Sesquiterpene constituents in Petasites hybridus

Article abstract of DOI:10.1016/S0031-9422(01)00489-7

The essential oil of the rhizomes of Petasites hybridus (Asteraceae) was investigated by gas chromatography, mass spectrometry, 1- and 2-dimensional NMR techniques and chemical correlations. Two new sesquiterpene hydrocarbons, petasitene and pethybrene, c

Full text of DOI:10.1016/S0031-9422(01)00489-7

Phytochemistry 59 (2002) 795–803
www.elsevier.com/locate/phytochem
Sesquiterpene constituents in Petasites hybridus
Yucel Saritas, Stephan H. von Reub, Wilfried A. Konig*
Institut fur Organische Chemie, Universitat Hamburg, Martin-Luther-King-Platz 6, D-20146 Hamburg, Germany
¨
¨
Received 10 October 2001; received in revised form 30 November 2001
Abstract
The essential oil of the rhizomes of Petasites hybridus (Asteraceae) was investigated by gas chromatography, mass spectrometry,
1- and 2-dimensional NMR techniques and chemical correlations. Two new sesquiterpene hydrocarbons, petasitene and pethy-
brene, could be identified. Petasitene is the parent sesquiterpene hydrocarbon to the known norsesquiterpene albene. The absolute
configuration of petasitene could be assigned by conversion of natural albene to petasitene by partial synthesis. Pethybrene is a
tricyclic sesquiterpene hydrocarbon, which rearranges to the structurally related a-isocomene under acidic conditions. Several ses-
quiterpenes were isolated from the hydrodistillation products of Petasites hybridus and investigated by spectroscopic methods and
chemical correlations # 2002 Elsevier Science Ltd. All rights reserved.
Keywords: Petasites hybridus; Sesquiterpene hydrocarbons; Petasitene; Albene; Pethybrene; Chemical correlation
1. Introduction
this compound, which was named petasitene (12), is
described in this paper. In addition, a second compound
with a mass spectrum with the unusual m/z 149 as its
base peak was identified and named pethybrene (9).
The rhizomes of Petasites hybridus and the related
species P. albus are well known since ancient times for
their spasmolytic and analgetic effects and extracts are
applied for treatment of gastrointestinal disturbances,
bronchial asthma and many other diseases (Willuhn,
1997). The pharmacological effects are mainly attrib-
uted to functionalized eremophilane type sesqui-
terpenes, which, among many other compounds, were
identified in previous investigations of extracts of P.
hybridus (Aebi et al., 1955; Stoll et al., 1956; Neuensch-
wander et al., 1979) and by headspace analysis in flow-
ers of P. albus (Brunke et al., 1992). Both Petasites
species are rich in sesquiterpene hydrocarbons (Fig. 1).
In addition, a trinorsesquiterpene, (À)-albene (3), was
identified as a constituent (Hochmannova et al., 1962).
Its absolute configuration was determined by Kreiser et
al. (1979) and reconfirmed by Baldwin et al. (1986) and
a biogenetic pathway for its biosynthesis was proposed
(Kreiser and Janitschke, 1979) with a hypothetic inter-
mediate 12 which is identical with the compound which
has now been isolated. The structure determination of
2. Results and discussion
A hydrodistillate of P. hybridus rhizomes was investi-
gated by GC–MS and exhibited a complex sesqui-
terpene hydrocarbon fraction (Fig. 1). Most of the
constituents could be identified by comparing their mass
spectra and retention indices with a spectral library
which was established under identical experimental
conditions (Joulain and Konig, 1998). Two unknown
sesquiterpene hydrocarbons, both with molecular ions
at m/z 204 (C15H24) were selected for isolation and
structural investigations.
2.1. Petasitene (12)
The isolation of 12 which is present in only 0.8% of
the total volatile constituents was carried out by dry-
column silica gel chromatography (Schwanbeck et al.,
1982) and repetitive preparative GC using packed col-
umns with different modified cyclodextrins as chiral
stationary phases (Hardt and Konig, 1994) (Fig. 2). The
mass spectrum with M+=204 suggested the elementary
* Corresponding author. Tel.: +49-40-42838-2824; fax: +49-40-
42838-2893.
E-mail address: wkoenig@chemie.uni-hamburg.de (W.A. Konig).
0031-9422/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved.
