Total Synthesis of (-)-Callystatin A

Article abstract of DOI:10.1021/ol0158922

(matrix presented) An effective total synthesis of (-)-callystatin A (1), member of the leptomycin family of antibiotics, has been achieved. The synthesis features Evans extended aldol methodology to construct the northern polypropionate subunit and two s

Full text of DOI:10.1021/ol0158922

ORGANIC
LETTERS
2001
Vol. 3, No. 11
1685-1688
Total Synthesis of (−)-Callystatin A
Amos B. Smith, III,* and Benjamin M. Brandt
Department of Chemistry, UniVersity of PennsylVania,
Philadelphia, PennsylVania 19104
smithab@sas.upenn.edu.
Received March 26, 2001
ABSTRACT
An effective total synthesis of (−)-callystatin A (1), member of the leptomycin family of antibiotics, has been achieved. The synthesis features
Evans extended aldol methodology to construct the northern polypropionate subunit and two separate Julia olefinations to assemble the
conjugated dienes. The total synthesis proceeded in 2.3% overall yield with the longest linear sequence of 15 steps.
In 1997 Kobayashi and co-workers disclosed the isolation
and planar structure of (-)-callystatin A (1), a remarkably
potent cytotoxic agent (e.g., IC50 0.01 ng/mL in vitro against
the KB cancer cell line).1 Subsequently, the Kobayashi group
confirmed the relative and absolute stereochemistries of
(-)-1 via partial2 and total synthesis,3 and then quite recently
reported construction of several structural analogues4 which
provide insight on the structure-activity relationships (SAR).
Crimmins and King5 have also achieved an effective total
synthesis of (-)-callystatin A (1).6
(3), and several other antibiotics.11 Most similar to (-)-
callystatin A (1), however, is leptomycin B12a (4), an
antitumor agent shown to exhibit a similar cytotoxic profile.12b
(7) Paterson, I.; Hulme, A. N. J. Org. Chem. 1995, 60, 3288-3300.
(8) (a) Umezawa, H.; Takaaki, A.; Uotani, K.; Hamada, M.; Takeuchi,
T.; Takahashi, S. J. Antibiot. 1980, 33, 1594-1596. (b) Uotani, K.;
Naganawa, H.; Kondo, S.; Aoyagi, T.; Umezawa, H. J. Antibiot. 1982, 35,
1495-1499. (c) Uotani, K. Ph.D. Thesis, Institute of Microbial Chemistry,
Tokyo, Japan.
(9) Kobayashi, S.; Tsuchiya, K.; Harada, T.; Nishide, M.; Kurokawa,
T.; Nagakawa, T.; Shimada, N.; Kobayashi, K. J. Antibiot. 1994, 47, 697-
702.
(-)-Callystatin A (1) possesses a number of structural
features in common with the enzyme inhibitor (-)-ebelactone
A,1,7,8 (2) (Figure 1), the immunosuppressant (-)-pironetin9,10
(10) Keck, G. E.; Knutson, C. E.; Wiles, S. A. Org. Lett. 2001, 3, 707-
710.
(11) Anguinomycin: (a) Hayakawa, Y.; Adachi, K.; Koneshima, N. J.
Antibiot. 1987, 40, 1349-1352. (b) Hayakawa, Y.; Sodha, K.; Shin-ya, K.;
Hidaka, T.; Seto, H. J. Antibiot. 1995, 48, 954-959. Kazusamycin: (c)
Komiyama, K.; Okada, K.; Oka, H.; Tomisaka, S.; Miyano, T.; Funayama,
S.; Umezawa, I. J. Antibiot. 1985, 38, 220-229. Leptofuranin: (d)
Hayakawa, Y.; Sohda, K.; Seto, H. J. Antibiot. 1996, 49, 980-984.
Ratjadone: (e) Schummer, D.; Gerth, K.; Reichenbach, H.; Ho¨fle, G. Liebigs
Ann. 1995, 685-688. (f) Christmann, M.; Bhatt, U.; Quitschalle, M.; Claus,
E.; Kallesse, M. Angew. Chem. 2000, 112, 4535-4538; Angew. Chem.,
Int. Ed. 2000, 39, 4364-4366. (g) Bhatt, U.; Christmann, M.; Quitschalle,
M.; Claus, E.; Kalesse, M. J. Org. Chem. 2001, 122, 1885-1893.
(12) (a) Hamamoto, T.; Seto, H.; Beppu, T. J. Antibiot. 1983, 36, 646-
650. (b) Murakami, N.; Sugimoto, M.; Nakajima, T.; Higuchi, K.; Aoki,
S.; Yoshida, M.; Kudo, N.; Kobayashi, M. Abstracts of Papers, 41st
Symposium on the Chemistry of Natural Products; Nagoya, October 1999;
p 229.
(1) Kobayashi, M.; Higuchi, K.; Murakami, N.; Tajima, H.; Aoki, S.
Tetrahedron Lett. 1997, 38, 2859-2862.
(2) Murakami, N.; Wang, W.; Aoki, M.; Tsutsui, Y.; Higuchi, K.; Aoki,
S.; Kobayashi, M. Tetrahedron Lett. 1997, 38, 5533-5536.
(3) Murakami, N.; Wang, W.; Aoki, M.; Tsutsui, Y.; Sugimoto, M.;
Kobayashi, M. Tetrahedron Lett. 1998, 39, 2349-2352.
(4) Murakami, N.; Sugimoto, M.; Nakajima, T.; Kawanishi, M.; Tsutsui,
Y.; Kobayashi, M. Bioorg. Med. Chem. 2000, 8, 2651-2661.
(5) Crimmins, M.; King, B. J. Am. Chem. Soc. 1998, 120, 9084-9085.
(6) For synthetic studies leading to the polypropionate and lactone
fragments, see: (a) Marshall, J.; Fitzgerald, R. J. Org. Chem. 1999, 64,
4477-4481. (b) Quitshalle, M.; Christmann, M.; Bhatt, U.; Kalesse, M.
Tetrahedron Lett. 2001, 42, 1263-1265.
10.1021/ol0158922 CCC: $20.00 © 2001 American Chemical Society
Published on Web 04/17/2001

