Sesquiterpenes from the essential oil of the liverwort Conocephalum conicum

Article abstract of DOI:10.1016/S0031-9422(98)00769-9

Three new brasilane type sesquiterpenes related to the known alcohol conocephalenol and the sesquiterpene alcohol presilphiperfolan-1-ol were isolated from a chemotype of the liverwort Conocephalum conicum collected in southern Germany and identified by NMR investigations, enantioselective gas chromatography and chemical correlations. In addition ent-(-)-dactylol, the enantiomer of a compound so far only found as a constituent of the Caribbean sea hare Aplysia dactylomela, was identified.

Full text of DOI:10.1016/S0031-9422(98)00769-9

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Phytochemistry 51 (1999) 517±523
Sesquiterpenes from the essential oil of the liverwort
Conocephalum conicum
Stephanie Melching, Wilfried A. Konig*
Institut fur Organische Chemie, Universitat Hamburg, D-20146 Hamburg, Germany
È
È
Received in revised form 16 November 1998
Abstract
Three new brasilane type sesquiterpenes related to the known alcohol conocephalenol and the sesquiterpene alcohol
presilphiperfolan-1-ol were isolated from a chemotype of the liverwort Conocephalum conicum collected in southern Germany
and identi®ed by NMR investigations, enantioselective gas chromatography and chemical correlations. In addition ent-( )-
dactylol, the enantiomer of a compound so far only found as a constituent of the Caribbean sea hare Aplysia dactylomela, was
identi®ed. # 1999 Elsevier Science Ltd. All rights reserved.
Keywords: Conocephalum conicum; Hepaticae; Sesquiterpenes; Brasiladienes
1. Introduction
2. Results and discussion
Conocephalum conicum is a thallous liverwort widely
spread in Europe, North Africa, Asia and North
America. The leaves display a pleasant turpentine odor
upon squeezing, which is mainly due to the high
monoterpene contents. The occurrence of di€erent
chemotypes or chemical races of this liverwort has
been known for some time. They were di€erentiated
by their ¯avonoid constituents (Markham, Porter,
Mues, Zinsmeister, & Brehm, 1976; Porter, 1981) and
by speci®c volatile components (Toyota, Saito,
Matsunami, & Asakawa, 1997). Specimens of the
chemotype described in this work were collected near
Adelberg (Goppingen), in southern Germany and in
Devils Bridge (Wales, UK). It is one out of four dis-
tinctly di€erent chemotypes of C. conicum investigated
in our group.
The structures of the sesquiterpene constituents as
obtained by hydrodistillation (Fig. 1) and dichloro-
methane extraction of the liverwort were almost com-
pletely elucidated. Although di€ering peak areas are
observed in the hydrodistillation and solvent extraction
gas chromatograms (Fig. 1), all the components,
except compound 12, are present in both samples,
which indicates that the sesquiterpene hydrocarbons
are genuine constituents in the plant and not artefacts
formed under the conditions of sample preparation.
One of the major components of the essential oil and
of the dichloromethane extract is conocephalenol (1),
which was ®rst isolated and described by Connolly
(1990) in C. conicum collected in Scotland and has
been synthesized by Tori, Sono, Nakashima, Nakaki,
and Asakawa (1991). Somewhat later, it was also iso-
lated from C. conicum collected in Germany
(Asakawa, 1995). Brasilane type structures have pre-
viously only been known from marine organisms
(Stallard, Fenical, & Kittredge, 1978; Wright, Konig,
& Sticher, 1991). Most of the volatile constituents as
shown in Fig. 1 were isolated by preparative gas chro-
* Corresponding author. Tel.: +49-40-4123-2824; fax: +49-40-
4123-2893.
E-mail address: wkoenig@chemie.uni-hamburg.de (W.A. Konig)
0031-9422/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved.
PII: S0031-9422(98)00769-9