PII: S0031-9422(01)00489-7

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Y. Saritas et al. / Phytochemistry 59 (2002) 795–803
Fig. 1. Gas chromatographic separation of the volatiles of Petasites hybridus and peak assignment. 25 m Fused-silica capillary column with CPSil 5,
50 ꢀC, 3 ꢀC/min to 230 ꢀC.
composition C15H24. This is supported by 24 proton
1
24.17, 24.40, 34.89 and 50.63), 4 methine groups (ꢀ
27.42, 45.75, 51.39, 121.47) and 3 quaternary carbon
signals (ꢀ 47.84, 59.08, 156.91) could be assigned. A
detailed investigation of the 1H–1H-COSY and the
inspection of long-range coupling correlations in the
HMBC- and HMQC spectra allowed the construction
of partial structures A, B and C together with 2 methyl
groups connected to quaternary carbons (Fig. 3). How-
ever, due to overlapping signals and missing correlations
signals and 15 carbon signals in the H and 13C NMR,
1
respectively. The H NMR showed one olefinic methine
proton (ꢀ 5.38, t, 2.6 Hz) and together with the mole-
cular composition a tricyclic system had to be assumed.
DEPT experiments indicated 2 tertiary and 2 secondary
methyl groups (ꢀ 17.17, 21.14, 25.41, 25.61) which cor-
responded to proton signals at ꢀ 0.96 (s, 6 H), 1.11 (d, 3
H) and 1.12 (d, 3 H). In addition 4 methylene groups (ꢀ

Y. Saritas et al. / Phytochemistry 59 (2002) 795–803
797
While the relative configuration of 12 was derived
from a NOESY spectrum (Fig. 5), the absolute config-
uration was determined by a chemical correlation
(Fig. 6) with natural albene (3), which is present in the
hydrodistillation products of P. hybridus and could be
isolated in sufficient amounts. As shown in Fig. 6, (À)-
albene (3) was converted to albenone (28). After
hydrogenation of the olefinic double bond to albanone
(29) nucleophilic addition of isopropyl-lithium yielded
the tertiary alcohol 30, which lost a water molecule by
treatment with acidic ion exchange resin Amber-
lyst115. Both natural and synthetic 12 showed a nega-
tive optical rotation and displayed completely identical
NMR and mass spectra as well as identical GC reten-
tion times on chiral and non-chiral stationary phases.
2.2. Pethybrene (9)
The isolation of this new sesquiterpene hydrocarbon
was complicated as it is only a minor constituent with
0.9% of the total volatiles eluting between two larger
peaks from the GC column. A combination of preparative
GC and TLC with AgNO3-precoated plates finally resul-
ted in the isolation of compound 9 in 95% purity (Fig. 7).
From the mass spectrum and M+=204 the molecular
1
composition C15H24 was derived. From the H and 13C
NMR spectra proton signals for an exocyclic double bond
(ꢀ 4.80, s, and 5.04, s), and one secondary and two tertiary
methyl groups (ꢀ 1.09, d, 1.26, s, 1.31, s) could be derived,
but no other spin systems could be recognized because of
largely overlapping signals. In the DEPT NMR spectra 3
methyl groups (ꢀ 17.90, 19.53 and 24.89), 7 methylene
groups (ꢀ 21.26, 27.59, 30.31, 40.14, 44.00, 48.66, and
99.17), 1 methine group (ꢀ 37.03) and 4 quaternary carbon
signals (ꢀ 45.60, 48.95, 62.32 and 162.37) could be detec-
ted. From these data together with the 1H–1H-COSY and
HMQC coupling correlations 6 partial structures could be
derived (Fig. 8), which could be connected to a tricyclic
sesquiterpene skeleton by a careful interpretation of the
HMBC data (Fig. 9). The NOESY spectrum of 9 afforded
the relative configuration (Fig. 10).
Fig. 2. Gas chromatographic control of the isolation procedure for
petasitene (12) from the hydrodistillation product of P. hybridus. 25 m
Fused-silica capillary column with CPSil 5, 50 ꢀC, 3 ꢀC/min to 230 ꢀC.
In cases when the direct determination of the absolute
configuration is not possible, a chemical correlation by
acid catalyzed rearrangement can be conclusive if a
product of known absolute configuration is formed. By
treating 9 with acidic ion exchange resin for 1 hour at
room temperature only one product was obtained
almost quantitatively which turned out to be the known
(À)-a-isocomene (11). This sesquiterpene hydrocarbon
which is also present in P. hybridus has been isolated
before from Isocoma wrightii (Zalkov et al., 1977) and
Berkheya radula (Bohlmann et al., 1977) and is also a
constituent of Silphium perfoliatum (Bohlmann and
Jakupovic, 1980). A tentative rearrangement mechan-
ism from (À)-pethybrene (9) to (À)-a-isocomene (11) is
depicted in Fig. 11.
