Oxidation Products of Abietic Acid
J ournal of Natural Products, 2002, Vol. 65, No. 11 1533
chromatography on silica gel, eluting with cyclohexane-
EtOAc-glacial acetic acid (1:9:0.01-0:10:0.01) to obtain seven
fractions. An aliquot (0.5 g) of fraction 2 (ca. 6 g) was separated
by column chromatography on Sephadex-LH 20 with MeOH
as solvent (no polymerization products were detected). Further
purification of 240 mg of fraction 2 [Rf 0.47-0.72, cyclohexane-
EtOAc-glacial acetic acid (1:9:0.01)] on silica gel yielded 9a
and 9b (2 mg each). Another 2.5 g quantity of fraction 2 was
absorbed on silica gel and eluted with cyclohexane-EtOAc-
glacial acetic acid (6:4:0.01) and CH2Cl2-MeOH (1:1) to give
eight fractions. Purification of 1.5 g of the most nonpolar
fraction 1 [Rf 0.11-0.51, cyclohexane-EtOAc-glacial acetic
acid (4:6:0.01)] with column chromatography on silica gel with
cyclohexane-EtOAc-glacial acetic acid (1:9:0.01 and 6:4:0.01)
as solvents afforded 10 (6 mg).
Oxid a tion of 1 Ad sor bed on Silica Gel. Compound 1 (0.8
g) was dissolved in ethanol, adsorbed on silica gel (4 g), and
exposed to light and air for 5 months. Column chromatography
on silica gel using the solvent systems cyclohexane-EtOAc-
glacial acetic acid (9:1:0.01-0:10:0.01) and EtOAc-EtOH-
glacial acetic acid (5:5:0.1-1:9:0.1) gave seven fractions of
orange-yellow oils. Fraction 4 (30 mg) [Rf 0.77-0.87, cyclo-
hexane-EtOAc-glacial acetic acid (1:9:0.01)] was purified by
HPLC on RP-18 eluting with MeOH-H2O ((8:2) flow rate 1
mL/min, detection at 254 nm) to afford 2.3 mg of 11.
Meth yl 13â-eth oxy-7r-h ydr oxyabiet-8(14)-en oate (m eth -
yl [1R-(1r,4a â,4br,7â,9r,10a r)]-1,2,3,4,4a ,4b,5,6,7,9,10,10a -
d od eca h yd r o-7-eth oxy-9-h yd r oxy-1,4a -d im eth yl-7-(1-m e-
thylethyl)phenanthrene-1-carboxylate,3a):colorlessspherolites
(cyclohexane-EtOAc); mp 95-96 °C; [R]24D -3.3° (c 0.51, CH2-
Cl2); IR (KBr) νmax 3431, 2941, 1728, 1670, 1460, 1388, 1254,
1186, 1149, 1073, 1048, 964, 872, 816, 744 cm-1; 1H NMR data,
see Table 1; 13C NMR data, see Table 2; EIMS m/z 378 [M]+
(1), 335 [M - C3H7]+ (100), 317 [335 - H2O]+ (6), 289 (1), 275
(4), 257 (3), 247 (14), 229 (10), 201 (4), 173 (1), 159 (3), 123
(6), 107 (6), 55 (3), 43 (4), 32 (18); HREIMS (pos) m/z 335.2219
[M - C3H7]+ (calcd 335.2217).
involved in allergenic processes, compound 5 is an interest-
ing candidate for future dermatological screening. This is
also true for compound 10, which has close structural
similarities with known allergens, even though it does not
bear a hydroperoxy group.
