Total synthesis and stereochemical assignment of (-)-ushikulide A

Article abstract of DOI:10.1021/ja906056v

We report the determination of the full stereostructure of (-)-ushikulide A (1), a spiroketal containing macrolide by total synthesis. Ushikulide A (1) was isolated from a culture broth of Streptomyces sp. IUK-102 and exhibits potent immunosuppressant activity (IC₅₀ = 70 nM). To embark upon an ushikulide A synthesis, a tentative assignment was made based on analogy to cytovaricin (2), a related macrolide isolated from a culture of Streptomyces diastatochromogenes whose full structure was previously established via synthesis and X-ray crystallography. This report delineates studies on several key steps, namely a direct aldol reaction catalyzed by the dinuclear zinc ProPhenol complex, a metal catalyzed spiroketalization, as well as application of an unprecedented asymmetric alkynylation of a simple saturated aldehyde with methyl propiolate to prepare the nucleophilic partner for a Marshall-Tamaru propargylation. These studies culminated in the first total synthesis and stereochemical assignment of (-)-ushikulide A and significantly extended the scope of the above-mentioned methodologies.

Full text of DOI:10.1021/ja906056v

Published on Web 09/23/2009
Total Synthesis and Stereochemical Assignment of
(-)-Ushikulide A
Barry M. Trost,* Brendan M. O’Boyle, and Daniel Hund
Department of Chemistry, Stanford UniVersity, Stanford, California, 94305-5080
Received July 20, 2009; E-mail: bmtrost@stanford.edu
Abstract: We report the determination of the full stereostructure of (-)-ushikulide A (1), a spiroketal
containing macrolide by total synthesis. Ushikulide A (1) was isolated from a culture broth of Streptomyces
sp. IUK-102 and exhibits potent immunosuppressant activity (IC50 ) 70 nM). To embark upon an ushikulide
A synthesis, a tentative assignment was made based on analogy to cytovaricin (2), a related macrolide
isolated from a culture of Streptomyces diastatochromogenes whose full structure was previously established
via synthesis and X-ray crystallography. This report delineates studies on several key steps, namely a
direct aldol reaction catalyzed by the dinuclear zinc ProPhenol complex, a metal catalyzed spiroketalization,
as well as application of an unprecedented asymmetric alkynylation of a simple saturated aldehyde with
methyl propiolate to prepare the nucleophilic partner for a Marshall-Tamaru propargylation. These studies
culminated in the first total synthesis and stereochemical assignment of (-)-ushikulide A and significantly
extended the scope of the above-mentioned methodologies.
Introduction
this was clearly insufficient evidence to firmly assign the
structure of ushikulide, it appeared at the outset that eight of
In spite of the tremendous advances in modern spectroscopic
methods, organic synthesis continues to play a pivotal role in
elucidating the full structure of complex natural products.1 This
method has the advantage that, even in the absence of a firm
structural assignment, a combination of logic and spectroscopic
comparison can arrive at the correct structure. Previous work
in our group led to the assignment of amphidinolide A after a
total of 11 diastereomers were prepared synthetically.2 Thus,
we were encouraged to consider the synthesis of other natural
products of uncertain stereochemistry.
the stereocenters (carbons 16, 19, 20, 21, 23, 26, 27, and 29)
were either clearly analogous to cytovaricin or their configu-
ration should be apparent after the first synthetic diastereomer
was prepared. Specifically, stereochemistry at C16 could be lost
through base catalyzed isomerization of the ꢀ,γ-unsaturated
ketone yielding ushikulide B (which contains an E olefin
between C15 and C16), while the stereochemistry at C29 (which
is isolated from the macrolactone) could be assigned as long as
the spiroketal portion of the synthetic material had the correct
configuration. Only six centers remained, a task roughly
comparable to our successful synthesis of amphidinolide A.2
In the absence of significant quantities of a natural product for
degradation studies, synthesis is the only method to reliably
assign a structure in the event that X-ray crystallography proves
untenable, as was the case for 1.6 We hope that this full account7
will provide a deeper understanding of the chemistry employed
in this study while also outlining the power of synthesis for the
determination of structure.
Synthetic Planning. A flexible approach that could be
amended to prepare any diastereomer of 1 was envisioned
(Scheme 1). As the stereochemistry of the spiroketal was pivotal,
it seemed well advised to split the spiroketal (3) away from the
aliphatic portion (4) via Suzuki coupling and esterification.
While sp2-sp2 Suzuki coupling is a well-established discon-
nection for diene-containing members of this family,8 our
retrosynthesis relied on a less common sp2-sp3 Suzuki reaction9
to access the C14-C15 olefin of ushikulide A. For spiroketal
fragment 3, a Marshall-Tamaru propargylation would provide
the homopropargylic alcohol that could be elaborated to the
Ushikulide A (1), a newly isolated3 and stereochemically
undefined member of the oligomycin-rutamycin family, was
chosen out of many possible synthetic targets based on several
considerations. Due to the stereochemical complexity of such
natural products, randomly preparing diastereomers would lead
to an exceedingly tedious task (ushikulide A contains 14
stereogenic centers, leading to 214 ) 16384 stereoisomers).
Therefore, a reasonable amount of likely stereochemical infor-
mation was required before synthetic studies could be initiated.
1 has a remarkable resemblance to cytovaricin (2), a natural
product isolated from the same species of bacteria, whose full
structure was established through X-ray crystallography4 and
confirmed via synthesis.5 Furthermore, careful comparison of
the NMR spectra of these structures revealed close similarity
between the spiroketal regions of 1 and 2 (Figure 2). While
(1) Nicolaou, K. C.; Snyder, S. A. Angew. Chem., Int. Ed. 2005, 44, 1012.
(2) (a) Trost, B. M.; Harrington, P. E. J. Am. Chem. Soc. 2004, 126, 5028.
(b) Trost, B. M.; Wrobleski, S. T.; Chisholm, J. D.; Harrington, P. E.;
Jung, M. J. Am. Chem. Soc. 2005, 127, 13589. (c) Trost, B. M.;
Harrington, P. E.; Chishom, J. D.; Wrobleski, S. T. J. Am. Chem.
Soc. 2005, 127, 13598.
(3) Takahashi, K.; Yoshihara, T.; Kurosawa, K. J. Antibiot. 2005, 58, 420.
(4) Sakurai, T.; Kihara, T.; Isono, K. Acta Crystallogr. 1983, C39, 295.
(5) Evans, D. A.; Kaldor, S. W.; Jones, T. K.; Clardy, J.; Stout, T. J.
J. Am. Chem. Soc. 1990, 112, 7001.
(6) Attempts to prepare crystals of ushikulide A suitable for X-ray
diffraction were unsuccessful in this case. Takahashi, K. Personal
communication, July 22, 2008.
(7) Trost, B. M.; O’Boyle, B. M. J. Am. Chem. Soc. 2008, 130, 16190.
9
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A R T I C L E S
Trost et al.
Figure 1. Tentative Stereochemical Assignment of (-)-Ushikulide A.
Figure 2. Comparison of Cytovaricin and Ushikulide A.
C15-C14 olefin. The metal catalyzed spiroketalization, pioneered
by Utimoto,10 represents a highly atom-economical and redox-
neutral11 approach to the desired spiroketal ring system.
Utilization of an alkyne as an orthogonally reactive surrogate12
for the desired spiroketal is strategically advantageous as it
enables two carbon-carbon bond forming reactions without
requiring the use of additional protecting group manipulations
or activating groups, which certainly would have been required
if more conventional carbonyl chemistry had been employed.
Either 5 or 6 could potentially lead to the desired spiroketal;
however, only the former case appeared likely to give the desired
regioselectivity (see 6 to 7, Scheme 1). Application of our
dinuclear zinc aldol methodology13 was expected to simplify
the aliphatic fragment into ketone 9 and aldehyde 10. Due to
the similar steric bulk of the methyl and allyl groups at C10,
conventional aldol methodologies would not be expected to give
acceptable diastereoselectivity for this reaction. However, we
envisioned that through catalyst control this highly convergent
strategy would become tenable.
Synthesis of the Spiroketal Fragment. Our studies began with
the preparation of several aldehydes to fulfill the role of synthon
8 (16a-d, Scheme 2). To accomplish this, a Crimmins14 aldol
reaction between thiazolidinethione 14 and three known alde-
hydes (13a-c)15 was employed. Reaction of the (Z)-titanium
enolate derived from 14 with aldehydes 13a-c resulted in
essentially complete diastereoselectivity and quantitative yield
for each case. Reduction of aldol adduct 15a to the ꢀ-hydroxy
aldehyde was complicated by facile epimerization of the
R-stereogenic center on silica gel (not shown). However, after
protection of the secondary alcohol, monoreduction proceeded
(13) Methyl ketones: (a) Trost, B. M.; Ito, H. J. Am. Chem. Soc. 2000,
122, 12003R-Hydroxyketones: (b) Trost, B. M.; Ito, H.; Silcoff, E. R.
