Total Synthesis of Spiruchostatin A, a Potent Histone Deacetylase Inhibitor

Article abstract of DOI:10.1021/ja039258q

The total synthesis of spiruchostatin A was accomplished, unambiguously confirming its structure. Key steps included the use of the Nagao thiazolidinethione auxiliary for a diastereoselective acetate aldol reaction and as an activated acylating agent for amide formation, and macrolactonization by the Yamaguchi protocol. Spiruchostatin A is shown to have biological activity similar to that of FK228, a potent histone deacetylase (HDAC) inhibitor in clinical trials. The spiruchostatin A analogue, epimeric at the β-hydroxy acid, is inactive, highlighting the importance of stereochemistry at this position for interactions with HDACs. Copyright

Full text of DOI:10.1021/ja039258q

Published on Web 01/09/2004
Total Synthesis of Spiruchostatin A, a Potent Histone Deacetylase Inhibitor
Alexander Yurek-George,† Fay Habens,‡ Matthew Brimmell,‡ Graham Packham,‡ and A. Ganesan*,†
Schools of Chemistry and Medicine, UniVersity of Southampton, Southampton SO17 1BJ, United Kingdom
Received October 27, 2003; E-mail: ganesan@soton.ac.uk
Histone deacetylases (HDACs) are zinc metalloenzymes that
catalyze the hydrolysis of acetylated lysine residues. In histones,
this returns lysines to their normal protonated state and is a global
mechanism of eukaryotic transcriptional control, resulting in tight
packaging of DNA in the nucleosome. Additionally, reversible
lysine acetylation is an important regulatory process for non-histone
proteins. For these reasons, the modulation of HDACs is receiving1
intense scrutiny. Typical HDAC inhibitors are substrate mimics with
a zinc-binding site such as a hydroxamic acid in the natural product
trichostatin A2 (1) and the synthetic drug candidate SAHA3 (2),
while trapoxin B4 (3) exemplifies a family of cyclic tetrapeptides
with an epoxyketone. The natural product depsipeptide FR-9012285
(4, FK228, in phase II clinical trials for cancer) is structurally
unique, acting6 as a prodrug that undergoes disulfide reduction
within the cell to release a zinc-binding thiol. The difficulty of
modifying FK228 has severely hampered the discovery of any
structure-activity relationships. A de novo approach is thus
advocated, but Simon’s elegant total synthesis7 is the only example
in this area.
hurdles. First, the stereochemistry of the â-hydroxy acid and statine
was not established. We reasoned that the former was (S), as in
FK228. The choice of statine diastereomer was less obvious, but
was fortunately assigned8b as (3S,4R) during the course of our work.
The second challenge is synthetic manipulation of the statine while
avoiding â-elimination, protecting group migration, and intra-
molecular cyclization.
We envisioned preparation of the â-hydroxy acid by Simon’s
procedure. An achiral aldol uneventfully converted aldehyde 7
(Scheme 1) to rac-8, but neither the Carreira conditions9 used by
Simon nor the alternative Keck methodology10 for enantioselective
acetate aldols satisfactorily yielded enantiopure 8. Instead, reaction
of 7 with the N-acetylthiazolidinethione Nagao auxiliary11 under
Vilarrasa’s TiCl4 conditions12 was highly diastereoselective, yielding
the readily separable 9 and 10. The stereochemical assignment was
based on analogy to the literature and confirmed by hydrolysis of
9 to the known â-hydroxy acid prepared by Simon. Since
completion of our work, Wentworth and Janda have reported13
a
chloroacetyloxazolidinone Evans aldol and subsequent dechlori-
nation to prepare the same â-hydroxy acid in their total synthesis
of FR-901375.
Scheme 1
The statine was obtained by Claisen condensation of Boc-D-valine
pentafluorophenyl ester with methyl acetate,14 followed by diaste-
reoselective reduction of 12 and switching the ester functionality
(Scheme 2). The choice of statine protecting groups was highly
crucial to our ultimate success. Advanced intermediates with methyl
or tert-butyl esters could not be unraveled to the free acid under
various conditions, while the trichloroethyl ester is cleavable at
neutral pH. Meanwhile, the N-Boc group in 14 can be removed by
acid to provide a protonated amine, preventing undesirable in-
tramolecular cyclization to the γ-lactam. In situ neturalization and
condensation with activated D-cysteine yielded dipeptide 15.
Protection of the alcohol and amino acid homologation furnished
protected tripeptide 17.
In 2001, the structures of spiruchostatin A and B (5, 6), isolated
from a Pseudomonas extract, were disclosed.8a Although the
biological activity reported was TGF-â-induced gene expression,
the similarity to FK228 suggested underlying HDAC inhibition.
Given the successful progression of FK228 to clinical trials,
spiruchostatin A is an attractive synthetic target with two additional
† School of Chemistry.
‡ School of Medicine.
9
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J. AM. CHEM. SOC. 2004, 126, 1030-1031
10.1021/ja039258q CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
Scheme 2 a
presumably by a prodrug mechanism similar to that of FK228.
Spiruchostatin A inhibited the growth of breast cancer cells with
an IC50 of approximately 10 nM, compared to 100 nM for the
HDAC inhibitor trichostatin A. epi-Spiruchostatin A was essentially
inactive at 10 µM, highlighting the importance of (S) stereochem-
istry in the â-hydroxy acid for favorable interactions with residues
around the rim of HDAC active sites. This observation parallels2b
trichostatin A, where the unnatural enantiomer is a significantly
less active HDAC inhibitor.
Our synthesis unambiguously confirms the complete structure
of spiruchostatin A. A noteworthy feature is the dual role of the
Nagao auxiliary as a chiral auxiliary for accomplishing acetate
aldols and as an acylating agent. FK228, and more recently
spiruchostatin A, have reached clinical trials as anticancer agents,
although they are unlikely to be optimized by Nature for potency
or selectivity against human HDACs. Our route paves the way for
the preparation and testing of unnatural analogues.
a Reagents and conditions: (a) (i) PfpOH, EDAC‚HCl, DMAP, CH2Cl2,
0 °C, 30 min, 20 °C, 4 h; (ii) LiCH2CO2CH3, THF, -78 °C, 45 min (66%);
(b) KBH4, MeOH, -78 to 0 °C, 50 min (70%); (c) (i) LiOH, 4:1 THF/
H2O, 0 °C, 1 h; (ii) TceOH, DCC, DMAP, CH2Cl2, 0 °C to room
temperature, 18 h (95%); (d) (i) TFA, CH2Cl2, room temperature, 3 h; (ii)
Fmoc-STrt-D-Cys, PyBOP, i-Pr2NEt, CH3CN, 20 °C, 20 min (74%); (e)
TIPSOTf, 2,6-lutidine, CH2Cl2, room temperature, 3 h (93%); (f) (i) 5%
Et2NH/CH3CN, 20 °C, 3 h; (ii) Fmoc D-Ala, PyBOP, i-Pr2NEt, CH3CN,
20 °C, 1 h (82%).
Acknowledgment. We thank the Combinatorial Centre of
Excellence, Cancer Research UK, and the Leukaemia Research
Fund for funding. We are grateful to Professor Kazuo Shin-ya for
information regarding the stereochemistry of spiruchostatin A and
spectra of the natural product.
Scheme 3 a
Supporting Information Available: Chemical and biological
experimental procedures. This material is available free of charge via
the Internet at http://pubs.acs.org.
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We anticipated that 9 was sufficiently activated15 to directly
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