Tetrahydroxanthones by sequential Pd-catalyzed C-O and C-C Bond construction and use in the identification of the "antiausterity" pharmacophore of the kigamicins

Article abstract of DOI:10.1021/ol103103n

Readily available C-acylated cycloalkanones undergo efficient Pd catalyzed ring closure/cross-coupling providing 7-substituted tetrahydroxanthones in a single operation. One of the synthesized derivatives (depicted) is shown to selectively kill pancreatic

Full text of DOI:10.1021/ol103103n

ORGANIC
LETTERS
2011
Vol. 13, No. 5
1056–1059
Tetrahydroxanthones by Sequential
Pd-Catalyzed C-O and C-C Bond
Construction and Use in the Identification
of the “Antiausterity” Pharmacophore
of the Kigamicins
Penelope A. Turner,† Ellanna M. Griffin,† Jacqueline L. Whatmore,*,‡
and Michael Shipman*,†
Department of Chemistry, University of Warwick, Gibbet Hill Road, Coventry,
CV4 7AL, United Kingdom, and Peninsula Medical School, University of Exeter,
St Luke’s Campus, Heavitree, Exeter, EX1 2LU, United Kingdom
m.shipman@warwick.ac.uk; jackie.whatmore@pms.ac.uk
Received December 22, 2010
ABSTRACT
Readily available C-acylated cycloalkanones undergo efficient Pd catalyzed ring closure/cross-coupling providing 7-substituted tetra-
hydroxanthones in a single operation. One of the synthesized derivatives (depicted) is shown to selectively kill pancreatic cancer (PANC-1)
cells under conditions of nutrient deprivation indicating that the tetrahydroxanthone is responsible, in part, for the “antiausterity” effects of the
naturally occurring kigamicins.
Xanthones and partially hydrogenated di- and tetrahy-
droxanthones (THXs) are widely occurring classes of nat-
ural products (NPs) with considerable biological activity.1
Of particular interest are the complex polyketide derived
NPs including kigamicin C,2 kibdelone A,3 simaomicin R,4
and puniceaside B5 that display potent anticancer2-4 or
neuroprotective5 activities (Figure 1). These molecules all
possess THXs bearing an aryl substituent at C-7 (1,
Scheme 1).6 No detailed structure-activity relationships
are available within these compound classes, nor are their
modes of action well understood. That said, the fact that
they all possess a common 7-arylated THX subunit suggests
that this carbon framework may play an important role in
the origin of their bioactivities.
To test this hypothesis, we sought to develop efficient,
mild methods for the synthesis of 7-arylated THXs and
related derivatives and evaluate their bioactivity. The
presence of hydroxyl groups in the saturated rings of these
NPs demands that any such methods must be nonacidic in
† University of Warwick.
‡ University of Exeter.
(1) Chantarasriwong, O.; Batova, A.; Chavasiri, W.; Theodorakis,
E. A. Chem.;Eur. J. 2010, 16, 9944. Krohn, K.; Kouam, S. F.; Kuigoua,
ꢀ
G. M.; Hussain, H.; Cludius-Brandt, S.; Florke, U.; Kurtan, T.;
Pescitelli, G.; DiBari, L.; Draeger, S.; Schulz, B. Chem.;Eur. J. 2009,
15, 12121. Na, Y. J. Pharm. Pharmacol. 2009, 61, 707. Pinto, M.; Sousa,
M. E.; Nascimento, M. S. J. Curr. Med. Chem. 2005, 12, 2517 and
references therein.
r
10.1021/ol103103n
Published on Web 01/26/2011
2011 American Chemical Society

