Total synthesis of (+)-chaetocin and its analogues: Their histone methyltransferase g9a inhibitory activity

Article abstract of DOI:10.1021/ja101280p

Chemical Equation Presentation The first total synthesis of (+)-chaetorin has been accomplished in only nine steps starting from the known N-Cbz-N-Me-serie using radical α-bromlnatlon reaction of diketopiperazine 10 and Co(I)-mediated reductive dimerizati

Full text of DOI:10.1021/ja101280p

Published on Web 03/08/2010
Total Synthesis of (+)-Chaetocin and its Analogues: Their Histone
Methyltransferase G9a Inhibitory Activity
Eriko Iwasa,†,‡ Yoshitaka Hamashima,† Shinya Fujishiro,†,§ Eisuke Higuchi,† Akihiro Ito,†
Minoru Yoshida,† and Mikiko Sodeoka*,†,|
RIKEN AdVanced Science Institute, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan, Graduate School of Science
and Engineering, Saitama UniVersity, 255 Shimo-okubo, Sakura-ku, Saitama 338-8570, Department of
EnVironmental Materials Science, School of Engineering, Tokyo Denki UniVersity, 2-2 Kanda-Nishiki-cho,
Chiyoda-ku, Tokyo, 101-8457, Japan, and Sodeoka LiVe Cell Chemistry Project, ERATO, Japan Science and
Technology Agency
Received February 12, 2010; E-mail: sodeoka@riken.jp
Scheme 1. Dimeric DKPs and Retrosynthesis of (+)-Chaetocin (1)
(+)-Chaetocin (1) is a complex epidithiodiketopiperazine (ETP)
alkaloid produced by Chaetronium minutum, which was isolated
in 1970.1 In addition to its antibacterial and cytostatic activity,1,2
1 is known to be a potent inhibitor of lysine-specific histone
methyltransferases (HMTs).3 Because methylation of histone
proteins plays important roles in controlling gene expression
patterns,4 synthetic study of 1 and its analogues would be helpful
in developing biological tools for epigenetic research. However,
total synthesis of 1 has not been reported to date. We herein describe
the first total synthesis of (+)-1, as well as its antipode and sulfur-
deficient analogues. We also examined the biological activity of
these compounds toward H3K9 HMT G9a, which is a potential
therapeutic target in human cancer.5
Following the early report by Nakagawa and Hino,6a elegant
methodologies were developed to synthesize the dimeric octacyclic
carbon frameworks characteristic of this family, and several total
syntheses of related compounds have been reported (Scheme 1).6b-d
Although there had been no report on the chemical synthesis of
this subclass of the ETP family when we started our project,
Movassaghi and co-workers recently completed a landmark total
synthesis of dimeric ETP 4.7,8 Based on the total synthesis of 2,
they installed disulfide bridges into the octacyclic core through
R-hydroxylation with [Ag(py)2]MnO4 as a stoichiometric oxidant.
In spite of their successes, the chemical synthesis of 1 still poses
the additional problem of how to prevent facile ꢀ-elimination of
serine units, if serine is used as a starting material. Keeping a
proposed biosynthetic route in mind,7 we initially attempted the
R-oxidation of dimeric compound 5 (Scheme 1, route A). We
selected a radical bromination reaction9 for this purpose, since
enolate chemistry seemed inappropriate. However, the octacyclic
core was found to be extremely unstable under radical conditions,
resulting in complete decomposition.10 Therefore, we decided to
test the early stage oxidation of the diketopiperazine (DKP) unit
(route B), which is distinct from the bioinspired late-stage oxidation
strategy employed in the synthesis of 4. To our delight, oxidized
compound 6 was accessible via bromocyclization of the corre-
sponding DKP, followed by radical R-bromination (Vide infra). We
expected that the octacyclic core structure would be constructed if
the compound 6 reacted without problem in Movassaghi’s Co(I)-
mediated radical dimerization reaction.11
synthesized without epimerization in 69% yield (5 steps) starting
from known N-Cbz-protected N-methyl-D-serine (7)12 and com-
mercially available D-Try-OMe•HCl (8).13 The obtained DKP was
subjected to the bromocyclization reaction with NBS to afford 10
in 88% yield. This reaction was highly stereoselective, and no
appreciable amount of the other stereoisomer was detected.14 The
relative stereochemistry of 10 was unambiguously determined by
X-ray analysis after simple conversion, revealing that the bromine
atom is positioned trans to the R-protons of the DKP ring.13
Because retention of the stereochemistry is ensured in the Co(I)-
mediated radical dimerization reaction,11 the absolute stereochem-
istry at the benzylic position would be directly transferred to those
at the 3 and 3′ positions of (+)-1. Thus, the tetracyclic compound
10 derived from D-amino acids, and not from L-amino acids, can
serve as a key precursor to (+)-1.
From 10, the total synthesis was accomplished in an additional
three steps. While a standard radical bromination reaction using
AIBN at high temperature led to decomposition of 10, the reaction
at room temperature using V-7015 as a radical initiator gave the
desired tribromide 11 cleanly in a stereoselective manner,16 even
though the R-protons of the DKP ring point to the concave face of
the fused 5,5-ring system. After simple filtration to remove
precipitated succinimide, the obtained crude tribromide was treated
with phosphate buffer to afford diol 12 in 47% yield as a major
stereoisomer,17 accompanied with other three minor diastereomers
(12% in total).13 Since the obtained diol 12 was relatively unstable
in CDCl3, we thought that the protection of the resulting hemi-
aminals might be necessary before examining the dimerization
reaction. However, to our delight, the hemiaminal was not sensitive
According to the early stage oxidation strategy, the total synthesis
of (+)-1 was achieved as shown in Scheme 2. DKP 9 was
† RIKEN Advanced Science Institute.
‡ Saitama University.
§ Tokyo Denki University.
| Japan Science and Technology Agency.
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4078 J. AM. CHEM. SOC. 2010, 132, 4078–4079
10.1021/ja101280p  2010 American Chemical Society

