Stereochemical investigation and total synthesis of inuloidin, a biologically active sesquiterpenoid from Heterotheca inuloides

Article abstract of DOI:10.1016/j.tet.2014.03.057

The stereochemistry of inuloidin (1), which was a sesquiterpenoid that was characterized as a plant growth inhibitory substance from Heterotheca inuloides, was investigated. The modified Mosher's method coupled with a total synthetic study using osmium oxidation and Burgess dehydration as key steps were performed to clarify the stereochemistry of 1, which was determined to be a 2S,4R isomer.

Full text of DOI:10.1016/j.tet.2014.03.057

Tetrahedron 70 (2014) 3141e3145
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Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Stereochemical investigation and total synthesis of inuloidin,
a biologically active sesquiterpenoid from Heterotheca inuloides
Jumpei Kamatani a, Takehiro Iwadate b, Reiko Tajima a, Hideaki Kimoto a,
,b,
Yoichi Yamada c, Noriyoshi Masuoka d, Isao Kubo e, Ken-ichi Nihei a
*
a Department of Applied Biochemistry, Faculty of Agriculture, Utsunomiya University, Tochigi 321-0943, Japan
b Department of Applied Life Science, United Graduate School of Agricultural Science, Tokyo University of Agriculture and Technology,
Tokyo 183-8509, Japan
c Department of Chemistry, Faculty of Education, Utsunomiya University, Tochigi 321-0943, Japan
d Department of Life Science, Faculty of Science, Okayama University of Science, Okayama 700-0005, Japan
e Department of Environmental Science, Policy and Management, University of California, Berkeley, CA 94720-3114, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
The stereochemistry of inuloidin (1), which was a sesquiterpenoid that was characterized as a plant
growth inhibitory substance from Heterotheca inuloides, was investigated. The modified Mosher’s
method coupled with a total synthetic study using osmium oxidation and Burgess dehydration as key
steps were performed to clarify the stereochemistry of 1, which was determined to be a 2S,4R isomer.
Ó 2014 Elsevier Ltd. All rights reserved.
Received 20 February 2014
Received in revised form 13 March 2014
Accepted 17 March 2014
Available online 25 March 2014
Keywords:
Heterotheca inuloides
Sesquiterpenoid
Plant growth inhibitory substance
Modified Mosher’s method
Burgess dehydration
1. Introduction
compound possessed plant growth inhibitory activity at 500
in the lettuce seeding assay, while it did not show any antibacterial
activity up to 800 g/mL. On the other hand, 7-hydroxy-3,4-
mg/mL
The dried flower of Heterotheca inuloides (Asteraceae), which
m
ꢀ
is called ‘arnica’ in Mexico, is widely used as a folk remedy to
dihydrocadalin (2), a congener of 1, was isolated as a major
cadalin-type sesquiterpenoid from H. inuloides,7 and it proved
a potent antibacterial compound without plant growth inhibitory
activity.
treat contusions, sprains, rheumatic disorders, and skin in-
flammations.1,2 Several sesquiterpenoids and flavonoids have been
identified as biologically active substances in H. inuloides.3e5
A
cadalin-type sesquiterpenoid, inuloidin (1), was isolated in minute
The biological activity of 1 and 2 is governed by their slight
structural differences, such as the direction of the styryl olefin and
the presence of an allylic alcohol moiety. Although the selective
activity against plant and bacterial cells was critically interesting,6
the determination of the stereochemistry and chemical synthesis
of 1 have not yet been performed. This study describes the ste-
reochemical investigation of 1 by using the modified Mosher’s
method, NOE experiments and a synthetic approach using osmium
oxidation and Burgess dehydration as key reactions.8
amounts from the methanolic extract of H. inuloides (Fig. 1).6 This
2. Results and discussion
Fig. 1. Structure of 1 and 2 isolated from H. inuloides.
Compound 1, which is a minor constituent of H. inuloides, con-
tains a secondary allylic alcohol moiety in its structure. Thus, the
modified Mosher’s method can be applied to determine the
* Corresponding author. Tel.: þ81 28 649 5412; fax: þ81 28 649 5401; e-mail
address: nihei98@cc.utsunomiya-u.ac.jp (K. Nihei).
0040-4020/$ e see front matter Ó 2014 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2014.03.057

3142
J. Kamatani et al. / Tetrahedron 70 (2014) 3141e3145
stereochemistry of the molecule.9 To obtain MTPA esters of 1, the
selective protection of the phenolic hydroxy group was needed.
