Conformationally restricted anti-plasmodial chalcones

Article abstract of DOI:10.1016/j.bmcl.2005.07.012

Chalcones can exist as Z- or E-isomers and it is generally anticipated that both isomers are equipotent. In order to determine the active isomer of anti-plasmodial chalcones a series of analogues locked in the Z- or the E-form were prepared and evaluated

Full text of DOI:10.1016/j.bmcl.2005.07.012

Bioorganic & Medicinal Chemistry Letters 15 (2005) 4858–4861
Conformationally restricted anti-plasmodial chalcones
Mogens Larsen, Hasse Kromann,à Arsalan Kharazmi and Simon Feldbæk Nielsen*,§
Lica Pharmaceuticals A/S, Symbion Science Park, Fruebjergvej 3, DK-2100 Copenhagen, Denmark
Received 23 May 2005; revised 29 June 2005; accepted 6 July 2005
Available online 8 September 2005
Abstract—Chalcones can exist as Z- or E-isomers and it is generally anticipated that both isomers are equipotent. In order to deter-
mine the active isomer of anti-plasmodial chalcones a series of analogues locked in the Z- or the E-form were prepared and eval-
uated for their anti-plasmodial activity. It was shown that the Z-locked analogue was nearly inactive, whereas the E-locked
analogues were equipotent to the parent chalcones, indicating that the E-isomer is the active conformation.
Ó 2005 Elsevier Ltd. All rights reserved.
A large number of publications covering bioactive
chalcones appear every year.1 The compounds have
shown an array of pharmacological activities, such
as anti-protozoal,2–4 anti-inflammatory,5 immunomod-
ulatory,6 nitric oxide inhibition,7 inhibition of the
production of interleukin-18 and anti-cancer activi-
ties.9 Licochalcone A (LicA; Table 1) has been used
extensively as a model compound in anti-parasitic
research.2,10,16,25,39
influence on the degree of isomerisation.13,14 However,
these rules only apply for some very specific compounds,
substituents and positions. From our experience, having
synthesised more than 1000 different chalcones, it is
almost impossible to predict which chalcones are prone
to photoisomerisation and to what extent.
It has previously been anticipated that the two stereoiso-
mers are equipotent but no proof has been published. If
the E- and the Z-isomers are equipotent, the interpreta-
tion of the biological data should be straightforward,
since the degree of isomerisation would be without effect
on the activity of the compounds. On the contrary, if
there is a marked difference between the activity of the
E- and Z-forms, the observed activity is a combination
of the intrinsic activity as well as the photolability of
the active isomer. As isomerisation can be extremely fast
(seconds), the biological evaluation of the compounds
can be quite complicated.
Chalcones exist as either the E- or the Z-isomers (Chart
1). The E-isomer is in most cases the thermodynamically
most stable form and consequently, the majority of
the chalcones is isolated as the E-isomer; in general,
recrystallisation of an E–Z mixture yields the E-isomer
as the only stereoisomer. It has previously been
shown that the a,b-double bond is very labile for photo-
isomerisation in solution, giving a mixture of the E- and
Z-isomers.11,26–28 Indeed, we have observed this in
our in vitro studies (e.g., in an assay for microsomal
turn-over) as well as our vivo studies.
To the best of our knowledge, only the studies by Iwata
et al., 14 and Shibata15 have attempted to investigate the
biological activity of E- and Z-chalcones, respectively.
The two isomers were isolated by HPLC and assayed
for their biological activity. The authors observe a rela-
tively small difference in the activity of the isolated iso-
mers leading to the conclusion that the isomers are
equipotent. On the basis of the work presented in this
communication, we believe that the isolated isomers
must have converted back to an equilibrium mixture
during the time of the assay.
A few studies have addressed this problem. It has been
shown that the rate of the isomerisation and the equilib-
rium ratio depend on the substitution in the aromatic
rings and the solvent used.12–14 The data indicate that
electron donating groups and the chelating properties
of the substituents in the 20-position have a significant
Keywords: Chalcone isomers; Malaria.
*
Corresponding author. Tel.: +45 72263703; fax: +45 72263320;
e-mail: sfndk@leo-pharma.com
This study seeks to determine the anti-plasmodial activ-
ities of the E- and Z-isomers in order to identify the
active isomer. This is achieved by the synthesis and
Present address: H. Lundbeck A/S, Valby DK-2500, Denmark.
Present address: MedChem Aps, Copenhagen DK-2100, Denmark.
Present address: LEO Pharma, Ballerup DK-2750, Denmark.
à
§
0960-894X/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2005.07.012

