Total synthesis of (-)-(7S,10R)-calamenene and (-)-(7S,10R)-2- hydroxycalamenene by use of a ring-closing metathesis reaction. A comparison of the cis- and trans-isomers

Article abstract of DOI:10.3390/70700517

The title compounds have been synthesized starting from l-menthone by application of a ring-closing metathesis reaction to confirm their reported absolute and relative stereochemistry. Comparisons of the NMR spectra and specific rotations are also discuss

Full text of DOI:10.3390/70700517

Molecules 2002, 7, 517-527
molecules
ISSN 1420-3049
http://www.mdpi.org
Total Synthesis of (-)-(7S,10R)-Calamenene and (-)-(7S,10R)-
2-Hydroxycalamenene by Use of a Ring-Closing Metathesis
Reaction. A Comparison of the cis- and trans-Isomers
Katsuyuki Nakashima, Masashi Imoto, Masakazu Sono, Motoo Tori,* Fumihiro Nagashima and
Yoshinori Asakawa.
Faculty of Pharmaceutical Sciences, Tokushima Bunri University, Yamashiro-cho, Tokushima,
770-8514, Japan. Tel (+81) 88 622-9611, Fax (+81) 88 655-3051, homepage;
http://p.bunri-u.ac.jp/~tori/english/e-tori.html
* Author to whom correspondence should be addressed; e-mail: tori@ph.bunri-u.ac.jp
Received: 2 May 2002; in revised form: 1 July 2002 / Accepted: 8 July 2002 / Published: 31 July 2002
Abstract: The title compounds have been synthesized starting from l-menthone by
application of a ring-closing metathesis reaction to confirm their reported absolute and
relative stereochemistry. Comparisons of the NMR spectra and specific rotations are also
discussed.
Keywords: Calamenene, hydroxycalamenene, ring closing metathesis, absolute
configuration, sesquiterpene.
Introduction
The absolute configurations of both (7S,10R)-calamenene (1), isolated from Chamaecyparis
nootkatensis by Andersen et al. [1] and (+)-2-hydroxycalamenene (2) [2], isolated from Dysoxylum
acutangulum by Nishizawa et al. [3] were determined using CD spectra or X-ray analysis and chemical
transformations. The stereochemistry of 1 was later revised after X-ray analysis [4]. We have been
working on natural products found in the liverwort [5, 6] and have reported the isolation of several
calamenenes [7, 8], including 5-hydroxycalamenene, whose structure was determined by X-ray analysis
[8]. Up to now, total syntheses of these chiral substances have never been clearly reported because the
final products were always a mixture of cis- and trans-isomers [9]. We are currently working on the

Molecules 2002, 7
518
ring-closing metathesis reaction (RCM) [10, 11] and planned to synthesize these natural products
possessing trisubstituted double bonds, in optically active forms, by application of RCM. As shown in
Scheme 1, our synthetic plan is to construct the trisubstituted double bond by RCM [12-14] starting from
l-menthone (5).
Scheme 1
14
2
1
6
10
11
15
4
7
RCM
12
13
1
O
OH
OH
OH
3
4
5
2
Results and Discussion
l-Menthone (5) was first allylated with LDA and then a Grignard reaction of this ketone with
methallylmagnesium chloride afforded alcohol 4. No attempt was made to determine the stereochemistry
because the newly created chiral center of compound 4 will be lost at a later stage. Now, the diene
alcohol 4 was treated with Grubbs’ catalyst (5 mol%) in CH2Cl2 (10 mM) at r.t. overnight to produce
alkene 3 in 96% yield. The stereochemistry of alkene 3 was analyzed by NOESY, however, due to the
overlapping of the protons the cis/trans stereochemistry at the ring junction was not known. Dehydration
of alcohol 3 with POCl3 afforded calamenene (1) and a small amount of diene 6. The diene 6 was
oxidized by DDQ to afford only cadalene (7) and no calamenene (1) was obtained. The alcohol 3 was
also transformed into enone 8 by allylic oxidation and then dehydration of 8 produced phenol 2, although
the yield was low (Scheme 2).
The spectral data of 1 were identical in all aspects to those reported in the literature [1], including the
specific rotation. The spectral data of compound 2 were also identical to those found in the literature [3],
however, the sign of the rotation was opposite to that reported. Thus, we conclude that the synthetic
compound was the enantiomer of the natural product found by Nishizawa et al. [3] and the assigned
absolute configuration was correct.