However, the reaction failed when using alkylating reagents, such
as ethereal diazomethane and iodomethane. Instead, a dialkyl
product was obtained when excess amount of alkyl halide was
used, suggesting that the phenolic and allylic hydroxy groups in 1
may have a similar reactivity. Hence we envisioned furnishing the
MTPA diesters of 1 to determine its stereogenic center.10
The method of MTPA diesters was recently applied for the de-
termination of 1,3-diols, for which it has slight limitation.11,12 It is
mainly assumed that two MTPA esterification sites should be apart
from each other for avoiding their anisotropic interference. How-
ever, by observing the structure of 1, the phenolic hydroxy group is
located far from the allylic hydroxy group. Thus, the modification by
MTPA at C-2 would not affect the 1H NMR signals generated by the
atoms around the allylic alcohol moiety. As a model experiment,
indeed, the comparison of the 1H NMR chemical shifts of (S)- and
(R)-MTPA esters of 2 did not show significant anisotropic effects
around C-2 (Fig. 2).
room temperature. However, the coupling constants observed be-
tween H-2/H-3 and H-2/H-30 were 3.4 Hz and 7.8 Hz, respectively,
indicating that the high-accuracy estimate of the relative configu-
ration of 1 could not be possible only with NMR experiments.
Moreover, the total synthesis of 1 has not been reported to date.
Thus, we conducted a synthetic study of 1 to confirm its stereo-
structure as well as to achieve its total synthesis for the first time.
The synthetic plan for 1 is shown in Fig. 4. The reaction started
from a methylated cadalin 3, which is a derivative of 2.15 The key
reactions were osmium oxidation of the cadalin derivative and
selective dehydration of the tertiary hydroxy group. The synthetic
route carried out in this experiment to obtain the racemic 1 is
summarized in Scheme 1. According to previous reports,15,16
cadalin 3 was prepared in 16% yield via four steps including Frie-
deleCrafts and Grignard reactions from 2-methoxytoluene and
succinic anhydride as the starting materials. The removal of the
methyl group of 3 was conducted in poor yield (<34%) under sev-
eral acidic conditions, such as BBr3 in CH2Cl2 and pyridinium hy-
drochloride.17,18 Fortunately, by using ethanethiolate anion as
a nucleophile, this reaction progressed smoothly to give the race-
mic mixture of phenol 2 in 91% yield.19
This phenol was converted into silyl ether 4 using TBSCl in ex-
cellent yield (98%). The diastereomeric mixture of diol 5 was fur-
nished in 87% yield from 4 by typical osmium oxidation. The
diastereomeric excess (de) of this reaction, which was estimated by
the 1H NMR experiment, was 57%, although the diastereomeric
mixture of 5 could not be separated by silica gel column chroma-
tography. In the 1H NMR spectra, the coupling constants observed
between H-2/H-3 and H-2/H-30 in 5 were 2.3 Hz and 5.4 Hz, re-
spectively. On the other hand, the minor isomer of 5 possessed
3.7 Hz and 11.8 Hz of the coupling constants at H-2. These values
suggested that H-2 of 5 should be equatorial while H-2 of the iso-
mer of 5 projected to the axial direction. Using TBSCl and imidazole,
the mixture of 5 was transformed into a diastereomeric mixture of
silyl ether 6. From this reaction mixture, 6 were readily isolated in
53% yields. The coupling constants observed between H-2/H-3 and
H-2/H-30 in 6 were 1.9 Hz and 5.8 Hz, respectively, which were
similar to those of 5. The significant NOE correlations between
isopropyl proton and H-5, and methyl proton and H-2 were ob-
served in the NOESY spectra for 6. These results strongly supported
isopropyl and methyl moieties of 6 as well as 5 should be in
equatorial positions. Consequently, the 2,4-trans orientation of 5
and 6 was determined. Because the bulky osmium reagent was
preferentially approached from the opposite site of the isopropyl
group in 4,14,20 the compound 2,4-trans diol 5 was mainly obtained.
