Structure and absolute configuration of nyasol and hinokiresinol via synthesis and vibrational circular dichroism spectroscopy

Article abstract of DOI:10.1021/np0502995

The absolute configuration of the norlignan (+)-nyasol was determined to be S by comparison of the experimental vibrational circular dichroism data with first-principle calculations taking into account the eight lowest energy conformations. The established absolute configuration of (+)-nyasol enables establishment of the absolute configuration of (-)-hinokiresinol, which is concluded to be S. A total synthesis and resolution of hinokiresinol has been performed to resolve the conflicting reports of the coupling constant of the vinylic protons of the disubstituted double bond in this molecule. Racemic hinokiresinol was resolved. Both enantiomers possess the same antiplasmodial activity.

Full text of DOI:10.1021/np0502995

J. Nat. Prod. 2005, 68, 1603-1609
1603
Structure and Absolute Configuration of Nyasol and Hinokiresinol via
Synthesis and Vibrational Circular Dichroism Spectroscopy
Peter R. Lassen,†,# Dorthe Mondrup Skytte,‡,# Lars Hemmingsen,§ Simon Feldbæk Nielsen,|
Teresa B. Freedman, Laurence A. Nafie, and S. Brøgger Christensen*,‡
The Quantum Protein Centre, Department of Physics, Technical University of Denmark, DK 2800 Lyngby, Denmark,
Department of Medicinal Chemistry, The Danish University of Pharmaceutical Sciences, Universitetsparken 2, DK-2100
Copenhagen, Denmark, Department of Natural Sciences, Royal Veterinary and Agricultural University, Thorvaldsensvej 40,
DK-1871 Frederiksberg C, Denmark, Lica Pharmaceuticals, Fruebjergvej 3, DK-2100 Copenhagen Ø, Denmark, and
Department of Chemistry, Syracuse University, Syracuse, New York 13244
Received August 16, 2005
The absolute configuration of the norlignan (+)-nyasol was determined to be S by comparison of the
experimental vibrational circular dichroism data with first-principle calculations taking into account the
eight lowest energy conformations. The established absolute configuration of (+)-nyasol enables
establishment of the absolute configuration of (-)-hinokiresinol, which is concluded to be S. A total
synthesis and resolution of hinokiresinol has been performed to resolve the conflicting reports of the
coupling constant of the vinylic protons of the disubstituted double bond in this molecule. Racemic
hinokiresinol was resolved. Both enantiomers possess the same antiplasmodial activity.
Chart 1
Nyasol (1) and the E-isomer hinokiresinol (2) (Chart 1)
possessing the skeleton C6C5C6 are typical examples of
naturally occurring norlignans. The confusion concerning
the nomenclature of these two compounds should have been
settled by Minami et al.1 and Oketch-Rabah et al.,2 who
agreed that the Z-isomer should be named nyasol and the
E-isomer hinokiresinol. Unfortunately, confusion concern-
ing the magnitude of the coupling constant of the vinylic
protons of the disubstituted double bond in 2 has still not
been settled.3-5
Chart 2
The first report on the isolation of hinokiresinol from
Chamaecyparis obtusa did not specify the optical rotation,6
but a later report established that this plant forms (-)-
(2).1 Similarly, no rotation was reported for hinokiresinol
isolated from Libocedrus yateensis7 or from Agathis aus-
tralis.8 Both antipodes of nyasol are naturally occurring.