Molecules 2002, 7
519
Scheme 2
Br
Grubbs'
catalyst
(5 mol%)
(2.0eq)
LDA
1)
HMPA, THF
-78°C rt, overnight
MgCl
O
CH2Cl2 (10 mM)
rt, overnight
OH
2)
OH
96 %
5
THF, -15°C, 2 h
4
3
2 steps 23 %
+
POCl3
DDQ
pyridine
rt, 2 h
benzene
rt, overnight
33 %
13 %
17 %
POCl3
1
6
7
3
[ ] -73° (c 0.88, CHCl )
α
D
3
O
OH
tBuOOH,
PhH
pyridine
OH
rt, 2 h
,
0 °C rt, overnight
33 %
16 %
2
8
[ ] -24° (c 1.06, CHCl )
α
D
3
Figure 1. 1H-NMR spectra of compound 1 (trans)
(trans)
1

Molecules 2002, 7
520
Figure 1 (cont). 1H-NMR spectra of compound 20 (cis)
(cis)
20
Figure 2. 1H-NMR spectra of compound 2 (trans)
OH
(trans)
2

Molecules 2002, 7
521
Figure 2 (cont). 1H-NMR spectra of compound 21 (cis)
OH
(cis)
21
We next attempted the synthesis of compounds 18 or 19, which are potential intermediates for the
synthesis of tamariscol [15, 16] and other terpenoids.
Scheme 3
LDA, allyl Bromide
HMPA
+
+
THF
-78 ºC~rt,
O
O
O
O
9
11
10
12
[(CH 3)2CHO] 4 Ti
Grubbs' catalyst
LDA, allyl bromide
no reaction
CH 2 Cl2
THF
-78 oC~rt,
rt, overnight
12 %
O
13
O
14
N(C2 H5 )3
NaBH4, CeCl3•7 H2O
CF3 SO3 Si(C2H5 )3
14
CH 2 Cl2
MeOH
0 oC, 30 min
0 oC~rt, overnight
OTES
O
OH
54 %
21 %
15
17
16
19
Grubbs' catalyst
CH 2 Cl2
rt, overnight
OTES
O
6 %
18

Molecules 2002, 7
522
Thus, dihydrocarvone (9) was allylated with LDA and allyl bromide, however, the desired product 10
was only formed in minute amounts. Instead, compounds 11 and 12 were produced in larger quantities.
Therefore, allylation of carvone (13) itself was tried. Again the yield was low, however, the RCM was
attempted with Grubbs’ catalyst. No reaction occurred in the case of compound 14. In an alternate
approach, the carbonyl group in 14 was reduced and protected with a TES group to afford 16. Then,
compound 16 was treated with Grubbs’ catalyst, but the cyclized product 17 was produced only in 6%
yield (Scheme 3).
Conclusions
We have applied the RCM reaction to the construction of a trisubstituted alkene and thus synthesized
1
two calamenene-type natural products, 1 and 2. The H-NMR spectra of 1 and 2 were compared with
those of the cis-isomers (Figures 1 and 2) [8]. Confusion about the stereochemistry (Table 1) as well as
the different numbering systems and names used for these compounds [2], made it very difficult to
identify each compound unambiguously.
Table 1. Comparison of specific rotations
R
S
S
R
S
R
R
Compound
S
1
ent-1
20
ent-20
Synthetic product
(this work)
Reported data
-73
-22 [1, wrong]
+43 [4]
-96 [1]
+31 [7]
-31 [17]
-77 [18]
+33.4 [8]
OH
OH
OH
OH
R
S
S
R
Compound
S
R
R
S
2
ent-2
21
ent-21
Synthetic product
(this work)
-24
-
-
-
Reported data
-
+38 [3]
+40[20]
-

