The epimerization of sesamin and asarinin

Article abstract of DOI:10.1021/np050106d

Sesamin (1) and asarinin (2) are two C-7′ epimeric lignans. Molecular modeling by semiempirical methods indicated that 1 is more stable than 2 by about 2.5 kcal/mol. However, epimerization under acidic conditions led to a 44.8/55.2 equilibrium ratio of 1 and 2. Single-crystal X-ray diffraction analyses indicated that 1 was monoclinic with a = 10.0435(19) A, b = 6.9151(8) A, c = 11.8460(13) A, and 2 was triclinic with a= 5.595(5) A, b = 9.5910(18) A, c = 15.620(4) A. The unexpected equilibrium ratio of 1 and 2 indicated that structural changes are dependent on the conditions of the extraction processes.

Full text of DOI:10.1021/np050106d

1622
J. Nat. Prod. 2005, 68, 1622-1624
The Epimerization of Sesamin and Asarinin
Chia-Ying Li,† Tahsin J. Chow,‡ and Tian-Shung Wu*,†,§
Department of Chemistry, National Cheng Kung University, Tainan 701, Taiwan, Republic of China, Institute of Chemistry,
Academia Sinica, Taipei 115, Taiwan, Republic of China, and National Research Institute of Chinese Medicine,
Taipei 112, Taiwan, Republic of China
Received March 29, 2005
Sesamin (1) and asarinin (2) are two C-7′ epimeric lignans. Molecular modeling by semiempirical methods
indicated that 1 is more stable than 2 by about 2.5 kcal/mol. However, epimerization under acidic
conditions led to a 44.8/55.2 equilibrium ratio of 1 and 2. Single-crystal X-ray diffraction analyses indicated
that 1 was monoclinic with a ) 10.0435(19) Å, b ) 6.9151(8) Å, c ) 11.8460(13) Å, and 2 was triclinic
with a ) 5.595(5) Å, b ) 9.5910(18) Å, c ) 15.620(4) Å. The unexpected equilibrium ratio of 1 and 2
indicated that structural changes are dependent on the conditions of the extraction processes.
Furofuran lignans comprise a large group of natural
products characterized by coupling of two phenylpropane
(C6-C3) units. They are among the largest subclass of
lignans with a wide range of biological activities such as
antitumor, antimitotic, antimicrobial, and antioxidative.1
The majority of furofuran lignans have 2,6-diaryl substit-
uents on the exo face of the bicyclic skeleton (e.g., sesamin
1), although those with endo, exo aryl substitution (e.g.,
asarinin 2) and others with endo, endo substitution are also
Figure 1. Structures of sesamin (1) and asarinin (2).
known.2 Sesamin (1), which can be extracted exclusively
from sesame in large quantities, is intriguing due to its
Results and Discussion
highly efficient antioxidant activity.3 It was also reported
that sesamin is a specific inhibitor of ∆5-desaturase, which
Both sesamin (1) and asarinin (2) have a furofuran
catalyzes the conversion of dihomo-γ-linolenic acid to
arachidonic acid, in both microorganisms and animals,4 and
sesamin also exerts hypocholesterolemic activity through
the inhibition of cholesterol absorption and synthesis.5 The
other report indicated that sesamin prevented liver damage
caused by alcohol or carbon tetrachloride and showed a
suppressive effect against 7,12-dimethylbenz[a]anthracene-
induced rat mammary carcinogenesis and antihypertensive
effects.6 Asarinin has also been shown to have several
significant biological activities, including antitumor promo-
tion, antiallergic activity, and enhancement of the toxicity