The silyl ether 6 can be converted by selective dehydration into
olefin 7 as a significant synthetic precursor of 1. However, the steric
hindrance of the tertiary hydroxyl moiety might block the in-
troduction of the leaving group, such as methanesulfonyl or halo-
gen moieties. After several trials, it was found that the Burgess
reagent [(methoxycarbonylsulfamoyl) triethylammonium hydrox-
ide] could be used to dehydrate 6,8,21 although the reaction yield
was slightly poor (33%). The removal of the TBS group of 7 was
performed with TBAF to afford racemic 1 in 39% yield. The overall
yield of 1 from 3 was 5% via six steps. 1H and 13C NMR data of the
racemic mixture of synthesized 1 were fully consistent with those
of natural 1.6 Consequently, the total synthesis of racemic 1 was
achieved and the stereochemistry of natural 1 was determined to
be (2S,4R)-inuloidin.
Fig. 2. Dd values (ppm) obtained from the MTPA ester of 1 and 2.
The (S)- and (R)-MTPA diesters of 1 were synthesized in high
yield (81% and 95%, respectively) by using the commercially avail-
able MTPA chlorides. The 1H NMR chemical shifts of both di-
astereomers were carefully assigned and the differences (Dd dS-
¼
MTPA esteredR-MTPA ester) observed for selected protons are summa-
rized in Fig. 2, which according to the rules of the modified
Mosher’s method clearly shows that the absolute configuration at
C-2 is S.9
This configuration suggests that 1 has an S-stereogenic center at
C-4 because the relationship between the hydroxy and isopropyl
groups was previously established as cis by extensive NMR exper-
iments.6 However, the absolute configuration at C-4 in 2 from H.
inuloides has been already determined as R with the aid of X-ray
crystallographic analysis.13 This contradiction led to preform ad-
ditional studies to elucidate the relative configuration of 1 by using
NMR decoupling and NOE experiments (Fig. 3).
Fig. 3. Significant NOE correlations of 1.
Significant NOE correlations were observed between the allylic
oxymethine and isopropyl protons, indicating that the relative
configuration of the allylic hydroxy and isopropyl groups is trans. In
this case, the bulky isopropyl group is in axial orientation.
According to a previous study reporting on the conformational
analysis of the similar molecule to 2,14 the Boltzmann distribution
of the conformer having axial isopropyl group was predominant at
3. Conclusions
The present study describes the elucidation of the stereo-
chemistry of natural 1 by modified Mosher’s method using the di-
MTPA esters and reports on its synthesis, which includes two key
reactions, osmium oxidation and Burgess dehydration. Accordingly,

J. Kamatani et al. / Tetrahedron 70 (2014) 3141e3145
3143
Fig. 4. Synthetic plan for 1.
Scheme 1. Synthesis of 1. Reagents and conditions: (a) EtSNa, DMF, reflux, 5 h, 91%; (b) TBSCl, imidazole, DMF, rt, overnight, 98%; (c) OsO4, NMO, H2O, t-BuOH, acetone, 0 ꢀC to rt,
overnight, 87% (57% de); (d) TBSCl, imidazole, DMF, rt, overnight, 53%; (e) Burgess reagent, CH2Cl2, reflux, overnight, 33%; (f) TBAF, THF, rt, 2 h, 39%.
it was clarified that natural 1 possessed 2S and 4R stereogenic
centers. The concise synthesis of racemic 1 was additionally dem-
onstrated in short steps starting from cadalin 3. Although 1 is
a minor constituent of H. inuloides, an adequate amount of 2 was
easily isolated as a single stereoisomer.3 Thus, our synthetic route is
applicable for the semi-synthetic supply of chiral 1 from natural 2
to understand the unique activity of cadalins against plants and
microorganisms.
TLC (10% EtOAceHexane) without solvent extraction steps. The
corresponding (R)-MTPA diester (2.7 mg) was furnished in 95%
yield as a colorless solid.
1H NMR (750 MHz, CDCl3):
d
0.795 (3H, d, J¼6.8 Hz, i-Pr), 0.959
(3H, d, J¼6.8 Hz, i-Pr), 2.039 (3H, s, Me), 2.073 (2H, m, H-3), 2.172
(1H, m, i-Pr), 2.768 (1H, m, H-4), 3.429 (3H, s, OMe), 3.669 (3H, s,
OMe), 5.194 (1H, s, CH2), 5.525 (1H, s, CH2), 5.928 (1H, br s, H-2),
7.015 (1H, s, H-6), 7.127 (1H, s, H-8), 7.309 (3H, m, Ph), 7.436 (5H, m,
Ph), 7.664 (2H, m, Ph).
4. Experimental
4.4. (S)-MTPA diester of 1
4.1. General experimental procedures
According to the same procedure, (S)-MTPA diester (2.3 mg) was
furnished by using (R)-MTPACl in 81% yield as a colorless solid.