M. Larsen et al. / Bioorg. Med. Chem. Lett. 15 (2005) 4858–4861
Table 1. In vitro anti-malarial activity29 of the prepared analogues
4859
R
R
against P. falciparum 3D7a
1: R=OMe
2: R=H
Compound
Structure
IC50 (lg/ml)
P. falciparum
O
HO
OH
R
LicA
2.2
8.9
O
3: R=OMe
4: R=H
O
O
O
O
O
O
12,17
R
a
O
R
518
>100
O
5: R=OMe
6: R=H
O
O
O
O
O
O
1133
13.3
O
O
R
O
O
Scheme 1. Reagents and conditions: (a) CH3COOH, concd HCl,
50 °C.
1631
6.0
6.7
22
The Z-locked analogues 5 and 6 of the chalcones 1 and 2
were prepared from the diketones 3 and 4, respectively,
by acid-catalysed cyclization using concentrated hydro-
chloric acid in acetic acid (Scheme 1). The E-locked ana-
logues were prepared in two steps. Treatment of an
appropriate substituted 2-hydroxy-benzaldehyde with
acrylonitrile in the presence of catalytic amount of
DABCO gave 7–10.20,21 Subsequent treatment with aryl
magnesium bromide gave the E-locked chalcone ana-
logues 11–15 (Scheme 2).22
O
O
630
35
O
O
1222
4.7
7.6
O
O
Table 1 summarises the anti-plasmodial activity of two
series of E- and Z-conformationally restricted analogues
and their parent chalcones against Plasmodium
falciparum.
1732
a Each value represents the mean of three experiments.
Parent chalcones 1 and 2 show good activity against the
Plasmodium parasites. On the contrary, the conforma-
tionally restricted analogues 5 and 6 are nearly inactive.
Unfavourable substitution of the a,b-double bond or
conformational factors can explain the loss of activity
of the compound. Previously it was shown that a- or
b-substitution with alkyl groups only marginally influ-
ences the anti-parasitic activity of the chalcones.23 These
results strongly indicate that the loss of activity is due to
the Z-conformation of the molecule.
H
H
4'
3'
4
3
H
2'
2
O
H
O
E-isomer
Z-isomer
Chart 1.
Similar to the Z-locked analogues 5 and 6, we have pre-
pared the corresponding conformationally restricted
E-locked analogues (11, 12). The data in Table 1 show
that the E-locked analogues are as potent as the parent
chalcones against Plasmodium parasites, thereby indicat-
ing that the bioactive isomer of anti-parasitic chalcones
is the E-isomer.
testing of conformationally restricted Z- and E-ana-
logues of different known anti-plasmodial chalcones.
The chalcones (1, 2, 18–20) were prepared by classical
Claisen–Schmidt condensation using a catalytic amount
of sodium hydroxide in ethanol.19