Scheme 1. “One-Pot” Route to 7-Aryl THXs using Tandem
Catalysis
Miyaura-Suzuki cross-coupling with boronic acid 3 using
the same Pd catalyst (Scheme 1).
In this way, libraries of simple, “drug-like” 7-arylated
THXs could be produced in a single operation through
application of tandem catalysis.9-11 In this communica-
tion, we successfully demonstrate the feasibility of this
approach to 7-substituted THXs. Moreover, using one of
the derived synthetic compounds, we confirm that this
heterocyclic scaffold is indeed responsible, at least in part,
for the bioactivity of one of these complex polyketide
derived NPs.
To ascertain if THXs could be made by Pd catalyzed
C-O bond construction, 2-(2-bromo-benzoyl)-cyclohex-
anone (4a) was synthesized in 65% yield in a single opera-
tion by deprotonation of cyclohexanone with LDA fol-
lowed by C-acylation of the resulting lithium enolate with
2-bromobenzoyl chloride (Table 1).12 The corresponding
chloride and iodide basedsubstrates 4band 4crespectively,
were made in the same way using the appropriate acid
chlorides. Next, we investigated a range of catalytic con-
ditions for ring closure to the THX nucleus.
Figure 1. Bioactive polyketide derived natural products con-
taining the 7-aryl tetrahydroxanthone (THX) nucleus.
nature to prevent facile aromatization to the corresponding
xanthones through elimination of water.7 In considering
possible routes to these molecules, we were drawn to the
ideaof using palladium catalysis toconstruct the THX ring
through selective C-O bond construction using the enolate
derived from a bromoketone such as 2.8 Importantly, by
careful orchestration of the reaction conditions, introduc-
tion of the 7-substituent might be realized concurrently by
(2) (a) Kunimoto, S.; Lu, J.; Esumi, H.; Yamazaki, Y.; Kinoshita, N.;
Honma, Y.; Hamada, M.; Ohsono, M.; Ishizuka, M.; Takeuchi, T.
J. Antibiot. 2003, 56, 1004. (b) Kunimoto, S.; Someno, T.; Yamazaki, Y.;
Lu, J.; Esumi, H.; Naganawa, H. J. Antibiot. 2003, 56, 1012. (c) Lu, J.;
Kunimoto, S.; Yamazaki, Y.; Kaminishi, M.; Esumi, H. Cancer Sci.
2004, 95, 547. (d) Someno, T.; Kunimoto, S.; Nakamura, H.; Naganawa,
H.; Ikeda, D. J. Antibiot. 2005, 58, 56. (e) Masuda, T.; Ohba, S.;
Kawada, M.; Ohsono, M.; Ikeda, D.; Esumi, H.; Kunimoto, S.
J. Antibiot. 2006, 59, 209.
(3) Ratnayake, R.; Lacey, E.; Tennant, S.; Gill, J. H.; Capon, R. J.
Chem.;Eur. J. 2007, 13, 1610.
(4) Koizumi, Y.; Tomoda, H.; Kumagai, A.; Zhou, X.-P.; Koyota, S.;
Sugiyama, T. Cancer. Sci. 2009, 100, 322.
(5) Du, X.-G.; Wang, W.; Zhang, S.-P.; Pu, X.-P.; Zhang, Q.-Y.; Ye,
M.; Zhao, Y.-Y.; Wang, B.-R.; Khan, I. A.; Guo, D.-A. J. Nat. Prod.
2010, 73, 1422.
Encouragingly, subjection of 4a to Pd2(dba)3, Xantphos
and Cs2CO3 in refluxing toluene furnished 5 in 76% yield
(Table 1, entry 1), whose spectroscopic data matched those
previously reported.7a,13 Further improvements were
achieved by screening a variety of phosphine ligands and
solvents (Table 1, entries 2-10). The use of Xphos as ligand
and dioxane as solvent proved most effective with 5 pro-
duced in an excellent 93% yield (Table 1, entry 10). The
cyclization proceeded with similar efficiency when the bro-
mide was replaced with a chloride, although appreciably
lower yields were observed using the corresponding iodide
(Table 1, entries 11-12).
The scope of this new route to THXs was investigated
through the preparation of derivatives 6a-i (Figure 2). In
most cases, the reactions were conducted using bromide
based substrates although for the preparation of 6h, the
corresponding chloride was used. Using the optimized
conditions, good to excellent yields were observed and a
variety of groups shown to be tolerated including acid
sensitive functional groups (e.g., 6c). The formation of
(6) The numbering system adopted is based on that of the parent
heterocycle, namely 1,2,3,4-tetrahydro-9H-xanthen-9-one.
(7) Existing routes to THXs rely on the use of strong acids, see: (a)
Watanbe, T.; Katayama, S.; Nakashita, Y.; Yamauchi, M. J. Chem.
Soc., Perkin Trans. 1 1978, 726. (b) Singh, O. V; Kapil, R. S; Garg, C. P;
Kapoor, R. P. Tetrahedron Lett. 1991, 32, 5619. (c) Kostakis, I. K.;
Tenta, R.; Pouli, N.; Marakos, P.; Skaltsounis, A.-L; Pratsinis, H.;
Kletsas, D. Bioorg. Med. Chem. Lett. 2005, 15, 5057.
(8) For the synthesis of benzofurans using C-O bond formation, see:
(a) Willis, M. C.; Taylor, D.; Gillmore, A. T. Org. Lett. 2004, 6, 4755. (b)
Willis, M. C.; Taylor, D.; Gillmore, A. T. Tetrahedron 2006, 62, 11513.
(9) For a monograph, see: Tietze, L. F.; Brasche, G.; Gericke, K.
Domino Reactions in Organic Chemistry; Wiley-VCH: Weinheim, 2006.
(10) For reviews, see: Bonne, D.; Coquerel, Y.; Constantieux, T.;
Rodriguez, J. Tetrahedron: Asymmetry 2010, 21, 1085. Barluenga, J.;
Rodriguez, F.; Fananas, F. J. Chem. Asian. J. 2009, 4, 1036. D’Souza,
D. M.; Muller, T. J. J. Chem. Soc. Rev. 2007, 36, 1095. Chapman, C. J.;
Frost, C. G. Synthesis 2007, 1. Ajamian, A.; Gleason, J. L. Angew. Chem.
Int. Ed. 2004, 43, 3754.
(11) For recent examples, see: Bryan, C. S; Braunger, J. A; Lautens,
M. Angew. Chem., Int. Ed. 2009, 48, 7064. Wu, X.-F; Neumann, H.;
Beller, M. Angew. Chem., Int. Ed. 2010, 49, 5284. Gandeepan, P.;
Parthasarathy, K.; Cheng, C.-H. J. Am. Chem. Soc. 2010, 132, 8569.
Kim, H.; Lee, K.; Kim, S.; Lee, P. H. Chem. Commun. 2010, 46, 6341.
Spergel, S. H.; Okoro, D. R.; Pitts, W. J. Org. Chem. 2010, 75, 5316.
Bararjanian, M.; Balalaie, S.; Rominger, F.; Movassagh, B.; Bijanzadeh,
H. R. J. Org. Chem. 2010, 75, 2806.
(12) Although drawn as 1,3-dicarbonyl compounds, the substrates
used in this study exist predominantly in the enol form in CDCl3, as
determined by 1H NMR spectroscopy (see Supporting Information).
ꢀ
ꢀ
(13) Patonay, T.; Levai, A.; Rimanm, E.; Varma, R. S. Arkivoc 2004,
183.
Org. Lett., Vol. 13, No. 5, 2011
1057

