Total synthesis and configurational assignment of (-)-dictyostatin, a microtubule-stabilizing macrolide of marine sponge origin

Article abstract of DOI:10.1002/anie.200460589

A flexible and modular approach was used in the convergent and highly stereocontrolled synthesis of the antimitotic agent dictyostatin. This first total synthesis establishes its full stereochemistry and should be amenable to producing useful quantities a

Full text of DOI:10.1002/anie.200460589

Angewandte
Chemie
Natural Products Synthesis
Total Synthesis and Configurational Assignment
of (ꢀ)-Dictyostatin, a Microtubule-Stabilizing
Macrolide of Marine Sponge Origin**
Ian Paterson,* Robert Britton, Oscar Delgado,
Arndt Meyer, and Karine G. Poullennec
Dictyostatin (1, Scheme 1) is a potent cytotoxic macrolide
first isolated by Pettit et al.[1] from a marine sponge of the
genus Spongia sp., collected in the Republic of Maldives, and,
subsequently, isolated by Wright and co-workers[2] from a
lithistida sponge of the family Corallistidae, harvested at great
depths off the Jamaican coast. The planar structure of this
unsaturated 22-membered macrolactone, featuring 11 stereo-
genic centers, an endocyclic 2Z,4E dienoate and a pendant Z-
diene moiety at C21, was deduced by the Pettit group,
primarily on the basis of 2D NMR spectroscopic data.[1]
Recently, we proposed a full stereochemical assignment for
dictyostatin, as indicated in 1, based on the use of extensive
high-field NMR experiments, including the Murata J-based
configuration analysis, in combination with molecular model-
ing.[3] Although it was not possible to ascertain the absolute
configuration, our assignment was based on dictyostatin being
biogenetically related to discodermolide.[4]
Notably, dictyostatin displays powerful growth-inhibitory
activity against a number of murine and human cancer cells at
low nanomolar concentrations and, importantly, retains
[*] Prof. Dr. I. Paterson, Dr. R. Britton, Dr. O. Delgado, A. Meyer,
Dr. K. G. Poullennec
University Chemical Laboratory
Lensfield Road, Cambridge, CB2 1EW (UK)
Fax: (+44)1223-336-362
E-mail: ip100@cam.ac.uk
[**] This research was supported by the EPSRC, the NSERC-Canada
(R.B.), the Gobierno de Canarias, the Cambridge European Trust
(O.D.), the EC (Network HPRN-CT-2000-18 and IE Marie Curie
Fellowship to K.G.P.), Novartis Pharma AG, and the Merck Research
Council. We thank Dr. Amy Wright (Harbor Branch Oceanographic
Institution) for helpful discussions and for providing an authentic
sample of dictyostatin, and Kate Ashton (Cambridge) for molecular
modeling assistance.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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DOI: 10.1002/anie.200460589
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Communications
Herein, we disclose the first total synthesis of dictyostatin (1)
and establish unequivocally the relative and absolute config-
uration of this rare polyketide metabolite. Furthermore, the
modular synthetic approach employed is flexible, highly
convergent, and stereocontrolled, and thus offers the poten-
tial to provide useful quantities of dictyostatin as well as a
range of structural derivatives to initiate structure–activity
relationship studies.
