Total synthesis of spiruchostatin B, a potent histone deacetylase inhibitor, from a microorganism

Article abstract of DOI:10.1039/b718310k

The first total synthesis of spiruchostatin B, a potent histone deacetylase inhibitor, was achieved in a convergent manner; the synthesis established stereochemistry at the C5″ position. The Royal Society of Chemistry.

Full text of DOI:10.1039/b718310k

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COMMUNICATION
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Total synthesis of spiruchostatin B, a potent histone deacetylase
inhibitor, from a microorganismw
Toshiya Takizawa, Kazuhiro Watanabe, Koichi Narita, Takamasa Oguchi,
Hideki Abe and Tadashi Katoh*
Received (in Cambridge, UK) 27th November 2007, Accepted 29th January 2008
First published as an Advance Article on the web 5th February 2008
DOI: 10.1039/b718310k
The first total synthesis of spiruchostatin B, a potent histone
deacetylase inhibitor, was achieved in a convergent manner; the
synthesis established stereochemistry at the C500 position.
coupling of segment 5 with segment 6 while avoiding epimer-
ization at the C2 position (D-alanine part) in 6. Segment 5
would be produced through an aldol reaction of D-allo-iso-
leucine derivative 77 with ethyl acetate (8) and subsequent
condensation with D-cysteine derivative 9.8 However, segment
6 would be prepared through Julia–Kocienski olefination9 of
sulfone 10 accessible from 1,3-propanediol with aldehyde 11
available from L-malic acid,10 and subsequent condensation
with D-alanine methyl ester (12).
Spiruchostatins A (1) and B (2) (Fig. 1), isolated from a culture
broth of Psuedomonas sp. by Shin-ya and co-workers1 in 2001,
exhibit potent histone deacetylase (HDAC) inhibitory activ-
ity.2 HDAC inhibitors have been reported to exhibit promi-
nent antitumor activity against various types of mammalian
solid tumors;3 therefore, these natural products are expected
to be promising candidates for novel molecular-targeted anti-
cancer agents. Structurally, 1 and 2 are bicyclic depsipeptides
consisting of (3S,4R)-statine (blue-colored part), D-cysteine
(orange-colored part), D-alanine (green-colored part),
(3R,4E)-3-hydroxy-7-mercapto-4-heptenoic acid (red-colored
part), and disulfide bond linkage. These structures are similar
to that of FK228 (FR901228) (3), a powerful HDAC inhibitor
isolated from the fermentation broth of Chromobacterium
violaceum by Fujisawa Pharmaceutical Co. Ltd. (now Astellas
Pharm Inc.).2
We initially pursued the synthesis of segment 5 as shown in
Scheme 2. Aldol reaction of the lithium enolate of ethyl acetate
(8) with the known N-Boc-D-allo-isoluecinal (7)7 produced the
desired product 14 (31%) and the undesired stereoisomer 13
(62%). Conversion of 13 to 14 was successfully performed by
inversion of the hydroxy group (77% yield in two steps); the
sequence involved Jones oxidation and stereoselective reduc-
tion with KBH4 (14 : 13 = 15 : 1).y Ethyl ester 14 was then
transformed to allyl ester 15 via a four-step sequence involving
TBS protection of the secondary hydroxy group (96%),
saponification of the ester moiety (80%), formation of an allyl
ester from the liberated carboxylic acid (98%), and deprotec-
tion of the N-Boc group (92%). Subsequent condensation of
amine 15 with N-Boc-S-trityl-D-cysteine (9)8 furnished the
desired product 16 in 86% yield. Ultimately, deprotection of
the N-Boc group in 16 afforded the requisite segment 5 in a
quantitative yield.
The remarkable biological properties and attractive struc-
tural features prompted us to undertake a project directed
toward the total synthesis of 1–3. Till date, only one total
synthesis of FK228 (3) has been reported by Simon and co-
workers,4az and two total syntheses of spiruchostatin A (1)
have been reported by Ganesan and co-workers5 followed by
Doi, Takahashi et al.6 However, total synthesis of 2 has not
yet been mentioned in the literature, and the stereochemistry
at C500 (spiruchostatin numbering) of 2 has not been clearly
assigned.
We then performed the synthesis of segment 6, as described
in Scheme 3. Sulfone 10, a crucial substrate for the Julia–
Kocienski olefination,9 was efficiently prepared from the
known 3-(4-methoxybenzyloxy)propan-1-ol (17)11 via a four-
step operation involving the formation of S-tetrazole
In this communication, we describe the first total synthesis
of 2, which established the stereochemistry at C500 as described
in Scheme 1.