(+)-Nyasol has been isolated from Asparagus africanus2
and A. cochinchinensis,9 and (-)-nyasol has been obtained
from Anemarrhena asphodeloides.10,11 A chiral analysis
revealed that the nyasol with a specific rotation of [R]D
-67° consisted of 77% of the levorotatory enantiomer and
23% of the dextrorotatory enantiomer, corresponding to a
specific rotation of -124° for enantiomeric pure (-)-nyasol.1
The absolute configuration of hinokiresinol (2) isolated
from Agathis australis has been assigned as S on the basis
of correlation with 3 and 4 (Chart 2).8 However, no specific
rotation for the isolated hinokiresinol (2) was reported,
which makes the determined absolute configuration less
authoritative.8 The correlation is based on comparison of
the ORD curve of the dimethyl ether of 2, formed by
thermal conversion of an orthoformate of the dimethyl
ether of agatharesinol (4), with the ORD curve of the free
phenol 2.8 The specific rotation of the phenol 2 isolated from
Chamaecyparis obtusa is -3,1 whereas the dimethyl ether
is reported to have a specific rotation of +8.4.12 Comparison
of the ORD curves of these two compounds, thus, should
take this shift of sign of the rotation into consideration.
The thermal conversion of the orthoformate apparently
isomerizes the disubstituted double bond into the Z-isomer,
3
since later studies showed that the J coupling of the two
vinylic protons at the disubstituted double bond is 12 Hz.4,5
Such a coupling constant is similar to that reported for
nyasol,2,9 whereas the coupling constant in 2 is 15.9 Hz.1
This isomerization further questions the conclusion con-
cerning the configuration.
The poor basis for the suggested absolute configuration
of hinokiresinol (2) and nyasol (1) has encouraged us to
reinvestigate the absolute configuration of (+)-2, isolated
from Asparagus africanus, by vibrational circular dichroism
spectroscopy (VCD). This technique permits comparison of
a recorded spectrum with a spectrum calculated from first-
principle quantum mechanical methods, i.e., in a nonem-
pirical way.13 A total synthesis of 2 and comparison of the
1H NMR spectra of 1 and 2 should finally settle the
confusion of the magnitude of the coupling constant. Finally
advantage was taken of access to the two optical antipodes
of 2 to reveal if the antiplasmodial activity was related to
the configurational variations. (+)-Nyasol is a modest
antiplasmodial agent,2 but a possible relationship between
configuration and antiplasmodial activity has not been
investigated.
* To whom correspondence should be addressed. Tel: +45 3530 6253.
Fax: +45 3530 6041. E-mail: sbc@dfuni.dk.
† The Quantum Protein Centre.
‡ The Danish University of Pharmaceutical Sciences.
§ Royal Veterinary and Agricultural University.
|
Lica Pharmaceuticals. Present address: Leo Pharma, Industriparken
55, DK-2710 Ballerup, Denmark.
Syracuse University.
# These two authors have contributed equally to the article.
10.1021/np0502995 CCC: $30.25
© 2005 American Chemical Society and American Society of Pharmacognosy
Published on Web 10/21/2005

1604 Journal of Natural Products, 2005, Vol. 68, No. 11
Lassen et al.
Table 1. Calculated Energies of Gas-Phase Nyasol Conformers
(kJ/mol), Relative to the Lowest Energy Structure, as Found
Using the MMFF Force Field and Two DFT Methods, B3LYP/
6-31G** and B3LYP/AUG-cc-pVDZa
of theoretical VCD signals are given in Table 2 and Figure
2.
The challenge arises from the fact that the VCD signals
from the various conformers tend to extinguish one an-
other, putting high demands on the signal-to-noise ratio
of the instrument. There are, however, spectral regions
where the first-principle calculations predict that most
conformers give similar VCD signals, allowing determina-
tion of the absolute configuration of nyasol (Figure 3).
In particular the strong positive signal at 1510 cm-1
observed for nyasol dissolved in DMSO-d6 (Figure 4) and
the spectral signatures at 800-1000 and 1590-1650 cm-1
observed for nyasol in a KBr pellet (Figure 5) are such
regions.