Table 1. Optimization of Pd-Catalyzed C-O Bond Forming
Conditions to THX (5)
Figure 2. Evaluation of scope of Pd-catalyzed reaction for
the synthesis of tetrahydroxanthones and related structures.
Reaction conditions: haloketone (0.35 mmol), Pd2(dba)3
(2.5 mol %), Xphos (6 mol %), Cs2CO3 (2.2 molar equiv),
1,4-dioxane (1 mL), 101 °C, 18 h.
entry
ketone
liganda
solvent
temp (°C)
yieldb
1
4a
4a
4a
4a
4a
4a
4a
4a
4a
4a
4b
4c
Xantphos
Xantphos
Xantphos
Xantphos
Xantphos
DPEphos
DPEphos
DPPB
toluene
THF
111
66
76%
76%
<25%
62%
87%
79%
86%
83%
82%
93%
88%
54%
2
3
H2O
100
153
101
111
101
101
101
101
101
101
4
DMF
5
dioxane
toluene
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
6
7
Scheme 2. Synthesis of THXs by Sequential Pd(0) Catalyzed
C-O and C-C Bond Formation
8
9
JohnPhos
Xphos
10
11
12
Xphos
Xphos
a Reaction conditions: ketone (1 molar equiv), Pd2(dba)3 (2.5 mol %),
ligand (6 mol %), Cs2CO3 (2.2 molar equiv) in solvent (0.41 M) for 18 h at
stated temperature. b Yield of isolated product after column chromato-
graphy.
THX 6e in good yield by selective insertion into the C-Br
bond adjacent to the carbonyl group14 was especially
encouraging in the context of the proposed sequential
C-O and C-C bond forming sequence (Scheme 1). Deri-
vatives containing additional carbocyclic (e.g., 6a) or hetero-
cyclic rings (e.g., 6g) are readily accessed. Moreover, the
synthesis of ring expanded systems (e.g., 6b) can be realized
through use of cycloheptanone in place of cyclohexanone
in the initial C-acylation reaction (cf Table 1). However,
a poor yield was observed in the formation of ring con-
tracted derivative 6d.
The formation of THXs can be achieved using less
expensive Cu (I) catalysis.15 For example, treatment of
4a with copper iodide (10 mol %), DMEDA (20 mol %)
and Cs2CO3 (2 equiv) in refluxing THF for 18 h provided 5
in anunoptimized54% yield. However, asthis method was
deemed to be less useful in the context of sequential bond
forming sequences, further optimization of this reaction
was not pursued.
Next, we explored the development of the tandem
process. When dibromide 7 was subjected to Pd catalysis
in the presence of PhB(OH)2, 8a was produced in 78%
yield by sequential C-O and C-C bond construction
(Scheme 2). Time-course GC-MS analysis revealed rapid
consumption of 7 leading to 6e by intramolecular C-O
bond formation. Further conversion to 8a was observed
through subsequent intermolecular Miyaura-Suzukicross-
coupling. No evidence for products arising from C-C
bond formation prior to ring closure could be detected
by this method. A small library of arylated derivatives
namely 8a-f were made by use of different ketones and
arylboronic acids in this sequential process. In the case of
(14) In related substrates bearing two C-Br bonds, the C-Br bond
ortho to the carbonyl group has been shown to undergo regioselective Pd
catalyzed reactions. For examples, see: (a) Ye, Y. Q.; Koshino, H.;
Onose, J.-I.; Yoshikawa, K.; Abe, N.; Takahashi, S. Org. Lett. 2009, 11,
5074 (Miyaura-Suzuki). (b) Iwasawa, N.; Otsuka, M.; Yamashita, S.;
Aoki, M.; Takaya, J. J. Am. Chem. Soc. 2008, 130, 6328 (Sonagashira).
(c) Houpis, I. N.; Van Hoeck, J.-P.; Tilstam, U. Synlett 2007, 2179
(Kumada).
(15) For copper catalyzed C-O bond construction of related benzo-
pyrans see: Fang, Y.; Li, C. J. Org. Chem. 2006, 71, 6427.
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Org. Lett., Vol. 13, No. 5, 2011

