Synthesis and screening of 3-substituted thioxanthen-9-one-10,10-dioxides

Article abstract of DOI:10.1016/j.bmcl.2007.07.103

This manuscript describes methods appropriate for the parallel synthesis of libraries based on the tricyclic thioxanthen-9-one-10,10-dioxide scaffold. The novel compounds were synthesized from previously reported 3-chlorothioxanthen-9-one-10,10-dioxide and commercially available 3-carboxylic acid thioxanthen-9-one-10,10-dioxide. The library members were screened for activity in a fluorescence polarization assay for inhibitors of BRCT domains of breast cancer gene 1 and in cell-based secreted alkaline phosphatase reported replicon system for activity against hepatitis C virus.

Full text of DOI:10.1016/j.bmcl.2007.07.103

Bioorganic & Medicinal Chemistry Letters 17 (2007) 5940–5943
Synthesis and screening of 3-substituted
thioxanthen-9-one-10,10-dioxides
Pedro M. J. Lory, Maria E. Estrella-Jimenez, Matthew J. Shashack, Ganesh L. Lokesh,
Amarnath Natarajan and Scott R. Gilbertson*
Department of Pharmacology and Toxicology,Chemical Biology Program, University of Texas Medical Branch,
301 University Boulevard, Galveston, TX 77555-0650, USA
Received 10 July 2007; accepted 23 July 2007
Available online 25 August 2007
Abstract—This manuscript describes methods appropriate for the parallel synthesis of libraries based on the tricyclic thioxanthen-9-
one-10,10-dioxide scaffold. The novel compounds were synthesized from previously reported 3-chlorothioxanthen-9-one-10,10-diox-
ide and commercially available 3-carboxylic acid thioxanthen-9-one-10,10-dioxide. The library members were screened for activity
in a fluorescence polarization assay for inhibitors of BRCT domains of breast cancer gene 1 and in cell-based secreted alkaline phos-
phatase reported replicon system for activity against hepatitis C virus.
Ó 2007 Elsevier Ltd. All rights reserved.
The generation of novel structures amenable to rapid
and efficient lead optimization constitutes an important
strategy in modern drug discovery. This paper reports
the utilization of thioxanthen-9-one-10,10-dioxides as
such in structure. Thioxanthenones and thioxanthen-9-
one-10,10-dioxides have been shown to possess a num-
ber of potentially useful biological activities including
anti-tumor,1,2 anti-allergic3–6 and monoamine oxidase
(MAO) inhibitory activity.7–9 However, some of the
most promising drug candidates arising from this class
of compounds have shown to have adverse toxic proper-
ties in Phase I clinical trials.10,11 Because of the potential
demonstrated by this scaffold, an approach to the syn-
thesis of these types of structures utilizing parallel
synthesis methods was developed. The molecules synthe-
sized by this approach were screened for their activity in
a Hep C replicon assay12 as well as for their ability to
inhibit the BRCT(BRCA1)-BACH1 interaction known
to have a role in tumor suppression, cell cycle regula-
tion, and DNA repair.13
the solubility of this class of compounds but would also
retain the postulated pharmacophoric motif required for
biological activity. With the goal of synthesizing a small
focused library of such molecules, a solution-phase par-
allel synthesis protocol for the synthesis of 10,10-dioxo-
3-piperidin-1-yl/piperizin-1-yl-thioxanthen-9-one
1
as
well as 10,10-dioxo-3-carboxamide derivatives
(Fig. 1) was developed.
2
The availability of 3-chloro-10,10-dioxide-thioxanthen-
9-one 7 proved to be crucial for the development of a
microwave-assisted protocol for the synthesis of a fo-
cused library of 36 novel 10,10-dioxo-3-piperidin-1-yl/
piperizin-1-yl-thioxanthen-9-one
derivatives
(1).14
Under basic conditions the nucleophilic substitution of
thiophenol 3 with the suitably substituted 2-iodo-3-chlo-
robenzoic acid building block 4 in the presence of a cat-
alytic amount of copper for 8 h affords the desired
coupled sulfide 5 in virtually quantitative yield (Scheme 1).
Based on preliminary biological activity data, it was
envisioned that introduction of suitably functionalized
amino as well as amide derivatives at the three-position
of the thioxanthenone scaffold would not only increase
O
S
O
S
H
N
N
R
O
O
O
O
X
O
R
Keywords: Thioxanthen-9-one; Microwave; Amine; Amide.
*
1, X = C, N
2
Corresponding author. Tel.: +1 409 772 9703; fax: +1 409 772
9700; e-mail: srgilber@utmb.edu
Figure 1.
0960-894X/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2007.07.103

