Design, synthesis, and biological evaluations of aplyronine A-mycalolide B hybrid compound

Article abstract of DOI:10.1021/ol300182r

A hybrid compound consisting of aplyronine A and mycalolide B was synthesized, and its biological activities were evaluated. The hybrid compound was found to have somewhat more potent actin-depolymerizing activity than aplyronine A. In contrast, the hybri

Full text of DOI:10.1021/ol300182r

ORGANIC
LETTERS
2012
Vol. 14, No. 5
1290–1293
Design, Synthesis, and Biological
Evaluations of Aplyronine AÀMycalolide
B Hybrid Compound
Kenichi Kobayashi, Yusuke Fujii, Yuichiro Hirayama, Shinichi Kobayashi,
Ichiro Hayakawa, and Hideo Kigoshi*
Department of Chemistry, Graduate School of Pure and Applied Sciences,
University of Tsukuba, Tennodai, Tsukuba 305-8571, Japan
kigoshi@chem.tsukuba.ac.jp
Received January 23, 2012
ABSTRACT
A hybrid compound consisting of aplyronine A and mycalolide B was synthesized, and its biological activities were evaluated. The hybrid
compound was found to have somewhat more potent actin-depolymerizing activity than aplyronine A. In contrast, the hybrid compound
possessed about 1000-fold less cytotoxicity than aplyronine A. These results indicated that there is no direct correlation between actin-
depolymerizing activity and cytotoxicity.
Actin isamajor proteinofthe cytoskeletonineukaryotic
cells. Recently, various actin-binding macrolideshavebeen
isolated from marine sources.1 Among these compounds,
aplyronine A (1), a marine macrolide isolated from the
Japanese sea hare Aplysia kurodai, shows potentantitumor
activities in vivo in addition to its actin-binding property
and is expected to be a new type of anticancer drug
candidate (Figure 1).2 In our previous studies, the side
chain part in aplyronine A (1) proved to be crucial for both
cytotoxicity and actin-depolymerizing activity.3 In con-
trast, the macrolide moiety in 1 significantly emphasizes its
cytotoxicity but is not so important to actin-depolymeriz-
ing activity. Since mycalolide B (2),4 a macrolide isolated
from a Japanese sponge, possesses a similar side chain to
that of aplyronine A (1) and the artificial analogue only
consisting of the side chain part of mycalolide B exhibits
stronger actin-depolymerizing activity than does that of
aplyronine A,5 hybrid compound 3, consisting of the
macrolactone part in aplyronine A (1) and the side chain
part in mycalolide B (2), might be expected to possess more
potent actin-depolymerizing activity and cytotoxicity than
aplyronine A (1). Thus, we planned to synthesize
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(b) Suenaga, K.; Kimura, T.; Kuroda, T.; Matsui, K.; Miya, S.;
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r
10.1021/ol300182r
Published on Web 02/22/2012
2012 American Chemical Society

aplyronine AÀmycalolide B hybrid compound 3 and to
evaluate its biological activity.
Scheme 1. A Retrosynthetic Pathway of Aplyronine
AÀMycalolide B Hybrid Compound 3
synthetic studies on acid 5, we designed ligand 9 for an
asymmetric NHK coupling and found this to be effective
for the asymmetric construction of the trisubstituted sec-
ondary allylic alcohol precursor of 5.7 For the NHK
coupling of 7 and 8, ligand 9 also worked successfully to
give the best yield and stereoselectivity among several
known sulfonamide ligands. As the primary asymmetric
NHK coupling product 10a has the desired stereochem-
istry for the C20ÀC34 segment 6, elaboration to 6 was
readily achieved. Thus, catalytic hydrogenation of 10a,
protection of the hydroxy group as a DMBOM ether, and
subsequent selective desilylation afforded a primary alco-
hol. Oxidation of the resulting primary hydroxy group
gave an aldehyde, which was subjected to Takai olefina-
tion11 followed by desilylation to furnish the targeted
C20ÀC34 segment 6.
Figure 1. Design of aplyronine AÀmycalolide B hybrid com-
pound 3.
Our retrosynthetic pathway of hybrid compound 3 is
shown in Scheme 1. The macrolactone part in 3 would be
constructed by an intramolecular NozakiÀHiyamaÀKishi
(NHK) reaction6 of compound 4, which could be derived
by an intermolecular esterification of C1ÀC19 segment 5
and C20ÀC34 segment 6. We previously reported the
synthesis of the C1ÀC19 segment 57 by an asymmetric
NHK coupling.8 The C20ÀC34 segment 6 could also be
assembled from iodoolefin 7 and aldehyde 8 by an asym-
metric NHK coupling.
The C21ÀC28 and C29ÀC34 segments 7 and 8 were
synthesized using an asymmetric aldol reaction9 and
Roush crotylboration10 as key steps (see Supporting
Information). An asymmetric NHK coupling8 between
fragments 7 and 8 was next attempted (Scheme 2). In our
Scheme 2. Synthesis of C20ÀC34 Segment 6
(6) (a) Jin, H.; Uenishi, J.; Christ, W. J.; Kishi, Y. J. Am. Chem. Soc.
1986, 108, 5644. (b) Takai, K.; Tagashira, M.; Kuroda, T.; Oshima, K.;
Utimoto, K.; Nozaki, H. J. Am. Chem. Soc. 1986, 108, 6048.
(7) Kobayashi, K.; Fujii, Y.; Hayakawa, I.; Kigoshi, H. Org. Lett.
2011, 13, 900.
(8) (a) Guo, H.; Dong, C.-G.; Kim, D.-S.; Urabe, D.; Wang, J.; Kim,
J. T.; Liu, X.; Sasaki, T.; Kishi, Y. J. Am. Chem. Soc. 2009, 131, 15387.
(b) Liu, X.; Henderson, J. A.; Sasaki, T.; Kishi, Y. J. Am. Chem. Soc.
2009, 131, 16678 and references cited therein.
(9) Crimmins, M. T.; King, B. W.; Tabet, E. A.; Chaudhary, K. J.
Org. Chem. 2001, 66, 894.
(10) Roush, W. R.; Palkowitz, A. D.; Ando, K. J. Am. Chem. Soc.
1990, 112, 6348.
Org. Lett., Vol. 14, No. 5, 2012
1291

