Syntheses and biological evaluation of irciniastatin A and the C1-C2 alkyne analogue

Article abstract of DOI:10.1021/ol1000389

Chemical equation presented Syntheses of both natural (+)- and unnatural (-)-irciniastatin A (aka psymberin) as well as a C1-C2 alkyne analogue of (+)-irciniastatin A have been achieved. The key features of the syntheses include a highly regioselective epoxide-opening reaction and a late-stage assembly of C1-C6, C8-C16, and C17-C25 fragments. (+)-Alkymberin retained a high level of cytotoxicity, whereas (-)-irciniastatin A showed almost no activity. These results suggest that (+)-alkymberin could be a useful enantio-differential probe for mode-of-action study.

Full text of DOI:10.1021/ol1000389

ORGANIC
LETTERS
2010
Vol. 12, No. 5
1040-1043
Syntheses and Biological Evaluation of
Irciniastatin A and the C1-C2 Alkyne
Analogue
Tsubasa Watanabe,† Takamichi Imaizumi,† Takumi Chinen,‡ Yoko Nagumo,‡
Masatoshi Shibuya,† Takeo Usui,‡ Naoki Kanoh,† and Yoshiharu Iwabuchi*,†
Department of Organic Chemistry, Graduate School of Pharmaceutical Sciences,
Tohoku UniVersity, Aoba, Sendai 980-8578, Japan, and Graduate School of Life and
EnVironmental Sciences, UniVersity of Tsukuba, Tennodai, Tsukuba 305-8572, Japan
iwabuchi@mail.pharm.tohoku.ac.jp
Received January 7, 2010
ABSTRACT
Syntheses of both natural (+)- and unnatural (-)-irciniastatin A (aka psymberin) as well as a C1-C2 alkyne analogue of (+)-irciniastatin A
have been achieved. The key features of the syntheses include a highly regioselective epoxide-opening reaction and a late-stage assembly of
C1-C6, C8-C16, and C17-C25 fragments. (+)-Alkymberin retained a high level of cytotoxicity, whereas (-)-irciniastatin A showed almost
no activity. These results suggest that (+)-alkymberin could be a useful enantio-differential probe for mode-of-action study.
In 2004, (+)-irciniastatin A (1)1 and psymberin,2 new
pederin-type natural products, were isolated by the Pettit
group from marine sponge Ircinia ramosa and by the Crews
group from marine sponge Psammocinia sp. (Figure 1). In
addition to (+)-irciniastatin A (1), the Pettit group also
isolated the C11 ketone analogue, named (-)-irciniastatin
B. (+)-Irciniastatin A (1) has been shown to exhibit
extremely potent and selective cytotoxicity against certain
human cancer cell lines.1,2
The promising therapeutic potential coupled with the
limited availability of these natural products has attracted
significant attention from the synthetic community. In 2005,
the first total synthesis of (+)-psymberin was achieved by
† Tohoku University.
‡ University of Tsukuba.
(1) Pettit, G. R.; Xu, J.-P.; Chapuis, J.-C.; Pettit, R. K.; Tackett, L. P.;
Figure 1. Pederin-type natural products.
Doubek, D. L.; Hooper, J. N. A.; Schmidt, J. M. J. Med. Chem. 2004, 47,
1149.
the De Brabander group, who demonstrated that (+)-
irciniastatin A and (+)-psymberin are identical, as repre-
(2) Cichewicz, R. H.; Valeriote, F. A.; Crews, P. Org. Lett. 2004, 6,
1951.
10.1021/ol1000389  2010 American Chemical Society
Published on Web 02/08/2010