4860
M. Larsen et al. / Bioorg. Med. Chem. Lett. 15 (2005) 4858–4861
HO
O
that the E-locked analogues were equipotent to the
parent chalcones, supporting the above conclusion. In
addition, a novel and promising class of anti-plasmodial
compounds has been discovered.
R2
H
We believe that the data shown in this report are an
important finding in regard to the biological properties
of chalcones. Clearly, our findings provide insight into
the potential difficulties of working with chalcones that
can easily undergo isomerisation on exposure to light.
We recommend that scientists working with chalcones
take the presented results into account and make sure
that the compounds are not exposed to light and to keep
solutions in dark glass vials.
a
O
7: R2=7-OMe
8: R2=H
9: R2=6-Br
10: R2=8-OMe
R2
N
We have prepared conformationally restricted analogues
of anti-plasmodial chalcones. The analogues with the
double bond in the Z-conformation were nearly inactive,
whereas the corresponding analogues being locked in the
E-conformation were equipotent to the parent chalcones
against Plasmodium parasites. This supports the hypoth-
esis that the E-isomer of the chalcones is the active one
with regard to anti-plasmodial activity.
b
11: R1= 4'-OMe; R2=7-OMe
12: R1= H; R2=H
13: R1= 4'-Me; R2=6-Br
14: R1= 2'-Me; R2=6-Br
15: R1= 2'-Me; R2=8-OMe
O
R2
R1
O
Scheme 2. Reagents and conditions: (a) acrylonitrile (4.5 equiv),
DABCO (0.1 equiv), neat, 80 °C, 18 h; (b) i: Ar-MgBr (1.2 equiv)
diethyl ether, reflux, 6 h. ii: 2 M HCl.
References and notes
1. Dimmock, J. R.; Elias, D. W.; Beazely, M. A.; Kandepu,
N. M. Curr. Med. Chem. 1999, 6, 1125.
To further support this observation, three more E-con-
formationally locked analogues of known anti-plasmo-
dial chalcones were prepared (Table 2). The data show
2. Nielsen, S. F.; Christensen, S. B.; Cruciani, G.; Kharazmi,
A.; Liljefors, T. J. Med. Chem. 1998, 41, 4819.
3. Li, R.; Kenyon, G. L.; Cohen, F. E.; Chen, X.; Gong, B.;
Dominguez, J. N.; Davidson, E.; Kurzban, G.; Miller, R.
E.; Nazum, E. O.; Rosenthal, P. J.; McKerrow, J. H.
J. Med. Chem. 1995, 38, 5031.
4. Liu, M.; Wilairat, P.; Go, M.-L. J. Med. Chem. 2001, 44,
4443.
5. Hsieh, H. K.; Tsao, L. T.; Wang, J. P.; Lin, C. N.
J. Pharm. Pharmacol. 2000, 52, 163.
Table 2. In vitro anti-malarial activity29 of the prepared analogues
against P. falciparum 3D7a
Compound
Structure
IC50 (lg/ml)
P. falciparum
6. Barfod, L.; Kemp, K.; Hansen, M.; Kharazmi, A. Int.
Immunopharmacol. 2002, 2, 545.
7. Rojas, J.; Dominguez, J. N.; Charris, J. E.; Lobo, G.;
Paya, M.; Ferrandiz, M. L. Eur. J. Med. Chem. 2002, 37,
699.
O
1824
10.0
Br
O
O
O
O
8. Batt, D. G.; Goodman, R.; Jones, D. G.; Kerr, J. S.;
Mantegna, L. R.; McAllister, C.; Newton, R. C.; Nurn-
berg, S.; Welch, P. K.; Covington, M. B. J. Med. Chem.
1993, 36, 1434.
9. Ducki, S.; Forrest, R.; Hadfield, J. A.; Kendall, A.;
Lawrence, N. J.; McGown, A. T.; Rennison, D. Bioorg.
Med. Chem. Lett. 1998, 8, 1051.
10. Chen, M.; Christensen, S. B.; Zhai, L.; Rasmussen, M. H.;
Theander, T. G.; Frøkjaer, S.; Steffansen, B.; Davidsen, J.;
Kharazmi, A. J. Infect. Dis. 1997, 176, 1327.
11. Noyce, D. C.; Pryor, W. A.; King, P. A. J. Am. Chem. Soc.
1959, 81, 5423.
12. Lutz, R. E.; Jordan, R. H. J. Am. Chem. Soc. 1950, 72,
4090.
13. Jurd, L. Tetrahedron 1969, 25, 2367.
1334
1937
1435
2038
8.6
9.3
Br
Br
Br
O
O
O
13.5
11.0
O
O
O
O
14. Iwata, S.; Nishino, T.; Inoue, H.; Nagata, N.; Satomi, Y.;
Nishino, H.; Shibata, S. Biol. Pharm. Bull. 1997, 20, 1266.
15. Shibata, S. Stem Cells 1994, 12, 44.
16. Kharazmi, A.; Christensen, S. B.; Ming, C.; Theander, T.
G. PCT application WO 9317671.
O
O
1536
15.1
17. Richardson, E. M.; Reid, E. E. J. Am. Chem. Soc. 1940,
62, 413.
a Each value represents the mean of three experiments.