C O M M U N I C A T I O N S
Scheme 2. Total Synthesis of (+)- and ent-Chaetocina
These results clearly indicate that the sulfur functionality is crucial
for the biological activity, while G9a is not sensitive to the 3D
structure of 1.13
In summary, the first total synthesis of (+)-chaetocin has been
accomplished in only nine steps starting from the known N-Cbz-
N-Me-serine, taking advantage of two radical reactions as key
reactions. The structure-inhibitory activity relationship of the
synthesized compounds should be helpful for the future design of
biological tools. Further structure-activity studies are under way.
Acknowledgment. This work was in part supported by Project
Funding from RIKEN. We thank Dr. D. Hashizume of RIKEN for
X-ray analysis.
Supporting Information Available: Complete ref 3c, detailed
experimental procedures, full characterization of new compounds,
copies of 1H and 13C NMR spectral data. This material is available
free of charge via the Internet at http://pubs.acs.org.
References
a Conditions: (a) NBS, MeCN, -30 °C, 10 h; (b) NBS, V70, CCl4, rt,
5 h; then pH7 phosphate buffer/MeCN ) 1/1, rt, 3 h; (c) CoCl(PPh3)3,
acetone, rt, 1.5 h; (d) H2S, BF3•Et2O, CH2Cl2, -78 °C to rt, sealed tube,
1.5 h; then I2.
(1) (a) Hauser, D.; Weber, H. P.; Sigg, H. P. HelV. Chim. Acta 1970, 53, 1061.
(b) Weber, H. P. Acta Crystallogr. 1972, B28, 1945.
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(3) (a) Greiner, D.; Bonaldi, T.; Eskeland, R.; Roemer, E.; Imhof, A. Nat. Chem.
Biol. 2005, 1, 143. For other HMT inhibitors: (b) Kubicek, S.; O’Sullivan,
R. J.; August, E. M.; Hickey, E. R.; Zhang, Q.; Teodoro, M. L.; Rea, S.;
Mechtler, K.; Kowalski, J. A.; Homon, C. A.; Kelly, T. A.; Jenuwein, T.
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to a reductive coupling reaction using a Co(I) complex,11 so that
the unprotected diol 12 could be directly used to furnish the desired
octacyclic tetraol 13 as a single isomer in 55% yield.
With the dimeric tetraol in hand, the stage was set for the
construction of the disulfide bridges. The reaction of 13 with
condensed H2S (bp -60.7 °C) was carried out at -78 °C in the
(6) (a) Nakagawa, M.; Sugumi, H.; Kodato, S.; Hino, T. Tetrahedron Lett.
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1
presence of BF3•OEt2. Examination of the H NMR spectrum of
the crude product indicated that the sulfur nucleophile was delivered
to the putative iminium ions stereoselectively, probably mainly from
the outer surface of the double-decker structure. After aqueous
workup, the crude mixture in ethyl acetate was treated with I2, and
pure (+)-1 was isolated in 44% yield. The obtained compound was
spectroscopically identical to a natural sample of (+)-1. It should
be noted that no less than ten bond-forming and cleaving events,
namely four substitution reactions (OHfSH), deprotection of four
Lewis acid-sensitive protecting groups (TBS and Boc groups), and
two S-S bond formations, occurred in the final step.
This success prompted us to investigate the influence of the
absolute stereochemistry and the sulfur functionality of 1 on its
biological activity. Thus, based on the established route, we
synthesized the antipode of (+)-1 (ent-1) and the sulfur-deficient
analogues 14 and ent-14,13 and the inhibitory activity of these
compounds against G9a was examined using a known ELISA
method.3b Interestingly, 1 and ent-1 inhibited G9a equally ef-
fectively (IC50: 2.4 and 1.7 µM, respectively). In contrast, sulfur-
deficient analogues 14 and ent-14 were inactive (IC50 >50 µM).
´
A. R. Chem.sEur. J. 2009, 28, 9928.
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(8) For selected examples of the synthesis of ETPs: (a) Fukuyama, T.; Kishi,
Y. J. Am. Chem. Soc. 1976, 98, 6723. (b) Fukuyama, T.; Nakatsuka, S.;
Kishi, Y. Tetrahedron 1981, 37, 2045. (c) Williams, R. M.; Rastetter, W. H.
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Org. Lett. 2007, 9, 5267. and references therein.
(9) Trown, P. W. Biochem. Biophys. Res. Commun. 1968, 33, 402.
(10) A similar phenomenon is also reported in ref 7.
(11) Movassaghi, M.; Schmidt, M. A. Angew. Chem., Int. Ed. 2007, 46, 3725.
(12) Aurelio, L.; Box, J. S.; Brownlee, R. T. C.; Hughes, A. B.; Sleebs, M. M.
J. Org. Chem. 2003, 68, 2652.
(13) See Supporting Information for details and further discussion.
(14) We speculate that the strong stereoselectivity might arise from the effect
of the N-methyl group on the DKP ring (see Supporting Information (J)
for further discussion). The selectivity varied depending on the substituents
of DKPs in previous examples. See refs 6c and 7.
(15) Kita, Y.; Sano, A.; Yamaguchi, T.; Oka, M.; Gotanda, K.; Matsugi, M.
Tetrahedron Lett. 1997, 38, 3549.
(16) A model study indicated that the bulky bromine atom at the benzylic position
was important to minimize overbromination that possibly occurs at the
methylene part of the tryptophan unit.13
(17) The stereochemistry was tentatively assigned based on the experimental
results described in the Supporting Information (L).
JA101280P
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