As outlined retrosynthetically in Scheme 1, our approach
relies on a late-stage reduction of the enone 2, controlled by
the macrocyclic conformation adopted by the 22-membered
ring; molecular modeling studies[8] indicated that hydride
addition should occur selectively to install the requisite C9
hydroxyl-bearing stereocenter. The 10Z alkene moiety in 2
was planned to arise from a Still–Gennari-type olefination
between the b-keto phosphonate 3 and aldehyde 4, in
conjunction with a Stille cross-coupling with vinyl stannane
5 to deliver the 2Z,4E dienoate. In principle, either of these
two reactions could be employed to close the macrocycle as
an alternative to a more conventional Yamaguchi macro-
lactonization, thus offering considerable flexibility in the
synthesis. In turn, the C11–C26 subunit 4 should be accessible
by a Horner–Wadsworth–Emmons (HWE) coupling between
aldehyde 6 and phosphonate 7, which contains the terminal
Z diene. As these latter segments share a common stereo-
chemical triad, it was anticipated that both 6 and 7 could be
prepared from common intermediate 8, which is available in
multigram quantities through efficient aldol methodology
reported previously by our group.[9]
Synthesis of the C11–C26 subunit 4, as shown in Scheme 2,
commenced with conversion of 1,3-diol 8 into bis-TBS ether 9
and liberation of the primary hydroxy group under mildly
acidic conditions. The resulting alcohol 10 was readily
converted into iodide 11 through a modified Mitsunobu
protocol (PPh3, I2, imid).[10] Subsequent alkylation of the
lithium enolate of the Myers propionamide 12[11] effected a
three-carbon homologation, delivering 13 with excellent
diastereoselectivity and yield (19:1 d.r., 88%).[12] Reductive
removal of the pseudoephedrine auxiliary (LiNH2BH3)[11]
afforded a primary alcohol, which was, in turn, oxidized
with Dess–Martin periodinane to provide aldehyde 6. The
HWE coupling partner 7 was synthesized by addition of
(MeO)2P(O)CH2Li to aldehyde 14, which was prepared from
1,3-diol 8 as described previously.[9,13] The resulting epimeric
mixture of alcohols was converted into b-ketophosphonate 7
(Dess–Martin periodinane, 83% over two steps). Following a
HWE coupling between aldehyde 6 and phosphonate 7,
employing Ba(OH)2 in wet THF, enone 15 was isolated in
92% yield.[14] With the entire C11–C26 backbone assembled,
we next focused on the stereocontrolled reduction of the C19
ketone directed by the proximal hydroxy-bearing stereo-
center through metal chelation. To this end, conjugate
reduction of enone 15 with the Stryker reagent
{[Ph3PCuH]6}[15] and oxidative removal of both PMB groups
with DDQ provided the intermediate b-hydroxyketone,
which underwent 1,3-syn-selective reduction when treated
with a cold (ꢀ308C) ethereal solution of Zn(BH4)2,[16]
generating triol 16 (> 20:1 d.r.).[17] Finally, the synthesis of
subunit 4 was completed by a three-step sequence involving
Scheme 1. Retrosynthetic analysis for dictyostatin (1) leading to key
building blocks 3, 6, and 7.
activity against taxol-resistant cancer cells that express active
P-glycoprotein. Preliminary studies into the mechanism of
action demonstrated that dictyostatin, in a similar manner to
taxol (paclitaxel), arrests cells in the G2/Mphase by potently
inducing tubulin polymerization and suppressing normal
microtubule dynamics, leading to apoptosis.[2] Hence, dictyo-
statin currently represents a promising antimitotic natural
product lead for development in cancer chemotherapy,
joining an elite group of microtubule-stabilizing polyketides
that include laulimalide,[5] peloruside A,[6] and discodermo-
lide.[7] Unfortunately, evaluation of its antitumor properties
has been precluded so far by the very low natural abundance.
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investigate the mild Still–Gennari modification of the HWE
olefination.[19] The requisite F3CCH2O-substituted phospho-
nate 3 was prepared from alcohol 19, which was obtained with
95% ee and 20:1 d.r. through a Brown asymmetric crotylation
of aldehyde 20 (Scheme 3).[20] Silyl ether formation
Scheme 3. Synthesis of C4–C10 subunit 3. a) TBSOTf, 2,6-lutidine,
CH2Cl2, ꢀ788C, 30 min; b) 1) O3, CH2Cl2, ꢀ788C; 2) Ph3P, ꢀ788C!
RT, 2 h; c) CrCl2, CHI3, THF, dioxane, 08C, 18 h; d) TBAF, AcOH, THF,
room temperature, 14 h; e) DMP, py, CH2Cl2, room temperature, 3 h;
f) NaClO2, NaH2PO4, 10:1 tBuOH/H2O, 08C, 2 h;
g) 1) Me2C=C(Cl)NMe2, CH2Cl2, room temperature, 15 min;
2) (F3CCH2O)2P(O)CH2Li, THF, ꢀ1008C, 1 h.
(TBSOTf), ozonolysis, and Takai methylenation[21] of the
intermediate aldehyde provided E vinyl iodide 21 in 71%
yield. After selective cleavage of the primary silyl ether,
oxidation of the resulting alcohol afforded acid 22, which was
transformed into its acid chloride by using the Ghosez reagent
(Me2C = C(Cl)NMe2).[22] Next, the acid chloride was added to
a solution of (F3CCH2O)2P(O)CH2Li in THF,[23] generated at
ꢀ1008C, to afford phosphonate 3 containing the required
functionality for the pivotal fragment assembly.
