A total synthesis of spiruchostatin A

Article abstract of DOI:10.1016/j.tetlet.2005.12.031

We achieved the total synthesis of the histone deacetylase inhibitor spiruchostatin A, as the prelude to the preparation of a combinatorial library of its analogues. Two key reactions were an asymmetric acetate aldol reaction using a Zr-enolate and macrol

Full text of DOI:10.1016/j.tetlet.2005.12.031

Tetrahedron
Letters
Tetrahedron Letters 47 (2006) 1177–1180
A total synthesis of spiruchostatin A
Takayuki Doi,a,* Yusuke Iijima,a Kazuo Shin-ya,b,c A. Ganesand and Takashi Takahashia,*
aDepartment of Applied Chemistry, Tokyo Institute of Technology, 2-12-1, Ookayama, Meguro, Tokyo 152-8552, Japan
bInstitute of Molecular and Cellular Biosciences, The University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-0032, Japan
cBiological Information Research Center (BIRC), National Institute of Advanced Industrial Science and Technology (AIST),
Protein Network Team, Low M.W. Chemical Laboratory, AIST Tokyo Waterfront Bio-IT Research Building,
2-42 Aomi, Koto-ku, Tokyo 135-0064, Japan
dSchool of Chemistry, University of Southampton, Southampton SO17 1BJ, UK
Received 31 October 2005; revised 28 November 2005; accepted 2 December 2005
Available online 28 December 2005
Abstract—We achieved the total synthesis of the histone deacetylase inhibitor spiruchostatin A, as the prelude to the preparation of
a combinatorial library of its analogues. Two key reactions were an asymmetric acetate aldol reaction using a Zr-enolate and macro-
lactonization using the Shiina method.
ꢀ 2005 Published by Elsevier Ltd.
Spiruchostatin A (1a) and B (1b), isolated from Pseudo-
monas sp., exhibit potent inhibitory activity against his-
tone deacetylases (HDACs).1 Spiruchostatin A is a
cyclic depsipeptide consisting of (3S,4R)-statine, D-cys-
teine, D-alanine, and (E)-3-hydroxy-7-thio-4-heptenoic
acid. The two thiol-groups were connected as a disulfide
forming a bicyclic depsipeptide. This structure is similar
to FR-901228 (FK-228),2 a HDAC inhibitor currently
in advanced clinical trials as an anticancer agent. One
total synthesis of FK-228 has been reported,3 while the
total synthesis of 1a was recently achieved.4 Potent
activity was observed for 1a in models for anticancer
activity and cardiac hypertrophy. In order to discover
analogues of 1, which are more potent and selective
HDAC inhibitors, we planned a total synthesis of 1a
in order to validate the route before proceeding with
combinatorial library synthesis.
D-alanine 5, and b-hydroxy acid 6 by sequential cou-
pling. We selected allyl ester in 3 because it endures under
the selective deprotection of N-Boc and N-Fmoc groups
at the N-terminus for the homologation of the peptide.
At the same time, the allyl ester can be selectively
removed by palladium-catalyzed reaction under mild
conditions in the presence of S-trityl groups to afford
seco-acid 2. Successful model studies can then be applied
to solid-phase synthesis using a similar allyl linker.
In the synthesis of b-hydroxy acid 6 (Scheme 2), the ace-
tate aldol reaction of enal 85 is a key step. Since acetate
aldols proceed with poor diastereoselectivity with the
classical EvansÕ oxazolidin-2-one auxiliary, previous
syntheses have employed alternative approaches: an
aldol with a chiral Ti-catalyst,3 an oxazolidinethione
auxiliary,5 an oxazolidinone derivative from chloroacetic
acid,5 and a thiazolidinethione auxiliary.4 We were inter-
ested in investigating the potential of SeebachÕs N-acet-
yl-oxazolidin-2-one 76 for this reaction, although only
a few examples of its acetate aldol reactions were repor-
ted.6a,7 Addition of the Li-enolate of 7 to enal 8 afforded
a 77:23 mixture of aldols 9 and 10 in 89% combined
yield (entry 1). The Zr-enolate,8 generated by transmetal-
ation of the Li-enolate with Cp2ZrCl2, effectively
increased the stereoselectivity of 9 up to 93% in 55%
yield (entry 3), while the Ti-enolate9 generated in situ
with TiCl(Oi-Pr)3 did not affect the selectivity (entry
2). The samarium-Reformatsky reaction of 7b
(X = Br) gave 9 with 87% diastereoselectivity in 57%
An outline of the synthetic strategy is illustrated in
Scheme 1. Spiruchostatin A (1a) can be synthesized
through macrolactonization of seco-acid derivative 2
and disulfide formation. Compound 2 would be pre-
pared by the sequential coupling of D-valine-derived
(3S,4R)-statine allyl ester 3b (R = allyl), D-cysteine 4,
Keywords: Spiruchostatin; Cyclic depsipeptide; Natural product syn-
thesis; Asymmetric aldol reaction.
*
Corresponding authors. Tel.: +81 3 5734 2120; fax: +81 3 5734
2884; e-mail addresses: doit@apc.titech.ac.jp; ttak@apc.titech.ac.jp
0040-4039/$ - see front matter ꢀ 2005 Published by Elsevier Ltd.
doi:10.1016/j.tetlet.2005.12.031

