Total synthesis of (-)-reidispongiolide A, an actin-targeting marine macrolide

Article abstract of DOI:10.1002/anie.200702178

(Chemical Equation Presented) Actin class: A stereoselective synthesis of the microfilament-destabilizing cytotoxic macrolide (-)-reidispongiolide A, isolated from the marine sponge Reidispongia coerulea, uses a convergent aldol-based strategy to construc

Full text of DOI:10.1002/anie.200702178

Angewandte
Chemie
DOI: 10.1002/anie.200702178
Natural Product Synthesis
Total Synthesis of (À)-Reidispongiolide A, an Actin-Targeting Marine
Macrolide**
Ian Paterson,* Kate Ashton, Robert Britton, Giuseppe Cecere, Galle Chouraqui,
Gordon J. Florence, and Jonathan Stafford
Reidispongiolide A (1), isolated by DꢀAuria et al. from the
sponge Reidispongia coerulea, which was collected off the
coast of New Caledonia,[1] and sphinxolide B (2)[2] are
representative members of a structurally unique family of
cytotoxic marine macrolides.[3] Their interaction with actin in
the cell cytoskeleton leads to microfilament destabilization,
and they show potent antiproliferative activity (for example,
IC50 = 0.04 and 0.01 mgmLÀ1 against human colon carcinoma
HT29) and the ability to circumvent multidrug resistance.
This profile makes these macrolides valuable molecular
probes in cell biology and promising lead compounds for
the development of novel chemotherapeutic agents that
target actin.[4,5] Their structures comprise a highly oxygenated
26-membered macrolactone, containing a d-lactone ring, and
appended with an elaborate side chain at C25 which
terminates in an N-vinylformamide group. Through the
combination of degradation fragment[2d,6,7] synthesis and
detailed NMR analysis, we determined the stereochemistry
of the entire reidispongiolide macrolide to be that shown in 1
(Scheme 1).[8] Subsequently, Rayment and co-workers[9]
reported the X-ray crystal structure of actin-bound reidis-
pongiolide A, assigning the complete configuration and
revealing its intriguing mechanism of microfilament destabi-
lization. The sparse natural supply of the reidispongiolides[1]
and sphinxolides[2] makes total synthesis of paramount
importance, not only to support further biological applica-
tions but also to enable structure–activity relationship (SAR)
studies. Herein, we report the first total synthesis of reidis-
pongiolide A based on a highly convergent modular assembly
process that evolved from our stereochemical analysis
groundwork.[7a,8]
C30–C36 side-chain segment 3, which incorporates the
sensitive N-vinylformamide functionality, through a suitable
aldol coupling with the macrolactone aldehyde 4. We elected
to disassemble the macrolide 4 into key subunits 5 (C14–C29)
and 6 (C4–C13) based on an envisaged second aldol coupling
to introduce the stereocenter at C13. The remaining subunit 7
As outlined in Scheme 1, our synthetic strategy for
reidispongiolide A involves a late-stage introduction of the
[*] Prof. Dr. I. Paterson, Dr. K. Ashton, Dr. R. Britton, G. Cecere,
Dr. G. Chouraqui, Dr. G. J. Florence, J. Stafford
University Chemical Laboratory
Lensfield Road, Cambridge, CB21EW (UK)
Fax: (+44)1223-336-362
E-mail: ip100@cam.ac.uk
Homepage: http://www-paterson.ch.cam.ac.uk/
[**] This work was supported by the EPSRC (GR/S19929/01), Emmanuel
College, Cambridge (Fellowship to G.J.F.), NSERC-Canada (Fellow-
ship to R.B.), Merck Research Laboratories (Scholarship to G.C.),
and AstraZeneca. We thank Professor M. V. D’Auria (University of
Naples “Federico II”) for providing an authentic sample of natural
redispongiolide A and helpful discussions throughout, and Dr. Ray
Finlay (AstraZeneca) for assistance.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Scheme 1. Synthetic strategy for reidispongiolide A (1) that involves
the key building blocks 3, 5, and 6.
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would then provide inherent flexibility whereby macrolide
the intermediate boron aldolate with LiBH4, afforded the 1,3-
syn diol 13 (93%, 6:1d.r.). Having installed all the required
stereochemistry of subunit 5, a chain extension to the enone
was now needed. Bis-O-methylation of 13 with NaH and MeI,
followed by selective cleavage of the TES ether gave alcohol
14. Dess–Martin oxidation of 14 gave the corresponding
aldehyde that was combined with dimethyl (2-oxopropyl)-
phosphonate to install the E-enone motif in 5 (90%, E:Z
> 20:1). By using this approach, the C14–C29 subunit 5 was
synthesized efficiently on a multigram scale in 13 steps and
20% overall yield from (S)-9.
