Serendipitous discovery of a cascade approach to perhydrodibenzofuranones related to the natural product incarviditone

Article abstract of DOI:10.1055/s-0033-1341081

The unexpected discovery of an eliminative deprotection and cyclodimerization cascade reaction sequence led to the preparation of a series of perhydrodibenzofuranones that bear a structural resemblance to the natural product incarviditone. One of the novel dimers was found to show significant antiproliferative activity towards a non-small-cell lung cancer cell line, and represents a useful lead compound for the discovery of more potent anti-cancer agents.Georg Thieme Verlag Stuttgart. New York.

Full text of DOI:10.1055/s-0033-1341081

LETTER
▌1263
lSeteterr endipitous Discovery of a Cascade Approach to Perhydrodibenzofuranones
Related to the Natural Product Incarviditone
Cascade Approach to Perhydrodibenzofuranones
Stephania Christou,a Alyn C. Edwards,a Robin G. Pritchard,a Peter Quayle,a Ian J. Stratford,b Roger C. Whitehead*a
a
School of Chemistry, University of Manchester, Oxford Road, Manchester, M13 9PL, UK
Fax +44(161)2754598; E-mail: roger.whitehead@manchester.ac.uk
b
Manchester Pharmacy School, University of Manchester, Oxford Road, Manchester, M13 9PL, UK
Received: 17.02.2014; Accepted after revision: 07.03.2014
velopment of a synthetic approach to the antipodal series
Abstract: The unexpected discovery of an eliminative deprotection
of compounds (ent-5) by using the cyclohexylidene-pro-
and cyclodimerization cascade reaction sequence led to the prepa-
tected lactone quinide 6 as the starting material.5 The
base-lability of several of the intermediates on the syn-
ration of a series of perhydrodibenzofuranones that bear a structural
resemblance to the natural product incarviditone. One of the novel
dimers was found to show significant antiproliferative activity to- thetic route has presented a significant challenge during
wards a non-small-cell lung cancer cell line, and represents a useful
lead compound for the discovery of more potent anti-cancer agents.
this venture, one particularly problematic transformation
being the eliminative deprotection of adducts 86 to give
Key words: natural products, medicinal chemistry, cascade reac-
tions, heterocycles, biomimetics
cyclohex-2-en-1-ones 9 (Scheme 2).
In 1989, Danishefsky and co-workers reported a high-
yielding conversion of the acetonide-protected
dihydroxycyclohexanone 10a into the enone 12 by using
1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) at elevated
temperatures in the presence of tert-butyldimethylsilyl
chloride (TBSCl) as a trapping reagent for the incipient al-
lylic alcohol 11 (Scheme 3).7 These investigators also de-
scribed the instability of alcohol 11 due, in part, to its
propensity to undergo tautomeric decomposition to cyclo-
hexane-1,4-dione. In our hands, cyclohexylidene-protect-
ed compound 10b could also be converted into 12, albeit
in a slightly lower yield than that reported for the corre-
sponding conversion of 10a.5 Unfortunately, however, ex-
posure of aryl-substituted cyclohexanones 8 to identical
The α-oxyalkylcyclohex-2-en-1-one-containing natural
products [(3R,4R,5R)-3,4,5-trihydroxy-6-oxocyclohex-1-
en-1-yl]methyl (2E)-but-2-enoate (1; COTC)1,2 and an-
theminone A (2)3 both demonstrate notable toxicity to-
wards a variety of cancer cell lines, and have therefore
attracted significant attention from the scientific commu-
nity. By using the butane–diacetal protected methylqui-
nate 3 as the key building block, we have recently
completed the synthesis of an array of hybrid analogues of
1 and 2 of general structure 5, which display potent anti-
tumor activities (Scheme 1).4
Encouraged by the promising biological activities of these reaction conditions did not meet with the same success,
new hybrid analogues, we are currently engaged in the de- and little if any of the expected TBS-protected compounds
O
O
O
HO
HO
OH
O
HO
OH
OH
COTC (1)
antheminone A (2)
O
HO
CO2Me
O
O
O
4 steps
OMe
5 steps
O
HO
R
O
R
OMe
O
O
OH
MeO
MeO
5
3
4
R = alkyl, aryl
Scheme 1 α-Oxyalkylcyclohex-2-enone-containing natural products, and an approach to the synthesis of their analogues
SYNLETT 2014, 25, 1263–1266
Advanced online publication: 03.04.2014
0
9
3
6
-
5
2
1
4
1
4
3
7
-
2
0
9
6
DOI: 10.1055/s-0033-1341081; Art ID: ST-2014-D0133-L
© Georg Thieme Verlag Stuttgart · New York

