(-)-Quinic acid: a versatile precursor for the synthesis of analogues of 2-crotonyloxymethyl-(4R,5R,6R)-4,5,6-trihydroxycyclohex-2-enone (COTC) which possess anti-tumour properties

Article abstract of DOI:10.1016/j.tetlet.2008.02.059

Syntheses of three novel analogues of the Streptomyces metabolite COTC are described, using the versatile chiral pool starting material, (-)-quinic acid. The results of bioassays of the target compounds against two lung cancer cell lines, A549 and H460, a

Full text of DOI:10.1016/j.tetlet.2008.02.059

Available online at www.sciencedirect.com
Tetrahedron Letters 49 (2008) 2410–2413
(ꢀ)-Quinic acid: a versatile precursor for the synthesis of analogues
of 2-crotonyloxymethyl-(4R,5R,6R)-4,5,6-trihydroxycyclohex-2-
enone (COTC) which possess anti-tumour properties
Claire L. Arthurs a, Katharine F. Lingley a, Michela Piacenti a, Ian J. Stratford b, Tanja Tatic a,
a,
b
*
Roger C. Whitehead , Natasha S. Wind
a School of Chemistry, University of Manchester, Oxford Road, Manchester M13 9PL, UK
b School of Pharmacy and Pharmaceutical Sciences, University of Manchester, Oxford Road, Manchester M13 9PL, UK
Received 15 January 2008; revised 6 February 2008; accepted 11 February 2008
Available online 14 February 2008
Abstract
Syntheses of three novel analogues of the Streptomyces metabolite COTC are described, using the versatile chiral pool starting
material, (ꢀ)-quinic acid. The results of bioassays of the target compounds against two lung cancer cell lines, A549 and H460, are
presented.
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords: Anti-cancer; Natural product; Quinic acid; Cyclopropane
2-Crotonyloxymethyl-(4R,5R,6R)-4,5,6-trihydroxycyclo-
hex-2-enone (COTC, 1) was isolated in 1975 from the cul-
tures of Streptomyces griseosporeus and was demonstrated
to possess cytotoxic and cyterostatic activity (Fig. 1). Both
1 and its synthetic analogue, 2-crotonyloxymethyl-cyclo-
hex-2-enone (COMC, 2), possess potent activity against
murine and human cancer cell lines in culture as well as
in tumour-bearing mice.1–3 A principal mechanism of
action of the cyclohexenones proceeds via enzyme-cataly-
sed conjugation of glutathione (GSH) to the enone moiety
of 1/2, thereby generating an exocyclic enone of type 3.
Subsequent alkylation of intracellular proteins and/or
DNA by this reactive entity then leads to cell death.4
A focus of our recent research effort has been the prep-
aration of analogues of 1 with general structure 4 wherein
there are four loci for diversification. Initial studies have
concentrated on analogues which differ from each other
in the extent of oxygenation of the cyclohexenone core
(Fig. 2). Thus, dihydroxylated compound 5, which pos-
sesses inverted stereochemistry at C5 compared with
COTC, was found to display low toxicity towards lung
cancer cell lines A549 and H460. Tri-hydroxylated com-
pound 6-epi-COTC (6) was equipotent with 5 towards the
lung cancer cell lines whereas racemic mono-hydroxylated
compound 7 was substantially more toxic than 5, 6 and
COMC (2), towards these cell lines.5 Previously, COMC
O
O
2
O
O
O
O
3
1
6
4
5
HO
OH
OH
COMC
COTC
2
1
GS
O
3
Fig. 1.
*
Corresponding author. Tel.: +44 161 275 7856; fax: +44 161 275 4939.
E-mail address: roger.whitehead@manchester.ac.uk (R. C. Whitehead).