J. Am. Chem. Soc. 2001, 123, 3367. Methyl ynones: (c) Trost, B. M.;
Fettes, A.; Shireman, B. T. J. Am. Chem. Soc. 2004, 126, 2660. Methyl
vinyl ketone: (d) Trost, B. M.; Shin, S.; Sclafani, J. A. J. Am. Chem.
Soc. 2005, 127, 8602. Acetone and 2-butanone: (e) Trost, B. M.;
Silcoff, E. R.; Ito, H. Org. Lett. 2001, 3, 2497. Ethyl diazoacetate
with dinuclear magnesium ProPhenol complex: (f) Trost, B. M.;
Malhotra, S.; Fried, B. A. J. Am. Chem. Soc. 2009, 131, 1674.
(14) (a) Crimmins, M. T.; She, J. Synlett 2004, 8, 1371. (b) Crimmins,
M. T.; King, B. W.; Tabet, E. A.; Chaudhary, K. J. Org. Chem. 2001,
66, 894.
(8) References 7, 8a, and b use a similar Suzuki/macrolactonization
strategy, whereas refs 8c and 8d employ a related Stille/macrolacton-
ization strategy: (a) White, J. D.; Hanselmann, R.; Jackson, R. W.;
Porter, W. J.; Ohba, Y.; Tiller, T.; Wang, S. J. Org. Chem. 2001, 66,
5217. (b) Evans, D. A.; Ng, H. P.; Rieger, D. L. J. Am. Chem. Soc.
1993, 115, 11446. (c) Panek, J. S.; Jain, N. F. J. Org. Chem. 1998,
63, 4572. (d) Panek, J. S.; Jain, N. F. J. Org. Chem. 2001, 66, 2747.
(9) Chemler, S. R.; Trauner, D.; Danishefsky, S. J. Angew. Chem., Int.
Ed. 2001, 40, 4544.
(10) Utimoto, K. Pure Appl. Chem. 1983, 55, 1845.
(11) Burns, N. Z.; Baran, P. S.; Hoffmann, R. W. Angew. Chem., Int. Ed.
2009, 48, 2854.
(15) (a) Barry, C. S.; Bushby, N.; Harding, J. R.; Willis, C. L. Org. Lett.
2005, 7, 2683. (b) Dickschat, J. S.; Bode, H. B.; Mahmud, T.; Mu¨ller,
R.; Schulz, S. J. Org. Chem. 2005, 70, 5174. (c) Hatakeyama, S.;
Kawamura, M.; Shimanuki, E.; Saijo, K.; Takano, S. Synlett 1992, 2,
114.
(12) For previous examples that employ the alkyne as a spiroketal surrogate,
see: (a) Li, Y.; Zhou, F.; Forsyth, C. J. Angew. Chem., Int. Ed. 2007,
46, 279. (b) Trost, B. M.; Weiss, A. H. Angew. Chem., Int. Ed. 2007,
46, 7664.
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Synthesis and Assignment of (-)-Ushikulide A
A R T I C L E S
Scheme 1. Retrosynthetic Analysis of Ushikulide A
Scheme 2. Preparation of Several Aldehyde Partners through a Crimmins Aldol Reaction
Scheme 3. Preparation of the Alkyne Fragment
1
* Isolated yields after purification by flash column chromatography. Diastereomeric ratios were determined by H NMR analysis of the crude reaction
mixture prior to purification.
uneventfully to yield the desired aldehydes (16a-d) in good
yield over three steps.
Preparation of an alkyne fragment to fulfill the role of synthon
11 began with a known Noyori reduction of methyl-3-oxopen-
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A R T I C L E S
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Scheme 4. Attempted Diastereoselective Alkyne Addition
1
* Isolated yields after purification by flash column chromatography. Diastereomeric ratios were determined by H NMR analysis of the crude reaction
mixture prior to purification.
tanoate (Scheme 3).16 Formation of the p-methoxybenzyl ether and
monoreduction with diisobutylaluminum hydride gave aldehyde
17 in good yield. Addition of the lithium anion of TMS acetylene
in the presence of a chelating Lewis acid (Et2AlCl) gave a mixture
of propargyl alcohols 18a and 18b in low diastereoselectivity and
yield (∼2:1 at C27, Scheme 3). Separation of these epimers on a
preparative scale was prohibitively difficult, particularly at this early
stage in the synthesis. As an alternative, a number of crotylations
were surveyed, starting with the conditions of Keck,17 which
afforded a mixture of predominantly two diastereomers (2:1, 19a:
19b) that were also inseparable by flash chromatography on silica
gel. Varying the Lewis acid (SnCl4, CH2Cl2, Entry 2) led to only
a negligible increase in diastereoselectivity. Fortunately, ample
precedent suggested that this problem could be overcome by
reagent control.18 In our case, using either antipode of Brown’s19
(Z)-crotyl-B(Ipc)2 reagent led to excellent control of stereochemistry
for the preparation of 19a or 19b in >20:1 d.r. by 1H NMR analysis
for both cases. Preparation of alkyne 20 was completed in three
additional steps (silylation, hydroboration-iodination, and nucleo-
philic substitution). Because no chromatographic separation of
diastereomers was required, this route proved to be highly scalable
and more than 10 g of 20 were ultimately prepared.
groups, but exposure of either 22a or 22b to hydrochloric acid
yielded either epimeric diol in good yield (23a or 23b).
The stereochemistry of each triol was assigned by the method
of Rychnovsky (eq 3).21 Protection of 1,3-diols 23a and 23b
was accomplished with 2-methoxypropene and PPTS, yielding
acetonides 24 and 25. gHSQC analysis of either acetonide
confirmed the relative stereochemistry with syn acetonide 24
displaying large differences in the acetonide 13C NMR shifts,
consistent with a chair conformation, while anti acetonide 25
displayed nearly equivalent 13C NMR shifts consistent with a
half-chair conformation.
The stereochemistry of 19a was also confirmed via formation
of a cyclic para-mehoxybenzyl acetal (eq 1). Exposure of 19a
to DDQ in the presence of molecular sieves20 furnished acetal
21, which could be cleanly separated from an initial mixture of
diastereomers in 60% isolated yield. NOE irradiation of the
benzylic acetal proton showed strong enhancements at C26 and
C29, consistent with the indicated chair form and demonstrating
the 1,3 diol to be of an anti stereochemical relationship.
With the two fragments in hand, our attention turned to
nucleophilic addition of alkyne 20 to the aldehyde fragment 16c
(eq 2). The addition proceeded with low diastereoselectivity;
however, separation of the C21 epimers was readily accomplished
by silica gel chromatography. Desilylation with fluoride (HF•pyridine
or n-Bu4NF) proceeded only very slowly at one of the two silyl
Attempts were also made to improve the diastereoselectivity
of the alkyne addition reaction (Scheme 4). Moderate Felkin-Anh
selectivity could be obtained in the presence of lithium bromide
and molecular sieves (6:1 d.r., Entry 2). However, obtaining
the chelation controlled product (22b) proved not to be feasible
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Synthesis and Assignment of (-)-Ushikulide A
A R T I C L E S
Scheme 5. Modification of the Spiroketalization Substrate
under the reaction conditions examined. Switching the C19 tert-
butyldimethylsilyl group to a methoxymethyl ether (16d, Entry
4) gave some improvement of the diastereoselectivity; unfor-
tunately, the resulting epimers (27a and 27b) were very
recalcitrant to chromatographic separation, causing this route
to be impractical.
(eq 4). When acetone was substituted as solvent with the same
catalyst, a mixture of acetonide 25 along with an unidentified
product was obtained. All other PdII catalyzed conditions that
were examined afforded only decomposition after prolonged
heating.
The low selectivity of the alkynylation led us to consider two
possible alternatives. The first would be addition of the alkyne
to a Weinreb amide, followed by diastereoselective Noyori
reduction,22 while the second would be inversion of the syn
1,3-diol to converge both epimers to the anti stereochemistry.
Because of the ease of chromatographic separation, we ulti-
mately elected to pursue the latter course (Scheme 5). Alterna-
tives to the 1,3-dithiane protecting group were also examined
as the sulfur atoms it contained might cause catalyst poisoning
during the key spiroketalization reaction. Three aldehydes
(16a-c) were subjected to alkyne addition with the lithium
acetylene derived from 20. After separation of the resulting
epimers, the anti product was esterified with retention (BzCl,
pyridine), while the syn product was esterified under conditions
leading to inversion (PPh3, DEAD, BzOH, toluene). All three
pairs of epimers converged, yielding propargyl benzoates
28a-c. The TBDPS group (28a) proved too acid-labile and
yielded predominantly triol 29a when subjected to the depro-
tection conditions developed previously (HCl, H2O, THF).
Fortunately, both 28b and 28c could be cleanly desilylated
yielding diols 29b and 29c respectively.
Further investigation of the spiroketalization reaction focused
on triol 30b and propargyl benzoate 29b as precursors to
spiroketal 31a (Scheme 6). Palladium catalysis again provided
only limited reactivity (Entry 1), prompting us to examine
platinum23 and gold.24 While platinum also failed (Entry 2),
gold(I)-chloride gave complete conversion at room temperature,
but almost none of the desired spiroketal (31a) was observed.