of certain insecticides.7,8
The radix of Asarum heterotropoides Fr. var. mandshu-
ricum Kitag. (Xixin) is a traditional herbal medicine listed
officially in the Chinese Pharmacopoeia and used as an
analgesic, antitussive agent, and expectorant for treatment
of influenza, headache, rheumatic pain, and asthma. It is
also used as a drug for invigorating blood circulation and
eliminating blood stasis.9 Furofuran lignans have been
reported to be widely contained in this plant.10 In our
processes of the isolation of sesamin and asarinin previ-
ously, different amounts of 1 and 2 were obtained due to
different extraction methods, e.g., soaking at room tem-
perature or vacuum distillation. Several reports also
indicated that sesamin epimerized to asarinin during acid-
catalyzed conditions. Given the varied biological activities
displayed by these two furofuran lignans and their inter-
esting conformational behavior, a more in-depth study on
the epimerization process was undertaken.
backbone, which is similar to those of norbornane and
bicyclo[3.3.0]octane. It is well-known that the exo isomer
is more stable than the endo isomer. Sesamin has two
substituents in exo positions, and asarinin has one sub-
stituent in exo and the other in endo position (Figure 1).
Therefore, the conformation of sesamin is supposed to be
more stable than that of asarinin. However, in normal
isolation processes from A. heterotropoides Fr. var. mand-
shuricum Kitag., the relative ratio of the two depends on
the methods of extraction. The quantity of 2 was close to
that of 1 when soaked in acetone at room temperature.
However, when they were isolated by vacuum distillation
(10 mmHg, 140 °C, 15 min) or by steam distillation (110
°C, 3 h), the quantity of 2 was 2-fold that of 1. These results
were quite unexpected, and it was suggested that the
variations occurred due to acidic conditions, when the plant
was extracted at high temperature. A controlled experi-
ment was therefore executed starting from pure samples
of 1 and 2. An amount of pure 1 or 2 was refluxed in acidic
MeOH for several hours, while the reaction was monitored
periodically to determine the ratio of 1 and 2. High-
performance liquid chromatography (HPLC) was utilized
for structure identification using a Cosmosil 5C18-Ms (5 µm,
4.6 mm × 250 mm) column with MeOH/H2O (65:35) as the
mobile phase. The signals were detected by UV at a
wavelength of 240 nm. The results are shown in Figure 2.
In these experiments, both pure sesamin and asarinin gave
the same ratio of 44.8/55.2 of 1 versus 2. It appeared that
asarinin was a slightly more favored product in these acid-
catalyzed epimerization reactions. It was therefore of our
interest to examine the crystal structures in order to
confirm their molecular geometry.
* To whom correspondence should be addressed. Tel: +886-6-2757575,
ext. 65333. Fax: +886-6-2740552. E-mail: tswu@mail.ncku.edu.tw.
† National Cheng Kung University.
Crystal structures of the two isomers are shown in
Figures 3 and 4. The furofuran structure of sesamin was
‡ Institute of Chemistry, Academia Sinica.
§ National Research Institute of Chinese Medicine.
10.1021/np050106d CCC: $30.25
© 2005 American Chemical Society and American Society of Pharmacognosy
Published on Web 11/28/2005

