A concise approach to benzoic acid derivatives bearing an α,β-unsaturated ketone substituent: Synthesis of methyl taboganate

Article abstract of DOI:10.1055/s-2005-921933

A concise route to benzoic acid derivatives bearing an α,β- unsaturated ketone moiety and its application to the synthesis of the natural product methyl taboganate (1) is described. The methodology is based on the addition of a Grignard reagent to an appr

Full text of DOI:10.1055/s-2005-921933

LETTER
3131
A Concise Approach to Benzoic Acid Derivatives Bearing an a,b-Unsaturated
Ketone Substituent: Synthesis of Methyl Taboganate
S
ynthesis of Methy
n
l
T
aboganat
a
e
stasia Detsi,a Vassilios Roussis,b Andrew Tsotinis,c Christos Roussakis,d Theodora Calogeropoulou*a
a
Institute of Organic and Pharmaceutical Chemistry, National Hellenic Research Foundation, 48 Vas. Constantinou Ave., 116 35 Athens,
Greece
Fax +30(210)7273831; E-mail: tcalog@eie.gr
b
University of Athens, Faculty of Pharmacy, Dept. of Pharmacognosy and Chemistry of Natural Products, Panepistimiopoli Zografou,
15771 Athens, Greece
c
University of Athens, Faculty of Pharmacy, Dept. of Pharmaceutical Chemistry, Panepistimiopoli Zografou, 15771 Athens, Greece
Laboratoire de Pharmacologie Marine, Faculté de Pharmacie, 1 rue G. Veil, 44035 Nantes, France
d
Received 12 October 2005
were isolated from the leaves of P. murrayanum, an en-
Abstract: A concise route to benzoic acid derivatives bearing an
a,b-unsaturated ketone moiety and its application to the synthesis of
the natural product methyl taboganate (1) is described. The method-
ology is based on the addition of a Grignard reagent to an appropri-
ately substituted salicylaldehyde followed by oxidation of the
resulting allylic alcohol to the corresponding a,b-unsaturated ke-
tone. The new compounds were tested for their cytotoxic activity.
demic species of the Jamaican flora.4
More recent examples of natural products of this structure
include lanceaefolic acid (5) and its methyl ester (6),
which possesses antifungal properties, both isolated from
the methanol extract of the leaves of the species P. lance-
aefolium,5 and aduncumene (7), which exhibits antifungal
activity and was identified among the major secondary
metabolites from P. aduncum.6
Key words: methyl taboganate, a,b-unsaturated ketones, cytotox-
icity
OH
O
The genus Piper (Piperaceae) includes over 700 species,
which are distributed all over the world and especially in
tropical and subtropical regions. They have been the sub-
ject of extensive phytochemical investigations since they
are a rich source of new natural products, such as benzoic
acids, amides, lignans, phenols, chromenes, flavonoids
and triterpenes, possessing potential antitumor, antimicro-
bial, antifungal and insecticidal activities.1 Members of
the Piperaceae family have been used in the folklore med-
icine of Central and South America: the Kuna Indians of
Panama traditionally use the plant Piper dilatatum as a
constituent in a tonic bath for various diseases, while a de-
coction of the leaves of Piper lancaefolium Kunth is used
in southern Colombia to treat skin infections.
COOR
methyl taboganate, R = CH3 (1)
taboganic acid, R = H (2)
OH
O
OH
O
3
COOH
HO
4
COOH
CH3O
O
OH
O
The structurally diverse bioactive compounds isolated
from plants of the genus Piper include several benzoic
acid derivatives bearing an a,b-unsaturated ketone sub-
stituent. In 1990 methyl taboganate (1, Figure 1), a
prenylated compound showing significant activity against
the leafcutter repellency bioassay, was isolated from the
leaves of the Panamanian shrub P. taboganum.2 A few
years later, in 1998, the same compound was identified
from the dichloromethane extract of the leaves of P. di-
latatum along with the corresponding taboganic acid (2).3
Both exhibited antifungal activity against the plant patho-
genic fungus Cladosporium cucumerinum. In addition,
the E- and Z-isomers of substituted benzoic acids 3 and 4,
COOCH3
COOR
lanceaefolic acid, R = H (5)
aduncumene (7)
lanceaefolic acid methyl ester, R = CH3 (6)
Figure 1 Benzoic acid derivatives bearing an a,b-unsaturated
ketone substituent isolated from species of the genus Piper
The interesting antifungal properties of compounds 1–7
prompted us to develop a synthetic methodology towards
methyl taboganate and other benzoic acid analogues bear-
ing the a,b-unsaturated ketone moiety. We could thus gain
access to analogues in order to probe the structural re-
quirements for biological activity in compounds of this
type.