Instead elimination product 32 was isolated in 61% yield (Entry
3). Examination of other gold catalysts (such as AuClPPh3 with
or without added silver salts) failed to resolve this difficulty.
Under nearly identical conditions, propargyl benzoate 29b gave
a 17% yield of spiroketal 31b along with 47% of elimination
product 32 (Entry 4). Examination of several Brønsted acid
additives showed that the distribution of products could be
biased toward the formation of 31b in the presence of PPTS
(Entry 5). In the presence of stronger acid (CSA, Entry 6), the
elimination problem was significantly exacerbated and the
overall yield decreased significantly. Variation of the solvent
caused significant perturbation to the ratio of products. Moving
from MTBE to THF (Entry 7) significantly improved the yield
of 31b, but continuing to an even more polar solvent, such as
methanol, increased the elimination problem relative to THF.
Out of the conditions surveyed, the best were those outlined in
Entry 7, and these were used for all further spiroketalization
reactions.
With the triol 23b in hand, we examined application of a
metal catalyzed spiroketalization for the preparation of spiroketal
26. Coordination of a carbophilic Lewis acid (PdII) to the alkyne
present in the starting material should lead to oxypalladation,
generating an enol-ether and ultimately producing the desired
spiroketal. Unfortunately, Utimoto’s conditions (Pd[CH3CN]2Cl2,
THF, CH3CN, reflux)10 failed entirely for the preparation of 26
(16) Kitamura, M.; Tokunaga, M.; Ohkuma, T.; Noyori, R. Org. Synth.
1998, 71, 589.
(17) (a) Keck, G. E.; Abbott, D. E.; Wiley, M. R. Tetrahedron Lett. 1987,
28, 139. (b) Keck, G. E.; Boden, E. P. Tetrahedron Lett. 1984, 25,
265. (c) Keck, G. E.; Abbott, D. E. Tetrahedron Lett. 1984, 25, 1883.
(d) Keck, G. E.; Abbott, D. E.; Boden, E. P.; Enholm, E. J. Tetrahedron
Lett. 1984, 25, 3927.
(18) (a) Nicolaou, K. C.; Li, Y.; Fylaktakidou, K. C.; Mitchell, H. J.; Wei,
H.-X.; Weyershausen, B. Angew. Chem., Int. Ed. 2001, 40, 3849. (b)
Chen, S.-H.; Horvath, R. F.; Joglar, J.; Fisher, M. J.; Danishefsky,
S. J. J. Org. Chem. 1991, 56, 5834.
(19) Brown, H. C.; Bhat, K. S. J. Am. Chem. Soc. 1986, 108, 5919.
(20) Nakatsuka, M.; Ragan, J. A.; Sammakia, T.; Smith, D. B.; Uehling,
D. E.; Schreiber, S. L. J. Am. Chem. Soc. 1990, 112, 5583.
(21) Rychnovsky, S. D.; Rogers, B.; Yang, G. J. Org. Chem. 1993, 58,
3511.
(23) (a) Dieguez-Vazquez, A.; Tzschucke, C. C.; Lam, W. Y.; Ley, S. V.
Angew. Chem., Int. Ed. 2008, 47, 209. (b) Liu, B.; De Brabander,
J. K. Org. Lett. 2006, 8, 4907.
(22) Matsumura, K.; Hashiguchi, S.; Ikariya, T.; Noyori, R. J. Am. Chem.
Soc. 1997, 119, 8738.
(24) (a) Antoniotti, S.; Genin, E.; Michelet, V.; Geneˆt, J. P. J. Am. Chem.
Soc. 2005, 127, 9976.
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Scheme 6. Optimization of the Spiroketalization Reaction
1
* Yields for Entries 1-6 were determined through H NMR analysis of the crude reaction mixture using benzyl benzoate as an internal standard. Entry
7 is the average isolated yield of two reactions, after separation of 31b and 32 by column chromatography. Both reactions were performed on greater than
150 mg scale.
Scheme 7. Mechanism of the Spiroketalization Reaction
Aponick25 reported the preparation of unsaturated 1,7-
dioxaspiro[5.5]undec-4-ene spiroketals via a gold catalyzed
dehydrative cyclization of monopropargylic triols. Thus, in
retrospect, it is not surprising that triol 30b was so prone to
elimination. However, the leaving group ability of benzoate
appears to be greater than a hydroxyl group, at least in a
thermodynamic sense (pKa 15.7 vs 4.3 in water).26 At first, we
were somewhat puzzled by this counterintuitive observation.
This issue is also convoluted by the fact that it is unclear whether
the gold cation is lost via protonaton prior to or following the
elimination. The mechanism below (Scheme 7) assumes loss
of gold cation prior to formation of the spiroketal. While we
cannot rule out that enol ether 33 or 34 could induce elimination
directly after protonolysis of the gold cation, the heightened
propensity of the hydroxy group to eliminate suggests that gold
is involved in the elimination step prior to protonolysis. This
premise is consistent with the fact that weak acid disfavors
elimination, as acid should favor protonolysis of the gold cation
before elimination can occur. The length of the reaction time
(25) Aponick, A.; Li, C.-Y.; Palmes, J. A. Org. Lett. 2009, 11, 121.
(26) (a) Bordwell, F. G. Acc. Chem. Res. 1988, 21, 456. (b) Harned, H. S.;
Robinson, R. A. Trans. Faraday Soc. 1940, 36, 973.
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Synthesis and Assignment of (-)-Ushikulide A
A R T I C L E S
did not alter the ratio of 31b and 32, implying that 31b does
not convert to 32 directly via reionization and solvolysis. The
first report of a gold catalyzed spiroketalization24 presented
several reactions in which 5-exo-dig cyclization dominated over
a possible 6-endo-dig pathway. Therefore, it seems more likely
that the C27 hydroxyl group attacks first via a 6-exo dig pathway
to form intermediate 33 or 34, although we cannot rule out a
6-endo cyclization of the C19 hydroxyl group, as it would lead
to the same product. Nucleophilic addition to C23 in preference
to C22 is enforced by the benzoyloxy or hydroxy group at C21,
which through its inductive electron withdrawing nature disfa-
vors a cationic intermediate at C22.27 Calculations have indicated
that this oxy-metalation consists of a syn addition across the
alkyne, resulting from prior coordination of the hydroxy group
to the gold Lewis acid (see intermediates 33 and 34).28 One
possible rationalization for the benzoyl groups lessened pro-
pensity toward elimination would be formation of a 6-membered
chelate (35), in which the σ* orbital of the C21-O bond is
orthogonal to both the π-system and the Au-C σ bond, leading
to a kinetically inert conformation. Intermediate 34 instead
encourages elimination through coordination of the hydroxyl
group to the gold center, resulting in formation allenyl enol ether
37, which could undergo reversible protonation to form oxonium
38, explaining the formation of the cis olefin geometry required
for 32.
edented before our communication.7 Implementation of our zinc
alkynylation method31 led to the isolation of 43 in high yield
and excellent enantioselectivity, the first such transformation
reported for a simple aliphatic aldehyde. Under identical
conditions, TMS acetylene fails entirely due to its increased
basicity, which causes destructive polymerization of the alde-
hyde by a competing aldol pathway. Methyl propiolate serves
as a very effective acetylene equivalent as saponification and
decarboxylation were easily accomplished,32 yielding 44 after
mesylation. Propargylation of 45 with 44 under the conditions
of Marshall33 afforded a 3.7:1 mixture of separable propargyl
epimers (C17) in a combined 90% yield. The stereochemistry
of 46a is assigned by Marshall’s model in which propargyl
mesylate 44 undergoes oxidative addition with palladium(0) with
inversion, followed by transmetalation to diethyl zinc with
retention. The resulting allenylzinc undergoes nucleophilic
addition to aldehyde 45 via coordination of the aldehyde to zinc.
Thus, stereochemistry at C16 is completely controlled, whereas
C17 is formed as a mixture favoring 46a through a Zimmerman-
Traxler transition state in which the aldehyde group occupies a
pseudoequatorial position. Both epimers were independently
carried forward to the complete spiroketal fragment, as the C17
stereochemistry is ultimately irrelevant and will be removed by
oxidation later in the synthesis. After some experimentation, it
proved best to incorporate the sensitive vinyl iodide last, giving
the completed spiroketal fragment (47) in 16 linear steps from
methyl 3-oxopentanoate.
Spiroketal product 41 was examined extensively by various
NMR experiments (gCOSY, gHSQC, gHMBC, ROESY, Figure
3). The expected NOE correlations (H19 to H27 and H19 to H21)
and coupling constants (JC26-C27 ) 2.5 Hz [equatorial-axial],
JC21-C22 ) 4.5 Hz [axial-equatorial] 12.0 Hz [axial-axial],
JC19-C20 ) 2.5 Hz [axial-equatorial]) were observed within the
bicyclic framework of spiroketal 41 consistent with the indicated
structure. The strong resemblance of these data to the NMR
spectrum of authentic ushikulide A (Figure 2) was taken to be
a very positive sign and improved the likelihood that our
synthetic spiroketal had the same relative stereochemistry as
the natural product.
