Synthesis-guided structure revision of the monoterpene alcohol isolated from Mentha haplocalyx

Article abstract of DOI:10.1080/09168451.2018.1549936

The monoterpene isolated from Mentha haplocalyx, 3,3,5-trimethyl-2-oxabicyclo[2.2.2]oct-5-en-4-ol, was synthesized according to its proposed structure. However, the NMR data of the synthetic sample were not in agreement with those reported for the natural product. After considerable efforts, the genuine structure was confirmed as (1R*,2R*)-4-(1′-hydroxy-1′-methylethyl)-1-methyly-cyclohex-3-ene-1,2-diol.

Full text of DOI:10.1080/09168451.2018.1549936

Bioscience, Biotechnology, and Biochemistry
ISSN: 0916-8451 (Print) 1347-6947 (Online) Journal homepage: http://www.tandfonline.com/loi/tbbb20
Synthesis-guided structure revision of the
monoterpene alcohol isolated from Mentha
haplocalyx
Shunsuke Konishi, Naoki Mori, Hirosato Takikawa & Hidenori Watanabe
To cite this article: Shunsuke Konishi, Naoki Mori, Hirosato Takikawa & Hidenori Watanabe
(2018): Synthesis-guided structure revision of the monoterpene alcohol isolated from Mentha
haplocalyx, Bioscience, Biotechnology, and Biochemistry, DOI: 10.1080/09168451.2018.1549936
To link to this article: https://doi.org/10.1080/09168451.2018.1549936
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BIOSCIENCE, BIOTECHNOLOGY, AND BIOCHEMISTRY
https://doi.org/10.1080/09168451.2018.1549936
Synthesis-guided structure revision of the monoterpene alcohol isolated from
Mentha haplocalyx
Shunsuke Konishia,b, Naoki Mori a, Hirosato Takikawaa and Hidenori Watanabea
aDepartment of Applied Biological Chemistry, Graduate School of Agricultural and Life Sciences, The University of Tokyo, Tokyo, Japan;
bTechnical Research Institute, R&D Center, T. Hasegawa Co., Ltd., Kawasaki, Kanagawa, Japan
ABSTRACT
ARTICLE HISTORY
Received 10 October 2018
Accepted 5 November 2018
The monoterpene isolated from Mentha haplocalyx, 3,3,5-trimethyl-2-oxabicyclo[2.2.2]oct-5-en-4-ol,
was synthesized according to its proposed structure. However, the NMR data of the synthetic
sample were not in agreement with those reported for the natural product. After considerable
efforts, the genuine structure was confirmed as (1R*,2R*)-4-(1′-hydroxy-1′-methylethyl)-1-methyly-
cyclohex-3-ene-1,2-diol.
KEYWORDS
Structural revision;
synthesis-guided;
monoterpene; Mentha
haplocalyx
In 2010, Liu et al. reported the isolation of a novel
monoterpene alcohol from the Chinese herb Mentha
haplocalyx [1]. On the basis of NMR and mass spectral
analyses, the structure of the natural product was pro-
posed to be 3,3,5-trimethyl-2-oxabicyclo[2.2.2]oct-
5-en-4-ol (1) (Figure 1). Inspired by the unique struc-
ture of 1, we initiated studies toward its synthesis as
a continuation of our synthetic studies on biologically
active and/or structurally unique terpenoids [2,3].
Herein, we report the following step-by-step: 1) syn-
thetic disproof of the proposed structure 1; 2) reconsi-
deration of the genuine structure based on NMR
spectroscopy; 3) synthesis of the candidate structures
(2, 3, cis- and trans-4); and 4) structural confirmation of
the naturally occurring monoterpene isolated from
Mentha haplocalyx.
assignation of 1 as the structure of the natural
product is incorrect. We therefore decided to clar-
ify the genuine structure of the monoterpene iso-
lated from Mentha haplocalyx through the careful
examination of the NMR data reported for the
natural product and those of various candidate
compounds
with
structural
similarlities.
Unexpectedly, we found that the reported
1
13C and H NMR data (in CD3OD) were in good
agreement with those reported for three other
natural compounds, i.e., asiasarinol (2) [5],
*
*
comosoxide B (3) [6,7], and (1R ,2S )-4-(1′-
hydroxy-1′-methylethyl)-1-methylycyclohex-3-ene-
1,2-diol (cis-4) [8]. The close similarities among
them strongly suggest that these isolated natural
products [1,5,6,8] would have identical structure.
This prompted us to undertake the synthesis of 2,
3, and cis-4, which had not previously been
synthesized.
Results and discussion
Our synthetic route to 1 is shown in Scheme 1.
A commercially available ketoester 5 was converted
into the corresponding enol phosphonate 6 (70%).
After the conversion of 6 into 7 by treatment with
Gilman reagent, 7 was exposed to MeLi to afford 8
(57% in 2 steps). Oxidation of 8 with singlet oxygen
in the presence of methylene blue produced the per-
oxide 9 (61%), which was then reduced with zinc
powder [4] in AcOH to furnish 10. Finally, intramo-
lecular etherification in SN2 manner was successfully
mediated by TsCl/DMAP in pyridine to afford the
initial target compound 1 (71% in 2 steps).