The pronounced similarities between the recorded spec-
trum of (+)-nyasol and the calculated spectrum for (S)-
nyasol in these regions therefore show that (+)-nyasol
possesses the S-configuration. There are discrepancies
between experimental and calculated spectra, particularly
in the region from 1100 to 1300 cm-1, for both the IR
absorption and VCD signals. The most intense signals in
this region originate from vibrations involving the two
hydroxy groups of nyasol, indicating that the broadening
and changes in frequencies and relative intensities of the
signals are due to nyasol-solvent and/or nyasol-nyasol
interactions. These intermolecular interactions tend to
broaden the signal from the C-O stretching vibration at
1270 cm-1 and almost extinguish the O-H bending at 1170
cm-1. Their combined vibration at 1620 cm-1 is less
affected. The quenched O-H bending has been reproduced
theoretically by calculating spectra with one explicit DMSO
molecule next to each of the two hydroxy groups, putting
the qualitative interpretation on more firm theoretical
grounds. UV-vis absorption and conventional (electronic)
CD spectra were also recorded for nyasol from A. africanus
(see Figure 6), and given the assignment derived from the
VCD experiments, we conclude that the CD spectrum
corresponds to (S)-nyasol.
no.
MMFF
B3LYP/6-31G**
B3LYP/AUG-cc-pVDZ
1
0.330200
0.000000
0.402573
0.729980
2.226303
3.039871
2.559570
2.368820
5.528778
5.008957
6.133102
5.669357
14.325104
14.631180
15.171234
14.964020
19.457092
16.850830
19.126968
19.592728
18.936859
19.799881
19.873138
14.858643
16.962799
17.427505
0.0000
0.0105
0.1444
0.1864
0.2547
0.9321
1.0502
1.2025
4.3426
4.4450
4.5631
4.9753
5.4190
5.4715
5.6002
5.6632
19.1898
19.2134
20.7913
21.0276
21.3007
21.5002
24.8189
24.9974
25.3807
25.6984
0.0567
0.0000
0.2133
0.1895
1.5526
1.8797
1.9229
2.1405
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
a The conformations are numbered according to the B3LYP/6-
31G** energies. The DFT results include thermal free energy
contributions calculated using B3LYP/6-31G**.
Minami et al.1 have similarly utilized CD data in the
assignment of absolute configuration of nyasol and hi-
nokiresinol, in agreement with the data presented in
Figure 6. Their findings prove that (-)-nyasol is hydroge-
nated to the antipode of the tetrahydro derivative of (-)-
hinokiresinol. Consequently the parent compounds (-)-
nyasol and (-)-hinokiresinol must possess opposite absolute
configurations as well. Since (+)-nyasol is S-configured, (-)-
nyasol must possess the R-configuration. (-)-Hinokiresinol
has the opposite configuration, that is, S, and consequently
(+)-hinokiresinol possesses the R-configuration as previ-
ously concluded.8
Synthesis of Hinokiresinol (2). Some synthetic meth-
ods for the dimethyl ether of 2 have been published.6,14 The
synthesis of the dimethyl ether of hinokiresinol is initiated
by an addition of a Grignard reagent to an oxo compound,
preventing the direct synthesis of hinokiresinol with free
phenolic groups.
In the only reported synthesis of the free diphenol 215
the phenolic hydroxyl groups were protected as tert-
butyldimethylsilyl ethers (TBDMS). The TBDMS group
was cleaved during the initial Claisen-Schmidt condensa-
tion and had to be reintroduced in a modest yield. This
problem was overcome by protecting the phenolic groups
as tetrahydropyranyl ethers. This protecting group sur-
vived the condensation of the ketone and the aldehyde to
give 5, which smoothly reacted with vinylmagnesium
bromide to give 6. Addition of copper(I) iodide ensured 1,4-
addition of the Grignard reagent. Sodium borohydride
Figure 1. DFT-optimized structures of four (S)-nyasol conformers (top
left: 1, top right: 5, bottom left: 9, bottom right: 13; numbered
according to relative energies, see Table 1), representing the 16 lowest
energy conformers. Each structure represents a group of four conform-
ers, which differ only in the relative orientation of the hydroxy groups.
Results and Discussion
Despite the moderate size of 1, a considerable number
of conformers exist at room temperature (Table 1 and
Figure 1), with the eight lowest energy structures within
2.1 kJ/mol at the B3LYP/AUG-cc-pVDZ level of theory.