P. M. J. Lory et al. / Bioorg. Med. Chem. Lett. 17 (2007) 5940–5943
5941
Cl
up, trace amounts of piperazine (<5%) were found to
be present by both H NMR and LC–MS analysis.
1
HO2C
KOH/H2O
+
SH
cat. Cu
120 ºC
99% yield
I
Cl
S
Given the general good yields at which these reactions
proceed, screening of alternative reaction conditions
was not actively pursued. Primary amines such as iso-
propyl amine, benzylamine, 4-(2-aminoethyl)morphol-
ine, and cyclopentyl amine were examined. In all cases
the expected products were presented in reaction mix-
tures as judged by LC–MS and proton NMR but further
purification was required.
CO2H
4
3
5
O
O
S
H2O2-AcOH
H2SO4
100 ºC,
4 h
90 ºC
70% yield
from 5 after
S
Cl
Cl
O
O
6
7
recrystalliztion
Taking advantage of the commercially available 9-oxo-
9H-thioxanthene-3-carboxylic acid 10,10-dioxide (8) a
small number of 3-substituted carboxylamides thioxan-
then-9-one-10,10-dioxide 9 (9a–9k) were synthesized.
Substituted carboxylic amides at the three-position of
the thioxanthen-9-one-10,10-dioxide have been reported
to be selective inhibitors of monoamine oxidases.4
Although Harfenist et al. have reported the synthesis
of various 3-substituted carboxylamides thioxanthen-9-
one 10,10-dioxide from the acid chloride derived from
the 3-substituted carboxylic acid, we opted to use cou-
pling reagents to synthesize the desired amides. Several
common coupling reagents, bases, and solvents were
screened, ranging from polymer-supported reagents
such as PS-piperidinomethyl as base as well as PS-
DCC as coupling reagent. Ultimately, the best results
were obtained using diisopropylethylamine (DIPEA)
as base, methylene chloride or DMF as solvents, and
BOP or HBTU as coupling reagents with methylene
chloride generally being the preferred solvent. In both
cases, using either BOP or HBTU, automated flash
chromatography was employed in the purification of
the desired amides (Table 2).
Scheme 1.
Treatment of 5 with concentrated sulfuric acid at 100 °C
over 4 h affords the Friedel–Crafts adduct, thioxanthe-
none 6. Upon pouring the reaction mixture onto ice,
the product precipitates out as an off-white solid. Oxida-
tion of 6 with hydrogen peroxide at 90 °C provided the
desired sulfone 7, which could be purified by recrystalli-
zation from ethyl acetate–hexanes.
Literature precedent for the synthesis of similar amino
derivatives requires a multi-step synthesis of the 3-
amino substrate followed by appropriate functionali-
zation of the amino moiety or a low yielding acidic
hydrolysis of the 3-tetrazole to the corresponding
3-amino product.8 The latter can then be further func-
tionalized only under vigorous basic conditions due to
inherent lack of reactivity of the amino functionality.8
The approach reported here makes use of this ring
system’s electron-withdrawing properties (carbonyl
and sulfone moieties) which allow for efficient aro-
matic nucleophilic displacement at the 3-chloro position
by a variety of commercially available piperidines and
piperazines.
The compounds were screened for activity in a cell-
based secreted alkaline phosphatase reported replicon
system for activity against hepatitis C assay12 and a fluo-
rescence polarization assay for inhibition of the BRCT–
BACH1 interaction.13
Treatment of a solution of 3-chloro-10,10-dioxide-thio-
xanthen-9-one in DMF with K2CO3 (1.2 equiv) followed
by the addition of the corresponding piperidine or piper-
azine (1.2 equiv) under microwave conditions led to the
formation of the corresponding 3-piperidin-1-yl/piperi-
zin-1-yl-thioxanthen-9-ones in good to excellent yields
(68–99%) (Table 1). Purification of the final products
was achieved in a very practical and efficient manner
by simple aqueous work-up using citric acid (1 M solu-
tion) and dichloromethane as extraction solvent. This
purification protocol proved equally adaptable to the
more basic piperazine products (e.g. 1k–1t), albeit
replacement of citric acid by hydrochloric acid (0.5 M
solution) was found to be necessary for a more efficient
removal of unreacted or slight excess of piperazine. It is
also worth noting that this slightly modified acidic
work-up resulted in only small amounts of product
(<5%) going into the mildly acidic water layer, as mon-
itored by LC–MS. Because of its greater basicity, com-
pound 1k could not be purified by this simple acidic
work-up protocol. Instead, it was purified by automated
flash chromatography. In some cases within the pipera-
zine series of compounds, and despite the acidic work-
In the case of the hepatitis C replicon system no signif-
icant activity was observed. While in the BRCT–
BACH1 assay system three compounds 1h, 1i, and 1b
exhibited moderate Ki values 38 1, 30 6, and
39 3 lM, respectively. We are currently synthesizing
bifunctional versions of these molecules for further
testing in the BRCT–BACH1 system.
In summary, a focused library of 3-substituted thioxan-
thenones was synthesized by the facile nucleophilic aro-
matic substitution and amide bond formation. In
general, this reaction was found to work well with sec-
ondary amines providing products in greater than or
equal to 90% purity after simple extraction. A small
collection of amides was also obtained in comparable
purity from the HBTU or BOP coupling of the corre-
sponding acid with an amine. All of the synthesized
compounds were screened for biological activity in two
different assays with three compounds providing moder-
ate inhibition of the BRCT–BACH1 protein–protein
interaction.