Asianarinol (2) was isolated from the traditional
medicine Asiasarum sieboldii in 1999, and its struc-
ture was proposed as 2 [5]. As depicted in Figure 1,
compound 2 consists of a 7-oxabicyclo[2.2.1]hept-
2-ene framework. As a starting point for the synthesis
of 2 (Scheme 2), we performed the Diels–Alder reac-
tion [9,10] between compound 11 [11] and 2-methyl-
furan, which was followed by hydrogenation to afford
bromoester 13 (43% in 2 steps). The reductive
removal of bromine from 13 was performed with
zinc powder to give α,β-unsaturated ester 14 in 79%
yield. Finally, 1,2-addition of MeMgCl in the pre-
sence of CeCl3 furnished the target compound 2 in
excellent yield. It is worth noting that only the
1,4-addition product was obtained in the absence of
CeCl3 [12].
As shown in Table 1, there are discrepancies
between the 13C and 1H NMR spectral data
obtained for the synthetic compound 1 and those
for the natural product, which suggest that the
CONTACT Hirosato Takikawa
atakikawa@mail.ecc.u-tokyo.ac.jp
© 2018 Japan Society for Bioscience, Biotechnology, and Agrochemistry

2
S. KONISHI ET AL.
Figure 1. Structures of 1 and the related monoterpenes.
Scheme 1. Synthesis of 1.
1
Table 1. 13C and H NMR data (in CD3OD) of the natural product, synthetic 1, and those of the related compounds.
Natural [1]
Synthetic 1
2 [5]
3 [6]
cis-4 [8]
δC
δH
δC
δH
δC
δH
δC
δH
δC
δH
21.8
24.1
1.15 (Me)
16.6
23.8
0.92 (Me)
1.24 (Me)
21.9
24.2
1.15
21.9
24.2
1.15
21.9
24.3
1.14
1.27
1.29
(2xMe)
1.29
1.62
1.29
1.64
25.5
26.3
1.85 (Me)
1.32 (1H)
29.0
29.1
29.0
29.1
29.1
29.2
29.0
(x2)
1.60–1.70
(2H)
1.60–
1.70
34.7
72.5
28.1
68.6
1.44 (1H)
34.7
72.5
1.69
2.10
34.7
72.5
1.69
2.11
34.8
72.7
2.12 (1H)
2.10–
2.20
1.90–2.00
(2H)
2.22 (1H)
3.92 (1H)
72.9
74.7
76.3
77.8
72.9
74.7
2.24
3.92
72.8
74.7
2.24
3.92
73.1
74.9
4.76 (1H)
5.94 (1H)
3.90
5.58
122.3
147.0
5.59 (1H)
123.6
148.3
122.2
146.9
5.59
122.3
147.1
5.60
122.4
147.2
Meanwhile, comosoxide B (3) was isolated from the
Thai herbal medicine Curcuma comosa in 2008, and the
structure 3 depicted in Figure 1, which exhibits a popular
p-menthane framework with an epoxy ring, was pro-
posed [6]. As shown in Scheme 2, our synthesis of 3
commenced with the Diels–Alder reaction between die-
nophile 15 [13] and isoprene in the presence of AlCl3.
Then, following the reported procedure [14], dehydro-
bromination with DBU was conducted to afford diene 17.
Regioselective epoxidation of 17 with mCPBA produced
18 (73% in 3 steps), which was then treated with MeLi to
furnish the desired epoxide 3 (32%). This epoxyalcohol 3
was found to be highly unstable in protic solvents and on
silica gel; this prevented us from obtaining 13C NMR data
of sufficient quality in CD3OD.
According to the former two studies, the cis-relative con-
figuration was determined based on NOE experiments. It
should be noted that only reference [8] reported the
NMR data measured in CD3OD. For the synthesis of
cis-4, we started from intermediate 17. Through treat-
ment with OsO4, NMO, and DABCO, 17 was dihydroxy-
lated to give cis-diol 19 (50%), which was converted into
the target cis-4 by treating with MeLi in 90% yield.
1
Table 2 summarizes the 13C and H NMR data of
the synthetic compounds 2, 3, and cis-4, where obvious
discrepancies with those of the natural product can be
observed. After a careful examination of all the spectral
data, it seemed reasonable to assign trans-4 as the
genuine structure of the monoterpene isolated from
Mentha haplocalyx, on the basis of the considerable
degree of similarity between the spectral data of the
natural product and those of the synthetic cis-4.
Notably, to the best of our knowledge, no report has
described the synthesis or isolation of trans-4.
*
*
(1R ,2S )-4-(1′-Hydroxy-1′-methylethyl)-1-methyl-
cyclohex-3-ene-1,2-diol (cis-4) was isolated from Riella
helicophylla in 1999 [15], from Protium heptaphyllum in
2002 [8], and also from Asarum sieboldii in 2012 [16].

SYNTHESIS-GUIDED STRUCTURE REVISION OF A MONOTERPENE
3
Scheme 2. Synthesis of 2, 3 and cis-4.
1
Table 2. 13C and H NMR data (in CD3OD) of the synthetic 2,
3, and cis-4.
72% yield with a small amount of cis-4 (18%). The NMR
data (in CD3OD) of the synthetic trans-4 were almost
identical to those reported for the natural products
[1,5,6,8].
