Syntheses of structurally-simplified and fluorescently-labeled neovibsanin derivatives and analysis of their neurite outgrowth activity in PC12 cells

Article abstract of DOI:10.1016/j.bmcl.2012.01.006

The syntheses of several neovibsanin derivatives were carried out in order to elucidate the simple structure required for displaying neurite outgrowth activity. In addition, a fluorescent probe molecule was synthesized and the analysis of its behavior in the PC12 cell line showed that the neovibsanins accumulate on the outer edge of the cell at the site of formation of prominences.

Full text of DOI:10.1016/j.bmcl.2012.01.006

Bioorganic & Medicinal Chemistry Letters 22 (2012) 2089–2093
Contents lists available at SciVerse ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Syntheses of structurally-simplified and fluorescently-labeled neovibsanin
derivatives and analysis of their neurite outgrowth activity in PC12 cells
Hiroshi Imagawa a, , Hayato Saijo a, Hitomi Yamaguchi a, Ken Maekawa a, Takahiro Kurisaki a,
⇑
Hirofumi Yamamoto a, Mugio Nishizawa a, Masataka Oda b, Michiko Kabura b, Masahiro Nagahama b,
Jun Sakurai b, Miwa Kubo c, Megumi Nakai c, Kosho Makino c, Mitsuko Ogata d, Hironobu Takahashi d,
Yoshiyasu Fukuyama c
a Chemistry of Functional Molecule, Faculty of Pharmaceutical Sciences, Tokushima Bunri University, Yamashiro-cyo, Tokushima, Japan
b Department of Microbiology, Faculty of Pharmaceutical Sciences, Tokushima Bunri University, Yamashiro-cyo, Tokushima, Japan
c Physical Chemistry, Faculty of Pharmaceutical Sciences, Tokushima Bunri University, Yamashiro-cyo, Tokushima, Japan
d Institute of Pharmacognosy, Tokushima Bunri University, Yamashiro-cyo, Tokushima, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
The syntheses of several neovibsanin derivatives were carried out in order to elucidate the simple struc-
ture required for displaying neurite outgrowth activity. In addition, a fluorescent probe molecule was
synthesized and the analysis of its behavior in the PC12 cell line showed that the neovibsanins accumu-
late on the outer edge of the cell at the site of formation of prominences.
Received 8 October 2011
Revised 28 December 2011
Accepted 4 January 2012
Available online 11 January 2012
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Neovibsanin derivative
Neurite outgrowth activity
PC12 cells
Fluorescent probe molecule
Structure–activity correlation
Neovibsanins, novel polyfunctionalized diterpenoid compounds
were originally isolated from the shrub Viburnum awabuki by
Fukuyama’s group in 1996.1 These natural products were found
to display neurite outgrowth activity in PC12 cells, suggesting neo-
vibsanin-type compounds could be promising candidates for the
development of novel therapeutic agents to treat Alzheimer’s dis-
ease.2 However, the structural factors of the neovibsanin skeleton
that are essential for exerting its biological activity were unknown.
In 2009, Williams et al. reported neurite outgrowth activities of
unnatural 4,5-bi-epi-neovibsanins and their synthetic intermedi-
ates. During the course of this research, they found that the stereo-
chemistry at the 4 and 5 positions on the neovibsanin skeleton has
very little effect on biological activity.3 In 2009, we reported the
first total synthesis of ( )-neovibsanin B (2, Fig. 1) based on an
intramolecular Diels–Alder reaction accelerated with 1,3-di-
methyl-2-imidazolidinone and a subsequent oxy-Michael addi-
tion-lactonization reaction.4 Furthermore we revealed that the
neurite outgrowth activity of racemic 2 is almost the same as that
of natural (+)-2.4 Here, we describe the syntheses of structurally-
simplified neovibsanin derivatives, such as 3, based on the
synthetic method developed previously for the total synthesis of
( )-2. We have also synthesized a fluorescent analog 4 that
H
R2
R1
5
4
H
O
2
O
O
18
O
RO
O
O
3
R1 = MeO, R2 = Me: Neovibsanin A (1)
R1 = Me, R2 = OMe: Neovibsanin B (2)
H
O
MeO
O
O
O
O
4
N
O
⇑
Corresponding author.