1178
T. Doi et al. / Tetrahedron Letters 47 (2006) 1177–1180
O
BocHN
R
O
FmocHN
TrtS
OH
O
OH
HN
N
H
HN
N
H
STrt
4
OH
O
O
S
OH
S
O
O
STrt
RO
HO
3
NH
NH
O
O
O
STrt
HO
OH
O
O
NHFmoc
HO
O
2
spiruchostatin A (1a) (R = Me)
spiruchostatin B (1b) (R = Et)
OH
5
6
Scheme 1. Retro-synthesis of spiruchostatin A (1a).
O
O
O
X
Table 1
O
N
+
Ph
Ph
H
STrt
a, b, c
OR
OH
O
BocHN
d
BocHN
8
7a (X = H)
7b (X = Br)
OH
O
3a (R = H)
3b (R = allyl)
R1 R2
O
O
NaOH
O
6
O
N
STrt
Ph
Ph
g, h
OH
e, f
FmocN
N
H
STrt
9 (R1 = OH, R2 = H)
10 (R1 = H, R2 = OH)
O
AllylO
11
Scheme 2. Asymmetric aldol reactions of acetate derivatives 7 with 8.
O
yield (entry 4).7 After simple purification by silica gel
column chromatography, hydrolysis of 9 afforded 6 in
76% yield.10 The absolute configuration was determined
as S by the modified Mosher method11 after converting
into its methyl ester (Table 1).
HN
N
H
g, i, j
OH
O
2
O
STrt
NFmoc
AllylO
12
The D-valine-derived (3R,4S)-statine 3a (R = H) was
prepared from N-Boc-D-Val-OH by condensation with
ethyl magnesium malonate,12 followed by stereoselective
reduction with NaBH413 and hydrolysis of ethyl ester in
34% overall yield (Scheme 3). After formation of allyl
ester 3b (R = allyl) and removal of the Boc group, cou-
pling with 4 (EDCI–HOBt) provided 11 in 76% yield.
Removal of the Fmoc group with Et2NH, followed
by coupling with 5 afforded 12 in 79% yield. After
removal of the Fmoc group, condensation of 6 was
achieved using PyBOP. The allyl ester was removed by
Scheme 3. Synthesis of cyclization precursor 2. Reagents and condi-
tions: (a) carbonyldiimidazole, (EtO2CCH2CO2)2Mg, THF; (b)
NaBH4, THF–MeOH (84:16), (c) LiOH, THF–H2O; recrystallization
(34% in three steps); (d) K2CO3, allyl bromide, (e) 4 M HCl–EtOAc; (f)
N-Fmoc-D-Cys(S-Trt)-OH (4), EDCI–HOBt, DIEA (76% in three
steps); (g) Et2NH; (h) N-Fmoc-D-Ala-OH (5), EDCI–HOBt, DIEA
(79% in two steps); (i) 6, PyBOP, DIEA (65%); (j) Pd(PPh3)4,
morpholine, MeOH (87%). EDCI = 1-ethyl-3-(30-dimethylaminoprop-
yl)carbodiimide, HOBt = 1-hydroxybenzotriazole, DIEA = N,N-
diisopropylethylamine,
PyBOP = benzotriazole-1-yl-oxy-tris-pyrro-
lidino-phosphonium hexafluorophosphate.
Table 1. Asymmetric aldol reaction of 7 with 8
Entry
Substrate
Additive
Temp/ꢁC
Yield/%
Ratio (9:10)d
1a
2a
3a
4b
7a
7a
7a
7b
None
TiCl(Oi-Pr)3
Cp2ZrCl2
None
ꢀ78
89
94
55
57
77:23
76:24
93:7
c
ꢀ78 to ꢀ40
ꢀ78 to rt
ꢀ78
c
87:13
a Compound 7a was treated with BuLi in THF forming enolate.
b Compound 7b was treated with SmI2 in THF forming the samarium enolate.
c 1.2 equiv.
d The ratio was determined by 1H NMR (400 MHz).

