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F. Epifano et al. / Bioorg. Med. Chem. Lett. 21 (2011) 769–772
OH
COOMe
CHO
OH
a
F
F
a
HO
HO
X
X
8-13
I
CHO
b,c
3
4
Scheme 1. Reagents and conditions: (a) hexamethylenetetramine (1 equiv),
CF3COOH (1 equiv), 100 °C, 24 h.
COOH
O
OH
OH
X
14-19
Cl
a
Scheme 4. Reagents and conditions: (a) (MeO)3POCH2COOMe (3 equiv), Na
(3 equiv) MeOH, 70 °C, 24 h; (b) isopentenyl bromide (1 equiv), K2CO3 (1 equiv),
acetone , 80 °C, 2 h; (c) NaOH 2 N (aq), reflux, 30 min, acid/base workup
CHO
CHO
5
6
ears to quantify oedema as weight difference between the two
plugs. The anti-inflammatory activity was expressed as percent
of oedema reduction in mice treated with the compounds under
test with regard to control mice. Oedema values, expressed as
means standard error of the mean, were analysed by one way
analysis of variance followed by Dunnett’s test for multiple com-
parison of unpaired data. A probability level lower than 0.05 was
considered as significant. Experiments complied with the Italian
D.L. n. 116 of January 1992 and associated guidelines in the Euro-
pean Communities Council Directive of 24 November 1986 (86/609
ECC) concerning animal welfare and appendix A of the European
Convention ETS 123.
Scheme 2. Reagents and conditions: (a) NCS (1 equiv), CHCl3, 60 °C, 12 h.
Benzaldehydes substituted in position 3 with a CH3– and a
NO2– group were commercially available.
Synthesized and commercially available 3-substituted-4-hydro-
xybenzaldehyde were then submitted to a Wittig–Horner olefin-
ation9 by reaction with methyl trimethylphosphonacetate and Na
in MeOH at 70 °C for 24 h to obtain the corresponding cinnamic
acid methyl esters 8 (X = –F, 86% yield), 9 (X = –Cl, 99% yield), 10
(X = –Br, 99% yield), 11 (X = –I, 81% yield), 12 (X = –Me, 89% yield),
and 13 (X = –NO2, 99% yield).10 All these derivatives were in turn
alkylated in position 40 by reaction with isopentenyl bromide pro-
moted by K2CO3 as the base in acetone at 80 °C for 2 h, followed by
basic hydrolysis in the same vessel and acid/base workup to afford
pure isopentenyloxycinnamic acids 14 (X = –F, 76% yield), 15
(X = –Cl, 99% yield), 16 (X = –Br, 80% yield), 17 (X = –I, 71% yield),
18 (X = –Me, 87% yield), and 19 (X = –NO2, 97% yield) (Scheme 4).
Finally the last product, 3-amino-4-isopentenyloxycinnamic
acid 20 was obtained in 59% yield from the nitro-derivative 19
by reduction with SnCl2 in EtOH at 70 °C for 30 min. (Scheme 5).
All the semi-synthetic isopentenyloxycinnamic acids are re-
ported herein for the first time.
The results on the anti-inflammatory activity screening of com-
pounds 1, 2, and 14–20, administered at the dose of 0.3 l
mol/cm2,
are reported in the Table 1, in comparison to those of the same dose
of the non-steroidal anti-inflammatory drug (NSAID) indomethacin
(a cycloxygenases [COXs] inhibitor), and nordihydroguaiaretic acid
(a [5-LOX] inhibitor), used as references. Each compound provoked
a significant oedema inhibition, which ranged from 30% (2) to 78%
(17 and 19). The natural compounds, boropinic acid 1 and 40-isopen-
tenyloxycinnamic acid 2 induced 38% and 30% oedema reduction,
respectively. While the semi-synthetic derivatives 16 and 18 pro-
voked less than 20% oedema reduction, the other semi-synthetic
compounds induced oedema reductions ranging from 46% (20) to
78% (17 and 19). The reference compounds indomethacin and
nordihydroguaiaretic acid induced 62% and 36% oedema inhibition,
respectively.
The topical anti-inflammatory activity of compound 1 and its
derivatives 2, and 14–20 was evaluated as inhibition of the Croton
oil-induced ear dermatitis in mice.11
Thus, the natural compounds 1 and 2 as well as the most active
semi-synthetic ones (15, 17, 19, 20) were further investigated for
their dose–activity relationship, in comparison to indomethacin
and nordihydroguaiaretic acid.
Male CD-1 mice (28–32 g, Harlan Laboratories, Udine, Italy)
were anaesthetised with ketamine hydrochloride (145 mg/kg,
intraperitoneally; Virbac, Milan, Italy). Inflammation was induced
on the right ear (surface: about 1 cm2) by application of 80
lg of
As reported in the Table 2, boropinic acid 1 and its derivatives
Croton oil (Sigma Chemical Co., St. Louis, USA) dissolved in acetone.
The left ear remained untreated as preliminary experiments
showed that the vehicle did not affect the inflammatory response
or induce irritation. Control mice received only the irritant solu-
tion, whereas the others received both the irritant and the com-
pounds under test dissolved in acetone. Six hours later, mice
were sacrificed and a plug (6 mm Ø) was excised from both the
15, 17, 19 and 20 (0.1–1 l
mol/cm2) exerted a dose-dependent
anti-inflammatory effect. The relevant ID50 values (dose inducing
50% oedema inhibition), as index of anti-inflammatory potency,
showed that the anti-inflammatory activity of compound
(ID50 = 0.72
mol/cm2) was slightly increased by its demethoxyla-
tion to 2 (ID50 = 0.50
1
l
l
mol/cm2). In addition, the semi-synthetic
derivatives of 2 substituted in position 30 with a chlorine (15) or
iodine (17) atoms, but also with a nitro (19) or amino (20) func-
tional groups, were about twice more active than 2 (ID50 ranging
from 0.18 to 0.33
potency of these compounds was comparable to that of indometh-
acin (ID50 = 0.23
mol/cm2) and about twice higher than that of
nordihydroguaiaretic acid (ID50 = 0.54
mol/cm2).
OH
I
l
mol/cm2). Moreover, the anti-inflammatory
a
5
l
CHO
l
Depicting preliminary structure–activity relationships (SAR),
we have already revealed by docking studies that an unfuctional-
ized isopentenyloxy side chain in position 4 of the aromatic ring
7
Scheme 3. Reagents and conditions: (a) KI/I2 (1 equiv), NH4OH 30%, rt, 6 h.