Scheme 2
hyde to furnish the Evans17 syn-aldol in 88% yield with
complete diastereoselectivity.18 Subsequent Parikh-Doering
oxidation furnished the â-ketoamide (+)-1118 (85%). The
titanium enolate derived from (+)-11 was then treated with
aldehyde (+)-12, the latter prepared in one step from
commercially available (S)-(-)-2-methyl-1-butanol, to pro-
vide the mismatched aldol (+)-13, with a syn/anti selectivity
of 4:1 (65%).19 Reduction with diisobutylaluminum hydride
(DIBAl-H) next led, in 65% yield, to lactol (+)-14, the
product resulting from stereoselective reduction of the C(17)
carbonyl, removal of the chiral auxiliary, and lactol forma-
tion. As an added benefit, lactol (+)-14 proved to be
crystalline; the structure was confirmed by single-crystal
X-ray analysis.
Figure 1. (-)-Callystatin A (1), a member of the Leptomycin
family of antibiotics, shares certain structural characteristics with
(-)-ebelactone B (2), (-)-pironetin A (3), and (+)-discodermolide
(5).
Our interest in (-)-callystatin A (1) derived from the
structural similarity to (+)-discodermolide (5), a tubulin
stabilizer.13 To differentiate these modes of action, we
embarked on a total synthesis that would provide sufficient
quantities of (-)-callystatin A (1) for detailed biological
analysis.
Wittig reaction of (carbethoxyethylidene)triphenylphos-
phorane with lactol (+)-14 (Scheme 3) next furnished the
R,â-unsaturated ester (+)-15 in 96% yield with 10:1 E:Z
selectivity. To remove the minor olefinic isomer, the mixture
was subjected to lactonization (catalytic camphorsulfonic acid
in CHCl3), thereby permitting ready removal of the Z isomer
by column chromatography. Selective protection of (+)-15
was then achieved with 10:1 chemoselectivity via exposure
to tert-butyldimethylsilyl triflate at -78 °C, to furnish the
C(19) mono silyl ether in 45% yield. Higher conversions
led to bis-silylation. Although the observed chemoselectivity
was fortuitous, the result permitted orthogonal silylation of
the remaining C(17) hydroxyl with TMSOTf and 2,6-lutidine
to furnish the fully protected ethyl ester (+)-16. Recycling
expedited material advancement. DIBAl-H reduction and
MnO2 oxidation then secured (+)-6 for the proposed late-
stage Julia coupling (68% yield; two steps).
Our retrosynthetic plan (Scheme 1) called for late-stage
installation of the C(12-13) E-olefin via Julia14 coupling
between enal 6 and sulfone 7. Further analysis of diene 7
suggested a second Julia coupling between enal 8 and
Kocienski15 sulfone 9 to set the C(6-7) E-olefin geometry.
The synthesis of advanced enal 6 (Scheme 2) began with
the addition of acyloxazolidinone (+)-1016 to propionalde-
Scheme 1
(13) Schreiber, S.; Chen, J.; Hung, D. Chem. Biol. 1996, 3, 287-293.
(14) Julia, M.; Launay, J.; Verpeaux, S.; Verpeaux, J. Tetrahedron Lett.
1982, 23, 2465-2472.
(15) Blakemore, P. R.; Cole, W. J.; Kocienski, P. J.; Morely, A. Synlett
1998, 26-28.
(16) Evans, D. A. Aldrichemica Acta 1982, 15, 23-32.
(17) Evans, D.; Bartroli, J.; Shih, T. J. Am. Chem. Soc. 1981, 103, 2127-
2129.
(18) Evans, D.; Ng, H.; Clark, S.; Rieger, D. Tetrahedron 1992, 48,
2127-2142.
(19) Steric demands of the chiral auxiliary and the C(16) methyl group
prevent enolization at C(16), see: Evans, D. A.; Clark, J. S.; Metternich,
R.; Novak, V. J.; Sheppard, G. S. J. Am. Chem. Soc. 1990, 112, 866-868.
1686
Org. Lett., Vol. 3, No. 11, 2001