Molecules 2002, 7
523
The original assignment by Andersen et al. [1] was later revised by Croft et al. [4] and other syntheses
have always afforded mixtures of cis and trans products. We now offer reliable data for the trans
derivatives and also give NMR data for comparison with those of cis-isomers [7, 8, 18, 19]. The
13C-NMR spectrum for compound 20 in ref. [18] was measured with an 80 MHz machine, while our
spectrum was taken with a 400 MHz spectrometer (Table 2). Because some peaks are congested in a
narrow region, some problems could have resulted with the assignments and our data have not yet been
fully assigned [20]. It is noteworthy that a condensed cyclopentene ring was not easy to construct by the
RCM reaction, presumably due to the fact that stereochemistry was not correct for cyclization.
Table 2. 13C-NMR data for the calamenes
ent-2
[3]
1
this work*
140.1
126.8
126.2
134.5
128.8
140.0
43.8
1
20
20
2
this work*
126.1
153.2
113.4
135.1
123.0
141.3
43.0
21
No.
[18]
[20]*
139.7
128.5
126.3
134.5
128.7
139.9
43.6
[18]
[20]*
126.7
153.0
112.8
135.7
120.8
141.3
43.3
1
2
140.3
127.0
126.4
134.6
129.0
140.3
44.1
21.3
31.0
32.6
32.1
17.5
21.6
22.3
21.3
140.0
128.5
126.5
134.6
128.9
140.2
43.8
23.3
28.9
31.2
32.7
17.8
21.5
22.5
21.2
126.5
153.2
113.8
135.1
123.1
141.3
43.2
19.2
27.2
26.7
33.2
19.7
21.1
22.2
21.3
3
4
5
6
7
8
21.1
23.3
19.1
16.4
9
30.8
28.7
27.1
28.8
10
11
12
13
14
15
32.5
31.1
26.5
26.5
31.9
32.5
33.1
30.9
17.3
17.6
19.6
17.3
21.5
21.4
21.0
20.5
22.3
19.7
22.1
21.2
21.3
21.2
21.1
21.1
*assignment is tentative.
Acknowledgements
We thank Dr. Masami Tanaka and Ms. Yasuko Okamoto (our university) for measurement of the NMR
and MS spectra, respectively.

Molecules 2002, 7
524
Experimental
General
The IR spectra were measured with a JASCO FT/IR-500 spectrophotometer. The 1H- and 13C-NMR
spectra were recorded on a JEOL ECP-400, a Varian Unity 200, or a Varian Gemini 200 spectrometer.
Deuteriochloroform was used for NMR and chemical shifts were expressed in ppm and the coupling
constant in Hz. The mass spectra, including high-resolution mass spectra, were taken with a JEOL
AX-500 spectrometer. The specific rotation was measured with a JASCO DIP-100 polarimeter. Silica
gel BW-300 (200-400 mesh, Fuji silycia) was used for column chromatography, and silica gel 60F254
plate (0.25 mm, Merck) were used for TLC. All reactions were carried out under an argon atmosphere.
THF was distilled from LiAlH4 and then from Na-benzophenone prior to use. RCM reagent was weighed
in a dry box and was used without purification. Anhydrous dichloromethane and benzene used for the
reactions were purchased from Kanto Chemical, Japan and were used without further purification.
Preparation of [1ξ,2ξ,3R,6S]-2-allyl-6-isopropyl-3-methyl-1-(2-methylpropyl)cyclohexan-1-ol (4).
l-Menthone (300 mg, 1.95 mmol) was treated with LDA (n-BuLi, 1.5 eq.; iPr2NH, 1.5 eq.) in THF at
-78ûC for 1 h, then, allyl bromide (0.39 mL, 2.0 eq.) was added. The temperature was gradually raised to
r.t. overnight. Water was added and the solvent was removed. The mixture was extracted with ether and
the organic layer was washed with 1M HCl and brine, dried (MgSO4), and evaporated to afford a residue
which was purified by silica gel column chromatography (hexane-EtOAc, 0-25%) to give
2-allylmenthone (65.3g, 17%); FTIR: 1720 cm-1; 1H-NMR (200 MHz) δ 0.84 (3H, d, J = 6.4 Hz), 0.89
(3H, d, J = 6.4 Hz), 1.04 (3H, d, J = 6.4 Hz), 1.47 (3H, m), 1.87 (1H, m), 2.06 (4H, m), 2.34 (2H, m), 4.94
(1H, br d, J = 10.4 Hz), 5.00 (1H, br d, 16.8 Hz), 5.84 (1H, ddt, J = 16.8, 10.4, 5.6 Hz); 13C-NMR (50
MHz) δ 18.8 (CH3), 20.5 (CH3), 21.4 (CH3), 26.1 (CH), 29.1 (CH2), 30.4 (CH2), 34.7 (CH2), 39.9 (CH),
56.9 (CH), 57.5 (CH), 115.5 (CH2), 137.3 (CH), 212.0 (C); MS (EI) m/z 194 [M]+ 194 (base), 179, 151,
138, 123, 109, 95, 81, 69, 55; HRMS (EI) Found m/z 194.1664 [M]+. C13H22O requires 194.1670.
The Grignard reagent was prepared from methallyl chloride (1 mL, 10 mmol) and Mg (243 mg, 10
mmol) in THF (8 mL) with aid of dibromoethane (0.05 mL, 0.1 eq.) at -15ûC. One half of this Grignard
reagent was introduced into a solution of 2-allylmenthone (100 mg, 0.52 mmol) in THF at -15ûC over 2
h. Saturated NH4Cl solution and water were added and the mixture was extracted with ether. The organic
layer was washed with sat. NH4Cl soln. and brine, dried (MgSO4), and was evaporated to afford a residue
which was purified by silica gel column chromatography (hexane-EtOAc, 0-10%) to give 4 (122 mg.
95%). 4; FTIR: 3600, 1640 cm-1; 1H-NMR (200 MHz) δ 0.92 (9H, m), 1.29 (2H, m), 1.41-1.58 (4H, m),
1.80 (1H, m), 1.83 (3H, m), 2.24 (1H, m), 2.36-2.72 (4H, m), 4.84 (1H, br s), 4.94 (1H, br s), 5.02 (1H, d,
J = 9.6 Hz), 5.18 (1H, d, J = 17.4 Hz), 6.08 (1H, m); 13C-NMR (50 MHz) δ 18.2 (CH3), 20.4 (CH3), 20.9
(CH3), 23.2 (CH3), 25.1 (CH3), 26.6 (CH), 31.7 (CH), 32.4 (CH2), 35.9 (CH2), 44.3 (CH2), 48.3 (CH),
49.8 (CH), 79.6 (C), 114.7 (CH2), 116.1 (CH), 138.7 (CH), 141.9 (C); MS (EI) m/z 250 [M]+ 250, 217,