Natural [6]a
Synthetic 2
Synthetic 3
Synthetic cis-4
δC
δH
δC
δH
δH
δC
δH
21.9
24.2
29.0
29.1
1.15
20.0
28.9
29.5
29.6
1.35
1.37
1.72
1.25
1.26
1.39
23.6
25.3
28.9
29.0
1.17
1.28
1.29
1.54
1.29
1.64
1.55–1.65
1.28–
1.50
34.7
72.5
1.69
2.11
32.6
70.6
32.6
70.7
1.82
2.05
Conclusion
2.00–2.17
72.8
74.7
122.3
147.1
2.24
3.92
5.60
78.5
88.5
129.5
148.3
1.94
4.17
6.02
72.6
73.0
121.3
148.0
2.28
3.78
5.65
The first racemic synthesis of the incorrectly proposed
structure for the monoterpene isolated from Mentha
haplocalyx (1) was achieved. We also found that the
reported NMR data of four independently isolated and
characterized natural compounds, i.e., 1, asiasarinol (2),
comosoxide B (3), and cis-4, were almost identical. By
synthesizing these compounds, we proved that all the
proposed structures were incorrect. Finally, the genuine
structure of not only the monoterpene isolated from
Mentha haplocalyx but also those of asiasarinol, comos-
oxide B and the monoterpene isolated from Protium
heptaphyllum was confirmed to be trans-4. Studies
toward the asymmetric synthesis of trans-4 are currently
in progress and will be reported in due course.
3.13
5.92
a
NMR data reported in ref [6]. were shown as the representative.
It should be mentioned that the NMR data of the
synthetic cis-4 recorded in “CDCl3” were in good
agreement with those of the monoterpene isolated
from Riella helicophylla reported in ref [15]. It
means that the monoterpene isolated from
R. helicophylla possesses the structure cis-4, unlike
that from Protium heptaphyllum [8]. We also con-
verted cis-4 into cis-20, the NMR data of which were
identical to those of the naturally occurring cis-20
isolated from Asiasari Radix [17].
We also tackled the synthesis of trans-4, which was
achieved through inversion of the secondary hydroxy
group in cis-4 as follows (Scheme 3). After oxidation of
cis-4 to the corresponding ketone 21 [18], reduction of 21
under Luche conditions produced the desired trans-4 in
Experimental
General procedures
All air- and/or water-sensitive reactions were carried
out under Ar atmosphere in dry solvents. Solvents

4
S. KONISHI ET AL.
Scheme 3. Synthesis of trans-4.
were dried as follows; THF and ether over sodium-
benzophenone, dichloromethane over P2O5. All melt-
ing points (mps) were uncorrected. Melting points
were recorded on a Yanaco Melting Point Apparatus.
IR spectra were measured with a Jasco FT/IR-230
spectrophotometer. 1H NMR (300, 400 and
500 MHz) and 13C NMR (75, 100 or 125 MHz) data
were recorded by JEOL JNM AL300, JEOL ECS400 or
JEOL JNM LA500, respectively. Chemical shifts (δ)
were referenced to the residual solvent peak as the
internal standard (CDCl3: δH = 7.26, δC = 77.0; C6D6:
δH = 7.15, δC = 128.4; CD3OD: δH = 3.30, δC = 49.0).
Mass spectra were recorded on JEOL JMS SX102 or
JEOL JMS-T100GCV. Column chromatography was
performed on Merck silica gel 60 (0.060–0.200 mm),
TLC was carried out on Merck glass plates pre-coated
with silica gel 60 F254 (0.25 mm) and preparative TLC
was carried out on Merck glass plates pre-coated with
silica gel 60 F254 (0.5 mm).
16.08, 22.47, 22.92, 51.43, 64.58, 64.66, 110.70, 123.51,
123.54, 136.41, 149.98; HR-FIMS m/z calcd for C12H19
O6P [M]+ 290.0919, found 290.0935.
Methyl 2-methylcyclohexa-1,3-diene-
1-carboxylate (7)
To a suspension of CuI (1.16 g, 6.09 mmol) in ether
(25 mL) was added MeLi (1.07 M in ether; 11.4 mL,
12.2 mmol) under Ar at 0 °C. After stirring for 5 min,
a solution of 6 (1.18 g, 4.07 mmol) in ether (10 mL)
was added to the reaction mixture at −78 °C. After
stirring for 10 h at −60 °C, the reaction mixture was
poured into sat. aq. NH4Cl and extracted with ether.
The organic layer was dried over MgSO4 and con-
centrated in vacuo to give crude 7 (0.66 g), which was
used in the next step without further purification. An
analytical sample of 7 was obtained by column chro-
matography on silica gel (hexane-EtOAc, 50/1) as
a colorless oil. IR (film) νmax 3037, 2949, 2881, 2832,
1
1708, 1575, 1434, 1268, 1218, 1076 cm−1; H NMR
Methyl 2-diethoxyphosphoryloxycyclohexa-
1,3-diene-1-carboxylate (6)
(300 MHz, CDCl3) δ 2.14 (3H, s), 2.10–2.20 (2H, m),
2.41 (2H, dd, J = 9.0, 9.6 Hz), 3.73 (3H, s), 5.90 (1H,
d, J = 9.3 Hz), 6.09 (1H, dt, J = 9.3, 4.5 Hz); 13C NMR
(75 MHz, CDCl3) δ 20.43, 22.92, 23.12, 51.09, 121.16,
130.90, 132.33, 142.93, 168.88; HR-FIMS m/z calcd
for C9H12O2 [M]+ 152.0837, found 152.0851.
To a suspension of NaH (60% oil dispersion; 288 mg,
7.20 mmol) in ether (10 mL) was added a solution of 5
(904 mg, 5.86 mmol) in ether (3 mL) under Ar at 0 °C.
