Total synthesis of rhizopodin

Article abstract of DOI:10.1002/anie.201201946

Symmetry helps: The total synthesis of the potent actin-targeting C ₂-symmetric myxobacterial macrolide rhizopodin (see scheme) is accomplished by the convergent assembly of three building blocks of similar complexity, a concise macrocyclization strategy, and a late-stage introduction of the labile side chains. Copyright

Full text of DOI:10.1002/anie.201201946

Angewandte
Chemie
DOI: 10.1002/anie.201201946
Natural Products Synthesis
Total Synthesis of Rhizopodin**
Michael Dieckmann, Manuel Kretschmer, Pengfei Li, Sven Rudolph, Daniel Herkommer, and
Dirk Menche*
Dedicated to Professor Gerhard Bringmann on the occasion of his 60th birthday
Rhizopodin (1, Scheme 1) is an architecturally unique poly-
ketide macrolide that was originally isolated from the
myxobacterium Myxococcus stipitatus by the groups of
Hçfle and Reichenbach.[1,2] It shows potent antifungal and
antiproliferative cytotoxicity against a range of tumor cell
lines with IC50 values in the low nanomolar range.[1] On
a molecular level, 1 disrupts the cytoskeleton of actin by
binding specifically to a few critical sites of G-actin.[3] Actin
presents one of the two major components of the cytoskeleton
in eukaryotic cells, in which it plays a critical role in the
determination of cell shape and a variety of cellular processes,
including cell motility, division, adhesion, and intracellular
transportation.[4] Actin-binding agents are becoming increas-
ingly important as molecular probes to understand the
organization and unveil the cellular functions of actin, and
as potential lead structures for the development of novel
chemotherapeutic anticancer agents.[4] Rhizopodin has also
been shown to reduce phagocytosis efficiency of yeast cells.[5]
While rhizopodin was initially reported to be a planar
monomeric structure,[1,3] its revised structure was reported
to be a C2-symmetric dimer, which is distinguished by a 38-
membered macrolide ring, two conjugated diene systems in
combination with two disubstituted oxazole systems, and two
side chains, which are terminated by N-vinylformamide
Scheme 1. Retrosynthetic analysis of rhizopodin. HWE=Horner–Wads-
worth–Emmons reaction, TBS=tert-butyldimethylsilyl, TES=triethyl-
silyl.
moieties.[6] In total, it contains 18 stereogenic centers in the
[*] Dipl.-Chem. M. Dieckmann,[+] Dipl.-Chem. M. Kretschmer,[+]
Dr. P. Li,[$] Dr. S. Rudolph[#]
carbon backbone. The full stereochemistry was independently
assigned by our group through high-field NMR experiments
in combination with molecular modeling and chemical
derivatization,[7] and by Schubert and co-workers with an X-
ray structure of an actin–rhizopodin complex.[8] The impor-
tant biological properties of rhizopodin and its rare occur-
rence in nature, together with its unique and intriguing
molecular architecture, renders this natural product an
attractive synthetic target, and several syntheses of fragments
have been reported,[9] including the preparation of the
originally proposed monomeric structure.[9d] Herein, we
present the first total synthesis of rhizopodin and unequiv-
ocally confirm its relative and absolute configuration.
Scheme 1 outlines our retrosynthetic analysis. The sym-
metry of the molecule allows the indicated double discon-
nections and the adoption of a highly convergent synthetic
plan by using three building blocks of similar complexity, that
is, the macrocyclic fragments 2 and 3, and the side chain 4. In
detail, sequential disconnection at the C6–C8 diene linkages
implies a cross-coupling toward a macrocycle and a conven-
Universitꢀt Heidelberg, Organisch-Chemisches Institut
INF 270, 69120 Heidelberg (Germany)
Dipl.-Chem. D. Herkommer, Prof. Dr. D. Menche
Kekulꢁ-Institut fꢂr Organische Chemie und
Biochemie der Universitꢀt Bonn
Gerhard-Domagk-Str. 1, 53121 Bonn (Germany)
E-mail: dirk.menche@uni-bonn.de
[$] Present address: Xi’an Jiaotong University, Xi’an, Shaanxi Province
(PR China)
[#] Present address: ASM Research Chemicals, Hannover (Germany)
[+] These authors contributed equally to this work.