8c, single crystal X-ray diffraction was used in conjunction
with NMR spectroscopy to unambiguously confirm its
identity.
With easy access to a range of 7-substituted THXs, we
sought to ascertain if they exhibit useful biological effects.
Esumi has recently pioneered a new anticancer strategy
based upon “antiausterity” through the discovery of the
naturally occurring kigamicins.2 Five different kigamicins
are known which vary only in the extent of glycosidation at
C-14.2b Kigamicin D shows significant antitumor effects
against a number of human pancreatic cancer xenografts
in nude or scid mice through targeting cancer cells’ tolerance
to nutrient starvation.2c,e Cancer cell lines such as PANC-1
have adapted such that they normally survive for long
periods of time under conditions of extreme nutrient
starvation. However, they are selectively killed upon addi-
tion of kigamicins.2a In contrast, most conventional anti-
cancer drugs show weaker cytotoxicity to cancer cells grown
under nutrient-deprived conditions.2c
To ascertain if the 7-aryl THX nucleus is the pharma-
cophore responsible for the antiausterity effects of the
kigamicins, we evaluated the effect of a representative
member of the synthesized THXs against human pancreatic
cancer (PANC-1) cells grown separately in nutrient rich
media (NRM) and nutrient deprived media (NDM).
Commercially sourced kigamicin C was used as a positive
control.16 Significantly, THX 8a displays the same “anti-
austerity” activity as the NPs, inhibiting PANC-1 cells
survival at >10 times lower concentrations in nutrient
deprived conditions than in normal culture (Figure 3).17
Indeed, synthetic 8a demonstrated greater selectivity for
cancer cells cultured in NDM than the NP itself.18 That
said it is clear that 8a is appreciably less active (>100 fold)
thankigamicin C, suggesting thatotherstructural elements
of this class of NP have a role to play in eliciting the
biological response.
Figure 3. Effects of (a) synthetic THX (8a) and (b) kigamicin C
on PANC-1 (pancreatic cancer) cells grown in nutrient deprived
media (;);) and nutrient rich media (;(;). Cell viability
determined using LDH assay after incubation with agent
(0-100 μg/mL) for 24 h. Measurements were performed at
least in triplicate. Full details are in the Supporting Information.
*p = e0.05, Mann-Whitney, n = 3-6.
heterocyclic ring and simultaneously introduce the 7-aryl
substituent in a controlled manner. Moreover, we have
successfully established that the THX nucleus of 8a is the
minimum pharmacophore of the kigamicins. Future
studies will be focused on: (i) using this methodology to
optimize and refine the potency of these small-mole-
cules with a view to producing medicinal agents that act
by this new anticancer strategy and (ii) ascertaining if
the bioactivity of related NPs3-5 also resides in the
7-aryl THX nucleus.
Acknowledgment. We thank Cancer Research UK
(C1252/A9196) for financial support of this project. We
are grateful to Dr. Guy J. Clarkson for the X-ray crystal-
lographic analysis, performed on X-ray facilities obtained,
through Birmingham Science City with support from
Advantage West Midlands (AWM).
In conclusion, we have developed a simple, efficient route
to THXs that exploits palladium catalysis to construct the
(16) Kigamicin C was purchased from bioaustralis fine chemicals
(http://www.bioaustralis.com).
Supporting Information Available. Experimental pro-
cedures, characterization data, copies of NMR spectra for
4a, 5-8, and “antiausterity” assay. This material is available
free of charge via the Internet at http://pubs.acs.org.
(17) IC50 = 24 μM in NDM, IC50 > 0.36 mM in NRM.
(18) Against PANC-1 cells, the IC50 value (0.089 μM in NDM) and
selective cytotoxicity [selectivity = IC50 (in NRM)/IC50 (in NDM) = 9]
determined in this study for kigamicin C are broadly consistent with
previously published values (see refs 2a and 2e).
Org. Lett., Vol. 13, No. 5, 2011
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