Exp er im en ta l Section
Gen er a l Exp er im en ta l P r oced u r es. Melting points were
determined with an Electrothermal IA 9200 digital melting
point apparatus and are not corrected. Optical rotations were
measured with a 241 MC polarimeter (Perkin-Elmer). IR
spectra were obtained using a 2000 FTIR infrared spectrom-
eter system (Perkin-Elmer). NMR spectra were recorded on
Varian Unity Inova 400, Bruker AMX-300, and Bruker DRX-
500 instruments. Chemical shifts were reported with reference
to the respective residual solvent peaks (δH 7.24 and δC 77.0
ppm for CDCl3). EIMS data were recorded on a Varian MAT
711 spectrometer, at 70 eV, and ESIMS on a TSQ 7000
spectrometer. HREIMS and HRMALDIMS were performed on
a VG-TRIBRID spectrometer and an IonSpec Ultima FT mass
spectrometer, respectively. HPLC was performed with
a
Merck-Hitachi L6200 Intelligent Pump connected to a Rheo-
dyne 7125 injector, a Merck-Hitachi L-4000 UV detector, a
Merck-Hitachi D-2500 Chromo-Integrator, a Kauer HPLC
precolumn (4 × 4 mm, LiChrosorb RP-18, 10 µm, Merck), and
a Knauer HPLC column (250 × 8 mm, Spherisorb ODS 2, 5
µm, Waters Spherisorb). Materials for column chromatogra-
phy: silica gel 60 (64-200 µm, pore diameter 60 Å, Merck)
and Sephadex LH-20 (Pharmacia). Materials for thin-layer
chromatography (TLC): TLC aluminum sheets silica gel 60
F254 (0.2 mm, 200 × 200 mm, Merck) and TLC RP-18 F254
aluminum sheets (0.2 mm, 200 × 200 mm Merck). Materials
for preparative TLC: PSC plates and silica 60 F254 (1 mm, 200
× 200 mm, Merck). Compounds were detected under UV light
at 254 nm, as well as by spraying with phosphomolybdic acid
reagent [12 MoO3 × H3PO4, 20% (w/w) in ethanol] and
subsequent heating to 105 °C. Solvents for column chroma-
tography were distilled prior to use, and HPLC grade solvents
were used for HPLC. Sacotan 90 was obtained from Krems
Chemie AG (Krems an der Donau, Austria).
Meth yl 13r-eth oxy-7r-h ydr oxyabiet-8(14)-en oate (m eth -
yl [1R-(1r,4a â,4br,7â,9r,10a r)]-1,2,3,4,4a ,4b,5,6,7,9,10,10a -
d od eca h yd r o-7-eth oxy-9-h yd r oxy-1,4a -d im eth yl-7-(1-m e-
th yleth yl)p h en a n th r en e-1-ca r boxyla te, 3b): colorless oil;
[R]24D -33.9° (c 0.049, CH2Cl2); IR (KBr) νmax 3436, 2948, 2871,
1
1726, 1459, 1387, 1249, 1191, 1104 cm-1; H NMR data, see
Table 1; 13C NMR data, see Table 2; EIMS m/z 335 [M - C3H7]+
(100), 317 [335 - H2O]+ (10), 289 (2), 275 (3), 257 (2), 247 (9),
229 (7), 201 (3), 181 (2), 159 (2), 139 (2), 123 (5), 100 (6), 81
(3), 55 (4), 43 (8), 32 (3); HRMALDIMS (pos) m/z 401.2662 [M
+ Na]+ (calcd 401.2662).
