Gram-scale synthesis of (+)-spongistatin 1: Development of an improved, scalable synthesis of the f-ring subunit, fragment union, and final elaboration

Article abstract of DOI:10.1021/ol801792k

(Chemical Equation Presented) In a quest to develop an effective, scalable synthesis of (+)-spongistatin 1 (1), we devised a concise, third-generation scalable synthesis of (+)-7, the requisite F-ring tetrahydropyran aldehyde, employing a proline-catalyzed cross-aldol reaction. Subsequent elaboration to (+)-EF Wittig salt (+)-3, followed by union with advanced ABCD aldehyde (-)-4, macrolactonization and global deprotection permitted access to >1.0 g of totally synthetic (+)-spongistatin 1 (1).

Full text of DOI:10.1021/ol801792k

ORGANIC
LETTERS
2008
Vol. 10, No. 19
4359-4362
Gram-Scale Synthesis of
(+)-Spongistatin 1: Development of An
Improved, Scalable Synthesis of the
F-Ring Subunit, Fragment Union, and
Final Elaboration
Amos B. Smith III,* Takashi Tomioka, Christina A. Risatti, Jeffrey B. Sperry, and
Chris Sfouggatakis
Department of Chemistry, Monell Chemical Senses Center, and Laboratory for
Research on the Structure of Matter, UniVersity of PennsylVania,
Philadelphia, PennsylVania 19104
smithab@sas.upenn.edu
Received August 1, 2008
ABSTRACT
In a quest to develop an effective, scalable synthesis of (+)-spongistatin 1 (1), we devised a concise, third-generation scalable synthesis of
(+)-7, the requisite F-ring tetrahydropyran aldehyde, employing a proline-catalyzed cross-aldol reaction. Subsequent elaboration to (+)-EF
Wittig salt (+)-3, followed by union with advanced ABCD aldehyde (-)-4, macrolactonization and global deprotection permitted access to >1.0
g of totally synthetic (+)-spongistatin 1 (1).
The spongipyrans comprise an architecturally unique
family of macrolides that display extraordinary cytotoxicity
against several highly chemo-resistant tumor cell lines.1
Among the natural congeners, (+)-spongistatin 1 (1) is one
of the most potent tumor cell growth inhibitors reported to
date (average GI50 values of 25-35 pM against the NCI
panel of 60 human cancer cell lines) (Scheme 1). Since their
independent isolation by the research groups of Pettit,2
Kitagawa,3 and Fusetani,4 the spongistatins have been the
focus of considerable attention in both the chemical and
biological communities.5
(1) (a) Pettit, G. A.; Cichacz, Z. A.; Gao, F.; Herald, C. L.; Boyd, M. R.;
Schmidt, J. M.; Hooper, J. N. J. Org. Chem. 1993, 58, 1302. (b) Pettit,
G. R. Pure Appl. Chem. 1994, 66, 2271.
(3) (a) Kobayashi, M.; Aoki, S.; Sakai, H.; Kawazoe, K.; Kihara, N.;
Sasaki, T.; Kitagawa, I. Tetrahedron Lett. 1993, 34, 2795. (b) Kobayashi,
M.; Aoki, S.; Sakai, H.; Kihara, N.; Sasaki, T.; Kitagawa, I. Chem. Pharm.
Bull. 1993, 41, 989.
(2) (a) Pettit, G. R.; Cichacz, Z. A.; Gao, F.; Herald, C. L.; Boyd, M. R.
J. Chem. Soc., Chem. Commun. 1993, 1166. (b) Pettit, G. R.; Cichacz, Z. A.;
Gao, F.; Herald, C. L.; Boyd, M. R. J. Chem. Soc., Chem. Commun. 1993,
1805.
(4) Fusetani, N.; Shinoda, K.; Matsunaga, S. J. Am. Chem. Soc. 1993,
115, 3977.