To our delight, when a mixture of aldehyde 4 and
phosphonate 3 was subjected to an excess of K2CO3 in the
presence of [18]crown-6 in toluene (Scheme 4),[19] the HWE
coupling proceeded smoothly to produce Z enone 23 effi-
ciently, with good levels of selectivity (Z/E = 5:1, 77%).
Notably, this constitutes one of the first examples of a Z-
selective intermolecular Still–Gennari olefination employing
such an elaborate b-ketophosphonate. Construction of the
dictyostatin carbon backbone was completed through a
copper-mediated, Liebeskind-type Stille coupling[24] (CuTC
in NMP) of vinyl iodide 23 and Z alkenyl stannane 5, which is
readily accessible from known acid 24.[25] Finally, TIPS ester
25 was converted into acid 26 (KF, THF/MeOH) in 83%
overall yield.
Scheme 2. Synthesis of C11–C26 subunit 4. a) TBSOTf, 2,6-lutidine,
CH2Cl2, 08C!RT, 15 min; b) p-TSA (30 mol%), 20:1 THF/H2O, 24 h;
c) PPh3, I2, imid, PhMe, 08C, 15 min; d) 1) 12, LiCl, LDA, THF,
ꢀ788C!RT; 2) 11, THF, 08C!RT, 18 h; e) 1) LDA, BH3NH3, THF,
08C; 2) 13, THF, 08C!RT, 18 h; f) DMP, NaHCO3, CH2Cl2, 08C, 2 h;
g) 1) nBuLi, (MeO)2P(O)Me, THF, ꢀ788C, 30 min; 2) 14, THF, ꢀ788C
; h) DMP, CH2Cl2, room temperature, 10 min; i) 1) 7, Ba(OH)2·8H2O,
THF, 1 h; 2) 6, 40:1 THF/H2O, 20 h; j) [Ph3PCuH]6, PhH, H2O, room
temperature, 2 h; k) DDQ, pH 7 buffer, CH2Cl2, 08C, 6 h; l) Zn(BH4)2,
Et2O, ꢀ308C, 2 h; m) TBSOTf, 2,6-lutidine, ꢀ78!08C, 30 min;
n) TBAF, AcOH, THF, room temperature, 24 h; o) TEMPO, PhI(OAc)2,
CH2Cl2, room temperature, 18 h. TBS=tert-butyldimethylsilyl, Tf=tri-
fluoromethanesulfonyl, p-TSA=p-toluenesulfonic acid, imid=imida-
zole, LDA=lithium diisopropylamide, DMP=Dess–Martin periodi-
nane, DDQ=2,3-dichloro-5,6-dicyano-1,4-benzoquinone, TBAF=tetra-
butylammonium fluoride, TEMPO=2,2,6,6-tetramethyl-1-piperidinyl-
oxy, free radical.
selective TBS protection of the C11 and C19 hydroxy groups
over that at C21 (TBSOTf), cleavage (TBAF, AcOH) of the
primary TBS ether in 17, and oxidation of resulting alcohol 18
(TEMPO/PhI(OAc)2).[18]
As our primary objective was an efficient and convergent
synthesis of dictyostatin, it was crucial to install the 10Z al-
kene function by employing advanced coupling partners.
Although a number of protocols exist to effect such coupling,
most require strongly basic conditions. Our concerns regard-
ing the epimerizable center at C12 in aldehyde 4 led us to
With 26 in hand, attention was now directed towards the
completion of the synthesis, as shown in Scheme 5. Seco acid
26 was subjected to a standard Yamaguchi macrolactonization
protocol (2,4,6-trichlorobenzoyl chloride, NEt3, DMAP,
PhMe, 608C, 2 h) to furnish the 22-membered macrocycle 2
cleanly in 77% yield.[26] Next, we investigated the reduction of
the C9 ketone, in a bid to exploit macrocyclic stereocontrol.[8]
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Gratifyingly, Luche reduction of enone 2 (NaBH4, CeCl3,
EtOH)[27] provided the expected alcohol 27 (70%). Finally,
global deprotection with HCl (3n) in methanol gave dictyo-
statin (1) in 87% yield. The spectroscopic data of our
synthetic material were in complete agreement with those
of an authentic sample obtained from the Corallistidae sponge
source (1H (700 MHz), 13C NMR (125 MHz), MS), and the
specific rotation of [a]D = ꢀ32.7 (c = 0.22, MeOH) agreed
with that measured in our laboratory for the authentic
sample,[28] thus allowing confident assignment of the relative
and absolute configuration of dictyostatin and validating our
earlier proposal.[3a]
Significantly, this configurational assignment for dictyo-
statin is fully consistent with a common biogenesis for the
structurally related, but open-chain, polyketide discodermo-
lide (28, Figure 1), presently in clinical development as a new-
Scheme 4. Synthesis of seco acid 26. a) K2CO3 (10 equiv), [18]crown-6
(25 equiv), toluene, room temperature, 48 h; b) CuTC, NMP, room
temperature, 1 h; c) KF, THF, MeOH, room temperature, 2 h;
d) TIPSCl, Et3N, CH2Cl2, room temperature, 45 min. CuTC=copper(i)
thiophenecarboxylate, NMP=1-methyl-2-pyrrolidinone, TIPS=triiso-
propylsilyl.