After stirring for 3 h at the same temperature, diethyl
chlorophosphate (1.04 mL, 7.23 mmol) was added and
the reaction mixture was stirred for 1 h at 0 °C. The
reaction mixture was poured into sat. aq. NH4Cl and
extracted with ether. The organic layer was washed with
sat. aq. NaHCO3 and brine, dried over MgSO4 and
concentrated in vacuo. The residue was chromato-
graphed over silica gel. EtOAc/EtOAc (1/1) gave 6
(1.18 g, 4.07 mmol, 70%) as a colorless oil. IR (film)
νmax 2987, 2952, 2838, 1717, 1646, 1299, 1029 cm−1;
1H NMR (300 MHz, CDCl3) δ 1.35 (3H, t, J = 7.2 Hz),
1.36 (3H, t, J = 7.2 Hz), 2.21–2.29 (2H, m), 2.53–2.61
(2H, m), 3.75 (3H, s), 4.22 (2H, q, J = 7.2 Hz), 4.24 (2H,
q, J = 7.2 Hz), 6.17 (1H, d, J = 9.9 Hz), 6.26 (1H, dt,
J = 9.9, 4.2 Hz); 13C NMR (75 MHz, CDCl3) δ 15.99,
2-(2′-Methylcyclohexa-1′,3′-dien-1′-yl)propan-
2-ol (8)
To a solution of crude 7 (0.63 g) in ether (15 mL) was
added MeLi (1.07 M in ether, 11.2 mL, 12.0 mmol)
under Ar at 0 °C. After stirring for 30 min, the reaction
mixture was poured into sat. aq. NH4Cl solution and
extracted three times with ether. The organic layer was
washed with brine, dried over MgSO4 and concentrated
in vacuo. The residue was chromatographed over silica
gel. EtOAc/EtOAc (10/1) gave 8 (338 mg, 2.22 mmol,
57% in 2 steps) as a yellow oil. IR (film) νmax 3419,

SYNTHESIS-GUIDED STRUCTURE REVISION OF A MONOTERPENE
5
3033, 2976, 2930, 2871, 2823, 1728, 1667, 1437,
colorless needles. mp 92–96 °C; IR (film) νmax 3373,
3038, 2968, 2939, 2872, 1650, 1440, 1126, 980 cm−1;
1H NMR (300 MHz, CD3OD) δ 0.93 (3H, s), 1.24 (3H,
s), 1.32 (1H, dd, J = 4.2, 10.8 Hz), 1.44 (1H, dt, J = 12.0,
1.8 Hz), 1.85 (3H, d, J = 1.8 Hz), 1.90–2.00 (2H, m), 4.17
(1H, m), 5.94 (1H, m); 13C NMR (75 MHz, CD3OD) δ
16.64, 23.78, 25.53, 26.34, 28.14, 68.56, 76.29, 77.80,
123.57, 148.35; HR-ESIMS m/z calcd for C10H16NaO2
[M+ Na]+ 191.1043, found 191.1037.
1
1362 cm−1; H NMR (300 MHz, CDCl3) δ 1.42 (6H,
s), 1.99 (3H, s), 2.00–2.15 (4H, m), 5.74 (2H, br s); HR-
FIMS m/z calcd for C10H16O [M]+ 152.1201, found
152.1184. The 13C NMR data of 8 were not of sufficient
quality due to its instability in CDCl3.
2-(6′-Methyl-2′,3′-dioxabicyclo[2.2.2]oct-5′-en-1′-
yl)propan-2-ol (9)
A solution of 8 (456 mg, 3.00 mmol) and methylene
blue (46 mg) in methanol (120 mL) was irradiated with
visible light for 40 min under O2 atmosphere at 0 °C.
After removal of MeOH in vacuo, the residue was
filtered through silica gel, and the filtrate was concen-
trated in vacuo. A solution of the residue in CH2Cl2
(10 mL) was treated with p-TsOH·H2O (10 mg) at
room temperature for 1 h. The resulting mixture was
chromatographed over silica gel. Elution with pentane/
ether (2/1) gave 9 (338 mg, 1.83 mmol, 61%) as a yellow
oil. This peroxide 9 was used for the next step imme-
diately. IR (film) νmax 3489, 2981, 2947, 1638, 1454,
Methyl 3-bromo-1-methyl-7-oxabicyclo[2.2.1]hepta-
2,5-diene-2-carboxylate (12a)
A solution of 2-methylfuran (15.7mL, 174 mmol) and
methyl 3-bromopropiolate 11 (7.10 g, 43.6 mmol) in
cyclohexane (100 mL) was refluxed for 1 d. After
removal of the solvent, the resulting mixture of 12a
and 12b was used in the next step without purification.
1
12a: H NMR (300 MHz, CDCl3) δ 1.89 (3H, s),
3.79 (3H, s), 5.21 (1H, d, J = 1.8 Hz), 6.99 (1H, d,
J = 5.1 Hz), 7.14 (1H, dd, J = 1.8, 5.1 Hz); 13C NMR
(75 MHz, CDCl3) δ 16.43, 51.65, 87.98, 91.49, 142.12,
146.96, 149.94, 153.89, 163.49.
1
1376, 1180, 933, 867, 674 cm−1; H NMR (400 MHz,
CDCl3) δ 1.37 (3H, s), 1.40 (3H, s), 1.46–1.51 (2H, m),
2.17 (3H, s), 2.22–2.28 (2H, m), 4.57 (1H, m), 6.32 (1H,
dd, J = 0.9, 3.9 Hz).