Thus, 1 is among the most flexible small organic mol-
ecules studied by VCD spectroscopy. Spectral assignments

Structure of Nyasol and Hinokiresinol
Journal of Natural Products, 2005, Vol. 68, No. 11 1605
Table 2. Assignment of Selected Peaks in the B3LYP/AUG-cc-pVDZ-Calculated IR and VCD Spectra for the Lowest Energy
Conformera
ν (cm-1
)
ꢀ (M-1 cm-1
)
104∆ꢀ (M-1 cm-1
)
assignment
phenol C-H oop bend
846
849
859
888
949
43.267
96.499
54.035
79.521
23.575
55.749
22.618
36.633
33.284
33.226
482.469
161.730
253.009
124.863
207.496
197.746
15.465
87.202
73.435
57.911
35.367
37.615
276.918
257.528
42.569
48.017
186.086
94.705
7.045
-263.076
323.353
-49.5495
-42.1694
26.0106
-4.53252
-4.70156
13.4051
-75.0412
-7.48434
96.6739
-92.2848
-23.8879
78.9772
-39.1544
-84.5487
97.8881
-40.1792
33.0594
-27.8639
-19.088
111.113
phenol C-H oop bend
phenol C-H oop bend
methine C-H bend
vinyl C-H oop bend
959
vinyl C-H oop bend
1005
1025
1120
1127
1184
1186
1189
1269
1273
1279
1322
1367
1369
1423
1434
1464
1535
1539
1625
1634
1653
1654
1689
1693
vinyl C-H oop bend
vinyl C-H ip bend
phenol C-H ip bend
phenol C-H ip bend
O-H bend
O-H bend
O-H bend
C-O stretch, methine C-H bend
C-O stretch, methine C-H bend
C-O stretch, methine C-H bend
vinyl C-H ip bend
phenol C-C stretch, C-H and O-H ip bend
Phenol C-C stretch, C-H and O-H ip bend
vinyl C-H ip bend
vinyl C-H ip bend
phenol C-H, O-H ip bend
phenol C-O stretch, C-H ip bend
phenol C-O stretch, C-H ip bend
phenol C-C stretch
22.9206
146.532
-55.4286
-5.691
phenol C-C stretch
2.0246
phenol C-C stretch
-5.97896
-25.3217
-76.3316
phenol C-C stretch
vinyl C-C stretch
38.819
vinyl C-C stretch
a Columns are wavenumbers ν, molar absorptivities ꢀ, differential absorptivities ∆ꢀ, and assignments. Abbreviations: ip ) in-plane,
oop ) out-of-plane.
Figure 2. Relative amplitudes of selected vibrational modes assigned in Table 2 for the lowest energy conformer, calculated using B3LYP/AUG-
cc-pVDZ. Each mode is labeled with the corresponding wavenumber (cm-1).
reduction of the ketone yielded a mixture of the diastere-
omeric alcohols 7. A further advantage of the tetrahydro-
pyranyl group was that a hydrochloric acid-catalyzed
elimination of water from 7 simultaneously cleaved the
protecting groups. In addition all the steps in the reaction
sequence in Scheme 1 afforded better yields than the
reaction sequence using the TBDMS protecting group. No
nyasol was detected in the reaction mixture. The vinylic
coupling constant was found to be 15.9 Hz, as expected for
a trans coupling.
Optical resolution was performed by chiral HPLC to give
1
the two enantiomers having identical H NMR spectra.
The antiplasmodial effect of the two enantiomers of 2
was determined. Surprisingly, the enantiomers were equi-

1606 Journal of Natural Products, 2005, Vol. 68, No. 11
Lassen et al.
Figure 3. Calculated IR absorption (lower half) and VCD spectra
(upper half) for the eight lowest energy conformers of (S)-nyasol, at
the B3LYP/AUG-cc-pVDZ level of theory. Frequencies are not scaled.
