A concise synthesis of (-)-lasonolide A

Article abstract of DOI:10.1021/ja411270d

Lasonolide A is a novel polyketide displaying potent anticancer activity across a broad range of cancer cell lines. Here, an enantioselective convergent total synthesis of the (-)-lasonolide A in 16 longest linear and 34 total steps is described. This approach significantly reduces the step count compared to other known syntheses. The synthetic strategy utilizes alkyne-bearing substrates as core building blocks and is highlighted by stitching together two similarly complex halves via a key Ru-catalyzed alkene-alkyne coupling and macrolactionization.

Full text of DOI:10.1021/ja411270d

Communication
pubs.acs.org/JACS
A Concise Synthesis of (−)-Lasonolide A
Barry M. Trost,* Craig E. Stivala, Kami L. Hull, Audris Huang, and Daniel R. Fandrick
Department of Chemistry, Stanford University, Stanford, California 94305-5580, United States
S
* Supporting Information
second was for the key alkene−alkyne coupling, which would
join fragments 2 and 3 while simultaneously forging the
skipped 1,4-diene. One unique aspect of the proposed coupling
is its propensity to generate branched 1,4-diene products,
whereas, for this application, a linear diene is required. While
use of this method to generate a linear diene has not been
demonstrated in a complex molecule synthesis, the prospect
that such regioselectivity may occur in some circumstances
(vide infra)24,25 led us to pursue this thought for the
aforementioned target. This analysis identified alkyne 2 and
alkene 3 as key building blocks, both containing similar levels of
complexity.
ABSTRACT: Lasonolide A is a novel polyketide display-
ing potent anticancer activity across a broad range of
cancer cell lines. Here, an enantioselective convergent total
synthesis of the (−)-lasonolide A in 16 longest linear and
34 total steps is described. This approach significantly
reduces the step count compared to other known
syntheses. The synthetic strategy utilizes alkyne-bearing
substrates as core building blocks and is highlighted by
stitching together two similarly complex halves via a key
Ru-catalyzed alkene−alkyne coupling and macro-
lactionization.
Alkyne 2 was prepared in a convergent manner, beginning
with construction of the tetrahydropyran ring (Scheme 2A).
asonolides are polyketides extracted from the Caribbean
Lmarine sponge, Forcepia sp. To date, seven lasonolides
(A−G) have been isolated from their natural source.1,2 In 1994,
lasonolide A was submitted to cytotoxicity testing and profiling
in the National Cancer Institute (NCI)’s 60-cell panel screen.
This screen confirmed that lasonolide A was highly potent
toward a broad range of cancer cell lines and that this
cytotoxicity stemmed from a unique mechanism of action,3
which still has not been fully elucidated.4 Due to its scarcity
from its natural source and its unique mechanism of action,
numerous synthetic studies have been directed toward the
lasonolides.5−14 These efforts have led to four successful total
syntheses of lasonolide A.15−20 Despite these efforts, research
into the molecule’s pharmacology has been hampered due to
the extremely limited availability of the sponge and difficulty in
performing lengthy chemical syntheses.21 These aspects make a
compelling case for the evolution of a more step22 and atom
economic23 synthesis of lasonolide.
Scheme 2. Preparation of Alkyne Fragment
Lasonolide A (1) consists of a 20-membered macrolide that
contains a skipped 1,4-diene, and two highly substituted
tetrahydropyran rings. The design of our synthetic plan
(Scheme 1) relied on the utilization of alkynes for the assembly
of two challenging subunits within the macrolide. The first was
the formation of the C12−C13 trisubstituted olefin, and the
The formation of each new stereocenter on the tetrahydropyr-
an ring was effected by utilizing the central C21 hydroxyl group
as a stereochemical handle. This stereocenter was formed by a
direct Zn/Prophenol-catalyzed aldol26 addition between ynone
5 and aldehyde 4 to generate a β-hydroxyynone 6 in 78% yield
and 99% ee. In a relay of stereochemical information, the C21
stereocenter directs the reduction of 6 with DIBAL-H to
furnish a 17:1 mixture of diastereomers favoring the syn-1,3-
diol.27,28 The resulting propargylic alcohol can be selectively
Scheme 1. Synthesis Plan for Lasonolide A
Received: November 4, 2013
© XXXX American Chemical Society
A
dx.doi.org/10.1021/ja411270d | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
protected as a TBDPS ether, affording acetonide 7 in 90% yield
over the two steps.