The ratio of 12a:12b was determined to be 11:1
based on 1H NMR analysis. The signal due to 1-Me of
12b was observed at δ = 1.74.
(1R*,4S*)-1-(1′-Hydroxy-1′-methylethyl)-
2-methylcyclohex-2-ene-1,4-diol (10)
Methyl 3-bromo-1-methyl-7-oxabicyclo[2.2.1]hept-
2-ene-2-carboxylate (13)
To a solution of 9 (27.6 mg, 0.150 mmol) in acetic
acid (0.5 mL) was added zinc powder (49 mg,
0.75 mmol) at room temperature. After stirring for
3 h, the reaction mixture was filtered through Celite
and the filtrate was concentrated in vacuo to give 10
(37 mg), which was used in the next step without
further purification. An analytical sample of 10 was
obtained by column chromatography on silica gel
(EtOAc) as a colorless oil. IR (film) νmax 3384, 2977,
2870, 1715, 1660, 1446, 1381 cm−1; 1H NMR
(400 MHz, CD3OD) δ 1.10 (3H, s), 1.18 (3H, s),
1.35–1.50 (2H, m), 1.60–2.00 (2H, m), 1.86 (3H, s),
3.95 (1H, br s), 5.55 (1H, br s); 13C NMR (100 MHz,
CD3OD) δ 21.85, 24.76, 25.87, 29.94, 32.38, 68.27,
76.08, 76.46, 134.45, 139.05; HR-ESIMS m/z calcd for
C10H18NaO3 [M+ Na]+ 209.1154, found 209.1156.
To a solution of a mixture of 12a and 12b in EtOAc
(100 mL) was added Pd/C (5%; 400 mg). The mixture
was stirred under H2 (0.1 MPa) at room temperature.
After stirring for 15 h, the reaction mixture was filtered
through a pad of Celite. The filtrate was concentrated
and chromatographed over silica gel. EtOAc/EtOAc
(20/1) gave 13 (4.62 g, 18.7 mmol, 43% in 2 steps) as
a yellow oil. IR (film) νmax 2982, 2951, 2871, 1715, 1602,
1
1435, 1327, 1267, 1078 cm−1; H NMR (300 MHz,
CDCl3) δ 1.49 (1H, m), 1.58 (2H, m), 1.76 (3H, s),
2.03 (1H, ddd, J = 5.1, 7.2, 16.5 Hz), 3.79 (3H, s), 4.83
(1H, d, J = 5.1 Hz); 13C NMR (75 MHz, CDCl3) δ 18.12,
27.29, 31.56, 51.58, 83.83, 89.17, 135.79, 138.39, 163.14;
HR-FIMS m/z calcd for C9H11BrO3 [M]+ 245.9892,
found 245.9907.
Methyl 1-methyl-7-oxabicyclo[2.2.1]hept-2-ene-
2-carboxylate (14)
3,3,5-Trimethyl-2-oxabicyclo[2.2.2]oct-5-en-4-ol (1)
To a solution of 10 (37 mg) in pyridine (0.5 mL) and CH2
Cl2 (1 mL) were added DMAP (55 mg, 0.45 mmol) and
p-TsCl (34 mg, 0.18 mmol) at 0 °C. After stirring for
1 day, the reaction mixture was concentrated in vacuo
and chromatographed over silica gel. EtOAc/EtOAc (5/
1) gave 1 (17.8 mg, 0.106 mmol, 71% in 2 steps) as
To a suspension of 13 (4.13 g, 16.7 mmol), zinc powder
(3.28 g, 50.2 mmol) in water (70 mL) was added AcOH
(10.0 g, 167 mmol) dropwise at room temperature. After
stirring for 1 h, the reaction mixture was neutralized with
NaHCO3 and filtered through Celite. The filtrate was
extracted with ether. The organic layer was dried over

6
S. KONISHI ET AL.
MgSO4 and concentrated in vacuo. The residue was
chromatographed over silica gel. Elution with pentane/
ether (8/1) gave 14 (2.22 g, 13.2 mmol, 79%) as a yellow
oil. IR (film) νmax 2980, 2951, 2872, 1719, 1597 cm−1;
1H NMR (300 MHz, CDCl3) δ 1.27–1.42 (2H, m), 1.55
(1H, m), 1.76 (3H, s), 2.04 (1H, m), 3.72 (3H, s), 4.93 (1H,
ddd, J = 0.9, 1.8, 5.1 Hz), 7.00 (1H, d, J = 0.9 Hz);
13C NMR (75 MHz, CDCl3) δ 17.67, 27.10, 30.01,
51.36, 77.45, 86.07, 141.53, 145.56, 164.03; HR-FIMS
m/z calcd for C9H12O3 [M]+ 168.0786, found 168.0788.
t, J = 6.9 Hz), 1.46 (3H, s), 1.64 (1H, m), 2.04–2.18
(2H, m), 2.53 (1H, dd, J = 6.0, 15.6 Hz), 3.16 (1H, d,
J = 4.2 Hz), 4.17 (2H, q, J = 6.9 Hz), 7.05 (1H, dd,
J = 3.0, 4.2 Hz); 13C NMR (75 MHz, CDCl3) δ 14.16,
20.63, 21.45, 26.50, 53.63, 60.67, 61.93, 133.77, 134.18,
166.15; HR-FIMS m/z calcd for C10H14O3 [M]+
182.0943, found 182.0943.