The bands are plotted as Lorentzians with half-widths of 6 cm-1. Half
of the spectra are offset for clarity.
potent [IC50 values: rac-2, 51 ( 10 µM; (+)-2, 55 ( 14 µM;
(-)-2, 80 ( 25 µM]. The antiplasmodial activity, thus, is
not dependent on the absolute configuration.
Figure 4. Comparison of observed and calculated IR absorption (lower
half) and VCD (upper half) spectra of nyasol. Experimental spectra
are measured in DMSO-d6, using a resolution of 8 cm-1 and 9 h
acquisition time, with the instrument optimized at 1400 cm-1. VCD
noise is shown in the uppermost (dashed) trace. Experimental IR
absorption spectra are corrected for the presence of solvent, and spectra
are offset for clarity. Calculated spectra are Boltzmann-weighted
averages of the eight lowest energy conformers of the S-enantiomer,
with three different DFT methods as indicated. The B3LYP/6-31G**
SCRF uses a continuum solvent model for DMSO.
Experimental Section
General Experimental Procedures. IR and VCD spectra
were obtained on a ChiralIR FT-VCD spectrometer (BioTools,
Inc., Wauconda, IL), equipped with two sources16 and two
photoelastic modulators17 optimized at 1400 cm-1. An optical
band-pass filter was used to allow only the spectral region
800-2000 cm-1 to reach the detector. Spectra were calibrated
automatically, using the standard calibration files.
MacroModel (Schro¨dinger, Portland, OR).18 Conformational
searches were made in the gas phase using systematic
torsional search (SUMM) and an energy window of 20 kJ/mol.
For each structure the structure was optimized using the
MMFF force field, with 500 steps, 1000 iterations, and a
threshold of 0.05. The (unique) structures from the conforma-
tional search were subsequently geometry optimized using
density functional theory (DFT) as implemented in Gaussian
03,19 using the B3LYP functional20 with the 6-31G** and AUG-
cc-pVDZ basis sets and default convergence criteria.21,22 Vi-
brational spectra were calculated with the same methods.
Dipole and rotational strengths from Gaussian were converted
into molar absorptivities13 (M-1 cm-1), and each spectrum was
plotted as a sum of Lorentzian bands with half-widths of 6
cm-1 for comparison with experiments. The relative energies
presented include free energy corrections from molecular
vibrations.
Nyasol Conformers. In both the gas phase and solvent,
the conformational searches of (S)-nyasol yielded 26-29
structures within the energy window. A few of these structures
converged to the same local minimum at the B3LYP/6-31G**
level of theory. All energies from gas-phase geometry optimi-
zations in MacroModel and Gaussian 03 are shown in Table
1, where the conformations are numbered in order of the
B3LYP/6-31G** energies. In most cases the MMFF force field
performs very well, but there are cases where the relative
energy deviates by up to 8 kJ/mol from the first-principle
calculations.
VCD spectra were recorded both in DMSO-d6 solution (5
mg of nyasol in 100 µL of DMSO-d6 in a BaF2 cell with 100
µm path length) and in KBr pellets (250 mg KBr, φ 13 mm ×
1 mm). The resolution was 8 cm-1, and the acquisition time
was 9 h for DMSO-d6 solution and 31 h for the KBr pellet. In
the latter case, spectra from two different orientations of the
KBr pellet were averaged to reduce the baseline offset. 1H
NMR spectra were recorded on a Varian Mercury spectrometer
or Varian Gemini 2000 spectrometer at 300 MHz using
tetramethylsilane as internal standard. Splitting patterns are
described as singlet (s), doublet (d), triplet (t), doublet of
doublets (dd), doublet of double doublets (ddd), and multiplet
(m). Uncorrected melting points were determined on a Bu¨chi
SMP-20 apparatus, and optical rotation was measured on a
Perkin-Elmer 241 polarimeter. Column chromatography was
performed on silica gel 60 (Merck 107734). For HPLC a Waters
6000A pump and a Shimadzu SPD 6A detector was used. THF
was distilled from LAH and kept over molecular sieves. UV-
vis absorption spectra were recorded on a Cary 5 spectrometer,
and conventional (electronic) CD spectra were recorded on a
JASCO-720 CD instrument using a cylindrical quartz cuvette
with a light path of 5 µm and a 61 mg/mL solution of nyasol
in MeCN. Using MeOH as solvent does not change the spectra
significantly. The instrument was purged with N2. All spectra
were baseline corrected using a baseline recorded with no cell
in the instrument and collected at room temperature.