[**] We thank the DFG (Me 2756/4-1), the DFG Graduiertenkolleg 850
(scholarship to M.K. and M.D.), the “Fonds der Chemischen
Industrie”, and the “Wild-Stiftung” for generous funding, and
Wiebke Ahlbrecht, Johannes Troendlin, Carolin Lang, and Nathalie-
Desirꢁe Costa Pinheiro for exploratory studies. Excellent assistance
with NMR analyses by Prof. Markus Enders and Beate Termin is
gratefully acknowledged. We thank Prof. Gꢂnter Helmchen for
helpful discussions and assistance with HPLC analyses.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201201946.
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tional cross-coupling as late steps in the synthesis. Two
esterification reactions, in turn, point to two common
precursors, a C1–C7 fragment 2 and a C8–C22 segment 3,
for both halves of the macrocycle of the target molecule.
Finally, introduction of the C23–C30 side-chain segment 4 was
planned through a suitable HWE-coupling/hydrogenation
sequence. In principle, several methodologies may be
employed for ring closure, thus offering considerable flexi-
bility in the synthesis. Importantly, the modular synthetic
approach employed is highly convergent and concise, and
thus offers the potential to provide a range of structural
derivatives for structure–activity relationship studies to
further explore the biological potential of this promising
macrolide antibiotic.
commercially available lactone 9 already features the
required hydroxy-bearing stereogenic center at C16 and was
thus transformed into the TBS-protected lactol 10, which was
functionalized to alkene 11 (3 steps, 72%) by a Peterson-
olefination sequence.[14] Oxidation of alkene 11 to acid 12
proceeded smoothly in three steps. This route proved superior
to a shorter sequence that involved an asymmetric allylation
of an aldehyde derived from 2,2-dimethyl-1,3-propandiol,
both in terms of price and scalability.[15] With multigram
quantities of 12 and 14 in hand, we proceeded with their
transformation to 15 under slightly modified Wipf condi-
Construction of vinyl iodide 2 started from d-malic acid
derived ester 5,[10] which was methylated and homologated by
a Claisen condensation with lithio tert-butyl acetate[11] to give
the corresponding b-keto ester together with minor amounts
of the tertiary alcohol arising from double addition
(Scheme 2). Asymmetric hydrogenation in the presence of
Scheme 2. Synthesis of the C1–C7 subunit 2. BINAP=2,2’-bis(diphe-
nylphosphino)-1,1’-binaphthyl, CSA=campher sulfonic acid,
DMP=Dess–Martin periodinane, LDA=lithium diisopropylamide,
Tf =trifluoromethanesulfonyl, TMS=trimethylsilyl.
Noyoriꢀs catalyst[12] gave the b-hydroxy ketone 6, in which the
TBS protecting groups was also removed, with excellent
diastereoselectivity (> 20:1). Selective protection of the
secondary alcohol was effected in two steps, which involved
double protection and subsequent liberation of the primary
alcohol 7. Homologation to building block 2 was achieved by
oxidation to the respective aldehyde, introduction of the vinyl
iodide by a Takai reaction[13] (89%, E/Z 7:1), and TMSOTf-
mediated cleavage of the ester (3 steps, 84%). The corre-
sponding methyl ester 8, which was required for selective
coupling, was readily available upon exposure to TMSCHN2.
For the construction of the C8–C22 subunit 3, we had
previously reported a route,[9e] which however proved to be
problematic during scale-up. Consequently, we developed an
alternative and more convergent strategy which relied on the
coupling of two segments of similar complexity, that is, our
previously described C8–C13 fragment 14 (derived from N-
Boc-d-serine (13) in 5 steps)[9e] and the C14–C18 fragment 12,
which arises from (À)-pantolactone (9, Scheme 3). In detail,
Scheme 3. Preparation of the C8–C22 subunit 3. 9-BBN=9-bora-
bicyclo[3.3.1]nonane, Boc=tert-butoxycarbonyl, DEPBT=3-(diethoxy-
phosphoryloxy)-1,2,3-benzotria-zin-4(3H)-one, DIBAlH=diisobutyl-
aluminium hydride, Cy=cyclohexyl, IBX=ortho-iodoxybenzoic acid,
LiHMDS=lithium hexamethyldisilazanide, PCC=pyridinium chloro-
chromate, pin=pinacolato, PMB=para-methoxybenzyl.