0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.02.059

C. L. Arthurs et al. / Tetrahedron Letters 49 (2008) 2410–2413
2411
R
O
TBSO
O
O
O
O
trimethylsulfoxonium iodide,
NaH, DMSO, rt, 1.5 h, 56%
O
X
O
Z
O
O
O
O
H3CO
H3CO
4
6
O
5
O
HO
OCH3
O
OCH3
12
13
Y
4
OH
5
O
O
O
H3CO
O
O
O
O
OCH3
14
HO
OH
HO
Scheme 1.
OH
6
7
by stereoelectronically favoured ‘axial attack’ at C3 of 14
by the ylid (Scheme 1).
Fig. 2.
Prompted by this observation, we embarked on a syn-
thesis of 9 proceeding via enone 14 as a key intermediate.
This latter compound was prepared in 67% overall yield
using a route modified slightly from that reported by
had been demonstrated to be the most potent anti-cancer
agent belonging to this structural class, so these findings
provide renewed stimulus for the synthesis and biological
assessment of compounds related to 1/2.
Bruckner and Gebauer for the synthesis of the cyclo-
¨
In this Letter, we describe the results of recent synthetic
chemistry endeavours which demonstrate the versatility of
the chiral pool material (ꢀ)-quinic acid (8) as a starting
material for the preparation of analogues of COTC. Our
target compounds were selected in order to probe several
key structure activity features: thus, a,b-cyclopropyl ketone
9, a pseudo-Michael acceptor, was chosen in order to ascer-
tain the absolute requirement for a conjugated ketone moi-
ety for anti-cancer activity. The unusual a,b-cyclopropyl-
a0,b0-enone 10, which has two potential sites available for
nucleophilic attack by GSH, was selected to probe whether
‘doubly-activated’ molecules of this type would display
enhanced anti-cancer properties compared with their
mono-activated counterparts. Finally, the enantiomerically
pure compound 11 was chosen to elucidate the influence of
the absolute stereochemistry at C4 on in vitro anti-cancer
activity (Fig. 3).
hexane-diacetal variant of 14 (Scheme 2).6 Morita–
Baylis–Hillman (M–B–H) reaction of the enone with
formaldehyde using the conditions reported by Rezgui
and El Gaied7 proceeded in excellent yield to give the
desired a-hydroxymethyl compound which was protected
as its tert-butyldimethylsilyl (TBS) ether to give 15. Pleas-
ingly, exposure of 15 to an excess of Corey’s ylid yielded
a single cyclopropanated compound 16, the stereochemical
assignment of which was supported by a significant NOE
from C(5)H (d 3.86 ppm) to the cyclopropyl methylene
endo hydrogen (d 1.40 ppm). Deprotection of the primary
hydroxyl group followed by crotonylation and acetal cleav-
age then yielded the target compound 9.8
The synthesis of a,b-cyclopropyl-a0,b0-enone 10 pro-
ceeded via cyclopropane 13 which can be prepared by the
treatment of either b-silyloxyketone 12 (vide supra) or
Our synthetic approach to cyclopropane 9 was founded
on a serendipitous observation arising during earlier inves-
tigations into the reactivity of ketone 12 towards various
nucleophiles (unreported data): thus, exposure of 12 to
Corey’s ylid (dimethylsulfoxonium methylide) yielded com-
pound 13 as the only isolable product. The formation of 13
was rationalised to occur via base-mediated elimination of
tert-butyldimethylsilanoxide to generate enone 14 followed
O
OH
HO
i) - iv)
CO2H
O
H3CO
HO
O
8
14
OH
OCH3
v), vi)
TBSO
OTBS
O
O
vii)
5
O
O
H3CO
H3CO
O
O
O
OH
HO
16
15
OCH3
OCH3
O
O
2
5
1
6
3
4
CO2H
viii), ix), x)
HO
HO
OH
9
OH
9
8
Scheme 2. Reagents and conditions: (i) butan-2,3-dione, (CH3O)3CH,
camphorsulfonic acid, CH3OH, D, 12 h, 98%; (ii) NaBH4, CH3OH, 0 °C
to rt, 24 h; (iii) NaIO4 on silica gel, CH2Cl2, 1.5 h, 70% over two steps; (iv)
Et3N, CH3SO2Cl, CH2Cl2, 3 h, 98%; (v) DMAP (cat.), H2CO, THF/H2O
(1:1), 40 °C, 24 h, 80%; (vi) TBSCl, DMAP (cat.), Et3N, CH2Cl2, 72 h,
98%; (vii) trimethylsulfoxonium iodide, NaH, DMSO, 48 h, 49%; (viii)
TBAF, THF, 0 °C, 98%; (ix) crotonic anhydride, pyridine, DMAP (cat.),
CH2Cl2, rt, 6 h, 46%; (x) TFA/H2O (7:1), 0 °C, 30 min, then HPLC, 99%.