Molecules 2002, 7
525
195 (base), 177, 139, 121, 109, 97, 83, 69, 55; HRMS (CI) Found m/z 251.2373 [M+ H]+. C13H22O
requires 251.2375.
Preparation of [1ξ,6ξ,7R,10S]-10-isopropyl-3,7-dimethylbicyclo[4.4.0]dec-3-en-1-ol (3).
To a stirred solution of 4 (350 mg, 1.4 mmol) in degassed CH2Cl2 (125 mL) was added a solution of
Grubbs reagent (57.6 mg, 5 mol%) in CH2Cl2 (15 mL) under Ar at room temperature. The mixture was
stirred overnight at r.t. The septum was removed and the mixture was stirred for further 30 min. and then
the solvent was removed. The residue was directly purified by silica gel column chromatography
(hexane-EtOAc, 0-20%) to afford 3 (298 mg, 96%): FTIR: 3450-3600 cm-1; 1H-NMR (600 MHz)δ 0.86
(3H, d, J = 6.6 Hz), 0.92 (3H, d, J = 6.9 Hz), 0.94 (3H, d, J = 6.9 Hz), 1.02 (2H, m), 1.08 (1H, m), 1.38
(1H, m), 1.50 (2H, m), 1.67 (3H, s), 1.71 (1H, m), 1.73 (1H, m), 1.96 (1H, br d, J = 18 Hz), 2.21 (3H, m),
5.44 (1H, br s); 13C-NMR (50 MHz) δ 18.2 (CH3), 19.9 (CH3), 20.3 (CH2), 23.7 (CH3), 23.7 (CH3), 25.5
(CH), 26.9 (CH2), 33.3 (CH), 35.5 (CH2), 42.8 (CH2), 46.8 (CH), 51.9 (CH), 72.5 (C), 120.2 (CH), 130.8
(CH); MS (EI) m/z 222 [M]+ 204, 189, 179, 161 (base), 139, 119, 105, 93, 84, 69, 55; HRMS (EI) Found
m/z 222.1991 [M]+. C15H26O requires 222.1983.
Preparation of (-)-calamenene (1) and [7S,10R]-2,5-dihydrocalamenene (6).
A solution of 3 (100 mg, 0.45 mmol) in pyridine (5 mL) was treated with POCl3 (0.084 mL, 2.0 eq.) at
r.t. overnight. Water was added and the mixture was extracted with ether. The organic layer was washed
with 1M HCl and brine, dried (MgSO4) and evaporated to afford a residue which was purified by silica
gel column chromatography (hexane-EtOAc, 0-20%) to give (-)-calamenene (1) (12.0 mg, 13%) and 6
(15.4 mg, 17%). 1: [α]D -73°(c 0.88, CHCl3); FTIR: 1510-1470 cm-1; 1H-NMR (200 MHz) δ 0.72 (3H,
d, J = 7.2 Hz), 1.00 (3H, d, J = 7.2 Hz), 1.26 (3H, d, J = 7.2 Hz), 1.39 (1H, m), 1.60 (1H, m), 1.81 (1H, m),
1.95 (1H, m), 2.22 (1H, m), 2.30 (3H, s), 2.70 (2H, m), 6.94 (1H, br d, J = 8.0 Hz), 7.02 (1H, br s), 7.12
(1H, d, J = 8.0 Hz); 13C-NMR (50 MHz) Table 1); MS (EI) m/z 202 [M]+ 202, 159 (base), 144, 129, 115,
115, 105, 91; HRMS (EI) Found m/z 202.1696 [M]+. C15HS requires 202.1722. 6; 1H-NMR (200 MHz)
δ 0.91(3H, d, J = 5.9 Hz), 0.93 (3H, d, J = 5.9 Hz), 0.98 (3H, d, J = 5.9 Hz), 1.68 (3H, br s), 5.42 (1H, m).
Preparation of [1ξ,6ξ,7S,10R]-6-hydroxy-7-isopropyl-4,10-dimethylbicyclo[4.4.0]dec-3-en-2-one (8).
A solution of 3 (100 mg, 0.45 mmol) in benzene (10 mL) was treated with Celite (1.05 g), tBuOOH
(0.31 mL), and PDC (1.2 g, 1.35 mmol) at 0ûC and the mixture was stirred overnight. The temperature
was gradually raised to room temperature overnight. A solution of Na2S2O3 was added and the mixture
was filtered through Celite. The filtrate was extracted with ether and the organic layer was washed with
brine, dried (MgSO4) and evaporated to afford a residue which was purified by silica gel column
1
chromatography (hexane-EtOAc, 0-30%) to give 8 (35.4 mg, 33%); FTIR: 3400, 1660 cm-1; H-NMR
(200 MHz) δ 0.90 (3H, d, J = 6.6 Hz), 0.95 (3H, d, J = 6.6 Hz), 0.95 (1H, m), 1.08 (3H, d, J = 5.8 Hz),

