Semisynthetic derivatives of sesquiterpene lactones by palladium-catalyzed arylation of the α-methylene-γ-lactone substructure

Article abstract of DOI:10.1021/jo901533e

(Chemical Equation Presented) The palladium-catalyzed arylation of different α-methylene-γ-lactone-containing sesquiterpene lactones was shown to produce E-olefin coupling products selectively in moderate to excellent yields. Biological evaluation of thes

Full text of DOI:10.1021/jo901533e

pubs.acs.org/joc
extended these studies to highly complex molecules contain-
Semisynthetic Derivatives of Sesquiterpene Lactones
by Palladium-Catalyzed Arylation of the
r-Methylene-γ-lactone Substructure
ing the R-methylene-γ-lactone substructure.5 Such work
would provide valuable details about the scope of these
synthetic methods.
Changho Han,† Francis J. Barrios,‡ Mark V. Riofski,† and
David A. Colby*,†,‡
†Department of Medicinal Chemistry and Molecular
Pharmacology, Purdue University, West Lafayette, Indiana
47907, and ‡Department of Chemistry, Purdue University,
West Lafayette, Indiana 47907
FIGURE 1. Products reported from Heck1 and cross-metathesis
reactions2,3 on R-methylene-γ-butyrolactone.
dcolby@purdue.edu
Received July 16, 2009
The sesquiterpene lactone class of natural products has a
diverse range of biological activities, including anticancer,
anti-inflammatory, and antiviral properties, and many con-
tain the R-methylene-γ-lactone motif.6 Recently, interest in
these natural products has dramatically increased due to
reports of anticancer stem cell activity.7 Although some
structural modifications to these natural products have been
made,6c,8-10 the direct homologation of the R-methylene-γ-
lactone substructure has not been explored. Such derivatives
would provideadditionalstructure-activitydata for this class
of natural products, because the electrophilic R-methylene-γ-
lactone is known to react with nucleophilic intracellular
thiols.6 Herein, we report our studies on the palladium-
catalyzed arylation of sesquiterpene lactones to provide
coupled products with exclusively the E-olefin geometry.
Although this stereochemical result conflicted with prior
work of Arcadi and co-workers,1 our data supports that the
The palladium-catalyzed arylation of different R-methy-
lene-γ-lactone-containing sesquiterpene lactones was
shown to produce E-olefin coupling products selectively
in moderate to excellent yields. Biological evaluation of
these semisynthetic sesquiterpene lactone derivatives in
HeLa cells showed interesting antiproliferative profiles
and provided initial structure-activity data.
(6) (a) Recent reports of anti-cancer activity: Liu, Z.; Liu, S.; Xie, Z.;
Pavlovicz, R. E.; Wu, J.; Chen, P.; Aimiuwu, J.; Pang, J.; Bhasin, D.; Neviani,
P.; Fuchs, J. R.; Plass, C.; Li, P.-K.; Li, C.; Huang, T. H. M.; Wu, L.-C.;
Rush, L.; Wang, H.; Perrotti, D.; Marcucci, G.; Chan, K. K. J. Pharmacol.
Exp. Ther. 2009, 329, 505–514. Gopal, Y. N. V.; Arora, T. S.; Van Dyke, M.
W. Chem. Biol. 2007, 14, 813–823. Steele, A. J.; Jones, D. T.; Ganeshaguru,
K.; Duke, V. M.; Yogashangary, B. C.; North, J. M.; Lowdell, M. W.;
Kottaridis, P. D.; Mehta, A. B.; Prentice, A. G.; Hoffbrand, A. V.;
Wickremasinghe, R. G. Leukemia 2006, 20, 1073–1079. Lindenmeyer,
M. T.; Hrenn, A.; Kern, C.; Castro, V.; Murillo, R.; Muller, S.; Laufer, S.;
Schulte-Monting, J.; Siedle, B.; Merfort, I. Bioorg. Med. Chem. 2006, 14,
2487–2497. (b) Anti-inflammatory activity: Wagner, S.; Hofmann, A.;
Siedle, B.; Terfloth, L.; Merfort, I.; Gasteiger, J. J. Med. Chem. 2006, 49,
2241–2252. Siedle, B.; Garcia-Pineres, A. J.; Murillo, R.; Schulte-Monting,
J.; Castro, V.; Rungeler, P.; Klaas, C. A.; Da Costa, F. B.; Kisiel, W.;
Merfort, I. J. Med. Chem. 2004, 47, 6042–6054. (c) Anti-viral activity:
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Howard, D. S.; Jordan, C. T. Blood 2005, 105, 4163–4169.
The conversion of R-methylene-γ-butyrolactone into
R-alkylidene-γ-butyrolactones via metal-catalyzed processes
has recently attracted attention to develop efficient routes to
biologically active natural products containing these func-
tional groups.1-5 Specifically, the palladium-catalyzed ary-
lation of R-methylene-γ-butyrolactone was reported to
provide a mixture of 3-benzylfuran-2(5H)-ones and R-ben-
zylidene-γ-butyrolactones with the Z-olefin geometry
(Figure 1).1 In contrast, ruthenium-catalyzed cross-metath-
esis protocols from Cossy2 and Howell3 provided the
R-alkylidene-γ-butyrolactones with excellent selectivity for
the E-olefin geometry. Thus far, no investigations have
(8) Recent examples: Nasim, S.; Crooks, P. A. Bioorg. Med. Chem. Lett.
2008, 18, 3870–3873. Srivastava, S. K.; Abraham, A.; Bhat, B.; Jaggi, M.;
Singh, A. T.; Sanna, V. K.; Singh, G.; Agarwal, S. K.; Mukherjee, R.;
Burman, A. C. Bioorg. Med. Chem. Lett. 2006, 16, 4195–4199. Macias, F. A.;
Velasco, R. F.; Alvarez, J. A.; Castellano, D.; Galindo, J. C. G. Tetrahedron
2004, 60, 8477–8488. Barrero, A. F.; Rosales, A.; Cuerva, J. M.; Oltra, J. E.
Org. Lett. 2003, 5, 1935–1938. (e) Kwok, B. H. B.; Koh, B.; Ndubuisi, M. I.;
Elofsson, M.; Crews, C. M. Chem. Biol. 2001, 8, 759–766.
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D’Ambrosio, M. Bioorg. Med. Chem. Lett. 2009, 19, 1858–1860. Neukirch,
H.; Kaneider, N. C.; Wiedermann, C. J.; Guerriero, A.; D’Ambrosio, M.
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H.; Vargas, D. J. Nat. Prod. 1993, 56, 90–98.
(1) Arcadi, A.; Chiarini, M.; Marinelli, F.; Berente, Z.; Kollar, L. Org.
Lett. 2000, 2, 69–72. Arcadi, A.; Chiarini, M.; Marinelli, F.; Berente, Z.;
ꢀ
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(2) Moise, J.; Arseniyadis, S.; Cossy, J. Org. Lett. 2007, 9, 1695–1698.
(3) Raju, R.; Allen, L. J.; Le, T.; Taylor, C. D.; Howell, A. R. Org. Lett.
2007, 9, 1699–1701.
(4) Brunner, H.; Cousturier de Courcy, N. L.; Genet, J. P. Tetrahedron
Lett. 1999, 40, 4815–4818.
(5) (a) One example of an intramolecular Heck reaction has been
reported. Ishibashi, H.; Ito, K.; Hirano, T.; Tabuchi, M.; Ikdea, M.
Tetrahedron 1993, 49, 4173–4182. (b) Combinatorial libraries have recently
beenassembled from somesubstitutedR-methylene-γ-butyrolactones. Elford,
T. G.; Ulaczyk-Lesanko, A.; De Pascale, G.; Wright, G. D.; Hall, D. G.
J. Comb. Chem. 2009, 11, 155–168.
(10) Review: Minnaard, A. J.; Wijnberg, J. B. P. A.; Groot, A. D.
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7176 J. Org. Chem. 2009, 74, 7176–7179
Published on Web 08/21/2009
DOI: 10.1021/jo901533e
r
2009 American Chemical Society