Fig. 3. Partial structures of petasitene (12) derived by NMR investi-
gations.
of the tertiary methyl groups the assignment of an
unambiguous total structure was still not possible.
Therefore, a small amount of 12 was converted to
epoxide 27 by treatment with m-chloroperbenzoic acid.
In this derivative the tertiary methyl signals could now
clearly be distinguished (ꢀ 0.83 (s, 3 H) and 20.98, C-16;
0.99 (s, 3 H) and 13.89, C-15) and from the long-range
coupling correlations in the HMBC spectrum (Fig. 4)
the structure of 27, and subsequently also of 12, could
be assigned.

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Y. Saritas et al. / Phytochemistry 59 (2002) 795–803
Fig. 4. Partial structures and HMBC correlations found for petasitene epoxide (27).
Fig. 5. NOESY correlations and relative configuration of petasitene (12).
3. Experimental
portions to hydrodistillation for 2.5 h to yield the
essential oil, which was collected in 1 ml of n-hexane
and not further quantified.
3.1. Plant material
Ten kilograms of Petasites hybridus rhizomes were
collected at the Alster in Hamburg (Germany) in May
1998 and were identified by Johannes Donath from the
Botanical Garden of the University of Hamburg. The
fresh material was washed, cut and submitted in 500 g
3.2. NMR spectroscopy
NMR measurements were carried out with a Bruker
WM 400 instrument for 1D spectra and with WM
500 instrument for 1D, H–1H-COSY, HMBC, HMQC
1

Y. Saritas et al. / Phytochemistry 59 (2002) 795–803
799
Fig. 6. Partial synthesis of petasitene (12) from (À)-albene (3). (a) Pyridinium dichromate, pyridine; (b) H2/Pd, ether; (c) isopropyl-lithium, n-hexane;
(d): H+
.
and NOESY spectra. TMS was used as an internal
standard.
3.6. Isolation of (À)-petasitene (12)
(À)-(1R,2S,6R,7S)-2,6-Dimethyl-3-isopropyltricyclo-
[5.2.1.0 2.6.]-dec-3-ene. After prefractionation of the raw
essential oil by dry column chromatography (elution
with hexane) the compound was enriched from the
hydrocarbon fraction by preparative GC on a 2,6-Me-3-
Pe-b-CD column and finally isolated (approx. 5 mg
total amount) through preparative GC with a 6-T-2,3-
3.3. GC–MS
Electron impact (70 eV) GC–MS measurements were
carried out on a Hewlett Packard HP 5890 gas chro-
matograph equipped with a 25 m CPSil 5 (Chrompack)
fused-silica capillary column coupled to a VG Analy-
ticalꢀ 70-250S mass spectrometer. Ion source temp.:
200 C.
1
Me-b-CD column. H NMR (400 MHz, C6D6): ꢀ=0.96
(6 H, s, 14-Me, 15-Me), 1.00 (1 H, d (br), J=9.7 Hz,
2
10-Ha), 1.11 (3 H, d, J=7.1 Hz, 12-Me), 1.12 (3 H, d,
J=6.6 Hz, 13-Me), 1.28 (1 H, m, 8-Ha), 1.34 (1 H, m, 9-
Ha), 1.56 (2 H, m, 8-Hb, 9-Hb), 1.74 (1 H, d (br), 10-Hb,
2J=9.7 Hz), 1.79 (1 H, d (br), J=3.6 Hz, 7-H), 1.95
(1 H, d (br), J=3.1 Hz, 1-H), 2.06 (1 H, qq, J1=J2=6.6
Hz, 11-H), 2.22 (2 H, d, J=2.6 Hz, 5-H), 5.38 (1 H, t,
J=2.6 Hz, 4-H). 13C NMR (125 MHz, C6D6) : ꢀ 17.17
(q, 14-Me), 21.14 (q, 15-Me), 24.17, 24.40 (2 C, t, 8-C, 9-
C), 25.41, 25.61 (2 C, q, 12-Me, 13-Me), 27.42 (d, 11-C),
34.89 (t, 10-C), 45.75 (d, 1-C), 47.84 (s, 6-C), 50.63 (t, 5-
C), 51.39 (d, 7-C), 59.08 (s, 2-C), 121.47 (d, 4-C), 156.91
(s, 3-C). MS (EI, 70 eV), m/z (rel.int.) : 204 (4) [M+],
189 (8), 161 (4), 147 (2), 137 (100), 121 (27), 105 (9), 95
(26), 79 (9), 77 (9), 75 (9), 67 (9), 57 (9), 41 (19).