NMR spectra were recorded in CD3OD or CDCl3 on a JEOL JNM-
ECS400 (1H at 400 MHz and 13C at 100 MHz) or a Varian INOVA-750
(1H at 750 MHz) spectrometer. Chemical shifts were recorded as
parts per million relative to the solvent signal for CD3OD (3.30 ppm
for 1H NMR, 49.0 ppm for 13C NMR) or for CDCl3 (7.24 ppm for 1H
NMR, 77.0 ppm for 13C NMR). HRMS spectra were measured on
a JEOL AccuTOF or an AB SCIEX TripleTOF 5600 mass spectrometer
fitted with an electrospray ion source in positive ionization mode.
IR spectra were measured with a PerkineElmer Paragon 1000 PC
FT-IR spectrometer. Compound 1 and 2 were isolated along with
previous works.6,7 Spectral data for 2 were consistent with those
reported previously.7
1H NMR (750 MHz, CDCl3):
d
0.714 (3H, d, J¼6.8 Hz, i-Pr), 0.932
(3H, d, J¼6.8 Hz, i-Pr), 1.918 (1H, m, H-3), 2.039 (1H, m, H-3), 2.073
(3H, s, Me), 2.263 (1H, m, i-Pr), 2.782 (1H, m, H-4), 3.391 (3H, s,
OMe), 3.672 (3H, s, OMe), 5.342 (1H, s, CH2), 5.578 (1H, s, CH2),
5.926 (1H, br s, H-2), 7.068 (1H, s, H-6), 7.176 (1H, s, H-8), 7.258 (2H,
m, Ph), 7.299 (1H, m, Ph), 7.333 (2H, m, Ph), 7.455 (3H, m, Ph), 7.670
(2H, m, Ph).
4.5. (R)-MTPA ester of 2
A CH2Cl2 (0.2 mL) solution of 2 (1.0 mg, 4.6
mmol) was treated
with DMAP (1.5 mg, 12.3 mol) and (S)-MTPACl (1.6 mg, 6.3
m
mmol)
4.2. Inuloidin (1)
at room temperature under a nitrogen atmosphere. After being
stirred overnight, the reaction mixture was purified by preparative
TLC (5% EtOAceHexane) without solvent extraction steps. The
corresponding (R)-MTPA diester (2.0 mg) was quantitatively fur-
nished as a colorless solid.
Spectral data for 1 were almost consistent with those reported
previously.6 However, corrected 1H NMR assignments were listed
as follows, 1H NMR (400 MHz, CD3OD):
d
0.773 (3H, d, J¼6.8 Hz, i-
Pr), 0.972 (3H, d, J¼6.8 Hz, i-Pr), 1.832 (1H, ddd, J¼6.3, 7.8, 13.2 Hz,
H-3), 1.935 (1H, ddd, J¼3.4, 5.8, 13.2 Hz, H-3), 2.153 (3H, s, Me),
2.164 (1H, oct, J¼6.8 Hz, i-Pr), 2.743 (1H, ddd, J¼5.8, 6.3, 6.8 Hz, H-
4), 4.534 (1H, dd, J¼3.4, 7.8 Hz, H-2), 5.158 (1H, s, CH2), 5.425 (1H, s,
CH2), 6.913 (1H, s, H-5), 6.971 (1H, s, H-8).
1H NMR (750 MHz, CDCl3):
d
0.784 (3H, d, J¼6.8 Hz, i-Pr), 0.870
(3H, d, J¼6.8 Hz, i-Pr), 1.849 (1H, m, i-Pr), 1.952 (3H, s, Me), 2.073
(3H, s, Me), 2.344 (3H, m, H-3, H-4), 3.696 (3H, s, OMe), 5.708 (1H,
br s, H-2), 6.836 (1H, s, H-5), 6.936 (1H, s, H-8), 7.453 (3H, m, Ph),
7.693 (2H, m, Ph).