2
2
À
formation might arise through sequential C(sp ) C(sp )
coupling and esterification of the hydroxy group at C25, in
either order. Another attractive aspect of this approach is that
the three subunits 3, 5, and 6 resemble the reidispongiolide
degradation fragments originally reported by DꢀAuria and co-
workers,[6] which we had initially targeted for determination
of the stereochemistry.[7a,8]
As shown in Scheme 2, the synthesis of the C14–C29
subunit 5 began with the construction of PMP-acetal 8,
Preparation of the C4–C13 subunit 6 (Scheme 3) began
with the opening of (S)-epoxide 15[14] with the lithium anion of
trimethylsilylacetylene in the presence of BF3·Et2O to afford
16 after desilylation. The conversion into methyl ether 17
(NaH, MeI) was followed by a Negishi carbometalation[15]
(Me3Al, [Cp2ZrCl2]) with an I2 quench to provide the (E)-
vinyl iodide 18 (91%). Desilylation of 18 using TBAF gave
Scheme 2. Synthesis of the C14–C29 subunit 5. a) DIBAL-H, TBME;
b) DMP, NaHCO3, CH2Cl2; c) (MeO)MeNC(O)CH2P(O)(OEt)2, LiCl,
Et3N, MeCN; d) DIBAL-H, THF, À788C; e) 1. (À)-Ipc2BCl, Et3N, Et2O,
08C; 2. 11, À788C; 3. LiBH4, À788C; f) NaH, MeI, THF, 08C to RT;
g) PPTS (cat.), MeOH; h) DMP, NaHCO3, CH2Cl2, 08C to RT;
i) MeC(O)CH2P(O)(OMe)2, LiCl, Et3N, MeCN. DIBAL-H=diisobutyl-
aluminum hydride, DMP=Dess–Martin Periodinane, Ipc=isopino-
campheyl, PMB=para-methoxybenzyl, PMP=para-methoxyphenyl,
PPTS=pyridinium para-toluenesulfonate, TBME=tert-butylmethyl
ether, TES=triethylsilyl, TIPS=triisopropylsilyl.
accessible in four steps from the ethyl ketone (S)-9[10] by using
our boron aldol methodology to install the required stereo-
chemistry, as reported previously.[7a] Regioselective DIBAL-
H opening of the acetal in 8 was best performed in TBME,
followed by Dess–Martin oxidation of the resulting primary
alcohol to give the corresponding aldehyde which underwent
olefination under LiCl/Et3N Horner–Wadsworth–Emmons
(HWE) conditions[11] to afford the Weinreb amide 10 (72%,
E:Z > 20:1). Reduction of 10 with DIBAL-H provided the
aldehyde 11 in readiness for a 1,4-syn boron aldol reac-
tion[7a,12] with the methyl ketone (S)-12.[10] Enolization of 12
with the matched chiral reagent (À)-Ipc2BCl and addition of
aldehyde 11 at À788C, followed by the in situ reduction[13] of
Scheme 3. Synthesis of the C4–C13 subunit 6. a) 1. nBuLi, HCCTMS,
BF3·Et2O, THF, À788C; 2. K2CO3, MeOH, RT; b) NaH, MeI, THF, 08C
to RT; c) 1. AlMe3, [Cp2ZrCl2], DCE, 608C; 2. I2, THF, À208C; d) TBAF,
THF; e) (COCl)2, DMSO, CH2Cl2; Et3N, À788C to RT; f) MeMgI, THF,
À78 to À508C; g) PCC, celite, CH2Cl2; h) 1. (À)-Ipc2BCl, Et3N, Et2O,
08C; 2. 21, À788C; i) 1. (EtO)2P(O)CH2CO2H, 2,4,6-Cl3(C6H2)COCl,
Et3N, DMAP, PhMe; 2. Ba(OH)2, wet THF; j) HF·py, py, THF, 08C to
RT; k) TEMPO, PhI(OAc)2, CH2Cl2. TBDPS=tert-butyldiphenylsilyl,
TMS=trimethylsilyl, DCE=1,2-dichloroethane, TBAF=tetrabutylam-
monium fluoride, PCC=pyridinium chlorochromate, DMAP=4-N,N’-
dimethylaminopyridine, DMSO=dimethylsulfoxide, py=pyridine,
TEMPO=4-amino-2,2,6,6-tetramethylpiperidine-1-oxyl.
