Synthesis and evaluation of heteroaromatic 6,7-diaryl-2,3,8,8a-tetrahydroindolizin-5(1H)-ones for cytotoxicity against the HCT-116 colon cancer cell line

Article abstract of DOI:10.1016/j.bmcl.2008.04.051

A heteroaromatic 6,7-diaryl-2,3,8,8a-tetrahydroindolizin-5(1H)-one analog library was prepared and tested for cytotoxic properties against the HCT-116 colon cancer cell line, thus providing additional information pertaining to structure-activity relations

Full text of DOI:10.1016/j.bmcl.2008.04.051

Available online at www.sciencedirect.com
Bioorganic & Medicinal Chemistry Letters 18 (2008) 3248–3250
Synthesis and evaluation of heteroaromatic
6,7-diaryl-2,3,8,8a-tetrahydroindolizin-5(1H)-ones
for cytotoxicity against the HCT-116 colon cancer cell line
F. Scott Kimball,a Richard H. Himesb and Gunda I. Georga,*
aDepartment of Medicinal Chemistry, University of Kansas, 1251 Wescoe Hall Drive, Lawrence, KS 66045, USA
bDepartment of Molecular Biosciences, University of Kansas, 1200 Sunnyside Avenue, Lawrence, KS 66045, USA
Received 6 March 2008; revised 16 April 2008; accepted 21 April 2008
Available online 25 April 2008
Abstract—A heteroaromatic 6,7-diaryl-2,3,8,8a-tetrahydroindolizin-5(1H)-one analog library was prepared and tested for cytotoxic
properties against the HCT-116 colon cancer cell line, thus providing additional information pertaining to structure–activity rela-
tionships for this class of compounds. The most active of the new analogs proved to be the C6 2-thiophene and 3-thiophene analogs
with IC50 values of 0.27 lM and 0.60 lM, respectively.
Ó 2008 Elsevier Ltd. All rights reserved.
OMe
Tyloindicine I (1, Fig. 1) was isolated from the aerial
parts of Tylophora indica and described by Ali et al.1,2
Preliminary analysis revealed potent nanomolar and
cancer cell-selective cytotoxic properties most likely
exerted through a novel mechanism of action.2,3 During
our efforts toward the total synthesis of tyloindicine I,
we found that a reaction intermediate toward the syn-
thesis of 1, 6-phenyl-7-(4-methoxyphenyl)-2,3,8,8a-tetra-
hydroindolizin-5(1H)-one (racemic 2, Fig. 1) displayed
selective cytotoxicity toward colon cancer cell lines,
was active in vivo in the mouse hollow fiber assay, and
presumably exerts its cytotoxic activity via an unknown
novel mechanism of action.3 Related studies were
reported by Sharma et al., who also investigated
2,3,8,8a-tetrahydroindolizin-5(1H)-ones as cytotoxic
agents.4
OMe
OMe
H
N
H
N
7
6
OH
OMe
O
OMe
1
(R)-2
Figure 1. Tyloindicine I (1) and lead 7-(4-methoxyphenyl)-6-phenyl-
2,3,8,8a-tetrahydroindolizin-5(1H)-one (2).
substitution well. In subsequent studies, we prepared
both enantiomers of 2, and found that only the (R)-2
enantiomer is cytotoxic.5
In an effort to expand the structure–activity relationship
information for these compounds, we sought to develop
an analog library to probe the ramifications of replacing
the southern phenyl ring with various heteroaromatic
rings. As the C6 aromatic ring of related analogs had
proved not to accommodate substitution well, we could
not explore the electronic ramifications of the southern
ring by substitution alone. With this limitation in mind,
a 14-membered heteroaromatic 6,7-diaryl-2,3,8,8a-tetra-
hydroindolizin-5(1H)-one analog library was prepared.
Ultimately, this library would serve to vary the elec-
tronic characteristics of the southern ring without
increasing the steric bulkiness by adding substituents
to the aromatic ring system.
Initial structure–activity studies from our laboratory
focused on altering the substitution patterns of both
the northern and southern aromatic rings attached to
the 2,3,8,8a-tetrahydroindolizin-5(1H)-one core.3 These
studies revealed that the lead compound, racemic 2,
was the most active in the library and indicated that
the southern aromatic ring at C6 did not accommodate
Keywords: Tetrahydroindolizinones; Synthesis; Heteroaromatic; Struc-
ture–activity studies; Cytotoxicity; HCT-116 colon cancer cell line.
*
Corresponding author. Tel.: +1 612 626 6320; fax: +1 612 626
6318.; e-mail: georg239@umn.edu
0960-894X/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2008.04.051