2-(4′-Methyl-3′,4′-epoxycyclohex-1′-en-1′-yl)-
propan-2-ol (3)
To a solution of MeLi (1.14 M in ether; 526 μL,
0.600 mmol) in THF (1 mL) was added a solution of
18 (36.4 mg, 0.200 mmol) in THF (0.8 mL) under Ar
at −78 °C. After stirring for 1 h at the same tempera-
ture, the reaction mixture was poured into sat. aq.
NH4Cl and extracted with EtOAc. The organic layer
was washed with sat. aq. NaHCO3, dried over Na2SO4
and concentrated in vacuo. The residue was chroma-
tographed over silica gel. Elution with hexane/EtOAc
(2/1) gave 3 (10.9 mg, 0.0648 mmol, 32%) as a colorless
oil. IR (film) νmax 3424, 2975, 2927, 2871, 1726, 1650,
1376, 1253 cm−1; 1H NMR (300 MHz, CD3OD) δ 1.25
(3H, s), 1.26 (3H, s), 1.39 (3H, s), 1.55–1.65 (2H, m),
2.00–2.11 (2H, m), 3.13 (1H, d, J = 4.5 Hz), 5.92 (1H,
dd, J = 2.7, 4.5 Hz); 1H NMR (300 MHz, C6D6) δ 1.06
(3H, s), 1.08 (3H, s), 1.20 (3H, s), 1.20 (1H, m),
1.76–1.86 (2H, m), 2.07 (1H, m), 2.85 (1H, d,
J = 4.2 Hz), 5.78 (1H, dd, J = 3.0, 4.2 Hz);13C NMR
(75 MHz, C6D6) δ 21.39, 21.67, 27.57, 28.05, 28.20,
54.29, 59.23, 71.69, 107.70, 114.82; HR-FIMS m/z calcd
for C10H16O2 [M]+ 168.1150, found 168.1146.
2-(1′-Methyl-7′-oxabicyclo[2.2.1]hept-2′-en-2′-yl)-
propan-2-ol (2)
To a solution of CeCl3 (4.33 g, 17.6 mmol) in THF
(30 mL) was added a solution of 14 (492 mg,
2.93 mmol) in THF (20 mL) at room temperature
under Ar. After stirring for 30 min, MeMgCl (3.0 M
in THF; 3.90 mL, 11.7 mmol) was added to the
mixture at −60 °C. After stirring for 40 min at the
same temperature, the reaction mixture was poured
into sat. aq. NH4Cl and extracted with ether. The
organic layer was washed with sat. aq. NaHCO3,
dried over MgSO4 and concentrated in vacuo. The
residue was chromatographed over silica gel. Elution
with hexane/EtOAc (2/1) gave 2 (483 mg, 2.87 mmol,
98%) as a colorless oil. IR (film) νmax 3418, 2978,
2944, 2871, 1650, 1461, 1384 cm−1; 1H NMR
(500 MHz, CD3OD) δ 1.35 (3H, s), 1.36 (3H, s),
1.40–1.53 (3H, m), 1.71 (3H, s), 1.94 (1H, dddd,
J = 4.0, 4.5, 9.0, 11.0 Hz), 4.76 (1H, dd, J = 2.0,
4.5 Hz), 6.02 (1H, d, J = 2.0 Hz); 13C NMR (125 MHz,
CD3OD) δ 19.96, 28.91, 29.50, 29.60, 32.60, 70.59,
78.49, 88.50, 129.52, 156.90; HR-FIMS m/z calcd for
C10H16O2 [M]+ 168.1150, found 168.1161.
Compound 3 was highly unstable against protic
solvents and decomposed in CD3OD or D2O within
0.5 h at room temperature.
Ethyl 4-methyl-3,4-epoxycyclohex-1-ene-
1-carboxylate (18)
Ethyl (3S*,4R*)-3,4-dihydroxy-4-methylcyclohex-
1-ene-1-carboxylate (19)
According to the reported procedure [13,14], the
known ester 17 was prepared from 15 in 2 steps,
which was used for this step without purification.
To a suspension of 17 (1.47 g, 7.48 mmol) and
NaHCO3 (0.75 g, 8.9 mmol) in CH2Cl2 (22 mL) was
added mCPBA (65%; 2.34 g, 8.81 mmol) portionwise
under Ar at 0 °C. After stirring for 1.5 h at the same
temperature, the reaction mixture was filtered
through a silica gel pad, and the filtrate was washed
with sat. aq. Na2S2O3, sat. aq. Na2CO3 and brine
successively, dried over MgSO4 and concentrated in
vacuo. The residue was chromatographed over silica
gel. Elution with hexane/EtOAc (15/1) gave 18
(1.03 g, 5.47 mmol, 73% in 3 steps) as a yellow oil.
IR (film) νmax 2981, 2930, 1714, 1643, 1263,
To a solution of 17 (166 mg, 1.00 mmol) in acetone
(5 mL) and water (5 mL) were added DABCO
(6.7 mg, 0.060 mmol), OsO4 (1% solution in
t-BuOH; 0.76 mL, 0.030 mmol) and NMO (50% aq.
solution; 234 mg, 1.00 mmol) successively at 0 °C.