Isola tion of Abietic Acid (1) a n d Syn th esis of Meth yl
Abieta te (2). Compound 1 was isolated from Sacotan 90, a
fraction of tall oil, one of the byproducts of cellulose manu-
facturing of softwood (from Pinus species). Altogether 55 g of
dipentylamine were added dropwise within 1 h to a refluxing
solution of 100 g of Sacotan 90 in 400 mL of acetone. After
stirring for an additional hour under reflux the reaction was
allowed to cool to room temperature. The precipitated am-
monium salt was recrystallized six times from acetone, dried
in vacuo, and suspended in diethyl ether, and 1 was liberated
Meth yl 7r-h ydr oper oxy-13r-h ydr oxyabiet-8(14)-en oate
or m eth yl 13r-h ydr oper oxy-7r-h ydr oxyabiet-8(14)-en oate
(m eth yl [1R-(1r,4a â,4br, 7r,9r,10a r)]-1,2,3,4,4a ,4b,5,6,7,9,-
10,10a -d od eca h yd r o-7-h yd r op er oxy-9-h yd r oxy-1,4a -d im -
et h yl-7-(1-m et h ylet h yl)p h en a n t h r en e-1-ca r boxyla t e or
m eth yl [1R-(1r,4a â,4br,7r,9r,10a r)]-1,2,3,4,4a ,4b,5,6,7,9,-
10,10a -d od eca h yd r o-9-h yd r op er oxy-7-h yd r oxy-1,4a -d im -
eth yl-7-(1-m eth yleth yl)p h en a n th r en e-1-ca r boxyla te, 5):
with 2 N H2SO4 and crystallized from EtOH-H2O.24 Com-
30
pound 2 was prepared using the method of Abad et al.31
Oxid a tion of 2 in Eth a n olic Solu tion . Compound 2 (1.99
g) was dissolved in 30 mL of EtOH and stirred vigorously at
room temperature under air. After 48 h the solvent was
evaporated and the obtained residue subjected to column
chromatography on silica gel with cyclohexane-EtOAc (68:
32), leading to the isolation of 3a /3b (317 mg/48 mg), 4a (46
mg), and 5 (40 mg).
colorless needles (cyclohexane-EtOAc); mp 83-84 °C; [R]24
D
-54.5° (c 0.23, CH2Cl2); IR (KBr) νmax 3545, 3393, 2954, 2867,
1706, 1661, 1466, 1436, 1387, 1305, 1257, 1193, 1149, 1132,
1104, 1080, 1057, 1041, 991, 968, 948, 888 (w, ν O-O, weak
1
hydroperoxide bond),19 851 cm-1; H NMR data, see Table 1;
13C NMR spectral data, see Table 2; EIMS m/z 366 [M]+ (3),
348 [M - H2O]+ (7), 333 [M - OOH]+ (9), 332 [M - H2O2]+
(29), 315 [m/z 333 - H2O]+ (8), 314 (22), 307 [M - COOCH3]+
(7), 289 (11), 263 (11), 255 (18), 247 (100), 229 (25), 211 (13),
199 (11), 183 (17), 165 (11), 159 (12), 146 (17), 123 (40), 121
(29), 109 (28), 107 (28), 95 (18), 81 (18), 79 (18), 55 (18), 43
(28), 41 (14); HRMALDIMS (pos) m/z 389.2295 [M + Na]+
(calcd 389.2298).
Oxid a tion of 1 in th e P r esen ce of H2SO4 a n d Su bse-
qu en t Meth yla tion . Compound 1 (30 g) was crystallized as
the dipentylammonium salt in acetone, and then 300 mL of 2
N H2SO4 was added. The volume was reduced on a rotary
evaporator at ca. 50 °C within 2-3 h. The liberated acids were
extracted with diethyl ether and converted into their methyl
esters. The orange oil (14 g) was purified by column chroma-
tography with stepwise gradients of cyclohexane-EtOAc (9:
1-0:10) and EtOAc-EtOH (5:1-2:1) and gave 6 (49 mg), 7
(28 mg), 8 (65 mg), and 5 (52 mg).
Meth yl 12-oxoa bieta te (m eth yl [1R-(1r,4a â,4br,10a r)]-
1,2,3,4,4a ,4b,5,6,10,10a -d eca h yd r o-6-oxo-1,4a -d im eth yl-7-
(1-m eth yleth yl)p h en a n th r en e-1-ca r boxyla te, 6): colorless
oil; 1H NMR data, see Table 1; 13C NMR data, see Table 2.
Other physical and spectral data, consistent with literature
values.22,23
Oxid a tion of P ow d er ed 1. Compound 1 (10 g) was heated
on an oil bath at 100 °C in a crystallizing dish for 5 weeks.
The sticky, yellow-brownish mixture was subjected to column