E-mail address: imagawa@ph.bunri-u.ac.jp (H. Imagawa).
Figure 1. Structure of neovibsanins and the derivatives.
0960-894X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2012.01.006

2090
H. Imagawa et al. / Bioorg. Med. Chem. Lett. 22 (2012) 2089–2093
H
H
H
H
H
bistetrahydropyranly moiety
MeO
MeO
EtO
n-PrO
n-BuO
O
O
O
O
O
3
H
MeO
O
O
O
O
O
O
O
remove side chains
5
O
O
α−MeO : 3a-α
β−MeO : 3a-β
80% α/β 1 : 2
α−EtO : 3b-α
β−EtO : 3b-β
66% α/β 1 : 3
α−PrO : 3c-α
β−PrO : 3c-β
82% α/β 1 : 2
α−BuO : 3d-α
β−BuO : 3d-β
88% α/β 1 : 1.6
side chains
H
H
MeO
O
O
O
O
O
H
H
O
the necessity of acetal
the necessity of double bond
O
H
O
O
6
7
O
O
No activity
No activity
Figure 2. Design of derivatives for structure–activity related studies.
95% α/β 1 : 2.7
86% α/β 1 : 3.2
3e
3f
maintains neurite outgrowth activity and can therefore act as a
molecular probe for mechanistic studies. In addition, we have char-
acterized the behavior of the fluorescent probe molecule in PC12
cell.
An elucidation of the essential structure of neovibsanins re-
quired for maintaining their biological activity is a vitally important
step towards a detailed mechanistic study of these molecules. To
clarify the structure required for an onset of neurite outgrowth
activity, we designed a number of derivatives, 3 and 5–7, which
either lack side chains from the neovibsanin skeleton or a part of
the skeleton (Fig. 2).
H
O
H
O
O
O
O
O
No activity
Low activity
3g 95% α/β 1 : 3.3
3h
74% α/β 1 : 3
Figure 3. The structures of acetal derivatives.
separable mixture of
a-methoxy 3a-a and b-methoxy isomers
Initially, we synthesized 3a in order to reveal the effect of the
side chains of neovibsanin on the biological activity. Although
the synthesis of the precursor for 3 was reported by Mehta et al.
in 2009,5 we attempted the synthesis of 3a on the basis of our ori-
ginal synthetic plan (Scheme 1). Enone alcohol 8, which was pre-
pared from cyclohexenone by Morita–Baylis–Hillman reaction
using formaldehyde with imidazole,6 was protected with TBS ether
affording 9 in 99% yield. Alkynylation of 9 with lithium acetylide
14 easily took place to give 10 in 79% yield. After reduction of
the triple bond in 10 with Red-Al,7 treatment of the resulting 11
with TBAF in THF induced oxy-Michael addition and subsequent
lactonization, giving rise to tricyclic lacton 6 in 92% yield.4 The
structure of 6 was confirmed by a X-ray crystallographic analysis
as shown with the Ortep drawing 12.8 The constriction of acetal
moiety was achieved by treatment with Tebbe reagent and subse-
quent expose to PPTS in MeOH. The acetal 3a was obtained as a
3a-b in 26% and 54%, respectively. In addition, several types of ace-
tals 3b–h were synthesized by using the corresponding alcohol in-
stead of MeOH. The hemiacetal 16 was prepared from 13 by
hydrolysis during slow silica gel column chromatography in 70%
yield. The saturated 7 was synthesized from 6 via hydrogenation
using Pd/C under H2, methylenation with Tebbe reagent and subse-
quent acetalization using PPTS in MeOH (Scheme 1, Fig. 3). The
truncated-bicyclic compound 5 is made up of a bistetrahydrofura-
nyl moiety, which is a characteristic structure of neovibsanins.
Compound 5 was synthesized from 4-hydroxybutan-2-one (17)
in six steps using the same synthetic methodology employed for
the other derivatives described above (Scheme 2).