Scheme 3. Synthesis of Alkene Fragment
An efficient way to establish the C22 quaternary stereocenter
utilizes the C21 hydroxyl group in a diastereoselective
transacetalization reaction.19 To achieve this objective, 7 was
treated with TFA and an excess of benzaldehyde in CHCl3 for
an extended period of time (18 h). As a result, the formation of
the desired acetal occurred in good yield with 5:1 chemo-
selectivity and 10:1 diastereoselectivity. Further, the undesired
isomers could be separated by column chromatography and
recycled to provide additional product. After two cycles, the
desired acetal, containing the newly formed C22 quaternary
stereocenter, could be isolated in 93% yield.
Continuing with the synthesis of 2, oxidation of the primary
neopentylic alcohol was accomplished using TPAP/NMO29 to
furnish aldehyde 8 in 76%. Removal of the silyl groups with
TBAF generated lactol 9 in excellent yield. Gratifyingly,
Horner−Wadsworth−Emmons olefination of 9 spontaneously
formed the desired tetrahydropyran ring as a single
diastereomer in 91% yield. The excellent diastereoselectivity
of this transformation reasonably arose from the reversible
nature of the conjugate addition, which allowed for the
formation of the thermodynamic product, the cis-2,6-
substituted tetrahydropyran. Direct reduction of the tert-butyl
ester with DIBAL-H delivered aldehyde 10 in 83% yield with
no issues of overreduction.
Synthesis of the side chain 15 (Scheme 2B) commenced with
the carbocupration of propargyl alcohol and iso-amylmagne-
sium bromide to generate allylic alcohol 12 in 78% yield. In
contrast to previous syntheses, which required multiple steps to
access this same compound, this approach demonstrates the
benefits of alkyne building blocks. Transesterification of known
ester 13 with 12 provided α-hydroxy allylic ester 14.19 The
secondary alcohol was subsequently converted to its TBS ether
and the phosphonium salt was obtained by nucleophilic
substitution of the alkylbromide with triphenylphosphine.
With phosphonium salt 15 in hand, Wittig olefination with
aldehyde 10 proceeded uneventfully, giving a high yield of a
single geometric isomer. Interestingly, removal of the
benzylidene acetal turned out to be more difficult than
anticipated. The acetal was resistant to hydrolysis with a variety
of Brønsted acids, which included HCl, CSA, and TsOH. On
the other hand, LiBF4 cleanly facilitated the removal of the
benzylidene acetal,30 along with the inconsequential cleavage of
the TBS ether, providing the target fragment alkyne 2 in 80%
yield.
propynylmagnesium bromide.32 (S)-CBS catalyzed reduction
of 21 afforded the syn-syn-stereotriad 22 in 71% yield and in
>20:1 diastereoselectivity. The use of freshly distilled catechol-
borane in nitroethane as solvent33 was critical to ensure high
diastereoselectivity and yield for this transformation. Addition-
ally, at this stage, the all-syn isomer could be isolated from the
undesired stereoisomers generated from the enzymatic
reduction.