1264
S. Christou et al.
LETTER
also at room temperature, which gave the desired alcohols
in yields of 57–81% (Scheme 3).
O
O
O
HO
O
It transpired that a major reason for the capricious nature
of the sodium hydroxide-mediated reaction was the ‘bipo-
lar reactivity’ of the products 9a–e under the basic reac-
tion conditions, which induced an unexpected
dimerization process.9 Indeed, when the conjugate ad-
ducts 8a–e were exposed to 0.5 M aqueous sodium hy-
droxide in tetrahydrofuran for a prolonged reaction time
(18 hours), the diasteroisomerically pure perhydrodiben-
zofuranones 13a–e were the only isolable products, ob-
tained in 55–86% yields.
conjugate
addition
3 steps
O
O
R
O
O
O
O
6
7
8
O
O
O
elimination/
deprotection
O
R
R
In the first instance, the structures of the dimeric products
were identified by means of extensive one- and two-di-
mensional NMR analyses (COSY, NOESY, and NOE);
typical chemical-shift values, together with the associated
geminal/vicinal coupling constants for the 4-iodophenyl-
substituted compound 13e are listed in Table 1. A signifi-
cant difficulty encountered during the assignment of the
data was the apparent absence of a three-bond coupling
between C(9)H and C(6)H. A long-range COSY experi-
ment in which cross-peaks arising from small coupling in-
teractions were enhanced highlighted the appropriate
signal, and a corresponding weak cross-peak was also
identified in the NOESY spectrum. By using the MM2
force field, an energy-minimized structure for 13e was
generated; this predicted an internuclear distance of ~2.8
Å between C(6)H and C(9)H, in accord with the NOESY
observation.
OH
OH
ent-5
9
Scheme 2 Synthetic approach to hybrid analogues of antheminone
A and COTC
could be isolated. After extensive investigations into this
reaction in both the presence and absence of a silyl trap-
ping agent, we discovered that conversion of aryl-substi-
tuted cyclohexanones 8a–e into the allylic alcohols 9a–e
could be accomplished in variable, sometimes quite un-
predictable, yields by using either triethylamine (two
equivalents) in 1,4-dioxane–water (10:1) at room temper-
ature or a catalytic quantity of aqueous sodium hydroxide
(0.5 M) in tetrahydrofuran at room temperature.8 The
most generally applicable and reproducible protocol,
however, involved the use of DBU in dichloromethane,
Table 1 1H NMR Spectroscopic Data for Dimer 13e
Hβ
O
O
O
O
Hα
H
O
Hβ
Hα
8
7
Hβ
Hα
H
i)
12
9
1
4
10
6
5
2
3
11
O
H
O
H
O
OH
11
OTBS
OH
12
I
I
13e
10a (acetonide protection): 87%
10b (cyclohexylidene protection): 80%
δ (ppm)
Coupling constant (Hz)
O
C(2)Hα
C(2)Hβ
2.38
2.88
J2
= 13.7
J2 ,3 = 3.9
α
β
,2
α
β
J2 ,3 = 13.7
O
ii)
R
C(3)H
C(4)H
C(5)H
C(6)H
3.19
4.39
4.74
3.05
OH
J
J
J
3,4 = 10.4
4,5 = 2.9
5,6 = 4.0
9a–e
R
O
iii)
O
O
O
C(8)Hα
C(8)Hβ
2.43
2.78
J8
= 16.2
α
β
,8
α
H
J8 ,9 = 5.8
H
H
A
β
J8 ,9 = 13.6
8a–e
B
C
R
O
C(9)H
C(10)H
C(11)H
3.41
4.32
3.32
J
J
9,10 = 7.7
10,11 = 9.8
H
R
a: R = 4-tert-butylphenyl
b: R = 4-biphenyl
c: R = 4-methoxphenyl
d: R = 4-fluorophenyl
e: R = 4-iodophenyl
OH
13a–e
C(12)Hα
C(12)Hβ
2.64
2.37
J12
= 17.8
J11,12α = 13.9
J11,12β = 3.4
α
β
,12
Scheme 3 Eliminative deprotection reactions of ketal-protected β,γ-
dihydroxycyclohexanones 10a, 10b, and 8a–e. Reagents and condi-
tions: (i) DBU, TBSCl, benzene, reflux, 6 h; (ii) DBU, CH2Cl2, r.t., 2
h; (iii) 0.5 M aq NaOH (a few drops), THF, r.t., 18 h.
Unfortunately, most of the novel dimers were isolated as
amorphous powders; however extensive recrystallization
Synlett 2014, 25, 1263–1266
© Georg Thieme Verlag Stuttgart · New York