Our initial efforts involved the route depicted in
Scheme 1. Methylation of 3-acetyl-4-hydroxybenzoic
acid (9), obtained by Fries rearrangement of 4-(acetyl-
SYNLETT 2005, No. 20, pp 3131–3135
Advanced online publication: 28.11.2005
DOI: 10.1055/s-2005-921933; Art ID: G33205ST
© Georg Thieme Verlag Stuttgart · New York
1
9
.1
2
.2
0
0
5

3132
A. Detsi et al.
LETTER
oxy)benzoic acid (8), gave the acetophenone derivative the corresponding allylic alcohols 19–21.12a,b The reaction
10. Mukaiyama aldol reaction of the corresponding tri- proceeded smoothly and, after approximately one hour,
methylsilyl enol ether 11 with acetone yielded the b-hy- was quenched at –78 °C using saturated aqueous NH4Cl
droxy ketone 12 which upon dehydration using solution. The applicability of our methodology was dem-
triethylamine and thionyl chloride provided the a,b-unsat- onstrated by using three different Grignard reagents,
urated ketone 13.7 However, deprotection of the hydroxyl which were either commercially available [e.g., 2-methyl-
group using a variety of conditions (BBr3, sodium ethane 1-propenyl-magnesium bromide, (16)] or freshly pre-
thiolate, iodo-BBN) led to 2,2-dimethyl-6-carboxychro- pared [(Z)-1-propenylmagnesium bromide (17) and vinyl-
man-4-one (14)2 instead of methyl taboganate (1). Further benzenemagnesium bromide (18)] using a slightly
attempts, using other protecting groups, were also unsuc- modified literature procedure.13 In the case of 18, the start-
cessful. However, we were gratified to find that com- ing b-bromostyrene contained approximately 15% of the
pound 13 exhibited significant antiproliferative and Z-isomer.
cytostatic activity in vitro, inducing irreversible prolifera-
OH
O
tion arrest in G1 phase of two non small lung cancer lines
R1
R2
namely, NSCLC-N6 and NSCLC-derived A549 cells.8,9
H
(i)
+
BrMg
O
16 R1 = R2 = CH3
COOCH3
17 R1 = CH3, R2 = H (Z only)
O
OH
O
OCH3 O
15
18 R1 = C6H5, R2 = H (E:Z = 5:1)
(i)
(ii)
R1
R1
R2
OH
O
OH OH
COOH
COOH
COOCH3
R2
(ii)
8
9
10
OSi(CH3)3
OCH3
OCH3 O
OH
COOCH3
COOCH3
19 R1 = R2 = CH3
20 R1 = CH3, R2 = H
21 R1 = C6H5, R2 = H
1
R1 = R2 = CH3
(iii)
(v)
(iv)
22 R1 = CH3, R2 = H (Z:E = 9/1)
23 R1 = C6H5, R2 = H (E:Z = 3/1)
COOCH3
12
COOCH3
11
Scheme 2 Synthetic approach to benzoic acid analogues bearing an
a,b-unsaturated ketone substituent. Reagents and conditions: (i) THF,
–78 °C, 1 h; (ii) MnO2, MeCN, 24 h
O
O
OCH3
(vi)
O
Attempted purification of allylic alcohol 19 by flash col-
umn chromatography on silica gel resulted to a mixture of
the more stable, conjugated allylic transposition product
24 and methyl 2,2-dimethyl-2H-1-benzopyran-6-
carboxylate14,15 (25, Scheme 3). The same was observed
in CDCl3 either due to its slightly acidic character or to the
presence of free radicals. This transformation has been
previously observed for analogous compounds upon acid-
ic treatment.16,17
COOCH3
COOCH3
14
13
Scheme 1 Synthetic route to the methylated analogue of methyl ta-
boganate, 13. Reagents and conditions: (i) AlCl3, 155 °C; (ii) MeI,
K2CO3, DMF; (iii) TMSCl, Et3N, NaI, MeCN, 35 °C; (iv)TiCl4,
CH3COCH3, CH2Cl2; (v) SOCl2, Et3N, CH2Cl2, 0 °C; (vi) BBr3 or so-
dium ethane thiolate or iodo-BBN
These results gave us an additional impetus to develop a
concise, efficient synthesis, for this class of compounds.
