Design, synthesis, and biological evaluation of largazole derivatives: Alteration of the zinc-binding domain

Article abstract of DOI:10.1016/j.tet.2014.05.078

A new series of largazole analogues in which the side chain was replaced with disulfide groups were synthesized, and their biological activities were evaluated. Compound 8 bearing an octyl moiety showed much better selectivity for HDAC1 over HDAC7 than largazole (320-fold). Structure-activity relationships suggested that the length in the disulfide chain of largazole is important for the selectivity toward HDAC1 over HDAC7.

Full text of DOI:10.1016/j.tet.2014.05.078

Tetrahedron xxx (2014) 1e7
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Tetrahedron
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Design, synthesis, and biological evaluation of largazole derivatives:
alteration of the zinc-binding domain
,
,
,
Jinyue Su a y, Yatao Qiu b y, Kun Ma c y, Yiwu Yao d, Zhen Wang d, Xianling Li d,
Dayong Zhang a , Zhengchao Tu , Sheng Jiang
,
d,
d,
*
*
*
a State Key Laboratory of Natural Medicines and Center of Drug Discovery, China Pharmaceutical University, Nanjing 210009, China
b School of Pharmaceutical Sciences, Sun Yat-Sen University, Guangzhou 510006, China
c Centre for Drug Evaluation, China Food and Drug Administration, Beijing 100038, PR China
d Laboratory of Medicinal Chemistry, Guangzhou Institute of Biomedicine and Health, Chinese Academy of Sciences, Guangzhou 510530, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A new series of largazole analogues in which the side chain was replaced with disulfide groups were
synthesized, and their biological activities were evaluated. Compound 8 bearing an octyl moiety showed
much better selectivity for HDAC1 over HDAC7 than largazole (320-fold). Structureeactivity relationships
suggested that the length in the disulfide chain of largazole is important for the selectivity toward HDAC1
over HDAC7.
Received 28 March 2014
Received in revised form 19 May 2014
Accepted 21 May 2014
Available online xxx
Ó 2014 Elsevier Ltd. All rights reserved.
Keywords:
HDAC inhibitor
Peptides
Macrocycles
largazole
Disulfide bridge
1. Introduction
Recently, two HDAC inhibitors (HDACis) suberoylanilide
hydroxamic acid (SAHA, vorinostat) and FK228 (romidepsin) have
Histone deacetylases (HDACs) are a family of enzymes that
control gene transcription via regulation of lysine acetylation in
chromatin.1 The 18 HDACs are classified into four groups on the
basis of their size, cellular localization, number of catalytic active
sites, and sequence homology to yeast HDAC proteins: class I,
HDAC1, -2, -3, and -8; class IIa, HDAC4, -5, -7, and -9; class IIb,
HDAC6 and -10; class III, sirtuin proteins; class IV, HDAC11.2 The
HDAC proteins are associated with basic cellular functions and
disease states such as cancer, but the importance of individual
isoforms cancer growth and inhibition is not well understood. Class
I isoforms are over expressed in solid and hematological tumors,
highly correlating with a worse prognosis,3e7 but not resting en-
dothelial cells and normal organs. Therefore, selective inhibition of
class I isoforms is currently a major area of research in cancer
chemotherapy.8
recently been licensed for the treatment of cutaneous T-cell lym-
phoma (CTCL). More than 11 new HDACis are at different stages of
clinical development for therapy of multiple cancer types.9 How-
ever, most pan-HDAC inhibitors exhibited side effects that might
limit their clinical potential.10 Therefore, looking for new HDAC
inhibitors that are specific-inhibition to one kind HDAC family is
extremely urgent and necessary.
Largazole is a natural macrocyclic depsipeptide isolated from
the cyanobacterium Symploca sp. by Luesch and co-workers, which
show promising HDAC1 inhibitory activity and selectivity (Fig. 1).11
These excellent properties of largazole have attracted several re-
search groups to complete its total synthesis and structureeactivity
relationship (SAR) studies.12e28 Mechanistic studies showed that
largazole is a prodrug that releases a free thiol function group of 3-
hydroxy-7-mercapto-4-heptenoic acid side chain through hydro-
lysis by cellular esterases or lipases.12,13 The free thiol function
group could chelate with the Zn2þ cation present at the active site.
Up to now, only two groups reported the results through modifi-
cation of thiol function of largazole.23,27 On the basis of their re-
sults, we envisioned that structural modifications in the zinc-
binding domain of largazole may modulate metal-binding affinity
and develop isoform-selective inhibitors, which could help in
* Corresponding authors. Tel./fax: þ86 (0)20 32015318 (S.J.); tel./fax: þ86 (0)20
32015324 (Z.T.); e-mail addresses: zhangdayong@cpu.edu.cn (D. Zhang), tu_zhengchao@
gibh.ac.cn (Z. Tu), jiang_sheng@gibh.ac.cn, jiangsh9@gmail.com (S. Jiang).
y
These authors contributed equally to the work.
0040-4020/$ e see front matter Ó 2014 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2014.05.078
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J. Su et al. / Tetrahedron xxx (2014) 1e7
HATU-mediated coupling to the thiazoline-thiazole carboxylic acid
29 gave the cyclization precursor 30. TFA-mediated removal of the
Boc and 2-(trimethylsilyl) ethanol groups, and subsequent macro-
lactamization with the use of HATU/HOAt/DIPEA in anhydrous DCM
provided 31 in 50% yield (two steps).32,33 Deprotection of trityl
group furnished the key intermediate 32 in 90% yield in the pres-
ence of TFA. Treatment of compound 32 with sulfinothioates 20e24
in different combinations in the presence of base at room tem-
perature gave the lagazole’s analogues 4e8 in high yields.
We evaluated biochemical activities of five largazole analogues
4e8 against HDACs 1 and 7 using largazole as the positive control.
The results were outlined in Table 1 and exhibited the interesting
HDACs isoform selectivity. Largazole showed weak selectivity for
the HDAC1 (IC50¼0.124
m
M) over the HDAC7 (IC50¼0.169
mM). For
compound 4, the selectivity between HDAC1 and HDAC7 is 961
fold more potent than that of largazole, in which the octanethioate
side chain of largazole may interact with the hydrophobic pocket
in the cap of HDAC1, which could increase the selectivity between
HDAC1 and HDAC7. Therefore, introducing a more hydrophobic
residue in the side chain part of largazole would be able to increase
isoform selectivity. The inhibition of compound 5 to HDAC1 in-
creased greatly, the selectivity between HDAC1 and HDAC7 is
much better than that of largazole. To evaluate whether the sub-
stitute group on the side chain part influences activity or not,
compound 6 was designed and synthesized, in which tert-butyl
group in the side chain. Compared with 5, the resulting compound
6 showed much less selectivity for HDAC1 over HDAC7. To our
delight, compound 8 with octyldisulfide side chain had great
Fig. 1. Structures of SAHA, romidepsin (FK228), and largazole.
understanding the role of different isoforms in disease states.