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tions.[16] This conversion involved amide formation with
3-(diethoxyphosphoryloxy)-1,2,3-benzotriazin-4(3H)-one
(DEPBT) and subsequent oxidation/cyclodehydration with
IBX and I2/PPh3, respectively, to afford 15 in good yield. Next,
the primary alcohol was liberated selectively and oxidized to
aldehyde 16, thus setting the stage for an aldol coupling with
methyl ketone 17. All attempts to install the stereocenter at
C18 selectively were however unsuccessful (Mukaiyama and
various boron-mediated aldol reactions, among others). After
extensive optimization, we found that use of LiHMDS as base
resulted in a mixture of the chromatographically separable
diastereomeric alcohols 18 and (18-epi)-18 (d.r. 1:1.7) in
excellent yield. Both products could be transformed into
building block 20 in a stereoconvergent manner, by a 1,3-anti
reduction[17] and methylation (18), and by a 1,3-syn-reduc-
tion,[18] methylation, and inversion of the configuration at
C-18 [(18-epi)-18], respectively. After introduction of the
terminal TBS ether (2 steps, 89%),[19] the required boronate 3
was obtained from 21 by cross-metathesis reaction with vinyl
boronate 19 in very good yield in the presence of the
Hoveyda-Grubbs II catalyst.[20]
Our synthesis of the side-chain fragment 4 was based on
an anti-aldol reaction of Masamuneꢀs chiral ephedrine-
derived ethyl ketone 22[21] with aldehyde 23, giving alcohol
24 in high yield and good selectivity (Scheme 4). After
methylation, the transformation to phosphonate 26 proved
Scheme 4. Assembly of the C23–C29 subunit 4. Bn=benzyl,
Mes=2,4,6-trimethylphenyl, PDC=pyridinium dichromate.
Scheme 5. Completion of the total synthesis. dppf=diphenylphosphi-
noferrocene, TCBC=2,4,6-trichlorobenzoyl chloride, TBAF=tetra-n-
butyl ammonium fluoride.
most reliable by using a four-step sequence, which relied on
the initial reductive removal of the auxiliary with LiAlH4 and
oxidation of the resulting primary alcohol with DMP.[22] After
addition of lithiated phosphonate 25, the intermediary b-
hydroxyphosphonate was oxidized with PDC to furnish 26 in
acceptable yield (37%) over five steps. Finally, liberation of
the primary alcohol and re-protection as the TES ether gave
the desired side-chain building block 4 in good yield.[23]
In a rationale to install the labile side chain, which
contains the sensitive N-vinylformamide functionality, in
a late stage of the synthesis, our strategy to unite the
fragments relied on closing the macrocyclic core first
(Scheme 5). After coupling of 3 and 8 by Suzuki reaction[24]
under the conditions described by Gopalarathnam and
Nelson,[25] the derived diene 27 was transformed to 28. To
this end, it was esterified with 2 using a Yamaguchi proce-
dure,[26] before the resulting terminal methyl ester was cleaved
selectively and another Yamaguchi esterification with 8 gave
28 in good yields. Finally, Suzuki macrocyclization proceeded
in 65% yield, thus giving the desired C2-symmetric macro-
cyclic core. Concomitant cleavage of the two terminal TBS
ethers was best effected with HCl/MeOH and gave diol 29.
Efforts were then directed to the challenging introduction of
the two full side chains of rhizopodin in a bidirectional
manner. Ultimately, the successful strategy involved
a Ba(OH)2-mediated coupling of the phosphonate 4 with
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the bis-aldehyde derived from 29,[27,28] followed by 1,4-
reduction of the intermediate enone with Strykerꢀs
reagent[29,30] and selective cleavage of the primary TES
ethers to furnish 30. For the introduction of the sensitive N-
vinylformamide units, we eventually found that 30 was best
oxidized to the respective aldehyde with DMP before
reaction with HNMe(CHO) and P2O5.[31] Since purification
of the resulting fully protected natural product was tedious,
the crude product was directly exposed to TBAF, thus giving
rhizopodin (1) in 14% yield over three steps after workup and
purification by reverse-phase HPLC. The spectroscopic data
(1H NMR, 13C NMR) and specific rotation of our synthetic
material were in agreement with those published for an
isolated sample of rhizopodin, thus confirming the relative
and absolute configuration of rhizopodin.
