to DF-10
(1 lM each), and NGF (positive control: 100 ng/mL) were adminis-
tered to rat cortical neurons on the third day after the treatment
with Ab, and the dendrite and axon lengths were gauged after
further incubation for 5 days. As shown in Figure 3, the dendrite
extension was evidently observed in the Ab-exposed cells treated
with novel DFs. Among them, DF-10 displayed the most significant
effects on dendrite extensions to realize the comparable level with
those of previously reported, potent anti-Alzheimer’s disease candi-
dates sominone4e and denosomin.5 Furthermore, the axon lengths
in the damaged cells were apparently re-extended by the DF-8 to
DF-10. Especially, it was remarkable that DF-10 expressed more
powerful axonal extension activity than the positive control NGF.
In conclusion, we accomplished the synthesis of novel
mono-protected catechol derivatives of perhydrophenanthrenes
via phenolic oxidation with PhI(OAc)2 on the basis of the o-quino-
dimethane chemistry. To the best of our knowledge, this is the first
successful application of the phenolic oxidation on functionalized
benzocyclobutenes. Furthermore, we disclosed the promising phar-
macophore with unprecedented structural characteristic for the
development of anti-Alzheimer’s disease drugs. It is an important
note that the requisite property of the substituent for exerting the
neuronal effects is orthogonal to those for the other bioactivities.6
Taking advantage of the synthetic viability of dihydrofuran-fused
perhydrophenanthrenes, the structure-activity-relationship study
and the elucidation on the mechanism of action are ongoing in
our laboratory.
Acknowledgments
This work was financially supported by the Ministry of Educa-
tion, Culture, Sports, and Technology, Japan, the KAKENHI, a
Grant-in-Aid for Scientific Research (C: 19590099).
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.bmcl.2011.10.127.
References and notes
1. (a) Terry, R. D.; Masliah, E.; Salmon, D. P.; Butters, N.; De Teresa, R.; Hill, R.;
Hansen, L. A.; Katzman, R. Ann. Neurol. 1991, 30, 572; (b) Canning, D. R.;
Mckeon, R. J.; DeWitt, D. A.; Perry, G.; Wujek, J. R.; Frederickson, R. C.; Silver, J.
Exp. Neurol. 1993, 124, 289; (c) Knowles, R. B.; Wyart, C.; Buldyrev, S. V.; Cruz,
L.; Urbanc, B.; Hasselmo, M. E.; Stanley, H. E.; Hyman, B. T. Proc. Natl Acad. Sci.
U.S.A. 1999, 96, 5274; (d) Simmons, L. K.; May, P. C.; Tomaselli, K. J.; Rydel, R. E.;
Fuson, K. S.; Brigham, E. F.; Wright, S.; Lieberburg, I.; Becker, G. W.; Brems, D.
N.; Li, W. Y. Mol. Pharmacol. 1994, 45, 373; (e) Bobinski, M.; Wegiel, J.;
Tarnawski, H. M.; Bobinski, M.; Reisberg, B.; De Leon, M. J.; Miller, D. C.;
Wisniewski, H. M. J. Neuropath. Exp. Neurol. 1997, 56, 414.
2. (a) Salloway, S.; Sperling, S.; Gilman, S.; Fox, N. C.; Blennow, K.; Raskind, M.;
Sabbagh, M.; Honig, L. S.; Doody, R.; van Dyck, C. H.; Mulnarld, R.; Barakos, J.;
Gregg, K. M.; Liu, E.; Lieberburg, I.; Schenk, D.; Black, R.; Grundman, M.
Neurology 2009, 73, 2061; (b) Henley, D. B.; May, P. C.; Dean, R. A.; Siemers, E. R.
Expert Opin. Pharmacother. 2009, 10, 1657.
3. Sharma, P. V. Classical Uses of Medicinal Plants; Chaukhambha Visvabharati:
Varanasi, India, 1996.
