Heteroannelated and 7-deoxygenated goniofufurone mimics with antitumour activity: Design, synthesis and preliminary SAR studies

Article abstract of DOI:10.1016/j.bmcl.2013.08.069

Cytotoxic (+)-goniofufurone mimic such as benzoxepane 2 was preferentially formed after the treatment of 7-O-benzoyl-5-O-benzyl (+)-goniofufurone derivative 6 with titanium(IV) fluoride. However, the corresponding 7-epimer 5 (derivative of 7-epi-goniofufu

Full text of DOI:10.1016/j.bmcl.2013.08.069

Bioorganic & Medicinal Chemistry Letters 23 (2013) 5507–5510
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Heteroannelated and 7-deoxygenated goniofufurone mimics with
antitumour activity: Design, synthesis and preliminary SAR studies
a
a
a
a
Velimir Popsavin a, , Jovana Francuz , Bojana Sreco Zelenovic , Goran Benedekovic , Mirjana Popsavin ,
⇑
´
´
´
b
b
´
´
Vesna Kojic , Gordana Bogdanovic
a
´
Department of Chemistry, Biochemistry and Environmental Protection, Faculty of Sciences, University of Novi Sad, Trg Dositeja Obradovica 3, 21000 Novi Sad, Serbia
b Oncology Institute of Vojvodina, Put doktora Goldmana 4, 21204 Sremska Kamenica, Serbia
a r t i c l e i n f o
a b s t r a c t
Article history:
Cytotoxic (+)-goniofufurone mimic such as benzoxepane 2 was preferentially formed after the treatment
of 7-O-benzoyl-5-O-benzyl (+)-goniofufurone derivative 6 with titanium(IV) fluoride. However, the cor-
responding 7-epimer 5 (derivative of 7-epi-goniofufurone) under the similar reaction conditions gave
mainly 7-deoxy derivative 7 as a result of an unexpected 1,5-hydride shift. Extension of this methodology
to the enantiomer ent-6 provided cytotoxic (ꢀ)-goniofufurone mimics ent-2 and ent-7. Synthesized com-
pounds showed diverse growth inhibitory effects against selected tumour cell lines, but were devoid of
any significant toxicity towards the normal foetal lung fibroblasts (MRC-5). A SAR study reveals the struc-
tural features of these lactones that are beneficial for their antiproliferative activity, such as presence of
an additional oxepane ring, the absolute stereochemistry and the presence of a deoxy function at the C-7
position.
Received 26 July 2013
Revised 13 August 2013
Accepted 14 August 2013
Available online 22 August 2013
Keywords:
Antitumour styryl lactones
Goniofufurone mimics
SAR
Titanium(IV) fluoride
Analogue synthesis
Ó 2013 Elsevier Ltd. All rights reserved.
Natural products based drug discovery has become a major
strategy in modern pharmaceutical research and development,
and roughly half of the currently used drugs are directly or indi-
rectly derived from natural products.1 The styryl lactones from
Goniothalamus (Blume) Hook. f. & Thomson (family Annonaceae
Juss.) are an interesting class of naturally occurring compounds,
many of which were found to exhibit impressive biological activi-
ties.2 In 1990, McLaughlin and co-workers isolated goniofufurone
(1, Fig. 1), a novel cytotoxic styryl lactone from the Southeast Asian
medicinal treelet Goniothalamus giganteus Hook. f. & Thomson.3
Due to its unique and intriguing structure, as well as its promising
antitumour activity this natural product along with a number of its
analogues has attracted the attention of many synthetic groups.4,5
We have also been involved in the synthesis of 1 and related com-
pounds.6,7 Thus, we have recently found that treatment of gon-
iofufurone derivative 5 (Scheme 1) with TiCl4 in dry CH2Cl2 gave
a mixture of epimeric 7-chloro-7-deoxy goniofufurone derivatives
(structures not shown) in 52% combined yield, along with a minor
amount of 2 (17%)8 that was presumably formed by an intramolec-
ular Friedel–Crafts reaction.
from fact that parent compound 1 has a restricted geometry of the
C5–C7 segment, due to an intramolecular H-bond formed between
the 5-OH and the 7-OH groups, as established by X-ray analysis.3
Alternatively, an automated molecular overlay of 3D structures 1
and 2 generated by X-ray diffraction8 revealed a good structural
mach of their furano-furanone segments and styryl moieties
(Fig. 2).9 Synthesis and in vitro antitumour screening of lactones
3 and 4 are also planned as they represent ring-opened analogues
of 2.
Herein, we want to report synthesis of novel goniofufurone mi-
mic 2 along with its ability to inhibit the growth of selected human
tumour cell lines. In order to achieve a more efficient synthesis of 2
we have decided to replace the catalyst (TiCl4) for the intramolec-
ular Friedel–Crafts reaction of 5 or 610 with a stronger Lewis acid
such as TiF4 (Scheme 1). However, the treatment of 5 with TiF4
in dry CH2Cl2, for 7 days at ꢀ18 °C, unexpectedly gave 7-deoxy
derivative 7 (44%) as the major product accompanied with a minor
amount of benzoxepane 2 (19%) as a result of an intramolecular
Friedel–Crafts alkylation process.8
The mechanism of deoxygenation of 5 at C-7 may be explained
by the several successive transformations shown in Scheme 1. In
the first step TiF4 coordinates the O-5 and O-7 of 5 to afford 5a with
activated benzylic carbon atom. The C7–O7 bond of 5a is then
cleaved with simultaneous transannular 1,5-hydride migration to
furnish 5b. A similar 1,5-hydride shift has recently been observed
As molecule 2 represents a conformationally restricted ana-
logue of 1 it was of interest to prepare it in a more efficient way,
and to evaluate its antitumour activity against selected human tu-
mour cell lines. The rationale underlying the preparation of 2 arises
in the Lewis acid-catalyzed construction of
a benzopyran
⇑
Corresponding author. Tel.: +381 21 485 27 68; fax: +381 21 454 065.
E-mail address: velimir.popsavin@dh.uns.ac.rs (V. Popsavin).
0960-894X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.bmcl.2013.08.069