Because we had assumed that triol 30b would be a superior
spiroketalization substrate, it had not been anticipated that the
benzoyl group would be present in the product. This led to
unexpected protecting group difficulties as the benzoyl group
was not compatible with conditions known to selectively cleave
benzyl groups in the presence of p-methoxybenzyl groups
(lithium 4,4′-di-tert-butylbiphenyl, THF, -78 °C).29 Hydro-
genolysis was unsatisfactory, and after surveying a fair number
of conditions, only a 40% yield of the desired alcohol (39) could
be isolated (eq 5).
Synthesis of the Aliphatic Fragment. A first generation
approach toward an aliphatic fragment to fulfill the role of
synthon 4 began from the known dibromide 48 (available in
three steps from the (S)-Roche ester, Scheme 9).20,34 Exposing
(27) This electronic bias has been observed in the mechanistically related
Wacker oxidation of terminal alkenes with allylic alkoxy substituents: (a)
Kang, S.-K.; Jung, K.-Y.; Chung, J.-U.; Namkoong, E.-Y.; Kim, T.-
H. J. Org. Chem. 1995, 60, 4678.
(28) Evidence for syn oxymetalation in gold-catalyzed hydration: (a) Teles,
J. H.; Brode, S.; Chabanas, M. Angew. Chem., Int. Ed. 1998, 37, 1415.
And in a related palladium-catalyzed Wacker-type cyclization: (b)
Hayashi, T.; Yamasaki, K.; Mimura, M.; Uozumi, Y. J. Am. Chem.
Soc. 2004, 126, 3036.
As a result, the 1,3-dithiane group was reinvestigated, and
to our delight, it proved to be an effective substrate for the
spiroketalization (Scheme 8). In analogy to the spiroketalization
with the benzyl ether, employing triol 30c in the spiroketalization
led almost exclusively to elimination product 42. In contrast,
benzoate 29c led, at slightly elevated temperature and increased
catalyst loading (50 °C, 10 mol % AuCl, THF), to spiroketal
41 in 63% isolated yield. Discrimination of the protecting groups
was now trivial and methyl iodide in buffered aqueous aceto-
nitrile gave aldehyde 45 directly.
(29) Crimmins, M. T.; McDougall, P. J.; Ellis, J. M. Org. Lett. 2006, 8,
4079.
(30) The method of Carreira is the most general, however, its catalytic
implementation, particularly for R-mono-substituted or acidic alde-
hydes is somewhat unreliable: (a) Anand, N. K.; Carreria, E. M. J. Am.
Chem. Soc. 2001, 123, 9687. At the time of our communication, 43
was the first report of enantioselective methyl propiolate addition to
an R-mono-substituted aldehyde in >90% ee. Since then additional
examples have been disclosed: (b) Tulington, M.; DeBerardinis, A. M.;
Pu, L. Org. Lett. 2009, 11, 2441.
(31) Trost, B. M.; Weiss, A. H.; von Wangelin, A. J. J. Am. Chem. Soc.
2006, 128, 8.
Our need to access mesylate 44 prompted us to examine the
asymmetric alkynylation of isovaleraldehyde with methyl pro-
piolate, a very challenging transformation30 that was unprec-
(32) Trost, B. M.; Weiss, A. H. Org. Lett. 2006, 8, 4461.
(33) Marshall, J. A.; Schaaf, G. M. J. Org. Chem. 2001, 66, 7825.
(34) Mulzer, J.; Berger, M. J. Org. Chem. 2004, 69, 891.
9
J. AM. CHEM. SOC. VOL. 131, NO. 41, 2009 15067

A R T I C L E S
Trost et al.
Scheme 8. Completion of the Spiroketal Fragment
dibromide 48 to n-BuLi led to the expected Fritsch-Buttenberg-
Wiechell rearrangement,35 and the resulting lithium acetylide
was quenched with N-methoxy-N-methylacetamide to yield
ynone 49 in 76% yield. This ketone gave good reactivity in the
direct zinc aldol reaction13 with aldehyde 5013d to yield aldol
adduct 51 as a single diastereomer by 1H NMR analysis.
Unfortunately, hydrosilylation36 of this product under the
standard conditions (Cp*Ru(CH3CN)3PF6, BDMSH, acetone)
caused deprotection of the diethoxyketal. Changing the solvent
to dichloromethane failed to remedy the problem. Therefore,
aldehyde 53 was prepared in enantiopure form via an allylation
reported by Mukaiyama.37 This aldehyde also gave good yield
and diastereoselectivity in the zinc aldol reaction to afford aldol
adduct 54, again as a single diastereomer. The alkyne moiety
of 54 was reduced in a syn fashion via a palladium-catalyzed
reduction,38 yielding enone 55. Lindlar reduction was also
attempted on ynone 54 or the corresponding syn 1,3-diol after
reduction of the ketone function. Both substrates gave prefer-
ential reduction of the terminal double bond, demonstrating the
superior selectivity of homogeneous palladium catalysis in this
case. Enone 55 was then reduced to the syn diol and protected
as a cyclic acetonide (syn stereochemistry of 56 and 64 was
consistent with Rychnovsky’s 13C shift analysis).21 Epoxidation
of 56 gave no diastereoselectivity and modest chemoselectivity
for the more substituted olefin to yield epoxides 57 and 58 as
a separable but unassigned mixture of diastereomers. Before
any attempt was made to remedy this diastereoselectivity issue,
the epoxide opening had to be demonstrated. Unfortunately,
several attempts to accomplish this transformation failed to yield
59, instead cleaving the p-methoxybenzyl group. While this
general strategy still might have provided the completed aliphatic
fragment,39 a more direct strategy was already showing promise
(Vide infra).
One potential method to prepare the aliphatic fragment would
be to couple a fully saturated aldehyde and ketone through the
direct aldol methodology. Utilization of such an unstabilized
ketone enolate with nearly 1:1 stoichiometry had not previously
been demonstrated in this reaction (acetone and acetophenone
are commonly employed as solvent or in excess [5-10
equivalents]). The requisite methyl ketone 61 was prepared
easily via Wacker oxidation40 of the known41 homoallylic
alcohol 60 (Scheme 10). By using a preformed lithium enolate
of ketone 61 with aldehyde 53, the expected aldol product could
be generated, but low stereoselectivity was observed as expected
Figure 3. NOE and Coupling Constant Analysis of the Spiroketal.
9
15068 J. AM. CHEM. SOC. VOL. 131, NO. 41, 2009

Synthesis and Assignment of (-)-Ushikulide A
A R T I C L E S
Scheme 9. First Approach Toward the Aliphatic Fragment
Scheme 10. Second Generation and Completion of the Aliphatic Fragment
1
* Isolated yields after purification by flash column chromatography. Diastereomeric ratios were determined by H NMR analysis of the crude reaction
mixture prior to purification. Entry 6 is based on recovered starting material (b.o.r.s.m.) after three total reactions.
(∼1:1 d.r., Entry 1). The standard zinc aldol conditions failed
entirely to give the desired product and only the reduced primary
alcohol (63) and recovered ketone (61) were observed (Entry
2). Presumably this reduction must proceed via a Meerwein-
Ponndorf-Verley pathway. Employing t-BuOH instead of
i-PrOH resulted in some yield of the desired product (62, Entry
3), while switching the solvent to 1,4-dioxane and increasing
the catalyst loading gave an improved yield of aldol 62 in
excellent stereoselectivity (Entry 4). Stoichiometric ProPhenol
and diethylzinc failed to increase this yield and instead led to
contamination of the product with various side products (Entry
5). Fortunately, the aldehyde and ketone could be recovered
and resubjected to the reaction conditions, leading to an
acceptable yield (65%) after three total runs. With ketone 62 in
hand syn reduction and acetonide formation gave 64, which was
converted to aliphatic fragment 65 in three additional steps
including a Horner-Wadsworth-Emmons olefination.
Further attempts to optimize the aldol reaction were fruitless,
and short of modifying the ligand, there seemed little hope for
(36) Trost, B. M.; Ball, Z. T. J. Am. Chem. Soc. 2005, 127, 17644.
(37) Yamada, K.; Tozawa, T.; Nishida, M.; Mukaiyama, T. Bull. Chem.
Soc. Jpn. 1997, 70, 2301.
(38) Trost, B. M.; Braslau, R. Tetrahedron Lett. 1989, 30, 4657.
(39) Torres, W.; Rodriguez, R. R.; Prieto, J. A. J. Org. Chem. 2009, 74,
2447.
(35) (a) Corey, E. J.; Fuchs, P. L. Tetrahedron Lett. 1972, 36, 3769. (b)
Orita, A.; Otera, J. Chem. ReV. 2006, 106, 5387.
9
J. AM. CHEM. SOC. VOL. 131, NO. 41, 2009 15069

A R T I C L E S
Trost et al.