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S. Melching, W.A. Konig / Phytochemistry 51 (1999) 517±523
È
matography (GC) and investigated by 1- and 2-dimen-
sional NMR spectroscopy.
Brasila-5,10-diene (4) with a molecular mass of 204
From this methylene signal a coupling to the only ole-
®nic proton at d 5.46 (broad singlet) can be observed.
Similarly, as in the case of 4, the C-2 methylene pro-
tons absorb at high ®eld (multiplet at d 1.03 and 2.29).
Brasila-1(6),5(10)-diene (6), molecular mass 204, is
very labile in the isolated state and usually decom-
posed in the refrigerator in less than a week, while it
appeared to be stable in the mixture of the essential
1
shows H NMR signals at d 4.88 and 5.02 typical for
an exocyclic double bond with a coupling correlation
to a singlet at d 1.82 indicationg an isopropenyl resi-
due. Correspondingly, in the 13C NMR spectrum the
triplet of an ole®nic methylene group is detected at d
112.6. Three quaternary ole®nic carbon signals are
indicated by singlets at d 129.6, 137.9 and 146.4 for
1
oil. In the H NMR spectrum the geminal C-13/C-14
methyl groups resonate at d 0.96 and 0.98. The signals
of the C-11- and C-12 methyl groups next to the ole®-
nic double bond appear at d 1.15 and 1.90, respect-
ively. A methyl doublet signal representing C-15 is
observed at d 1.01. No ole®nic protons were found.
Dehydration of a sample of conocephalenol (1) with
phosphoryl chloride yielded products 4±6 (Fig. 2),
giving proof of the assigned structures and stereoche-
mical correlations.
1
carbons C-5, C-6 and C-10. In the H NMR spectrum
two geminal methyl singlets can be observed at d 0.94
and 0.99 showing a coupling correlation in the 1H1H
COSY spectrum. A third methyl doublet signal for C-
15 is observed at d 1.00, which couples with the meth-
ine multiplet in the range of d 1.15±1.30. A methylene
signal at unusually high ®eld of d 0.86 for H-2a
couples with H-2b at d 1.56.
Brasila-5(10),6-diene (5), molecular mass 204, was
1
Finally, in addition to the brasiladienes, 3,4-
dimethoxystyrene (7), a constituent, which also occurs
in a northern German chemotype of C. conicum, was
identi®ed by comparison of its spectroscopic properties
also isolated by preparative GC. In the H NMR ®ve
methyl signals are present. For two high ®eld singlets
at d 0.91 and 0.97 a coupling correlation can be
1
detected in the H1H COSY spectrum, indicating two
with a synthetic reference (Peterson, Russell, &
geminal methyl groups C-13 and C-14. Another two
signals at lower ®eld (d 1.69 and 1.9) also couple with
each other and can be assigned to methyl groups C-11
and C-12. A methyl doublet at d 1.08, representing the
methyl group C-15, has a coupling correlation to a
methine proton at C-9, which again couples with the
widely split methylene signal of C-8 at d 2.10 and 2.53.
Everson, 1991). Compound 7 was previously identi®ed
as a constituent of the ether extract of an unknown
Malaysian liverwort of Asterella or Mannia species
(Asakawa, Toyota, Tanaka, Hashimoto, & Joulain,
1995).
The main component of the essential oil of C. coni-
cum is ( )-selin-11-en-4-ol (8), a compound identi®ed

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S. Melching, W.A. Konig / Phytochemistry 51 (1999) 517±523
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519
before as a constituent of Podocarpus dacrydioides
(San Feliciano, Medarde, Del Rey, Del Corral, &
Barrero, 1990) and of the liverwort Riccardia jackii
(Asakawa, 1995). Compound 8 shows spectroscopic
data identical to those published (San Feliciano et al.,
1990). (+)-Selina-4,11-diene (9) can be considered as a
dehydration product of 8 and was identi®ed by com-
parison of its mass and NMR spectra with published
data (Maurer, & Grieder, 1977). Dehydration of 8
gave (+)-9, ( )-a-selinene (10) and (+)-b-selinene (11)
(Fig. 3). The absolute con®guration of all products
was determined by enantioselective GC with
heptakis(6-O-t.butyldimethylsilyl-2,3-di-O-methyl)-
b-cyclodextrin as stationary phases.
Konig, unpublished results) and the spectroscopic data
correspond to those published (Ganûer, Pollak, &
Berger, 1995). Known compounds are also ( )-bicyclo-
germacrene (13), ( )-isolepidozene (14) (Konig et al.,
1996) and bicyclogermacren-14-al (15). The spectro-
scopic data of the latter compound were identical with
those published by Connolly (1990). The NMR spec-
tral data of compound 3 matched with those published
for (+)-dactylol, a sesquiterpene alcohol isolated from
the Caribbean sea hare Aplysia dactylomela (Schmitz,
Hollenbeak,
&
Vanderah, 1978). However, we
measured a negative optical rotation and the presence
of the ent-( )-dactylol can be concluded.
The NMR spectroscopic data of 2 (presilphiperfo-
lan-1-ol) indicated a tricyclic sesquiterpene alcohol
(C15H26O) without an ole®nic double bond. Four
methyl groups (three singlets and one doublet) could
be assigned: d 1.32 corresponding to CH3-14, d 0.80
and 0.94, which showed a coupling correlation in the
1H1H COSY spectrum, corresponding to CH3-12 and
Peak 12 (Fig. 1) could be assigned to ( )-germacra-
dien-11-ol (12), a component also found in the liver-
wort
Dumortiera
hirsuta
(Toyota,
Yoshida,
Matsunami, & Asakawa, 1997). The con®guration is
identical to a sample of 12, which was isolated from
the liverwort Porella obtusata (C. Fricke and W.A.

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Fig. 1. Gas chromatographic separation of the dichloromethane extract (top) and the hydrodistillate (bottom) of C. conicum. 25 m CP Sil 5; col-
1
umn temperature 508C, 38C min to 2308C.
CH3-13, and a doublet at d 0.89, corresponding to
CH3-15. The latter couples to a multiplet in the
absorption range of d 1.52±1.65. A methylene singlet
at d 1.50 shows a coupling correlation to one of the
1
high-®eld methyl singlets in the H1H COSY spectrum.
In the HMBC spectrum coupling correlations to
Fig. 2. Dehydration of conocephalenol (1) with phosphoryl chloride.