Han et al.
JOCNote
TABLE 1. Heck Couplings with Parthenolide and Aryl Iodidesa
FIGURE 2. Structures of two sesquiterpene lactones.
preferential selectivity for the E-isomer in the Heck reaction
extends to R-methylene-γ-lactones as well. Growth inhibition
studies on cancer cells were subsequently performed to eval-
uate the biological activity of these semisynthetic sesquiter-
pene derivatives. Also, an analysis of the structure-activity
data for these arylated R-methylene-γ-lactones is described.
We began our investigations to determine the results of
palladium-catalyzed arylation reactions with substituted
R-methylene-γ-lactones. Using sesquiterpene lactone, parthe-
nolide (1), as a representative structure (Figure 2), we conducted
a series of reactions with readily available aryl iodides using the
previously reported Heck reaction conditions1 (Table 1). We
hypothesized that only an exocyclic olefin would be produced
because the C11-C13 insertion should occur opposite to the C7
proton which renders this proton unavailable for β-hydride
elimination.11 Indeed, when 5 mol % of Pd(OAc)2 with Et3N in
DMF at 80 °C was used, a single product with an exocyclic
olefin was isolated in each case after purification in moderate to
good yields (57%-85%). The substitution pattern on the
aromatic ring and the presence of electron-donating or elec-
tron-withdrawing substituents did not affect the yields or the
preference for a single olefin geometry in the isolated product.
We also strategically selected aryl iodides that contained pri-
marily fluorine substituents as well as a variety of election-
donating and -withdrawing groups in order to understand the
structure-activity relationships of substituents on the aromatic
ring of the analogues. Fluorinated compounds can have en-
hanced biological profiles and serve as metabolic probes. The
assignment of the C11-C13 olefin geometry (parthenolide
numbering) of 3 was determined using 1H, COSY, and NOESY
NMR experiments to be the E-olefin. Specifically, NOESY
crosspeaks were readily apparent between the phenyl ring
protons and the protons attached to C7 and C8 on the macro-
cycle. Also, the C13 vinyl proton had a chemical shift of
7.68 ppm that supports the assignment as an E-olefin. For
4-11, the assignment of the E-olefin geometry was accom-
plished by the diagnostic chemical shift of the C13 proton.12 The
assignment of the E-olefin for the compounds 8 and 11 was
further verified following determination of an X-ray crystal
structure of each (Figure 3).13 These data support that the
preferential selectivity for the E-isomer (over the Z-isomer)
from the Heck reaction11 extends to R-methylene-γ-lactones
and renders the prior assignment of the Z-olefin-containing
products questionable.1,14 The conformation of the macrocycle
of 8 and 11 did not change under the reaction conditions and is
similar to the reported X-ray structure of 1.15 Also, no structural
reorganizations of the bicyclic ring system in the parthenolide
derivatives were observed.9
aAll reactions were carried out for 24 h. bAll yields refer to isolated,
pure products.
FIGURE 3. X-ray structures of sesquiterpene lactones: (A) parthe-
nolide15 (1), (B) compound 8, (C) compound 11.
(11) Beletskaya, I. P.; Cheprakov, A. V. Chem. Rev. 2000, 100, 3009–
3066.
(12) See Table S1 in the Supporting Information.
(13) See the Supporting Information.
(14) See Table S2 in the Supporting Information.
(15) Quick, A.; Rogers, D. J. Chem. Soc., Perkins Trans. 2 1976, 465–469.
To explore the scope of this arylation protocol further, the
sesquiterpene lactone, R-santonin (2), was modified to install
J. Org. Chem. Vol. 74, No. 18, 2009 7177