Scheme 3
Scheme 5
Turning to advanced sulfone 7 (Scheme 1), we reasoned
that 7 should be available via union of enal 8 with
Kocienski15 sulfone 9. The synthesis of 8 began with
protection of the commerically available (R)-(-)-Roche ester
(17) as the TES ether (Scheme 4); reduction with DIBAl-H
the corresponding primary alcohol (+)-22.23 Purification of
(+)-22 (90%) was best performed by bulb-to-bulb distillation.
A Mitsunobu protocol24 efficiently installed the phenyl
tetrazole sulfide moiety (99% yield); oxidation with hydrogen
peroxide-ammonium molybdate completed construction of
sulfone (+)-9 in 69% yield. X-ray analysis secured the
structure of (+)-9.
Scheme 4
Assembly of diene (+)-23 was achieved by treatment of
a DME solution of (+)-8 and (+)-9 with NaHMDS in the
presence of HMPA (Scheme 6). Although the yield of (+)-
Scheme 6
at -78 °C then furnished aldehyde (-)-18 in 86% yield.
Still-Gennari Wittig20 coupling of phosphonate 1921 with
(-)-1822 led to (+)-20 in 79% yield; the Z:E selectivity was
8:1. A two-step reduction/oxidation sequence completed the
synthesis of (+)-8.
To construct acetal (+)-2123 (Scheme 5), the starting point
for sulfone 9, we exploited the known asymmetric hetero-
Diels-Alder reaction. Acetal (+)-21 was then reduced to
(20) Still, W. C.; Gennari, C. Tetrahedron Lett. 1983, 24, 4405-4408.
(21) Phosphonate 19 was synthesized from the commercially available
bis(2,2,2-hexafluoroethyl)(methoxycarbonylethyl) phosphonate. See: Morim-
oto, Y.; Matsuda, F.; Shirahama, H. Tetrahedron 1996, 32, 10609-10630.
(22) Barrett, A. G. M.; Edmunds, J. J.; Hendrix, J. A.; Horita, K.;
Parkinson, C. J. J. Chem. Soc., Chem. Commun. 1992, 1238-1240.
(23) Bauer, T.; Chapuis, C.; Jezewski, A.; Kozak, J.; Jurczac, J.
Tetrahedron: Asymmetry 1996, 7, 1391-1404.
23 was at best modest (ca. 35%), the E-olefin was the
exclusive product. The low yield was due to facile â-elim-
ination of sulfone (+)-9 leading to decompostition. Use of
(24) Mitsunobu, O. Synthesis 1981, 1-28.
Org. Lett., Vol. 3, No. 11, 2001
1687