Herein, we report our efforts to modify the metal-binding domain
of largazole with the goal of increasing selectivity for HDAC1 over
other isoforms (Fig. 2).
binding affinity against HDAC1 (IC50¼0.009
mM) and showed much
better selectivity for HDAC1 over HDAC7 to that of largazole (320-
fold), which may be beneficial for future development of specific
therapeutic agents.
The antiproliferative activities of largazole and its analogues
4e9 were evaluated against human tumor cell lines, such as Molt-4
(Human acute lymphoblastic leukemia cell line), U937 (human
leukemic monocyte lymphoma cell line), MCF-7 (human breast
adenocarcinoma cell line), and BGC823 (human stomach cancer cell
line), with SAHA as the positive control. The results are summarized
in Table 2 and all analogues 4e8 show very good GI50 values against
Molt-4, U937, and BGC823 cancer cell lines, and are more active
than SAHA. It is remarkable that compounds 7 and 8, especially in
view of its higher HDAC1 activities, have much better anti-cancer
activities than that of other analogues 4e6 against Molt-4 and
U937 cancer cell lines.
3. Conclusion
Fig. 2. Design of new analogues of largazole targeted to Zn2þ binding motif.
In summary, a series of new largazole analogues 4e8 have been
designed on the basis of alteration of metal-binding domains, and
synthesized in high yields. Biological results showed that alteration
of octanethioate side chain of largazole had various effects to the
selectivity toward HDAC1 over HDAC7. To our delight, compound 8
was identified to show much higher selectivity against HDAC1 over
HDAC7 in comparison with largazole. These results clearly in-
dicated that modification of metal-binding domains may be the
way forward to the development of isoform-selective HDAC
inhibitors.
2. Results and discussion
The synthesis of key intermediates 20e24 was shown in
Scheme 1. Hexan-1-ol 9 and octan-1-ol 10 were masked with
tosylates 11 and 12, which further reacted smoothly with TrtSH at
room temperature in the presence of potassium tert-butoxide in
THF, giving compounds 13 and 14 in parallel in 84e85% yield. The
ester group of 13 and 14 were oxidized with I2 to establish the
disulfide bridge, which afforded intermediates 15 and 16. Sub-
sequent reaction of the disulfides 15e19 with m-CPBA generated
sulfinothioates 20e24 in quantitative yields.29e31
The syntheses of the designed analogues 4e8 were illustrated in
Scheme 2. Compound 26 was obtained from a commercially
available (ꢀ) malic acid 25 by following the well-established pro-
cedures.22 Coupling of the alcohol 26 with enantiomerically pure
4. Experimental section
4.1. General
1H NMR and 13C NMR spectra were recorded on Bruker Avance
ARX-400 or 500 MHz. Mass spectra were performed on Kompact
Axima-CFR MALDI mass spectrometers. Optical rotations were
recorded on a Perkin Elmer 341 polarimeter. Anhydrous solvents
amino acid Fmoc-
L-valine in the presence of EDCI and HOAt at room
temperature gave compound 27. Removal of the Fmoc group and
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J. Su et al. / Tetrahedron xxx (2014) 1e7
3
Scheme 1. Reagents and conditions: (a) TsCl, Et3N, DMAP, DCM, 93e95%; (b) TrtSH, t-BuOK, THF, 84e85%; (c) I2, MeOH, 76%e78%; (d) m-CPBA, DCM, in quantitative yields.
NHFmoc
OH
O
OH
O
lit.22
.
a
HO
SiMe3
STrt O
O
O
OH
TrtS
O
SiMe3
O
O
(-)-Malic acid 25
26
27
S
N
O
S
O
N
N
NH2
O
NH
NH
d
c
b
S
S
N
O
STrt O
O
S
O
STrt
O
O
STrt
O
O
NHBoc
SiMe3
SiMe3
O
N
H
O
31
30
28
N
O
S
N
O
4: R=Et
S
f
NH
e
S
NH
5: R=n-Bu
6: R=t-Bu
7: R=n-Hex
8: R=n-Oct
S
N
N
20-24
S
N
N
O
HS
O
O
O
O
O
O
OH
BocHN
N
S
29
N
H
H
R
S
32
Scheme 2. Reagents and conditions: (a) Fmoc-L-valine, EDCI, HOAt, DIPEA, CH2Cl2, room temperature, 79%; (b) piperidine, DMF, 20 min, room temperature, 96%; (c) 27, HATU, HOAt,
DIPEA, DMF, room temperature, 85%; (d) 1. trifluoroacetic anhydride, CH2Cl2, 24 h; 2. HATU, HOAt, DIPEA, DMF, 2 days, room temperature, 50% (2 steps); (e) TFA, TES, CH2Cl2, 86%; (f)
Et3N, DCM, 73e83%.
were obtained as follows: THF by distillation from sodium and
benzophenone; dichloromethane from CaH2. All other solvents
were reagent grade. All moisture sensitive reactions were carried
out in flame dried flask under argon atmosphere.
hexyl alcohol (1.58 mL, 10 mmol) in DCM (20 mL) was added
dropwise at 0 ꢁC, and the mixture was allowed warm to rt and
stirred for 1 h, the solvent was evaporated in vacuo, the residue was
dissolved by ethyl acetate (EA) and washed with satd aq NaHCO3,
the organic layer was dried with Na2SO4, filtered, concentrated, and
purified by column chromatography to afford 11 as yellow solid
4.2. Hexyl 4-methylbenzenesulfonate (11)
(2.7 g, 95%). 1H NMR (400 MHz, CDCl3):
d
7.78 (d, J¼8.4 Hz, 2H), 7.34
To a solution of Tscl (2.86 g, 15 mmol) and DMAP (166 mg,
1 mmol) in DCM (30 mL), triethylamine (2.1 mL, 15 mmol) and
(d, J¼8.0 Hz, 2H), 4.02 (t, J¼6.4 Hz, 2H), 2.44 (s, 3H), 1.66e1.59 (m,
2H), 1.33e1.17 (m, 6H), 0.84 (t, J¼7.0 Hz, 3H) ppm.