LETTER
Cascade Approach to Perhydrodibenzofuranones
1265
efforts eventually generated a sample of the tert-butylphe- [(+) with (+) / (–) with (–)] gave rise to 15, whereas ‘hetero-
nyl compound 13a that was suitable for X-ray crystallo- cyclodimerization’ [(+)-14 with (–)-14] gave 16, possibly
graphic analysis, and this confirmed the structural identity through intramolecular aldol cyclization of an initially
of the compound (Figure 1).10
formed perhydrodibenzofuranone 17. The structures of
the novel compounds 13a–e reported here, which are sim-
ilar to that of 15 and which arise from a ‘homocyclodimer-
ization’ process, are in accord with this conclusion.
Both rengyolone (14) and incarviditone (15) have been re-
ported to demonstrate toxicity towards cancer cell lines,
so, prompted by this finding, we assessed the toxicity of a
selection of the novel dimers, as well as one of their im-
mediate precursors (enone 9a), towards the A549 non-
small-cell lung cancer cell line. The cells were exposed to
varying concentrations of the compounds for 96 hours,
and toxicity was assessed by using the 3-(4,5-dimethylthi-
azol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay
(Table 2).14
Table 2 Assessment of the Toxicities of Analogues of Incarviditone
and Rengyolone toward the Non-Small-Cell Lung Cancer Cell Line
A549
Compound
IC50a (μM)
Figure 1 Crystal structure of dimer 13a with ellipsoids at 50% prob-
ability
9a
1.96 ± 0.38
The syn–anti–syn arrangement of C(5)H/C(6)H,
C(6)H/C(9)H, and C(9)H/C(10)H is consistent with a tan-
dem ‘suprafacial–suprafacial’ oxa-Michael/carba-Mi-
chael cyclodimerization sequence (Scheme 4). This is
initiated by conjugate addition of an oxyanion derived
from one molecule of the starting material to the C3 posi-
tion of another molecule, anti to the C5 aromatic substit-
uent of the latter. A related cyclodimerization sequence,
albeit under more strongly basic conditions, was recently
used to great effect by Wu and co-workers11 during their
biosynthetically inspired synthesis of the natural product
incarviditone (15).12 They found that treatment of a race-
mic sample of rengyolone (14)13 with one equivalent of
sodium hydride in dichloromethane gave 15 in 40% iso-
lated yield, together with a 38% yield of incarvileatone
(16). They concluded that ‘homocyclodimerization’ of 14
13a
13c
13d
13e
12.01 ± 1.6
nontoxic at 100 μM
nontoxic at 100 μM
102.61 ± 41.62
a IC50 = concentration required to reduce proliferation by 50%. Tox-
icity experiments were repeated in triplicate and data within individ-
ual experiments were derived from four separate observations;
average values are given.
Perhaps unsurprisingly, enone 9a proved to be the most
potent of the compounds tested. Being a Michael accep-
tor, this compound would be expected to alkylate intracel-
lular nucleophiles, such as protein thiols, leading to cell
damage. Of the dimers that we assayed, only the tert-
O
O
O
H
H
O
homo-
dimerization
O
O
O
O
H
HO
NaH
CH2Cl2
r.t.
R
O-
H
HO
R
( )-incarviditone (15)
H
(revised structure)
oxa-Michael/
O
carba-Michael
cyclodimerization
O
HO
hetero-
O
O
O
( )-rengyolone (14)
dimerization
H H
O
H
H
OH
O
9
10
6
5
O
O
O
O
R
O
H
O
H
HO
R
OH
H
OH
( )-incarvilleatone (16)
13
17
Scheme 4 Cyclodimerization reactions to give perhydrodibenzofuranones
© Georg Thieme Verlag Stuttgart · New York
Synlett 2014, 25, 1263–1266