Molecules 2002, 7
526
1.30 (3H, m), 1.55 (1H, m), 1.85 (1H, m), 1.89 (3H, s), 2.10 (2H, m), 2.40 (1H, br d, J = 17.6 Hz), 2.65
13
(1H, d, J = 17.6 Hz), 5.79 (1H, s); C-NMR (50 MHz) δ 18.1 (CH3), 19.6 (CH2), 22.1 (CH3), 23.7
(CH3X2), 25.5 (CH), 27.3 (CH), 34.9 (CH2), 44.2 (CH2), 52.0 (CH), 60.7 (CH), 77.1 (C), 126.4 (CH),
153.7 (C) 200.5 (C); MS (EI) m/z 236 [M]+, 236, 221, 193, 175, 165, 151, 137 (base), 125, 111, 95, 83,
69, 55; HRMS (EI) Found m/z 236.1770 [M]+. C15H24O2 requires 236.1777.
Preparation of (-)-[7S,10R]-2-hydroxycalamenene (2).
A solution of 8 (63 mg, 0.27 mmol) in pyridine (2 mL) was treated with POCl3 (0.13 mL, 5 eq.) at 0ûC.
The mixture was stirred at r.t. overnight. Water was added and the mixture was extracted with ether. The
organic layer was washed with 1M HCl, and brine, dried (MgSO4) and evaporated to afford a residue
which was purified by silica gel column chromatography (hexane-EtOAc, 0-50%) to give 2 (18.7 mg,
16%); [α]D -24°(c 1.06, CHCl3); FTIR: 3450, 1620, 1580 cm-1; 1H-NMR (200 MHz) δ 0.82 (3H, d, J =
7.2 Hz), 0.99 (3H, d, J = 7.2 Hz), 1.20 (3H, d, J = 7.2 Hz), 1.51 (1H, m), 1.81 (1H, m), 1.99 (2H, m), 2.24
(3H, s), 2.46 (1H, m), 3.05 (1H, s), 4.64 (1H, s), 6.43 (1H, s), 6.58 (1H, s); 13C-NMR (50 MHz) (Table 1);
MS (EI) m/z 218 [M]+, 218, 175 (base), 160, 147, 121, 115, 105, 91; HRMS (EI) Found m/z 218.1691
[M]+. C15H22O requires 218.1671.
References and Notes
1. Andersen, N. H.; Syrdal, D. D.; Graham, C. The absolute stereochemistry of calamenene.
Tetrahedron Lett. 1972, 905-908.
2. Connolly, J. D.; Hill, R. A. Dictionary of Terpenoids; Chapman & Hall: London, 1991. The name
and numbering used followed those proposed in this source.
3. Nishizawa, M.; Inoue, A.; Sastrapradja, S,; Hayashi, Y. (+)-8-Hydroxycalamenene: A fish-poison
principle of Dysoxylum acutangulum and D. alliaceum. Phytochemistry 1983, 22, 2083-2085.
4. Croft, K. D.; Ghisalberti, E. L.; Hocart, C. H.; Jefferies, P. R.; Raston, C. L.; White, A. H. Absolute
configuration of the (+)-calamenenes: Crystal structure of 7-hydroxycalamenene. J. Chem. Soc.
Perkin 1 1978, 1267-1270.
5. Asakawa, Y. In Progress in the Chemistry of Organic Natural Products, Herz, W.; Grisebach, H.;
Kirby, G. W., Ed.; Springer-Verlag: Wien, 1982; Vol. 42, p 1.
6. Asakawa, Y. In Progress in the Chemistry of Organic Natural Products, Herz, W.; Kirby, G. W.;
Moor, R. E.; Steglich, W.; Tamm, Ch., Ed.; Springer-Verlag: Wien, 1995; Vol. 65, p 1.
7. Nagashima, F.; Suda, K.; Asakawa, Y. Cadinane-type sesquiterpenoids from the liverwort Scapania
undulata. Phytochemistry 1994, 37, 1323-1325.
8. Nagashima, F.; Momosaki, S.; Watanabe, Y.; Takaoka, S.; Huneck, S.; Asakawa, Y.
Sesquiterpenoids from the liverworts Bazzania trilobata and Porella canariensis. Phytochemistry
1996, 42, 1361-1366.