Next, we measured the neurite outgrowth activity of these syn-
thetic derivatives using PC12 cells (JCRB0733).9 Our results showed
that 3a-a and 3a-b significantly promote NGF (20 ng/mL)-mediated
neurite outgrowth at the same level as that of natural 2. These
findings infer that the side chains of 2 are unimportant in terms
of the onset of neurite outgrowth activity (Fig. 4). Quantitative-
activity evaluation for derivatives 3a–d revealed that increasing
the bulkiness of the acetal moiety leads to decreased activity of
the corresponding derivatives (Fig. 5). Moreover, the activity of
O
O
O
OMe
OTBS
HO
the corresponding
a-isomer was slightly greater than that of the
a
c
OH
O
OTBS
b
b-isomer (Fig. 5). Compound 3h showed only very weak activity
and 3e–g were inactivity any more.13 Interestingly, hemiacetal 16
shows significant cytotoxicity as well as weak neurotrophic activ-
ity. The bicyclic derivative 5, intermediate lactone 6 and saturated
compound 7 were found to be inactive. From these results, we con-
cluded that a cyclic acetal with double bond, which is represented
as structure 3a or 3b were essential factors for neurite outgrowth
activity.
8
9
10
H
O
O
OMe
HO
O
g
d
OTBS
6
H
11
15
The Ortep drawing
e
e, f
12
In order to elucidate the mechanism for the neurite outgrowth
activity of neovibsanins, we attempted to synthesize fluorescently-
labeled derivatives based on the active derivative 3a as a probe
molecule. To determine a suitable positioning 3a for the installa-
tion of a fluorescent functional group, we synthesized several
derivatives of 3a with alkyl or aryl group at two positions. The syn-
thesis of 2-position10 substituted 3a derivatives was achieved
starting from alkylation of known ketone 2111 using the corre-
sponding alkyl lithium or Grignard reagent. After hydrolysis of an
acetal moiety 22a–c with 1 M HCl, the dehydration reaction took
place cleanly to give enone 23a–c in good yield.11 The resulting
H
H
7
HO
O
O
f
h
3a-h
O
O
16
13
Scheme 1. Syntheses of 3a and the acetal derivatives. Reagents and conditions: (a)
TBSCl (1.2 equiv), Et3N (2.5 equiv), DMAP (0.2 equiv), CH2Cl2, rt, 2 h, 99% yield; (b)
LiCCCOOMe (14, 1.2 equiv), THF, À78 °C, 30 min, 79% yield; (c) Red-Al (2.0 equiv),
THF, À78 °C, 30 min, 96% yield; (d) TBAF (3.0 equiv), THF, rt, 30 min, 92% yield; (e)
Tebbe reagent (2.0 equiv), pyridine, THF/toluene, rt, 2 h; (f) PPTS (0.4 equiv), ROH,
rt, 5 h, 66–95% yield in 2 steps; (g) Pd/C, MOH, H2, rt, 12 h, 83% yield; (h) silica gel
chromatography, 76% yield.

H. Imagawa et al. / Bioorg. Med. Chem. Lett. 22 (2012) 2089–2093
2091
O
OMe
O
OMe
OTBS
H
H
MeO
OTBS
O
O
O
O
OH
a, b
c
d
e, f
HO
HO
O
O
17
18
19
20
5
Scheme 2. Synthesis of bicyclic compound 5. Reagents and conditions: (a) TBSCl (1.2 equiv), Et3N (3.0 equiv), DMAP (0.2 equiv), CH2Cl2, rt, 2 h, 98% yield; (b) LiCCCOOMe
(2.0 equiv), THF, À78 °C, 30 min, 85% yield; (c) Red-Al (2.0 equiv), THF, À78 °C, 30 min, 91% yield; (d) TBAF (3.0 equiv), THF, rt, 12 h, 73% yield; (e) Tebbe reagent (1.0 equiv),
pyridine, THF/toluene, rt, 1 h; (f) PPTS (0.2 equiv), MeOH, rt, 12 h, 72% yield in 2 steps.
Figure 4. The neurite outgrowth activities of 3a-
observed. Middle picture: A notable neurite growth was observed in the presence of 10
observed in the presence of 10 M 3a- and 20 ng/mL NGF.
a
and 3a-b. Left picture: PC12 cells in the presence of 20 ng/mL of NGF as control. No remarkable morphological change was
lM of 3a-b and 20 ng/mL of NGF. Right picture: A significant neurite growth was
l
a
Figure 5. Quantitative assays for neurite outgrowth activities of acetal derivatives. PC12 cells were cultured in two 24-well plates in DMEM + 10% HS and 5% FBS for 24 h at a
density of 2 Â 103 cells/cm2. The medium was then changed to DMEM + 2% HS and 1% FBS supplemented with NGF (10 ng/mL) in the absence or presence of test compound.