Thus, we evolved a two-step synthesis, consisting of the
addition of a Grignard reagent to an appropriately substi-
tuted salicylaldehyde, followed by oxidation of the result-
ing alcohol to the desired product (Scheme 2).
OH OH
O
OH
OH
H+
H+
COOCH3
COOCH3
COOCH3
19
25
24
H+
The required 5-carbomethoxysalicylaldehyde (15) was
synthesized via the Duff reaction, suitable for the prepara-
tion of aromatic aldehydes bearing free hydroxyl
groups.10,11 Monoformylation of methyl p-hydroxyben-
zoate to aldehyde 15 was achieved in 35% yield by heat-
ing one equivalent of methyl p-hydroxybenzoate and one
equivalent of hexamethylenetetramine (HMTA) in neat
TFA at 70 °C overnight. Attempts to improve the yield of
the reaction (longer reaction times, reflux) were not suc-
cessful. Slow addition of a THF solution of the Grignard
reagents 16–18 (2.5 equiv) to a solution of 5-car-
bomethoxy-salicylaldehyde (15, 1 equiv) at –78 °C gave
Scheme 3 Allylic transposition of alcohol 19 and cyclization to
compound 25
Thus, the crude allylic alcohols 19–21 were subjected im-
mediately to oxidation, using an excess of MnO2 in MeCN
to yield the desired a,b-unsaturated ketones 1, 22 and 23
in 40–50% yields (calculated on the basis of the starting
salicylaldehyde).18 Chalcone 23 was isolated as a mixture
of the corresponding geometrical isomers in a ratio of
E:Z = 3:1, as calculated from the 1H NMR spectrum. The
Synlett 2005, No. 20, 3131–3135 © Thieme Stuttgart · New York

LETTER
Synthesis of Methyl Taboganate
3133
major E-isomer was separated from the mixture by HPLC
analysis using a chiral stationary phase.19 In the case of
compound 22 a small amount of the E-isomer was also ob-
tained along with the expected Z-isomer (Z:E = 9:1).
References
(1) Parmar, V. S.; Jain, S. C.; Bisht, K. S.; Jain, R.; Taneja, P.;
Jha, A.; Tyagi, O. D.; Prasad, A. K.; Wengel, J.; Olsen, C.
E.; Boll, P. M. Phytochemistry 1997, 46, 597.
(2) Roussis, V.; Ampofo, S. A.; Wiemer, D. F. Phytochemistry
1990, 29, 1787.
(3) Terreaux, C.; Gupta, M. P.; Hostettmann, K. Phytochemistry
1998, 49, 461.
(4) Seeram, N. P.; Jacobs, H.; McLean, S.; Reynolds, W. F.
Phytochemistry 1996, 43, 863.
(5) Lopez, A.; Ming, D. S.; Neil Towers, G. H. J. Nat. Prod.
2002, 65, 62.
In order to examine the effect of the prenyl moiety on cy-
tostatic activity, we hydrogenated methyl taboganate us-
ing H2 and 10% Pd/C as a catalyst at one atmosphere
(Scheme 4). The reaction proceeded smoothly in one hour
providing analogue 26, lacking the a,b-double bond.20
OH
O
OH O
(6) Lago, J. H. G.; Ramos, C. S.; Casanova, D. C. C.; de
Morandim, A. A.; Bergamo, D. C. B.; Cavalheiro, A. J.; da
Bolzani, V. S.; Furlan, M.; Guimaraes, E. F.; Young, M. C.
M.; Kato, M. J. J. Nat. Prod. 2004, 67, 1783.
H2, 10% Pd/C
EtOAc, 1 atm
COOCH3
COOCH3
(7) 1-(2-Methoxy-5-carboxymethylphenyl)-1-trimethyl-
silyloxyethylene (11).