5942
P. M. J. Lory et al. / Bioorg. Med. Chem. Lett. 17 (2007) 5940–5943
Table 1. Synthesis of 10,10-dioxo-3-piperidin-1-yl/piperizin-1-yl-thioxanthen-9-onesa,b
O
O
S
DMF, K2CO3
S
Cl
HN
R'
O
O
X
O
O
R
Microwave, 30 min.
7
1a-1jj
Yield (%)b
Yield (%)b
Yield (%)b
O
S
O
S
O
O
N
N
1a 68
1m 98
1n 94
1y 45d
S
O
S
N
N
O
O
O
O
O
O
O
S
N
N
N
N
O
O
O
O
O
O
N
Cl
O
1z 49d
Cl
1b 99
1c 83
O
O
N
N
S
N
N
Cl
O
O
S
O
S
1o 80
1aa 45d
N
N
S
N
O
O
O
N
N
N
N
N
O
S
O
S
O
S
O
1d 99
1p 82
1q 70
1bb 57d
1cc 82d
N
N
N
S
N
N
O
O
O
O
O
O
O
O
N
N
N
N
O
O
O
O
N
N
OMe
N
O
O
O
S
1e 100
S
N
N
O
O
O
O
1f 98
1g 99
1h 99
1i 91
1r 88
1s 98
1t 98
1u 87
1dd 18d
1ee 40d
1ff 6d
S
N
N
S
N
N
S
N
N
N
O
O
O
O
O
O
O
O
O
O
O
O
O
N
N
O
O
O
O
O
S
O
S
S
O
S
O
S
O
S
N
N
N
N
O
Ph
NC
O
N
N
N
N
N
OCH3
S
N
N
O
O
O
O
O
O
O
O
O
O
N
N
N
N
N
N
O
O
O
O
Bn
CN
O
S
O
1gg 40d
S
O
NH2
N
N
O
O
O
O
O
1j 93
1k 72c
1l 73
1v 47d
1w 25d
1x 30d
1hh 76d
1ii 12d
1jj 74d
S
S
N
N
S
N
N
N
O
O
O
O
O
O
O
O
O
O
N
O
O
O
O
S
S
N
N
S
N
N
N
O
O
O
O
NH
N
O
O
NC
O
S
N
S
O
O
N
S
N
N
N
N
F3C
a Reactions were run using chloro-sulfone 7, 1.2 equiv of amine and 1.2 equiv of K2CO3 and 6.0 mL of solvent in a CEM Discovery microwave
system. A microwave irradiation power of 300 W, ramp time of 2.0 min. with a run time of 30 min at 155 °C and simultaneous cooling (powermax
mode) was used.
b Unless noted isolated crude yields are reported.
c 2.2 equiv of K2CO3 were used for the synthesis of this product.
d Isolated yields after column chromatography.

P. M. J. Lory et al. / Bioorg. Med. Chem. Lett. 17 (2007) 5940–5943
Table 2. Synthesis of 10,10-dioxo-3-carboxamides-thioxanthen-9-onesa
5943
O
O
HBTU or BOP, DIEPA
NH2R, CH2Cl2
H
N
S
CO2H
S
R
O
O
O
O
O
8
9a-9k
b
R
Product
Yield (%)b
36
R
Product
Yield (%)
N
H
N
9a
9g
29
67
57
56
71
H
CF3
OCH3
N
H
9b
9c
9d
9e
9f
46
24
30
31
66
9h
9i
HN
OCH3
O
O
O
N
H
N
H
O
OCH3
N
H
N
H
9j
N
H
9k
N
H
OCF3
N
H
a Reactions were run using carboxylic acid-sulfone 8, 1.1 equiv of amine, 1.1 equiv of HBTU or BOP and 5.0 equiv of DIPEA (Diisopropylethyl
amine) in 5.0 mL of solvent (CH2Cl2) at rt.
b Isolated crude yields are reported.
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Acknowledgments
This work was supported by NIH P41 GM079588.
Financial assistance provided by the Robert A. Welch
foundation is gratefully acknowledged. M.E.E-J.
acknowledges support by a training fellowship from
the Keck Center Pharmacoinformatics Training Pro-
gram of the Gulf Coast Consortia (NIH Grant #
T90DK071504-03).
Supplementary data
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Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/
j.bmcl.2007.07.103.
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