10.1021/ol801792k CCC: $40.75
Published on Web 08/28/2008
2008 American Chemical Society

Crimmins,10 Heathcock,11 and the Ley12 laboratories. In
addition, several synthetic approaches to the spongistatins
have been disclosed.13
Scheme 1
However, even with these seminal synthetic achievements,
the scarcity of the spongistatins has prohibited further
biological testing. Indeed, a reisolation by the Pettit group
afforded only 35 mg of the natural product from 13 tons! of
wet sponge.14 Given the limited supply from nature, our
group initiated an ambitious program to develop a scalable
approach to (+)-spongistatin 1 (1), capable of delivering ca.
1 g, not only for further biological development but also as
an integral part of a program to design simpler congeners
possessing potent tumor cell growth inhibitory activity.
To this end, we recently reported an effective synthesis
of the EF fragment of (+)-spongistatin 1 (1) via (+)-7
(Scheme 2) exploiting the Petasis-Ferrier union/rearrange-
Scheme 2a
The spongistatins possess a striking array of structural
features, including a 42-membered macrolactone incorporat-
ing two spiroketals, in conjunction with a hemiketal, a fully
substituted tetrahydropyran unit, and a highly unsaturated
side chain. The relative and absolute stereochemistries, first
deduced by Kitagawa,3 were confirmed by the Evans total
synthesis of spongistatin 2 (2)6 and the Kishi total synthesis
of spongistatin 1 (1).7 More recently, successful total
syntheses have been also achieved by the Smith,8 Paterson,9
aIncluding reagent preparations.
ment,15 a synthetic tactic employed extensively in our
laboratories to access cis-2,6-disubstituted tetrahydropyrans.16
This approach successfully provided more than 700 mg of
the EF Wittig salt, which eventually led to 80 mg of
spongistatin 1 (from 450 mg of the EF Wittig salt).17 Shortly
thereafter, the MacMillan group reported an elegant two-
step synthesis of carbohydrates,18 combining a highly enan-
tioselective proline-catalyzed cross-aldol reaction of oxy-
(5) (a) Catassi, A.; Cesario, A.; Arzani, D.; Menichini, P.; Alama, A.;
Bruzzo, C.; Imperatori, A.; Rotolo, N.; Granone, P.; Russo, P. Cell. Mol.
Life Sci. 2006, 63, 2377. (b) Pettit, R.; Woyke, T.; Pon, S.; Cichacz, Z.;
Pettit, G.; Herald, C. Med. Mycol. 2005, 43, 453. (c) Pettit, R. K.; McAllister,
S. C.; Pettit, G. R.; Herald, C. L.; Johnson, J. M.; Cichacz, Z. A. Int. J.
Antimicrob. Agents 1998, 9, 147. (d) Bai, R.; Taylor, G. F.; Cichacz, Z. A.;
Herald, C. L.; Kepler, J. A.; Pettit, G. R.; Hamel, E. Biochemistry 1995,
34, 9714.
(6) (a) Evans, D. A.; Coleman, P. J.; Dias, L. C. Angew. Chem., Int.
Ed. Engl. 1997, 36, 2738. (b) Evans, D. A.; Trotter, B. W.; Coˆte´, B.;
Coleman, P. J. Angew. Chem., Int. Ed. Engl. 1997, 36, 2741. (c) Evans,
D. A.; Trotter, B. W.; Coˆte´, B.; Coleman, P. J.; Dias, L. C.; Tyler, A. N.
Angew. Chem., Int. Ed. Engl. 1997, 36, 2744. (d) Evans, D. A.; Trotter,
B. W.; Coleman, P. J.; Coˆte´, B.; Dias, L. C.; Rajapakse, H. A.; Tyler, A. N.
Tetrahedron 1999, 55, 8671.
(10) Crimmins, M. T.; Katz, J. D.; Washburn, D. G.; Allwein, S. P.;
McAtee, L. F. J. Am. Chem. Soc. 2002, 124, 5661.
(11) Heathcock, C. H.; McLaughlin, M.; Medina, J.; Hubbs, J. L.;
Wallace, G. A.; Scott, R.; Claffey, M. M.; Hayes, C. J.; Ott, G. R. J. Am.
Chem. Soc. 2003, 125, 12844.