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4
J. Su et al. / Tetrahedron xxx (2014) 1e7
Table 1
then satd aq sodium thiosulfate (10 ml) was added, the methanol
IC50 values for HDAC1 and HDAC7 inhibition (m
M)a
was evaporated under reduce pressure and diluted with water
(20 ml), extracted with EA (3ꢂ30 ml), the combined organic phases
was dried with Na2SO4, filtered, concentrated, and purified by silica
gel chromatography to afford 15 (1.74 g, 76%) as a yellow liquid. 1H
Sample
IC50 (uM)
HDAC1
Selectivity
HDAC7
HDAC1/HDAC7
Largazole
0.124
0.0246
0.0338
0.131
0.015
0.009
0.169
22.2
5.57
4.0
7.1
2.92
0.961
0.001
0.006
0.033
0.002
0.003
NMR (400 MHz, CDCl3):
d 3.01e2.85 (m, 4H), 1.78e1.65 (m, 4H),
4
5
6
7
8
1.42e1.25 (m, 12H), 0.91e0.88(m, 6H) ppm.
4.7. Octyl(trityl)sulfane (16)
a
Largazole was used as a positive control. Values are means of three different
experiments, standard error of the IC50 was generally less than 10%.
The procedure was the same as described above for the syn-
thesis of 15, compound 16 was obtained as yellow liquid (1.8 g,
78%). 1H NMR (400 MHz, CDCl3):
(m, 4H), 1.40e1.28 (m, 20H), 0.88 (t, J¼6.8 Hz, 6H) ppm.
d
2.68 (t, J¼2.4 Hz, 4H), 1.71e1.63
Table 2
Antiproliferative activities of largazole and analogues 4e8aee
4.8. (S,E)-2-(Trimethylsilyl)ethyl 3-hydroxy-7-(tritylthio)hept-
Sample
GI50 (mM)
4-enoate (26)
Molt-4
U937
MCF-7
BGC823
Largazole
4
5
6
7
8
0.027
0.098
0.071
0.033
0.045
0.051
0.77
0.033
0.28
0.46
0.066
0.068
0.070
1.67
2.35
2.22
2.25
4.87
2.61
2.11
13.6
0.145
0.50
0.39
0.20
0.25
0.29
4.69
Compound 26 was synthesized according to the literature.22 1
H
NMR (400 MHz, CDCl3):
d 7.42e7.39 (m, 6H), 7.30e7.28 (m, 6H),
7.22e7.19 (m, 3H), 5.61e5.54 (m, 1H), 5.41 (dd, J¼15.6, 6.0 Hz, 1H),
4.46e4.40 (m, 1H), 4.19 (t, J¼8.4 Hz, 2H), 2.88 (d, J¼4.0 Hz, 1H),
2.47e2.44 (m, 2H), 2.20 (t, J¼7.2 Hz, 2H), 2.07 (dd, J¼14.4, 7.2 Hz,
2H), 1.01e0.96 (m, 2H), 0.04 (s, 9H).
SAHA
a
SAHA was used as a positive control.
Molt-4: human acute lymphoblastic leukemia cell line; U937: human leukemic
b
4.9. (S,E)-2-(Trimethylsilyl)ethyl 3-(((S)-2-((((9H-fluoren-9-yl)
methoxy)carbonyl) amino)-3-methylbutanoyl)oxy)-7-(tri-
tylthio)hept-4-enoate (27)
monocyte lymphoma cell line; MCF-7: human breast adenocarcinoma cell line;
BGC823: human stomach cancer cell line.
c
Inhibition of cell growth by the listed compounds was determined by using
CCK8 assay.
d
NA, not active up to the highest concentration tested.
Standard error of the GI50 was generally less than 10%.
e
To a solution of 26 (526 mg, 1.02 mmol), Fmoc-L-valine (1.033 g,
3.05 mmol), and DMAP (25 mg, 0.203 mmol) in DCM (30 mL),
DIPEA(0.50 mL, 3.045 mmol), EDCI (584 mg, 3.045 mmol), and
HOAt (414 mg, 3.045) in DCM (20 mL) was added at 0 ꢁC, and the
mixture was allowed warm to rt and stirred for 12 h, the solvent
was washed with satd aq NaHCO3, the organic layer was dried with
Na2SO4, filtered, concentrated, and purified by column chroma-
tography to afford 2712 (673 mg, 79%). 1H NMR (400 MHz, CDCl3)
4.3. Octyl 4-methylbenzenesulfonate (12)
The procedure was the same as described above for the syn-
thesis of 11, compound 12 was obtained as yellow solid (3.0 g, 93%).
1H NMR (400 MHz, CDCl3):
d
7.79 (d, J¼8.4 Hz, 2H), 7.34 (d,
J¼8.0 Hz, 2H), 4.02 (t, J¼6.6 Hz, 2H), 2.45 (s, 3H), 1.66e1.58 (m, 2H),
d
7.73 (d, J¼7.5 Hz, 2H), 7.56 (m, 2H), 7.21e7.39 (m, 19H), 5.58e5.71
1.29e1.21 (m, 10H), 0.86 (t, J¼6.8 Hz, 3H) ppm.
(m, 2H), 5.35 (dd, J¼7.5, 15.5 Hz, 1H), 5.25 (d, J¼9.0 Hz, 1H),
4.31e4.42(m, 2H), 4.17e4.29 (m, 2H), 4.07e4.15 (m, 2H), 2.65 (dd,
J¼8.0, 16.0 Hz, 1H), 2.53 (dd, J¼5.5, 12.0 Hz, 1H), 2.08e2.20 (m, 3H),
2.01 (m, 2H), 0.90e0.98 (m, 2H), 0.89 (d, J¼6.5 Hz, 3H), 0.78 (d,
4.4. Hexyl(trityl)sulfane (13)
A solution of triphenymethyl mercaptan (3.94 g, 14.25 mmol) in
THF (25 ml) was added dropwise to a solution of compound 11
(2.7 g, 9.5 mmol) and potassium tert-butanolate (3.2 g, 28.5 mmol)
in THF (25 ml) at 0 ꢁC, then the mixture was warmed to rt and
stirred for 3 h before the solution was adjusted to pH¼2 by 1 M HCl,
the solvent was evaporated in vacuo and redissolved by EA and
washed with water, the organic phases dried with Na2SO4, filtered,
concentrated, and purified by silica gel chromatography to afford as
J¼7.0 Hz, 3H), 0.01 (s, 9H); 13C NMR (125 MHz, CDCl3)
d 171.0, 169.8,
156.3, 145.0, 144.8,144.1, 144.0, 141.5, 134.2, 129.7, 128.0, 127.9, 127.2,
126.8,125.3, 120.1, 72.0, 67.2, 66.8, 63.3, 58.9, 47.4, 39.8, 31.6,
31.5,31.3, 19.1, 17.5, 17.5, ꢀ1.4.