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V. Popsavin et al. / Bioorg. Med. Chem. Lett. 23 (2013) 5507–5510
Figure 2. Superimposed structures of 1 (green) and 2 (pink). Green dashed line
represents intramolecular H-bond in 1.
conformer 5a0 thus favouring a transannular 1,5-hydride shift in
the TiF4-complex of 5. However, in the case of 6a0 and 6a00, the
equilibrium is shifted to the chair-like conformer 6a0 whose ben-
zylic hydrogens are far more distant from the C-7 atom. Such an
arrangement disfavoured the transannular 1,5-hydride shift in
6a. These speculations are confirmed by molecular modelling.
Namely, the conformational search12 performed on both interme-
diates 5a and 6a confirmed a higher stability of conformer 5a0
(favourable geometry for the hydride shift) over 5a00 (unfavourable
geometry for the hydride shift). A higher stability of 6a0 (unfavour-
able geometry for the hydride shift) over 6a00 (favourable geometry
for the hydride shift) was also confirmed. These findings appear to
convincingly explain the experimental results.
7-Deoxy derivative 7 that was isolated as the major product
after treatment of 5 with TiF4 was recently prepared in our labora-
tory and evaluated for its antiproliferative activity against several
tumour cell lines.7 As a consequence of the in vitro antitumour
activity of this product, we were especially keen to prepare and
evaluate the opposite enantiomer of 7 (structure ent-7, Scheme 2),
since it is well known that enantiomers of certain biologically
active molecules may show improved potencies,13,14 or even differ-
ent biological activities altogether.15
Figure 1. Chemical structure of (+)-goniofufurone (1) and analogues 2–4.
skeleton.11 Final hydrolysis of 5b gave the deoxygenated product 7
and benzaldehyde.
In an attempt to extend this methodology to a similar substrate,
but with opposite stereochemistry at the C-7 position, lactone 6
was treated under the same conditions. Quite unexpectedly, the
treatment of 6 with titanium(IV) fluoride in dichloromethane gave
oxepane 2 (48%) as a major product, accompanied with a minor
amount of 7-deoxy derivative 7 (31%). Accordingly, it appears that
(7R) stereochemistry of 5 directs the course of the reaction toward
preferential formation of 7-deoxygenated derivative 7, while (7S)
stereochemistry of 6 favours the preferential formation of ben-
zoxepane 2. These differences in reactivity could be well rational-
ized by taking into consideration the stereochemical properties of
TiF4-complexes 5a and 6a (Fig. 3).
To promote the desired 1,5-hydride shift, conformers 5a0 and
6a00, whose benzylic hydrogens were located close to the electro-
philic C-7 atom, should be considered. In the case of 5a0 and 5a00,
the conformational equilibrium is largely shifted to the twist-boat
Scheme 1. Reagents and conditions: (a) TiF4, CH2Cl2, ꢀ18 °C, 7 days for 5, 14 days
for 6, 19% of 2, 44% of 7 (from 5), 48% of 2, 31% of 7 (from 6).
Figure 3. Stereochemical properties of titanium(IV) complexes 5a and 6a.