3.4. Preparative GC
Preparative GC was carried out with a modified Var-
ian 2800 gas chromatograph equipped with a stainless
steel column (Silcosteel, Amchro) with 10% SE 30 on
chromosorb W-HP, (1.85 m  4.3 mm), 5% hepta-
kis(2,6-di-O-methyl-3-O-pentyl)-b-cyclodextrin
– OV
1701 (1:1; w/w) on Chromosorb W-HP (1.85 m  4.3
mm) [2,6-Me-3-Pe-b-CD], or 6.4% heptakis-(6-O-tert-
butyldimethylsilyl-2,3-di-O-methyl)-b-cyclodextrin – SE
52 (1:1; w/w) on Chromosorb W-HP (1.95 m  5.3 mm)
[6-T-2,3-Me-b-CD]. Helium was used as carrier gas at a
flow rate of 240 ml/min and a temperature program
ꢀ
ꢀ
applied (initial: 80 C, 2 C/min, to 160 ꢀC).
3.7. Synthesis of (+)-(3R)-petasitenepoxide (27)
3.5. Polarimetry
(+)-(1R,2S,3R,5S,6R,7S)-2,7-Dimethyl-3-isopropyl-
4-oxa-tetracyclo-[6.2.1.02.7.03.5.]-undecane. To 3 mg of
(À)-petasitene (12) in 2 ml CHCl3 5 mg of m-chloro
perbenzoic acid were added. After 24 h the soln. was
A Perkin-Elmer 241 polarimeter was used. To avoid
inaccuracies only the sense of the optical rotation was
determined due to the small quantity of isolated material.

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Y. Saritas et al. / Phytochemistry 59 (2002) 795–803
J=7.1 Hz, 12-H), 3.34 (1 H, d, J=2.0 Hz, 5-H). 13C
NMR (125 MHz, CDCl3) : ꢀ 13.89 (q, 15-C), 18.31,
19.69 (2C, q, 13-C, 14-C), 20.98 (q, 16-C), 21.45 (t, 9-C),
22.31 (t, 10-C), 24.76 (d, 12-C), 33.26 (t, 11-C), 42.62 (d,
1-C), 44.53 (t, 6-C), 48.91 (s, 7-C), 50.01 (d, 8-C), 52.93
(s, 2-C), 62.57 (d, 5-C), 77.61 (s, 3-C). MS (EI, 70 eV) m/
z (rel.int.) : 220 (2) [M+], 205 (4), 187 (2), 177 (4), 153
(100), 135 (10), 121 (5), 107 (16), 93 (11), 79 (10), 67
(10), 55 (10), 41 (17).
3.8. Isolation of (À)-albene (3)
(À)-(1S,2S,6S,7R)-2,6-Dimethyltricyclo-[5.2.1.02.6]-dec-
3-ene. After prefractionation of the raw essential oil by
dry column chromatography (Schwanbeck et al., 1982)
with hexane as eluent the compound (approx. a total
amount of 50 mg) was isolated from the hydrocarbon
fraction by preparative GC using a 2,6-Me-3-Pe-b-CD
column. 1H NMR (400 MHz, CDCl3) : ꢀ 0.94 (6 H, s, 2-
Me, 6-Me), 0.98 (1 H, d (br), 2J=9.5 Hz, 10-Ha), 1.29 (2
H, m, 8-Ha, 9-Ha), 1.56 (2 H, m, 8-Hb, 9-Hb), 1.63 (1
2
H, d (br), J=9.5 Hz, 10-Hb), 1.78 (1 H, d (br), J=5.1
Hz, 1-H), 1.80 (1 H, d (br), J=4.1 Hz, 7-H), 2.23 (2 H,
dd, 5-H, J=5.6 Hz, J=2.4 Hz, 5-H), 5.26 (1 H, dt,
J=6.1 Hz, J=5.6 Hz, 4-H), 5.56 (1 H, dt, J=6.1 Hz,
J=2.4 Hz, 3-H). 13C NMR (100 MHz, CDCl3) : ꢀ 18.09
(q, 2-Me), 20.65 (q, 6-Me), 23.77, 23.79 (2C, t, 8-C, 9-C),
34.16 (t, 10-C), 46.55 (s, 6-C), 47.02 (d, 7-C), 50.27 (d,
1-C), 51.75 (t, 5-C), 56.33 (s, 2-C), 128.30 (d, 4-C),
139.59 (d, 3-C). MS (EI, 70 eV), m/z (rel.int.) : 162 (2)
[M+], 147 (4), 133 (2), 119 (7), 105 (9), 95 (100), 94 (41),
79 (24), 67 (13), 53 (7), 41 (17).