4.3. (R)-MTPA diester of 1
4.6. (S)-MTPA ester of 2
A CH2Cl2 (0.2 ml) solution of 1 (1.0 mg, 4.3
m
mol) was treated
According to the same procedure, (S)-MTPA ester (1.9 mg) was
furnished by using (R)-MTPACl in 94% yield as a colorless solid.
with DMAP (3.0 mg, 24.6 mol) and (S)-MTPACl (3.2 mg, 12.6
m
mmol)
at room temperature under a nitrogen atmosphere. After being
stirred overnight, the reaction mixture was purified by preparative
1H NMR (750 MHz, CDCl3):
d
0.781 (3H, d, J¼6.8 Hz, i-Pr), 0.871
(3H, d, J¼6.8 Hz, i-Pr), 1.850 (1H, m, i-Pr), 1.956 (3H, s, Me), 2.071

3144
J. Kamatani et al. / Tetrahedron 70 (2014) 3141e3145
(3H, s, Me), 2.344 (3H, m, H-3, H-4), 3.694 (3H, s, OMe), 5.707 (1H,
br s, H-2), 6.840 (1H, s, H-5), 6.938 (1H, s, H-8), 7.453 (3H, m, Ph),
7.695 (2H, m, Ph).
(100 MHz, CDCl3):
d
ꢂ4.27 (TBS), ꢂ4.14 (TBS), 16.2 (Me), 16.7 (i-Pr),
18.2 (TBS), 20.7 (i-Pr), 25.8 (TBS), 26.0 (C-3), 28.8 (Me), 31.1 (i-Pr),
37.6 (C-4), 71.8 (C-1), 73.6 (C-2), 116.0 (C-8), 128.1 (C-6), 129.3 (C-5),
129.8 (C-10), 140.2 (C-9), 152.4 (C-7). 13C NMR (100 MHz, CDCl3) for
4.7. 7-Hydroxy-3,4-dihydrocadalin (2)
the diastereomer:
d
ꢂ4.29 (TBS), ꢂ4.05 (TBS), 15.6 (Me), 16.8 (i-Pr),
16.72 (Me), 18.2 (TBS), 20.9 (i-Pr), 25.8 (TBS), 26.4 (C-3), 29.4 (i-Pr),
41.5 (C-4), 71.5 (C-1), 73.9 (C-2), 116.7 (C-8), 128.7 (C-5), 129.0 (C-6),
131.0 (C-10), 139.2 (C-9), 152.3 (C-7).
To a solution of ether 3 (0.20 g, 0.87 mmol)15 and ethanethiol
(1.89 ml, 26 mmol) in DMF (20 ml) was slowly added 55% sodium
hydride, dispersion in paraffin liquid (1.62 g, 37 mmol), and the
resultant solution was heated at reflux for 5 h. The reaction mixture
was cooled to room temperature, treated with saturated aqueous
NH4Cl solution (50 ml) and extracted with EtOAc (3ꢁ20 ml). The
organic layer was washed with brine (3ꢁ10 ml) and dried over
Na2SO4. Filtration and concentration followed by silica gel chro-
matography (2e8% EtOAc in hexane) gave 0.17 g (0.79 mmol, 91%)
of cadalin 2 as a colorless solid. Spectral data were complete
agreement with those previously reported.7
4.10. 2,7-Bis(tert-butyldimethylsiloxy)-3,4-dihydro-1-hydroxy
cadalin (6)
TBSCl (1.00 g, 6.64 mmol) and imidazole (0.60 g, 8.81 mmol)
were added to a solution of the diastereomixture of 5 (0.80 g,
2.19 mmol) in DMF (20 ml) at room temperature. After being stirred
overnight, the reaction mixture was treated with H2O (50 ml) and
extracted with EtOAc (3ꢁ20 ml). The organic layer was washed
with brine (3ꢁ10 ml) and dried over Na2SO4. Filtration and con-
centration followed by silica gel chromatography (1e4% EtOAc in
hexane) gave 0.56 g (1.17 mmol, 53%) of silyl ether 6 as colorless
solid. IR (film) nmax 3565, 2956, 1504, 1254, 1087, 837 cmꢂ1. 1H NMR
4.8. 7-tert-Butyldimethylsiloxy-3,4-dihydrocadalin (4)
TBSCl (1.04 g, 6.90 mmol) and imidazole (0.57 g, 8.47 mmol)
were added to a solution of cadalin 2 (0.58 g, 2.68 mmol) in DMF
(20 ml) at room temperature. After being stirred overnight, the
reaction mixture was treated with H2O (50 ml) and extracted with
EtOAc (3ꢁ20 ml). The organic layer was washed with brine
(3ꢁ10 ml) and dried over Na2SO4. Filtration and concentration
followed by silica gel chromatography (1e4% EtOAc in hexane) gave
0.87 g (2.63 mmol, 98%) of silyl ether 4 as a colorless oil. IR (film)
(400 MHz, CDCl3): d 0.101 (6H, s, TBS), 0.202 (6H, s, TBS), 0.661 (3H,
d, J¼6.8 Hz, i-Pr), 0.815 (9H, s, TBS), 0.988 (3H, d, J¼6.8 Hz, i-Pr),
0.989 (9H, s, TBS), 1.356 (3H, s, Me), 1.704 (1H, ddd, J¼1.9, 10.4,
14.1 Hz, H-3), 1.872 (1H, dt, J¼14.1, 5.8 Hz, H-3), 2.149 (3H, s, Me),
2.401 (1H, dsept, J¼4.7, 6.8 Hz, i-Pr), 2.860 (1H, m, H-4), 3.953 (1H,
dd, J¼1.9, 5.8 Hz, H-2), 6.926 (1H, s, H-5), 7.004 (1H, s, H-8). 13C
NMR (100 MHz, CDCl3):
d
ꢂ4.94 (TBS), ꢂ4.25 (TBS), ꢂ4.23 (TBS),
nmax 2956, 1504, 1254, 1142, 887, 838 cmꢂ1
.