1
26
3-Acetyl-4-methoxybenzoic acid methyl ester (10, 4.16 g,
0.02 mol) was dissolved in Et3N (3.8 mL, 0.027 mol)
followed by the addition of TMSCl (3.4 mL, 0.027 mol) and
the resulting solution was heated at 35 °C. A solution of NaI
(4.05 g, 0.027 mol) in MeCN (25 mL) was added at such a
rate to keep the temperature at 35–40 °C without external
heating. After completion of the addition, the mixture was
stirred at r.t. for 2 h and was subsequently poured onto cold
H2O (350 mL) and extracted with Et2O. The organic layer
was dried (K2CO3) and the solvent was evaporated in vacuo
to afford 1-(2-methoxy-5-carboxymethylphenyl)-1-
trimethylsilyloxyethylene (11) as an oil (5.13 g, 92%). 1H
NMR (300 MHz, CDCl3): d = 8.18 (d, J = 2.14 Hz, 1 H),
7.92 (dd, J = 8.73, 2.17 Hz, 1 H), 6.89 (d, J = 8.71 Hz, 1 H),
4.98 (s, 1 H), 4.68 (s, 1 H), 3.90 (s, 3 H), 3.85 (s, 3 H), 0.25
(s, 9 H).
Scheme 4 Hydrogenation of methyl taboganate (1)
The new compounds were evaluated against three cell
lines namely, NSCLC-N6-L16, A549 and SRA.21 As
shown in Table 1, methyl taboganate (1), 22, and the hy-
drogenated derivative 26 did not show any activity at the
concentrations tested (<30 mg/mL). The methylated deriv-
ative 13, as previously reported, was active against all
three cell lines, while the new chalcone analogue 23 was
even more potent than 13. The inhibitory effects of 13 and
23 were dose-dependent and only observed after continu-
ous drug exposure. In addition, this effect is irreversible
since similar results were obtained with discontinuous ex-
posure of the cells to the two compounds.
1-(2-Methoxy-5-carboxymethylphenyl)-3-hydroxy-3-
methyl-1-butanone (12).
Table 1 IC50 Values (mg/mL), after Continuous Cell Exposure to
TiCl4 (2 mL, 0.018 mol) was added to ice-chilled CH2Cl2 (26
mL) followed by the dropwise addition of a solution of
CH3COCH3 (1.5 mL, 0.021 mol) in CH2Cl2 (5.5 mL). Upon
completion of the addition a solution of 1-(2-methoxy-5-
carboxymethylphenyl)-1-trimethylsilyloxyethylene (11,
5.13 g, 0.018 mol) in CH2Cl2 (2.5 mL) was added and the
resulting solution was stirred for 15 min. The reaction
mixture was poured onto ice water with vigorous stirring and
the organic layer was separated. The aqueous layer was
extracted with CH2Cl2. The combined organic extracts were
washed with a 1:1 mixture of sat. aq NaHCO3 and H2O, brine
and were dried (Na2SO4). The solvent was evaporated in
vacuo and the residue was purified by flash column
chromatography (PE–EtOAc, 1:1) to afford 1-(2-methoxy-
5-carboxymethylphenyl)-3-hydroxy-3-methyl-1-butanone
(12) as a viscous oil (3.35 g, 69%). 1H NMR (300 MHz,
CDCl3): d = 8.31 (d, J = 2.14 Hz, 1 H), 8.15 (dd, J = 8.73,
2.18 Hz, 1 H), 6.99 (d, J = 8.71 Hz, 1 H), 3.96 (s, 3 H), 3.90
(s, 3 H), 3.15 (s, 2 H), 1.31 (s, 6 H).
the Compoundsa
IC50 (mg/mL)
Compound
NSCLC-N6-L16
Inactive
16.3
A549
SRA
1
Inactive
12.7
Inactive
12.4
13
22 (Z:E = 9:1)
23 (E:Z = 3:1)
26
Inactive
<3.3
Inactive
<3.3
Inactive
6.9
Inactive
Inactive
Inactive
a IC50 is the concentration of the compound required to reduce cell vi-
ability by 50%.
In conclusion, a concise approach to benzoic acid deriva-
tives bearing an a,b-unsaturated ketone substituent has
been developed giving access to new derivatives with cy-
tostatic activity. Further studies to evaluate the biological
activity of the new compounds and the impact of the dou-
ble bond geometry are in progress.
Methyl 4-Methoxy-3-(3-methyl-2-butenoyl)benzoate
(13).
To an ice-cold solution of 1-(2-methoxy-5-carboxymethyl-
phenyl)-3-hydroxy-3-methyl-1-butanone (12, 2.85 g, 0.012
mol) in Et3N (5.3 mL, 0.038 mol) and CH2Cl2 (5.3 mL) was
added dropwise SO2Cl (1.60 mL, 0.022 mol) during 30 min.