Molecules 2002, 7
527
9. Ladwa, P. H.; Joshi, G. D.; Kulkarni, S. N. Stereochemical disposition of isopropyl group in
synthetic (+)-calamenen. Indian J. Chem. 1978, 16B, 853-855.
10. Nakashima, K.; Ito, R.; Sono, M.; Tori, M. Olefin metathesis reactions of some aromatic dienes with
ortho- and meta-disubstitution. Formation of 10-, 12-, 14-, and 17-membered cyclic compounds and
isomerization of an allylic alcohol. Heterocycles 2000, 53, 301-314.
11. Nakashima, K.; Imoto, M.; Miki, T.; Miyake, T.; Fujisaki, N.; Fukunaga, S.; Mizutani, R.; Sono,
M.; Tori, M. Ring closing metathesis reaction of dienes with acrylate moiety leading to 5- to
7-membered lactones and cyclization to 14-membered rings. Heterocycles 2002, 56, 85-89.
12. Grubbs, R. H.; Chang, S. Recent advances in olefin metathesis and its application in organic
synthesis. Tetrahedron 1998, 54, 4413-4450.
13. Fürstner, A. Alkene Metathesis in Organic Synthesis; Springer: Heidelberg, 1998.
14. Fürstner, A. Olefin metathesis and beyond. Angew. Chem. Int. Ed. 2000, 39, 3012-3043.
15. Tori, M.; Sono, M.; Asakawa, Y. Absolute configuration and synthesis of the liverwort
sesquiterpene alcohol tamariscol, J. Chem. Soc. Perkin Trans. 1 1990, 2849-2850.
16. Tori, M.; Sono, M.; Nishigaki, Y.; Nakashima, K.; Asakawa, Y. Studies on the liverwort
sesquiterpene alcohol tamariscol. Synthesis and absolute configuration. J. Chem. Soc. Perkin
Trans. 1 1991, 435-445.
17. Kim, Y. –K.; Cool, L. G.; Zavarin, E. cis-Calamenene-related sesquiterpenoids from Cupressus
bakeri foliage. Phytochemistry 1994, 36, 961-965.
18. Bunko, J. D.; Ghisalberti, E. L.; Jefferies, P. R. (1R,4S)-Calamenene. Chemical correlation with the
diterpene analogues of Eremophila. Aust. J. Chem. 1981, 34, 2237-2242.
19. Toyota, M.; Asakawa, Y.; Takemoto, T. Sesquiterpenes from Japanese liverworts. Phytochemistry
1981, 20, 2359-2366.
20. Nagashima, F. et al., unpublished data.
Sample Availability: Samples are not available.
© 2002 by MDPI (http://www.mdpi.org). Reproduction is permitted for noncommercial purposes.