1H NMR (400 MHz,
ꢂ4.18 (TBS), 16.4 (Me), 16.7 (i-Pr), 18.1 (TBS), 18.2 (TBS), 20.8 (i-Pr),
25.8 (TBS), 26.6 (C-3), 28.9 (Me), 30.0 (i-Pr), 37.9 (C-4), 71.4 (C-1),
74.8 (C-2),116.5 (C-8),127.2 (C-6), 128.5 (C-5), 130.2 (C-10),141.3 (C-
9), 151.8 (C-7). ESIHRMS m/z 501.3144 [MþNa]þ (calcd for
CDCl3): 0.203 (3H, s, TBS), 0.220 (3H, s, TBS), 0.780 (3H, d,
d
J¼6.8 Hz, i-Pr), 0.859 (3H, d, J¼6.8 Hz, i-Pr), 1.013 (9H, s, TBS), 1.833
(1H, oct, J¼6.8 Hz, i-Pr), 1.956 (3H, s, Me), 2.175 (3H, s, Me), 2.275
(1H, m, H-4), 2.311 (2H, m, H-3), 5.643 (1H, s, H-2), 6.646 (1H, s, H-
C27H50NaO3Si2, 501.3196).
8), 6.841 (1H, s, H-5). 13C NMR (100 MHz, CDCl3):
d
ꢂ4.3 (TBS), ꢂ4.1
(TBS), 16.6 (Me), 18.2 (TBS), 19.0 (i-Pr), 20.2 (Me), 21.4 (i-Pr), 25.77
(C-3), 25.81 (TBS), 30.4 (i-Pr), 43.5 (C-4), 113.3 (C-8), 123.2 (C-2),
126.0 (C-6), 130.9 (C-5), 131.3 (C-10), 131.5 (C-9), 133.9 (C-1), 152.0
(C-7). ESIMS m/z 331.2 [MþH]þ.
4.11. 2,7-Bis(tert-butyldimethyl)-inuloidin (7)
Under Ar atmosphere, Burgess reagent (0.13 g, 0.55 mmol) was
added to a solution of silyl ether 6 (88 mg, 0.18 mmol) in dried
CH2Cl2 (8 ml) at room temperature. The resultant solution was
heated at reflux overnight, treated with H2O (20 ml) and extracted
with EtOAc (3ꢁ10 ml). The organic layer was washed with brine
(3ꢁ10 ml) and dried over Na2SO4. Filtration and concentration
followed by silica gel chromatography (3e8% Et2O in hexane) gave
4.9. 7-tert-Butyldimethylsiloxy-3,4-dihydro-1,2-
dihydroxycadalin (5)
Silyl ether 4 (0.87 g, 2.63 mmol) and 4-methylmorpholine N-
oxide (0.62 g, 5.29 mmol) were dissolved in acetone (20 ml), t-
BuOH (10 ml), and H2O (10 ml). To the cold (0 ꢀC) and stirred so-
lution was added dropwise 4.0% OsO4 solution (1.00 ml, 0.16 mmol),
and resultant mixture was stirred overnight at room temperature.
After 10% aqueous Na2SO3 solution (30 ml) was added, resultant
mixture was stirred for 1 h and extracted with EtOAc (3ꢁ20 ml).
