The first total synthesis and neurotrophic activity of clusiparalicoline A, a prenylated and geranylated biaryl from Clusia paralicola

Article abstract of DOI:10.1016/S0040-4039(02)01630-1

The first synthesis of clusiparalicoline A, a prenylated and geranylated biphenyl compound isolated from the roots of Clusia paralicola, has been achieved by applying the sequential palladium-catalyzed Stille and Suzuki reactions to the formation of all t

Full text of DOI:10.1016/S0040-4039(02)01630-1

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 6919–6923
The first total synthesis and neurotrophic activity of
clusiparalicoline A, a prenylated and geranylated biaryl from
Clusia paralicola
Shigeki Takaoka, Kousuke Nakade and Yoshiyasu Fukuyama*
Institute of Pharmacognosy, Faculty of Pharmaceutical Sciences, Tokushima Bunri University, Tokushima 770-8514, Japan
Received 11 July 2002; revised 31 July 2002; accepted 2 August 2002
Abstract—The first synthesis of clusiparalicoline A, a prenylated and geranylated biphenyl compound isolated from the roots of
Clusia paralicola, has been achieved by applying the sequential palladium-catalyzed Stille and Suzuki reactions to the formation
of all the CꢀC bonds on electron-rich aryl bromide and triflate. Clusiparalicoline A has been found to exhibit a potent neurite
outgrowth promoting activity at 1.0 mM in a primary culture of fetal rat cortical neurons. © 2002 Elsevier Science Ltd. All rights
reserved.
Clusiparalicolines A (1) and B (2) were isolated as
DNA strain-session active compounds from the roots
of Clusia paralicola and their structures were elucidated
by NMR analyses (Fig. 1). They exhibit weak cytotoxi-
city against the KB human cancer cell.1 Clusiparalicol-
ine A is a simple biaryl on which the prenyl and geranyl
groups are attached. We previously reported that some
biphenyl natural products, i.e. mastigophorenes A and
B,2,3 and honokiol,4 showed intriguing neurotrophic
activities on a primary culture of rat cortical neurons.
Our interests in evaluating neurotrophic activity of allyl
and/or prenyl biphenyl natural products led us to
undertake synthesis of 1. In this letter, we report the
first total synthesis of clusiparalicoline A (1) by apply-
ing sequential palladium-catalyzed cross-coupling reac-
tions and the evaluation of its neurotrophic activity in
a primary culture of rat cortical neurons.
OH
OH
HO
We have envisioned that synthesis of biaryls could be
readily accomplished through the Suzuki–Miyaura
reaction.5,6 Because clusiparalicoline A (1) is not quite
symmetrical, application of such an approach to the
synthesis of 1 would require preparation of two geranyl-
ated and prenylated phenols bearing different substitu-
tion patterns (3 and 4). In terms of retrosynthesis, as
outlined in Scheme 1, the right half of clusiparalicoline
A is readily traced back to a bromophenyl triflate 5,
and assembly of the key intermediate 3 from 5 could be
based on palladium-catalyzed Stille7 and/or Suzuki–
Miyaura reactions by utilizing different reactivity of the
oxidative addition of aryl bromide and aryl triflate to
Pd(0). On the other hand, the left half 4 could be
derived from a phloroglucinol 6 by directed ortho
metalation.8
OH
1
OH
OH
O
OH
2
Figure 1. Structures of clusiparalicolines A (1) and B (2).
Keywords: clusiparalicoline A; Stille reaction; Suzuki reaction; palla-
dium; cytotoxicity; neurotrophic activity.
* Corresponding author. Tel.: +81 88 622 9611 (ext. 5911); fax: +81
88 655 3051; e-mail: fukuyama@ph.bunri-u.ac.jp
Our synthesis of the right half intermediate began with
the commercially available 3,4-dihydroxybenzaldehyde
(Scheme 2). After protection of the catechol group of 7
0040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)01630-1