Computational Methods. Conformational searches and
geometry optimizations of (S)-nyasol were performed with

Structure of Nyasol and Hinokiresinol
Journal of Natural Products, 2005, Vol. 68, No. 11 1607
Figure 5. Observed IR absorption (lower half) and VCD (upper half)
spectra of nyasol in a KBr pellet, compared to the DFT calculation for
(S)-nyasol using B3LYP/AUG-cc-pVDZ. The experimental resolution
is 8 cm-1, with the instrument optimized at 1400 cm-1. The total
acquisition time is 31 h, and spectra from two different orientations
of the KBr pellet were averaged to reduce the baseline offset. VCD
noise is shown in the uppermost (dashed) trace.
A representative selection of the gas-phase structures are
shown in Figure 1, arranged in order of increasing energy. The
conformational energy displays gaps of about 3 kJ/mol between
conformers 8 and 9 and of about 14 kJ/mol between conformers
16 and 17. In the analysis of the VCD spectra we have
therefore included only the eight lowest energy conformers.
Furthermore, the conformational energies of the 16 lowest
energy conformers fall into groups of four having rather similar
energies, because both hydroxy groups can point in two almost
equally favorable orientations. Within each group of four
conformers, the carbon skeleton is basically fixed, and only
the hydroxy group orientations differ. No considerable geom-
etry changes in the gas-phase structures occur when DFT
optimization includes a self-consistent reaction field (SCRF)
solvent model (polarizable continuum model)23 for DMSO.
Infrared and VCD Spectra. Figure 3 depicts the calcu-
lated IR and VCD spectra for the eight lowest energy struc-
tures, calculated using the B3LYP functional and AUG-cc-
pVDZ basis set. The spectra of conformers 5-8 are offset for
clarity. All of the conformations have very similar IR spectra,
differing mostly in frequency and intensity of some minor
peaks. However, in the calculated VCD spectra, considerable
differences arise between the conformers To determine the
absolute configuration, we identified the spectral regions in
Figure 3 with the least variability. These regions will be
optimal for determinations of the absolute configurations of
nyasol, as the signals from different conformers do not
extinguish each other to any significant extent. These regions
are 800-1000, 1250-1350, and 1510 cm-1, and the negative
amplitude signals around 1650-1700 cm-1. To distinguish
conformers in the observed spectra, we note that the band
around 1530 cm-1 has a different shape for the two groups of
conformers (1-4 and 5-8). Additionally, around 1300 and 1650
cm-1, there seem to be consistent differences between these
two groups.
Figure 6. UV-vis absorption (bottom) and CD (top) of (+)-(S)-nyasol
in MeCN (61 mg/mL), path length of 5 µm. Based on the assignment
derived from the VCD spectra, the CD spectrum represents (S)-nyasol.
using B3LYP/6-31G**, B3LYP/6-31G** with an SCRF con-
tinuum solvent model for DMSO, and B3LYP/AUG-cc-pVDZ,
respectively. The solvent DMSO-d6 obscures the spectrum
below 1100 cm-1. In the 1250-1350 cm-1 region the VCD
signals are too weak to reliably determine the absolute
configuration, but above 1400 cm-1 the experimental spectrum
of (+)-nyasol and the calculated spectrum for S-nyasol compare
well. Similarly, there are considerable differences between the
calculated and observed IR absorption signals in the 1180-
1300 cm-1 region. In Figure 5 the (+)-nyasol containing a KBr
pellet measurement is compared to the B3LYP/AUG-cc-pVDZ
calculation for S-nyasol, which shows very good agreement in
the region 800-1000 cm-1 and reasonable agreement above
1300 cm-1. The nyasol concentration differs between the
DMSO-d6 solution and KBr pellet experiments, and conse-
quently different spectral regions have optimal IR absorption
for VCD measurements. Thus the two measurements comple-
ment each other by probing different spectral regions, and
consequently, VCD signal intensities should not be compared
between the two observed spectra. KBr pellets can easily show
birefringence artifacts, which is probably the case in the 1000-
1100 cm-1 region, and possibly other spectral regions as well.