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[15] Asymmetric Brown allylation of an aldehyde derived from
mono-PMB-protected 2,2-dimethyl-1,3-propandiol proceeded in
small scale in good selectivity (92% ee) and yield (71%). On
large scale, removal of the chiral iso-pinocamphenyl ligand
proved difficult.
[16] P. Wipf, C. P. Miller, J. Org. Chem. 1993, 58, 3604.
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In conclusion, this first total synthesis of rhizopodin
proceeds in 31 steps (longest linear sequence) and confirms
unambiguously the relative and absolute configuration that
was determined through our earlier stereochemical analysis[7]
and with the X-ray structure of the actin complex reported by
Schubert and co-workers.[8] Notable features include a con-
vergent preparation of the oxazole rings from advanced
fragments, a concise assembly of the macrocyclic core,
involving a Suzuki macrocyclization of a highly elaborated
substrate and a late stage introduction of the labile side chains
in a bidirectional manner based on an HWE-coupling/hydro-
genation sequence. In combination with the available struc-
tural data on actin-bound rhizopodin[8] and related macro-
lides,[4] the design of novel analogues with improved func-
tional properties and/or simplified core structures can now be
envisaged.
[18] I. Paterson, M. Donghi, K. Gerlach, Angew. Chem. 2000, 112,
3453; Angew. Chem. Int. Ed. 2000, 39, 3315.
[19] This sequence was necessary, because attempts to disconnect the
PMB group from the fully formed macrocycle only led to
complete decomposition. We tracked this behavior back to
a comparable reactivity of the diene units. However, when the
TBS-protected congener of 17 was used in the respective aldol
reaction, separation of the TBS-protected analogues of 18
proved impossible.
[20] C. Morrill, R. H. Grubbs, J. Org. Chem. 2003, 68, 6031.
[21] A. Abiko, J.-F. Liu, S. Masamune, J. Am. Chem. Soc. 1997, 119,
2586.
Received: March 13, 2012
Published online: && &&, &&&&
[22] Attempts to remove the auxiliary from 24 directly with lithiated
25 were unsuccessful in this case, see: J. Li, D. Menche, Synlett
2009, 2417.
[23] The TES-protected analogue of 23 was also employed as starting
material to avoid the transformation of 26 to 4. It turned out,
however, that the yield in the first aldol reaction was significantly
lower.
[24] N. Miyaura, A. Suzuki, Chem. Rev. 1995, 95, 2457.
[25] A. Gopalarathnam, S. G. Nelson, Org. Lett. 2006, 8, 7.
[26] J. Inanaga, K. Hirata, H. Saeki, T. Katsuki, M. Yamaguchi, Bull.
Chem. Soc. Jpn. 1979, 52, 1989.
[27] I. Paterson, K.-S. Yeung, J. B. Smaill, Synlett 1993, 774.
[28] Alternatively, the fully formed side chain incorporating the
sensitive N-vinylformamide may be introduced by a boron-
mediated aldol reaction of a methyl ketone equivalent to 4. This
sequence, however, proved less reliable, see: I. Paterson, K.
Ashton, R. Britton, G. Cecere, G. Chouraqui, G. J. Florence, J.
Stafford, Angew. Chem. 2007, 119, 6279; Angew. Chem. Int. Ed.
2007, 46, 6167.
Keywords: actin · macrolides · natural products · polyketides ·
total synthesis
.
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4
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Angew. Chem. Int. Ed. 2012, 51, 1 – 5
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Angewandte
Chemie
Communications
Natural Products Synthesis
Symmetry helps: The total synthesis of
the potent actin-targeting C2-symmetric
myxobacterial macrolide rhizopodin (see
scheme) is accomplished by the conver-
gent assembly of three building blocks of
similar complexity, a concise macrocycli-
zation strategy, and a late-stage intro-
duction of the labile side chains.
M. Dieckmann, M. Kretschmer, P. Li,
S. Rudolph, D. Herkommer,
D. Menche*
&&&& — &&&&
Total Synthesis of Rhizopodin
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These are not the final page numbers!