Synthesis of optically active methyl 7 β-hydroxykaurenoate with potent neuroprotective activity

Article abstract of DOI:10.1248/cpb.52.1153

(-)-Methyl 7β-hydroxykaurenoate (3) and its 4-demethyl acetate (-)-4 were both synthesized via methods that contained radical cyclization and intramolecular Diels-Alder reactions as key steps. Both compounds displayed potent neuroprotective activity again

Full text of DOI:10.1248/cpb.52.1153

September 2004
Communications to the Editor
Chem. Pharm. Bull. 52(9) 1153—1154 (2004)
1153
dic acids, a suitable chiral precursor was sought. The half
ester 5, which can be readily synthesized in large quantities
with high optical purity from the corresponding diester using
porcine liver esterase (PLE),8) was eventually selected. Com-
pound 5, which has an optical purity of 95% ee, was then
treated with isobutene in the presence of concentrated sulfu-
ric acid, followed by reaction with sodium hydroxide in
methanol (Chart 1). This generated the t-butyl ester 6 in 67%
overall yield. A subsequent Grignard reaction and treatment
with diluted sulfuric acid produced the lactone 7 in an overall
yield of 45%. Optically active (ꢀ)-7 was transformed into
the kaurene-type diterpene 3 utilizing the same procedure
previously described for the racemate synthesis.6) The optical
purity of product 7 was determined at a later stage.
Synthesis of Optically Active Methyl
b
7 -Hydroxykaurenoate with Potent
Neuroprotective Activity
Masahiro TOYOTA,a Kiminori MATSUURA,a
Masahiro YOKOTA,a Manami KIMURA,b
Masahiro YONAGA,b Hachiro SUGIMOTO,b
and Masataka IHARA*,a
a Department of Organic Chemistry, Tohoku University, Graduate
School of Pharmaceutical Sciences; Aobayama, Sendai 980–8578,
Japan: and b Tsukuba Research Laboratories, Eisai Co. Ltd.;
Tsukuba 300–2635, Japan.
Received June 11, 2004; accepted July 8, 2004
Introduction of a propargyl group into lactone 7 produced
8 in 92% yield and this product was then subjected to radical
cyclization. The desired tricyclic compound 9 was success-
fully obtained in 93% yield following treatment with silica
gel. Reduction of the lactone 9 with lithium aluminum hy-
dride generated the diol product 10 in 97% yield, which was
then converted into the corresponding (R)-methoxy(trifluo-
(ꢀ)-Methyl 7b-hydroxykaurenoate (3) and its 4-demethyl ac-
etate (ꢀ)-4 were both synthesized via methods that contained
radical cyclization and intramolecular Diels-Alder reactions as
key steps. Both compounds displayed potent neuroprotective ac-
tivity against N-methyl-D-aspartate toxicity in cultured cortical
neurons.
1
romethyl)phenylacetate 11, the H-NMR spectrum of which
Key words kaurene diterpenoid; neuroprotective activity; chiral synthe-
sis; radical cyclization
indicated an optical purity of 94% ee. This therefore indi-
cated that no racemization occurred during the transforma-
tion reactions.
It has been suggested by a number of previous studies1—4)
that excess activation of glutamate receptors causes severe
and irreversible damage to the mammalian central nervous
system (CNS). Akaike and coworkers have recently reported
the isolation of atisane diterpenes, serofendic acids A (1) and
B (2) (Fig. 1), from fetal calf serum and described their po-
tent protective activity in cortical neurons against both nitric
oxide donor and glutamate cytotoxicity.5) These observations
prompted us to examine the possible neuroprotective activity
of our synthetic compounds that have a variety of bridged
skeletons. Here we report that kaurene derivatives act as po-
tential neuroprotective agents.
Among the many compounds that we tested, racemates of
7b-hydroxykaurenoate (3)6) and its demethyl acetate 46) dis-
play potent activity for lactate dehydrogenase (LDH)7) in-
duced by N-methyl D-aspartate (NMDA) in cortical cell cul-
tures (Fig. 2). The activity was comparable to that of meman-
tine, which received US FDA approval for the treatment of
moderate to severe Alzheimer’s disease.
Following acetylation of 10, the resulting acetate 12, ob-
tained in 99% yield, was treated with phosphorous oxychlo-
ride in pyridine to produce the olefin 13 in 93% yield (Chart
2). This compound was further converted into aldehyde 15 in
two steps. The subsequent addition to 15 of an anion9) pro-
duced from 3-triethylsilyloxy-1,4-pentadiene, followed by
acetylation, generated a 3 : 1 mixture of two epimeric ac-
etates 16 in 81% overall yield. The trans-fused tetracyclic
compound 17 was then obtained in an overall yield of 42%
by the intramolecular Diels–Alder reaction of the epimeric
mixture 16, conducted in toluene in a sealed tube at 200 °C,
followed by treatment with tetrabutylammonium fluoride. An
optical purity of 95% ee was further confirmed by the con-
version of 17 into 19 through 18.
A Corey–Chaykovsky reaction10) of 17 produced the epox-
ide 20 as a single stereoisomer in 60% yield, which was sub-
sequently converted into the ester 21 in three steps in an
To elucidate further the mechanisms of their biological ac-
tivity, the corresponding optically active compounds were
then synthesized. For the purposes of preparing chiral com-
pounds that have the same absolute configuration as serofen-
Fig. 1. Structures of Serofendic Acids A (1) and B (2)
Reagents and conditions: (i) isobutene, concentrated H2SO4, Et2O, room tempera-
ture; (ii) NaOH, MeOH, room temperature; (iii) MeMgBr, THF, then 10% H2SO4; (iv)
LDA, propargyl bromide, HMPA, THF, room temperature; (v) Bu3SnH, AIBN, ben-
zene, reflux, then SiO2; (vi) LiAlH4, Et2O; (vii) (R)-MTPACl, DMAP, Et3N, CH2Cl2,
room temperature; (viii) Ac2O, pyridine, room temperature.
Fig. 2. Structures of Kaurene Derivatives with Potent Neuroprotective Ac-
tivity
Chart 1
* To whom correspondence should be addressed. e-mail: mihara@mail.pharm.tohoku.ac.jp
© 2004 Pharmaceutical Society of Japan