Fig. 7. Gas chromatographic control of the isolation procedure for
pethybrene (9) from the hydrodistillation product of P. hybridus. 25 m
Fused-silica capillary column with CPSil 5, 50 ꢀC, 3 ꢀC/min to 230 ꢀC.
3.9. Synthesis of (À)-albenone (28)
washed with 2 ml satd. Na2SO3 soln., dried over MgSO4
and purified by preparative GC with a 2,6-Me-3-Pe-b-
CD column to give 2 mg of (+)-(3R)-petasitenepoxide
(À)-(1R,2S,6S,7S)-2,6-Dimethyltricyclo-[5.2.1.02.6]-dec-
4-en-3-one. To 20 mg of (À)-albene (3) in 2 ml of pyri-
dine 700 mg of pyridinium dichromate and 10 mg of
molecular sieve 3 A were added. After heating for 24 h
under reflux 2 ml of H2Owere added. The reaction
product was extracted by shaking with Et2O, washed
with 1 M HCl, NaHCO3 and satd. NaCl soln., dried
over MgSO4, evaporated and purified by column chro-
matography (hexane–Et2O, 10:1) to give 19 mg of (À)-
albenone (28). 1H NMR (500 MHz, C6D6): ꢀ 0.62 (3H,s,
6-Me), 0.66 (1 H, d (br), 2J=10.4 Hz, 10-Ha), 0.85 (3 H,
1
(27). H NMR (400 MHz, CDCl3) : ꢀ 0.83 (3 H, s, 16-
H), 0.92 (6 H, d, J=7.1 Hz, 13-H, 14-H), 0.99 (3 H, s,
2
15-H), 1.09 (1 H, d (br), J=9.7 Hz, 11-Ha), 1.24 (1 H,
m, 9-Ha), 1.32 (1 H, m, 10-Ha), 1.43 (1 H, m, 9-Hb),
2
1.55 (1 H, m, 10-Hb), 1.62 (1 H, d (br), J=9.7 Hz, 11-
Hb), 1.71 (1 H, dd, J=14.6 Hz, J=2.0 Hz, 6-H), 1.77 (1
H, d (br), J=3.6 Hz, 8-H), 1.82 (1 H, d, J=14.6 Hz, 6-
H), 2.18 (1 H, d (br), J=2.5 Hz, 1-H), 2.25 (1 H, sept,
Fig. 8. Partial structures of pethybrene (9) derived by NMR investigations.

Y. Saritas et al. / Phytochemistry 59 (2002) 795–803
801
Fig. 9. Partial structures and HMBC correlations found for pethybrene (9).
passed through the soln. After 4 h the soln. was filtered
to give 19 mg of (À)-albanone (29). 1H NMR (500
MHz, C6D6) : ꢀ 0.68 (3H, s, 6-Me), 0.75 (3 H, s, 2-Me),
2
0.84 (1 H, d (br), J=10.4 Hz, 10-Ha), 1.10 (3 H, m, 8-
Ha, 9-Ha, 10-Hb), 1.27 (4 H, m, 8-Hb, 9-Hb, 5-H), 1.65
(1 H, d (br), J=2.5 Hz, 7-H), 1.89 (1 H, dt, 2J=17.9 Hz,
J=6.3 Hz, 4-H), 2.01 (1 H, dt, 2J=17.7 Hz, J=11.4 Hz,
4-H), 2.35 (1 H, d (br), J=3.2 Hz, 1-H). 13C NMR (100
MHz, C6D6) : ꢀ 16.13 (q, 2-Me), 20.93 (q, 6-Me), 22.79
(t, 8-C), 23.65 (t, 9-C), 35.29 (t, 5-C), 36.18, 36.33 (2C, t,
4-C, 10-C), 46.03 (d, 1-C), 46.10 (s, 6-C), 51.10 (d, 7-C),
56.41 (s, 2-C), 223.01 (s, 3-C). MS (EI, 70 eV), m/z
(rel.int.) : 178 (20) [M+], 163 (6), 156 (6), 135 (4), 122
(44), 111 (22), 94 (100), 79 (50), 77 (28), 67 (37), 53 (28),
41 (61).
