Total synthesis of aplyronine C

Article abstract of DOI:10.1021/ol401327r

A highly stereocontrolled total synthesis of the cytotoxic marine macrolide aplyronine C is described. The route exploits aldol methodology to install the requisite stereochemistry and features a crucial boron-mediated aldol coupling of an N-vinylformamide-bearing methyl ketone with a macrocyclic aldehyde to introduce the full side chain. The synthesis of two novel C21-C34 side chain analogs is also reported.

Full text of DOI:10.1021/ol401327r

ORGANIC
LETTERS
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Vol. XX, No. XX
000–000
Total Synthesis of Aplyronine C
Ian Paterson,* Sarah J. Fink, Lydia Y. W. Lee, Stephen J. Atkinson, and
Simon B. Blakey
University Chemical Laboratory, University of Cambridge, Lensfield Road,
Cambridge CB2 1EW, U.K.
ip100@cam.ac.uk
Received May 13, 2013
ABSTRACT
A highly stereocontrolled total synthesis of the cytotoxic marine macrolide aplyronine C is described. The route exploits aldol methodology to
install the requisite stereochemistry and features a crucial boron-mediated aldol coupling of an N-vinylformamide-bearing methyl ketone with a
macrocyclic aldehyde to introduce the full side chain. The synthesis of two novel C21ꢀC34 side chain analogs is also reported.
Actin, the most common protein in eukaryotic cells, is
involved in numerous vital cellular functions including cell
shape maintenance, division, locomotion, and adhesion.1
Actin dynamics are normally tightly controlled by a number
of actin-binding proteins; dysregulation has been implicated
in diseases including stroke, cystic fibrosis, and cancer.1a,2
Aplyronines AꢀH3 (Figure 1) comprise a family of
actin-binding marine macrolides that can serve as small
molecule mimics of actin-binding proteins. They were
isolated in low yield (10ꢀ5ꢀ10ꢀ7% based on wet weight)
from the Japanese sea hare Aplysia kurodai by Yamada
and co-workers based on their potent cytotoxicity against
HeLa-S3 cells.4 Notably, the antiproliferative efficacy
of aplyronine A (1) has been demonstrated in vivo
against P388 leukemia (T/C 545%, 0.08 mg/kg) and Lewis
lung carcinoma (T/C 556%, 0.04 mg/kg), leading to its
identification as a promising anticancer drug candidate.5
Aplyronine A forms a 1:1 complex with globular actin (G-
actin, Kd = 100 nM), inhibiting polymerization, and also
depolymerizes fibrous actin (F-actin).6 X-ray analysis of
the actinꢀaplyronine A crystal structure,7 structureꢀ
activity relationship studies,8 and photoaffinity studies9
have highlighted the importance of the C24ꢀC34 tail
region in the strong actin depolymerizing activity. The
mechanism of cytotoxicity for these macrolides remains
unelucidated. Recent work has shown that aplyronine A
causes caspase-dependent apoptosis with rapid disassem-
bly of the actin cytoskeleton and dephosphorylation of
focal adhesion kinase10 and has suggested an interaction
between the aplyronineꢀactin complex and a secondary
biomolecule such as Arp2/3.11
(1) Reviews: (a) Allingham, J. S.; Klenchin, V. A.; Rayment, I. Cell
Mol. Life Sci. 2006, 63, 2119. (b) Yeung, K.ꢀS.; Paterson, I. Angew.
Chem., Int. Ed. 2002, 41, 4632.
(2) (a) Pantaloni, D.; le Clainche, C.; Carlier, M. F. Science 2001, 292,
1502. (b) Pollard, T. D.; Borisy, G. G. Cell 2003, 112, 453.