1154
Vol. 52, No. 9
Reagents and conditions: (i) POCl3, pyridine, room temperature; (ii) K2CO3, MeOH,
room temperature; (iii) Et3N, DMSO, SO3·Py, CH2Cl2, room temperature; (iv) 3-tri-
ethylsilyloxy-1,4-pentadiene, s-BuLi, THF, ꢀ78 °C; (v) Ac2O, DMAP, CH2Cl2, room
temperature; (vi) 200 °C, toluene, then TBAF, THF, room temperature; (vii) K2CO3,
MeOH, 50 °C; (viii) (R)-MTPACl, DMAP, Et3N, ClCH2CH2Cl, room temperature.
Chart 2
Fig. 3. Effects of Methyl 7b-Hydroxykaurenoate (3) and Its Demethyl Ac-
etate 4 on NMDA Toxicity in Cortical Neurons
Lightly shaded columns (base) indicate NMDA-untreated cells and darkly shaded
columns indicate NMDA-treated cells. A, racemate of methyl 7b-hydroxykaurenoate
(ꢂ)-(3); B, (ꢂ)-4; C, (ꢀ)-methyl 7b-hydroxykaurenoate (ꢀ)-(3); D, (ꢀ)-4. Each value
represents the meanꢂS.E.M. (nꢃ3—6). ** pꢄ0.01 and *** pꢄ0.001 vs. NMDA-
treated control cells. Data were analyzed using ANOVA and Dunnett’s multiple com-
parison methods.
ated currents in cortical neurons, despite its pronounced ac-
tivity in preventing glutamate neurotoxicity.11) Based upon
these findings, it was speculated that the neuroprotective ef-
fects of these compounds do not involve the inhibition of
glutamate receptor channel activities. The neuroprotective
mechanisms of kaurene derivatives are still unknown. How-
ever, from our results showing that compounds (ꢀ)-3 and
(ꢀ)-4 displayed more potent neuroprotective effects than the
corresponding racemate products, we postulate that a recep-
tor other than the NMDA receptor might be involved in the
Reagents and conditions: (i) Me3SꢁOIꢀ, NaH, DMSO, 45 °C; (ii) BF·OEt2, toluene;
(iii) NaClO2, KH2PO4, 2-methyl-2-butene, t-BuOH, H2O, room temperature; (iv) MeI,
DBU, MeCN, room temperature; (v) K2CO3, MeOH, 50 °C.; (vi) TMSOTf, 2,6-lutidine,
CH2Cl2, room temperature; (vii) MeI, LDA, HMPA, THF, ꢀ78 °C; (viii) TBAF, THF,
room temperature.
Chart 3
overall yield of 62% (Chart 3). Following the exchange of the neuroprotective effects of these molecules.
protecting group for a hydroxyl group via (ꢀ)-4, the result-
Acknowledgments We thank Professor A. Akaike, Kyoto University,
for his generous and helpful discussions.
ing silyl ether 22 was methylated to generate 23 in an overall
yield of 89% with high diastereoselectivity. Removal of the
silyl group of 23 synthesized the compound (ꢀ)-3 in 93%
yield.
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