4.10. (S,E)-2-(Trimethylsilyl)ethyl 3-(((S)-2-amino-3-
methylbutanoyl)oxy)-7-(tritylthio)hept-4-enoate (28)
yellow solid 13 (3.1 g, 84%). 1H NMR (400 MHz, CDCl3):
d 7.42e7.41
Piperidine (0.25 mL, 2.44 mmol) was add to a solution of 27
(682 mg, 0.81 mmol) in DMF (8 ml) and stirred for 2 h, the solvent
was evaporated in vacuo and purified by column chromatography
(m, 6H), 7.40e7.39 (m, 6H), 7.29e7.28 (m, 3H), 2.13 (t, J¼7.6 Hz, 2H),
1.42e1.37 (m, 2H), 1.35e1.14 (m, 6H), 0.79 (t, J¼6.8 Hz, 3H) ppm.
to afford 28 (482 mg, 96%). 1H NMR (400 MHz, CDCl3):
d 7.40e7.38
4.5. Octyl(trityl)sulfane (14)
(m, 6H), 7.29e7.25 (m, 6H), 7.22e7.18 (m, 3H), 5.69e5.57 (m, 2H),
5.36 (dd, J¼15.4, 7.4 Hz, 1H), 4.16e4.09 (m, 2H), 3.21 (d, J¼4.8 Hz,
1H), 2.64 (dd, J¼15.6, 8.4 Hz, 1H), 2.53 (dd, J¼15.6, 5.2 Hz, 1H),
2.19e2.15 (m, 2H), 2.07e1.98 (m, 2H), 1.97e1.93 (m, 1H), 0.98e0.93
(m, 2H), 0.92 (d, J¼6.8 Hz, 3H), 0.81 (d, J¼6.8 Hz, 3H), 0.03 (s, 9H).
The procedure was the same as described above for the syn-
thesis of 13, compound 14 was obtained as yellow solid (3.3 g, 85%).
1H NMR (400 MHz, CDCl3):
d 7.33e7.28 (m, 11H), 7.25e7.22 (m, 4H),
2.55 (t, J¼7.2 Hz, 2H), 1.67e1.60 (m, 2H), 1.37e1.30 (m, 10H), 0.81 (t,
J¼6.8 Hz, 3H) ppm.
4.11. (R)-20-(((tert-Butoxycarbonyl)amino)methyl)-4-methyl-
4.6. 1,2-Dihexyldisulfane (15)
4,5-dihydro-[2,40-bithiazole]-4-carboxylic acid (29)
To a solution of I2 in MeOH (20 ml) was added 13 (3.055 g,
7.87 mmol) in MeOH (20 ml) dropwise at rt and stirred for 10 min,
Compound 29 was synthesized according to the literature.22 1
NMR (400 MHz, MeOD): 7.96 (s, 1H), 5,41 (br, 1H), 4.62 (d,
H
d
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J. Su et al. / Tetrahedron xxx (2014) 1e7
5
J¼5.2 Hz, 2H), 3.89 (d, J¼11.6 Hz, 1H), 3.34 (d, J¼11.6 Hz, 1H), 1.68 (s,
(q, J¼7.2 Hz, 2H), 2.39e2.35 (m, 2H), 2.15e2.01 (m, 2H), 1.86 (s, 3H),
0.69 (d, J¼6.9 Hz, 3H), 0.52 (d, J¼6.9 Hz, 3H).
3H), 1.46 (s, 9H).
4.12. (S,E)-2-(Trimethylsilyl)ethyl 3-(((S)-2-((R)-20-(((tert-bu-
toxycarbonyl)amino)methyl)-4-methyl-4,5-dihydro[2,40-bi-
thiazole]-4-carboxamido)-3-methylbutanoyl)oxy)-7 (tri-
tylthio)hept-4-enoate (30)12
4.15. (5R,8S,11S)-11-((E)-4-(Ethyldisulfanyl)but-1-en-1-yl)-8-
isopropyl-5-methyl-10-oxa-3,17-dithia-7,14,19,20-
tetraazatricyclo[14.2.1.12,5]icosa-1(18),2(20),16(19)-triene-
6,9,13-trione (4)
To a solution of 28 (927 mg, 1.5 mmol) and 29 (533.5 mg,
1.5 mmol) in DCM (30 mL) at 0 ꢁC, HATU (856 mg, 2.25 mmol), HOAt
(306 mg, 2.25 mmol), and DIPEA (0.74 mL, 4.5 mmol) were added,
and the mixture was allowed warm to rt and stirred for 12 h, the
solvent was washed with satd aq NaHCO3, the organic layer was
dried with Na2SO4, filtered, concentrated, and purified by column
chromatography to afford 30 (1.22 g, 85%). 1H NMR (400 MHz,
To a solution of ethyl disulfide (3 equiv) in DCM (10 mL), m-
chloroperbenzoic acid (3 equiv) was added at 0 ꢁC. Then the re-
action mixture was stirred at 0 ꢁC for 2 h, then a saturated solution
of NaHCO3 was added slowly, the mixture was separated and or-
ganic layer was washed with brine, dried, and concentrated. The
concentrate was dissolved in DCM (5 mL) and Et3N (3 equiv) was
added to the solution. A solution of 32 (1 equiv) in DCM (5 mL) was
dropwise added to the above solution and stirred at rt for 4 h. The
reaction mixture was washed with brine, dried, concentrated, and
CDCl3):
d 7.91 (s, 1H), 7.36e7.41 (m, 6H), 7.16e7.29 (m, 9H),
5.59e5.71 (m, 2H), 5.36 (dd, J¼7.5, 15.3 Hz, 1H), 5.30 (s, 1H), 4.62 (d,
J¼6.0 Hz, 2H), 4.48 (dd, J¼4.8, 9.3 Hz, 1H), 4.12e2.18 (m, 2H), 3.77
(d, J¼11.4 Hz, 1H), 3.32 (d, J¼11.4 Hz, 1H), 2.69 (dd, J¼8.1, 15.9 Hz,
1H), 2.54 (dd, J¼5.1, 15.9 Hz,1H), 2.03e2.18 (m, 5H),1.57 (s, 3H),1.47
(s, 9H), 0.93e0.99 (m, 2H), 0.81 (d, J¼6.9 Hz, 3H), 0.74 (d, J¼6.9 Hz,
chromatographic purification afforded the desired product 4 as
20
white solid (88 mg, 79%). [
(400 MHz, CDCl3):
a
]
D
8.14 (c 0.172, CHCl3). 1H NMR
d
7.76 (s, 1H), 7.18 (d, J¼9.6 Hz, 1H), 6.50 (dd,
J¼9.2, 2.8 Hz, 1H), 5.91e5.85 (m, 1H), 5.70e5.66 (m, 1H), 5.54 (dd,
J¼15.6, 6.8 Hz, 1H), 5.28 (dd, J¼17.6, 9.6 Hz, 1H), 4.60 (dd, J¼9.4,
3.4 Hz, 1H), 4.27 (dd, J¼17.6, 3.2 Hz,1H), 4.04 (d, J¼11.2 Hz, 1H), 3.28
(d, J¼11.2 Hz, 1H), 2.86 (dd, J¼16.2, 10.2 Hz, 1H), 2.73e2.64 (m, 5H),
2.44 (dd, J¼14.4, 7.2 Hz, 2H), 2.14e2.06 (m, 1H), 1.86 (s, 3H), 1.31 (t,
3H), 0.02 (s, 9H). 13C NMR (125 MHz, CDCl3):
d 174.6, 170.6, 169.9,
155.8, 148.8, 145.0, 134.2, 129.7, 128.1, 128.0, 126.8, 121.7, 85.3, 80.7,
77.5, 72.0, 66.8, 63.4, 57.0, 42.5, 41.7, 39.9, 31.5, 31.4, 31.3, 28.5, 25.0,
19.3, 17.7, 17.5, ꢀ1.3.