F. Scott Kimball et al. / Bioorg. Med. Chem. Lett. 18 (2008) 3248–3250
3249
appropriate heteroaryl acetic acid to provide the aldol
precursors 4a–4n. The final step called for a basic intra-
molecular aldol condensation to construct the heteroar-
omatic library of analogs 3a–3n. As previously
illustrated, the acidic BOC-deprotection and basic
intramolecular aldol conditions led to the retro-Mi-
chael–Michael racemization.5 The yields and substitu-
tion patterns for the heteroaryl analog library
developed in Scheme 2 are shown in Table 1. In addi-
tion, the phenyl parent compounds 4a and 4i were
carried through the synthesis as well so as to have a
standard for cytotoxicity comparison purposes.
R
R
N
O
Ar
N
Ar
O
O
3a-3n
4a-4n
OH
O
O
N
t-Bu
O
5
The heteroaromatic indolizidine analog library was
tested for cytotoxicity against the HCT-116 colon cancer
cell line. The results are summarized in Table 2. It is
clear that the pyridyl analogs 3b–3d of the parent com-
pound 3a are the least active of the compounds tested,
with the 2- and 3-pyridyl derivatives having much
reduced activity. The latter two compounds were not
pursued further as the thiomethyl derivatives. The thio-
methyl complement of the 4-pyridyl derivative 3j, also
had the lowest activity of the thiomethyl subgroup of
Scheme 1. Retrosynthesis for 3a–3n.
The retrosynthetic analysis for the analog library is
patterned after the stereospecific synthesis developed re-
cently in our laboratories to generate both enantiomers
of 2 (Scheme 1).5 Although in this case, without any
stereocontrolling element elsewhere in the molecule,
the stereocenter is destroyed due to a retro-Michael–
Michael racemization.5 Regardless, the use of N-BOC
L-homoproline (5) as the starting substrate was conve-
nient and the synthesis was otherwise sound for our
purposes of probing the heteroaromatics effects on bio-
logical activity. Thus, title compounds 3a–3n can be
envisioned as coming from amides 4a–4n following an
intramolecular aldol condensation of the amide with
the aryl ketone. In turn, amides 4a–4n may be derived
from 5 through a Grignard reaction followed by a
BOC-deprotection/peptide bond formation sequence.
Table 1. Synthetic yields for analog library5
Analog
R
Ar
Yield (%)
6a
6b
OMe
SMe
n/a
n/a
80a
77a
Methoxy acylation
4a
4b
4c
4d
4e
4f
OMe
OMe
OMe
OMe
OMe
OMe
OMe
OMe
Phenyl
74a
2-Pyridine
3-Pyridine
4-Pyridine
2-Thiophene
3-Thiophene
2-Furan
78a
See 3cb
45a
The synthesis toward 3a–3n was initiated with the Wein-
reb amidation of (S)-2-(1-(tert-butoxycarbonyl)-pyrroli-
din-2-yl)acetic acid (5, Scheme 2). The subsequent
Grignard addition into the Weinreb amide supplied aryl
ketones 6a–6b. An acidic BOC-deprotection of ketones
6a–6b formed stable hydrochloride amine salts, which
were subjected to an EDCI coupling protocol with the
93a
86a
4g
4h
77a
3-Furan
59a
Methoxy intramolecular aldol condensation
3a ((+/À)-2)
OMe
OMe
OMe
OMe
OMe
OMe
OMe
OMe
Phenyl
30
89
57b
82
68
79
48
65
3b
3c
3d
3e
3f
2-Pyridine
3-Pyridine
4-Pyridine
2-Thiophene
3-Thiophene
2-Furan
R
OH
O
O
a, b
c, d
3g
3h
N
O
3-Furan
t-Bu
N
O
O
t
-Bu
Thiomethyl acylation
4i
O
SMe
SMe
SMe
SMe
SMe
SMe
Phenyl
58a
44a
87a
77a
77a
53a
6a-6b
5
4j
4-Pyridine
2-Thiophene
3-Thiophene
2-Furan
R
4k
4l
R
4m
4n
e
3-Furan
N
O
Ar
N
Thiomethyl intramolecular aldol condensation
Ar
O
3i
SMe
SMe
SMe
SMe
SMe
SMe
Phenyl
66
52
51
81
36
60
O
3j
4-Pyridine
2-Thiophene
3-Thiophene
2-Furan
3a-3n
4a-4n
3k
3l
Scheme 2. Synthesis of 3a–3n.5 Reagents and conditions: (a) N,O-
dimethylhydroxylammonium chloride, EDCI, NMM, dry DCM; (b)
Mg0 turnings, cat. I2 crystals, p-bromoanisol for 6a or p-bromothio-
anisole for 6b, dry THF; (c) 4 M HCl/dioxane; (d) heteroaryl acetic
acid, EDCI, NMM, dry DCM; (e) 7.7% KOH(EtOH), reflux.
3m
3n
3-Furan
a Yield over two steps.
b Yield over three steps.