(7) (a) Guo, J.; Duffy, K. J.; Stevens, K. L.; Dalko, P. I.; Roth, R. M.;
Hayward, M. M.; Kishi, Y. Angew. Chem., Int. Ed. 1998, 37, 187. (b)
Hayward, M. M.; Roth, R. M.; Duffy, K. J.; Dalko, P. I.; Stevens, K. L.;
Guo, J.; Kishi, Y. Angew. Chem., Int. Ed. 1998, 37, 192.
(12) Ball, M.; Gaunt, M. J.; Hook, D. F.; Jessiman, A. S.; Kawahara,
S.; Orsini, P.; Scolaro, A.; Talbot, A. C.; Tanner, H. R.; Yamanoi, S.; Ley,
S. V. Angew. Chem., Int. Ed. 2005, 44, 5433.
(8) (a) Smith, A. B., III; Doughty, V. A.; Lin, Q.; Zhuang, L.; McBriar,
M. D.; Boldi, A. M.; Moser, W. H.; Murase, N.; Nakayama, K.; Sobukawa,
M. Angew. Chem., Int. Ed. 2001, 40, 191. (b) Smith, A. B., III; Lin, Q.;
Doughty, V. A.; Zhuang, L.; McBriar, M. D.; Kerns, J. K.; Brook, C. S.;
Murase, N.; Nakayama, K. Angew. Chem., Int. Ed. 2001, 40, 196. (c) Smith,
A. B., III; Zhu, W.; Shirakami, S.; Sfouggatakis, C.; Doughty, V. A.;
Bennett, C. S.; Sakamoto, Y. Org. Lett. 2003, 5, 761. (d) Smith, A. B., III;
Doughty, V. A.; Sfouggatakis, C.; Bennett, C. S.; Koyanagi, J.; Takeuchi,
M. Org. Lett. 2002, 4, 783.
(13) (a) Ciblat, S.; Kim, J.; Stewart, C. A.; Wang, J.; Forgione, P.; Clyne,
D.; Paquette, L. A. Org. Lett. 2007, 9, 719. (b) Allais, F.; Cossy, J. Org.
Lett. 2006, 8, 3655. (c) Holson, E. B.; Roush, W. R. Org. Lett. 2002, 4,
3723. (d) Holson, E. B.; Roush, W. R. Org. Lett. 2002, 4, 3719. (e) Lemaire-
Audoire, S.; Vogel, P. J. Org. Chem. 2000, 65, 3346. (f) Zuev, D.; Paquette,
L. A. Org. Lett. 2000, 2, 679.
(14) Pettit, G. R. Arizona State University, personal communication.
(15) Smith, A. B., III; Sfouggatakis, C.; Gotchev, D. B.; Shirakami, S.;
Zhu, W.; Doughty, V. A. Org. Lett. 2004, 6, 3637.
(9) Paterson, I.; Chen, D. Y.-K.; Coster, M. J.; Acena˜, J. L.; Bach, J.;
Gibson, K. R.; Keown, L. E.; Oballa, R. M.; Trieselmann, T.; Wallace,
D. J.; Hodgson, A. P.; Norcross, R. D. Angew. Chem., Int. Ed. 2001, 40,
4055.
(16) Smith, A. B., III; Fox, R. J.; Razler, T. M. Acc. Chem. Res. 2008,
41, 675.
(17) Sfouggatakis, C. PhD Thesis, University of Pennsylvania, 2004.
(18) Northrup, A. B.; MacMillan, D. W. C. Science 2004, 305, 1753.
4360
Org. Lett., Vol. 10, No. 19, 2008

aldehydes19 with a Mukaiyama aldol reaction. This approach
to carbohydrates via the intermediacy of a lactone held the
promise of an even more concise approach to (+)-7 pos-
sessing the requisite cis-tetrahydropyran F-ring of (+)-
spongistatin 1.20 The potential benefits of this strategy would
include: (1) a shorter reaction sequence, (2) excellent cost
of goods, (3) stable intermediates, (4) simple operations,
including purifications, (5) high reproducibility and ef-
ficiency, and most important, (6) scalability.