After 4 days the length of the neurites were quantified. Data are expressed as the mean SE (n = 149).⁄⁄P <0.01 versus control; Dunnett’s/test.
H
H
H
MeO
MeO
MeO
O
O
R
O
O
2
O
O
O
O
O
O
O
O
OH
a
b
24a-c
R
Me
Bu
Ph
HO
22a-c
24a
24b
24c
21
23a-c
Scheme 3. Syntheses of 2-potition substituted derivatives of 3. Reagents and conditions: (a) MeLi or BuLi or PhMgCl, THF, À78 °C, 20 mins; (b) 1 M HCl, 57–69% yield in 2
steps.
enones were converted to 2-position substituted derivatives 24–c
by a similar strategy to that used for the synthesis of 3 (Scheme
3). The synthesis of 18-substituted derivatives 29 and 30 were
accomplished from known enone 2510,12 in six steps according to
the established route for the syntheses of the other derivatives.
The diastereoisomers 27 and 28 were generated by alkynylation
to 26 and then separated. Isomers 27 and 28 were subsequently
converted to 29 and 30, respectively (Scheme 4).
Next, we investigated the neurite outgrowth activities of these
2- or 18-substituted derivatives. Our experiments showed that

2092
H. Imagawa et al. / Bioorg. Med. Chem. Lett. 22 (2012) 2089–2093
O
OEt
H
MeO
O
O
OH
O
OTBS
OTBS
HO
O
b
a
18
O
OEt
OTBS
29
H
+
MeO
25
26
27
(27/28 1 : 1.4)
O
HO
O
28
30
Scheme 4. Syntheses of 18-potition substituted derivatives 29 and 30. Reagents and conditions: (a) TBSCl (1.5 equiv), imidazole (3 equiv), CH2Cl2, rt, 12 h, quant.; (b)
LiCCC(OEt)3 31, THF, 0 °C to rt, 30 min, 2 steps 73% yield.
substitution at the 2- or 18-position,13 which have no functional
group in the natural product, totally eliminates the biological activ-
ity. Therefore introduction of the fluorescent group at these posi-
tions was found to be unsuitable for maintaining the function of
the probe molecule. Consequently, we focused on the synthesis
of 4, which has a fluorescent label at the position 10 where a side
chain exists in the natural product 2. A cyclohexene core was syn-
thesized by Diels-Alder cycloaddition of acrolein and diene 32.14 By
applying a Witting reaction to cycloadduct 33,15 C1-extended alde-
hyde 34 was generated in two steps (72% yield). Treatment with
Figure 6. Result of the neurite outgrowth activity assay for 4. PC12 cells cultured in
DMEM/2% HS + 1% FBS and treated for 96 h, Left picture: PC12 cells under 10 ng/mL
of NGF as control. No remarkable morphological change was observed. Right
DIBAL, and primary-selective protection with 2-methoxy benzyl
ether (here after 2-MPM)16 via stannyl acetal,17 followed by oxida-
tion resulted in the formation of enone 36 in a reasonable yield. To
36, a hydroxy methyl group was introduced by Morita–Baylis–Hill-
man reaction, and the resulting alcohol was protected with a TBS
group to give 37 in good yield. An alkynylation of 37 with lithium
acetylide took place cleanly, giving rise to a mixture of stereoiso-
picture: significant neurite growth was observed in the presence of 40
10 ng/mL of NGF.
lM 4 and
in Figure 6. Despite substitution with a sterically bulky functional
group, compound 4 retained a moderate biological activity. There-
fore we began to analyze the behavior of neovibsanin derivatives
inside the PC12 cell line by using 4 as a fluorescent molecular
probe.