Han et al.
JOCNote
TABLE 2. Heck Couplings with 12 and Aryl Iodidesa
TABLE 3. Antiproliferative Assay in HeLa Cellsa
compd
IC50, μM
compd
IC50, μM
1
3
4
5
6
7
8
9
7.8 ( 1.3
10
11
12
13
14
15
16
>200
>200
12.1 ( 1.5
>200
>200
>200
>200
>200
21.6 ( 1.3
49.8 ( 1.7
32.7 ( 1.5
>200
15.2 ( 1.5
>200
aAntiproliferative assays were conducted in the HeLa (cervical
cancer) cell line. All values represent the average of n = 3 ( standard
deviation.
of R-arylation of this functional group is not known. In our
assay, parthenolide 1 shows activity similar to previously
reported values in HeLa cells (IC50 = 8 μM) and served as a
positive control.19 The 11,13-dehydrosantonin 12 is 2-fold
less active than parthenolide. The most potent arylated
derivatives of 1 or 12 is the parthenolide analogue 8 contain-
ing the m-trifluoromethyl substituent with an IC50
=
15.2 μM. This semisynthetic derivative 8 is 2-fold less potent
than the parent compound, despite the presumed steric
hindrance on the electrophilic R-methylene-γ-lactone caused
by the R-aryl group during a thiol approach. Analysis of
structure-activity data shows the parthenolide analogues
4-6 containing a para substituent on the aryl ring retain
some activity. However, all derivatives (9-11) containing
ortho-substituents show no activity in the assay. Also, the
arylated R-santonin derivatives 13-16 were inactive. In
contrast, the semisynthetic parthenolide derivatives 7-9
with a meta substituent displayed a dramatic range of
activity. Overall, the analogues with electron-withdrawing
substituents at the meta- and para-positions retain activity,
and this observation may imply that analogues with multiple
electron-withdrawing substituents at these sites will provide
additional improvements in biological activity.
In summary, we have demonstrated the utility of the Heck
reaction to generate R-benzylidene-γ-lactones with selectiv-
ity for the E-olefin geometry from sesquiterpene lactones
with R-methylene-γ-lactone substructures. This strategy
provides products in good yields and is amenable to assem-
bling derivatives of molecules containing the R-methylene-γ-
lactone substructure. A preliminary biological evaluation of
these new semisynthetic sesquiterpene lactones in HeLa cells
led to the identification of the novel m-trifluoromethyl
compound 8 and provided structure-activity data about
the role of substituents on the R-benzylidene-γ-lactone func-
tional group. Additional studies to improve the biological
activity of 8 through the incorporation of multiple substit-
uents on the aromatic ring and by replacing the phenyl ring
with heteroaromatic rings are underway.
aAll reactions were carried out for 20-24 h. bAll yields refer to
isolated, pure products.
the requisite R-methylene-γ-lactone, according literature
precedent (eq 1).16
The 11,13-dehydrosantonin 12 presents three R,β-unsatu-
rated carbonyl systems for the investigation of chemoselec-
tivity under the Heck reaction conditions. The reaction
proceeded exclusively at the exo-methylene functional group
of 12 and, similar to 1, the E-olefin products were isolated in
good yields (73%-81%) with several aryl iodides (Table 2).
Again, the substituents on the aromatic ring did not affect
the reaction yields. The assignment of E-olefin geometry
was analogously accomplished by the diagnostic C13 vinyl
proton.12 Similar to the arylation of 1, no structural reorga-
nization of the tricyclic ring system was observed in the
derivatives of 2.17
Because the sesquiterpene lactones are bioactive natural
products, we conducted growth inhibition assays with HeLa
(cervical cancer) cells18 on sesquiterpene lactones 3-16 to
determine the antiproliferative action (Table 3). We selected
HeLa cells because they have been widely studied with
sesquiterpene lactones.6a,19 Although the electrophilic
R-methylene-γ-lactone is known to trap nucleophilic intra-
cellular thiols, such as cysteine residues,6 the biological effect
Experimental Section
General Procedure for Palladium-Catalyzed Arylation. Synthesis
of (E)-13-Phenylparthenolide (3). A mixture of parthenolide
(10 mg, 0.04 mmol), triethylamine (17 μL, 0.12 mmol), and
iodobenzene (5 μL, 0.045 mmol) in DMF (200 μL) was treated
with palladium(II) acetate (0.5 mg, 0.002 mmol) and then heated
at 80 °C under air. After 24 h, the reaction mixture was allowed to
cool to rt, water (2 mL) was added, and the resultant mixture was
extracted with Et2O (2 mL ꢀ 5). The organics were dried over
Na2SO4 and concentrated under reduced pressure. SiO2 flash
(16) Rossi, C.; Ambrogi, V.; Grandolini, G.; Scarcia, V. J. Pharm. Sci.
1986, 75, 784–786.
(17) Barton, D. H. R. Helv. Chim. Acta 1959, 42, 2604–2616.
(18) Sun, H.-X.; Zheng, Q.-F.; Tu, J. Bioorg. Med. Chem. 2006, 14, 1189–
1198.
(19) Fonrose, X.; Ausseil, F.; Soleilhac, E.; Masson, V.; David, B.; Pouny,
I.; Cintrat, J.-C.; Rousseau, B.; Barette, C.; Massiot, G.; Lafanechere, L.
Cancer Res. 2007, 67, 3371–3378.
7178 J. Org. Chem. Vol. 74, No. 18, 2009