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alcohol 19 (93%), which was converted into methyl ketone 20
(58%, 3 steps). Enolization of 20 with (À)-Ipc2BCl/Et3N,[12b]
followed by addition of aldehyde (R)-21 at À788C, provided
the separable adducts 22 and 23 in 43% and 27% yield,
respectively. In this demanding mismatched situation, the
combination of Felkin–Anh induction from the a-chiral
aldehyde 21 and Ipc-ligand influence imparted only moderate
levels of aldol stereocontrol in favor of 22, albeit overturning
the inherent 1,5-anti stereoinduction[8,16] arising from the b-
methoxy group in 20, which is directed towards formation of
the epimeric adduct 23. Treatment of 22 with diethylphos-
phonoacetic acid generated the corresponding ester[8] that
underwent Ba(OH)2-promoted HWE cyclization[17] to give
the dihydropyrone 24 (65%). Cleavage of the TBS ether with
HF·py and oxidation[18] of the resulting alcohol with TEMPO
and PhI(OAc)2 completed the C4–C13 subunit 6 in 11 steps
and 10% overall yield from (S)-15.
isomer (52%, Z:E 1.5:1). Iodine-mediated isomerization then
provided the desired (E)-alkenylformamide 3 cleanly (82%;
obtained as a mixture of rotamers by NMR analysis), which
completed the C30–C36 side-chain segment in 6 steps and
19% overall yield from (S)-25.
With the three key subunits in hand, attention was now
focused on their union, which began with a Mukaiyama aldol
reaction[22] of 5 and 6 to introduce the stereocenter at C13 that
relied on Felkin–Anh induction from the aldehyde
6
(Scheme 5). Enolization of methyl ketone 5 with LiHMDS
at À788C and trapping with TMSCl provided silyl enol ether
30. Exposure of 6 and 30 to BF3·Et2O in CH2Cl2 at À958C
provided the desired (13S)-adduct 31 preferentially (70%,
3:1d.r.). Evans–Saksena reduction [23] of 31 with Me4NBH-
(OAc)3 then gave the 1,3-anti diol 32 (95%, > 95:5 d.r.).
Conversion of 32 into the corresponding bismethyl ether 33[24]
with Me3O·BF4 (97%) enabled the chromatographic removal
of the minor diastereomer that arose from the Mukaiyama
aldol coupling. In preparation for macrocycle formation,
oxidative cleavage of the PMB ether at C25 in 33 with DDQ
then gave 34 (75%).
The synthesis of the remaining subunit 3 utilized our
lactate aldol chemistry[19] to install the anti relationship at
C32/C33 (Scheme 4). Thus, enolization of ethyl ketone (S)-
Installation of the C1–C4 linker subunit 7 was now
needed. Initial studies focused on the esterification of 34 with
7, but led to degradation and/or recovery of starting materials
under a variety of conditions. Gratifyingly, reversing the order
of this coupling sequence proved successful. In practice, the
Pd-mediated Stille coupling[25] of vinyl stannane 7 with the
vinyl iodide 34 installed the E,E diene in seco-acid 35, which
underwent Yamaguchi macrolactonization[26] to provide the
desired 26-membered macrocycle 36 (32% over 2 steps). At
1
this stage, the H and 13C NMR spectra of 36 showed good
correlations with the macrocyclic region of reidispongioli-
de A, further supporting our proposed stereostructure.
Cleavage of the TIPS ether at C29 with HF·py and Dess–
Martin oxidation of the resulting alcohol 37 provided the
macrocyclic aldehyde 4 (62%). Attention was now directed
on the challenging introduction of the full side chain of
reidispongiolide A. After careful optimization, it was found
that addition of 4 to the boron enolate of 3 (cHex2BCl/Et3N)
provided the aldol adduct 38 cleanly as a single isomer (70%),
attributable to 1,5-anti stereoinduction[16] from the b-methoxy
group in 3 as well as 1,4-syn induction.[27] Finally, dehydration
of this b-hydroxy ketone with the Burgess reagent[28] (88%),
followed by 1,4-reduction[29] of the intermediate E enone with
[{Ph3PCuH}6] gave reidispongiolide A (1) in 75% yield. All
spectroscopic data (1H and 13C NMR, IR, MS) for the
synthetic material were in excellent agreement with that
reported[1,30] for natural reidispongiolide A, and correlated
with an authentic sample, including HPLC comparison. As
reidispongiolide A has a relatively low magnitude of specific
rotation ([a]D = À10.0 (c = 0.02, MeOH) compared with
[a]D = À4.8 in Ref. [1]), we also carried out a chiroptical
correlation using circular dichroism spectra,[30] thereby con-
clusively defining the relative and absolute configuration.
