Design, synthesis and antiproliferative activity of two new heteroannelated (-)-muricatacin mimics

Article abstract of DOI:10.1016/j.bmcl.2008.08.097

Two new (-)-muricatacin mimics bearing a furano-furanone ring and an oxygen isostere in the side chain have been designed and synthesized and their in vitro antiproliferative activity was evaluated against several human tumour cell lines. Both analogues showed an increased activity against HL-60 cells with 17- and 185-fold higher potency than (-)-muricatacin. A straightforward synthesis of (-)-muricatacin is also disclosed.

Full text of DOI:10.1016/j.bmcl.2008.08.097

Bioorganic & Medicinal Chemistry Letters 18 (2008) 5182–5185
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Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Design, synthesis and antiproliferative activity of two new heteroannelated
(À)-muricatacin mimics
a
a
a
a
Velimir Popsavin a, , Bojana Sreco , Goran Benedekovic , Mirjana Popsavin , Jovana Francuz ,
*
´
´
b
b
´
´
Vesna Kojic , Gordana Bogdanovic
a Department of Chemistry, Faculty of Sciences, University of Novi Sad, Trg D. Obradovi´ca 3, 21000 Novi Sad, Serbia
b Institute of Oncology Sremska Kamenica, Institutski put 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:
Two new (À)-muricatacin mimics bearing a furano-furanone ring and an oxygen isostere in the side chain
have been designed and synthesized and their in vitro antiproliferative activity was evaluated against
several human tumour cell lines. Both analogues showed an increased activity against HL-60 cells with
17- and 185-fold higher potency than (À)-muricatacin. A straightforward synthesis of (À)-muricatacin
is also disclosed.
Received 11 July 2008
Revised 24 August 2008
Accepted 26 August 2008
Available online 29 August 2008
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords:
Muricatacin mimics
Wittig reaction
Goniofufurone mimics
Bioisostere
Antitumour activity
Annonaceous acetogenins, which have been isolated from a
number of tropical and subtropical plants of the Annonaceae family,
have attracted much attention due to their interesting biological
activities.1 (À)-Muricatacin (1, Scheme 1) is a simple member of
the annonaceous acetogenins family. It was isolated as the major
component of a pseudo-racemic mixture (ca. 25% ee based on opti-
cal rotation) from the seeds of Annona muricata and has received a
great deal of attention due to its diverse biological profile.2 Notably,
both 1 and its enantiomer exhibit a potent antiproliferative activity
towards several human tumour cell lines with SAR studies showing
that activity is influenced significantly by the nature of the side
chain.3 The biological potential of muricatacin has prompted many
syntheses.4 However, development of general and flexible strate-
gies for the preparation of new side chain analogues and more
highly functionalized mimics remains a goal. A number of muricat-
acin analogues has also been synthesized,3,5,6 but only a few were
evaluated for their antitumour activity.3,6 Previous studies in our
laboratory showed that a mimic of (À)-muricatacin in which a
methylene group from the side chain has been replaced by an ether
function (compound 2) exhibited in vitro antitumour activity
against several human cancer cell lines.7 As an extension of our pre-
vious work, we report herein on the synthesis and biological evalu-
ation of furanolactones 3 and 4 as conformationally constrained
(À)-muricatacin analogues. Compound 3 represents a heteroann-
elated mimic of 1 with constrained rotation around the C4–C5 and
C5–C6 bonds, while the molecule 4 represents a one-carbon higher
homologue of 3. In the same time, both furanolactones 3 and 4
might also be considered as non-styryl analogues of (À)-goniofufu-
rone (5), the opposite enantiomer of naturally occurring cytotoxic
lactone (+)-goniofufurone.8 Apart from the synthesis of 3 and 4, a
novel route for the preparation of (À)-muricatacin (1) is also
disclosed in order to provide a sample of 1 that should serve as a
positive control in antitumour assays.