The mixture was then stirred for 2 h, poured onto 53 g of
crushed ice and extracted with EtOAc. The combined
organic extracts were washed with 1 M HCl, H2O, sat. aq
NaHCO3, brine, then dried (Na2SO4), and concentrated
under vacuum. The crude product was purified by flash
column chromatography (PE–EtOAc, 85:15) as eluent to
Synlett 2005, No. 20, 3131–3135 © Thieme Stuttgart · New York

3134
A. Detsi et al.
LETTER
give the title compound (2.53 g, 90%) as an off-white solid;
mp 47–48 °C. Anal. Calcd for C14H16O4: C, 67.73; H, 6.50.
Found: C, 68.01; H, 6.64. 1H NMR (300 MHz, CDCl3): d =
8.19 (d, J = 2.20 Hz, 1 H), 8.08 (dd, J = 8.70, 2.20 Hz, 1 H),
6.95 (d, J = 8.75 Hz, 1 H), 6.53 (d, J = 2.50 Hz, 1 H), 3.91
(s, 3 H), 3.88 (s, 3 H), 2.21 (s, 3 H), 1.96 (s, 3 H). 13C NMR
(75 MHz, CDCl3): d = 191.8, 166.3, 161.0, 156.1, 133.8,
131.8, 131.1, 124.9, 122.8, 111.3, 56.0, 51.9, 27.9, 21.3.
4-Hydroxy-3-(3-methylbut-2-enoyl)benzoic Acid Methyl
Ester (Methyl Taboganate, 1).
According to the general procedure from 62 mg of crude
allylic alcohol 19 (obtained from 50 mg, 0.28 mmol of
aldehyde 15), 1 was obtained (25 mg, 40% overall yield,
after purification by flash column chromatography using
PE–EtOAc, 92:8) as a light yellow crystalline solid; mp 92–
94 °C (lit.2 mp 91–95 °C). 1H NMR (300 MHz, CDCl3): d =
13.30 (s, 1 H), 8.46 (d, J = 1.83 Hz, 1 H), 8.04 (dd, J = 8.80,
2.10 Hz, 1 H), 6.94 (d, J = 8.50 Hz, 1 H), 6.84 (s, 1 H), 3.87
(s, 3 H), 2.21 (s, 3 H), 2.05 (s, 3 H).
(8) Tsotinis, A.; Calogeropoulou, T.; Roussakis, C.; Roussis, V.
PCT/EP99.02835, WO99/54278, 1999; Chem. Abstr. 1999,
131, 286260.
(9) Carbonelle, D.; Jaquot, C.; Lanco, X.; le Dez, G.; Tomasoni,
C.; Briand, G.; Tsotinis, A.; Calogeropoulou, T.; Roussakis,
C. Int. J. Cancer 2001, 92, 388.
(10) Lindoy, L. F.; Meecham, G. V.; Svenstrup, N. Synthesis
1998, 1029.
(11) Diaz, P.; Michel, S.; Charpentier, B. Bioorg. Med. Chem.
Lett. 1997, 7, 2289.
(12) (a) Shimizu, H.; Okamura, H.; Iwagawa, T.; Nakatami, M.
Tetrahedron 2001, 57, 1903. (b) General Procedure for
the Synthesis of Allylic Alcohols 19–21.
3-But-2-enoyl-4-hydroxybenzoic Acid Methyl Ester (22).
Following the general procedure, from 220 mg of crude
alcohol 20 (obtained from 150 mg, 0.83 mmol of aldehyde
15), 22 was obtained as a yellow solid [90 mg, 50% overall
yield after purification by flash column chromatography
(PE–EtOAc, 90:10)]. Anal. Calcd for C12H12O4: C, 65.45; H,
5.49. Found: C, 65.42; H, 5.33. Ratio of cis:trans = 9:1. 1H
NMR (300 MHz, CDCl3): d = 13.18 (s, 0.9 H), 13.07 (s, 0.1
H), 8.55 (d, J = 1.83 Hz, 0.9 H), 8.50 (d, J = 2.43 Hz, 0.1 H),
8.10 (dd, J = 8.85, 2.13 Hz, 1 H), 7.33–7.11 (m, 1.8 H),
7.04–6.94 (m, 0.1 H), 6.98 (d, J = 8.55 Hz, 1 H), 6.61–6.52
(m, 0.1 H), 3.93 (s, 2.7 H), 3.90 (s, 0.3 H), 2.18 (dd, J = 7.32,
1.83 Hz, 0.3 H), 2.06 (d, J = 6.09 Hz, 2.7 H). 13C NMR (75
MHz, CDCl3): d = 193.8, 167.2, 166.1, 147.3, 136.9, 132.6,
132.4, 125.0, 124.9, 123.5, 120.8, 118.8, 118.7, 52.2, 18.8.