mers of 38 in 80% yield (b–OH/a–OH 1: 5.5). Tricyclic lactones
40a and 40b were constructed by treatment with TBAF for 39,
and a deprotection of the 2-MPM group with DDQ under bilayer
condition.18 After isolation of a desired 40b by silica gel column
chromatography, 42 was synthesized via C1 extension and acetal
formation from 40b. Finally, a TBS group in 42 was removed by a
treatment with TBAF, and an immediate esterification of the result-
ing unstable alcohol with a coumarin moiety19 led to the target
fluorescently-labeled derivative 4 in 85% yield in 2 steps (Scheme
5). The result of a neurite outgrowth-activity assay for 4 is shown
When PC12 cells were incubated with 4 (40 lg/mL) in the pres-
ence of NGF (10 ng/mL) no alteration in cell morphology was
observed during the initial 24 h period. The probe molecule dis-
persed uniformly into the cell to generate many small vesicles
(Fig. 7, left picture). After 48 h, short prominences appeared at
the outer edge of the cell. Intriguingly, the fluorescent vesicles
HO
2-MPMO
2-MPMO
O
O
OAc
OAc
OH
O
O
O
OAc
32
a
b, c
d
e, f
g, h
H
OTBS
H
33
34
35
36
37
O
O
H
O
H
O
OEt
O
O
OEt
(β−ΟΗ/α−ΟΗ 1:5.5)
2-MPMO
HO
HO
O
O
O
2-MPMO
i
j
k, l
OTBS
OTBS
+
HO
HO
1 : 5
38
39
40a
40b
H
H
MeO
O
O
n, o
p, q
m
O
O
4
TBSO
TBSO
ex: 400 nm / em: 410 nm
41
42
Scheme 5. Synthesis of fluorescently-labeled derivatives. Reagents and conditions: (a) BF3-Et2O, CH2Cl2, À78 °C, 86% yield; (b) Ph3PCH2OMe, LHMDS, THF, À78 °C; (c) 1 M
HCl/THF (10:1), rt, 2 steps 72% yield; (d) DIBAL, toluene, À78 °C, 98% yield; (e) Bu2SnO, toluene, reflux with Dean–Stark apparatus, then n-Bu4NI, 2-methoxybenzyl chloride, rt,
92% yield; (f) TPAP, NMO, CH2Cl2, rt, quant.; (g) Et3P, 37% COH2 in H2O, rt, 69% yield; (h) TBSOTf, 2,6-lutidine, CH2Cl2, 98% yield; (i) LiCCCOOEt, toluene À78 °C, 80% yield (
ab
1:5.5); (j) Red-Al, toluene, 0 °C; (k) TBAF, THF, rt, 2 steps 85% yield; (l) DDQ, CH2Cl2, pH 7 phosphate buffer, 61% yield; (m) TBSCl, imidazole, rt, quant.; (n) Tebbe reagent,
pyridine, À78 °C; (o) PPTS, MeOH, rt, 2 steps 66% yield; (p) TBAF, THF, rt, 30 min, (q) EDCI, DMAP, 2-(7-(dimethylamino)-2-oxo-2H-chromen-4-yl)acetic acid, CH2Cl2, rt, 2
steps 85% yield.

H. Imagawa et al. / Bioorg. Med. Chem. Lett. 22 (2012) 2089–2093
2093
Figure 7. An analysis of the localization of neovibsanin derivatives inside a PC12 cell by using 4 as a fluorescent molecular probe. Left picture: PC12 cell after 24 h of the
culture with 40 M 4 and 10 ng/mL NGF. Compound 4 was identified as small-blue particles. Middle picture: PC12 cell after 48 h. Right picture: PC12 cell after 72 h. Note that
l
the fluorescent molecule 4 (blue particles) has accumulated inside each prominence.
Kawazu, K.; Fukuyama, Y. Tetrahedron Lett. 1999, 40, 6261; (c) Kubo, M.; Chen,
I.-S.; Fukuyama, Y. Chem. Pharm. Bull. 2001, 49, 242; (d) Fukuyama, Y.; Kubo,
M.; Minami, H.; Yuasa, H.; Matsuo, A.; Fujii, T.; Morisaki, M.; Harada, K. Chem.
Pharm. Bull. 2005, 53, 72; (e) Fukuyama, Y.; Fujii, H.; Minami, H.; Takahashi, H.;
Kubo, M. J. Nat. Prod. 2006, 69, 1098; (f) Fukuyama, Y.; Kubo, M.; Esumi, T.;
Harada, K.; Hioki, H. Heterocycles 2010, 81, 1571; (g) Fykuyama, Y.; Kubo, M.