The synthesis of 1 is shown in Scheme 2. The sequence started
from the known dialdose 6 that is readily available from D-xylose
in five steps.7 Wittig olefination of aldehyde 6 with the appropriate
C11-ylide gave the corresponding Z-olefine 7 as the only isolable
isomer in 50% yield. Catalytic hydrogenation of 7 over PtO2, fol-
lowed by hydrolytic removal of the cyclohexylidene protective
group in 8, gave the expected lactol 9. Oxidative cleavage of 9 with
Scheme 1. Structures of (À)-muricatacin (1), (À)-goniofufurone (5) and (À)-muricatacin
mimics 2, 3 and 4.
* Corresponding author. Tel.: +381 21 385 27 68; fax: +381 21 454 065.
E-mail address: popsavin@ih.ns.ac.yu (V. Popsavin).
0960-894X/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2008.08.097

V. Popsavin et al. / Bioorg. Med. Chem. Lett. 18 (2008) 5182–5185
5183
Scheme 2. Reagents and conditions: (a) [Ph3PC11H23]+BrÀ, n-BuLi, THF, À78 °C ? rt, then rt for 46 h, 50%; (b) H2-PtO2, MeOH, rt, 16 h, 90%; (c) 70% AcOH, reflux, 18 h, 57%;
(d) aq NaIO4, silica gel, Et2O, rt, 24.5 h, then Ph3P:CHCO2Me, rt, 3 h, 68%; (e) H2-Pd/C, MeOH, rt, 3 h; (f) 2:1 TFA/H2O, rt, 1.5 h, 92% (from 10).
sodium periodate on silica gel followed by Wittig olefination of the
resulting aldehyde with a stabilized C2-ylide (Ph3P = CHCO2Me)
afforded the unsaturated ester 10 as a mixture of the correspond-
ing Z- and E-isomers. The mixture was not separated, but was sub-
sequently subjected to catalytic hydrogenation over 10% Pd/C to
afford the saturated ester 11, which was isolated in pure form after
the usual work-up and used in the next step without any further
purification. Subsequent treatment of crude 11 with aq trifluoro-
acetic acid gave (À)-muricatacin (1) in 92% yield (from 10). A syn-
in the reaction scheme). The IR, 1H and 13C NMR data are consistent
with structure 14. An acid-catalyzed methanolysis of 14, in the
presence of catalytic amount of sulphuric acid gave the known10
furano-lactone 15 in 79% yield. The 1H and 13C NMR spectral data
of thus prepared sample 15 were in full agreement with the re-
ported data. Hydrolytic removal of the dimethyl acetal protection
in 15 followed by a subsequent NaBH4 reduction of the resulting
aldehyde 16 gave the corresponding primary alcohol 17. This pro-
cedure provided the key intermediate 17 in 32.3% overall yield
with respect to the starting compound 12. Alcohol 17 readily re-
acted with an excess of nonyl bromide and silver oxide in ether,
in the presence of a catalytic amount of silver triflate, to give the
expected 7-O-nonyl derivative 18 in 33% yield. Hydrogenolytic re-
moval of the benzyl ether protective group in 18 furnished 313 in
82% yield. Finally, by using the former two-step sequence, and
the decyl bromide as an alkylation agent, compound 17 was con-
verted to the target 4.14
Compounds 2, 3 and 4 were evaluated for their in vitro antitu-
mour activity against the following human cell lines: myeloge-
nous leukaemia (K562), promyelocytic leukaemia (HL-60), T cell
leukaemia (Jurkat), cervix carcinoma (HeLa) and normal foetal
lung fibroblasts (MRC-5). Cell growth inhibition was evaluated
after 72-h cells treatment by using the MTT assay. The (À)-Muri-
catacin (1) was used as a reference compound. The results are
shown in Table 1.