(3) (a) Yamada, K.; Ojika, M.; Ishigaki, T.; Yoshida, Y.; Ekimoto,
H.; Arakawa, M. J. Am. Chem. Soc. 1993, 115, 11020. (b) Yamada, K.;
Ojika, M.; Kigoshi, H.; Suenaga, K. In Drugs from the Sea; Fusetani, N.,
Ed.; Karger: Basel, Switzerland, 2000; pp 59ꢀ73. (c) Ojika, M.; Kigoshi,
H.; Suenaga, K.; Imamura, Y.; Yoshikawa, K.; Ishigaki, T.; Sakakura,
A.; Mutou, T.; Yamada, K. Tetrahedron 2012, 68, 982.
(4) Aplyronine A: IC50 = 0.45 nM; aplyronine C: IC50 = 22.4 nM;
aplyronine D: IC50 = 0.071 nM against HeLa-S3 cells (ref 3).
(5) Crews, P.; Gerwick, W. H.; Schmitz, F. J.; France, D.; Bair,
K. W.; Wright, A. E.; Hallock, Y. Pharm. Biol. 2003, 41, 39.
Figure 1. Structures of selected aplyronines.
r
10.1021/ol401327r
XXXX American Chemical Society

The aplyronineshaveelicitedsignificantinterestfortheir
potent antitumor activities and novel actin-binding proper-
ties, as well as their unique structures.11ꢀ16 Moreover, they
hold great potential for the development of biomolecular
probes and novel actin-targeting therapeutic agents. It
is a testament to the severe challenge presented by these
complex polyketides that, despite considerable effort
from several groups,12ꢀ16 only one prior total synthesis
of aplyronines AꢀC has been achieved, as reported by
Yamada and Kigoshi.16aꢀc
Scheme 1. Previous Work and Revised Strategy
In our previous work toward the aplyronines (Scheme 1),
we reported the synthesis of an advanced macrocyclic
intermediate 412a,b and β-ketophosphonate 512c in pursuit
of an aborted HWE fragment coupling strategy. Here, we
describe the synthesis of a more suitable aldol coupling
partner 6, enabling completion of the total synthesis of
aplyronine C (2) and providing access to some novel
C21ꢀC34 tail analogs.
Our revised strategy hinges on a key aldol fragment
coupling between the C1ꢀC27 aldehyde 7, derived from
previously synthesized macrocycle 4, with the (E)-N-
methyl-N-vinylformamide-bearing methyl ketone 6. The
strategic decision to incorporate the terminal (E)-vinylfor-
mamide into the coupling fragment was influenced by
our recent total syntheses of reidispongiolide A17 and
rhizopodin,18 and contrasts with more established ap-
proaches in which this sensitive moiety is introduced by
a testing, late-stage condensation reaction with a highly
functionalized C34 aldehyde.
The synthesis of the C28ꢀC34 methyl ketone 6 is outlined
in Scheme 2. It commenced with a Sn(II)-mediated aldol
reacton19 between known (R)-Roche ester-derived ketone
812a and acetaldehyde to generate the all syn aldol adduct 9
with high yield and selectivity (97%, 15:1 dr).20 A directed
1,3-anti reduction under EvansꢀTishchenko conditions21
set the C31 stereocenter and concomitantly capped the
C29 alcohol as the ester (10).22 Following silyl protection,
the C29 and C33 alcohols were revealed to produce diol 11.
Oxidation of diol 11 to the corresponding keto-aldehyde
required that the primary and secondary alcohols be
oxidized concurrently, lest intramolecular cyclization onto
the nascent aldehyde form an undesired hemiacetal. This
was achieved using a double Swern oxidation.23 Following
our Wittig protocol for the synthesis of N-methyl-N-
vinylformamides,24 the ylide of phosphonium salt 12
(LiHMDS) reacted selectively with the aldehyde to intro-
duce the N-vinylformamide terminus in 13, predominantly
in the (Z)-configuration (75% over two steps, 8:1 Z/E). Two
further steps introduced the C31 acetate moiety; isomeriza-
tion to (E)-vinylformamide 6then proceeded smoothly in the
presence of stoichiometric iodine under light-free condi-
tions.24 While all β-acetoxy ketone intermediates were prone
to elimination and required careful handling, introduction
of the C31 acetate moiety at this early stage proved
crucial. Attempted isomerization of fragments bearing a
C31-OTES (13) or -OPMB group failed to provide the
desired (E)-vinylformamide under a variety of conditions.