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S. Takaoka et al. / Tetrahedron Letters 43 (2002) 6919–6923
OR
OR
OR
OR
R'
B
Br
R'
OTf
3
5
1
OR
OMe
TfO
OR
HO
OMe
4
6
Scheme 1. Synthetic plan of clusiparalicoline A (1).
OMOM
OMOM
OMOM
OMOM
OH
1) allylbromide, K2CO3
acetone
1) MOMCl, iPr2NEt
OH
2) mCPBA, KHCO3
3) LiAlH4,THF
2) NBS, CH3CN
61%
Br
70%
OH
O
CHO
7
8
OMOM
OMOM
2.0 eq.
Bu3Sn
OMOM
Br
1) Pd(Ph3P)4, Et2NH
CH2Cl2
10 mol% Pd(Ph3P)4, LiCl
DMF, 80ºC, 52%
OMOM
10
2) Tf2O, Et3N
CH2Cl2
Br
OTf
2.5 eq.
Bu3Sn
OMOM
65%
9
OMOM
10 mol% PdCl2(Ph3P)2
DMF, 80ºC, 53%
TfO
11
Scheme 2. Preparation of geranylated 2,4,5-trihydroxybenzene derivatives (10 and 11).
equiv.)13 in DMF at 80°C for 20 h, the Stille reaction
took place on the CꢀOTf bond, giving rise to the
desired intermediate 10 in 52% as a E/Z mixture (2:1).14
On the other hand, a combination of PdCl2(Ph3P)2 (10
mol%), geranyl tributyltin (2.5 equiv.) without an addi-
tive such as TBAC and LiCl in DMF at 80°C for 2 h
brought about the cross-coupling reaction on the CꢀBr
bond to yield the other desired coupling compound 11
as the sole product in 53% yield.15
by reaction with MOMCl and iPr2NEt, Baeyer–Villiger
oxidation of the aldehyde, followed by reduction with
LiAlH4, afforded 7 in 70% yield. After the formed
phenolic group was protected as an allyl ether, a result-
ing allyl ether was brominated with NBS to give rise to
8 as the sole product in 61% yield.9 Deallylation of 8
with 15 mol% of Pd(Ph3P)4 in the presence of an excess
of Et2NH10 and then reaction with Tf2O and Et3N
produced 9 which was substituted with a bromine and
a triflate group ortho to each other in 65% yield over
two steps. With the requisite compound 9 in hand, we
concentrated on finding out the palladium-catalyzed
cross-coupling conditions leading to the right half inter-
mediate appended with a geranyl group. Thus, we
Preparation of the left half intermediate was started
from O-dimethylphloroglusinol and MOM derivative
13 was readily derived in 70% yield over five steps by
the process, as shown in Scheme 3. While the directed
ortho lithiation of 13 failed to introduce a prenyl group
on the position ortho to the both MOM groups, alkyla-
tion on this particular position was carried out through
a bromine–metal exchange. At first, compound 13 was
attempted the Stille reaction of
9 with geranyl
tributyltin11 under various conditions. After extensive
optimization, we found that chemoselective cross-cou-
pling (C–Br versus C–OTf)12 of 9 with geranyl tri-
butyltin relied on combination of the Pd catalyst, sol-
vent and additive. When 9 was reacted with Pd(Ph3P)4
(10 mol%), geranyl tributyltin (2.0 equiv.) and LiCl (3
17
brominated with NBS in CHCl3 to afford the desired
product 14 and its regioisomer 15 in 74 and 21% yield,
respectively. Treatment of 14 with n-BuLi in the pres-

S. Takaoka et al. / Tetrahedron Letters 43 (2002) 6919–6923
6921
1) 10% NaOH, MeOH
OMOM
OMOM
OMe
1) BzCl
2) BBr3, CH2Cl2
2) TBDMSCl, imidazol
DMF
3) MOMCl, iPr2NEt
DMF
70%
100%
TBDMSO
OMOM
HO
OMe
BzO
OMOM
12
13
OMOM
OMOM
Br
Br
TBDMSO
NBS, CHCl3, r.t.
+
TBDMSO
OMOM
OMOM
15 (21%)
14 (74%)
1) nBuLi, CuBr·SMe2
DMPU, THF, -78ºC
2)
Br
-78ºC to r.t.
96%
OMOM
OMOM
1) TBAF, THF
2) Tf2O, Et3N
CH2Cl2
TfO
OMOM
TBDMSO
OMOM
100%
16
17
Scheme 3. Preparation of prenylated 2,4,6-trihydroxybenzene derivative (17).
ence of CuBr·SMe2 at −78°C,18 followed by adding
prenyl bromide, afforded the desired C-alkylation
product 16 in 96% yield. Removal of the TBDMS and
subsequent triflation of 16 produced the left half inter-
mediate 17 with a triflate group quantitatively (Scheme
3).
boronic ester to the triflate 11 using bis(pinaco-
late)diborane were not effective. However, this protocol
could provide direct access to a pinacol boronic ester
from the bromide 10 using PdCl2dppf (5 mol%), dppf
(30 mol%), KOAc (3.0 equiv.) in the presence of
bis(pinacolate)diborane when dioxane was used as sol-
vent, resulting in the formation of the boronic ester 18
in 75% yield. Under conditions similar to the prepara-
tion of 18, next Suzuki coupling between 17 and 18
proceed smoothly to afford the coupling product 19 in
high yield.20 Finally, all MOM groups were hydrolyzed
Now, we are in a position to apply Suzuki–Miyaura
reaction to the cross-coupling between the right half
unit 10 or 11 and the left half unit 17 (Scheme 4). The
Suzuki–Miyaura protocol19 for direct introduction of a
O
B
O
OMOM
OMOM
17
2
10
5 mol% PdCl2dppf, 30 mol% dppf
3.0 eq. K3PO4, dioxane, 80ºC
90%
O
5 mol% PdCl2dppf, 30 mol% dppf
3.0 eq. KOAc, dioxane, 80ºC
75%
B
O
18
OMOM
OMOM
Clusiparalicoline A (1)
1) CSA, MeOH
2) HPLC
MOMO
+
OH
OH
OMOM
19
HO
OH
1a
Scheme 4. Synthesis of clusiparalicoline A (1).