The noise level does not identify such artifacts.
Synthesis of Hinokiresinol. (E)-1,3-Bis(4-[(tetrahydro-
2H-pyran-2-yl)methyl]phenyl)pent-4-en-1-one (6). A solu-
tion of vinylmagnesium bromide (1.0 M solution in THF, 60.0
mmol, 60.0 mL), in an inert nitrogen atmosphere, was added
to a suspension of cuprous iodide (2.7 mmol, 0.520 g) in THF
(60 mL) and cooled to 0 °C. After stirring for 15 min at 0 °C a
solution of (E)-1,3-bis(4-[tetrahydro-2H-pyran-2-yloxy]phenyl)-
prop-2-en-1-one (5)24 (11.13 g, 27.3 mmol) in THF (30 mL) was
added to the reaction mixture, and the mixture was stirred
for an additional 2 h at 0 °C. After quenching with an aqueous
saturated NH4Cl solution the mixture was diluted with Et2O
(400 mL) and successively washed twice with water (400 mL)
Assignments for the lowest energy conformer are shown in
Table 2.
In Figure 4 the recorded spectrum (DMSO-d6 solution) of
(+)-nyasol isolated from A. africanus as previously reported8
is compared to the Boltzmann weighted average of the
calculated spectra for the eight lowest energy conformers,

1608 Journal of Natural Products, 2005, Vol. 68, No. 11
Lassen et al.
Scheme 1
and once with brine (200 mL). The organic layer was dried
over Na2SO4, filtered, and concentrated in vacuo. The product
was purified by column chromatography using toluene-EtOAc
(20:1) as an eluent to give 6 (7.47 g, 75%) as pale yellow
crystals, mp 92-94 °C (MeOH): 1H NMR (CDCl3, 300 MHz)
δ 1.55-2.05 (12H, m, CH2 groups from THP); 3.26 (1H, dd, J
) 6.6, 16.8 Hz, H-8); 3.35 (1H, dd, J ) 7.8, 16.8 Hz, H-8); 3.56-
3.64 (2H, m, OCH2 groups from THP); 3.81-3.94 (2H, m, OCH2
groups from THP); 4.07 (1H, ddd, J ) 7.2 Hz, H-7′); 4.99 (1H,
d, J ) 17.4 Hz, H-9′); 5.03 (1H, d, J ) 10.2 Hz, H-9′); 5.37
(1H, ddd, J ) 3.0 Hz, O2CH group from THP); 5.51 (1H, ddd,
J ) 3.0 Hz, O2CH group from THP); 6.01 (1H, ddd, J ) 6.6,
10.8, 17.4 Hz, H-8′); 6.98 (2H, d, J ) 8.7 Hz, H-3′, H-5′); 7.06
(2H, d, J ) 8.7 Hz, H-3, H-5); 7.16 (2H, d, J ) 8.4 Hz, H-2′,
H-6′); 7.90 (2H, d, J ) 8.4 Hz, H-2, H-6).