Scheme 11. Alternative Aldol Disconnection in the Synthesis of the Aliphatic Fragment
solving the problem of catalyst turnover that required several
recycling iterations. To this end, we considered another aldol
disconnection, wherein the C7-C8 bond instead of the C8-C9
would be formed in the aldol reaction. To attempt this,
preparation of methyl ketone 67 was required rather than
aldehyde 53. The ideal method to accomplish this would be a
stereoselective allylation of 2,3-butanedione (eq 6). This reaction
was unknown in the literature, and a number of attempts were
made to accomplish it, which are outlined below. Unfortunately,
the best enantioselectivity obtained was 45% ee, (Entry 5) too
low to carry forward with the desired route.
strong NOE correlations were observed to H5 and H7. Coupling
constant analysis (JH5-H6 ) 2.0 Hz [axial-equatorial] and JH6-H7
) 2.0 Hz [axial- equatorial]) was also consistent with the chair
form indicated, further supporting that the expected stereo-
chemistry had been observed in both aldol reactions.
Completion of the Synthesis. Completion of the synthesis
rested on two key bond-forming reactionssan sp3-sp2 Suzuki
coupling44 and an esterification. While a great many syntheses
have relied on macrolactonization,45 only a few have utilized
Suzuki coupling (particularly an sp3-sp2 coupling) for the
formation of macrocycles.46 We therefore elected to attempt
the Suzuki macrocyclization first to explore its potential in this
arena (Scheme 12). Yamaguchi esterification47 allowed for the
direct coupling of 47a and 65 to yield ester 72. Hydroboration
of 72 (9-BBN, THF) gave the expected preference for reaction
of the terminal olefin as observed by 1H NMR analysis.
However, under a variety of conditions, which were later shown
to be viable in an intermolecular reaction, macrocycle 73 could
not be detected or isolated. The borate described above was
injected via syringe pump to a solution containing palladium
catalyst, base, and solvent (palladium sources: Pd[PPh3]4,
PdCl2•(dppf), and Pd2(dba)3•CHCl3; bases: Cs2CO3, TlOEt;
solvents: dioxane, THF, DMF, each with cosolvent of water).
In many of these attempts the vinyl iodide was clearly
consumed, unfortunately a number of products were formed,
none of which corresponded to the macrocycle 73, which was
successfully isolated later on.
The alternative pathway (Suzuki coupling then macrolacton-
ization) was attempted. From the unsaturated acid (65), hy-
droboration was feasible with 2.5 equiv of 9-BBN. Immediate
gas evolution gave evidence that the carboxylic acid was reacting
with 1 equiv of 9-BBN; however, this fast reaction was followed
by consumption of the terminal olefin. The crude borate (74)
was added along with vinyl iodide 47a to a flask containing
PdCl2•(dppf), triphenylarsine, and cesium carbonate.48 At room
temperature, this gave seco-acid 75 directly.
Alternatively, aldehyde 53 was transformed to silyl ether 68
in three steps. Addition of methyl magnesium bromide gave a
mixture of secondary alcohols, which were oxidized without
further purification. Finally, formation of the lithium enolate
and silylation afforded the desired silyl enol ether.
Aldehyde partner 69 was prepared via oxidative cleavage of
60. Moderate yield and diastereoselectivity were obtained in
the Mukaiyama, aldol reaction42 using a slight excess of 68 in
toluene with boron trifluoride diethyl etherate to yield the
Felkin-Anh aldol product (70).43 Reduction of this hydroxy
ketone afforded the syn-diol, which was converted to an
acetonide (64) identical to that prepared in our previous strategy.
Alternatively, the PMB group was cyclized to the p-methox-
yphenyl acetal (71). Upon irradiation of the benzylic proton,
Several conditions were applied to the seco-acid (75) to effect
macrolactonization. Yonemitsu’s modification of the Yamaguchi
esterification protocol8b failed to give any product (2,4,6-
(44) Chemler, S. R.; Trauner, D.; Danishefsky, S. J. Angew. Chem., Int.
Ed. 2001, 40, 4544.
(45) Parenty, A.; Moreau, X.; Campagne, J. M. Chem. ReV. 2006, 106,
911.
(46) (a) White, J. D.; Tiller, T.; Ohba, Y.; Porter, W. J.; Jackson, R. W.;
Wang, S.; Hanselmann, R. Chem. Commun. 1998, 79. (b) Chemler,
S. R.; Danishefsky, S. J. Org. Lett. 2000, 2, 2695. (c) Kallan, N. C.;
Halcomb, R. L. Org. Lett. 2000, 2, 2687. (e) Mohr, P. J.; Halcomb,
R. L. J. Am. Chem. Soc. 2003, 125, 1712. Williams, D. R.; Walsh,
M. J.; Miller, N. A. J. Am. Chem. Soc. 2009, 131, 9038.
(47) Fu¨rstner, A.; Kattnig, E.; Lepage, O. J. Am. Chem. Soc. 2006, 128,
9194.
(40) Tsuji, J. Synthesis 1984, 369.
(41) (a) Keck, G. C. Abbott, D. E. Boden, E. P. Enhom, E. J. Tetrahedron
Lett. 1984, 25, 3927. For detailed experimental conditions see ref 10.
(42) Mahrwald, R. Chem. ReV. 1999, 99, 1095.
(43) Mitchell, I. S.; Pattenden, G.; Stonehouse, J. Org. Biomol. Chem. 2005,
3, 4412.
9
15070 J. AM. CHEM. SOC. VOL. 131, NO. 41, 2009

Synthesis and Assignment of (-)-Ushikulide A
Scheme 12. Completion of the Synthesis
A R T I C L E S
trichlorobenzoyl chloride, Et3N, 4-DMAP, benzene). Applying
the Wasserman-Kita esterification to macrolaconization49 gave
the first successful isolation of macrocycle 73 in ∼30% yield,
but attempts to improve the yield were unsuccessful (ethoxy
acetylene, 2 mol % [RuCl2(p-cymene)]2, toluene, then 10 mol
% CSA). The conditions of Shiina50 gave the best result (65%)
for preparation of macrolactone 73. Following cyclization, the
secondary tert-butyldimethylsilyl ether was cleaved employing
buffered HF•pyridine and the resulting alcohol was oxidized
with Dess-Martin periodinane. The final deprotection required
DDQ to oxidatively cleave the p-methoxybenzyl ethers and then
acid to hydrolyze the cyclic acetonide. It proved to be essential
to perform the oxidative deprotection first, as prior deprotection
of the acetonide led to undesired cyclization during oxidative
removal of the PMB group (PMP acetal or p-methoxybenzoate
formation). When the PMB groups were removed first, the
resulting triol could be cleanly isolated without interference from
neighboring groups. In the final acid catalyzed deprotection, the
starting acetonide triol quickly interconverted to two other
compounds of similar polarity as evidenced by TLC analysis
(presumably these were migration of the acetonide to the C5 or
C10 oxygens, respectively); however, on prolonged stirring at
room temperature (∼4 h), these three compounds converged to
a single spot of indistinguishable Rf when compared to an
authentic sample of ushikulide A. Upon isolation, this compound
MeOH)}. In total, these data confirmed our tentative assignment
and led us to assign the complete structure of (-)-ushikulide A
as depicted in Scheme 12.
Summary
We have developed a flexible synthetic route, which estab-
lishes the full structure of (-)-ushikulide A for the first time.
This route, which requires 21 linear and 40 total steps, provides
the natural product in 2.2% yield over the longest linear
sequence, a significant accomplishment given the complexity
of 1. The key gold-catalyzed spiroketalization to yield 41 is
the first example of a metal-catalyzed spiroketalization in which
regioselectivity is controlled by the electronic bias of the
alkyne. This implementation extends the methodology to 1,7-
dioxaspiro[5.5]undecane ring systems, a motif found in many
natural products. Application of our zinc aldol methodology to
aldehyde 53 and ketone 61 gave excellent control of diastereo-
selectivity, although catalyst turnover still needs to be improved.
Control of diastereoselectivity in this manner allowed for an
otherwise unavailable synthetic disconnection resulting in the
preparation of aliphatic fragment 65 in only 11 linear steps.
Finally, effective use of Suzuki coupling, Shiina macrolacton-
ization, and the Marshall-Tamaru propargylation all contributed
to the overall success of this synthetic endeavor.
This synthesis confirmed the “educated guess” that was made
when we began our synthesis; however, it is likely that almost
any diastereomer of ushikulide might have been prepared via
this general strategy. Furthermore, a large number of unassigned
natural products could be targeted by this method. In total, there
are at least 12 natural products as close in similarity to ushikulide
as cytovaricin,51 and many other unknown natural products
could be approached in such a manner. Each specific case would
lead to a slightly different implementation of this method. For
example, access to greater amounts of material would allow
for degradation studies, whereas a less conformational flexible
1
exhibited identical H, 13C, IR, HRMS, and HPLC properties
when compared to the same authentic sample. Measurement of
the optical rotation {[R]D (24 °C): -12° (c 0.28, MeOH)}
confirmed that the absolute stereochemistry was the same as
that of natural ushikulide A {[R]D (24 °C): -13° (c 0.50,
(48) (a) Miyaura, N.; Suzuki, A. Chem. ReV. 1995, 95, 2457. (b) Kapur,
M.; Khartulyari, A.; Maier, M. E. Org. Lett. 2006, 8, 1629.
(49) Trost, B. M.; Chisholm, J. D. Org. Lett. 2002, 4, 3743.