Our synthetic plan is outlined in Scheme 1. Spiruchostatin B
(2) should be synthesized by macrolactonization of seco-acid 4
followed by a disulfide bond formation according to the
protocols described by previous literature.5,6 The major chal-
lenge of this scheme is a convergent assembly of 4 by amide
Laboratory of Synthetic Medicinal Chemistry, Department of
Chemical Pharmaceutical Science, Tohoku Pharmaceutical University,
4-4-1 Komatsushima, Aoba-ku, Sendai, 981-8558, Japan. E-mail:
katoh@tohoku-pharm.ac.jp; Fax: +81-22-727-0135; Tel: +81-22-
727-0135
w Electronic supplementary information (ESI) available: Experimental
procedures and characterization data for all new compounds along
with copies of 1H and 13C NMR spectra. See DOI: 10.1039/b718310k
Fig.
1 Structures of spiruchostatins A (1), B (2) and FK228
(FR901228) (3).
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Scheme 2 Synthesis of segment 5. Reagents and conditions: (a) LDA,
EtOAc (8), THF, ꢁ78 1C; add. 7, 62% for 13, 31% for 14 (13 : 14 = 2 :
1); (b) Jones’ reagent, acetone, rt, 86%; (c) KBH4, MeOH, ꢁ40 1C,
90% for 14, 6% for 13 (14 : 13 = 15 : 1); (d) TBSCl, imidazole, DMF,
rt, 96%; (e) 1M NaOH, EtOH, rt, 80%; (f) allyl bromide, K2CO3,
DMF, rt, 98%; (g) TMSOTf, 2,6-lutidine, CH2Cl2, rt; MeOH, rt,
92%; (h) 9, PyBOP, i-Pr2NEt, MeCN, rt, 86%; (i) TMSOTf, 2,6-
lutidine, CH2Cl2, rt, 99%. TMSOTf = trimethylsilyl trifluoro-
methanesulfonate, PyBOP (benzotriazol-1-yloxy)tripyrrolidino-
=
phosphonium hexafluorophosphate.
dilute solution of 4 in CH2Cl2 (0.001 M) with 2-methyl-6-
nitrobenzoic anhydride (MNBA) (1.3 equiv.) and DMAP (3.0
equiv.) at room temperature for 15 h produced the desired
macrocycle 23 in high yield (89%). Finally, disulfide bond
Scheme 1 Synthetic plan for spiruchostatin B (2). TBS = tert-
butyldimethylsilyl, Tr (trityl) = triphenylmethyl, PMB = 4-methoxy-
benzyl, Boc = tert-butoxycarbonyl, PMP = 4-methoxyphenyl.
product12 (95%), Molybdenum-mediated oxidation,9b,13 de-
protection of the PMB group (94% in two steps), and forma-
tion of the S-trityl product (96%). The crucial Julia–Kocienski
olefination of 10 with the known aldehyde 11,10 readily pre-
pared from L-malic acid, produced the desired product 19 as an
inseparable mixture of E/Z-stereoisomers (E : Z = 5 : 1 by 400
MHz 1H NMR) in 66% yield. The mixture 19 was subjected to
regioselective acetal opening with DIBAL14 at 0 1C; the desired
E-olefinic alcohol 20a was produced as a major product (60%)
along with the undesired Z-olefinic isomer 20b (12%). Twofold
oxidation of 20a afforded carboxylic acid 21 (66% in two
steps), which was finally converted to the requisite segment 6
(88% in two steps) by condensation with D-alanine methyl ester
(12) and saponification of the ester function.
With the key segments 5 and 6 synthesized, we next in-
vestigated the synthesis of spiruchostatin B (2) by assembling
the two segments as shown in Scheme 4. Initial attempts to
achieve the pivotal condensation of 5 with 6 under conven-
tional conditions15 (e.g., PyBOP, EDCI/HOBT, or HATU, rt)
failed; the condensation product was produced in good yield
(B80%) but with considerable epimerization at the C2 stereo-
genic center (D-alanine part). After screening several reaction
conditions, we solved this problem using a combination of
HATU and HOAt at low temperature. Treatment of 5 and 6
with HATU (1.3 equiv.) and HOAt (1.3 equiv.) in the presence
of i-Pr2NEt (2.5 equiv.) in CH2Cl2 at ꢁ30 1C for 2 h produced
the desired condensation product (94%) without appreciable
epimerization at C2. The resulting product was then converted
to seco-acid 4 (84% in two steps), a crucial substrate for
macrolactonization, by deprotection of the PMB and allyl
groups. The critical macrolactonization of 4 was best achieved
by employing the Shiina protocol.16z Thus, treatment of a
Scheme 3 Synthesis of segment 6. Reagents and conditions: (a)
1-phenyl-1H-tetrazole-5-thiol, DEAD, PPh3, THF, rt, 95%; (b)
Mo7O24(NH4)6ꢂ4H2O, 30% H2O2, EtOH, rt; (c) DDQ, CH2Cl2/
H2O, rt, 94% (2 steps); (d) TrSH, DEAD, PPh3, CH2Cl2, reflux,
96%; (e) LiN(SiMe3)2, DMF, ꢁ60 1C; at ꢁ60 1C, add. 11, ꢁ60 to 0 1C,
66% (E : Z = 5 : 1); (f) DIBAL, toluene, 0 1C, 60% for 20a, 12% for
20b; (g) Dess–Martin periodinane, NaHCO3, CH2Cl2, rt, 88%; (h)
NaClO2, NaH2PO4, DMSO–H2O, rt, 75%; (i) D-alanine methyl ester
(12), PyBOP, i-Pr2NEt, CH2Cl2, 0 1C to rt, 90%; (j) 1 M LiOH,
MeOH, rt, 98%. DEAD = diethyl azodicarboxylate, DDQ = 2,3-
dichloro-5,6-dicyano-1,4-benzoquinone, DIBAL = diisobutylalumi-
nium hydride.