Figure 1. Overlay of preferred solution conformation[3a] of dictyostatin
(in yellow, C1/C2 s-trans rotamer) in water with conformation of disco-
dermolide (28) obtained from the X-ray crystal structure (in blue).
generation microtubule-stabilizing anticancer agent.[29] There
is an exact configurational match of the C19–C26 and C6–C14
regions of dictyostatin with those at C17–C24 (shown in red)
and C4–C12 (shown in blue) of discodermolide, respectively.
As shown by the overlay in Figure 1, the preferred confor-
mation for dictyostatin in solution[3a] resembles closely that
determined for discodermolide in the solid state, suggesting
that they may interact in a similar fashion with the taxol-
binding site on b-tubulin.[30] As a conformationally con-
strained macrocyclic analogue of discodermolide, dictyostatin
represents a highly attractive template for rational drug
design.
Scheme 5. Completion of the synthesis of dictyostatin (1). a) 2,4,6-tri-
chlorobenzoyl chloride, NEt3, DMAP, PhMe, 608C, 2 h; b) NaBH4,
CeCl3·7H2O, EtOH, ꢀ308C; c) HCl (3n), MeOH, room temperature,
6 h. DMAP=4-dimethylaminopyridine.
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[12] The 1,3-syn stereochemistry of amide 13 was confirmed by NOE
In conclusion, this expedient total synthesis of (ꢀ)-
dictyostatin proceeds in 27 steps and 3.8% overall yield
(longest linear sequence from the Roche ester[9]) and
establishes unequivocally the relative and absolute configu-
ration. Key transformations include an unprecedented Still–
Gennari-type HWE coupling with a complex b-ketophosph-
onate, a Liebeskind-type Stille cross-coupling to install the
2Z,4E dienoate, a facile Yamaguchi macrolactonization to
construct the macrocycle, and subsequent reduction of the C9
ketone under macrocyclic stereocontrol. Importantly, this
modular, convergent synthesis should be amenable to pro-
ducing useful quantities and designed analogues of this novel
microtubule-stabilizing agent, enabling extensive exploration
of its anticancer properties.
enhancements on lactone B, obtained by a two-step sequence
(HCl (2n), EtOH; TEMPO, PhI(OAc)2) from alcohol A.
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Received: May 7, 2004
Published Online: July 1, 2004
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[17] The relative stereochemistry of 16 was confirmed unequivocally
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[18] A. De Mico, R. Margarita, L. Parlanti, A. Vescovi, G. Piancatelli,
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Keywords: antitumor agents · conformation analysis ·
macrocycles · natural products · total synthesis
.
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[4] The structural similarities between discodermolide and dictyos-
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whereas that reported by Pettit et al.[1] was ꢀ20 (c = 0.12,
MeOH). For copies of 1H and 13C NMR spectra, see the
Supporting Information.
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[30] Note added in proof: Preliminary biological evaluation suggests
that dictyostatin is also a ligand for the taxoid binding site on
b.tutublin and binds more strongly than taxol. We thank
Dr. Fernando Dꢀaz and Ruben M. Buey (Centro de Investiga-
ciones Biolꢁgicas, CSIC, Madrid) for carrying out this study with
synthetic dictyostatin.
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[9] 1,3-Diol 8 was prepared on a multigram scale in five steps and
52% yield from methyl (S)-3-hydroxy-2-methylpropionate
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