(50) (a) Shiina, I.; Kubota, M.; Oshiumi, H.; Hashizume, M. J. Org. Chem.
2004, 69, 1822. (b) Shiina, I. Chem. ReV. 2007, 107, 239.
9
J. AM. CHEM. SOC. VOL. 131, NO. 41, 2009 15071

A R T I C L E S
Trost et al.
(c ) 0.93, CHCl3); IR (neat): 3419, 2959, 2237, 1719, 1436, 1254,
1070 cm-1;1H NMR (500 MHz, CDCl3): δ 4.52 (t, 1H, J ) 7.0
Hz), 3.77 (s, 3H), 2.70 (s, broad, 1H), 1.89-1.81 (m, 1H), 1.70
(ddd, J ) 7.0, 7.5, 13.5 Hz, 1H), 1.60 (ddd, J ) 7.0, 7.0, 14.0 Hz,
target would allow for more computational analysis than was
possible here. The success of this project is a testament to the
power of organic synthesis in the structural elucidation of
complex natural products. Further studies to apply and improve
upon this strategy in the synthesis of other members of the
oligomycin-rutamycin family are underway and will be reported
in due course.
1H), 0.94 (d, J ) 7.0 Hz, 3H), 0.94 (d, J ) 7.0 Hz, 3H) ppm; 13
C
NMR (125 MHz, CDCl3) δ 153.9, 88.8, 75.9, 60.5, 52.8, 45.6, 24.4,
22.4, 22.1 ppm.
(2S,3S,4R,6R,8S)-2-((2R,3S)-3-Ethynyl-2-hydroxy-5-methylhexyl)-
8-((R)-2-(4-methoxybenzyloxy)butyl)-3,9-dimethyl-1,7-dioxaspiro-
[5.5]undecan-4-yl Benzoate (46). Aldehyde 45 (132.9 mg, 0.25
mmol) and propargyl mesylate 44 (71.5 mg, 0.38 mmol) are
dissolved in dry THF (1.7 mL) and cooled to -78 °C. A solution
of palladium acetate (5.6 mg, 25.1 µmol) and triphenylphosphine
(6.6 mg, 25.1 µmol) in THF (1 mL) is transferred into the reaction
flask via syringe, followed by a solution of diethyl zinc in hexanes
(0.75 mL, 1M, 0.75 mmol). The reaction is allowed to warm to
-20 °C for a period of 14 h, after which time it is poured into a
saturated solution of ammonium chloride. The organic phase is
diluted with ethyl acetate, separated and dried over sodium sulfate.
Following filtration and concentration, the crude residue is purified
by flash chromatography (19:1 toluene/EtOAc) yielding the major
diastereomer 46a (113.2 mg, 71%) followed by its C-17 epimer
46b (30.0 mg, 19%).
Experimental Section
(2S,3S,4S,6R,8S)-2-((1,3-Dithian-2-yl)methyl)-8-((R)-2-(4-meth-
oxybenzyloxy)butyl)-3,9-dimethyl-1,7-dioxaspiro[5.5]undecan-4-
yl Benzoate (41). A solution of diol 29c (1.1492 g, 1.82 mmol) in
dry THF (0.05M, 36 mL) is degassed (2× freeze/pump/thaw) and
allowed to warm under argon. Gold(I) chloride (42.3 mg, 0.18
mmol) and PPTS (45.7 mg, 0.18 mmol) are added, and degassing
is repeated for 2 additional cycles. The reaction is then warmed to
50 °C for a period of 22 h. After this time, the solvent is removed
under reduced pressure, a drop of triethyl amine is added, and the
residue is purified by flash chromatography (9:1 hexane/THF)
yielding unsaturated spiroketal 41 (325.6 mg, 28%) followed by
the desired spiroketal product 42 (729.6 mg, 63%).
Data for 41. Rf ) 0.23 (9:1 Hexane/THF); [R]D (23 °C): -5.2°
(c ) 0.71, CHCl3); IR (neat): 2936, 1719 (sharp) cm-1;1H NMR
(500 MHz, C6D6): δ 8.17-8.16 (m, 2H), 7.39 (d, J ) 8.5 Hz, 2H),
7.16-7.07 (m, 3H), 6.85 (d, J ) 8.5 Hz, 2H), 5.83 (dt, 1H, J )
4.7, 12.1 Hz), 4.56 (d, J ) 11.4 Hz, 1H), 4.50 (d, J ) 11.4 Hz,
1H), 4.34 (ddd, 1H, J ) 3.4, 3.6, 11.4 Hz), 4.26 (dd, 1H, J ) 5.9,
8.5 Hz), 4.19 (dt, J ) 2.4, 6.8 Hz, 1H), 3.71-3.68 (m, 1H), 3.31
(s, 3H), 2.43-2.24 (m, 4H), 2.09 (t, J ) 7.0 Hz, 1H), 1.92-1.84
(m, 3H), 1.73-1.58 (m, 4H), 1.50-1.20 (m, 8H), 1.14 (t, J ) 7.4
Hz, 3H), 0.98 (d, J ) 6.9 Hz, 3H), 0.87 (d, J ) 6.9 Hz, 3H); 13C
NMR (125 MHz, C6D6): δ 165.3, 159.4, 132.8, 132.1, 129.8, 129.1,
113.9, 97.7, 78.3, 71.0, 69.9, 67.4, 54.7, 44.4, 39.3, 37.5, 36.0, 35.8,
30.5, 30.3, 30.2, 29.9, 29.9, 27.4, 26.8, 25.9, 11.4, 9.6, 5.4 ppm;
HRMS (ESI+) calcd for C35H48NaO6S2 (M + Na)+ 651.2790, found
651.3694.
Data for 46a. Rf ) 0.20 (19:1 Toluene/EtOAc); [R]D (23 °C):
-69° (c ) 0.93, CHCl3); IR (neat): 3505 (broad), 3305, 2955, 1718
1
(sharp), 1514 cm-1; H NMR (500 MHz, CDCl3): δ 8.04 (d, J )
7.2 Hz, 2H), 7.57 (t, J ) 7.4 Hz, 1H), 7.45 (t, J ) 7.6 Hz, 2H),
7.29 (d, J ) 8.5 Hz, 2H), 6.85 (d, J ) 8.6 Hz, 2H), 5.54 (td, J )
12.0, 4.8 Hz, 1H), 4.50 (d, J ) 11.4 Hz, 1H), 4.45 (d, J ) 11.2
Hz, 1H), 4.17 (m, 1H), 4.02-3.95 (m, 1H), 3.87 (s, 1H), 3.78 (s,
3H), 3.73 (dd, J ) 9.7, 3.0 Hz, 1H), 3.64 (dt, J ) 10.0, 5.4 Hz,
1H), 2.34-2.22 (m, 1H), 2.17-2.10 (m, 1H), 2.04 (d, J ) 2.3 Hz,
1H), 1.99-1.90 (m, 3H), 1.84-1.73 (m, 2H), 1.73-1.41 (m, 9H),
1.38 (dd, J ) 14.36, 1.78 Hz, 1H), 1.17-1.05 (m, 1H), 0.97 (d, J
) 6.9 Hz, 3H), 0.96 (d, J ) 7.3 Hz, 3H), 0.95-0.90 (m, 6H), 0.84
(d, J ) 6.6 Hz, 3H) ppm; 13C NMR (125 MHz, CDCl3): δ 165.7,
158.8, 132.9, 131.3, 130.5, 129.5, 128.7, 128.4, 113.7, 98.2, 84.6,
76.9, 73.0, 71.6, 70.8, 70.6, 69.3, 68.2, 55.2, 39.0, 37.0, 36.6, 36.2,
35.7, 30.1, 29.1, 26.6, 26.2, 25.8, 23.6, 21.1, 11.2, 9.2, 5.6 ppm;
HRMS (ESI+) calcd for C39H54O7Na (M + Na)+ 657.3766 found
657.3767.
(R)-Methyl 4-Hydroxy-6-methylhept-2-ynoate (43). A flask
containing (S,S) ProPhenol (479.1 mg, 0.75 mmol) dissolved in
dry toluene (50 mL) is sequentially treated with methyl propiolate
(1.87 mL, 21.0 mmol) followed by dimethyl zinc (1.2 M in hexane,
17.5 mL, 21.0 mmol). After 30 min, isovaleraldehyde is injected
(804 µL, 7.5 mmol). The reaction is transferred to the cold room
(ca. 4 °C) and stirred for a period of 60 h. Concentrated aqueous
ammonium chloride (50 mL) and diethyl ether (50 mL) are added,
and the reaction is transferred to a separatory funnel. The organic
phase is dried over magnesium sulfate, filtered, and concentrated
under reduced pressure (ca. 100 Torr). The resulting solution is
purified by flash chromatography (4:1 PE/Et2O) and concentrated
under reduced pressure (∼100 Torr) to yield propargyl alcohol 44a
as a clear oil (1.1193 g, 88%).
