2840 J. Agric. Food Chem., Vol. 44, No. 9, 1996
Jime´nez-Estrada et al.
Ta ble 1. Effect of P iqu er ol A (Ia ) a n d Com p ou n d s Ib, II,
IIIa , a n d IVa on th e Ra d icle Gr ow th of Am a r a n th u s
h ypoch on d r ia cu s a n d Ech in och loa cr u sga lli
radicle growth, %
compd
amount, ppm
A. hypochondriacus
E. crusgalli
Ia
10
30
100
10
30
100
10
30
100
10
30
100
10
123.0a
70.1a
0.0a
82.0
43.8a
17.1a
85.0a
62.0a
25.0a
111.8
96.0
Ib
45.8a
35.3a
0.0a
II
71.8a
72.8a
49.7a
50.5a
28.5a
0.0a
90.7
IIIa
IVa
a
110
78.6a
59.5a
92.0
71.8a
71.3a
35.0a
30
100
92.3
81.5
F igu r e 1.
P ) 0.001 (significance). Control growth ) 100%.
Obten tion of Com p ou n d IVa . A 150 mg (0.009 mol)
sample of piquerol A was dissolved in ethanol (2 mL), and
pieces of metalic sodium (300 mg) were added slowly until a
dough was obtained. This material was dissolved in AcOEt
and acidified with 10% HCl (10 mL). The resulting mixture
was extracted with hexane, hexane/CH2Cl2, and CH2Cl2. The
resulting organic extracts were combined and concentrated to
yield an oily residue, which was purified by preparative TLC
to afford IVa as a yellow oil (15 mg); Rf 0.62 (9:1 hexane-
AcOEt). IR (cm-1): νmax 3602 (-OH phenolic), 3080, 1578
(aromatic ring). 1H NMR 7.0 (1H, t, H5), 6.7 (1H, dd, J ) 8
Hz and J ) 2 Hz, H4) and 6.6 (1H, dd, J ) 8 Hz and J ) 2 Hz,
H6), 5.16, 4.8 (2H, m, vinylic), 2.23 (3H, s, methyl aromatic),
2.0 (3H, t, Me-vinylic). MS: m/ z (rel intens) 148 (100).
Obt en t ion of Com p ou n d s IIIa a n d IVa w it h P d /C.
Piquerol A (Ia ) 200 mg (0.001 mol) was mixed with 200 mg
Pd/C (5%) in EtOAc (20 mL) and refluxed for 3 h. The mixture
was filtered and concentrated and distilled in vacuo. An oily
compound was obtained (198 mg), which was purified by
preparative TLC to yield a crystalline compound (20 mg) which
was identified as IIIa ; mp 89-91 °C, Rf 0.33 (8:2 petroleum
ether-AcOEt). IR (cm-1): νmax 3605, 3533 (OH, phenol), 1276
(CsO). 1H NMR (ppm): δ 6.63 (2H, s, H5, H6), 5.51, 5.0 (2H,
m, vinylics), 2.65 (3H, s, aromatic-Me), 2.0 (3H, c, vinylic-Me).
EM: M+, m/ z ) 164 (100%). 13C NMR (CDCl3, ppm (Cn, in-
tensity)): δ 147 (C1, 39), 145 (C4, 33), 141 (C8, 44), 130 (C6,
16), 121 (C5, 40), 118 (C3, 239), 114 (C2, 204), 112 (C10, 189),
23 (C9, 141), 13 (C7, 100). In addition, 63 mg of IVa , Rf 0.6
(9:1 petroleum ether-EtOAc), was obtained as a volatile
colorless oil. IR (cm-1): νmax 3522 (-OH phenol), 1276 (C-
O). 1H NMR (ppm): δ 7.06 (1H, t, H5), 6.7 (1H, dd, J ) 8 Hz
and J ) 2 Hz, H4) and 6.6 (1H, dd, J ) 8 Hz and J ) 2 Hz,
H6), 5.5, 5.05 (2H, m, vinylics), 2.2 (3H, s, aromatic-Me), 2.0
(3H, t, vinylic-Me).
Compounds IIIa (18 mg) and IVa (15 mg) were separated,
dissolved in pyridine (0.5 mL) and acetic anhydride (0.5 mL).