An equivalent of a trans hydro-alkylation under development
in our laboratories was envisioned to access the C12−C13 (Z)-
trisubstituted alkene. The sequence began with a Ru-catalyzed
hydrosilylation34 of propargylic alcohol 22 to generate the
desired trisubstituted (Z)-vinylsilane 23 with high levels of
geometric selectivity (>15:1), in 86% yield. Formation of the
substituted tetrahydropyran ring was effected through a
simultaneous Hoveyda−Grubbs second-generation catalyzed
cross metathesis/oxa-Michael sequence between alkene 23 and
crotonaldehyde.35 The reaction proceeded with high diaster-
eoselectivity (dr >20:1), and the desired cis-2,6-substituted
tetrahydropyran (24) was isolated in 63% yield.36
Elaboration of the aldehyde into an (E,E)-dienoate moiety
was envisioned using a Horner−Wadsworth−Emmons olefina-
tion. Initial attempts using KHMDS as a base provided the
desired dienoate accompanied with extensive epimerization at
C7 (dr = 1:1), presumably arising from a retro-Michael/
Michael reaction. Changing the base to LDA reduced the
epimerization and 25 was obtained as a 85:15 mixture of
diastereomers at C7. Gratifyingly, it was found that the use of
LiOH and 4 Å molecular sieves20 was superior in minimizing
substrate epimerization and the desired dienoate could be
obtained as a 10:1 mixture of diastereomers.37 The Pd-
catalyzed sp2−sp3 Hiyama coupling38−40 between the vinyl
silane 25 and allyl acetate delivered 1,4-diene 26 in 85% yield.
Finally, saponification of the ethyl ester with LiOH furnished
the desired alkene fragment (3).
Having both alkene 3 and alkyne 2 in hand,we explored the
key alkene−alkyne coupling. Insights from model studies,
employing the [CpRu(MeCN)3]PF6 catalyst, suggested that
there was a significant solvent effect associated with both
reactivity and the linear to branched selectivity. Reactions run
in chlorinated solvents (CH2Cl2 and dichloroethane) typically
The synthesis of alkene 3 was pursued as illustrated in
Scheme 3. Initial efforts were dedicated to establishing the
absolute stereochemistry of the fragment by an enzymatic
dynamic kinetic asymmetric reduction.31 After extensive
screening it was found that β-ketoester 18, readily available
from a Blaise reaction between α-bromo ester 17 and allyl
cyanide 16, could be reduced to the corresponding β-
hydroxyester 19 as a 4:1 mixture of syn:anti diastereomers
(75%). The enantiomeric excess of the major syn diastereomer
was measured to be >95% by chiral GC analysis. It is worth
noting that careful control of the reaction pH (4.5) proved to
be essential in order to avoid the undesired olefin isomer-
ization.
β-Hydroxyester 19 was converted to ynone 21 in three
straightforward steps, which included TIPS protection of the
secondary alcohol, conversion of the ethyl ester into a Weinreb
amide, and formation of the ynone by addition of 1-
B
dx.doi.org/10.1021/ja411270d | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
Scheme 4. Completion of the Synthesis of (−)-Lasonolide A
showed little to no evidence of alkene−alkyne coupling. When
DMF was used as solvent, the desired coupling reaction
proceeded; however, poor linear-to-branched product ratios
were observed. Acetone was found to be the optimal solvent
generally delivering the products in good yield with reasonable
linear to branched ratios. In this way, an approximately 2:1
linear:branched product was obtained as an acetonide between
the C21 and C30 hydroxyl groups in 69% yield (82% brsm).
Formation of the acetonide was determined to precede the
alkene−alkyne coupling event and most likely results from the
Lewis acidity of the cationic ruthenium catalyst. The excellent
chemoselectivity of this process is illustrated by the tolerance of
the dense array of functionality on alkyne 2 and alkene 3 under
the reaction conditions, which is a testament to the mildness of
the Ru-catalyzed alkene−alkyne coupling reaction.
group, albeit in a low unoptimized 10% yield. The identity of
this macrolactone was verified by an independent synthesis.