(2S,3R,4S,7R,8R)-1-(Tert-butyldimethylsilyloxy)-7-hydroxy-3,8-
bis(4-methoxybenzyloxy)-2,4,8-trimethylundec-10-en-5-one (62). A
solution of (S,S) ProPhenol (590.2 mg, 0.92 mmol) in dioxane (8
mL) is treated with diethyl zinc (1 M in hexanes, 1.84 mL) at
ambient temperature. A separate flask is charged with ketone 61
(913.6 mg, 2.31 mmol), aldehyde 53 (759.5 mg, 3.24 mmol) and
activated 4 Å powdered molecular sieves (500 mg). After 30 min,
the catalyst solution is transferred into the reaction flask at ambient
temperature. The reaction is stirred for 45 h and quenched with
0.1 N HCl (50 mL) and diluted with ethyl acetate (100 mL). The
solution is filtered and the organic phase is washed with an addition
portion of 0.1 N HCl followed by sodium bicarbonate (25 mL
portions). The organic phase is dried over sodium sulfate, filtered
and concentrated. Flash chromatography (85:15 Hexane/EtOAc)
gives aldol adduct 62 (518.5 mg, 35%) as a single diastereomer
along with a 1.4:1 mixture of the aldehyde and ketone (1.026 g,
60%). Repetition of the procedure above 2 additional times
(identical relative stoichiometry) with recovered starting material
gives a total of 65% yield based on recovered starting material.
Data for 62. Rf ) 0.33 (4:1 Hexane/EtOAc); [R]D (23 °C): +28°
(c ) 0.28, CH2Cl2); IR (neat): 3525 (broad), 2931, 1703 (sharp),
Data for 43.52 Enantiomeric excess is determined to be 95% on
Chiracel OD column; 98:2 heptane/isopropanol; Flow rate ) 0.8
mL/min; Detector set to 220 nm; Rt(major S): 21.6 min, Rt(minor
R): 23.67 min; Rf ) 0.30 (2:1 Hexanes/Et2O); [R]D (23 °C): 13.8°
(51) The Dunaimycins are the largest class of related natural products: (a)
Karwowski, J. P.; Jackson, M.; Maus, M. L.; Kohl, W. L.; Humphrey,
P. E.; Tillis, P. M. J. Antibiot. 1991, 44, 1312. (b) Hochlowski, J. E.;
Mullally, M. M.; Brill, G. M.; Whittern, D. N.; Buko, A. M.; Hill, P.;
McAlpine, J. B. J. Antibiot. 1991, 44, 1318. (c) Burres, N. S.;
Premachandran, U.; Frigo, A.; Swanson, S. J.; Mollison, K. W.; Fey,
T. A.; Krause, R. A.; Thomas, V. A.; Lane, B.; Miller, L. N.;
McAlpine, J. B. J. Antibiot. 1991, 44, 1331. (d) Yamazaki, M.;
Yamashita, T.; Harada, T.; Nishikiori, T.; Saito, S.; Shimada, N.; Fujji,
A. J. Antibiot. 1992, 45, 171. (e) Kirst, H. A.; Mynderse, J. S.; Martin,
J. W.; Patrick, J. B.; Paschal, J. W.; Steiner, J. L. R.; Lobkovsky, E.;
Clardy, J. J. Antibiot. 1996, 49, 162. (f) Hosotani, N.; Kumagai, K.;
Nakagawa, H.; Shimatani, T.; Saji, I. J. Antibiot. 2005, 58, 409.
(52) 43 has been synthesized previously: Rajaram, A. R.; Pu, L. Org. Lett.
2006, 8, 2019.
1
1514 cm-1; H NMR (500 MHz, CDCl3): δ 7.21 (d, J ) 9.3 Hz,
4H), 6.84 (d, J ) 8.7 Hz, 4H), 5.94-5.82 (m, 1H), 5.20-5.05 (m,
2H), 4.43-4.39 (m, 4H), 4.19-4.06 (m, 1H), 3.88-3.71 (m, 7H),
3.54-3.40 (m, 2H), 3.04-2.82 (m, 2H), 2.63-2.48 (m, 2H),
2.45-2.32 (m, 1H), 1.73-1.69 (m, 1H), 1.21 (s, 3H), 1.14 (d, J )
6.7 Hz, 3H), 0.85 (d, J ) 6.8 Hz, 3H), 0.90 (s, 9H), 0.03 (s, 3H),
0.03 (s, 3H) ppm; 13C NMR (125 MHz, CDCl3): δ 213.9, 159.1,
9
15072 J. AM. CHEM. SOC. VOL. 131, NO. 41, 2009

Synthesis and Assignment of (-)-Ushikulide A
A R T I C L E S
133.7, 131.2, 130.8, 129.3, 128.8, 117.9, 113.8, 113.7, 79.7, 78.5,
73.9, 71.7, 65.5, 63.8, 55.3, 50.0, 43.5, 39.0, 38.8, 29.7, 25.9, 19.6,
18.2, 12.6, 11.5, -5.4, -5.5; HRMS (ESI+) calcd for C36H56-
O7NaSi (M + Na)+ 651.3693 found 651.3694.
130.9, 129.4, 129.2, 128.6, 128.2, 122.2, 113.7, 113.7, 113.6, 98.4,
97.4, 84.4, 78.2, 77.9, 74.2, 73.9, 73.4, 70.7, 69.7 (3C), 67.4, 63.7,
55.2 (3C), 47.1, 42.3, 42.1, 37.5, 36.0, 34.3, 33.7, 33.0, 30.2, 30.1,
29.9, 29.7, 26.9, 26.4, 26.0, 25.5, 24.3, 22.6, 21.3, 20.3, 19.5, 18.1,
17.7, 10.9, 9.1, 7.8, 4.7, -4.2, -4.4 ppm; HRMS (ESI+) calcd for
C73H112O13NaSi (M + Na)+ 1247.7770 found 1247.7766.
(-)-Ushikulide A (1). A plastic vial is charged with THF (10
mL), pyridine (5.7 mL), and HF•pyridine (Aldrich, 3.0 mL). To a
second plastic vial containing macrocycle 73 (10.6 mg, 8.7 µmol)
is added the stock solution prepared above (5 mL). The reaction is
stirred under nitrogen for 26 h and diluted with ethyl acetate (100
mL) and diluted with water (25 mL). Finally, the biphasic mixture
is neutralized with saturated sodium bicarbonate (∼20 mL). The
organic phase is then washed with copper sulfate, water, and brine
(30 mL portions). The organic phase is then dried over sodium
sulfate, filtered, and concentrated. Flash chromagraphy (9:1 toluene/
EtOAc) gives the alcohol as a white foam.
The alcohol prepared above is immediately dissolved in dichlo-
romethane (1 mL) and Dess-Martin reagent is added (15.0 mg).
The milky white solution is stirred for 4 h and quenched with
sodium thiosulfate (5 mL) and diluted with diethyl ether (40 mL).
The organic phase is washed with sodium bicarbonate (2 × 15 mL),
dried over sodium sulfate, concentrated, and used directly in the
next step without further purification.
Seco Acid (75). Carboxylic acid 65 (64.8 mg, 0.109 mmol) and
9-BBN (33.1 mg, 0.271 mmol) are dissolved in THF (1 mL) and
stirred under a nitrogen atmosphere for a period of 14 h. The solvent
is removed under reduced pressure and water is injected (60 µL).
In a separate flask vinyl iodide 47 (109.5 mg, 0.141 mmol) is
dissolved in DMF (2 mL) and the solvent is degassed (2× freeze/
pump/thaw). PdCl2(dppf) (7.9 mg, 10.9 µmol), cesium carbonate
(106.5 mg, 0.327 mmol) and triphenyl arsine (3.3 mg, 11.0 µmol)
are added to the flask containing 74. The DMF solution of 47 is
transferred into the reaction flask and the reaction is stirred at
ambient temperature under argon for 21 h. The reaction is then
cooled in an ice bath, and 30% aqueous peroxide (2 mL) is injected
dropwise. After warming to ambient temperature for 30 min, the
reaction is poured into sodium thiosulfate (5 mL) acidified with 1
M sodium bisulfate (pH ) 2, ∼15 mL) and further diluted with
water (30 mL). The aqueous solution is washed with ethyl acetate
(3 × 25 mL), and the combined organic extracts are dried over
sodium sulfate, filtered, and concentrated. Flash chromatography
(gradient 10 to 20 to 35% chloroform/EtOAc with 1% methanol)
gives seco-acid 75 as a white foam (89.1 mg, 67%).
Data for 75. Rf ) 0.35 (2:1 chloroform/EtOAc with 1%
The crude ketone is dissolved in dichloromethane (3 mL) and
water (0.5 mL). DDQ (10.2 mg) is added at ambient temperature,
and the red solution is stirred vigorously for 45 min. Silica gel
(300 mg) is added along with additional dichloromethane (10 mL).