J¼7.2 Hz, 3H), 0.69 (d, J¼6.8 Hz, 3H), 0.52 (d, J¼6.8 Hz, 3H) ppm. 13
C
4.13. (5R,8S,11S)-8-Isopropyl-5-methyl-11-((E)-4-(tritylthio)
but-1-en-1-yl)-10-oxa-3,17-dithia-7,14,19,20-tetraazatricyclo
[14.2.1.12,5]icosa-1(18),2(20),16(19)-triene-6,9,13-trione (31)
NMR (125 MHz, CDCl3): d 173.5, 169.4, 168.9, 167.9, 164.6, 147.5,
132.8, 128.3, 124.1, 84.4, 72.0, 57.8, 43.3, 41.1, 40.5, 37.8, 34.1, 32.7,
31.9, 24.2, 18.9, 16.7, 14.4 ppm. MS (EI, m/z): 557 (Mþþ1). HRMS
(ESI): calcd for C23H32N4O4S4: [MþH]þ, 557.1379; found, 557.1380.
To a solution of 30 (700 mg, 0.731 mmol) in CH2Cl2 (15 mL) was
added TFA (3 mL) at 0 ꢁC, and the mixture was stirred at rt for 24 h.
The reaction mixture was concentrated in vacuo and azeotropically
dried with toluene to give the crude amino acid. This amino acid
was then dissolved in DMF (731 mL), and HATU (833 mg,
2.193 mmol), HOAt (298 mg, 2.193 mmol), and DIPEA (1.09 mL,
6.58 mmol) were added to the solution successively. The reaction
mixture was stirred for 48 h at rt and then the solvent was evap-
orated under reduced pressue and dissolved in ethyl acetate
(20 ml). This solution was washed with sat aq NaCl and the aqueous
layer was extracted with ethyl acetate (3ꢂ20 mL). The combined
organic layer was dried over anhydrous Na2SO4 and evaporated in
vacuo to give an oil, which was subjected to column chromatog-
raphy to give 31 (270 mg, 50%) as a white solid. 1H NMR (400 MHz,
4.16. (5R,8S,11S)-11-((E)-4-(Butyldisulfanyl)but-1-en-1-yl)-8-
isopropyl-5-methyl-10-oxa-3,17-dithia-7,14,19,20-
tetraazatricyclo[14.2.1.12,5]icosa-1(18),2(20),16(19)-triene-
6,9,13-trione (5)
The procedure was the same as described above for the syn-
thesis of 4, compound 5 was obtained as white solid (51 mg, 83%).
[a] d 7.76 (s, 1H),
20 1.12 (c 0.179, CHCl3). 1H NMR (400 MHz, CDCl3):
D
7.18 (d, J¼8.4 Hz,1H), 6.51 (dd, J¼9.2, 2.8 Hz,1H), 5.92e5.85 (m,1H),
5.70e5.65 (m, 1H), 5.54 (dd, J¼15.4, 6.6 Hz, 1H), 5.31e5.25 (m, 1H),
4.60 (dd, J¼9.4, 3.4 Hz, 1H), 4.27 (dd, J¼17.6, 3.2 Hz, 1H), 4.04 (d,
J¼11.2 Hz, 1H), 3.28 (d, J¼11.2 Hz, 1H), 2.86 (dd, J¼16.4, 10.0 Hz, 1H),
2.72e2.66 (m, 5H), 2.44 (dd, J¼14.4, 7.2 Hz, 2H), 2.12e2.08 (m, 1H),
1.86 (s, 3H), 1.68e1.61 (m, 2H), 1.41 (dd, J¼15.0, 7.4 Hz, 2H), 0.92 (t,
CDCl3): d 7.84 (s, 1H), 7.31e7.28 (m, 12H), 7.26e7.19 (m, 3H), 7.11 (d,
J¼9.2 Hz, 1H), 6.63 (d, J¼8.0 Hz, 1H), 5.77e5.70 (m, 1H), 5.63e5.59
(m, 1H), 5.41 (dd, J¼15.6, 6.8 Hz, 1H), 5.22 (dd, J¼17.4, 9.4 Hz, 1H),
4.56 (dd, J¼9.2, 3.6 Hz, 1H), 3.35 (d, J¼11.2 Hz, 1H), 2.86 (dd, J¼16.4,
10.0 Hz, 1H), 2.64 (dd, J¼16.2, 3.0 Hz, 1H), 2.22e2.19 (m, 3H),
2.11e2.07 (m, 3H), 1.86 (s, 3H), 0.68 (d, J¼6.8 Hz, 3H), 0.52 (d,
J¼6.8 Hz, 3H).