The C28ꢀC34 fragment 6 was thus accessed by an efficient
10 step sequence in 60% overall yield.
(6) Saito, S.; Watabe, S.; Ozaki, H.; Kigoshi, H.; Yamada, K.;
Fusetani, N.; Karaki, H. J. Biochem. (Tokyo) 1996, 120, 552.
(7) 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.
(8) Kigoshi, H.; Suenaga, K.; Takagi, M.; Akao, A.; Kanematsu, K.;
Kamei, N.; Okugawa, Y.; Yamada, K. Tetrahedron 2002, 58, 1075.
(9) Kuroda, T.; Suenaga, K.; Sakakura, A.; Handa, T.; Okamoto, K.;
Kigoshi, H. Bioconjugate Chem. 2006, 17, 524.
(10) (a) Kita, M.; Yoneda, K.; Hirayama, Y.; Yamagishi, K.; Saito,
Y.; Sugiyama, Y.; Miwa, Y.; Ohno, O.; Morita, M.; Suenaga, K.;
Kigoshi, H. ChemBioChem 2012, 13, 1754. (b) Ohno, O.; Morita, M.;
Kitamura, K.; Teruya, T.; Yoneda, K.; Kita, M.; Kigoshi, H.; Suenaga,
K. Bioorg. Med. Chem. Lett. 2013, 23, 1467.
(11) (a) Kita, M.; Hirayama, Y.; Sugiyama, M.; Kigoshi, H. Angew.
Chem., Int. Ed. 2011, 50, 9871. (b) Kita, M.; Hirayama, Y.; Yamagishi,
K.; Yoneda, K.; Fujisawa, R.; Kigoshi, H. J. Am. Chem. Soc. 2012, 134,
20314.
(12) (a) Paterson, I.; Cowden, C. J.; Woodrow, M. D. Tetrahedron
Lett. 1998, 39, 6037. (b) Paterson, I.; Woodrow, M. D.; Cowden, C. J.
Tetrahedron Lett. 1998, 39, 6041. (c) Paterson, I.; Blakey, S. B.; Cowden,
C. J. Tetrahedron Lett. 2002, 43, 6005.
(13) (a) Calter, M. A.; Guo, X. Tetrahedron 2002, 58, 7093. (b) Calter,
M. A.; Zhou, J. Tetrahedron Lett. 2004, 45, 4847.
(14) Marshall, J. A.; Johns, B. A. J. Org. Chem. 2000, 65, 1501.
(15) (a) El-Awa, A.; Fuchs, P. Org. Lett. 2006, 8, 2905. (b) Noshi,
M. N.; El-Awa, A.; Torres, E.; Fuchs, P. L. J. Am. Chem. Soc. 2007, 129,
11242. (c) Hong, W. P.; Noshi, M. N.; El-Awa, A.; Fuchs, P. L. Org.
Lett. 2011, 13, 6342.
(16) (a) Kigoshi, H.; Ojika, M.; Ishigaki, T.; Suenaga, K.; Mutou, T.;
Sakakura, A.; Ogawa, T.; Yamada, K. J. Am. Chem. Soc. 1994, 116,
7443. (b) Suenaga, K.; Ishigaki, T.; Sakakura, A.; Kigoshi, H.; Yamada,
K. Tetrahedron Lett. 1995, 36, 5053. (c) Kigoshi, H.; Suenaga, K.;
Mutou, T.; Ishigaki, T.; Atsumi, T.; Ishiwata, H.; Sakakura, A.; Ogawa,
T.; Ojika, M.; Yamada, K. J. Org. Chem. 1996, 61, 5326. (d) Kobayashi,
K.; Fujii, Y.; Hayakawa, I.; Kigoshi, H. Org. Lett. 2011, 13, 900.