The synthesis of the precursor 4 for an intramolecular
NHK reaction is illustrated in Scheme 3. The esterification
between the C1ÀC19 segment5 and theC20ÀC34 segment
6 under Yamaguchi conditions12 gave ester 12. Selective
removal of the C19 silyl ether proceeded cleanly to give
alcohol 13, which was then oxidized to provide the cycliza-
tion precursor, aldehyde 4.
Table 1. Intramolecular NHK Reaction of 4
Scheme 3. Synthesis of Precursor 4 for an Intramolecular NHK
Reaction
yield (%)a
concn
solvent (mM) 14a
entry
1
conditions
14b
NiCl2 (10 mol %),
CrCl2 (20 equiv)
DMSO
DMSO
CH3CN
1.0
10
1.0
46
46
49
50
2
3
NiCl2 (10 mol %),
CrCl2 (5 equiv)
49
27
NiCl2(dppp) (4 mol %),
CrCl2 (10 equiv), proton-
sponge (10 equiv), ent-9
(10 equiv)
a The stereochemistry was confirmed by modified Mosher’s
method.16
We next examined the crucial construction of the macro-
lactone ring by intramolecular NHK reaction (Table 1).6
Treatment of 4 with CrCl2/NiCl2 in DMSO at a dilute
concentration (c = 1.0 mM) efficiently afforded the
expected cyclization product 14a (46%) and its C19 epimer
14b (50%) (entry 1). This macrocyclization also proceeded
smoothly even at a higher concentration (c = 10 mM) to
give 14a (46%) and 14b (49%) (entry 2). In our aplyronine
A synthesis,13 construction of a similar macrolactone via
the Yamaguchi method12 required high-dilution condi-
tions (c = 0.39 mM), which makes this intramolecular
NHK reaction without the use of high-dilution conditions
much more convenient.14 In contrast, when we tried the
asymmetric intramolecular NHK reaction with ent-9
(entry 3), the stereoselectivity was not very good, as
expected, and the yield of the product was moderate.
Conversion of the undesired C19 epimer 14b into the
desired isomer 14a was achieved by following the proce-
dure reported by Paterson.15
N-methylformamide group at C34, the O,O-dimethylglyceric
ester group at C29, and the N,N,O-trimethylserine ester
groups at C717 afforded aplyronine AÀmycalolide B
hybrid compound 3.
Scheme 4. Synthesis of Aplyronine AÀMycalolide B Hybrid
Compound 3
In our conversion of 14a into hybrid compound 3, we
followed a similar strategy for the synthesis of aplyronine
A by our group (Scheme 4).13 Thus, introduction of the
(11) Takai, K.; Nitta, K.; Utimoto, K. J. Am. Chem. Soc. 1986, 108,
7408.
(12) Inanaga, J.; Hirata, K.; Saeki, H.; Katsuki, T.; Yamaguchi, M.
Bull. Chem. Soc. Jpn. 1979, 52, 1989.
(13) Kigoshi, H.; Ojika, M.; Ishigaki, T.; Suenaga, K.; Mutou, T.;
Sakakura, A.; Ogawa, T.; Yamada, K. J. Am. Chem. Soc. 1994, 116,
7443.
(14) Namba and Kishi reported a catalytic intramolecular NHK reac-
tion without the use of high-dilution techniques: Namba, K.; Kishi, Y.
J. Am. Chem. Soc. 2005, 127, 15382.
(15) Paterson, I.; Cowden, C. J.; Woodrow, M. D. Tetrahedron Lett.
1998, 39, 6037.
1292
Org. Lett., Vol. 14, No. 5, 2012