3250
F. Scott Kimball et al. / Bioorg. Med. Chem. Lett. 18 (2008) 3248–3250
Table 2. Cytotoxicity of the heteroaromatic indolizidine analog library
against the HCT-116 colon cancer cell line
library (3a–3n in Table 2) revealed that the combination
of a C6-thiophene substituent with a C7-(4-thio-
methyl)phenyl moiety provided analogs (3k and 3l) that
displayed cytotoxicities comparable to the lead com-
pound 3a.
Analog
R
Ar
IC50 (lM)
3a ((+/À)-2)
OMe
OMe
OMe
OMe
OMe
OMe
OMe
OMe
SMe
SMe
SMe
SMe
SMe
SMe
Phenyl
0.39
104
79
3b
3c
3d
3e
3f
2-Pyridine
3-Pyridine
4-Pyridine
2-Thiophene
3-Thiophene
2-Furan
19
5.0
1.5
Acknowledgments
3g
3h
3i
6.0
3.0
These studies were supported by the National Cancer
Institute (CA 90602 and N01-CM-67259). We thank
Deborah A. Bohlander for conducting the cytotoxicity
assays.
3-Furan
Phenyl
1.0
6.0
3j
4-Pyridine
2-Thiophene
3-Thiophene
2-Furan
3k
3l
0.27
0.60
3.0
3m
3n
References and notes
3-Furan
1.1
1. Ali, M.; Ansari, S. H.; Qadry, J. S. J. Nat. Prod. 1991, 54,
1271.
2. Ali, M.; Ansari, S. H.; Grever, M. R. Pharmazie 2001, 56,
188.
compounds. In general, there was little difference in
activity among the furan and thiophene derivatives of
the parent or thiomethyl compounds, although the thio-
methyl thiophene derivatives 3k and 3l were the most
active of all the derivatives tested and were equal in
potency to the parent compound 3a.
3. Kimball, F. S.; Dutta, D.; White, J. M.; Himes, R. H.;
Georg, G. I. Bioorg. Med. Chem. Lett. 2007, 17, 4703.
4. Sharma, V. M.; Seshu, K. V. A.; Krishna, C. V.; Prasanna,
P.; Sekhar, V. C.; Venkateswarlu, A.; Rajagopal, S.;
Ajaykumar, R.; Deevi, D. S.; Mamidic, N. V. S. R.;
Rajagopalanb, R. Bioorg. Med. Chem. Lett. 2003, 13, 1679.
5. Kimball, F. S.; Turunen, B. J.; Ellis, K. C.; Himes, R. H.;
Georg, G. I. Bioorg. Med. Chem. Lett. 2008, 16, 4367, All
intermediates and analogs in Table 1 were prepared
utilizing the experimental conditions reported in this paper.
The spectroscopic data for all compounds in Table 1 were
found to be in agreement with their structures.
In conclusion, a heteroaromatic analog library was
designed and prepared using a synthetic sequence devel-
oped in our laboratories.5 Since the southern aromatic
ring did not accommodate substitution well, we could
not explore the electronic ramifications of the southern
ring by substitution, and therefore prepared a heteroar-
omatic analog library. The evaluation of said compound