DME as solvent with 1 equiv of HMPA proved essential to
minimize this competing process. Removal of the TES group
(catalytic PPTS/methanol) followed by iodination (PPh3,
DEAD, iodomethane)25 furnished (-)-24 (91%, two steps),
which was then converted to sulfone (+)-7 in 65% yield via
displacement of the iodide with the anion derived from
phenyl methyl sulfone.
led to 25 in 92% yield as a mixture of E:Z olefinic isomers
(3.5:1), favoring the desired E-isomer. Exposure of this
mixture to aqueous acetic acid in THF effected hydrolysis
of both the C(17) trimethylsilyl ether and C(1) methyl acetal
to furnish the corresponding lactol in 72% yield. Although
the olefinic isomers proved separable at this juncture, we
found that better results could be obtained when the
separation was performed on the subsequent, more stable
ketolactone. To arrive at the ketolactone, several oxidation
protocols were examined. Ultimately we discovered that PCC
activated with acetic acid provided the best yield (72%) of
the penultimate precursor for callystatin A (1). Completion
of the (-)-callystatin A (1) synthetic venture was then
achieved by removal of the TBS ether (HF‚Pyr, 79% yield);
the spectral data observed for synthetic (-)-callystatin A (1)
were identical in all respects to those reported for the natural
With the stage set to assemble the complete callystatin
carbon skeleton (Scheme 7), addition of the lithium anion
Scheme 7
1
material (e.g., H and 13C NMR, HRMS, optical rotation,
and IR).
In summary, an effective synthesis of (-)-callystatin A
(1) has been achieved, with the longest linear sequence
requiring 15 steps. The overall yield was 2.3% with an
average yield of 78% per step. Studies to probe the mode of
action of (-)-callystatin A (1) are currently underway.
Acknowledgment. Financial support was provided by the
National Institutes of Health (National Institute of General
Medical Sciences) through Grant GM-29028. In addition,
we thank Drs. George T. Furst, Patrick J. Carroll, and Rakesh
Kohli of the University of Pennsylvania Spectroscopic
Service Center for assistance in securing and interpreting
high-field NMR spectra, X-ray crystal structures, and mass
spectra, respectively. Finally, we thank Dr. R. Michael
Corbett for his assistance in preparing this manuscript.
Supporting Information Available: Spectroscopic and
analytical data for compounds 1, 7, 14, 20, 23, and 25 as
well as experimental procedures. This material is available
free of charge via the Internet at http://pubs.acs.org.
derived from sulfone (+)-7 (nBuLi, HMPA) to aldehyde
(+)-6 followed by acetic anhydride afforded a mixture of
acetates in 76% yield. Sodium amalgam reduction cleanly
(25) Loibner, H.; Zbiral, E. HelV. Chim. Acta 1976, 59, 2100-2113.
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