The organic layer was washed with brine (3ꢁ10 ml) and dried over
Na2SO4. Filtration and concentration followed by silica gel chro-
matography (5e20% EtOAc in hexane) gave 0.83 g (2.28 mmol, 87%,
57% de) of the diastereomixture of diol 5 as a colorless oil, which
was used in the next step without further purification. 1H NMR
27 mg (59
nmax 2957, 1497, 1254, 1094, 888, 837 cmꢂ1
CDCl3): 0.036 (3H, s, TBS), 0.095 (3H, s, TBS), 0.199 (3H, s, TBS),
m
mol, 33%) of vinylidene 7 as a colorless oil. IR (film)
.
1H NMR (400 MHz,
d
0.217 (3H, s, TBS), 0.769 (3H, d, J¼6.8 Hz, i-Pr), 0.867 (9H, s, TBS),
0.982 (3H, d, J¼6.8 Hz, i-Pr), 1.005 (9H, s, TBS), 1.835 (1H, ddd,
J¼5.8, 8.5, 13.2 Hz, H-3), 1.879 (1H, ddd, J¼4.2, 5.8, 13.2 Hz, H-3),
2.153 (1H, m, i-Pr), 2.164 (3H, s, Me), 2.796 (1H, quart, J¼5.8 Hz, H-
4), 4.562 (1H, dd, J¼4.2, 8.5 Hz, H-2), 5.131 (1H, br s, CH2), 5.289
(1H, br s, CH2), 6.929 (2H, m, H-5, H-8). 13C NMR (100 MHz, CDCl3):
d
ꢂ4.57 (TBS), ꢂ4.55 (TBS), ꢂ4.30 (TBS), ꢂ4.10 (TBS), 16.9 (Me),
18.2 (TBS), 18.25 (TBS), 18.32 (i-Pr), 21.6 (i-Pr), 25.8 (TBS), 25.9
(TBS), 31.8 (i-Pr), 33.1 (C-3), 41.2 (C-4), 69.9 (C-2), 106.8 (CH2),
114.4 (C-8), 128.6 (C-6), 130.4 (C-5), 132.0 (C-10), 132.7 (C-9), 147.7
(C-1), 151.9 (C-7). ESIHRMS m/z 461.3262 [MþH]þ (calcd for
(400 MHz, CDCl3): d 0.201 (3H, s, TBS), 0.222 (3H, s, TBS), 0.643 (3H,
d, J¼6.8 Hz, i-Pr), 0.994 (3H, d, J¼6.8 Hz, i-Pr), 1.000 (9H, s, TBS),
1.400 (3H, s, Me), 1.788 (1H, ddd, J¼2.3, 10.0, 14.4 Hz, H-3), 1.951
(1H, ddd, J¼5.4, 6.8, 14.4 Hz, H-3), 2.151 (3H, s, Me), 2.364 (1H,
dsept, J¼4.6, 6.8 Hz, i-Pr), 2.879 (1H, ddd, J¼4.6, 6.8, 10.0 Hz, H-4),
3.902 (1H, dd, J¼2.3, 5.4 Hz, H-2), 6.965 (1H, s, H-5), 7.010 (1H, s, H-
C
27H49O2Si2, 461.3271).
4.12. Inuloidin (1)
8). 1H NMR (400 MHz, CDCl3) for the diastereomer:
d 0.201 (3H, s,
TBS), 0.222 (3H, s, TBS), 0.635 (3H, d, J¼6.8 Hz, i-Pr), 1.001 (9H, s,
TBS), 1.056 (3H, d, J¼6.8 Hz, i-Pr), 1.571 (3H, s, Me), 1.864 (1H, ddd,
J¼3.7, 4.9, 12.5 Hz, H-3), 1.967 (1H, m, H-3), 2.166 (3H, s, Me), 2.492
(1H, m, i-Pr), 2.734 (1H, dt, J¼11.3, 4.9 Hz, H-4), 3.598 (1H, dd, J¼3.7,
11.8 Hz, H-2), 6.976 (1H, s, H-8), 7.047 (1H, s, H-5). 13C NMR
TBAF trihydrate (0.16 g, 0.51 mmol) was added to a solution of
silyl ether 6 (10 mg, 22 mmol) in THF (1 ml) at room temperature.
The resultant solution was stirred for 2 h, treated with saturated
aqueous NH4Cl solution (10 ml) and extracted with EtOAc (3ꢁ5 ml).
The organic layer was washed with brine (3ꢁ5 ml) and dried over

J. Kamatani et al. / Tetrahedron 70 (2014) 3141e3145
3145
4. Haraguchi, H.; Ishikawa, H.; Sanchez, Y.; Ogura, T.; Kubo, Y.; Kubo, I. Bioorg. Med.
Chem. 1997, 5, 865.