The mixtures were heated in a steam bath for 1 h and worked
up as usual to obtain IIIb and IVb, respectively. Compound
IIIb was oily; Rf 0.5 (8:2 petroleum ether-EtOAc). IR (cm-1):
νmax 1758 (CdO) and 1164 (CsO). 1H NMR (CDCl3, ppm):
δ 6.91 (2H, s, H5, H6), 5.23, 4.78 (2H, dd, vinylics), 2.3, 2.25
(6H, s, methyl acetate), 2.1 (3H, s, aromatic-Me), 1.9 (3H, s,
vinylic-Me). Compound IVb (15 mg) was also an oil; Rf 0.30
(9.5:0.5 petroleum ether-EtOAc). IR (cm-1): νmax 1760 (CdO),
1189 (CsO). 1H NMR (CDCl3, ppm): δ 7.06 (1H, t, H5), 6.91
(1H, dd, J ) 8 Hz and J ) 1.9 Hz, H4), 6.85 (1H, dd, J ) 8 Hz
and J ) 1.9 Hz, H6), 5.25, 4.8 (2H, m, vinylics), 2.23 (3H, s,
methyl acetate), 2.26 (3H, s, aromatic-Me), 1.98 (3H, c, vinylic-
Me).
dd, J ) 8 Hz and J ) 2 Hz, H6), 5.29, 4.80 (2H, m, vinylics),
3.81 (3H, s, O-Me), 2.6 (3H, s, aromatic-Me), 1.98 (3H, c,
vinylic-Me).
P r ep a r a tion of Com p ou n d V. A 241 mg (0.009 mol)
sample of piquerol A diacetate (Ib) was dissolved in THF (10
mL), and NaH (400 mg) was added slowly at room temperature
for 4 h. The mixture was then heated (steam bath) for 10 min.
The reaction mixture was poured slowly in water (20 mL) and
saturated with NaCl. From this mixture an oily volatil
compound V (18 mg) was obtained by extraction (CH2Cl2); Rf
0.65 (8:2 petroleum ether-EtOAc). IR (cm-1): νmax 3600, 3343
(OH). 1H NMR (ppm): δ 7.02 (1H, t, H5), 6.71 (2H, m, H4,
H6), 5.18, 4.83 (1H, m, vinylics), 2.19 (3H, s, aromatic-Me), 2.01
(3H, c, vinylic-Me). EM: m/ z ) 148 (11%). 13C NMR (ppm
(Cn, intensity)): δ 143 (C1,145), 112-126 (C2-C6 and C10), 24
(C9, 54), 12 (C7, 42).
Bioa ssa ys w ith Seed s. Bioassay experiments were de-
signed to evaluate the effect of piquerol A and aromatic
derivatives at 10, 30, and 100 µ/mL on the radicle growth of
Amaranthus hypochondriacus (A. hypochondriacus) (Amaran-
thaceae) and Echinochloa crusgalli (E. crusgalli) (Poaceae). A
completely randomized block design with four repititions for
each target species was used. Ten seeds of each species were
sown on Whatman No. 4 qualitative filter paper in petri dishes.
Distilled water (1.5 mL) was added to each dish. The dishes
were incubated in darkness at 27 °C. The radicle length was
measured after 24 h in the case of A. hypochondriacus and 48
h in the case of E. crusgalli. The data were analyzed by
ANOVA (Table 1).
RESULTS AND DISCUSSION
In this study the natural product piquerol A (Ia ) was
transformed into aromatic derivatives. Firstly, piquerol
A was transformed into benzylic alcohol II by reacting
piquerol A with HCl in water.
Secondly, piquerol A was aromatized by treatment
with Pd/C 5% in EtOAc. As a result of this procedure
two derivatives were obtained. The more polar deriva-
tive was purified by preparative TLC and was charac-
terized as 2,3-dialkylhydroquinone IIIa by IR, 1H NMR,
and MS techniques. Compound IIIa was diacetylated
with pyridine/acetic anhydride and gave IIIb, thus
confirming the structure of 2,3-dialkylhydroquinone.
The less polar compound obtained by TLC was a
colorless volatile oil. The structure IVa was deduced
1
Obten tion of Com p ou n d IVc. A 40 mg (0.002 mol)
sample of IVa was dissolved in THF (3 mL) and flushed with
argon. The mixture was stirred for 10 min, and then sodium
hydride (138 mg) and CH3I (1 mL) were added slowly at room
temperature during 2 h. The mixture was saturated with
NaCl and extracted with CHCl3 to yield IVc as an oil (13 mg);
Rf 0.58 (9.5:0.5 petroleum ether-EtOAc). IR (cm-1): νmax 2854
(-OMe), 1256 (dCOC). 1H NMR (CDCl3, ppm): δ 7.06 (1H,
t, H5), 6.85 (1H, dd, J ) 8 Hz and J ) 2 Hz, H4) and 6.75 (1H,
by IR, H NMR, and MS data. The acetyl derivative
IVb was obtained. Compound IVa was also methylated
with MeI/NaH and gave IVc (Flores, 1992). In this
piquerol A aromatization, two other products were
obtained with a very low yield and it was not possible
to determine their structural characteristics.
The aromatization of piquerol A to yield compound
IVa was performed by treatment with metallic sodium.