thetic sample of 1 {mp 68–69 °C, [
a]D = À22.0 (c 1.3, CHCl3)
literature5d mp 68–70 °C, [
a]
D = À19.0 (c 1.8, CHCl3)} exhibited
1H and 13C spectral data identical to those previously reported in
the literature.5d
The synthesis of analogues 3 and 4 is shown in Scheme 3. The
sequence commenced with the formation of the protected alde-
hydo-lactone 16 from the known9
D-Glucose derivative 12 by a
slight modification of a literature method.10 The terminal isopro-
pylidene domain in 12 was directly converted to an aldehyde
group in a single step using 1.25 M equivalents of periodic acid
in dry ethyl acetate.11 The resulting crude aldehyde 13 was used
in the next step without any purification. The Z-selective Wittig
reaction with the stabilized C2-ylide (Ph3P = CHCO2Me), that was
performed in dry methanol at low temperature,12 afforded pre-
dominantly the Z-enoate 14 (73% from 12), accompanied with a
minor amount (10%) of the corresponding E-isomer (not shown
Scheme 3. Reagents and conditions: (a) H5IO6, EtOAc, rt, 1.5 h; (b) Ph3P:CHCO2Me, MeOH, 0 °C for 0.5 h, then rt for 1.5 h, 73% from 12; (c) i—H2SO4, MeOH, reflux, 2 h,
ii—NaHCO3, MeOH, 35 °C, 1 h, 79%; (d) 9:1 TFA/H2O, rt, 0 °C for 0.5 h, then rt for 0.5 h; (e) NaBH4, MeOH, rt, 1.5 h, 56% from 15; (f) C9H19Br, Ag2O, AgOTf, Et2O, reflux, 28 h, 33%;
(g) C10H21Br, Ag2O, AgOTf, Et2O, reflux, 32 h, 54%; (h) H2-Pd/C, MeOH, rt, 18 h for 18, 82% of 3, 21 h for 19, 80% of 4.

5184
V. Popsavin et al. / Bioorg. Med. Chem. Lett. 18 (2008) 5182–5185
sized in our laboratory).7 Analogue 4 exhibited the same growth
Table 1
Antiproliferative activities of 1, 2, 3 and 4
inhibition pattern as (À)-muricatacin (1) itself, being completely
inactive against the Jurkat cell line, but highly active towards the
K562, HL-60 and HeLa malignant cells. However, compounds 2
and 3 demonstrated high antiproliferative activity against the all
tested malignant cell lines. Both analogues 3 and 4 showed selec-
tive increase of activity against HL-60 cells with 17- and 185-fold
higher potency than (À)-muricatacin, respectively. Remarkably,
neither of the lactones 1–4 exhibited any activity towards the nor-
mal foetal lung MRC-5 cells. Based upon these biological results,
we believe that the analogues 2–4 may serve as important leads
in the synthesis of more potent and selective antitumour agents
derived from the natural product 1. Further optimization of the
structures, including the preparation of novel muricatacin and gon-
iofufurone mimics is currently underway, and results will be re-
ported in due course.
Compound
IC50 (l
M)a
K562
HL-60
Jurkat
HeLa
MRC-5
1
2
3
4
0.04
0.13
8.61
1.25
25.85
0.15
1.53
0.14
>100
3.01
6.64
>100
0.17
21.45
9.59
>100
>100
>100
>100
0.30
a
IC50 is the concentration of compound required to inhibit the cell growth by 50%
compared to an untreated control. Values are means of three independent experi-
ments done in quadriplicates. Coefficients of variation were <10% (range: 0.13–
9.89%).
All three (À)-muricatacin mimics (2–4) retained the selectivity
between the normal human cells (MRC-5) and tumour cells (K562,
HL-60, Jurkat and HeLa). Among 2–4, only the analogue 4 exhibited
the same growth inhibition pattern as (À)-muricatacin (1) and rep-
resents a bioisostere of 1. As (À)-muricatacin itself, compound 4
was inactive against T cell leukaemia (Jurkat), but showed a potent
antiproliferative activity towards K562, HL-60 and HeLa malignant
cells. On the contrary to 1 and 4, both analogues 2 and 3 exhibited
significant antiproliferative activities towards the all tested cell
lines, including a notable activity against the Jurkat cells. (À)-Muri-
catacin (1) was the most potent compound in the K562 cell line, as
it exhibited 1–2 order of magnitude higher antiproliferative effect
when compared to 2, 3 and 4. The most pronounced antiprolifera-
tive activity of analogues 2–4 was recorded against the HL-60 cell
line. Compounds 2 and 4 exhibited 172- and 185-fold, stronger
activities when compared to the parent compound 1. The analogue
3 also showed a remarkable antiproliferative activity towards this
cell line, being almost 17-fold, more potent than (À)-muricatacin.