J¼7.4 Hz, 3H), 0.69 (d, J¼6.8 Hz, 3H), 0.52 (d, J¼6.8 Hz, 3H) ppm. 13
C
NMR (125 MHz, CDCl3):
d 173.5, 169.4, 168.8, 167.9, 164.5, 147.5,
132.8, 128.3, 124.1, 84.4, 72.0, 57.8, 43.3, 41.1, 40.5, 38.8, 37.3, 34.1,
31.9, 31.3, 24.2, 21.6, 18.8, 16.7, 13.7 ppm. MS (EI, m/z): 585 (Mþþ1).
HRMS (ESI): calcd for C25H36N4O4S4: [MþH]þ, 585.1692; found,
585.1690.
4.14. (5R,8S,11S)-8-Isopropyl-11-((E)-4-mercaptobut-1-en-1-
yl)-5-methyl-10-oxa-3,17-dithia-7,14,19,20-tetraazatricyclo
[14.2.1.12,5]icosa-1(18),2(20),16(19)-triene-6,9,13-trione (32)
4.17. (5R,8S,11S)-11-((E)-4-(tert-Butyldisulfanyl)but-1-en-1-
yl)-8-isopropyl-5-methyl-10-oxa-3,17-dithia-7,14,19,20-tetra-
azatricyclo[14.2.1.12,5]icosa-1(18),2(20),16(19)-triene-6,9,13-
trione (6)
To a solution of 31 (187.8 mg, 0.254 mmol) in DCM at 0 ꢁC, TES
(0.12 mL, 0.762 mmol) and TFA (0.85 mL) were added, the mixture
was allowed warm to rt and stirred for 2 h before concentrated,
then the crude was subjected to column chromatography to give 32
as a white foam solid (108 mg, 86%). 1H NMR (400 MHz, CDCl3)
The procedure was the same as described above for the syn-
thesis of 4, compound 6 was obtained as white solid (46 mg, 73%).
[a] d 7.76 (s, 1H),
20 1.35 (c 0.148, CHCl3). 1H NMR (400 MHz, CDCl3):
D
7.17 (d, J¼9.6 Hz, 1H), 6.52 (d, J¼7.2 Hz, 1H), 5.94e5.86 (m, 1H),
5.70e5.66 (m, 1H), 5.56 (dd, J¼15.6, 6.8 Hz, 1H), 5.28 (dd, J¼17.8,
9.4 Hz, 1H), 4.60 (dd, J¼9.2, 3.6 Hz, 1H), 4.27 (dd, J¼17.4, 3.0 Hz, 1H),
4.04 (d, J¼11.6 Hz, 1H), 3.28 (d, J¼11.2 Hz, 1H), 2.92e2.84 (m, 2H),
2.74e2.66 (m, 2H), 2.51 (dd, J¼14.0, 7.2 Hz, 2H), 2.12e2.08 (m, 1H),
1.86 (s, 3H), 1.38 (s, 9H), 0.69 (d, J¼6.8 Hz, 3H), 0.52 (d, J¼6.8 Hz,
d
7.76 (s, 1H), 7.18 (d, J¼9.4 Hz, 1H), 6.52 (d, J¼9.2 Hz, 1H), 5.83 (dt,
J¼14.5, 6.8 Hz, 1H), 5.68 (t, J¼8.2 Hz, 1H), 5.54 (dd, J¼15.5, 6.8 Hz,
1H), 5.27 (dd, J¼17.6, 9.3 Hz, 1H), 4.61 (dd, J¼9.3, 3.2 Hz, 1H), 4.28
(dd, J¼17.2, 4.0 Hz, 1H), 4.04 (d, J¼11.2 Hz, 1H), 3.28 (d, J¼11.4 Hz,
1H), 2.87 (dd, J¼16.3, 10.1 Hz, 1H), 2.71 (dd, J¼6.8, 6.8 Hz, 1H), 2.57
Please cite this article in press as: Su, J.; et al., Tetrahedron (2014), http://dx.doi.org/10.1016/j.tet.2014.05.078

6
J. Su et al. / Tetrahedron xxx (2014) 1e7
3H) ppm. 13C NMR (125 MHz, CDCl3):
d
173.5, 169.4, 168.8, 167.9,
350e380 nm and detection of emitted light in the range
440e460 nm. In order to compensate for HDAC inhibition activity
of the DMSO solvent, all values were normalized in comparison
with DMSO as a solvent control. GraphPad Prism software 5.0 was
used to calculate the doseeresponse curve via non-linear re-
gression analysis.
164.5, 147.5, 132.6, 128.5, 124.1, 84.4, 72.0, 57.8, 48.9, 43.3, 41.1, 40.5,
38.0, 34.1, 31.9, 31.5, 24.2, 22.7,18.8,16.7, 14.1 ppm. MS (EI, m/z): 585
(Mþþ1). HRMS (ESI): calcd for C25H36N4O4S4: [MþH]þ, 585.1692;
found, 585.1695.
4.18. (5R,8S,11S)-11-((E)-4-(Hexyldisulfanyl)but-1-en-1-yl)-8-
isopropyl-5-methyl-10-oxa-3,17-dithia-7,14,19,20-
tetraazatricyclo[14.2.1.12,5]icosa-1(18),2(20),16(19)-triene-
6,9,13-trione (7)
4.20.2. Cell culture and cell viability assay. Cell lines, Molt-4, U937,
MCF-7, and BGC823 were purchased from Shanghai Cell Bank,
Chinese Academy of Sciences. Cells were routinely grown and
maintained in medium RPMI or DMEM with 10% FBS and with 1%
penicillin/streptomycin. All cell lines were incubated in a Thermo/
Forma Scientific CO2 Water Jacketed Incubator with 5% CO2 in air at
37 ꢁC. Cell viability assay was determined by the CCK8 (DOjinDo,
Japan) assay. Cells were seeded at a density of 400e800 cells/well
in 384 well plates and treated with various concentration of com-
pounds or solvent control. After 72 h incubation, CCK8 reagent was
added, and absorbance was measured at 450 nm using Envision
2104 multi-label Reader (Perkin Elmer, USA). Doseeresponse
curves were plotted to determine the GI50 values using Prism 5.0
(GraphPad Software Inc., USA).
The procedure was the same as described above for the syn-
thesis of 4, compound 7 was obtained as white solid (70 mg, 75%).
