Enantioselective syntheses of FR901464 and spliceostatin A: Potent inhibitors of spliceosome

Article abstract of DOI:10.1021/ol4024634

Enantioselective syntheses of FR901464 and spliceostatin A, potent spliceosome inhibitors, are described. The synthesis of FR901464 has been accomplished in a convergent manner in 10 linear steps (20 total steps). The A-tetrahydropyran ring was constructed from (R)-isopropylidene glyceraldehyde. The functionalized tetrahydropyran B-ring was synthesized utilizing a Corey-Bakshi-Shibata reduction, an Achmatowicz reaction, and a stereoselective Michael addition as the key steps. Coupling of A- and B-ring fragments was accomplished via cross-metathesis.

Full text of DOI:10.1021/ol4024634

ORGANIC
LETTERS
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Vol. XX, No. XX
000–000
Enantioselective Syntheses of FR901464
and Spliceostatin A: Potent Inhibitors of
Spliceosome
Arun K. Ghosh* and Zhi-Hua Chen
Department of Chemistry and Department of Medicinal Chemistry, Purdue University,
560 Oval Drive, West Lafayette, Indiana 47907, United States
akghosh@purdue.edu
Received August 26, 2013
ABSTRACT
Enantioselective syntheses of FR901464 and spliceostatin A, potent spliceosome inhibitors, are described. The synthesis of FR901464 has been
accomplished in a convergent manner in 10 linear steps (20 total steps). The A-tetrahydropyran ring was constructed from (R)-isopropylidene
glyceraldehyde. The functionalized tetrahydropyran B-ring was synthesized utilizing a CoreyÀBakshiÀShibata reduction, an Achmatowicz
reaction, and a stereoselective Michael addition as the key steps. Coupling of A- and B-ring fragments was accomplished via cross-metathesis.
In 1996, Nakajima and co-workers from the Fujisawa
Pharmaceutical Co. isolated FR901464 (1) (Scheme 1)
from the fermentation broth, Pseudomonas sp. No. 2663.1
FR901464 exhibited enhanced transcriptional activity of
promoter SV40 at a very low concentration (10 nM). It
exhibited remarkable antitumor activity, displaying IC50
values ranging from 0.6 to 3.4 nM against multiple human
cancer cell lines. Furthermore, it showed significant effec-
tiveness against human solid tumors implanted in mice
atadoserangeof 0.056À1 mg/kg.1b Subsequently, Yoshida
and co-workers reported that a more stable methylated
derivative of FR901464, named spliceostatin A (2), re-
tained similar potent antitumor activity as FR901464.2
More significantly, both FR901464 and spliceostatin A
potently inhibited in vitro splicing and promoted pre-
mRNA accumulation by binding to SF3b, a ribonuclear
protein in the spliceosome.2b Thus, structural analogues of
FR901464 may have potential clinical applications with a
novel mechanism of action. The biology and chemistry of
FR901464 attracted much interest among the synthetic
community. The first total synthesis of FR901464 was
accomplished by Jacobsen and co-workers.3 Two other
syntheses were reported later by Kitahara and co-workers
and Koide and co-workers.4,5 The reported syntheses were
carried out in a total of 29À41 chemical steps. In our
continuing interest in natural products6 that inhibit splicing
activity, we sought to develop a convergent synthesis of
FR901464 and spliceostatin A in an effort to facilitate the
synthesis of structural variants. Herein, we report a concise,
(1) (a) Nakajima, H.; Sato, B.; Fujita, T.; Takase, S.; Terano, H.;
Okuhara, M. J. Antibiot. 1996, 49, 1196–1203. (b) Nakajima, H.; Hori,
Y.; Terano, H.; Okuhara, M.; Manda, T.; Matsumoto, S.; Shimomura,
K. J. Antibiot. 1996, 49, 1204–1211. (c) Nakajima, H.; Takase, S.;
Terano, H.; Tanaka, H. J. Antibiot. 1997, 50, 96–99.
(2) (a) Motoyoshi, H.; Horigome, M.; Ishigami, K.; Yoshida, T.;
Horinouchi, S.; Yoshida, M.; Watanabe, H.; Kitahara, T. Biosci.
Biotechnol. Biochem. 2004, 68, 2178–2182. (b) Kaida, D.; Motoyoshi,
H.; Tashiro, E.; Nojima, T.; Hagiwara, M.; Ishigami, K.; Watanabe, H.;
Kitahara, T.; Yoshida, T.; Nakajima, H.; Tani, T.; Horinouchi, S.;
Yoshida, M. Nature Chem. Biol. 2007, 3, 576–583. (c) Zhang, F.; He,
H.-Y.; Tang, M.-C.; Tang, Y.-M.; Zhou, Q.; Tang, G.-L. J. Am. Chem.