The actin-depolymerizing activity and cell growth in-
hibitory effects of 3 and 1 were evaluated against HeLa S3
cells (Table 2). Hybrid compound 3was found to have some-
what more potent actin-depolymerizing activity (EC50
1.0 μM) than aplyronine A (1) (EC50 = 1.4 μM). In
contrast, the cytotoxicity of hybrid compound 3 (IC50
=
=
12 nM) was about 1000-fold weaker than that of aplyronine
A (1) (IC50 = 0.010 nM). These results indicate that there
is no direct correlation between actin-depolymerizing
activity and cell growth inhibitory activity. Thus, we
discuss the reasons for the considerably reduced cytotoxi-
city of 3. From our previous structureÀactivity relation-
ship study,3b the acyl group at C29 was found to be
unimportant to cytotoxicity. For this reason, displace-
ment of a N,N-dimethylalanine ester with an O,O-
dimethylglycerate would not be a reasonable explanation
for the fairly low cytotoxicity. Therefore, we assume that
the differences in configuration at C25 and the degree of
substitution at C24 between 1 and 3 significantly influ-
enced their cytotoxicities. These should cause a change in
the conformational relationship or angle between the
macrolactone and the side chain part, which can be
deduced by comparing X-ray crystallographic structures
between the actinÀaplyronine A (1) complex18 and actinÀ
kabiramide C (20, mycalolide B-related compound) com-
plex19 (Figure 2). Consequently, we presume that both hybrid
compound 3 and aplyronine A (1) bind readily with actin;
nevertheless, 3 cannot maintain a strong interaction with the
other target biomolecules of aplyronine A (1) to exhibit cyto-
toxicity. In this regard, actin-related proteins (Arp2 and 3)
have also been found as target biomolecules of aplyronine A
(1) by our group.20 However, the role of Arp2 and Arp3 in the
strong antitumor activity of aplyronine A (1) is still unclear.
Figure 2. Conformational structures of aplyronine A (1) and
kabiramide C (20) based on X-ray analyses of actinÀaplyronine
A complex and actinÀkabiramide C complex.
and the side chain part of mycalolide B (2), using two
asymmetric NHK couplings and an intramolecular NHK
reaction as the key steps, and we have biologically eval-
uated this compound. Hybrid compound 3 retained potent
actin-depolymerizing activity, whereas its cytotoxicity
against HeLa S3 cells was considerably reduced compared
to that of aplyronine A (1). These findings suggest that
actin-depolymerization is not directly related to the ob-
served cytotoxicity and that the side chain of mycalolide B
(2) is not suitable for 3 to exhibit strong cytotoxicity.
Further investigations regarding the design and synthesis
of aplyronine A analogues are currently underway in our
group.
Table 2. Actin-Depolymerizing Activity and Cytotoxicity of
Hybrid Compound 3 and Aplyronine A (1)
actin-depolymerizing cytotoxicity against
compound
activity (EC50
)
HeLa S3 cells (IC50)
hybrid compound 3
aplyronine A (1)
1.0 μM
1.4 μM
12 nM
0.010 nM
Acknowledgment. This work was supported by Grants-
in-Aid for Scientific Research, from the Ministry of Edu-
cation, Culture, Sports, Science and Technology (MEXT),
Japan, and by a grant from the Uehara Memorial Foun-
dation (H.K.). I.H. thanks The Society of Synthetic Or-
ganic Chemistry, Japan (Meiji Seika Award in Synthetic
Organic Chemistry, Japan) and Suntory Institute for
Bioorganic Research (SUNBOR GRANT) for financial
support.
In conclusion, we have synthesized a hybrid compound
3, comprised of the macrolactone part of aplyronine A (1)
(16) Ohtani, I.; Kusumi, T.; Kashman, Y.; Kakisawa, H. J. Am.
Chem. Soc. 1991, 113, 4092.
(17) Esterification of alcohol 18 with (S)-N,N,O-trimethylserine gave
a 3:1 mixture of (S)- and (R)-trimethylserine esters 19, whereas ester-
ification of alcohol 18 with (R)-N,N,O-trimethylserine afforded only
(R)-ester 19.
(18) Hirata, K.; Muraoka, S.; Suenaga, K.; Kuroda, T.; Kato, K.;
Tanaka, H.; Yamamoto, M.; Takata, M.; Yamada, K.; Kigoshi, H. J.
Mol. Biol. 2006, 356, 945.
(19) Kabiramide C (20) is a cytotoxic marine macrolide, which binds to
actin at the 1,3-cleft, as well as aplyronine A (1), and shows actin-
depolymerizing activity: Klenchin, V. A.; Allingham, J. S.; King, R.;
Tanaka, J.; Maririott, G.; Rayment, I. Nat. Struct. Mol. Biol. 2003, 10,
1058.
Supporting Information Available. Experimental pro-
cedures and spectroscopic data for new compounds. This
material is available free of charge via the Internet at
http://pubs.acs.org.
(20) Kita, M.; Hirayama, Y.; Sugiyama, M.; Kigoshi, H. Angew.
Chem., Int. Ed. 2011, 50, 9871.
The authors declare no competing financial interest.
Org. Lett., Vol. 14, No. 5, 2012
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