V. Popsavin et al. / Bioorg. Med. Chem. Lett. 23 (2013) 5507–5510
5509
9 with benzyl bromide in anhydrous DMF, in the presence of so-
dium hydride as a catalyst, gave the expected 5-O-benzyl deriva-
tive 10 in 98% yield. Hydrolytic removal of the cyclohexylidene
protective group in 10 with aqueous acetic acid afforded the ex-
pected lactol 11 (75%), which upon treatment with Meldrum’s acid
in anhydrous DMF, in the presence of triethylamine, afforded 4 in
80% yield. Both newly synthesized analogues 3 and 4 were fully
characterized by the corresponding physical and spectral data.
The biological activities of the synthesized derivatives 2, ent-2,
3, 4, 7 and ent-7 were evaluated by an in vitro cytotoxicity test car-
ried out with a panel of five human tumour cell lines (myelogenous
leukaemia K562, promyelocytic leukaemia HL-60, Jurkat T cells
leukaemia, Raji Burkitt’s lymphoma, cervix carcinoma HeLa) and
one normal cell line (foetal lung fibroblasts MRC-5). Cell growth
inhibition was evaluated after 72-h cells treatment by using the
MTT test. (+)-Goniofufurone (1) and the commercial antitumour
agent doxorubicin (DOX) were used as the positive controls in this
assay.
Scheme 2. Reagents and conditions: (a) BzCl, Py, 0 °C ? rt, 18 h; (b) TiF4, CH2Cl2,
ꢀ18 °C, 14 days.
As shown in Table 1, a number of goniofufurone mimics exhibit
potent in vitro anticancer activities, with IC50 values in the low
micromolar range. Moreover, all of them including the lead 1 were
completely inactive toward normal MRC-5 cells, in contrast to the
commercial drug DOX that showed a potent cytotoxic activity to-
The known13 5-O-benzyl derivative ent-8 served as a conve-
nient starting material for this part of the work. Alcohol ent-8 read-
ily reacted with benzoyl chloride under standard conditions to give
the corresponding 7-O-benzoyl derivative ent-6 in 68% yield. Treat-
ment of ent-6 with TiF4 under the same reaction conditions as that
applied for the transformation of 6, furnished the expected prod-
ucts ent-2 and ent-7 in 42 and 33% respective yields. The physical
and spectroscopic data of thus obtained samples ent-2, ent-6 and
ent-7 were identical to those previously reported for the opposite
enantiomers (2, 6 and 7).7,8
ward these cells (0.10 lM). Additionally, styryl lactones 1 and 2
show selective cytotoxicities toward certain cancer cell lines,
whereas analogues ent-2, 3, 4, 7 and ent-7 are broadly toxic against
all tumour cell lines under evaluation. The most potent antiprolif-
erative activity of lactones 4 and ent-7 were recorded in the K562
(0.14 and 0.02
bly more active than the commercial antitumour agent doxorubi-
cin against these cell lines. Analogue demonstrated
submicromolar activity against HL-60 cells with IC50 value
(0.78 M), similar to that recorded for DOX (0.92 M) in the same
cell line. The highest potency of 2 was recorded in Jurkat cell line
(0.074 M), although this analogue exhibited 2.5-fold lower activ-
ity than DOX (0.03 M). Compounds 2 and 7 showed the most po-
lM) and HL-60 cells (0.56 and 0.17 lM), being nota-
Preparation of ring-opened analogues 3 and 4 is outlined in
Scheme 3. Compound 3 was prepared directly from 2 by catalytic
hydrogenolysis over 10% Pd/C as a catalyst. The reaction was com-
pleted for 8 days, at room temperature and normal pressure of
hydrogen, to afford a good yield of 3 (70%). Analogue 4 however,
was prepared by a three-step sequence starting from secondary
2
a
l
l
l
alcohol 9 that was easily accessible from D
-xylose.7 Treatment of
l
tent antiproliferative activity against Raji cells, with respective IC50
values being 6.5- and 3-fold lower than that recorded for the com-
mercial cytostatic doxorubicin in the same cell line. Raji cells were
the most sensitive to ent-7. This analogue was almost 100-fold
more active in Raji cells than control compound (DOX). Addition-
ally, analogue ent-7 appeared to be the most potent cell-growth
inhibitor synthesized in this work with IC50 values in the range
0.02–1.21
growth inhibitory activity against HeLa cell line (1.21
although it demonstrated 17-fold lower activity than DOX
(0.07 M) in the same cell culture.
lM. Lactone ent-7 also exhibited the most potent
lM),
l
In a previous attempt to correlate the structures of some bioac-
tive lactones with their cytotoxic activities, we have observed that
Table 1
Antiproliferative activities of 1, 2–4, 7, ent-2, ent-7 and DOX
a
Compds
IC50
(l
M)
K562
HL-60
Jurkat
Raji
HeLa
MRC-5
1
2
0.41
1.43
1.21
1.56
0.14
0.51
0.02
0.25
>100
0.78
11.69
11.67
0.56
43.81
0.17
0.92
32.45
0.074
18.45
21.36
1.01
2.52
0.12
0.03
18.45
0.46
15.88
5.67
9.48
1.03
0.03
2.98
8.32
>100
6.56
11.40
4.32
4.42
1.21
0.07
>100
>100
>100
>100
>100
>100
>100
0.10
ent-2
3
4
7
ent-7
DOX
a
IC50 is the concentration of compound required to inhibit the cell growth by 50%
Scheme 3. Reagents and conditions: (a) H2, Pd/C, MeOH, rt, 8 days; (b) BnBr, NaH,
DMF, 0 °C ? rt, 1.5 h; (c) 70% aq AcOH, reflux, 3 h; (d) Meldrum’s acid, Et3N, 46 °C,
49 h.
compared to an untreated control. The values are means of three independent
experiments done in quadruplicates. Coefficients of variation were <10%.