O
O
O
O
O
O
HO
HO
10
11
Fig. 3.

2412
C. L. Arthurs et al. / Tetrahedron Letters 49 (2008) 2410–2413
O
resulting vicinal diol gave b-hydroxy ketone 22. Dehydra-
O
i)
tion of 22 to give enone 23 could be achieved at slightly
elevated temperature using methanesulfonyl chloride and
Et3N, however the reaction was sluggish and the transfor-
mation was better accomplished at room temperature via
an intermediate trifluoromethanesulfonate and in the pres-
ence of pyridine. Catalytic hydrogenation of 23 proceeded
with complete chemoselectivity to give cyclohexanone 24
and subsequent base-mediated elimination of the cyclo-
hexylidene protecting group with concomitant silylation
gave 25, which was identical in all respects to the material
reported by Danishefsky and co-workers.11 DMAP-cataly-
sed Morita–Baylis–Hillman reaction of 25 with formalde-
hyde yielded hydroxymethyl compound 26 which was
converted in the usual manner (vide supra) to target
O
O
H3CO
H3CO
O
O
OCH3
OCH3
ii)
14
13
iii), iv)
O
O
OH
TBSO
HO
18
17
v), vi)
10
Scheme 3. Reagents and conditions: (i) trimethylsulfoxonium iodide,
NaH, DMSO, rt, 1.5 h, 61%; (ii) TFA/H2O (7:1), rt, 12 h, then K2CO3,
H2O, CH3OH, rt, 30 min, 99%; (iii) TBSCl, DMAP (cat.), Et3N, CH2Cl2,
rt, 6 d, 78%; (iv) imidazole (cat.), H2CO, THF, Na2CO3(aq), 40 °C, 7 d,
23%; (v) crotonic anhydride, pyridine, DMAP (cat.), CH2Cl2, rt, 24 h,
48%; (vi) TFA/H2O (7:1), 0 °C, 1 h, 94%.
20
compound 11 (½aꢁD ꢀ23.0 (c 0.45, CH2Cl2)). The enantio-
meric purity of this material was confirmed by chiral
GC analysis using the essentially racemic compound 75a
for comparison.
enone 14 with Corey’s ylid (Scheme 3). Overnight exposure
of 13 to aqueous TFA resulted in eliminative acetal
removal to give alcohol 17 in good yield, and subsequent
silylation of the free hydroxyl group in 17 followed by
imidazole-catalysed M–B–H reaction9 then gave hydroxy-
methyl compound 18. Esterification of the primary hydro-
xyl of 18 followed by acid-catalysed removal of the silyl
protecting group provided target compound 10 in ten steps
from (ꢀ)-quinic acid (8).10
The results of bioassays of the analogues of COTC
against two lung cancer cell lines (A549 and H460) are
shown in Table 1. The assays were carried out by exposing
cells to varying concentrations of each compound for 4
days. The number of surviving cells was then determined
by the use of the MTT assay.12
The results of bioassay of the novel compounds 9–11
allow several conclusions to be reached regarding the struc-
tural features influencing the anti-cancer properties of
COTC analogues towards the two cell lines: (i) an a,b-
enone moiety appears to be vital for anti-cancer activity
(data for 9); (ii) the absolute configuration at C4 has no
observable influence on potency (comparative data for 7
and 11); (iii) incorporation of an additional electrophilic
site does not improve potency (comparative data for 10
and 7/11).