sented by (+)-1.3 To date, several total4-6 and formal7
syntheses as well as SAR studies8,9 have been reported.
Among them, the Schering-Plough group has reported that
the substituents at C4 and C5 are important for the cytotox-
icity, and that the C1-C2 double bond is not essential for
activity.9 Also, preliminary biological studies using natural
and synthetic samples have suggested that (+)-irciniastatin
A (1) might have a different mode-of-action from that of
other pederin family members (Figure 1).2,8
synthesized by the regioselective ring-opening of those of epoxy
alcohol 7, which could be easily obtained using an SAE strategy.
The synthesis of 3 commenced with regioselective ring-
opening of known epoxy alcohol (+)-711 with MeOH (Scheme
2). Initially, we tried Sharpless condition using Ti(OiPr)4 as
Scheme 2. Synthesis of the Acyclic Side Chain (-)-3
Intrigued by these results, we, several years ago, started a
synthetic program that would enable acquisition of possible
isomers and analogues. As a part of this program, we herein
describe the syntheses and biological evaluation of both
enantiomers of irciniastatin A (1), and (+)-“alkymberin”, a
C1-C2 alkyne analogue of natural (+)-1.
Retrosynthetically, (+)-irciniastatin A (1) was divided into
the C1-C6 acyclic side chain 3 and protected hemiaminal 2,
which in turn disconnected in a retro-aldol fashion into
C17-C25 aldehyde fragment 4 and C8-C16 tetrahydropyran
fragment 5 (Scheme 1). It should be noted that intermediates 3
Lewis acid,12 but the yield and selectivity were unsatisfactory
(54%, 1,2-diol:1,3-diol ) 3:1). To improve this situation, we
screened various Lewis acids (BF3·OEt2, MgBr2 etc.) and finally
found that Eu(OTf)3 gave the desired 1,2-diol 6, which is
inseparable from the corresponding 1,3-diol, in high yield and
selectivity (>20:1). The loading of Eu(OTf)3 could be reduced
to a catalytic amount when it was used with 0.2 equiv of 2,6-
di-tert-butyl-4-methylpyridine (DTBMP), giving comparable
selectivity (18:1).13 The obtained 1,2-diol 6 was converted via
the usual three steps to the primary alcohol (+)-9, which was
oxidized using 1-Me-AZADO14 to furnish carboxylic acid (-)-3.
The synthesis of 5 began with a known epoxy alcohol (+)-
10,15 which was derived from commercially available (-)-
pantolactone (Scheme 3). After protection of the primary
Scheme 1. Retrosynthetic Analysis of (+)-Irciniastatin A (1)
Scheme 3. Synthesis of C8-C16 Fragment (+)-5
and 5 could be derived from epoxy alcohols 7 and 8,
respectively. To synthesize not only fragments 3 and 5 but also
their isomers for SAR study, we planned to utilize a Sharpless
asymmetric epoxidation (SAE) chemistry10 as a key reaction.
For example, enantiomers and diastereomers of 6 could be
(3) Jiang, X.; Garcia-Fortanet, J.; De Brabander, J. K. J. Am. Chem.
Soc. 2005, 127, 11254.
(4) Huang, X.; Shao, N.; Palani, A.; Aslanian, R.; Buevich, A. Org.
Lett. 2007, 9, 2597
.
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.
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11, 3990
.
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(8) Jiang, X.; Williams, N.; De Brabander, J. K. Org. Lett. 2007, 9, 227
.
(9) Huang, X.; Shao, N.; Huryk, R.; Palani, A.; Aslanian, R.; Seidel-
Dugan, C. Org. Lett. 2009, 11, 867
.
(10) Gao, Y.; Hanson, R. M.; Klunder, J. M.; Ko, S. Y.; Masamune,
H.; Sharpless, K. B. J. Am. Chem. Soc. 1987, 109, 5765.
Org. Lett., Vol. 12, No. 5, 2010
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hydroxyl group in (+)-10, the epoxide was opened regioselec-
tively with lithium acetylide and BF3·OEt216 to give alcohol
11. Deprotection and subsequent TEMPO oxidation afforded
aldehyde (+)-12.
the presence of Et3B in MeOH provided the 1,3-syn diol (dr
>20:1),20 which was converted to lactone (+)-19 in 78%
yield in 2 steps. Hydrogenolysis of benzyl ester followed
by a Curtius sequence using 2-(trimethylsilyl)ethanol as a
nucleophile gave Teoc-protected hemiaminal (+)-2 in high
yield.21,22
We then focused on the diastereoselective allylation of
(+)-12. After extensive experimentation, we found that the
reaction using allyltributylstannane and MgBr2 proceeded in
a highly diastereoselective manner to give diol (+)-13.17
Protecting group manipulation and trans- reduction of the
alkyne moiety provided the corresponding allyl alcohol,
which was then subjected to SAE to give (+)-8 in 94% yield
and >20:1 diastereoselectivity. After protection of the primary
hydroxyl group, treatment of the epoxide with CSA in
MeOH-CH2Cl2 effected the deprotection of two TES groups
and formation of the desired tetrahydropyran core.
Coupling of (+)-2 with the acyclic side chain fragment
proved to be a difficult task. Initially, we examined the
coupling reaction of (+)-2 with several derivatives of the
carboxylic acid (-)-3, which never yielded the desired
product. After intensive effort, we realized that Teoc-
protected hemiaminal (+)-2 and pivaloate 20 were most
suitable for this coupling reaction.5 Finally, global depro-
tection using TASF provided (+)-irciniastatin A (1).
With a highly convergent and flexible route to access (+)-
irciniastatin A (1) in hand, we then synthesized “alkymberin”
(29), which bears an alkyne moiety at the C1-C2 position.
As described, the C1-C2 olefin moiety has been reported
to be unnecessary for cytotoxicity,9 and the alkyne part is
expected to be a useful handle for introducing several reporter
tags using click chemistry.23 Moreover, we decided to
synthesize (-)-irciniastatin A (1) to examine whether ircini-
astatin A acts as a “ligand” or “chemical reagent” in cells.
For example, the acyl aminal at C8 could be a good
electrophilic reagent (i.e., acylimine) when the methoxy
group at C8 was eliminated.
Selective methylation of the C8-hydroxyl group was
accomplished using Me2SO4 to give methyl ether (+)-15 in
78% yield. Silylation of the remaining secondary alcohol in
(+)-15 followed by oxidative cleavage of the terminal olefin
(OsO4, NMO; NaIO4) provided the corresponding aldehyde
16. Treatment of crude 16 with ethylmagnesium bromide
followed by oxidation of the resultant secondary alcohol
using 1-Me-AZADO gave ketone (+)-17 in 88% for 4 steps.
Finally, cleavage of BOM ether, oxidation of the resultant
alcohol using 1-Me-AZADO+BF4 /NaClO2,18 and protection
-
provided benzyl ester (+)-5 in high yield.
Aldehyde 4 was prepared based on De Brabander’s
protocol.3 The union of 4 with (+)-5 was achieved by mixing
the Z-boron enolate of (+)-5 with aldehyde 4 at -78 °C to
give the aldol product (+)-18 in a highly diastereoselective
manner (Scheme 4).19 Reduction of (+)-18 with NaBH4 in
For the synthesis of (+)-alkymberin, we prepared alkyne
side chain 28 based on the synthetic route previously
established (Scheme 5). In the course of the synthesis, we
Scheme 5
.
Synthesis of (+)-Alkymberin [(+)-29]
Scheme 4. Total Synthesis of (+)-Irciniastatin A (1)
-
found that the 1-Me-AZADO+BF4 /NaClO2 system was
more effective for the oxidation of alcohol (+)-27. The
resultant carboxylic acid was activated as mixed anhydride
28, which was successfully coupled with (+)-2 to furnish
(+)-alkymberin [(+)-29].
To synthesize (-)-irciniastatin A (1), each enantiomer of
the C1-C6 acyclic side chain [i.e., ent-20] and the C8-C16
1042
Org. Lett., Vol. 12, No. 5, 2010