5510
V. Popsavin et al. / Bioorg. Med. Chem. Lett. 23 (2013) 5507–5510
heteroannelation of natural leads may increase the cytotoxicities of
the resulting conformationally restricted goniofufurone mimics.13
It was also found that the opposite enantiomers of certain bioactive
styryl lactones may exhibit more potent cytotoxicity with respect
to the parent compounds.13,14a These findings have prompted us
to perform a preliminary SAR study16 of the analogues 2, ent-2, 3,
4, 7 and ent-7. The first structural feature considered in the SAR
study was the influence of heteroannelation of leads 1 and ent-1.
The results obtained from the treatment of cancer cells with 2
demonstrated that the introduction of a new benzoxepane ring in-
creases the potency originally displayed by lead 1 against three of
five tested malignant cell lines. This effect is even more pro-
nounced if the structure of ent-1 is extended with a benzoxepane
ring (analogue ent-2). These findings are in agreement with data
shown in Table 1, which revealed that the opening of seven-mem-
ber ring by disconnection of C10–O bond decreases the activities of
resulting (+)-goniofufurone mimic 3 against the majority of tu-
mour cell lines under evaluation. However, the formal cleavage
of C7–C8 bond of the benzoxepane ring in 2, increases the potency
originally displayed by 2, against three of five tested malignant cell
lines. The next structural element considered was the absolute ste-
reochemistry of analogues. A comparison of IC50 values for (+)- and
(ꢀ)- enantiomers reveals that ent-2 had more profound effect only
against HeLa cells, while the analogue 2 was completely inactive
against this cell line. However, (ꢀ)-enantiomer ent-7 demonstrated
a superior cytotoxicity against all cell lines under evaluation being
4- to 258-fold more potent than (+)-enantiomer 7. Finally, the most
pronounced antiproliferative activities were observed with deoxy-
genated analogues 4, 7 and ent-7. These findings indicated that
introduction of a deoxy function at the C-7 position increases the
antiproliferative activity of the analogues against majority of tu-
mour cell lines under evaluation.
To summarize, 7-O-benzoylated styryl lactones such as 5 or 6,
upon treatment with titanium(IV) fluoride in dichloromethane
preferentially gave the cytotoxic 7-deoxy derivative 7 or benzoxe-
pane 2, respectively. On the contrary to both titanium(IV) chloride
and titanium(IV) bromide that were found to catalyse the displace-
ment of the C-7 benzylic ester function in the same lactones 5 or 6
with the corresponding halogenide anions,8 titanium(IV) fluoride
served as a selective catalyst for an intramolecular Friedel–Crafts
alkylation process (conversion of 6 and ent-6 to 2 and ent-2), or
for a C-7 deoxygenation through a novel 1,5-hydride shift (conver-
sion of 5 to 7). All synthesized compounds showed diverse growth
inhibitory effects against the tested malignant cells, but were de-
void of any significant cytotoxicity toward the normal foetal lung
fibroblasts (MRC-5). A SAR study reveals that the following struc-
tural features are beneficial for the antiproliferative activity of syn-
thesized lactones: the presence of an additional oxepane ring, the
absolute stereochemistry, as well as the presence of a deoxy func-
tion at the C-7 position.
Acknowledgments
Financial support of the Ministry of Education, Science and
Technological Development of the Republic of Serbia is acknowl-
edged (Grant No. 172006).
Supplementary data
Supplementary data associated with this article can be found, in
the
online
version,
at
http://dx.doi.org/10.1016/
j.bmcl.2013.08.069.
References and notes
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