In conclusion, we have demonstrated the versatility of
(ꢀ)-quinic acid as a starting material for the synthesis of
analogues of the anti-tumour agent COTC. Three novel
target compounds have been prepared which were selected
in order to probe several important structure activity
features. The results of bioassay of the analogues provide
useful information which will guide future researchers in
The key intermediate in the synthesis of our final target
11 was TBS-ether 25, a large-scale synthesis of which has
been described by Danishefsky and co-workers.11 During
the course of these workers’ synthesis, isopropylidene pro-
tection was employed for the vicinal cis-diol in (ꢀ)-quinic
acid. In our hands, although this synthetic route was
successful, it proved to be a little capricious with regard
to the reproducibility of yields as well as the ease of isola-
tion of some intermediates. We opted, therefore, to modify
the route to 25 and to utilise cyclohexylidene protection for
the cis-diol moiety in 8 (Scheme 4).
Cyclohexylidene quinide 21 was prepared from 8 in
standard fashion and subsequent reductive ring-opening
of the c-lactone followed by oxidative cleavage of the
OH
O
O
O
O
O
O
O
O
O
v)
i)
ii), iii)
iv)
O
8
O
O
OH
O
21
22
23
24
vi)
HO
O
O
viii), ix)
TBSO
vii)
11
TBSO
26
25
Scheme 4. Reagents and conditions: (i) cyclohexanone, DMF/C6H6 (1:1), Amberlite IR 120 (H)Ò, reflux (Dean and Stark), 5 h, 60%; (ii) NaBH4, CH3OH,
0 °C to rt, 24 h; (iii) NaIO4, H2O, CH3OH, 0 °C, 1.5 h, 58% from 21; (iv) (CF3SO2)2O, pyridine, CH2Cl2, 0 °C to rt, 12 h, 82%; (v) H2, 10% Pd on C,
EtOAc, rt, 17 h, 92%; (vi) DBU, TBSCl, C6H6, reflux, 6 h, 80%; (vii) DMAP (cat.), H2CO, THF/H2O (1:1), 40 °C, 24 h, 52%; (viii) crotonic anhydride,
pyridine, DMAP (cat.), CH2Cl2, rt, 1.5 h, 68%; (ix) TFA/H2O (7:1), 0 °C, 1 h, 98%.

C. L. Arthurs et al. / Tetrahedron Letters 49 (2008) 2410–2413
2413
Table 1
Mannervik, B.; Houk, K. N.; Ganem, B.; Creighton, D. J. J. Am.
Chem. Soc. 2003, 125, 15049–15058; (f) Joseph, E.; Ganem, B.;
Eiseman, J. L.; Creighton, D. J. J. Med. Chem. 2005, 48, 6549–6552;
(g) Zheng, Z.-B.; Zhu, G.; Tak, H.; Joseph, E.; Eiseman, J. L.;
Creighton, D. J. Bioconjugate Chem. 2005, 16, 598–607.
Bioactivity of COTC analogues towards lung cancer cell lines
Compound
IC50 (lM)
A549
H460
COMC (2)5a
55
147
170
24
>200
18
40
158
158
10
>200
20
5. (a) Arthurs, C. L.; Wind, N. S.; Whitehead, R. C.; Stratford, I. J.