M. Larsen et al. / Bioorg. Med. Chem. Lett. 15 (2005) 4858–4861
4861
18. Clausen, C.; Wartchow, R.; Butenschon, H. Eur. J. Org.
Chem. 2001, 1, 93.
M.; Reynolds, E. E.; Rubin, R.; Tobias, B.; Hallak, H.;
Doherty, A. M. J. Med. Chem. 1997, 40.
19. Wattanasin, S.; Murphy, W. S. Synthesis 1980, 8, 647.
20. Rene, L.; Royer, R. Eur. J. Med. Chem. 1975, 10, 72.
21. Mouysset, G.; Payard, M.; Grassy, G.; Tronche, P.;
Dabire, H.; Mouille, P.; Schmitt, H. Eur. J. Med. Chem.
1987, 22, 539.
22. Shimizu, T.; Hayashi, Y.; Yamada, K.; Nishio, T.;
Teramura, K. Bull. Chem. Soc. Jpn. 1981, 54, 217.
23. Nielsen, S. F.; Kharazmi, A.; Christensen, S. B. Bioorg.
Med. Chem. 1998, 6, 937.
32. Li-Wen, X.; Lyi, L.; Chun-Gu, X.; Pei-Qing, Z. Helv.
Chim. Acta 2004, 87, 3080.
33. For 11: 1H NMR (CDCl3): d 7.66 (d, J = 9.0 Hz, 2H), 6.95
(d, J = 8.4 Hz, 1H), 6.90 (d, J = 8.8 Hz, 2H), 6.71 (d,
J = 1.3 Hz, 1H), 6.43 (dd, J = 8.4 Hz, J = 2.5 Hz, 1H),
6.39 (d, J = 1.8 Hz, 1H), 5.06 (d, J = 1.4 Hz, 2H), 3.82 (s,
3H), 3.74 (s, 3H).
34. For 13: 1H NMR(CDCl3): d 7.64 (2H, d, J = 8.3 Hz), 7.34
(1H, dd, J = 8.1 Hz, 2.4 Hz), 7.30 (2H, d, J = 8.3 Hz), 7.22
(1H, d, J = 2.4 Hz), 7.03 (1H, br s), 6.79 (1H, d,
J = 8.1 Hz) 5.18 (2H, br s), 2.42 (3H, s).
24. Raiford, L. C.; Tanzer, L. K. J. Org. Chem. 1941, 6,
722.
25. Chen, M.; Christensen, S. B.; Nadelmann, L.; Fich, K.;
Lemmich, E.; Blom, J.; Theander, T. G.; Kharazmi, A.
Antimicrob. Agents Chemother. 1993, 37, 2550.
26. Pina, F.; Melo, M. J.; Maestri, M.; Passaniti, P.; Camai-
oni, N.; Balzani, V. Eur. J. Org. Chem. 1999, 3199.
27. Moncada, M. C.; Pina, F.; Roque, A.; Parola, A. J.;
Maestri, M.; Balzani, V. Eur. J. Org. Chem. 2004, 304.
28. Ferreira, D.; Roux, D. G. J. Chem. Soc., Perkin Trans. 1
1977, 134.
29. Anti-plasmodial activity was measured as described by:
Ziegler, H.; Stærk, D.; Christensen, J.; Hviid, L.; Ha¨ger-
strand, H.; Jaroszewski, J. W. Antimicrob. Agents Chemo-
ther. 2002, 46, 1441.
35. For 14: 1H NMR(CDCl3): d 7.39 (1H, td, J = 7.4 Hz,
1.8 Hz), 7.33 (1H, dd, J = 8.8 Hz, 2.6 Hz), 7.30–7.23 (3H,
m), 7.17 (1H, d, J = 2.6 Hz), 6.82 (1H, br s), 6.77 (1H, dd,
J = 8.6 Hz, 0.6 Hz), 5.19 (2H, s), 2.36 (3H, s).
36. For 15: 1H NMR(CDCl3): d 7.38 (1H, td, J = 7.5 Hz,
1.6 Hz), 7.31 (1H, dd, J = 7.5 Hz, 1.6 Hz), 7.30–7.22 (2H,
m), 6.92–6.90 (2H, m), 6.86 (1H, t, J = 7.5 Hz), 6.68 (1H, dd,
J = 7.5 Hz, 1.5 Hz), 5.15 (2H, s), 3.90 (3H, s), 2.38 (3H, s).
37. For 19: 1H NMR(CDCl3): d 7.72 (1H, d, J = 16.3 Hz),
7.67 (1H, d, J = 2.6 Hz), 7.51 (1H, dd, J = 8.2 Hz, 1.6 Hz),
7.45 (1H, dd, J = 8.8 Hz, 2.6 Hz), 7.38 (1H, td, J = 7.3 Hz,
1.6 Hz), 7.3–7.24 (m, 2H), 7.20 (1H, d, J = 16.3 Hz), 6.82
(1H, d, J = 8.8 Hz) 3.82 (3H, s), 2.41 (3H, s).
30. (a) Hoffman, W. D.; McEwen, W. E.; Kleinberg, J.
Tetrahedron 1959, 5, 293; (b) For 6: 1H NMR (CDCl3):
d = 7.74 (d, J = 6.9 Hz, 2H), 7.41–7.32 (m, 1H), 30–7.20
(m, 2H), 7.18–7.05 (m, 5H), 3.08–2.86 (m, 4H), 2.20–2.05
unitet, 2H.
38. For 20: 1H NMR (CDCl3): d 7.81 (1H, d, J = 16.1 Hz),
7.81 (1H, bd, J ꢀ 8 Hz), 7.32 (1H, td, J = 7.9, 1.8 Hz),
7.30–7.20 (3H, m), 7.18 (1H, d, J = 16.1 Hz), 7.08 (1H, t,
J = 7.9 Hz), 6.96 (1H, dd, J = 8.0, 1.3 Hz), 3.87 (3H, s),
3.82 (3H, s), 2.46 (3H, s).
31. Patt, W. C.; Edmunds, J. J.; Repine, J. T.; Berryman, K.
A.; Reisdorf, B. R.; Lee, C.; Plummer, M. S.; Shahripour,
A.; Haleen, S. J.; Keiser, J. A.; Flynn, M. A.; Welch, K.
39. Chen, M.; Theander, T. G.; Christensen, S. B.; Hviid, L.;
Zhai, L.; Kharazmi, A. Antimicrob. Agents Chemother.
1994, 38, 1470.