To circumvent this difficulty, a different macrolactionization
precursor was designed. Protection of the least sterically
hindered alcohols of both 32 and 33 as their TBS ethers, and in
situ hydrolysis of the resulting TBS ester with HCl, delivered
seco acid 34 in good yield. To our delight, when 34 was
subjected to the Yamaguchi macrolactionization protocol, TBS-
protected lasonolide A (35) was generated in 40−62%.42 At
this stage, the linear and branched isomers, generated from the
alkene−alkyne coupling, were separable by column chromatog-
raphy, allowing for the isolation of pure TBS-protected
lasonolide A (35). Finally, global deprotection of the silyl
groups with HF·Pyr15,16 furnished the target molecule
(−)-lasonolide A (1) in 75% yield.
The synthetic (−)-lasonolide A along with the isomeric
macrolactone analogue, generated from this work, were
screened against a variety of cancer cell lines.44 IC50 values
for the DU145, HCT116, and MCF7 cell lines with our
synthetic lasonolide A were consistent with the values
previously reported from the NCI’s 60-cell panel screen.
Biological testing also revealed that lasonolide A inhibits A2058,
Adr-Res, BXPC3, H460, SK-BR-3, and KPL-4 cell lines at nM
concentrations. The isomeric lasonolide analogue A was
significantly less active than lasonolide A in terms of its
application to a broad range of cancer cell lines. Interestingly, it
did exhibit nM activity against the HCT116 cell line with IC50
values equal to that of lasonolide A.
Efforts to increase the linear-to-branched selectivity24,25 by
varying the reaction conditions proved ineffective. However, a
positive effect was observed by incorporating a TBS protecting
group on the C9 hydroxyl group, which increased the linear to
branched selectivity to 3:1 in 43% yield.
Completion of lasonolide A was accomplished from both 28
and 30 and each reaction sequence is presented in Scheme
4.41,42 Hydrolysis of the acetonide with CSA in methanol
afforded the corresponding triol 32 and tetraol 33. Unfortu-
nately, lasonolide A was never observed from the direct
macrolactionization of tetraol 32 utilizing several macro-
lactonization reagents (i.e., Yamaguchi and Mukaiyama)
under various reaction conditions. These reactions generally
resulted in the decomposition of the starting seco acid.
Interestingly, use of the Shiina reagent43 led to the selective
formation of a macrolide, A (Figure 1), arising from a
cyclization of the carboxylic acid onto the C28 hydroxyl
In conclusion, a concise synthesis of (−)-lasonolide A was
completed in only 16 linear steps and 34 total steps (1.6%
overall yield from allyl cyanide (16)) from commercially
available starting materials, drastically reducing the step count
compared to the previously known routes. This atom- and step-
economical approach stems from the key role of alkynes as
building blocks and intermediates. Furthermore, it features a
key Ru-catalyzed alkene−alkyne coupling leading to the less
common linear product as the major isomer, which is the first
demonstration of an intermolecular transformation of this type
that favors the linear product in such a complex setting.
ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental procedures and analytical data for all new
compounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
Figure 1. Lasonolide Analog A.
C
dx.doi.org/10.1021/ja411270d | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
(23) Trost, B. M. Science 1991, 254, 1471−1477.
AUTHOR INFORMATION
■
(24) Trost, B. M.; Toste, F. D.; Pinkerton, A. B. Chem. Rev. 2005,
101, 2067−2096.
Corresponding Author
bmtrost@stanford.edu
(25) Trost, B. M.; Frederiksen, M. U.; Rudd, M. T. Angew. Chem., Int.
Ed. 2005, 44, 6630−6666.
Notes
(26) Trost, B. M.; Fettes, A.; Shireman, B. T. J. Am. Chem. Soc. 2004,
126, 2660−2661.
The authors declare no competing financial interest.
(27) Kiyooka, S.-j.; Kuroda, H.; Shimasaki, Y. Tetrahedron Lett. 1986,
27, 3009−3012.