(17) Paterson, I.; Ashton, K.; Britton, R.; Cecere, G.; Chouraqui, G.;
Florence, G. J.; Knust, H.; Stafford, J. Chem.;Asian J. 2008, 3, 367.
(18) Dalby, S. M.; Goodwin-Tindall, J.; Paterson, I. Angew. Chem.,
Int. Ed. 2013 in press.
(19) Paterson, I.; Tillyer, R. D. Tetrahedron Lett. 1992, 33, 4233.
(20) The C29 configuration was confirmed by Mosher ester analysis;
see the Supporting Information.
(21) Evans, D. A.; Hoveyda, A. H. J. Am. Chem. Soc. 1990, 112, 6447.
(22) The relative configuration was confirmed by NMR analysis of
the corresponding acetonide; see the Supporting Information.
(23) Omura, K.; Swern, D. Tetrahedron 1978, 34, 1651.
(24) Paterson, I.; Cowden, C. J.; Rahn, V. S.; Woodrow, M. D.
Synlett 1998, 915.
B
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Scheme 2. Synthesis of C28ꢀC34 Ketone 6
Scheme 3. Synthesis of C1ꢀC27 Aldehyde 7
Scheme 4. C21ꢀC34 Side Chain Analogs
Aldehyde 7 was readily accessible in four steps from our
previously reported macrocyclic intermediate 4 (Scheme 3).
Accordingly, global deprotection with aqueous HF was
followed by TES protection of the resultant tetraol. Selective
unveiling of the C27 primary alcohol under mild conditions
(THF/H2O/AcOH) and subsequent oxidation provided the
desired aldehyde 7.
With C1ꢀC27 aldehyde 7 and C28ꢀC34 ketone 6 in
hand, attention turned to assembly of the full aplyronine
backbone.25 Owing tothe complexityofthealdehyde 7 and
the acid and base sensitivity of methyl ketone 6, especially
mild conditions were required. Initially, we rehearsed this
crucial fragment coupling and endgame using 14 as a
truncated model for aldehyde 7.26 Boron-mediated aldol
coupling conditions proved uniquely effective, leading
to two novel C21ꢀC34 tail analogs for the aplyronines
(15 and 16; Scheme 4).27
Application of the boron aldol coupling conditions to
real aldehyde 7 successfully formed the desired C27ꢀC28
bond (Scheme 5).17 After careful optimization, we were
able to perform this delicate aldol coupling using
c-Hex2BCl/Et3N for enolization of 6 at ꢀ10 °C followed
by slow addition of the enolate (3 equiv) to aldehyde 7
at ꢀ78 °C. Following a mild, nonoxidative workup,
β-hydroxy ketone 17 was obtained in 61% yield with good
recovery of the excess ketone 6.
Deoxygenation at C27 was achieved using a two-step
procedure:17 the β-hydroxy ketone 17 was dehydrated with
the Burgess reagent (Et3NSO2NCO2Me)28 to the corre-
sponding enone, which was subsequently reduced in a 1,4-
sense to ketone 18 using Stryker’s reagent.29 Under these
conditions, elimination of the potentially labile β-acetoxy
ketone was minimized, and complete selectivity for reduc-
tion of the enone over the sensitive R,β,γ,δ-unsaturated
dienoate was obtained.
From this point, all that remained to obtain aplyronine
C (2) was stereoselective reduction of the C29 ketone,
installation of the appropriate C29 amino ester, and global
deprotection. Screening reduction conditions on the
model system26 revealed that ketone 19 exhibits moderate
inherent diastereoselectivity for the desired C29 epimer
(NaBH4, 2:1 dr; entry 1, Table 1). Attempts to enhance this
selectivity using bulky reducing agents (entries 2ꢀ3) led
(25) As outlined in Scheme 1, our synthetic plan had initially
anticipated forging the C27ꢀC28 bond via an HWE reaction between
aldehyde 7 and β-ketophosphonate 5. While MasamuneꢀRoush condi-
tions (LiCl, DBU) were successful on a model system (ref 12c), attempts
with real aldehyde 7 led only to β-elimination of the C25 silyl ether to
give the corresponding enal.