At the outset of what now constitutes a third-generation
synthesis in the spongistatin area, we envisioned a five-step
approach to (+)-7 from known21 aldehyde 6 taking advantage
of the MacMillan protocol (Scheme 3). However, initial
orientations; facile separation of the diastereomeric triols was
thus possible. Treatment of the lactone diol (-)-12 with silver
oxide, benzyl bromide, and calcium sulfate in 1,2-dichloro-
ethane then provided (-)-10 in 78% yield. In this transfor-
mation, monobenzylation occurs first at the R-position at 40
°C over a 24 h time period; increasing the temperature to
60 °C, with additional silver oxide and benzyl bromide, leads
to the second benzylation after 8 h in 78% yield. Recovered
monobenzylated lactone (ca. 17-20%) can be converted to
the desired bis-benzylated product (-)-10 upon exposure to
the original benzylation conditions. In this fashion, a two-
step yield of 91% can be achieved. Continuing with the
synthesis, addition of the Grignard reagent derived from cis-
1-bromo-3-hexene to lactone (-)-10, followed by reduction
of the derived lactol with Et3SiH and BF3·Et2O, employing
the Kishi protocol,25 cleanly produced the desired cis-
tetrahydropyran (+)-13 in high yield (89%; two steps)
(Scheme 4). Removal of the BPS protecting group also
Scheme 3
Scheme 4
efforts indicated that the Mukaiyama aldol was not applicable
for large-scale production of 9 (>100 g scale). The issue
was attributed to the instability of the ꢀ-hydroxyaldehyde
(+)-8.22 As a result, an approach that permitted production
of (+)-7 on large scale had to be developed.
In keeping with the MacMillan aldol route, conversion of
6 to (+)-8 was easily performed on large scale (>100 g).
Exhaustive extraction with water eliminates >90% of both
the homoaldol byproduct and the reaction solvents (cf. DMF/
dioxane) without compromise in yield23 or the need for
chromatography. The resultant diastereomeric mixture of
aldehydes (5:1) could then be treated without separation with
(methoxycarbonyl-methylene)-triphenylphosphorane to fur-
nish unsaturated esters 11 (still a 5:1 mixture) in 94% yield.
Sharpless asymmetric dihydroxylation using AD-mix ꢀ,24
followed by cyclization with pyridinium p-toluenesulfonate,
successfully provided the corresponding lactone (-)-12 in
81% yield (based on the anti diastereomer). Pleasingly, the
only triol diastereomer to undergo lactone formation was the
anti isomer that places all substituents in pseudoequitorial
occurred during the reductive Et3SiH process.26 Final Parikh-
Doering oxidation27 of the primary hydroxyl provided the
F-ring aldehyde (+)-7 in 94% yield (50% overall yield from
6). To date, the third-generation route to the F-ring aldehyde
(+)-7 has provided more than 68 g.
Construction of Wittig salt (+)-3 from (+)-7 followed our
previous reported15 sequence (Scheme 5). This sequence
proved highly efficient and enabled the production of
multigram quantities of (+)-3. Specifically, conversion of
(+)-7 to EF Wittig salt (+)-3 begins by addition of dithiane
(-)-1415 under chelation controlled conditions to construct
the linear precursor of the E-ring of (+)-spongistatin 1, (+)-
15. After a 9-step sequence that includes an acid-catalyzed
spiroketalization, ozonolysis, and R-methylenation with
Eschenmoser’s salt, allyl iodide (+)-16 is obtained in 43%
(19) Northrup, A. B.; Mangoin, I. K.; Hettche, F.; MacMillan, D. W. C.
Angew. Chem., Int. Ed. 2004, 43, 2152.
(20) (a) Lactones comprise well known precursors of cis-tetrahydropy-
rans. Lemaire-Audoire, S.; Vogel, P. Tetrahedron Lett. 1998, 39, 1345. (b)
Zhao, G.-L.; Liao, W.-W.; Cordova, A. Tetrahedron Lett. 2006, 47, 4929.
(21) Choi, W. B.; Wilson, L. J.; Yeola, S.; Liotta, D. C.; Schinazi, R. F.
J. Am. Chem. Soc. 1991, 113, 9377.
(22) Ka¨llstro¨m, S.; Erkkila¨, A.; Pihko, P. M.; Sjo¨holm, R.; Sillanpa¨a¨,
R.; Leino, R. Synlett 2005, 751.