Fig. 10. NOESY correlations and relative configuration of pethybrene
(9).
s, 2-Me), 1.07 (1 H, m, 8-Ha), 1.13 (1 H, m, 9-Ha), 1.16
(1 H, d (br), 2J=10.4 Hz, 10-Hb), 1.32 (2 H, m, 8-Hb, 9-
Hb), 1.50 (1 H, d (br), J=2.5 Hz, 7-H), 2.14 (1 H, d (br),
J=2.5 Hz, 1-H), 6.04 (1 H, d, J=5.7 Hz, 4-H), 6.58
(1 H, d, J=5.7 Hz, 5-H). 13C NMR (100 MHz, C6D6): ꢀ
14.25 (q, 2-Me), 16.33 (q, 6-Me), 22.48 (t, 8-C), 23.30 (t,
9-C), 33.91 (t, 10-C), 44.07 (d, 7-C), 45.08 (d, 1-C), 52.94
(s, 2-C), 54.49 (s, 6-C), 134.08 (d, 4-C), 168.98 (d, 5-C),
213.53 (s, 3-C). MS (EI, 70 eV), m/z (rel.int.) : 176 (100)
[M+], 161 (63), 148 (31), 133 (41), 119 (28), 110 (68),
108 (68), 105 (41), 91 (41), 80 (41), 79 (35), 67 (31), 53
(16), 41 (31).
3.11. Synthesis of (+)-(3S)-petasitan-3-ol (30)
(+)-(1R,2S,3S,6R,7S)-2,6-Dimethyl-3-isopropyltricyclo-
[5.2.1.0 2.6]-decan-3-ol. Under an argon atmosphere 19
mg of (À)-albanone (29) in hexane were added dropwise
to 40 ml of boiling isopropyl-lithium (2.0 M) in hexane.
After heating under reflux for 60 h the reaction mixture
was slowly added to 100 g crushed ice. The reaction
product was isolated with hexane, dried over MgSO4,
and purified by preparative GC with a SE 30 phase to
give 2 mg of (+)-(3S)-petasitan-3-ol (30). 1H NMR (500
MHz, C6D6) : ꢀ 0.64 (3H, s, 14-Me), 0.78 (3 H, d, J=6.6
Hz, 12-Me), 0.82 (3 H, s, 15-Me), 0.86 (3 H, d, J=6.9
2
3.10. Synthesis of (À)-albanone (29)
Hz, 13-Me), 1.05 (1 H, d (br), J=9.8 Hz, 10-Ha), 1.13
(1 H, m, 4-Ha), 1.18 (1 H, m, 9-Ha), 1.22 (1 H, m, 5-
Ha), 1.30 (1 H, m, 8-Ha), 1.38 (1 H, m, 4-Hb), 1.41 (2 H,
m, 8-Hb, 9-Hb), 1.68 (1 H, qq, J1=J2=6.6 Hz, 11-H),
1.80 (1 H, d (br), J=4.1 Hz, 7-H), 1.84 (1 H, m, 5-Hb),
(À)-(1R,2S,6R,7S)-2,6-Dimethyltricyclo-[5.2.1.02.6]-
decan-3-one. To 19 mg of (À)-albenone (28) in 2 ml
hexane 20 mg of Pd-C (15%) were added and H2 was

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Y. Saritas et al. / Phytochemistry 59 (2002) 795–803
added. After standing at room temperature for 12 h the
mixture was filtered and the dehydration product iso-
lated through preparative GC with a SE 30 phase to
give 1 mg (À)-petasitene identical to the natural product
isolated from P. hybridus.