After stirring for 15 h at room temperature, the
reaction mixture was poured into sat. aq. Na2S2O3
and extracted with EtOAc. The organic layer was
dried over MgSO4 and concentrated in vacuo. The
residue was chromatographed over silica gel. Elution
with hexane/EtOAc (1/1) gave 19 (101 mg,
0.502 mmol, 50%) as colorless crystals. mp 76 °C;
IR (film) νmax 3404, 2975, 2938, 1696, 1651, 1445,
1
1375, 1256, 1137, 1037 cm−1; H NMR (300 MHz,
CD3OD) δ 1.23 (3H, s), 1.27 (3H, t, J = 7.2 Hz), 1.60
(1H, ddd, J = 6.0, 9.0, 14.1 Hz), 1.85 (1H, ddd, J = 4.2,
1
1194 cm−1; H NMR (300 MHz, CDCl3) δ 1.28 (3H,

SYNTHESIS-GUIDED STRUCTURE REVISION OF A MONOTERPENE
7
6.0, 14.1 Hz), 2.22 (1H, dddt, J = 4.2, 6.0, 18.3,
2.1 Hz), 2.45 (1H, dddd, J = 2.7, 6.0, 9.0, 18.3 Hz),
3.95 (1H, dt, J = 4.2, 2.7 Hz), 4.18 (2H, q, J = 7.2 Hz),
6.70 (1H, m); 13C NMR (75 MHz, CD3OD) δ 14.52,
23.01, 25.92, 33.78, 61.70, 70.24, 72.50, 132.61, 140.44,
168.50; HR-FIMS m/z calcd for C10H16O4 [M]+
200.1049, found 200.1047.
17.5, 2.0 Hz), 5.15 (1H, m), 5.61 (1H, dt, J = 3.0,
1.5 Hz); 13C NMR (125 MHz, CDCl3) δ 21.22, 22.32,
25.40, 28.76, 28.97, 33.07, 68.90, 72.52, 74.57, 116.35,
149.25, 170.80; HR-FIMS m/z calcd for C12H20O4
[M]+ 228.1362, found 228.1356.
The NMR data described above were almost iden-
tical with those reported in ref [16]. Therefore, the
structure of the monoterpene isolated from Riella
helicophylla was proven to be cis-4.
(1R*,2S*)-4-(1′-Hydroxy-1′-methylethyl)-1-
methylcyclohex-3-ene-1,2-diol (cis-4)
6-Hydroxy-3-(1′-Hydroxy-1′-methylethyl)-6-
methylcyclohex-2-en-1-one (21)
To a solution of MeLi (1.14 M in ether; 2.6 mL,
3.0 mmol) in THF (3 mL) was added a solution of
19 (98.9 mg, 0.494 mmol) in THF (1 mL) under Ar
at −20 °C. After stirring for 20 min at the same
temperature, the reaction mixture was poured into
sat. aq. NH4Cl and extracted with EtOAc. The
organic layer was dried over MgSO4 and concen-
trated in vacuo. The residue was chromatographed
over silica gel. Elution with CH2Cl2/MeOH (10/1)
gave cis-4 (83.0 mg, 0.446 mmol, 90%) as a white
solid. mp 120 °C; IR (film) νmax 3387, 2974, 2933,
To a solution of cis-4 (83.0 mg, 0.446 mmol) in
CH2Cl2 (8 mL) were added NaHCO3 (75 mg,
0.89 mmol) and DMP (227 mg, 0.535 mmol) at 0
°C. After stirring for 20 h at room temperature, the
reaction mixture was poured into sat. aq. Na2S2O3
and extracted with EtOAc. The organic layer was
washed with sat. aq. Na2CO3 and brine, dried over
MgSO4 and concentrated in vacuo. The residue was
chromatographed over silica gel. Elution with hex-
ane/EtOAc (1/1.5) gave 21 (62.1 mg, 0.337 mmol,
76%) as a yellow viscous oil. IR (film) νmax 3419,
2977, 2936, 2872, 1669, 1364, 1326, 1134 cm−1;
1H NMR (400 MHz, CDCl3) δ 1.28 (3H, s), 1.39
(3H, s), 1.40 (3H, s), 1.98 (1H, ddd, J = 5.2, 11.6,
12.8 Hz), 2.13 (1H, ddd, J = 2.8, 4.8, 12.8 Hz), 2.33
(1H, br s), 2.43 (1H, dddd, J = 2.4, 4.8, 11.6,
19.2 Hz), 2.56 (1H, ddd, J = 2.8, 5.2, 19.2 Hz),
3.71 (1H, br s), 6.16 (1H, d, J = 2.4 Hz); 13C NMR
(100 MHz, CDCl3) δ 23.85, 24.73, 28.56, 28.70,
35.73, 72.60, 72.63, 119.31, 171.35, 203.10;
1H NMR (500 MHz, CD3OD) δ 1.28 (3H, s), 1.35
(3H, s), 1.37 (3H, s), 1.99–2.02 (2H, m), 2.47 (1H,
dddd, J = 2.0, 5.5, 9.5, 19.0 Hz), 2.60 (1H, dt,
J = 19.0, 4.0 Hz), 6.08 (1H, d, J = 2.0 Hz);
13C NMR (125 MHz, CD3OD) δ 23.71, 25.57,
28.56, 28.71, 37.65, 73.12, 73.60, 120.90, 173.39,
204.04; HR-FIMS m/z calcd for C10H16O3 [M]+
184.1099, found 184.1103.