4. (a) Tohda, C.; Kuboyama, T.; Komatsu, K. Neuroreport 2000, 11, 1981; (b) Zhao,
J.; Nakamura, N.; Hattori, M.; Kuboyama, T.; Tohda, C.; Komatsu, K. Chem.
Pharm. Bull. 2002, 50, 760; (c) Kuboyama, T.; Tohda, C.; Zhao, J.; Nakamura, N.;
Hattori, M.; Komatsu, K. Neuroreport 2002, 13, 1715; (d) Kuboyama, T.; Tohda,
C.; Komatsu, K. Br. J. Pharmacol. 2005, 144, 961; (e) Kuboyama, T.; Tohda, C.;
Komatsu, K. Eur. J. Neurosci. 2006, 23, 1417; (f) Tohda, C.; Joyashiki, E. Br. J.
Pharmacol. 2009, 157, 1427; (g) Joyashiki, E.; Matsuya, Y.; Tohda, C. Int. J.
Neurosci. 2011, 121, 181.
5. Matsuya, Y.; Yamakawa, Y.; Tohda, C.; Teshigawara, K.; Yamada, M.; Nemoto, H.
Org. Lett. 2009, 11, 3970.
6. Biological evaluation of dihydrofuran-fused perhydrophenathrenes in our
group, see: (a) Matsuya, Y.; Sasaki, K.; Nagaoka, M.; Kakuda, H.; Toyooka, N.;
Imanishi, N.; Ochiai, H.; Nemoto, H. J. Org. Chem. 2004, 69, 7989; (b) Matsuya,
Figure 3. Effect of DF-8 to DF-10 on the dendritic and axonal extension in
Ab-damaged neurons.

452
K. Sugimoto et al. / Bioorg. Med. Chem. Lett. 22 (2012) 449–452
Y.; Sasaki, K.; Ochiai, H.; Nemoto, H. Bioorg. Med. Chem. 2007, 15, 424; (c)
dt, J = 10.7, 2.8 Hz), 4.04–3.98 (1H, m), 3.96–3.87 (4H, m), 3.18 (1H, dd, J = 15.9,
3.3 Hz), 3.11 (1H, dd, J = 15.9, 2.2 Hz), 2.80 (1H, dt, J = 14.8, 3.3 Hz), 2.58–2.47
(1H, m), 1.98–1.82 (2H, m); 13C NMR (75 MHz, CDCl3): d 155.0, 143.0, 132.0,
131.3, 127.0, 121.8, 115.5, 113.1, 110.2, 104.6, 79.8, 65.3, 63.7, 51.0, 35.8, 32.9,
31.5, 29.7; IR (KBr): 3396, 2232 cmꢁ1; MS (EI); m/z 311 (M+); HRMS (EI): calcd
for C18H17NO4: 311.1158 (M+), found 311.1152.
Matsuya, Y.; Imamura, Y.; Miyahara, T.; Ochiai, H.; Nemoto, H. Eur. J. Org. Chem.
2008, 1426; (d) Matsuya, Y.; Suzuki, N.; Kobayashi, S.; Miyahara, T.; Ochiai, H.;
Nemoto, H. Bioorg. Med. Chem. 2010, 18, 1477; (e) Yu, D.-Y.; Matsuya, Y.; Zhao,
Q.-L.; Ahmed, K.; Wei, Z.-L.; Nemoto, H.; Kondo, T. Apoptosis 2007, 12, 1523.
7. It was previously reported that p-quinone dimethylketals were produced by
the treatment of 4-unsubstituted phenols with 2 equiv of PhI(OAc)2 in MeOH.
In our case, since the substrates 2a and 2b were equipped with the
benzocyclobutene substituent on 4-positions, they afforded the 2,4-
dimethoxy cyclohexadienones 3a and 3b respectively. (a) Pelter, A.; Elgendy,
S. Tetrahedron Lett. 1988, 29, 677; For recent accounts and reviews on
hypervalent iodine-mediated phenol oxidation, see (b) Quideau, S.; Pouységu,
L.; Deffieux, D. Synlett 2008, 467; (c) Ciufolini, M. A.; Braun, N. A.; Canesi, S.;
Ousmer, M.; Chang, J.; Chai, D. Synthesis 2007, 3759; (d) Rodriguez, S.; Wipf, P.