In summary, we have completed a highly stereocontrolled
synthesis of reidispongiolide A that proceeds in 0.9% overall
yield with a longest linear sequence of 24 steps from 9, and
unequivocally established its relative and absolute configu-
ration as depicted in structure 1, in agreement with our earlier
Scheme 4. Synthesis of the C30–C36 side chain 3. a) 1. cHex2BCl,
=
Me2NEt, 08C; 2. H2C CHCH2CHO, À788C; b) Me3O·BF4, proton
sponge, CH2Cl2, 08C to RT; c) MeLi, Et2O, À78 to À208C; d) OsO4
(cat.), NaIO4, THF, H2O; e) LiHMDS, THF, À788C; f) I2 (cat.), CH2Cl2,
dark. Bz=benzoyl, cHex=cyclohexyl, LiHMDS=lithium hexamethyldi-
silazide.
25[19a,c] with cHex2BCl and Me2NEt followed by addition of
freshly prepared 3-butenal[20] provided the adduct 26 (91%,
> 20:1d.r.). Treatment of 26 with Me3O·BF4 afforded ketone
27 (92%), which underwent methyl addition (MeLi) with
concomitant benzoyl cleavage, followed by dihydroxylation
and in situ oxidative cleavage of both 1,2-diols to provide d-
ketoaldehyde 28 (60%). At this point, installation of the
sensitive N-vinylformamide functionality was required. By
using our Wittig protocol,[21] treatment of aldehyde 28 with
the ylide derived from phosphonium salt 29 (LiHMDS,
À788C) gave the (Z)-alkenylformamide as the predominant
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Scheme 5. Subunit assembly and completion of reidispongiolide A. a) LDA, TMSCl, Et3N, THF, À788C; b) BF3·Et2O, CaH2, CH2Cl2, À95 to À788C;
c) Me4NBH(OAc)3, MeCN, AcOH, À308C; d) Me3O·BF4, proton sponge, CH2Cl2, 08C; e) DDQ, pH 7 buffer, CH2Cl2; f) [Pd2(dba)3], iPr2NEt, NMP;
g) 1. 2,4,6-Cl3(C6H2)COCl, Et3N, PhMe; 2. DMAP; h) HF·py, py, THF; i) DMP, CH2Cl2; j) 1. cHex2BCl, Et3N, Et2O, 08C; 2. 4, À78 to 08C, 1 h;
k) Et3NSO2NCO2Me, THF; l) [{Ph3PCuH}6], PhMe, H2O. LDA=lithium diisopropylamide, DDQ=2,3-dichloro-5,6-dicyano-1,4-benzoquinone,
dba=dibenzylideneacetone, NMP=1-methyl-2-pyrrolidinone.
stereochemical analysis and the X-ray structure of the actin
[1] M. V. DꢀAuria, L. Gomez-Paloma, L. Minale, A. Zampella, J.-F.
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Verbist, C. Roussakis, C. Debitus, J. Patissou, Tetrahedron 1994,
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reidispongiolide/sphinxolide family of cytotoxic marine mac-
rolides. In combination with the available structural data on
actin-bound reidispongiolide and related marine macro-
lides,[5,9] the design of novel analogues with tailored functional
properties can now be envisaged.
[2] “The name sphinxolide, from the mysterious Egyptian Sphinx,
reflects our difficulties in defining the source and, for some time,
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Guella, I. Mancini, G. Chiasera, F. Pietra, Helv. Chim. Acta 1989,
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1993, 49, 8657; c) S. Carbonelli, A. Zampella, A, Randazzo, C.
Debitus, L. Gomez-Paloma, Tetrahedron 1999, 55, 1 4665; d) C.
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Received: May 17, 2007
Published online: July 6, 2007
[3] For reviews on marine macrolide synthesis, see: a) I. Paterson,
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[4] K.-S. Yeung, I. Paterson, Angew. Chem. 2002, 114, 4826; Angew.
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Keywords: actin · macrolides · natural products · polyketides ·
total synthesis
.
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[24] The stereochemical assignment of 33 and its minor 13,15-epi
diastereomer was aided by the related preparation of truncated
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C29 in 38 was assigned by analogy with the corresponding boron
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generated adduct IV (> 20:1d.r.), in which the configuration of
the resulting center that bears a hydroxy group was determined
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[30] Copies of NMR, CD spectra, and HPLC chromatograms for the
synthetic and natural reidispongiolide A, along with comparison
tables of 1H and 13C NMR data, are provided in the Supporting
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