1
2902, 1662, 1378, 1131 cm−1; H NMR (500 MHz,
CDCl3) δ 1.24 (3H, s), 1.34 (6H, s), 1.61 (1H, m),
1.85 (1H, dt, J = 13.5, 6.0 Hz), 2.07 (1H, dt, J = 18.0,
6.0 Hz), 2.31 (1H, m), 3.88 (1H, br s), 5.73 (1H, m);
1H NMR (300 MHz, CD3OD) δ 1.17 (3H, s), 1.28
(3H, s), 1.29 (3H, s), 1.54 (1H, dt, J = 13.2, 6.3 Hz),
1.82 (1H, ddd, J = 5.7, 6.6, 13.2 Hz), 2.05 (1H, dddt,
J = 5.7, 6.6, 18.0, 1.5 Hz), 2.28 (1H, dddt, J = 6.0,
7.2, 18.0, 1.8 Hz), 3.78 (1H, m), 5.65 (1H, dt, J = 3.6,
1.8 Hz); 13C NMR (75 MHz, CD3OD) δ 23.64,
25.25, 28.91, 29.03, 33.59, 70.68, 72.63, 72.98,
121.26, 147.95; HR-FIMS m/z calcd for C10H18O3
[M]+ 186.1256, found 186.1255.
(1R*,2S*)-1-Hydroxy-4-(1′-hydroxy-1′-
methylethyl)-1-methylcyclohex-3-en-2-yl acetate
(cis-20)
To a solution of cis-4 (16.0 mg, 0.0859 mmol) and
DMAP (11.5 mg, 0.0941 mmol) in pyridine (1 mL)
was added Ac2O (16.3 μL, 0.172 mmol) at 0 °C. After
stirring for 2 h at room temperature, the reaction
mixture was poured into dil. HCl and extracted
with EtOAc. The organic layer was washed with sat.
aq. NaHCO3, dried over MgSO4 and concentrated in
vacuo. The residue was chromatographed over silica
gel. Elution with dichloromethane/methanol (20/1)
gave cis-20 (16.2 mg, 0.0664 mmol, 77%) as a white
solid. mp 89–90 °C; IR (film) νmax 3419, 2976, 2933,
(1R*,2R*)-4-(1′-Hydroxy-1′-methylethyl)-1-
methylcyclohex-3-ene-1,2-diol (trans-4)
To a solution of 21 (258 mg, 1.40 mmol) in MeOH
(7 mL) was added a solution of CeCl3·7H2O (782 mg,
2.10 mmol) in MeOH (3.5 mL) under Ar at −60 °C.
After stirring for 5 min, NaBH4 (58.2 mg, 1.54 mmol)
was added to the resulting mixture at the same tem-
perature. After stirring for 15 min, water was added
and methanol was removed under reduced pressure.
The residual aqueous layer was saturated with NaCl
and extracted with EtOAc. The organic layer was
dried over MgSO4 and concentrated in vacuo. The
1
2851, 1716, 1372, 1254 cm−1; H NMR (500 MHz,
CDCl3) δ 1.21 (3H, s), 1.33 (6H, s), 1.65 (1H, ddd,
J = 6.0, 7.5, 13.5 Hz), 1.89 (1H, dt, J = 13.5, 6.0 Hz),
2.09 (1H, m), 2.12 (3H, s), 2.37 (1H, dddt, J = 6.0, 7.5,

8
S. KONISHI ET AL.
residue was chromatographed over silica gel. Elution
with CH2Cl2/MeOH (15/1 to 8/1) gave cis-4 (46.2 mg,
0.248 mmol, 18%) and trans-4 (187.4 mg, 1.01 mmol,
72%) as a white solid. mp 138 °C; IR (film) νmax 3288,
3211, 2980, 2930, 2873, 1451, 1358, 1311 cm−1;
1H NMR (500 MHz, CDCl3) δ 1.20 (3H, s), 1.34
(3H, s), 1.35 (3H, s), 1.68–1.80 (2H, m), 2.16 (1H,
dddt, J = 6.0, 9.0, 18.0, 2.0 Hz), 2.27 (1H, dddt,
J = 6.0, 7.5, 18.0, 1.5 Hz), 4.09 (1H, q, J = 2.5 Hz),
5.68 (1H, ddd, J = 1.5, 2.0, 2.5 Hz); 1H NMR
(500 MHz, CD3OD) δ 1.15 (3H, s), 1.29 (6H, s),
1.60–1.71 (2H, m), 2.11 (1H, dddt, J = 6.0, 8.0, 18.0,
2.0 Hz), 2.23 (1H, dtt, J = 18.0, 1.5, 6.0 Hz), 3.92
(1H, m), 5.59 (1H, dt, J = 3.0, 1.5 Hz); 13C NMR
(125 MHz, CD3OD) δ 21.85, 24.13, 29.01, 29.06,
34.71, 72.50, 72.90, 74.78, 122.31, 147.05; HR-FIMS
m/z calcd for C10H18O3 [M]+ 186.1256, found
186.1261.
Author Contribution
H.W. designed this study. S.K. carried out the experiments.
N.M. contributed to analytical works. S.K. and H.T. wrote
the manuscript with assistance from all authors.
Acknowledgments
Our thanks are due to T. Hasegawa Co., Ltd. for financial
support.
Disclosure statement
No potential conflict of interest was reported by the
authors.
ORCID
Naoki Mori
http://orcid.org/0000-0001-6478-1289
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