Synthesis 2004, 2767; applications in natural product synthesis, see (e)
Pouységu, L.; Deffieux, D.; Quideau, S. Tetrahedron 2010, 66, 2235.
8. (a) Toyooka, N.; Nagaoka, M.; Nemoto, H. Synlett 2001, 1123; (b) Toyooka, N.;
Nagaoka, M.; Sasaki, E.; Qin, H.; Kakuda, H.; Nemoto, H. Tetrahedron 2002, 58,
6097; (c) Matsuya, Y.; Qin, H.; Nagaoka, M.; Nemoto, H. Heterocycles 2004, 69,
207.
11. Spectral data for DF-9: 1H NMR (300 MHz, CDCl3): d 6.85 (1H, s), 6.77 (1H, s),
6.04 (1H, d, J = 2.2 Hz), 5.66 (1H, s), 5.28 (1H, ddd, J = 10.4, 3.6, 2.2 Hz), 4.06–
3.97 (1H, m), 3.95–3.82 (4H, m), 3.88 (3H, s), 3.19 (1H, dd, J = 15.9, 3.6 Hz), 3.05
(1H, dd, J = 15.9, 2.2 Hz), 2.84 (1H, ddd, J = 14.8, 14.8, 3.3 Hz), 2.55 (1H, ddd,
J = 14.8, 12.4, 5.5 Hz), 1.97–1.84 (2H, m); 13C NMR (75 MHz, CDCl3): d 145.71,
145.65, 143.2, 128.6, 124.4, 121.9, 116.6, 110.1, 108.4, 104.6, 79.8, 65.4, 63.7,
56.2, 51.1, 35.5, 33.3, 31.6, 30.0; IR (neat): 3536, 2229 cmꢁ1; MS (EI): m/z 341
(M+). HRMS (EI): calcd for C19H19NO5: 341.1263 (M+), found: 341.1297.
12. Spectral data for DF-10: 1H NMR (300 MHz, CDCl3): d 6.90 (1H, s), 6.76 (1H, s),
6.04 (1H, d, J = 2.2 Hz), 5.62 (1H, s), 5.28 (1H, ddd, J = 10.4, 3.3, 2.2 Hz), 4.13–
3.95 (1H, m), 3.94–3.82 (4H, m), 3.88 (3H, s), 3.22 (1H, dd, J = 15.9, 3.3 Hz), 3.06
(1H, dd, J = 15.9, 2.2 Hz), 2.80 (1H, ddd, J = 14.6, 3.3, 3.3 Hz), 2.50 (1H, ddd,
J = 14.6, 14.6, 4.4 Hz), 1.98–1.80 (2H, m); 13C NMR (75 MHz, CDCl3): d 146.3,
144.6, 142.8, 126.9, 123.0, 121.9, 112.4, 112.1, 110.3, 104.5, 79.7, 65.2, 63.6,
55.9, 50.9, 35.3, 33.4, 31.3, 29.7; IR (neat): 3444 cmꢁ1; MS (EI): m/z 341 (M+);
HRMS (EI): calcd for C19H19NO5: 341.1263 (M+), found: 341.1286.
9. A recent review on o-quinodimethanes, see, Segura, J. L.; Martin, N. Chem. Rev.
1999, 99, 3199.
10. Spectral data for DF-8: 1H NMR (300 MHz, CDCl3): d 7.12 (1H, d, J = 8.2 Hz), 6.86
(1H, d, J = 2.2 Hz), 6.74 (1H, dd, J = 8.2, 2.5 Hz), 6.03 (1H, d, J = 1.9 Hz), 5.29 (1H,