5. Delgado, G.; Olivares, M. S.; Chavez, M. I.; Ramírez-Apan, T.; Linares, E.; Bye, R.;
Espinosa-Carcía, F. J. J. Nat. Prod. 2001, 64, 861.
6. Kubo, I.; Ishiguro, K.; Chaudhuri, S. K.; Kubo, Y.; Sanchez, Y.; Ogura, T. Phyto-
chemistry 1995, 38, 553.
Na2SO4. Filtration and concentration followed by silica gel chro-
matography (10e50% EtOAc in hexane) gave 2.0 mg (8.6 mol, 39%)
m
ꢀ
of inuloidin 1 as a colorless oil. Spectral data were fully consistent
with those reported previously.6
7. Bohlmann, F.; Zdero, C. Chem. Ber. 1976, 109, 2021.
8. Burgess, E. M.; Penton, H. R.; Taylor, E. A. J. Am. Chem. Soc. 1970, 92, 5224.
9. Ohtani, I.; Kusumi, T.; Kashman, Y.; Kakisawa, H. J. Am. Chem. Soc. 1991, 113,
4092.
10. Ichikawa, A.; Takahashi, H.; Ooi, T.; Kusumi, T. Biosci. Biotechnol. Biochem. 1997,
61, 881.
Acknowledgements
This study is financially supported in part by JSPS KAKENHI
Grant Number 23510252. The authors are indebted to Professor K.
Tsujimoto, Japan Advanced Institute of Science and Technology,
Ishikawa, for invaluable discussion and technical assistance.
11. Konno, K.; Fujishima, T.; Liu, Z.; Takayama, H. Chirality 2002, 13, 72.
~
ꢀ
12. Seco, J. M.; Quinoa, E.; Riguera, R. Chem. Rev. 2012, 112, 4603.
13. Stipanovic, R. D.; Puckhaber, L. S.; Reibenspies, J. H.; Williams, H. J. Phyto-
chemistry 2006, 67, 1304.
14. Benincori, T.; Bruno, S.; Celentano, G.; Pilati, T.; Ponti, A.; Rizzo, S.; Sada, M.;
Supplementary data
ꢁ
Sannicolo, F. Helv. Chim. Acta 2005, 88, 1776.
15. McCormick, J. P.; Shinmyozu, T.; Pachlatko, J. P.; Schafer, T. R.; Gardner, J. W.;
Stipanovic, R. D. J. Org. Chem. 1984, 49, 34.
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.tet.2014.03.057.
16. Cocker, W.; Sainsbury, D. M. J. Chem. Soc. 1965, 3319.
17. Silcoff, E. R.; Sheradsky, T. New J. Chem. 1999, 23, 1187.
18. Gates, M.; Tschudi, G. J. Am. Chem. Soc. 1956, 78, 1380.
19. Bianco, G. G.; Ferraz, H. M. C.; Costa, A. M.; Costa-Lotufo, L. V.; Pessoa, C.; de
Moraes, M. O.; Schrems, M. G.; Pfaltz, A.; Silva, L. F. J. Org. Chem. 2009, 74, 2561.
20. Nabeta, K.; Katayama, K.; Nakagawara, S.; Katoh, K. Phytochemistry 1993, 32,
117.
21. Holton, R. A.; Kim, H. B.; Somoza, C.; Liang, F.; Biediger, R. J.; Boatman, P. D.;
Shindo, M.; Smith, C. C.; Kim, S.; Nadizadeh, H.; Suzuki, Y.; Tao, C.; Vu, P.; Tang,
S.; Zhang, P.; Murthi, K. K.; Gentile, L. N.; Liu, J. H. J. Am. Chem. Soc. 1994, 116,
1599.
References and notes
1. Heinrich, H.; Robles, M.; West, J. E.; Orliz de Montellano, B. R.; Rodriguez, E.
Annu. Rev. Pharmacol. Toxicol. 1998, 38, 539.
ꢀ
2. Gene, R. M.; Segura, L.; Adzet, T.; Marin, E.; Iglesias, J. J. Ethnopharmacol. 1998,
60, 157.
3. Kubo, I.; Muroi, H.; Kubo, A.; Chaudhuri, S. K.; Sanchez, Y.; Ogura, T. Planta Med.
1994, 60, 218.