4-Hydroxy-3-(3-phenylacryloyl)benzoic Acid Methyl
Ester (23).
To a 0.07 M solution of 3-formyl-4-hydroxy-benzoic acid
methyl ester (15, 1 equiv) in THF was added dropwise a
solution of the appropriate Grignard reagent 16–18 (0.5 M in
THF, 5 equiv) at –78 °C over a period of 10 min. After
stirring for 1 h, the reaction was quenched with sat. NH4Cl
at –78 °C, the mixture was extracted with EtOAc (3 × 15
mL), dried (Na2SO4) and the solvents were evaporated in
vacuo. The yellowish oily crude product was used for the
next reaction without purification.
According to the general procedure from 200 mg of crude
allylic alcohol 21 (obtained from 60 mg, 0.33 mmol of
aldehyde 15), the a,b-unsaturated ketone 23 was obtained as
a bright yellow solid [38 mg, 41% overall yield after
purification by flash column chromatography (PE–EtOAc,
95:5)]. Anal. Calcd for C17H14O4: C, 72.33; H, 5.00. Found:
C, 72.05; H, 5.21. Ratio of trans:cis = 3:1. 1H NMR (300
MHz, CDCl3): d = 13.32 (s, 0.75 H), 12.68 (s, 0.25 H), 8.67
(d, J = 1.83 Hz, 0.75 H), 8.49 (d, J = 2.43 Hz, 0.25 H), 8.15
(dd, J = 8.55, 1.83 Hz, 0.75 H), 8.10 (dd, J = 8.55, 2.13 Hz,
0.25 H), 7.98 (d, J = 15.00 Hz, 0.75 H), 7.76 (d, J = 15.00
Hz, 0.75 H), 7.73–7.71 (m, 2 H), 7.47–7.45 (m, 2 H), 7.29–
7.27 (m, 1 H), 7.13 (d, J = 12.81 Hz, 0.25 H), 7.05 (d,
J = 8.52 Hz, 0.75 H), 7.01 (d, J = 8.55 Hz, 0.25 H), 6.73 (d,
4-Hydroxy-3-(1-hydroxy-3-methylbut-2-enyl)benzoic
Acid Methyl Ester (19).
1H NMR (300 MHz, CD3COCD3): d = 7.91 (d, J = 2.40 Hz,
1 H), 7.79–7.76 (m, 1 H), 6.88 (d, J = 8.50 Hz, 1 H), 5.79 (d,
J = 9.15 Hz, 1 H), 5.41 (d, J = 9.15 Hz, 1 H), 3.83 (s, 3 H),
1.81 (s, 3 H), 1.73 (s, 3 H).
4-Hydroxy-3-(1-hydroxybut-2-enyl)benzoic AcidMethyl
Ester (20).
1H NMR (300 MHz, CD3COCD3): d = 7.96 (d, J = 2.40 Hz,
1 H), 7.78 (dd, J = 8.50, 2.40 Hz, 1 H), 6.89 (d, J = 8.50 Hz,
1 H), 5.92–5.83 (m, 1 H), 5.62–5.58 (m, 1 H), 3.84 (s, 3 H),
1.79 (d, J = 5.50 Hz, 3 H).
4-Hydroxy-3-(1-hydroxy-3-phenylallyl)benzoic Acid
Methyl Ester (21).
J = 12.81 Hz, 0.25 H), 3.94 (s, 2.25 H), 3.88 (s, 0.75 H). 13
NMR (75 MHz, CDCl3): d = 193.5, 167.2, 146.7, 166.1,
137.0, 134.3, 132.2, 131.3, 129.4, 129.1, 129.0, 128.4,
120.9, 119.4, 118.8, 52.2.