T. Doi et al. / Tetrahedron Letters 47 (2006) 1177–1180
1179
References and notes
O
HN
N
H
S
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OH
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2
NH
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HO
OH
O
b, a
13
1a
Scheme 4. Synthesis of 1a. Reagents and conditions: (a) I2, CH2Cl2–
MeOH (quant.); (b) MNBA (1.2 equiv), DMAP (2.4 equiv), CH2Cl2,
rt, 67%.
palladium-catalyzed reaction with morpholine to pro-
vide cyclization precursor 2.
5. Chen, Y.; Gambs, C.; Abe, Y.; Wentworth, P., Jr.; Janda,
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Removal of trityl groups of 2 followed by disulfide for-
mation3,14 provided 13 in quantitative yield (Scheme 4).
The macrolactonization of 13 failed under various
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(MMFF) suggested that there would be an appropriate
conformation for the cyclization within 3 kcal/mol from
the global minimum.15 On the other hand, the macro-
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2-methyl-6-nitrobenzoic
2
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anhydride
(MNBA)16,17
efficiently proceeded at room temperature (67%) without
protection of the hydroxy group in the statine unit,
whereas the reported Yamaguchi method18 required
80 ꢁC and proceeded in 40% yield.4 Finally disulfide for-
mation between the di-S-trityl moieties furnished spiru-
chostatin (1a) in quantitative yield.4 The synthetic 1a
exhibited 1H and 13C NMR spectral data as well as opti-
cal rotation identical to those published for the natural
product.1,19
1
10. Spectral data of 6: H NMR (400 MHz, CDCl3) d 7.41–
7.19 (m, 15H), 5.59 (ddd, J = 15.5, 6.8, 6.3 Hz, 1H), 5.42
(dd, J = 15.5, 6.3 Hz, 1H), 4.45 (m, 1H), 2.55 (m, 2H),
2.21 (m, 2H), 2.09 (m, 2H); 13C NMR (100 MHz, CDCl3)
d 176.5, 144.9, 131.6, 130.7, 129.6, 127.9, 126.6, 68.5, 66.6,
41.1, 31.4, 31.3; IR (neat) 3417, 3056, 3030, 2925, 1713,
1595, 1489, 1444, 1280, 1183, 1101, 1083, 1034, 1002, 972,
In summary, we have completed a total synthesis of spiru-
chostatin A. Compared to the first synthesis,4 the pres-
ent route illustrates several new features. We have
demonstrated an asymmetric aldol reaction using the
Zr-enolate of acetyl N-oxazolidin-2-one derivative that
proceeds with high diastereoselectivity. The synthesis
of the statine involves a malonate condensation under
mild conditions, and the use of an allyl ester protecting
group was found to be compatible with highly function-
alized intermediates. Finally, the macrolactonization
was efficiently achieved with the Shiina reagent, and
provides another example of the utility of this method-
ology for such reactions. Interestingly, the macrolacton-
ization failed if performed after disulfide bond
formation, and this suggests that subtle conformational
differences exist between 2 and 13. A solid-phase synthe-
sis based on these results and its application toward a
combinatorial library of spiruchostatin analogues is
underway in our laboratory.
25
743, 700, 676, 621, 506 cmꢀ1; ½aꢁD ꢀ4.99 (c 1.55, CH2Cl2)
20
[lit.3 ½aꢁD ꢀ5.0 (c 2.0, CH2Cl2)], HRMS (ESI-
TOF) calcd for [C26H26O3+Na]+ 441.1500, found 441.1495.
11. Ohtani, I.; Kusumi, T.; Kashman, T.; Kakisawa, H. J.
Am. Chem. Soc. 1991, 113, 4092–4096.
12. Brooks, D. W.; Lu, L. D.-L.; Masamune, S. Angew.
Chem., Int. Ed. Engl. 1979, 18, 72–74.
13. (a) Jouin, P.; Poucet, J.; Dufour, M.-N.; Pantaloni, A.;
Castro, B. J. Org. Chem. 1989, 54, 617–627; (b) Harris, B.
´
D.; Joullie, M. M. Tetrahedron 1988, 44, 3489–3500.
14. Kamber, B.; Hartmann, A.; Eisler, K.; Riniker, B.; RInk,
H.; Siber, P.; Rittel, W. Helv. Chim. Acta 1980, 63,
899–915.
15. Macromodel (version 6.0) Mohamadi, F.; Richards, N. G.
J.; Guida, W. C.; Liskamp, R.; Lipton, M.; Caufield, C.;
Chang, G.; Hendrickson, T.; Still, W. C. J. Comput. Chem.
1990, 11, 440–467.
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Acknowledgments
This work was supported by a Grant-In-Aid from the
Ministry of Education, Culture, Sports, Science and
Technology, Japan (No. 16350051).
17. Inoue, M.; Sasaki, T.; Hatano, S.; Hirama, M. Angew.
Chem., Int. Ed. 2004, 43, 6500–6505.