A sub-micromolar activity of 4 was also recorded against the HeLa
malignant cells, comparable to that observed for the reference
compound 1.
Acknowledgments
Financial support from the Ministry of Science of the Republic
of Serbia (Project No. 142005) is gratefully acknowledged.
References and notes
1. For
a recent review on isolation, synthesis and mechanism of action of
annonaceous acetogenins see: Bermejo, A.; Figadère, B.; Zafra-Polo, M.-C.;
Barrachina, I.; Estornell, E.; Cortes, B. L. Nat. Prod. Rep. 2005, 22, 269.
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617.
4. For recent syntheses of muricatacin, see: (a) Ferrié, L.; Reymond, S.;
Capdevielle, P.; Cossy, J. Synlett 2007, 2891; (b) Southard, J. M.; Sears, N. L.
Texas J. Sci. 2007, 59, 301; (c) Yokoyama, T.; Kutsumura, N.; Ohgiya, T.;
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Lett. 2004, 6, 4143. and references cited therein.
According to the above biological results, replacement of the
side chain methylene group in (À)-muricatacin with an oxygen
isostere increases the activity of the analogue (compound 2)
against the HL-60, and Jurkat cell lines, but significantly de-
creases its antiproliferative activity in K562 and HeLa cells. How-
ever, a comparison of biological data of 2 and 3 revealed that
introduction of a THF ring decreases the activity of annelated
(À)-muricatacin mimic 3 in K562, HL-60 and Jurkat cell lines,
but increases its activity against the HeLa cells. Finally, one-car-
bon homologation of the side chain in 3 increases the activity of
resulting homologue 4 against the K562, HL-60 and HeLa cell
lines. The respective IC50 values of 4 were found to be 7-, 10-
and 32-fold lower then those recorded for analogue 3. These re-
sults are in good agreement with previous findings that the
length of the side chain is crucial for antitumour activity of
muricatacin analogues.3 However, the mechanism of action is
still not understood. Since it has been proposed that acetogenins
of Annonaceae act as an inhibitor of complex I in the mitochon-
dria1 respiratory system,1 it is possible that muricatacin and
analogues act via an identical mechanism. The observed differ-
ences in the antiproliferative potencies in respect to the cancer
cell lines used may then be explained by a specificity difference
in the hosts’ mitochondrial complex I (NADH-ubiquinone oxido-
reductase), but more work is needed before any conclusion can
be made.
5. (a) Konno, H.; Hiura, N.; Yanaru, M. Heterocycles 2002, 57, 1793; (b) Andres, J.
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39, 1755; (d) Chang, S. W.; Hung, C. Y.; Liu, H. H.; Uang, B. J. Tetrahedron:
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Vanaar, M. P. M.; Thijs, L.; Zwanenburg, B. Tetrahedron 1995, 51, 11223; (i)
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1998, 19, 1016; (c) Yoon, S. H.; Moon, H. S.; Hwang, S. K.; Choi, S. R.; Kang, S. K.
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7. Popsavin, V.; Krstic, I.; Popsavin, M.; Sreco, B.; Benedekovic, G.; Kojic, V.;
Bogdanovic´, G. Tetrahedron 2006, 62, 11044.
8. Isolation of goniofufurone: (a) Fang, X. P.; Anderson, J. E.; Chang, C. J.; Fanwick,
P. E.; McLaughlin, J. L. J. Chem. Soc., Perkin Trans. 1 1990, 1655; for reviews on
biological activity of styryl lactones and analogues see: (b) de Fatima, A.;
Modolo, L. V.; Conegero, L. S.; Pilli, R. A.; Ferreira, C. V.; Kohn, L. K.; de Carvalho,
J. E. Curr. Med. Chem. 2006, 13, 3371; (c) Mareyala, H. B.; Joe, M. Curr. Med.
Chem. Anti-Cancer Agents 2001, 1, 293; (d) Blazquez, M. A.; Bermejo, A.; Zafra-
Polo, M. C.; Cortes, D. Phytochem. Anal. 1999, 10, 161.