1266
S. Christou et al.
LETTER
(6) Adducts 8 were prepared by conjugate addition to 7 of the
appropriate Gilman cuprate (stoichiometric) or the
appropriate arylboronic acid in the presence of a rhodium
catalyst.
(7) Audia, J. E.; Boisvert, L.; Patten, A. D.; Villalobos, A.;
Danishefsky, S. J. J. Org. Chem. 1989, 54, 3738.
(8) Barros, M. T.; Maycock, C. D.; Ventura, M. R. J. Org.
Chem. 1997, 62, 3984.
butylphenyl analogue 13a demonstrated significant anti-
proliferative activity. As in the case of incarviditone, the
biological mechanism of action of 13a is unknown; how-
ever, this compound represents a useful lead compound
for the discovery of more potent antiproliferative agents,
and investigations to achieve this goal are now underway.
(9) For two related dimerizations see: (a) Carreño, M. C.;
Ribagorda, M. Org. Lett. 2003, 5, 2425. (b) Paddock, V. L.;
Phipps, R. J.; Conde-Angulo, A.; Blanco-Martin, A.; Giró-
Mañas, C.; Martin, L. J.; White, A. J. P.; Spivey, A. C.
J. Org. Chem. 2011, 76, 1483.
Acknowledgment
We acknowledge, with thanks, the EPSRC (S.C.) for funding, and
we also thank the following undergraduate students at the Universi-
ty of Manchester for their valuable practical contributions to the in-
itial phase of this study: Emma Birkett, Connaugh Fallon, Hannah
Langford, Krishna Mistry, Sarah O’Keefe, Shanneal Readman, and
Jack Watling.
(10) Crystal data for 13a: C32H40O4; M = 488.64, space group
P21, a = 7.1087(4), b = 9.6046(9), c = 19.1101(16) Å,
β = 96.131(7)°, U = 1297.30(18) Å3, dcalcd = 1.251 Mg/m3.
Intensity data were collected by using an Agilent Supernova
diffractometer; 7390 reflections were collected, of which
3995 were unique, Rint = 0.0532. Data processing was
carried out by using CrysAlis PRO, and the structure was
solved by direct methods using SIR92. All nonhydrogen
atoms were refined anisotropically and hydrogen atoms
were refined isotropically. Refinement on F2 was carried out
by using SHELXL97: Final R1 = 0.0553, wR2 = 0.1155 for
data with; I > 2σ(I). Crystallographic data for compound 13a
have been deposited with the accession number CCDC
983059, and can be obtained free of charge from the
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; Fax: +44(1223)336033; E-mail:
deposit@ccdc.cam.ac.uk; Web site:
www.ccdc.cam.ac.uk/conts/retrieving.html.
(11) Zhao, K.; Cheng, G.-J.; Yang, H.; Shang, H.; Zhang, X.; Wu,
Y.-D.; Tang, Y. Org. Lett. 2012, 14, 4878.
(12) Chen, Y.-Q.; Shen, Y.-H.; Su, Y.-Q.; Kong, L.-Y.; Zhang,
W.-D. Chem. Biodiversity 2009, 6, 779.
(13) Endo, K.; Hikino, H. Can. J. Chem. 1984, 62, 2011.
(14) Carmichael, J.; DeGraff, W. G.; Gazdar, A. F.; Minna, J. D.;
Mitchell, J. B. Cancer Res. 1987, 47, 936.
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett.SnoIufproig
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Synlett 2014, 25, 1263–1266
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