[a] d 7.76 (s, 1H),
20 1.52 (c 0.132, CHCl3). 1H NMR (400 MHz, CDCl3):
D
7.18 (d, J¼9.2 Hz, 1H), 6.51 (d, J¼8.4 Hz, 1H), 5.93e5.86 (m, 1H),
5.71e5.66 (m, 1H), 5.57 (dd, J¼15.2, 6.4 Hz, 1H), 5.28 (dd, J¼17.6,
9.2 Hz, 1H), 4.60 (dd, J¼9.4, 3.4 Hz, 1H), 4.28 (dd, J¼17.4, 3.0 Hz, 1H),
4.04 (d, J¼11.6 Hz, 1H), 3.28 (d, J¼11.2 Hz, 1H), 3.03e2.84 (m, 5H),
2.73e2.68 (m, 1H), 2.54 (dd, J¼14.0, 6.8 Hz, 2H), 2.14e2.06 (m, 1H),
1.86 (s, 3H), 1.76 (dd, J¼7.6, 5.2 Hz, 2H), 1.33e1.25 (m, 6H), 0.89 (t,
J¼7.0 Hz, 3H), 0.69 (d, J¼7.2 Hz, 3H), 0.52 (d, J¼6.8 Hz, 3H) ppm. 13
C
NMR (125 MHz, CDCl3):
d 173.5, 169.3, 168.8, 167.9, 164.5, 147.5,
132.3, 128.8, 124.1, 84.5, 71.9, 57.8, 43.3, 41.1, 40.5, 39.5, 38.1, 34.1,
31.3, 29.7, 29.0, 28.1, 24.2, 22.5,18.8,16.7,14.0 ppm. MS (EI, m/z): 613
(Mþþ1). HRMS (ESI): calcd for C27H40N4O4S4: [MþH]þ, 613.2005;
found, 613.2007.
Acknowledgements
This work was supported by the National Key Program for Basic
Research (2009CB940900), the National Natural Science Founda-
tion of China (21172220), the Special Foundation of President, and
the Strategic Leading Science & Technology Programme of the
Chinese Academy of Sciences.
4.19. (5R,8S,11S)-8-Isopropyl-5-methyl-11-((E)-4-(octyldi-
sulfanyl)but-1-en-1-yl)-10-oxa-3,17-dithia-7,14,19,20-
tetraazatricyclo[14.2.1.12,5]icosa-1(18),2(20),16(19)-triene-
6,9,13-trione (8)
Supplementary data
The procedure was the same as described above for the syn-
Copies of 1H NMR, and 13C NMR spectra for the products. Sup-
plementary data associated with this article can be found in the
online version, at http://dx.doi.org/10.1016/j.tet.2014.05.078.
thesis of 4, compound 8 was obtained as white solid (66 mg, 76%).
[a] d 7.76 (s, 1H),
20 1.89 (c 0.106, CHCl3). 1H NMR (400 MHz, CDCl3):
D
7.18 (d, J¼9.6 Hz, 1H), 6.51 (d, J¼8.4 Hz, 1H), 5.94e5.85 (m, 1H),
5.70e5.66 (m, 1H), 5.60e5.51 (m, 1H), 5.28 (dd, J¼17.6, 9.6 Hz, 1H),
4.60 (ddd, J¼9.4, 3.4, 1.6 Hz, 1H), 4.27 (dd, J¼17.6, 3.2 Hz, 1H), 4.04
(d, J¼11.2 Hz, 1H), 3.28 (d, J¼11.2 Hz, 1H), 3.06e2.83 (m, 3H),
2.72e2.65 (m, 3H), 2.57e2.42 (m, 2H), 2.11e2.06 (m, 1H), 1.86 (s,
3H), 1.77e1.67 (m, 2H), 1.27e1.25 (m, 10H), 0.88 (t, J¼6.4 Hz, 3H),
0.69 (d, J¼7.2 Hz, 3H), 0.52 (d, J¼6.8 Hz, 3H) ppm. 13C NMR
References and notes
1. (a) Bolden, J. E.; Peart, M. J.; Johnstone, R. W. Nat. Rev. Drug Discov. 2006, 5,
769e784; (b) Minucci, S.; Pelicci, P. G. Nat. Rev. Cancer 2006, 6, 38e51; (c)
Haberland, M.; Montgomery, R. L.; Olson, E. N. Nat. Rev. Genet. 2009, 10, 32e42;
(d) Fass, D. M.; Kemp, M. M.; Schroeder, F. A.; Wagner, F. F.; Wang, Q.; Holson, E.
B. Histone Acetylation and Deacetylation In Epigenetic Regulation and Epi-
genomics: Advances in Molecular Biology and Medicine; Wiley-VCH: Weinheim,
Germany, 2012; pp 515e561.
2. (a) Johnstone, R. W. Nat. Rev. Drug Discov. 2002, 1, 287e299; (b) Dokmanovic,
M.; Clarke, C.; Marks, P. A. Mol. Cancer Res. 2007, 5, 981e989; (c) de Ruijter, A. J.;
van Gennip, A. H.; Caron, H. N.; Kemp, S.; van Kuilenburg, A. B. Biochem. J. 2003,
370, 737e749.
(125 MHz, CDCl3):
d 173.5, 169.4, 168.8, 167.9, 164.5, 147.5, 132.8,
128.3, 124.1, 84.5, 71.9, 57.8, 43.3, 41.1, 40.5, 39.5, 39.1, 38.1, 37.7,
34.1, 31.8, 29.7, 28.5, 28.4, 24.2, 22.6, 18.8, 16.7, 14.1 ppm. MS (EI, m/
z): 641 (Mþþ1). HRMS (ESI): calcd for C29H44N4O4S4: [MþH]þ,
641.2318; found, 641.2318.
3. Weichert, W.; Roske, A.; Gekeler, V.; Beckers, T.; Stephan, C.; Jung, K.; Fritzsche,
F. R.; Niesporek, S.; Denkert, C.; Dietel, M.; Kristiansen, G. Br. J. Cancer 2008, 98,
604e610.
4.20. Biological assays
4. Weichert, W.; Roske, A.; Niesporek, S.; Noske, A.; Buckendahl, A. C.; Dietel, M.;
Gekeler, V.; Boehm, M.; Beckers, T.; Denkert, C. Clin. Cancer Res. 2008, 14,
1669e1677.
5. Rikimaru, T.; Taketomi, A.; Yamashita, Y.; Shirabe, K.; Hamatsu, T.; Shimada, M.;
Maehara, Y. Oncology 2007, 72, 69e74.