The solution is concentrated to dryness and the residual solid is
loaded onto a short column of silica gel. The product elutes (single
fraction collected, 9:1 DCM/EtOAc) separating easily from the red
catechol byproduct. After concentration, the crude triol is suspended
in a 3:2 mixture of glacial acetic acid and water (3 mL), and the
reaction is stirred vigorously under nitrogen for 5 h. The solvent is
removed in Vacuo, and the residue is dissolved three times in toluene
and concentrated to dryness (for the first cycle a minimum amount
of methanol is added to give a homogeneous solution). The resulting
oil is purified by flash chromatography (10:1 chloroform/methanol)
yielding 1 as a white powder (3.3 mg, 52% over 3 steps).
methanol); [R]D (23 °C): -16° (c ) 0.26, benzene); IR (neat): 3419
(broad), 2956, 1697 cm-1 1H NMR (500 MHz, CDCl3): δ
;
7.37-7.18 (m, 6H), 7.10 (dd, J ) 15.7, 7.9 Hz, 1H), 6.91-6.85
(m, 6H), 5.83 (d, J ) 15.8 Hz, 1H), 5.40 (dd, J ) 14.3, 7.5 Hz,
1H), 5.19 (dd, J ) 15.2, 8.9 Hz, 1H), 4.63-4.34 (m, 7H), 4.25-4.13
(m, 1H), 3.88-3.76 (m, 12H), 3.71-3.63 (m, 1H), 3.51-3.38 (m,
2H), 2.82-2.68 (m, 1H), 2.27-2.15 (m, 1H), 2.12-2.00 (m, 2H),
1.95-1.81 (m, 2H), 1.82-1.44 (m, 17H), 1.39 (s, 1H), 1.36 (s,
3H), 1.20-1.30 (m, 2H), 1.14 (s, 3H), 1.12 (d, J ) 6.8 Hz, 3H),
0.98 (d, J ) 6.9 Hz, 3H), 0.98-0.93 (m, 9H), 0.88 (s, 9H), 0.82
(d, J ) 6.6 Hz, 3H), 0.81-0.77 (m, 6H), 0.07 (s, 6H) ppm; 13C
NMR (125 MHz, CDCl3): δ; 170.2, 159.1, 159.0, 158.7, 154.4,
132.4, 132.0, 131.1, 130.8, 129.2, 128.8, 128.3, 119.9, 113.7, 113.7,
113.7, 98.4, 97.5, 82.5, 77.9, 74.1, 73.7, 73.7, 73.5, 70.7, 69.6, 68.2,
67.3, 64.3, 55.2 (3C), 47.1, 41.1, 40.0, 39.4, 37.4, 36.4, 34.6, 30.3,
30.1, 29.8, 28.3, 27.8, 27.4, 27.0, 26.8, 26.5, 26.0, 25.6, 24.3, 22.6,
20.6, 19.3, 18.1, 17.5, 14.9, 13.6, 10.9, 9.8, 9.3, -4.1, -4.3 ppm;
HRMS (ESI+) calcd for C73H114O14NaSi (M + Na)+ 1265.7876
found 1265.7885.
Data for Synthetic (-)-Ushikulide A:53 Rf ) 0.30 (9:1 chloroform/
methanol); [R]D (23 °C): -12° (c ) 0.28, CH3OH); IR (neat): 3440
(broad), 1711 (sharp), 1463, 1276, 985 cm-1; 1H NMR (600 MHz,
CD3OD): δ 6.76 (dd, J ) 15.7, 10.1 Hz, 1H), 5.81 (d, J ) 15.7
Hz, 1H), 5.60 (ddd, J ) 15.0, 9.4, 4.2 Hz, 1H), 5.44 (dd, J ) 15.4,
8.8 Hz, 1H), 5.30 (ddd, J ) 11.6, 5.0, 5.0 Hz, 1H), 4.33 (t, broad,
J)8.2 Hz, 1H), 4.09 (m, 1H), 4.02 (m, 1H), 3.78 (m, 1H), 3.62 (J
) dd, 9.5, 1.5 Hz, 1H), 3.18 (J ) dd, 11.0, 2.0 Hz, 1H), 3.07 (ddd,
J ) 15.1, 9.0, 8.0 Hz, 1H), 2.78 (dd, J ) 17.6, 6.6 Hz, 1H), 2.73
(J ) dd, 17.4, 8.1 Hz, 1H), 2.46 (m, 1H), 2.23 (m, 1H), 2.19 (m,
1H), 2.15-2.00 (m, 3H), 1.80-1.40 (m, 18H), 1.28 (m, 1H), 1.14
(d, J ) 6.6 Hz, 3H), 1.06 (s, 3H), 0.98 (t, J ) 7.3 Hz, 3H), 0.94
(d, J ) 7.1 Hz, 3H), 0.91 (d, J ) 6.8 Hz, 3H), 0.87 (d, J ) 7.1 Hz,
Macrocycle (73). A solution of seco-acid 75 (84.3 mg, 68.8 µmol)
in dry DCE (5 mL) is transferred to a gastight syringe and injected
into a flask containing 2-nitrobenzoic anhydride (freshly recrystal-
lized, 38.6 mg, 0.105 mmol) and 4-DMAP (18.0 mg, 0.126 mmol)
in additional DCE (55 mL) over a period of 18 h. After an additional
2 h, the reaction is poured into a separatory funnel containing
saturated sodium bicarbonate (30 mL) and ethyl acetate (90 mL).
The aqueous phase is separated, and the organic phase is washed
with ammonium chloride and sodium bicarbonate (30 mL portions).
The organic phase is then dried over sodium sulfate, filtered, and
concentrated. Flash chromatography (95:5 toluene/ethyl acetate)
gives the macrocycle as a white foam (55.4 mg, 65%).
Data for 73. Rf ) 0.54 (9:1 toluene/EtOAc); [R]D (23 °C): -20°
(c ) 0.10, chloroform); IR (neat): 2926, 1718, 1514 cm-1; 1H NMR
(500 MHz, CDCl3): δ 7.29-7.21 (m, 6H), 6.88-6.82 (m, 6H), 6.83
(dd, J ) 15.6, 7.9 Hz, 1H), 5.83 (d, J ) 15.8 Hz, 1H), 5.54-5.28
(m, 2H), 5.19 (dd, J ) 15.2, 9.3 Hz, 1H), 4.58-4.35 (m, 6H),
4.00-3.95 (m, 1H), 3.85-3.83 (m, 1H), 3.82-3.75 (m, 10H),
3.67-3.61 (m, 1H), 3.48-3.34 (m, 2H), 3.18 (d, J ) 7.0 Hz, 1H),
2.78-2.61 (m, 1H), 2.12-1.91 (m, 4H), 1.90-1.77 (m, 2H),
1.79-1.42 (m, 19H), 1.39 (s, 3H), 1.36 (s, 3H), 1.28-1.22 (m,
2H), 1.14 (s, 3H), 1.12 (d, J ) 6.8 Hz, 3H), 0.98 (d, J ) 6.9 Hz,
3H), 0.96-0.90 (m, 6H), 0.88 (s, 9H), 0.85-0.79 (m, 3H),
0.78-0.74 (m, 6H), 0.07 (s, 3H), 0.03 (s, 3H) ppm; 13C NMR (125
MHz, CDCl3): δ 164.5, 159.1, 158.9, 158.7, 150.6, 132.5, 131.2,
3H), 0.85 (d, J ) 7.0 Hz, 3H), 0.79 (d, J ) 7.0 Hz, 3H) ppm; 13
C
NMR (125 MHz): δ 212.5, 166.5, 152.1, 136.6, 129.5, 122.7, 98.8,
81.3, 75.9, 75.4, 74.9, 71.2, 70.5, 69.0, 67.1, 57.8, 42.9, 41.6, 41.9,
41.3, 39.7, 38.4, 36.3, 36.3, 34.6, 34.5, 33.0, 32.0, 30.6, 27.5, 26.6,
24.1, 23.2, 22.7, 22.3, 17.9, 11.4, 10.2, 6.1, 4.2 ppm; HRMS (ESI+)
calcd for C40H68NaO10 (M + Na)+ 731.4710, found 731.4700.
Acknowledgment. We gratefully acknowledge K. Takahashi
for providing an authentic sample of ushikulide A and D. A.
(53) The 1H NMR of synthetic ushikulide A was compared to an authentic
sample generously provided by K. Takahashi. The authentic spectrum
was obtained on the same instrument (600 MHz) and at the same
concentration (∼2 mg/mL) as our synthetic sample. All protons are
within 0.01 ppm, and the coupling constants for all peaks also match.
The 13C spectra from the literature is in excellent agreement (within
0.1 ppm) with our synthetic sample. See Supporting Information for
further details and spectral data. For published data see ref 5.
9
J. AM. CHEM. SOC. VOL. 131, NO. 41, 2009 15073

A R T I C L E S
Trost et al.
Evans for providing spectra of cytovaricin. We also acknowledge
S. Lynch for his invaluable help in the acquisition of 2-D NMR
data. We thank the Stanford Graduate Fellowship Program and
NIH (GM 13598) for their financial support of our program,
Johnson Matthey for their supply of precious metal salts, and
Aldrich for ProPhenol.
Supporting Information Available: Detailed experimental
procedures, full characterization of all products, and comparison
NMR spectra. This material is available free of charge via the
Internet at http://pubs.acs.org.
JA906056V
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15074 J. AM. CHEM. SOC. VOL. 131, NO. 41, 2009