9. Bichard, C. J. F.; Wheatley, J. R.; Fleet, G. W. J. Tetrahedron: Asymmetry 1994, 5,
431.
10. Prakash, K. R. C.; Rao, S. P. Tetrahedron Lett. 1991, 32, 7473.
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1372.
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1987, 43, 1895.
13. Selected data for 3: mp 53 °C (from CH2Cl2–hexane); [
a
]
23 = À35.0 (c 0.5, in
D
CHCl3); 1H NMR (250 MHz, CDCl3): d 0.88 (t, 3H, J = 6.7 Hz, CH3), 1.11–1.39 (m,
12H, 6Â CH2), 1.59 (m, 2H, CH2), 2.65 (dd, 1H, J2a,2b = 18.8, J2a,3 = 1.1 Hz, H-2a),
2.76 (dd, 1H, J2a,2b = 18.8, J2b,3 = 5.4 Hz, H-2b), 3.51 (m, 2H, 2Â H-9), 3.88 (m,
2H, 2Â H-7), 4.11 (m, 1H, H-6), 4.22 (br s, 1H, exchangeable with D2O, OH),
4.53 (d, 1H, J4,5 = 3.2 Hz, H-5), 4.86 (d, 1H, J3,4 = 4.2 Hz, H-4), 5.02 (td, 1H,
J2a,3 = 1.1, J2b,3 = 5.4, J3,4 = 4.2 Hz, H-3); 13C NMR (62.9 MHz, CDCl3): d 14.0
(CH3), 22.6, 25.9, 29.6, 29.2, 29.3, 29.4 and 31.8 (7Â CH2), 36.0 (C-2), 69.5 (C-7),
72.6 (C-9), 76.0 (C-5), 76.8 (C-3), 78.6 (C-6), 88.2 (C-4), 175.4 (C-1); LRMS (ESI):
m/z 339 (M++K), 323 (M++Na), 301 (M++H).
In summary, two novel heteroannelated (À)-muricatacin mim-
ics bearing a furano-furanone ring and an oxygen isostere in the
side chain (3 and 4) have been designed and synthesized. Their
in vitro antitumour activities were evaluated and compared to
those recorded for the parent compound 1 (synthesized in this
work), and to those recorded for analogue 2 (previously synthe-

V. Popsavin et al. / Bioorg. Med. Chem. Lett. 18 (2008) 5182–5185
5185
14. Selected data for 4: mp 59–60 °C (from CH2Cl2–hexane); [
a
]
23 = À29.1 (c
(d, 1H, J5,6 = 3.0 Hz, H-5), 4.88 (d, 1H, J3,4 = 4.0 Hz, H-4), 5.03 (dd, 1H,
J2,3 = 5.4, J3,4 = 4.2 Hz, H-3); 13C NMR (62.9 MHz, CDCl3): d 14.0 (CH3), 22.6,
25.9, 29.2, 29.3, 29.4, 29.5 and 31.8 (8Â CH2), 36.1 (C-2), 69.6 (C-7), 72.6
(C-9), 76.1 (C-5), 76.9 (C-3), 78.6 (C-6), 88.2 (C-4), 175.3 (C-1); LRMS (CI):
m/z 315 (M++H).
D
0.97, in CHCl3); 1H NMR (250 MHz, CDCl3): d 0.88 (t, 3H, J = 6.6 Hz, CH3),
1.07–1.68 (m, 16H, 8Â CH2), 2.67 (d, 1H, J2a,2b = 18.7 Hz, H-2a), 2.77 (dd,
1H, J2a,2b = 18.7, J2b,3 = 5.4 Hz, H-2b), 3.54 (m, 2H, 2Â H-9), 3.86 (m, 2H, 2Â
H-7), 4.11 (m, 1H, H-6), 4.27 (br s, 1H, exchangeable with D2O, OH), 4.54