6. Miyake, K.; Yoshizumi, T.; Imura, S.; Sugimoto, K.; Batmunkh, E.; Kanemura, H.;
Morine, Y.; Shimada, M. Pancreas 2008, 36, e1e9.
7. Weichert, W. Cancer Lett. 2009, 280, 168e176.
8. Butler, K. V.; Kozikowski, A. P. Curr. Pharm. Des. 2008, 14, 505e528.
9. (a) Paris, M.; Porcelloni, M.; Binaschi, M.; Fattori, D. J. Med. Chem. 2008, 51,
1505e1529; (b) Arrowsmith, C. H.; Bountra, C.; Fish, P. V.; Lee, K.; Schapira, M.
Nat. Rev. Drug Discov. 2012, 11, 384e400.
10. Bruserud, O.; Stapnes, C.; Ersvaer, E.; Gjertsen, B. T.; Ryningen, A. Curr. Pharm.
Biotechnol. 2007, 8, 388e400.
11. Taori, K.; Paul, V. J.; Luesch, H. J. Am. Chem. Soc. 2008, 130, 1806e1807.
12. Bowers, A.; West, N.; Taunton, J.; Schreiber, S. L.; Bradner, J. E.; Williams, R. M. J.
Am. Chem. Soc. 2008, 130, 11219e11222.
13. Ying, Y.; Taori, K.; Kim, H.; Hong, J.; Luesch, H. J. Am. Chem. Soc. 2008, 130,
8455e8459.
4.20.1. HDAC assays. Purified recombinant HDAC enzymes 1 and 7
purchased from BPS Bioscience (catalog number 50051, 50002,
50003, 50004, 50005, 50006, 50007, 50008, 50009, 50010, 50011
MA, USA) were used in DAC activity assays according to the man-
ufacturer’s protocol. Reaction mixture containing 3
ml of HDAC as-
say buffer (BPS catalog number 50031), 1 l of 1 mg/ml BSA, 1
m
ml of
HDAC were added to a low binding 384 Corning black plate (catalog
number 3573). 10 nl of a series of serial dilutions of compounds was
transferred to the above 384 well plates using the Echo520 Liquid
Handler (Labcyte). Incubated at rt for 10 min. 4 ul of HDAC assay
buffer and 1
BPS catalog number 50040) were added. Incubated at 37 ꢁC for
60 min. Added 10
ml of 200 mM substrate (BPS catalog number 50037 or
m
l of HDAC assay developer (2ꢂ) (BPS catalog
number 50030) and incubated the plate at room temperature for
15 min. The fluorescent was detected with Envision 2104 multi-
label Reader capable of excitation at a wavelength in the range
14. Seiser, T.; Kamena, F.; Cramer, N. Angew. Chem., Int. Ed. 2008, 47, 6483e6485.
15. Ying, Y.; Liu, Y.; Byeon, S. R.; Kim, H.; Luesch, H.; Hong, J. Org. Lett. 2008, 10,
4021e4024.
Please cite this article in press as: Su, J.; et al., Tetrahedron (2014), http://dx.doi.org/10.1016/j.tet.2014.05.078

J. Su et al. / Tetrahedron xxx (2014) 1e7
25. Chen, F.; Gao, A.-H.; Li, J.; Nan, F.-J. ChemMedChem 2009, 4, 1269e1272.
7
16. Bowers, A. A.; Greshock, T. J.; West, N.; Estiu, G.; Schreiber, S. L.; Wiest, O.;
Williams, R. M.; Bradner, J. E. J. Am. Chem. Soc. 2009, 131, 2900e2905.
17. Ghosh, A. K.; Kulkarni, S. Org. Lett. 2008, 10, 3907e3909.
18. Nasveschuk, C. G.; Ungermannova, D.; Liu, X.; Phillips, A. J. Org. Lett. 2008, 10,
3595e3598.
26. Souto, J. A.; Vaz, E.; Lepore, I.; Poppler, A.-C.; Franci, G.; Alvarez, R.; Altucci, L.;
de Lera, A. R. J. Med. Chem. 2010, 53, 4654e4667.
27. Bhansali, P.; Hanigan, C. L.; Casero, R. A., Jr.; Tillekeratne, L. M. V. J. Med. Chem.
2011, 54, 7453e7463.
19. Ren, Q.; Dai, L.; Zhang, H.; Tan, W.; Xu, Z.; Ye, T. Synlett 2008, 2379e2383.
20. Numajiri, Y.; Takahashi, T.; Takagi, M.; Shin-ya, K.; Doi, T. Synlett 2008,
2483e2486.
21. Wang, B.; Huang, P.-H.; Chen, C.-S.; Forsyth, C. J. J. Org. Chem. 2011, 76,
1140e1150.
28. Li, X.; Tu, Z.; Li, H.; Liu, C.; Li, Z.; Sun, Q.; Yao, Y.; Liu, J.; Jiang, S. ACS Med. Chem.
Lett. 2013, 4, 132e136.
29. Clennan, E. L.; Wang, D.; Zhang, H.; Clifton, C. H. Tetrahedron Lett. 1994, 35,
4723e4726.
30. Freeman, F.; Lee, C. J. Org. Chem. 1988, 53, 1263e1266.
31. Waag, T.; Gelhaus, C.; Rath, J.; Stich, A.; Leippe, M.; Schirmeister, T. Bioorg. Med.
Chem. Lett. 2010, 20, 5541e5543.
22. Zeng, X.; Yin, B.; Hu, Z.; Liao, C.; Liu, J.; Li, S.; Li, Z.; Nicklaus, M. C.; Zhou, G.;
Jiang, S. Org. Lett. 2010, 12, 1368e1371.
23. Bowers, A. A.; West, N.; Newkirk, T. L.; Troutman-Youngman, A. E.; Schreiber, S.
L.; Wiest, O.; Bradner, J. E.; Williams, R. M. Org. Lett. 2009, 11, 1301e1304.
24. Benelkebir, H.; Marie, S.; Hayden, A. L.; Lyle, J.; Loadman, P. M.; Crabb, S. J.;
Packham, G.; Ganesan, A. Bioorg. Med. Chem. 2011, 19, 3650e3658.
32. Meng, Q.; Li, F.; Jiang, S.; Li, Z. ACS Med. Chem. Lett. 2013, 4, 858e862.
33. Jiang, S.; Li, Z.; Ding, K.; Roller, P. P. Curr. Org. Chem. 2008, 12, 1502e1542.
Please cite this article in press as: Su, J.; et al., Tetrahedron (2014), http://dx.doi.org/10.1016/j.tet.2014.05.078