Soc. 2011, 133, 2452–2462. (d) Fan, L.; Lagisetti, C.; Edwards, C. C.;
Webb, T. R.; Potter, P. M. ACS Chem. Biol. 2011, 6, 582–589.
(3) (a) Thompson, C. F; Jamison, T. F.; Jacobsen, E. N. J. Am. Chem.
Soc. 2000, 122, 10482–10483. (b) Thompson, C. F.; Jamison, T. F.;
Jacobsen, E. N. J. Am. Chem. Soc. 2001, 123, 9974–9983.
(4) (a) Horigome, M.; Motoyoshi, H.; Watanabe, H.; Kitahara, T.
Tetrahedron Lett. 2001, 42, 8207–8210. (b) Motoyoshi, H.; Horigome,
M.; Watanabe, H.; Kitahara, T. Tetrahedron 2006, 62, 1378–1389.
(5) (a) Albert, B. J.; Koide, K. Org. Lett. 2004, 6, 3655–3658. (b)
Albert, B. J.; Sivaramakrishnan, A.; Naka, T.; Koide, K. J. Am. Chem.
Soc. 2006, 128, 2792–2793. (c) Albert, B. J.; Sivaramakrishnan, A.;
Naka, T.; Czaicki, N. L.; Koide, K. J. Am. Chem. Soc. 2007, 129, 2648–
2659.
(6) (a) Ghosh, A. K.; Anderson, D. D. Org. Lett. 2012, 14, 4730–
4733. (b) Ghosh, A. K.; Li, J. Org. Lett. 2011, 13, 66–69.
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10.1021/ol4024634
XXXX American Chemical Society

enantioselective synthesis of FR901464 which has been
accomplished in 10 linear steps.
As shown in Scheme 1, we planned to utilize cross-
metathesis7 to couple the epoxy alcohol segment 3 and
the amide segment 4 at a late stage of the synthesis. A
related strategy was utilized by Koide and co-workers.5b
Scheme 2. Synthesis of Epoxy Alcohol Segment 3
Scheme 1. Retrosynthesis of FR901464
resulting dithiane with t-BuLi at À78 °C for 1 h followed
by reaction with (R)-isopropylidene glyceraldehyde pro-
vided a mixture (1:1) of diastereomers 12 and 13 in 61%
yield in two steps.11 This lack of stereoselectivity was
somewhat unexpected, especially given the presence of
chelating atoms at both R and β positions of (R)-isopro-
pylidene glyceraldehyde. In an attempt to improve anti-
diastereoselectivity, we investigated this addition reaction
in the presence of a number of Lewis acids such as CeCl3,12
ZnCl2,13 and MgBr214 in THF and ether. However, there
was no further improvement in the diastereomeric ratio.
The isomers were separated by silica gel chromatography.
The syn-isomer 12 was converted to desired anti-isomer 13
by a Mitsunobu reaction in the presence of p-nitrobenzoic
acid followed by NaOH-mediated hydrolysis of the benzo-
ate ester.15,16 The hydroxyl group of 13 was protected as a
PMB ether, and subsequent removal of the isopropylidene
group was carried out by the addition of p-TsOH in a one-
pot operation to provide diol 14. The primary alcohol was
selectively monotosylated using TsCl and Et3N in the
presence of dibutyltin oxide. Reaction of the resulting
monotosylate with an excess of CoreyÀChaykovsky di-
methylsulfonium methylide,17 prepared by treatment of
trimethylsulfonium iodide with n-BuLi, furnished allylic
The highly functionalized tetrahydropyran ring (A)
could be constructed by utilizing a versatile chiral pool
(R)-isopropylidene glyceraldehyde 5.8 The amide fragment
4 could be formed via coupling of amine 6 and acid 7.
Amine 6 could be obtained by a reductive amination of
pyranone derivative 8. The functionalized pyranone ring
(B) would be derived from furan derivative 9 via an
Achmatowicz rearrangement9 as the key step. This opti-
cally active furan derivative could be obtained by CBS-
reduction of a commercially available ketone. Acid 7 would
be derived from the known optically active alcohol 10.10
The synthesis of epoxy alcohol segment 3 is shown in
Scheme 2. Commercially available bromo ketone 11 was
protected as its dithiane derivative. Lithiation of the
(7) (a) Chatterjee, A. K.; Choi, T.-L.; Sanders, D. P.; Grubbs, R. H.
J. Am. Chem. Soc. 2003, 125, 11360–11370. (b) Prunet, J. Curr. Top.
Med. Chem. 2005, 5, 1559–1577.