tetrahydropyran fragment [(-)-5] is needed. The C1-C6
acyclic side chain ent-20 was prepared using (+)-DET by
SAE in the established route in Scheme 2. The C8-C16
tetrahydropyran fragment (-)-5 was synthesized from alde-
hyde (-)-12, which was prepared in 8 steps from (-)-
pantolactone (30) (Scheme 6).
as such, (+)-alkymberin [(+)-29] is a good candidate for an
enantio-differential probe24 for mode-of-action study.19
In summary, we have accomplished syntheses of (+)- and
(-)-irciniastatin A (1), as well as (+)-alkymberin (29), via
a convergent synthetic route. Biological evaluation of these
compounds suggested that (+)-alkymberin (29) can be a
good enantio-differential probe for analyzing mode-of-action
of (+)-irciniastatin A (1). Further studies on both SAR and
the mode-of-action of irciniastatins are now in progress, and
results will be reported in the near future.
Scheme 6. Synthesis of (-)-Irciniatstain A (1)
Acknowledgment. We thank Dr. Shinichi Nishimura
(Kyoto University) for his helpful suggestions and comments.
Supporting Information Available: Experimental pro-
cedures, characterization data, and copy of NMR spectra.
This material is available free of charge via the Internet at
http://pubs.acs.org.
OL1000389
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Commun. 2009, 1739.
Synthetic (+)- and (-)-irciniastatin A (1) and (+)-
alkymberin (29) were evaluated for their cytotoxicity against
HeLa cells. As expected, (+)-alkymberin (29) retained a high
level of cytotoxic activity [GI50 value of 1.2 nM for (+)-1,
0.2 nM for (+)-29]. In contrast, (-)-irciniastatin A (1)
showed almost no cytotoxic activity (GI50 > 1000 nM). These
results indicated that an enantio-differential recognition event
occurs between (+)-irciniastatin A (1) and its cellular target;
(19) Evans, D. A.; Calter, M. A. Tetrahedron Lett. 1993, 34, 6871.
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M. J. Tetrahedron Lett. 1987, 28, 155.
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