Curr. Top. Phytochem. 2011, 10, 39.
assembled at the neck of each small prominence (Fig. 7, middle
picture). Significant morphological changes were observed after
72 h. Elongation of the prominences, where the fluorescent mole-
cules accumulated, became particularly noticeable (Fig. 7, right
picture). These results show the neovibsanins accumulate in the
prominences.
In conclusion, the simple structure of neovibsanins required for
maintaining the neurite outgrowth activities and the localization
of neovibsanin derivatives inside the PC12 cell were revealed. We
are currently attempting to synthesize a photo-affinity labeled
probe for further mechanistic studies.
3. (a) Gallen, M. J.; Williams, C. M. Org. Lett. 2008, 10, 713; (b) Chen, A. P. J.;
Williams, C. M. Org. Lett. 2008, 10, 3441.
4. (a) Imagawa, H.; Saijo, H.; Kurisaki, T.; Yamamoto, H.; Kubo, M.; Fukuyama, Y.;
Nishizawa, M. Org. Lett. 2009, 11, 1253; (b) Esumi, T.; Mori, T.; Zhao, M.; Toyota,
M.; Fukuyama, Y. Org. Lett. 2010, 12, 888.
5. Mehta, G.; Bhat, B. A. Tetrahedron Lett. 2009, 50, 2474.
6. Gatri, R.; Gaïed, M. M. E. Tetrahedron Lett. 2002, 43, 7835.
7. Koide, K.; Meta, C. T. Org. Lett. 2004, 11, 1785.
8. Crystallographic data (excluding structure factors) for the structures of 6 have
been deposited with the Cambridge Crystallographic Data Centre as
supplementary publication numbers CCDC 832773. Copies of the data can be
obtained, free of charge, on application to CCDC, 12 Union Road, Cambridge CB2
1EZ UK (Fax: +44(0)-1223-336033 or e-mail: deposit@ccdc.cam.ac.uk).
9. Lloyd, A. G.; Arthur, S. T. Proc. Natl. Acad. Sci. U.S.A. 1976, 73, 2424.
10. This position was numbered based on the natural neovibsanin skeleton.
11. Smith, A. B., III; Dorsey, B. D.; Ohba, M.; Lupo, A. T., Jr.; Malamas, M. S. J. Org.
Chem. 1988, 53, 4314.
Acknowledgments
This study was financially supported by a Grant-in-Aid for Sci-
entific Research (C) (23590033) from the Ministry of Education,
Culture, Sports, Science and Technology of the Japanese Govern-
ment, and a MEXT.SENRYAKU.
12. Gatri, R.; Moncef, M.; Gaïed, E. Tetrahedron Lett. 2002, 43, 7835.
13. A mixture of stereoisomer at acetal position was used for the assay without
separation.
Supplementary data
14. An asymmetric version of this Diels-Alder reaction was recently reported.
Ahrendt, K. A.; Borths, C. J.; MacMillan, D. W. C. J. Am. Chem. Soc. 2000, 122,
4243.
15. Narasimhan, N. S.; Mali, R. S. Synthesis 1975, 12, 796.
16. In the previous synthetic study of neovibsanin B, we have found that 2-
methoxy benzyl ether, which have a coordination ability, is advantageous for
subsequent regioselective alkylation.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.bmcl.2012.01.006. These data
include MOL files and InChiKeys of the most important compounds
described in this article.
17. Guibe, F. Tetrahedron 1997, 53, 13509.
18. We developed this deprotection condition in previous total synthesis of 2 (Ref.
4a).
19. (a) Hermanand, B. A.; Fernandez, S. M. Biochem. Biophys. Res. Commun. 1981,
103, 1112; (b) Goya, S.; Takadate, A.; Fujino, H.; Otagiri, M.; Uekama, K. Chem.
Pharm. Bull. 1982, 30, 1363.
References and notes
1. (a) Fukuyama, Y.; Minami, H.; Takeuchi, K.; Kodama, M.; Kawazu, K.
Tetrahedron Lett. 1996, 37, 6767; (b) Fukuyama, Y.; Esumi, T. J. Org. Synth.
Chem. Jpn. 2007, 65, 585.
2. (a) Fukuyama, Y.; Minami, H.; Yamamoto, I.; Kodama, M.; Kawazu, K. Chem.
Pharm. Bull. 1998, 46, 545; (b) Kubo, M.; Minami, H.; Hayashi, E.; Kodama, M.;