3.14. Isolation of (À)-pethybrene (9)
(À)-(1S,2R,5S,8S)-7-methylen-2,5,8-trimethyltricyclo-
[6.3.0.01.5]-undecane. After prefractionation of the raw
essential oil by dry column chromatography (Schwan-
beck et al., 1982) with hexane as eluent the hydrocarbon
fraction was fractionated by preparative GC using a 2,6-
Me-3-Pe-b-CD column. The compound was enriched
from the first sesquiterpenoid fraction by preparative
TLC with AgNO3-silica gel (impregnated with ca. 0.5 g of
AgNO3 in 100 ml EtOH–H2O (1 : 4) for 1 h, dried at
110 ꢀC) with n-hexane as eluent and was finally purified by
preparative GC with a 6-T-2,3-Me-b-CD column. 1H
NMR (500 MHz, C6D6 ): ꢀ 1.09 (3 H, d, J=7.1 Hz, 14-
Me), 1.26 (3 H, s, 13-Me), 1.31 (3 H, s, 15-Me), 1.33 (1 H,
m, 3-H), 1.41 (1 H, m, 11-H), 1.50 (1 H, m, 4-H), 1.60 (1 H,
m, 9-H), 1.63 (2 H, 2s, 6-H), 1.68 (1 H, m, 4-H), 1.74 (1 H,
m, 9-H), 1.78 (2 H, m, 10-H), 1.89 (1 H, q, J=7.1 Hz, 2-
H), 2.07 (1 H, m, 3-H), 2.14 (1 H, m, 11-H), 4.80 (1 H, s,
12-CH), 5.04 (1 H, s, 12-CH). 13C NMR (100 MHz,
C6D6): ꢀ 17.90 (q, 14-Me), 19.53 (q, 13-Me), 21.26 (t, 10-
C), 24.89 (q, 15-Me), 27.59 (t, 3-C), 30.31 (t, 11-C), 37.03
(d, 2-C), 40.14 (t, 4-C), 44.00 (t, 9-C), 45.60 (s, 5-C), 48.66
(t, 6-C), 48.95 (s, 8-C), 62.32 (s, 1-C), 99.17 (t, 12-CH2),
162.37 (s, 7-C). MS (EI, 70 eV), m/z (rel.int.) : 204 (8)
[M+], 189 (22), 175 (7), 161 (15), 149 (100), 133 (15), 119
(15), 107 (19), 105 (19), 93 (19), 91 (19), 79 (15), 67 (9),
55 (16), 41 (28).
Fig. 11. Tentative rearrangement mechanism of pethybrene (9) to (À)-
a-isocomene (11).
1.87 (1 H, d (br), J=4.4 Hz, 1-H), 2.51 (1 H, d (br),
2J=9.8 Hz, 10-Hb). 13C NMR (100 MHz, C6D6) : ꢀ
17.61 (q, 12-Me), 19.00, 19.06 (2 C, q, 14-Me, 13-Me),
20.71 (q, 15-Me), 23.34 (t, 8-C), 25.27 (t, 9-C), 33.25 (d,
11-C), 36.58 (t, 4-C), 37.85 (t, 10-C), 38.58 (t, 5-C),
44.62 (d, 1-C), 46.41 (d, 7-C), 53.01 (s, 6-C), 53.59 (s, 2-
C), 88.55 (s, 3-C). MS (EI, 70 eV), m/z (rel.int.) : 222 (1)
[M+], 189 (1), 179 (16), 161 (7), 151 (1), 137 (11), 123
(100), 111 (16), 95 (24), 94 (33), 81 (34), 67 (34), 55 (24),
41 (85).
3.15. Rearrangement of (À)-pethybrene (9)
To 0.2 mg of (À)-pethybrene in 500 ml of n-hexane
0.5 mg of AmberlystTM 15 were added. After stirring
at room temperature for 2 h the solution was filtered
to give (À)-a-isocomene (11) identical to the natural
product from Silphium perfoliatum.
3.12. Preparation of isopropyl-lithium
Acknowledgements
To 2 g of Li in 30 ml boiling hexane 9 ml of 2-chloro-
propane in 20 ml hexane were added. After heating
under reflux for 3 h the reaction mixture was filtered to
give a 2.0 M isopropyl-lithium soln. (hexane was dried
over Na and freshly distilled under Ar; 2-chloropropane
was dried over P2O5 and freshly distilled under Ar).
We thank Dr. V. Sinnwell, University of Hamburg,
for his support in the NMR measurements and Mrs. A.
Meiners and Mr. M. Preusse for technical assistance.
The financial support of the Fonds der Chemischen
Industrie is gratefully acknowledged.
3.13. Synthesis of (À)-petasitene (12)
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