ACKNOWLEDGMENTS
■
We thank the NIH (GM33049) for generous support to our
programs. C.E.S., K.L.H., and A.H. are the recipients of NIH
Ruth L. Kirschstein National Research Service Awards
(GM101747, GM086093, and GM077840, respectively). We
also thank Johnson-Matthey for generous gifts of palladium and
ruthenium salts and Ranier Kalkofen for performing prelimi-
nary experiments. Finally, we are grateful to Codexis, Inc. for
the generous donation of enzymes and NADPH and
Genentech (Gail Phillips, Vishal Verma and John Flygare) for
screening the synthetic (−)-lasonolide A and lasonolide
analogue.
(28) Mohr, P. Tetrahedron Lett. 1991, 32, 2219−2222.
(29) Ley, S. V.; Norman, J.; Griffith, W. P.; Marsden, S. P. Synthesis
1994, 7, 639−666.
(30) (a) Lipshutz, B. H.; Harvey, D. F. Synth. Commun. 1982, 12,
267−277. (b) For a recent example of benzylidene acetal hydrolysis
using LiBF4 in a complex setting, see: Araoz, R.; Servent, D.; Molgo, J.;
Iorga, B. H.; Fruchart-Gaillard, C.; Benoit, E.; Gu, Z.; Stivala, C. E.;
Zakarian, A. J. Am. Chem. Soc. 2011, 133, 10499−10511.
(31) For examples of enzymatic dynamic kinetic asymmetric
reductions of β, see: (a) Fauve, A.; Veschambre, H. Tetrahedron
Lett. 1987, 28, 5037−5040. (b) Kawai, Y.; Takanobe, K.; Ohno, A.
Bull. Chem. Soc. Jpn. 1995, 68, 285−288.
(32) Williams, J. M.; Jobson, R. B.; Yasuda, N.; Marchesini, G.;
Dolling, U.-H.; Grabowski, E. J. J. Tetrahedron Lett. 1995, 36, 5461−
5464.
REFERENCES
■
(1) Horton, P. A.; Koehn, F. E.; Longley, R. E.; McConnell, O. J. J.
Am. Chem. Soc. 1994, 116, 6015−6016.
(33) Yu, C.-M.; Kim, C.; Kweon, J. H. Chem. Commun. 2004, 21,
2494−2495.
(2) Wright, A. E.; Chen, Y.; Winder, P. L.; Pitts, T. P.; Pomponi, S.
A.; Longley, R. E. J. Nat. Prod. 2004, 67, 1351−1355.
(3) Alley, M. C.; Scudiero, D. A.; Monks, A.; Hursey, M. L.;
Czerwinski, M. J.; Fine, D. L.; Abbott, B. J.; Mayo, J. G.; Shoemaker, R.
H.; Boyd, M. R. Cancer Res. 1988, 48, 589−601.
(4) Prommier et al. have indicated that lasonolide A is a reversible
inducer of chromosome condensation at all phases of the cell cycle.
However, they have not been able to identify the cellular target of
lasonolide A. Zhang, Y.-W.; Ghosh, A. K.; Pommier, Y. Cell Cycle
2012, 11, 4424−4435.
(34) Trost, B. M.; Ball, Z. T.; Joge, T. Angew. Chem., Int. Ed. 2003,
42, 3415−1418.
(35) Garber, S. B.; Kingsbury, J. S.; Gray, B. L.; Hoveyda, A. H. J. Am.
Chem. Soc. 2000, 122, 8168−8179.
(36) For an example of a cross metathesis/oxa-Michael addition to
from a tetrahydropyran, see: Fuwa, H.; Noto, K.; Sasaki, M. Org. Lett.
2012, 12, 1636−1639.
(37) These diastereomers become separable after the macro-
lactionization.
(38) Hiyama, T.; Hatanaka, Y. Pure Appl. Chem. 1994, 66, 1471−
1478.
(39) Trost, B. M.; Machacek, M. R.; Ball, Z. T. Org. Lett. 2003, 5,
1895−1898.
(40) Dey, R.; Chattopadhyay, K.; Ranu, B. C. J. Org. Chem. 2008, 73,
9461−9464.