Bioorg. Med. Chem. Lett. 2007, 17, 553–557; (b) Arthurs, C. L.;
Raftery, J.; Whitby, H. L.; Whitehead, R. C.; Wind, N. S.; Stratford,
I. J. Bioorg. Med. Chem. Lett. 2007, 17, 5974–5977.
55a
65b
75a
9
6. Gebauer, O.; Bruckner, R. Liebigs Ann. 1996, 1559–1563.
¨
7. Rezgui, F.; El Gaied, M. M. Tetrahedron Lett. 1998, 39, 5965–5966.
8. Selected analytical data for compound 9: A clear oil; ½aꢁ2D2 +101.5 (c 1.3,
CH2Cl2); mmax (film)/cmꢀ1 3434 (br s, OH), 2099 (w, CH), 1644 (br s,
C@O); dH (300 MHz; CDCl3) 1.22 (1H, dd, J 8.4, 6.3, cyclopropyl
CHexo), 1.43 (1H, t, J 6.0, cyclopropyl CHendo), 1.91 (3H, dd, J 6.9,
1.8, CH3CH@CH), 2.05–2.11 (1H, m, C(3)H), 2.31 (1H, dd, J 18.0,
10.2, C(6)Hb), 2.35 (2H, br, 2 ꢂ OH), 2.82 (1H, dd, J 18.0, 5.7,
C(6)Ha), 3.79–3.87 (1H, m, C(5)H), 3.95 (1H, d, J 11.7, one of
CH2OC@O), 4.14 (1H, q, J 4.2, C(4)H), 4.73 (1H, d, J 11.7, one of
CH2OC@O), 5.86 (1H, dq, J 15.6, 1.8, CH3CH@CH), 7.00 (1H, dq, J
15.6, 6.9, CH3CH@CH); dC (75 MHz; CDCl3) 13.8 (cyclopropyl
CH2), 18.0 (CH3CH@CH), 26.5 (C(3)H), 34.8 (C(2)), 43.4 (C(6)H2),
64.7 (CH2OC@O), 68.4 (C(5)H), 70.4 (C(4)H), 122.2 (CH3CH@CH),
145.4 (CH3CH@CH), 166.3 (CH2OC@O), 222.1 (C(1)O); m/z (CI/
NH3) 258 (MNH4þ, 10%), 241 (MH+, 100); (Found: 241.2523,
C12H17O5 requires 241.2524).
10
11
17
11
the design and synthesis of compounds belonging to this
structural class with improved anti-cancer activity.
Acknowledgements
We acknowledge, with thanks, the EPSRC (C.L.A.),
BBSRC (M.P.) and the MRC (I.S. and N.S.W) for funding
and AstraZeneca for the generous provision of a vacation
bursary (K.F.L.).
9. Luo, S.; Wang, P. G.; Chen, J.-P. J. Org. Chem. 2004, 69, 555–558.
References and notes
20
10. Selected analytical data for compound 10: A clear oil; ½aꢁD ꢀ84.2 (c
0.57, CH2Cl2); mmax(film)/cmꢀ1 3424 (br, OH), 1713 (s, C@O), 1666 (s,
C@O); dH (300 MHz; CDCl3) 1.11–1.34 (2H, m, cyclopropyl CH2),
1.93 (3H, dd, J 6.0, 3.0, CH3CH@CH), 2.07–2.12 (2H, m, C(5)H and
C(6)H), 2.60 (1H, br, OH), 4.72–4.87 (2H, m, CH2OC@O), 4.96–4.98
(1H, m, C(4)H), 5.90 (1H, dq, J 15.0, 3.0, CH3CH@CH), 6.35–6.36
(1H, m, C(3)H), 7.05 (1H, dq, J 15.0, 6.0, CH3CH@CH); dC (75 MHz;
CDCl3) 11.3 (cyclopropyl CH2), 18.4 (CH3CH@CH), 18.9 (C(5)H or
C(6)H), 24.3 (C(5)H or C(6)H), 60.7 (CH2OC@O), 64.9 (C(4)H),
122.3 (CH3CH@CH), 130.7 (C(2)), 141.9 (C(3)H), 146.1
(CH3CH@CH), 166.3 (CH2OC@O), 195.6 (C(1)O); m/z (CI/NH3)
240 (MNH4þ, 15%), 223 (MH+, 100); (Found: 223.0969, C12H15O4
requires 223.0965).
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