(26) Attempted formation of lithium enolates of such N-vinylforma-
mide-bearing ketones resulted only in decomposition. For full details of
the model fragment coupling and endgame, see the Supporting
Information.
(28) Burgess, E. M.; Penton, H. R.; Taylor, E. A. J. Org. Chem. 1973,
38, 26.
(29) Mahoney, W. S.; Brestensky, D. M.; Stryker, J. M. J. Am. Chem.
Soc. 1988, 110, 291.
(27) Perrins, R. D.; Cecere, G.; Paterson, I.; Marriott, G. Chem. Biol.
2008, 15, 287.
Org. Lett., Vol. XX, No. XX, XXXX
C

Scheme 5. Fragment Coupling
Scheme 6. Completion of Aplyronine C (2)
obtained with good selectivity by reduction with Zn(BH4)2
(90%, 10:1 dr). Esterification with (S)-N,N-dimethylalanine
under Keck conditions32 (DCC, DMAP, CSA), as prece-
dented by Yamada,16a was followed by global deprotection
using HF•py and pyridine to provide (þ)-aplyronine C (2).
To our satisfaction all 1H and 13C NMR spectroscopic data
for this synthetic material correlated with those reported for
natural aplyronine C.3a
Table 1. Model Studies of C29 Reduction
In conclusion, this highly stereocontrolled total synth-
esis of aplyronine C was completed in 28 steps (LLS) and
3.6% overall yield, by a route that is significantly shorter
than the previous synthesis (45 steps LLS).16b Alcohol 21
represents an advanced common intermediate from which
we can access other members of the aplyronine family.
Studies toward these congeners and other novel analogs
will be reported in due course.
entry
conditions
NaBH4, MeOH, rt
yield (%)
55
dr
1
2
3
4
5
2:1
ꢀ
a
L-selectride, THF, ꢀ78 °C
ꢀ
a
LiAlH(Ot-Bu)3, THF, ꢀ10 °C f rt
ꢀ
ꢀ
NaBH4, CeCl3 7H2O, MeOH, 0 °C
34
77
7:1
10:1
3
Zn(BH4)2, Et2O, 0 °C
Acknowledgment. This work was supported by the Dr.
Herchel Smith Fellowship from Williams College (S.J.F.),
the Croucher Foundation (L.Y.W.L.), the EPSRC (S.J.A.),
and the Commonwealth Trust (S.B.B.). Mass spectrometry
data were acquired at the EPSRC UK National Mass
Spectrometry Facility at Swansea University.
a Slow reaction rate, forming predominantly elimination and decom-
position products.
primarily to formation of elimination-related byproducts.
Luche conditions30 (entry 4) appeared more promising,
giving an improved 7:1 dr but a disappointingly low yield.
Ultimately, Zn(BH4)231 was found to provide the desired
alcohol 20 in 10:1 dr and 77% yield (entry 5).
Supporting Information Available. Experimental pro-
cedures, details of the model system, and full spectroscopic
data for all new compounds. This material is available free
of charge via the Internet at http://pubs.acs.org.
Pleasingly, this result transferred well to the real system
18 (Scheme 6). The desired diastereomer at C29 (21) was
(32) Boden, E. P.; Keck, G. E. J. Org. Chem. 1985, 50, 2394.
(30) (a) Luche, J. L. J. Am. Chem. Soc. 1978, 100, 2226. (b) Gemal,
A. L.; Luche, J. L. J. Am. Chem. Soc. 1981, 103, 5454.
(31) Oishi, T.; Nakata, T. Acc. Chem. Res. 1984, 17, 338.
The authors declare no competing financial interest.
D
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