Epimerization of Sesamin and Asarinin
Journal of Natural Products, 2005, Vol. 68, No. 11 1623
potential energy of 1 is lower than that of 2 by 2.32 kcal/
mol. A similar result can be obtained by the PM3 method,
where 1 is more stable than 2 by 2.74 kcal/mol. For
epimerization to occur, a process of protonation followed
by ring-opening must be involved. These processes depend
significantly on the polarity of solvents. The thermo-
dynamic equilibrium between 1 and 2 also depends on the
hydrogen-bonding nature of the solvents, as both structures
are highly oxygenated. These factors, which were not
considered thoroughly by the simple empirical modeling,
may override the small HF difference.
Figure 2. Acid-catalyzed epimerization of sesamin/asarinin.
In summary, we have presented evidence that either
asarinin or sesamin can epimerize under acidic conditions.
The crystal structures of both isomers were confirmed by
X-ray crystal diffraction analyses. Their heats of formation
were estimated by semiempirical methods, which showed
that 1 is more stable than 2 by about 2.5 kcal/mol. The
experimental equilibrium ratio of 44.8/55.2 between sesa-
min and asarinin was unexpected. This indicated that these
compounds can epimerize in an unexpected way during the
extraction processes. Indeed, previous studies revealed that
in the commercial preparation of sesamin epimerization
occurred during acid-clay bleaching in an oil refining
process and a mixture of sesamin and asarinin in almost
equal amounts was formed.11 As a result, hepatic fatty acid
oxidation, mainly ascribed to asarinin, increased. Thus, the
decoction process of herbal medicines should be carried out
more carefully, especially for the complex traditional
Chinese medicines.
Figure 3. Diagram of sesamin (1) by X-ray. Principal ellipses are
shown at the 50% probability level; H atoms, as small spheres of
arbitrary size.
Experimental Section
Materials. Asarum heterotropoides var. mamdshuricum
was obtained from the market in Taiwan, and its origin was
verified by Prof. C. S. Kuoh. A voucher specimen was deposited
in the herbarium of Cheng Kung University, Tainan, Taiwan.
The standard sesamin (1) and asarinin (2) were obtained from
A. heterotropoides. var. mamdshuricum, and the purities were
determined by HLPC analysis. All reagents were analytical
grade. MeOH and HCl were obtained from E. Merck (Ger-
many).
Figure 4. Diagram of asarinin (2) by X-ray. Principal ellipses are
shown at the 50% probability level; H atoms, as small spheres of
arbitrary size.
Epimerization Study. A sample (20 mg of sesamin 1 or
asarinin 2) was dissolved in 50 mL of 10% (w/w) HCl/EtOH.
The solution was stirred under reflux (about 85 °C). The
reaction was monitored by HPLC until equilibrium was
reached. After every hour 1 mL of the reaction solution was
diluted to 10 mL with MeOH and injected into HPLC for
quantitative analysis. The HPLC was equipped with a Hitachi
L-6200 pump, a Hitachi L-4000 UV detector, and a Cosmosil
5C18-MS (5 µm, 4.6 mm × 250 mm) column. The mobile phase
comprised a mixture of MeOH/H2O (65:35). The flow rate was
0.8 mL/min, and the detecting wavelength was 240 nm. The
reactions reached equilibrium at 1.5 h, where the mixture
contained 44.8% of 1 and 55.2% of 2.
Figure 5. Likely conformations of 1 and 2.
more twisted than that of asarinin, as both angles C7-
C8-C9′ (125.3°) and C9-C8′-C7′ (128.2°) in sesamin are
larger than those (114.9°, 113.6°) in asarinin (see Support-
ing Information) to reduce proton interactions at C7 and
C7′. Two phenyl substituents at C7 and C7′ of sesamin are
almost in the same plane as its central furofuran structure
with torsional angles C6-C1-C7-C8 ) 18.7° and C6′-
C1′-C7′-C8′ ) 43.9°. However, torsional angles C6-C1-
C7-C8 and C6′-C1′-C7′-C8′ in asarinin are 72.8° and
30.1°, respectively. In asarinin one of the phenyl substit-
uents is almost perpendicular to its central furofuran
structure and the substituent is located at the “endo”
position. The stability of asarinin can be explained by the
boat/chair conformation of its furofuran rings (Figure 5).
In this form the phenyl group occupies the equatorial-endo
position and steric hindrance is substantially reduced.
The heats of formation (HF) of the two isomers were
calculated by semiempirical methods. The HFs of 1 and 2
were estimated by AM1 with full geometry optimization
to be -151.96 and -149.64 kcal/mol, respectively. The
Computation. Molecular modeling was performed by
semiempirical methods AM1 and PM3 with full geometry
optimization. These methods were implanted in the Spartan
’04 1.0.1 software (Wavefunction Inc.) and were executed on a
personal computer.
Acknowledgment. We thank the National Science Coun-
cil, Republic of China, for financial support of this research
(NSC 91-2113-M-006-007).
Supporting Information Available: X-ray crystallographic data
of compounds 1 and 2. This material is available free of charge via
the Internet at http://pubs.acs.org.
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(3) Satoko, S. Y.; Masumi, K.; Yoshinao, O.; Yoko, T.; Takashi, I. J. Agric.
Food Chem. 2001, 49, 2647-2651.

1624 Journal of Natural Products, 2005, Vol. 68, No. 11
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