Han et al.
JOCNote
chromatography (19:1-7:3 hexanes/EtOAc) afforded the (E)-13-
phenylparthenolide product 3 as a solid (7.8 mg) in 60% yield: mp
196-198 °C; 1H NMR (500 MHz, CDCl3) δ 7.68 (d, J =
3.6 Hz, 1H), 7.46-7.35 (m, 5H), 5.29 (d, J = 11.5 Hz, 1H), 3.96
(dd, J = 8.8, 6.8 Hz, 1H), 3.32 (m, 1H), 2.84 (d, J = 8.9 Hz, 1H),
2.42 (m, 1H), 2.27-2.09 (m, 5H), 1.68 (s, 3H), 1.42 (m, 1H), 1.31
(s, 3H), 1.27 (m, 1H); 13C NMR (125 MHz, CDCl3) δ 170.9, 138.2,
134.8, 133.5, 129.8 (2), 129.7, 129.0, 128.5 (2), 125.0, 83.0, 66.5,
61.6, 46.8, 41.8, 36.1, 30.1, 24.3, 17.5, 17.4; IR (film) νmax 2928,
1753, 1644, 1193 cm-1; HRMS (EI) m/z calcd for C21H24O3 (M)þ
324.1725, found 324.1732; [R]25D þ142 (c 0.73, CHCl3).
Acknowledgment. We are grateful to Purdue University
and Purdue Alumni Association for funding. We acknowl-
edge the CCCB Cell Culture/Flow Cytometry Center,
Purdue University, as well as Phillip E. Fanwick and the
X-ray Crystallography Center, Purdue University.
Supporting Information Available: Additional experimental
details, full characterization data, 1H and 13C spectral reproduc-
tions, and X-ray data. This material is available free of charge
via the Internet at http://pubs.acs.org.
J. Org. Chem. Vol. 74, No. 18, 2009 7179