(23) Twenty water extractions successfully removed most major impuri-
ties, unreacted propanal, homoaldol byproduct, and reaction solvents (cf.
DMF/dioxane).
(25) Lewis, M. D.; Cha, J. K.; Kishi, Y. J. Am. Chem. Soc. 1982, 104,
4976.
(26) Yoshimitsu, T.; Song, J. J.; Wang, G.-Q.; Masamune, S. J. Org.
Chem. 1997, 62, 8978.
(24) Hermitage, S. A.; Murphy, A.; Nielsen, P.; Roberts, S. M.
Tetrahedron 1998, 54, 13185.
(27) Parikh, J. P.; Doering, W. E. J. Am. Chem. Soc. 1967, 89, 5505.
Org. Lett., Vol. 10, No. 19, 2008
4361

Scheme 5
Scheme 6
sequence requires 8 steps compared to the 12 steps in our
second-generation synthesis, and proceeds in 50% overall
yield from 6. On 10 to 100 g scales, the average yield for
each step was over 85%. With this achievement, the third-
generation synthesis of Wittig-salt (+)-3 now proceeds with
a longest linear sequence of 27 steps from commercially
available cis-1,4-butenediol (9.5% overall), and as such
represents a significant improvement, not only in yield, but
also scalability. For (+)-spongistatin, the longest linear
sequence is 31 steps (based on EF Wittig salt) and proceeds
with an overall yield of 3.1%. To date, employing the second-
and third-generation strategies, we have prepared 1.009 g
of totally synthetic (+)-spongistatin 1 (1).
overall yield.15 Alkylation of the latter with cyanohydrin 17
next successfully installs the chlorinated side-chain of (+)-
spongisatin 1. Final conversion to the EF Wittig salt is then
achieved in 4 steps and 68% yield. Employing the now
scalable approach to (+)-7, we had in hand >5.8 g of EF
Wittig salt (+)-3.
Completion of the third-generation synthesis of (+)-
spongistatin 1 began with fragment union of EF Wittig salt
(+)-3 with advanced aldehyde (-)-4.8d Wittig union em-
ploying the MeLi·LiBr conditions originally introduced by
Crimmins10 in their synthesis of (+)-spongistatin 1 and 2,
and subsequently utilized by Heathcock11 and Ley,12 pro-
vided alkene (+)-19 in 64% yield.
Acknowledgment. Financial support was provided by the
NIH (NCI) through Grant No. CA-70329. C.A.R. was
supported by the American Cancer Society-Michael Schmidt
Postdoctoral Fellowship and J.B.S. by a National Institutes
of Health, Ruth L. Kirschstein National Research Service
Award (FCA121716A) from the National Cancer Institute.
We also thank Drs. George Furst (University of Pennsylva-
nia) and Rakesh Kohli (University of Pennsylvania) for their
assistance in obtaining NMR spectra and high-resolution
mass spectra, respectively.
For final elaboration to (+)-spongistatin 1 (1), the two-
step deprotection/reprotection sequence utilized in our second-
generation synthesis to reveal seco-acid (+)-20 was not
applicable, due to the failure of KF to remove cleanly the
TES protecting groups (formally TMS groups).7b Use of
TBAF, as employed by Heathcock to remove selectively the
TES ethers at C(41) and C(42), and the TIPS ester11 was
therefore employed. Selective Yamaguchi macrolactonization
then provided the desired 42-membered macrolide in 65-80%
yield.15 Global deprotection employing 5 M HF in acetoni-
trile (1:1) completed the synthesis of (+)-spongistatin 1 (1)
in 87% yield. Synthetic (+)-spongistatin 1 was identical in
all respects with the natural product.
Supporting Information Available: Spectroscopic and
analytical data and selected experimental procedures. This
material is available free of charge via the Internet at
http://pubs.acs.org.
In summary, an effective, scalable synthesis of the F-ring
subunit of (+)-spongistatin 1 (+)-7 exploiting an L-proline
organocatalyzed cross-aldol reaction has been achieved; the
OL801792K
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Org. Lett., Vol. 10, No. 19, 2008