C
1H NMR (300 MHz, CD3COCD3): d = 8.07 (br s, 1 H),
7.83–6.55 (m, 10 H), 5.98–5.76 (m, 1 H), 3.82 (br s, 3 H).
(13) Chen, C.; Layton, M. E.; Shair, M. D. J. Am. Chem. Soc.
1998, 120, 10784.
(19) trans-4-Hydroxy-3-(3-phenylacryloyl)benzoic Acid
Methyl Ester.
(14) Alhuwalia, V. K.; Jolly, R. S.; Tehim, A. K. Tetrahedron
1982, 38, 3673.
(15) Diaz, P. P. D.; Arias, T. C.; Joseph-Nathan, P.
Phytochemistry 1987, 26, 809.
(16) Talley, J. H. Synthesis 1983, 845.
(17) Cardillo, G.; Cricchio, R.; Merlini, L.; Nasini, G. Gazz.
Chim. Ital. 1969, 99, 612.
Separated from the trans–cis mixture 25, using HPLC
(column DAICEL-CHIRACEL OD, 254 nm, 1.2 mL/min,
hexane–2-PrOH, 80:20, tR = 28 min). 1H NMR (300 MHz,
CDCl3): d = 13.31 (s, 1 H), 8.67 (d, J = 2.50 Hz, 1 H), 8.17
(dd, J = 9.15, 1.83 Hz, 1 H), 7.99 (d, J = 16.00 Hz, 1 H), 7.73
(d, J = 16.00 Hz, 1 H), 7.73–7.70 (m, 2 H), 7.48–7.45 (m, 3
H), 7.06 (d, J = 9.15 Hz, 1 H), 3.94 (s, 3 H).
(18) General Procedure for the Synthesis of a,b-Unsaturated
(20) Hydroxy-3-(3-methylbutyryl)benzoic Acid Methyl Ester
(26).
Ketones.
The crude allylic alcohols 19–21 were dissolved in dry
MeCN (1.3% w/v), activated MnO2 85% (5-fold w/w) was
added in one portion. After stirring the reaction mixture at
r.t. overnight, it was filtered through Celite, which was
washed with EtOAc (4 × 20 mL). Evaporation of the
solvents followed by purification by flash column
chromatography afforded pure a,b-unsaturated ketones 1, 22
and 23, respectively.
To a solution of 1 (20 mg, 0.085 mmol) in EtOAc (2 mL)
was added 10% Pd/C (2 mg). The suspension was stirred
under 1 atm of H2 for 1 h. The reaction mixture was filtered
through Celite, which was washed with EtOAc, and the
filtrate was concentrated in vacuo to afford compound26 (17
mg, 85%) as an off-white solid; mp 43–44 °C. Anal. Calcd
for C13H16O4: C, 66.09; H, 6.83. Found: C, 66.12; H, 6.89.
1H NMR (300 MHz, CDCl3): d = 12.88 (s, 1 H), 8.49 (d,
Synlett 2005, No. 20, 3131–3135 © Thieme Stuttgart · New York

LETTER
J = 1.83 Hz, 1 H), 8.11 (dd, J = 8.83, 2.14 Hz, 1 H), 7.00 (d,
Synthesis of Methyl Taboganate
3135
and A549 obtained from ATCC collection reference
J = 8.55 Hz, 1 H), 3.91 (s, 3 H), 2.91 (d, J = 6.69 Hz, 2 H),
2.34–2.27 (m, 1 H), 1.02 (d, J = 6.72 Hz, 3 H). 13C NMR (75
MHz, CDCl3): d = 206.7, 166.2, 166.0, 136.9, 132.6, 120.9,
119.1, 118.7, 52.2, 30.9, 25.3, 22.7.
CCL6185, was used for all experiments. Cells were seeded
into 96-well plates (105 cells/mL for NSCLC-N6, 2 × 104
cells/mL for A549 and 3 × 104 cells/mL for SRA) and
exposed to various concentrations of the compounds for 72
h. The cytotoxicity was determined using the MTT dye
reduction assay of Mosmann: Mosmann, T. J. Immunol.
Methods 1983, 65, 55.
(21) Cytotoxicity Assay Protocol. Cell Lines and Culture.
The NSCLC-N6-L16 cell line, derived from a human non-
small-cell bronchopulmonary carcinoma (moderately
differentiated, rarely keratinizing, classified as T2N0M0),
Synlett 2005, No. 20, 3131–3135 © Thieme Stuttgart · New York