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(no. 17035073) and a Grant-in-Aid for High Technology
Research Program from the Ministry of Education, Culture,
Sports, Science and Technology of Japan (MEXT).
Notes and references
z A total synthesis of FR901375, a structurally closely related bicyclic
depsipeptide isolated from a microorganism along with FK228 (3), has
been reported.4b
Note added at proof: After submission of this manuscript, we learned
that the second, improved total synthesis of 3 has been reported by
Williams and co-workers.18
y In this stereoselective reduction, several reducing agents such as
NaBH4, LiBH4 and KBH4 were examined; the best result was obtained
by the use of KBH4.
z In the previous two total syntheses of spiruchostatin A (1), Ganesan
and co-workers5 successfully achieved the crucial macrolactonization
using the Yamaguchi method (2,4,6-Cl3C6H2COCl, Et3N,
MeCN–THF, 0 to 20 1C; DMAP, toluene, 50 1C, 53%); on the other
hand, Doi, Takahashi et al.6 efficiently performed the macrolactoniza-
tion event with the Shiina method (MNBA, DMAP, CH2Cl2, rt, 67%).
8 By employing (2R,3R)-D-isoleucine derivative instead of (2R,3S)-D-
allo-isoleucine derivative 7, we have also synthesized 500-epi-spiruchos-
Scheme 4 Synthesis of spiruchostatin B (2). Reagents and conditions:
(a) HATU, HOAt, i-Pr2NEt, CH2Cl2, ꢁ30 1C, 94%; (b) DDQ,
CH2Cl2/H2O, rt, 85%; (c) Pd(PPh3)4, morpholine, THF, rt, 99%;
(d) MNBA, DMAP, CH2Cl2, rt, 89%; (e) I2, MeOH–CH2Cl2, rt, 94%;
(f) HFꢂpyridine, pyridine, rt, 93%. HATU = O-(7-azabenzotriazol-1-
yl)-N,N,N0,N0-tetramethyluronium hexafluorophosphate, HOAt =
20
tatin B, [a]D ꢁ49.3 (c = 0.58, MeOH), in the same manner as
described for the synthesis of spiruchostatin B (2). The 1H and 13C
NMR spectra of the synthesized 500-epi-spiruchostatin B did not match
those of natural spiruchostatin B (see ESIw).
1-hydroxy-7-azabenzotriazole, MNBA
anhydride, DMAP = 4-dimethylaminopyridine.
= 2-methyl-6-nitrobenzoic
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establishment of the C500 stereochemistry in
2 to be
(S)-configuration as depicted in Scheme 4.8
In conclusion, we have accomplished a total synthesis of
spiruchostatin B (2) in a convergent manner starting from
D-allo-isoleucinal 7, aldehyde 11 derived from L-malic acid,
and 1,3-propanediol derivative 17. The pivotal steps of the
synthesis involve (i) Julia–Kocienski olefination of sulfone 10
and aldehyde 11 to install the requisite (E)-olefin unit present
in the critical segment 6, (ii) condensation of segments 5 and 6
to directly assemble the crucial seco-acid 4, and (iii) macro-
lactonization of 4 using the Shiina reagent to efficiently con-
struct the desired macrocycle 23. The C500 stereochemistry of 2
was determined by the present synthesis.
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We are grateful to Dr Kazuo Shin-ya, National Institute of
Advanced Industrial Science and Technology, for providing us
with copies of the 1H and 13C NMR spectra of natural
spiruchostatin B (2). We also thank Associate Professor
Takayuki Doi, Tokyo Institute of Technology, and Associate
Professor Isamu Shiina, Tokyo University of Science, for
useful discussion and suggestion. This work was supported
by a Grant-in-Aid for Scientific Research on Priority Area
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This journal is The Royal Society of Chemistry 2008
Chem. Commun., 2008, 1677–1679 | 1679