(8) (R)-Isopropylidene glyceraldehyde is commercially available and
also can be conveniently prepared from D-mannitol in gram scale; see:
Organic Synthesis; Wiley: New York, 1998; Collect. Vol. IX, p 6; Org.
Synth. 1995, 72, 6.
(9) (a) Achmatowicz, O.; Bukowski, P.; Szechner, B.; Zwierzchowska,
Z.; Zamojski, A. Tetrahedron 1971, 27, 1973–1996. (b) Georgiadis, M. P.;
Albizati, K. F.; Georgiadis, T. M. Org. Prep. Proc. Int. 1992, 24,
95–118.
(11) The absolute stereochemistry of 12 and 13 were ultimately
identified by comparing the 2D-NMR spectroscopy of advanced inter-
mediate 16 and its C-4 epimer.
(12) Chen, Z.-H.; Tu, Y.-Q.; Zhang, S.-Y.; Zhang, F.-M. Org. Lett.
2011, 13, 724–727.
(13) Zhang, Y.; Rohanna, J.; Zhou, J.; Lyer, K.; Rainier, J. D. J. Am.
Chem. Soc. 2011, 133, 3208–3216.
(14) Williams, D. R.; Fultz, M. W. J. Am. Chem. Soc. 2005, 127,
14550–14551.
(15) Prasad, K. R.; Gholap, S. L. J. Org. Chem. 2008, 73, 2–11.
(16) For details, see the Supporting Information.
(10) Trost, B. M.; Quintard, A. Org. Lett. 2012, 14, 4698–4700.
B
Org. Lett., Vol. XX, No. XX, XXXX

alcohol 15 in 84% yield. A similar functional group
transformation was previously reported by Carreira and
co-workers.18 The dithiane group of 15 was then removed
by using an excess of Hg(ClO4)2 in methanol in the
presence of dry 2,6-lutidine. This condition resulted in
the formation of the corresponding methyl ketal as a
mixture of anomers, which upon treatment with a catalytic
amount of p-TsOH in methanol at 0 °C provided a single
diastereomer 16.19 Removal of the PMB group in 16 with
DDQ20 followed by alcohol directed epoxidation with
m-CPBA afforded the desired epoxy alcohol segment 3
stereoselectively as a white solid in 19% overall yield from
11 (8 steps). The methyl ketal 3 is quite stable and easy to
handle for subsequent reactions.
Scheme 4. Synthesis of Amide 4
The preparation of the Z-allylic acetate side chain 7 is
shown in Scheme 3. Optically active alcohol 10 was effi-
ciently prepared by utilizing a catalytic asymmetric addi-
tion protocol reported by Trost and co-workers to provide
10 in 98% ee.10 Saponification of methyl ester 10 with
aqueousLiOHfollowed by acetylation with acetylchloride
provided acetate 17 in excellent yield. Hydrogenation over
Lindlar’s catalyst afforded the desired cis-alkene 7.
Scheme 3. Synthesis of Acid 7
the protocol described by Kishi and co-workers.22 Our
subsequent synthetic plan required installation of the C20
(S)-methyl-bearing stereocenter. We elected to carry out
a 1,4-addition to enone 19. Accordingly, treatment of 19
with MeLi/CuBr Me2S at À78 °C for 2 h provided the
3
desired pyranone 8 in excellent yield (92%) and diaster-
eoselectivity (25:1 dr, by 1H and 13C NMR analysis). The
observed diastereoselectivity can be explained based upon
the conformational analysis of enone 19. The stereoche-
mical outcome of Michael addition can be rationalized
by assuming stereoelectronically favorable axial attack of
the cuprate as shown in the transition-state model 20.23
Further studies regarding the origin of high diastereoselec-
tivity is under investigation.
Pyranone 8 and known alkene 2124 were then subjected
to cross-metathesis conditions using Grubbs’ second gen-
eration catalyst25 to provide the corresponding terminal
tosylate. Treatment of the resulting tosylate with t-BuOK
in DMSO at 75 °C for 12 h resulted in diene 22 via base-
promoted elimination in 41% yield over two steps.26 For
the introduction of the amine group at C-14, we anticipated
that a reductive amination could proceed stereoselectively
via substrate control. Indeed, reductive amination of 22
The synthesis of amide segment 4 is shown in Scheme 4.