(E)-1,3-Bis(4-[(tetrahydro-2H-pyran-2-yl)methyl]phe-
nyl)pent-4-en-1-ol (7). NaBH4 (3.24 g, 85.6 mmol) was added
to a solution of 6 (7.47 g, 17.1 mmol) in EtOH (150 mL), and
the solution was stirred overnight at room temperature. The
reaction mixture was quenched with an aqueous solution of
NaOH (5%, 300 mL), and the mixture was extracted twice with
Et2O (150 mL). The combined organic layers were dried over
Na2SO4, filtered, and concentrated in vacuo. The product was
purified by column chromatography using toluene-EtOAc (9:
1) as an eluent to give a mixture of the diastereomers as a
yellow syrup (7.20 g, 96%): 1H NMR (CDCl3, 300 MHz) δ 1.56-
2.34 (14H, m, H-8 + CH2 groups from THP); 3.32 (ddd, J )
7.5 Hz); 3.44 (ddd, J ) 8.1 Hz) (1H all together, H7′); 3.56-
3.60 (2H, m, OCH2 groups from THP); 3.84-3.93 (2H, m, OCH2
groups from THP); 4.43 (dd, J ) 6.0 Hz); 4.54 (dd, J ) 6.0 Hz)
(1H all together, H-7); 4.96-5.06 (2H, m, H-9′); 5.36-5.40 (2H,
m, O2CH groups from THP); 5.89 (ddd, J ) 7.5, 9.9, 17.4 Hz);
5.96 (ddd, J ) 7.5, 9.9, 17.4 Hz) (1H all together, H-8′); 6.95-
7.23 (8H, m, H-2, H-3, H-5, H-6, H-2′, H-3′, H-5′, H-6′).
Hinokiresinol (2) (racemic). To a solution of the alcohol
7 (7.19 g, 16.4 mmol) in MeOH (150 mL) was added a 7%
aqueous solution of HCl (85 mL), and the mixture was refluxed
for 1 h. After cooling it was poured into a saturated aqueous
NaHCO3 solution (500 mL). The mixture was extracted twice
with CH2Cl2 (200 mL). The combined organic layers were
washed with brine (200 mL), dried over Na2SO4, filtered, and
concentrated in vacuo. The product was purified by column
chromatography using toluene-EtOAc (9:1) to give racemic 2
as colorless crystals, which over time turned red (1.93 g, 47%),
mp 69-71 °C (CHCl3): 1H NMR (acetone-d6, 300 MHz) δ 4.12
(1H, t, J ) 6.9 Hz, H-7′); 5.09 (2H, m, H-9′); 6.09 (1H, ddd, J
) 6.9, 9.9, 17.7 Hz, H-8′); 6.26 (1H, dd, J ) 6.9, 15.9 Hz, H-8);
6.36 (1H, d, J ) 16.2 Hz, H-7); 6.78 (2H, d, J ) 8.7 Hz, H-3′,
H-5′); 6.80 (2H, d, J ) 9.0 Hz, H-3, H-5); 7.10 (2H, d, J ) 8.7
Hz, H-2′, H-6′); 7.27 (2H, d, J ) 9.0 Hz, H-2, H-6); 8.24 (1H, s,
br, OH); 8.41 (1H, s, br, OH)
Optical Resolution of Hinokiresinol (2). Racemic hi-
nokiresinol was resolved over a Daicel Chiralcel OD 0.46 ×
25 cm chiral column using petroleum ether-2-propanol (94:
6) with added HOAc (0.4%) as an eluent and a flow of 1 mL/
min. The two enantiomers eluted with retention times of 64.2
min ([R]D25 -1.7° (c 0.27, acetone)), lit.25 -3°) and 71 min ([R]D
25
+4.4° (c 0.38, acetone)).
Antiplasmodial Assay. The antiplasmodial activity toward
the chloroquine-sensitive 3D7 Plasmodium falciparum strain
(IC50 24 nM) was determined as previously described.26 The
experiments were performed three times, each time in tripli-
cate.
Acknowledgment. This work was supported by the Dan-
ish Ministry of Science Technology and Innovation and by the
Lundbeck Foundation. P.-O. Norrby from the Technical Uni-
versity of Denmark is acknowledged for assistance with
MacroModel. P. W. Thulstrup, The Royal Veterinary and
Agricultural University, is acknowledged for help with the
electronic CD measurements. The Jmol development team
(http://jmol.sourceforge.net) is acknowledged for their molec-
ular visualization software.
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