1180
T. Doi et al. / Tetrahedron Letters 47 (2006) 1177–1180
18. Inanaga, J.; Hirata, K.; Saeki, H.; Katsuki, T.; Yama-
guchi, M. Bull. Chem. Soc. Jpn. 1979, 52, 1989–1993.
19. Spectral data of synthetic 1a: 1H NMR (400 MHz,
CDCl3) d 7.39 (d, J = 7.3 Hz, 1H), 6.72 (d, J = 9.2 Hz,
1H), 6.39 (m, 1H), 5.93 (br s, 1H), 5.66 (d, J = 15.5 Hz,
1H), 5.50 (br s, 1H), 4.92 (dt, J = 9.2, 3.8 Hz, 1H), 4.57
(m, 1H), 4.27 (dq, J = 7.3, 3.9 Hz, 1H), 3.37 (m, 2H), 3.23
(m, 1H), 3.15 (d, J = 5.9 Hz, 1H), 2.77–2.69 (m, 5H), 2.57
(d, J = 13.1 Hz, 1H), 2.50–2.35 (m, 2H), 1.51 (d, J =
7.3 Hz, 3H), 1.03 (d, J = 6.8 Hz, 3H), 0.93 (d, J = 6.8 Hz,
3H); 13C NMR (100 MHz, CDCl3) d 172.1, 171.2, 171.1,
169.3, 132.7, 129.6, 71.1, 68.7, 62.5, 56.1, 52.3, 40.9 · 2,
39.7, 39.3, 32.6, 29.9, 20.5, 19.8, 16.3; IR (neat) 3328, 2964,
2933, 1730, 1661, 1543, 1432, 1370, 1274, 1162, 1043, 985,
26
754, 666, 565 cmꢀ1
;
½aꢁD ꢀ61.1 (c 0.965, MeOH)
26
26
[lit.1 ½aꢁD ꢀ63.6 (c 0.14, MeOH), lit.4 ½aꢁD ꢀ61.1 (c 0.14,
MeOH)], HRMS (ESI-TOF) calcd for [C20H31N3O6S2+
Na]+ 496.1552, found 496.1547.