Enantioselective reduction of commercially available acet-
yl furan 18 with (S)-2-Me-CBS and BH3 Me2S afforded
3
chiral alcohol 9 in 94% yield (93% ee).21 An Achmatowicz
rearrangement was then carried out by treatment of alco-
hol 9 with tBuO2H in the presence of a catalytic amount
of VO(acac)2 to furnish a hemiketal,9 which was directly
reduced to enone 19 as a single diastereomer by employing
(17) (a) Corey, E. J.; Chaykovsky, M. J. Am. Chem. Soc. 1965, 87,
1353–1364. (b) Alcaraz, L.; Harnett, J. J.; Mioskowski, C.; Martel, J. P.;
Le gall, T.; Shin, D.-S.; Falck, J. R. Tetrahedron Lett. 1994, 35, 5449–
5452.
(18) Bode, J. W.; Carreira, E. M. J. Org. Chem. 2001, 66, 6410–6424.
(19) (a) Williams, D. R.; Jass, P. A.; Tse, H.-L. A.; Gaston, R. D.
J. Am. Chem. Soc. 1990, 112, 4552–4554. (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, 196–199.
(20) Horita, K.; Yoshioka, T.; Tanaka, T.; Oikawa, Y.; Yonemitsu,
O. Tetrahedron 1986, 42, 3021–3028.
(23) Woodward, S. Chem. Soc. Rev. 2000, 29, 393–401.
(24) Ghosh, A. K.; Nicponski, D. R. Org. Lett. 2011, 13, 4328–4331.
(25) Scholl, M.; Ding, S.; Lee, C. W.; Grubbs, R. H. Org. Lett. 1999,
1, 953–956.
(21) (a) Corey, E. J.; Roberts, B. E. J. Am. Chem. Soc. 1997, 119,
12425–12431. (b) Gazaille, J. A.; Abramite, J. A.; Sammakia, T. Org.
Lett. 2012, 14, 178–181.
(22) Lewis, M. D.; Cha, J. K.; Kishi, Y. J. Am. Chem. Soc. 1982, 104,
4976–4978.
(26) Paquette, L. A.; Gugelchuk, M.; McLaughlin, M. L. J. Org.
Chem. 1987, 52, 4732–4740.
Org. Lett., Vol. XX, No. XX, XXXX
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second-generation catalyst to afford spliceostatin A (2)
as a white solid in 57% isolated yield based upon one
recycle of unreacted 3 and 4 under the same conditions.
The removal of the methyl ketal in 2 was achieved by
exposure of 2 to PPTS in wet THF28 at 0 °C which
provided FR901464 (1) as a white powder in good
Scheme 5. Synthesis of FR901464 and Spliceostatin A
1
yield. The H and 13C NMR of our synthetic FR901464
[[R]23D À13.0 (c 0.45, CH2Cl2)] is identical to the reported
spectra of natural [[R]23 À12.0 (c 0.5, CH2Cl2)]1a and
D
synthetic3À5 FR901464.
In summary, we have accomplished a concise and
enantioselective strategy for the syntheses of FR901464
and spliceostatin A in 20 and 19 total steps with the longest
linear sequence of 10 and 9 steps, respectively. This repre-
sents a significant improvement over previously reported
routes. The key features in our syntheses include the use of
readily availalable chiral pool (R)-isopropylidene glycer-
aldyhyde 5 to form an A-ring fragment, a CBS reduction,
an Achmatowicz rearrangement, and a stereoselective
Michael addition for the construction of a B-ring frag-
ment, and a cross-metathesis reaction for coupling the
two fragments. The synthesis is short, convergent and
amenable to the synthesis of structural variants. Further
syntheses of structural variants and biological studies
are in progress.
with ammonium acetate and NaBH3CN afforded the cor-
responding primary amine 6 as a major product (6:1 dr, by
1H- and 13C NMR analysis).27 The crude amine 6 and its
epimer were directly treated with acid 7using standard amida-
tion conditions to give the amide 4 along with minor C-14
epimer, which were separated by column chromatography.
With the stereoselective syntheses of segments epoxy
alcohol 3 and amide 4, we then turned our attention to
construct the C6ÀC7 double bond of the target molecules.
As shown in Scheme 5, cross-metathesis of the two frag-
ments proceeded smoothly in the presence of Grubbs’
Acknowledgment. Financial support by the National
Institutes of Health is gratefully acknowledged.
Supporting Information Available. General experimen-
tal procedures, characterization data for all new products.
This material is available free of charge via the Internet
at http://pubs.acs.org.
(27) Rafferty, R. J.; Williams, R. M. J. Org. Chem. 2012, 77, 519–524.
(28) DeChristopher, B. A.; Loy, B. A.; Marsden, M. D.; Schrier,
A. J.; Zack, J. A.; Wender, P. A. Nature Chem. 2012, 4, 705–710.
The authors declare no competing financial interest.
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