6922
S. Takaoka et al. / Tetrahedron Letters 43 (2002) 6919–6923
Figure 2. Morphometric analysis of the neurons effected by 1 and 1a. After the neuronal cells (9000 cells cm−2) cultured for 6 days
in the presence of 0.5% EtOH, bFGF, 1, and 1a were fixed by 4% paraformaldehyde–PBS, the immunohistochemical staining for
MAP-2 was performed. Morphometric analysis was carried out on these neurons according to the criteria.4 The data are expressed
as means SE (n=80); Student’s test; ꢀꢀꢀP <0.005 versus control: Dunnet’s test; **P <0.01 versus control.
simultaneously by a catalytic amount of CSA21 in
MeOH at room temperature giving rise to a mixture of
clusiparalicoline A (1) and its Z isomer 1a in 68% yield.
Each of the isomers could be readily separated by
activity caused by clusiparalicoline A may not be
closely related to cytotoxicity.
We have accomplished the first synthesis of clusiparali-
coline A (1) by applying sequential palladium cross-
coupling reactions to all the CꢀC bond formations on
the electron-rich aryl bromide and triflate, and have
found that 1 and its Z isomer 1a have neurite out-
growth promoting activity on the primary culture of rat
cortical neurons. Further studies on structure–activity
relationship concerning substituted biphenyl molecules
are under way.
1
HPLC.22 The H and 13C NMR of 1 are superimposed
with those of natural one. Thus, the first total synthesis
of clusiparalicoline A has been accomplished by
sequential palladium-catalyzed Stille and Suzuki–
Miyaura reactions.
Search for non-peptidal small molecules with neuro-
trophic properties is of great significance because neu-
rotrophic factors such as the nerve growth factor
(NGF) must overcome intrinsic drawbacks associated
with polypetidal factors.23 As stated in introductory
remarks, our studies on neurotrophic natural products
implies that some biphenyl natural products with allyl
and alkenyl substituents may have neurotrophic prop-
erty.24 Hence, we examined how clusiparalicoline A (1)
and its Z isomer 1a effect on neurite outgrowth in a
primary culture of rat cortical neurons.4 As being anti-
cipated, compounds 1 and 1a have been found to
promote neurite outgrowth at 1 mM, as shown in Fig. 2.
Their activities are likely to be comparable with
bFGF25 at this concentration. When 10 mM of 1 or 1a
was applied to the neuronal cultures, however, all neu-
rons were killed during 6 day culture period. This result
is consistent with the concentration (EC50 8.6 mM) at
which clusiparalicoline A exhibits KB inhibitory activ-
ity.1 On the other hand, 1 and 1a have no effect on
morphology and viability of cortical neurons at a lower
concentration of 0.1 mM. Wall et al.1 indicated that the
presence of a catechol moiety and a long chain alkyl
group such as a geranyl group on the benzene ring were
responsible for cytotoxicty but the biaryl moiety was
not essential. It was also reported that some biaryls had
specific affinity to neuronal cells.26 Thus, the biaryl
moiety bearing alkenyl groups in clusiparalicoline A
may have some positive effect on development and
survival of neurons, but neurite outgrowth promoting
Acknowledgements
We wish to thank Dr. Mansukh C. Wani, Research
Triangle Institute, for his generous gift of the NMR
spectra of clusiparalicoline A. This work was supported
in a part by a Grant-in Aid for Scientific Research (No.
12480175) from the Ministry of Education, Culture,
Sports, Science, and Technology of Japan, and the
Open Research Center Fund from the Promotion and
Mutual Aid Corporation for Private School of Japan.
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