(41) For clarity, only the major (linear) isomer from the alkene−
alkyne coupling is drawn. In the case where R = H, a 2:1 ratio of
products is advanced, and in the case of R = TBS, a 3:1 ratio of
products is advanced. The linear and branched isomers become
separable after macrolactionization.
(42) See Supporting Information for details.
(43) Shiina, I.; Kubota, M.; Oshiumi, H.; Hashizume, M. J. Org.
Chem. 2004, 69, 1822−1830.
(44) IC50 values (nM) for lasonolide A (1): A2058, 16.6; Adr-Res,
414.1; BXPC3, 148.1; DU145, 29.2; HCT116, 9.8; H460, 20.7; MCF7,
47.4; SK-BR-3, 40.2; KPL-4, 39.4. IC50 values (nM) for lasonolide
analogue A: A2058, >1000; Adr-Res, >1000; BXPC3, >1000; DU145,
>1000; HCT116, 12.0; H460, >1000; MCF7, >1000; SK-BR-3, >1000;
KPL-4, >1000.
(5) Ghosh, A. K.; Ren, G.-B. J. Org. Chem. 2012, 77, 2559−2565.
(6) Deba, T.; Yakushiji, F.; Shindo, M.; Shishido, K. Synlett 2003, 10,
1500−1502.
(7) Yoshimura, T.; Bando, T.; Shindo, M.; Shishido, K. Tetrahedron
Lett. 2004, 45, 9241−9244.
(8) Gurjar, M. K.; Kumar, P.; Rao, B. V. Tetrahedron Lett. 1996, 37,
8617−8620.
(9) Gurjar, M. K.; Chakrabarti, A.; Rao, B. V.; Kumar, P. Tetrahedron
Lett. 1997, 38, 6885−6888.
(10) Nowakowski, M.; Hoffmann, H. M. R. Tetrahedron Lett. 1997,
38, 1001−1004.
(11) Beck, H.; Hoffmann, H. M. R. Eur. J. Org. Chem. 1999, 11,
2991−2995.
(12) Hart, D. J.; Patterson, S.; Unch, J. P. Synlett 2003, 9, 1334−
1338.
(13) Dalgard, J. E.; Rychnovsky, S. D. Org. Lett. 2005, 7, 1589−1591.
(14) Sawant, K. B.; Ding, F.; Jennings, M. P. Tetrahedron Lett. 2006,
47, 939−942.
(15) Lee, E.; Song, Y. H.; Kang, J. W.; Kim, D.-S.; Jung, C.-K.; Joo, J.
M. J. Am. Chem. Soc. 2002, 124, 384−385.
(16) Song, H. Y.; Joo, J. M.; Kang, J. W.; Kim, D.-S.; Jung, C.-K.;
Kwak, H. S.; Park, J. H.; Lee, E.; Hong, C. Y.; Jeong, S. W.; Jeon, K.;
Park, J. H. J. Org. Chem. 2003, 68, 8080−8087.
(17) Ghosh, A. K.; Gong, G. Org. Lett. 2007, 9, 1437−1440.
(18) Ghosh, A. K.; Gong, G. Chem. Asian J. 2008, 3, 1811−1823.
(19) Kang, S. H.; Kang, S. Y.; Kim, C. M.; Choi, H.-w.; Jun, H. S.;
Lee, B. M.; Park, C. M.; Jeong, J. W. Angew. Chem., Int. Ed. 2003, 42,
4779−4782.
(20) Yoshimura, T.; Yakushiji, F.; Kondo, S.; Wu, X.; Shindo, M.;
Shishido, K. Org. Lett. 2006, 8, 475−478.
(21) Isbrucker, R. A.; Guzman, E. A.; Pitts, T. P.; Wright, A. E. J.
Pharm. Exp. Ther 2009, 331, 733−739.
(22) Wender, P. A.; Verma, V. A.; Paxton, T. J.; Pillow, T. H. Acc.
Chem. Res. 2008, 41, 40−49.
D
dx.doi.org/10.1021/ja411270d | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX