An efficient approach to chiral nonracemic trans- and cis-decalin scaffolds for drimane and labdane synthesis

Article abstract of DOI:10.1016/j.tetasy.2004.07.064

Optically active trans- and cis-ring junction decalinic intermediates, which represent useful precursors for the synthesis of more complex natural targets, have been conveniently prepared starting from the β-ketoester 2 obtained by standard chemistry from

Full text of DOI:10.1016/j.tetasy.2004.07.064

Tetrahedron:
Asymmetry
Tetrahedron: Asymmetry 15 (2004) 3223–3231
An efficient approach to chiral nonracemic trans- and
cis-decalin scaffolds for drimane and labdane synthesis
Gian Piero Pollini,a,* Anna Bianchi,a Alberto Casolari,a Carmela De Risi,a
Vinicio Zaniratoa and Valerio Bertolasib
a
`
Dipartimento di Scienze Farmaceutiche, Universita di Ferrara, Via Fossato di Mortara 19, I-44100 Ferrara, Italy
`
Dipartimento di Chimica, Centro di Strutturistica Diffrattometrica, Universita di Ferrara, Via L. Borsari 46, I-44100 Ferrara, Italy
b
Received 12 July 2004; accepted 23 July 2004
Available online 1 October 2004
Abstract—Optically active trans- and cis-ring junction decalinic intermediates, which represent useful precursors for the synthesis of
more complex natural targets, have been conveniently prepared starting from the b-ketoester 2 obtained by standard chemistry from
b-ionone and dimethyl carbonate. The chiral auxiliary (ꢀ)-menthol, easily attached to 2 through DMAP-catalyzed transesterifica-
tion, allowed a clean separation of the diastereomers obtained in the key electrocyclization step, which were further elaborated to
chiral intermediates already taken to drimane and labdane sesquiterpenes.
ꢀ 2004 Elsevier Ltd. All rights reserved.
1. Introduction
ways, including enzymatic resolution,4–8 the use of enan-
tiomerically pure 1,2-diols as chiral auxiliaries for acetal
formation,9–12 derivatization of the secondary alcohol
obtained by sodium borohydride reduction via its (R)-
naphthylethylcarbamate13 or esterification of the pri-
mary alcohol with Boc-L-proline,14 or by traditional
salts formation with (ꢀ)-a-phenylethylamine or (ꢀ)-
ephedrine of the C-2 methylene carboxylic acid.15,16
A large number of naturally occurring compounds
exhibiting diverse biological activities present as a com-
mon structural feature a 4,4,10-trimethyl substituted
trans-decalin nucleus; representative examples are found
particularly in the large families of drimane and labdane
sesquiterpenes.1
Asymmetric syntheses of most of the members of these
families are commonly based on starting chiral synthons
derived by conversion of higher terpenes (e.g., sclareol,
manool, abietic acid, communic acid, and larixol), or
from simple terpenes [e.g., (ꢀ) and (+)-carvone or thuj-
one] as well as from well-established synthetic chiral
materials such as the Wieland–Miescher ketone.
CO2Me
O
H
( )-1
2. Results and discussion
Racemic b-ketoester 1, which was first prepared in 1957
by Eschenmoser et al.2 during their pioneering work on
biogenetic-type cyclization of polyenes and later by
White et al.3 through an improved procedure, has often
been exploited as the starting material for the synthesis
of most of these products.
As part of our continuous interest in the field,17 we here-
in report our recent synthetic efforts to obtain chiral
building blocks for the synthesis of this class of
compounds.
In 1989 Isoe et al.18 developed an efficient methodology
for the preparation of racemic octalone 6 starting from
the readily available b-ionone via the high-yield
sequence depicted in Scheme 1. Thus, enolacetate 3 of
the b-ketoester 2, obtained by standard chemistry
from b-ionone and dimethyl carbonate, was converted
Not surprisingly, the preparation of both enantiomers of
1 and related compounds was attempted in different
*
Corresponding author. Tel.: +39 0532 291286; fax: +39 0532
291296; e-mail: pol@unife.it
0957-4166/$ - see front matter ꢀ 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2004.07.064

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G. P. Pollini et al. / Tetrahedron: Asymmetry 15 (2004) 3223–3231
Ac2O, Py
β-ionone
CO2Me
CO2Me
O
OAc
3
2
hν
sensitizer
CO2Me
CO2Me
OAc
CO2Me
O
1. NaOMe
2. NaH
OAc
heat
H
4
( )-5
( )-6
Scheme 1.
2
through irradiation with a high-pressure mercury lamp,
in the presence of benzantrone as a sensitizer, into the
corresponding cis-enolacetate 4, which was then submit-
ted to the featuring thermal electrocyclization to pro-
duce quantitatively the bicyclic derivative 5. The latter
could be subsequently converted to the interesting build-
ing block 6 through a careful choice of basic conditions
to secure the desired relative stereochemistry in repro-
ducible yields.
(-)-menthol
throughout the paper
DMAP, toluene
R consists in (1R,2S,5R)-(-)-Menthol
O
Ac2O, Py
CO-R
O
CO-R
OAc
8
7
hν
sensitizer
CO-R
OAc
We were particularly intrigued by the possibility of per-
forming this sequence in the presence of a chiral auxil-
iary in order to prepare bicyclo[4.4.0]decane (decalin)
skeletons in enantiomerically pure form given that this
route could open the way to trans-decalins (Scheme 4)
as well as cis-decalins (Scheme 7).
CO-R
OAc
10 crystalline
thermal
electrocyclization
CO-R
OAc
9
To this end, we envisioned the use of the cheap commer-
cially available (ꢀ)-menthol as the chiral auxiliary,
which could be easily bound through DMAP-catalyzed
transesterification of 2.19 We anticipated that the chiral
auxiliary might act as an asymmetry inducer in the elec-
trocyclization step or it might simply allow conventional
separation of diastereomeric compounds 10 and 11
(Scheme 2). With this in mind, we submitted the (ꢀ)-
menthyl ester derivative 7 to the synthetic sequence pre-
viously described by the Japanese group.18 Thus, the
formation of enolacetate 8 and its irradiation proceeded
uneventfully to produce the menthyl cis-enolacetate
ester 9, which was submitted to thermal electrocycliza-
tion. This operation essentially generated the same
amount of two compounds, which could be cleanly sep-
arated by simple crystallization, one being a white crys-
talline compound, the second one a yellow oil.
11 oily
Scheme 2.
X-ray analysis (Fig. 1) allowed us to assign the stereo-
structure 10 to the crystalline derivative, the structure
of the oily stereoisomer consequently being assigned as
11.
In an attempt to obtain optically active trans-fused bicy-
clic compounds, following IsoeÕs directions, we first tried
the removal of the enolacetate group by treating crystal-
line compound 10 with NaOMe (Scheme 3).
Unexpectedly, the basic treatment led to the formation
of the rather unusual diosphenol 12, whose identity
was eventually confirmed by X-ray structure (Fig. 2).
Figure 1. ORTEP view of compound 10 showing the thermal ellipsoids
at 30% level of probability.

G. P. Pollini et al. / Tetrahedron: Asymmetry 15 (2004) 3223–3231
3225
CO-R
CO2Bn
O
CO-R
CO-R
O
O
OH
NaOMe
air
10
10 crystalline
1. NaOMe
2. NaH
PhCH2OH
DMAP
H
H
O
13
(+)-15
3. H2/cat.
toluene
12
CO2Bn
CO-R
Scheme 3.
O
O
11
oily
H
H
14
(-)-15
Scheme 4.
tional advantage of an ester moiety cleavable by
hydrogenolysis.
In order to gain further proof of the assigned structures
of enantiomeric 15, we decided to convert them into com-
pounds already described in the literature as pure stereo-
isomers, but obtained by different routes (Scheme 5).
CH2OH
OH
O
H
H
(-)-16
(+)-17
CO2Bn
O
H
H
(+)-15
CO2Bn
O
H2, Pd/C
LiAlH4
Figure 2. ORTEP view of compound 12 showing the thermal ellipsoids
at 30% level of probability.
CH2OH
OH
O
(-)-15
The formation of compound 12 can be accounted for by
a spontaneous oxidation of the deacetoxylated inter-
mediate, owing to the known20 propensity to aerial oxi-
dation of the active methylene groups in some
deconjugated octalones. The undesired formation of
diosphenol 12 was easily suppressed by working in an
oxygen free atmosphere during the basic treatments.
Thus, both the crystalline derivative 10 and the oily
derivative 11 were converted into the corresponding
conjugated ketones through sequential treatments under
argon with sodium methoxide and sodium hydride and
eventually submitted to catalytic reduction producing
the expected b-ketoesters 13 and 14 in satisfactory over-
all yields (Scheme 4).
H
H
(+)-16
(-)-17
Scheme 5.
Thus, lithium aluminum hydride reduction of the b-
keto-ester moiety cleanly converted (+)-15 and (ꢀ)-15
into enantiomeric diols (ꢀ)-16 and (+)-16, useful syn-
thetic intermediates for drimane sesquiterpenes and lab-
dane diterpenes.14 On the other hand, ketones (+)-17
and (ꢀ)-17, known fragrance chemicals with distinct
odor differences,21,22 could be easily obtained by a
one-pot hydrogenolysis/decarboxylation sequence of
(+)-15 and (ꢀ)-15, respectively.
The physical properties of the obtained trans-fused bicy-
clic compounds described in Scheme 5 perfectly agreed
with the reported ones, thus confirming the assigned
structure and stereochemistry.
Having diastereomeric compounds 13 and 14 in hand,
we tried unsuccessfully their conversion into the known
enantiomers of 1 through direct transesterification by
heating in methanol in the presence of a catalytic
amount of DMAP, using the same protocol described
for the preparation of 7. However, removal of the chiral
auxiliary was conveniently accomplished simply by sub-
stituting methanol with the higher boiling benzyl alcohol
through a clean transesterification producing the benzyl
esters (+)-15 and (ꢀ)-15, respectively, featuring the addi-
Alternatively, a number of chemical manipulations were
also performed on compounds still containing the chiral
auxiliary (ꢀ)-menthol, using 13 as the starting model
compound. In this way, we were able to prepare (ꢀ)-
albicanol 22 as well as the C-1 exo-methylene compound

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G. P. Pollini et al. / Tetrahedron: Asymmetry 15 (2004) 3223–3231
(+)-20 (Scheme 6), which have been previously
described, fully characterized and utilized as the key
intermediates in the synthesis of several important com-
pounds including the marine natural products zonarol13
and copalol.14
CH2OH
O
CO-R
O
1,2-bis(TMSO)ethane
TMS triflate
LiAlH4
13
O
O
H
H
18
19
Ph3P=CH2
CO-R
10% HCl
CH2OH
H
21
O
LiAlH4
H
H
(-)-22
(+)-20
Scheme 6.
Figure 3. ORTEP view of compound 23 showing the thermal ellipsoids
at 30% level of probability.
Interestingly, along the route to (+)-20 we needed to
protect the carbonyl group of 13 as the corresponding
1,3-dioxolane, but this trivial operation cannot be
achieved following usual directions (ethylene glycol in
toluene in the presence of acid catalyst as well as the sys-
tem triethyl orthoformate-ethylene glycol). However, a
clean ketalization could be accomplished using less then
usual conditions,23 providing compound 18 in good
yield. Reduction of the ester group with lithium alumi-
num hydride gave the corresponding primary alcohol
19, which was easily taken to the target compound
(+)-20 by a two step transformation, that is, restoration
of the C-2 keto group and acid promoted dehydration of
the crude b-hydroxy carbonyl derivative.
Also starting from 13, via Wittig olefination of the C-2
keto group and subsequent lithium aluminum hydride
reduction of compound 21, we obtained (ꢀ)-22, the
unnatural antipode of albicanol.24,25 Moreover, as
anticipated, we demonstrated that the diastereomeric
compounds 10 and 11, obtained in the thermal electro-
cyclization step, could be conveniently used to access
optically active cis-decalin systems (Scheme 7).
Figure 4. ORTEP view of compound (+)-24 showing the thermal
ellipsoids at 30% level of probability.
CO-R
OH
CH2OH
OH
Assuming that the addition of hydrogen on compound
10 occurred stereoselectively anti to the ester group,
enantiomer diol (ꢀ)-24 is likely to derive from the oily
diastereomer 11 as the starting chiron.
1. H2/cat.
2. NaOMe
LiAlH4
10
H
H
23
(+)-24
Scheme 7.
3. Conclusion
In fact, submitting the key intermediate 10 to catalytic
reduction followed by sodium methoxide promoted O-
deacetylation, we obtained hydroxy ester 23, which on
reduction with lithium aluminum hydride furnished
the enantiomerically pure diol (+)-24. The structures
of both new compounds have been confirmed by X-
ray analysis (Figs. 3 and 4).
In summary, the original route developed by Isoe and
co-workers18 for the synthesis of racemic decalin inter-
mediates, has been conveniently extended to the synthe-
sis of both optically active trans- and cis-decalin
synthons, which represent useful starting chiral materi-
als for the synthesis of biologically active terpenes.

G. P. Pollini et al. / Tetrahedron: Asymmetry 15 (2004) 3223–3231
3227
The racemic resolution has been achieved using as the
chiral auxiliary the cheap (ꢀ)-menthol, which has been
introduced and removed through DMAP-catalyzed
transesterification, providing a facile entry to the opti-
cally active b-ketoesters 15, which are valuable starting
materials for the preparation of more complex natural
targets.
(m, 1H); 5.52 (s, 1H); 5.92 (d, J = 16.2Hz, 1H); 6.64
(d, J = 16.2Hz, 1H); 2(Z) isomer d: 0.73–1.22 (m,
18H); 1.30–2.20 (m, 15H); 2.27 (s, 3H); 4.69–4.75 (m,
1H); 5.63 (s, 1H); 6.62 (d, J = 16.2Hz, 1H); 7.33–7.37
(d, J = 16.2Hz, 1H). Anal. Calcd for C26H40O4: C,
74.96; H, 9.68. Found: C, 74.81; H, 9.52.
4.4. 3-Acetyloxy-5-(20,60,60-trimethylcyclohex-10-enyl)-
penta-2,4-dienoic acid (ꢀ)-menthyl ester, 9
4. Experimental
4.1. General
A cooled (0ꢁC) solution of 8 (1.00g, 2.40mmol) in dry
THF (50mL) containing benzanthrone (160mg,
0.07mmol) was irradiated for 8h with a 125W, high-
pressure, mercury-vapor lamp. Evaporation of the sol-
vent followed by addition of hexane allowed removal
of benzanthrone by filtration. The filtrate was evapo-
rated and the residue purified by chromatography
(ether–petroleum ether 1:9) to give oily 9 (0.97g, 97%)
as a C-2 diastereomixture. IR (film) mmax: 1780, 1720,
Melting points were determined on a Buchi-Tottoli
¨
apparatus and are uncorrected. Infrared (IR) spectra
were recorded on a FT-IR Paragon 500 spectrometer.
1H nuclear magnetic resonance (1H NMR) spectra were
taken on a Bruker AC spectrometer at 200MHz and on
a Varian Mercury Plus spectrometer at 400MHz, for
solutions in CDCl3 unless otherwise noted. Chemical
shifts are given in parts per million downfield from tetra-
methylsilane as the internal standard while coupling
constants are given in hertz. Optical rotations were
measured with a Perkin–Elmer 241 MC Polarimeter.
Organic solutions were dried over anhydrous magne-
sium sulfate and evaporated with a rotary evaporator.
Petroleum ether refers to the fractions boiling in the
range 40–60ꢁC and ether to diethyl ether. Merck silica
gel (70–230 mesh) was used for column chromatogra-
phy. All reactions were performed under an N2 or Ar
atmosphere. Elemental analyses were effected by the
microanalytical laboratory of Dipartimento di Chimica,
University of Ferrara.
1440cmꢀ1 1H NMR (200MHz, CDCl3) d: 0.73–1.15
.
(m, 18H); 1.22–2.20(m, 15H); 2.27 (s, 3H); 4.72 (dt,
J = 4.4, 10.8Hz, 1H); 5.63 (s, 1H); 6.62 (d, J = 16.2Hz,
1H); 7.35 (d, J = 16.2Hz, 1H). Anal. Calcd for
C26H40O4 requires C, 74.96; H, 9.68. Found: C, 74.85;
H, 9.55.
4.5. (1R,8aS)- and (1S,8aR)-2-Acetyloxy-5,5,8a-tri-
methyl-1,5,6,7,8,8a-hexahydro-naphthalene-1-carboxylic
acid (ꢀ)-menthyl ester, 10 and 11
Compound 9 (1.00g, 2.45mmol) was heated at 180ꢁC
for 1.5h, then the cooled oily residue was purified by
chromatography (ether–petroleum ether 1:9) to give a
semisolid mass (0.51g, 50%), which was crystallized
from EtOH 95% to give 10 (0.21g) as white crystals.
The filtrate was evaporated to give the oily 11 (0.25g).
4.2. 3-Oxo-5-(20,60,60-trimethylcyclohex-10-enyl)-pent-4-
enoic acid (ꢀ)-menthyl ester, 7
20
Compound 10: Mp 89–90ꢁC. ½aŠ ¼ þ395:0 (c 1.00,
A mixture of b-ketoester 2 (1.00g, 4.00mmol), (ꢀ)-men-
thol (1.26g, 8.00mmol), and 4-dimethylaminopyridine
(50mg, 0.40mmol) in toluene (100mL) was heated at
reflux for 36h and then evaporated in vacuo. The resi-
D
CHCl3). IR (film) mmax: 1775, 1712, 1640cmꢀ1
.
1H
NMR (400MHz, CDCl3) d: 0.70–1.08 (m, 12H); 1.14
(s, 6H); 1.21–1.82 (m, 13H); 1.92–195 (m, 2H); 2.09 (s,
3H); 2.85 (s, 1H); 4.62 (dt, J = 4.4, 10.8Hz, 1H); 5.77
(s, 2H). Anal. Calcd for C26H40O4 requires C, 74.96;
H, 9.68. Found: C, 74.88; H, 9.60.
due was chromatographed on silica gel (ether–petroleum
ether 1:1) to give 7 (1.27g, 85%) as an oil. ½aŠ
20
D
¼
ꢀ48:2 (c 1.82, CHCl3). IR (film) mmax: 1740, 1640,
1580cmꢀ1 1H NMR (200MHz, CDCl3) d: 0.72–1.12
.
20
D
(m, 18H); 1.20–2.15 (m, 15H); 3.59 (s, 2H); 4.73 (dt,
J = 4.4, 10.8Hz, 1H); 6.19 (d, J = 16.4Hz, 1H); 7.37
(d, J = 16.4Hz, 1H). Anal. Calcd for C24H38O3: C,
76.96; H, 10.23. Found: C, 76.80; H, 10.05.
Compound 11: ½aŠ ¼ ꢀ305:0 (c 2.74, CHCl3). 1H
NMR (400MHz, CDCl3) d: 0.68–1.08 (m, 12H); 1.12–
1.82 (m, 19H); 1.83–2.20(m, 5H); 2.80(s, 1H); 4.57
(dt, J = 4.0, 10.4Hz, 1H); 5.75 (s, 2H). Anal. Calcd for
C26H40O4 requires C, 74.96; H, 9.68. Found: C, 74.78;
H, 9.58.
4.3. 3-Acetyloxy-5-(20,60,60-trimethylcyclohex-10-enyl)-
penta-2,4-dienoic acid (ꢀ)-menthyl ester, 8
4.6. (1S,4aR,8aR)-5,5,8a-Trimethyl-2-oxo-decahydro-
naphthalene-1-carboxylic acid (ꢀ)-menthyl ester, 13,
(1R,4aS,8aS)-5,5,8a-trimethyl-2-oxo-decahydro-naph-
thalene-1-carboxylic acid (ꢀ)-menthyl ester, 14 and
(8aS)-2-Hydroxy-5,5,8a-trimethyl-3-oxo-3,5,6,7,8,8a-
hexahydro-naphthalene-1-carboxylic acid (ꢀ)-menthyl
ester, 12
A solution of 7 (1.00g, 0.26mmol) in dry pyridine
(1mL) was treated with acetic anhydride (1.5mL,
0.16mmol), stirred at rt for 8h and then neutralized
by careful addition of 10% HCl. The mixture was ex-
tracted with ether (2 · 25mL), washed with saturated
sodium bicarbonate solution, dried, and evaporated to
give 8 (1.00g, 93%) as oily mixture of C-2 stereoisomers
(E:Z ratio = 2:3). IR (film) mmax: 1760, 1720, 1450cmꢀ1
.
A solution of 10 (1.00g, 0.24mmol) in MeOH (5mL)
containing NaOMe (0.22g, 0.41mmol) was stirred
under argon for 4h at 0ꢁC after which 10% phosphoric
1H NMR (200MHz, CDCl3): 2(E) isomer d: 0.73–1.12
(m, 18H); 1.30–2.20 (m, 15H); 2.22 (s, 3H); 4.69–4.75

3228
G. P. Pollini et al. / Tetrahedron: Asymmetry 15 (2004) 3223–3231
acid was added until pH = 6. After evaporation of most
of methanol, the residue was extracted with ether
(3 · 25mL), the extracts washed with saturated sodium
bicarbonate solution, dried, and evaporated. The resi-
due, dissolved in anhydrous benzene (10mL), was added
dropwise to a suspension of NaH (0.40g, 16.40mmol) in
anhydrous benzene (10mL) and heated at reflux for 1h.
To the cooled reaction mixture, 10% phosphoric acid
was added until pH = 6, the organic layer was separated
and the aqueous phase extracted with ether (2 · 30mL).
The combined organic extracts were washed with satu-
rated sodium bicarbonate solution, dried, and evapo-
rated. The residue was dissolved in EtOH (25mL) and
the obtained solution shaken for 4h at 20psi in a Parr
apparatus in the presence of 5% palladium on charcoal
(100mg). Filtration on Celiteꢂ, evaporation in vacuo,
C21H28O3 requires C, 76.79; H, 8.59. Found: C, 76.62;
H, 8.53. The spectral data of (+)-15 and (ꢀ)-15 were iden-
1
tical: IR (film) mmax: 1720, 1650, 1540cmꢀ1. H NMR
(400MHz, CDCl3) d: 0.89 (s, 3H); 0.96 (s, 3H); 1.13–
1.32 (m, 5H); 1.38–1.82 (m, 6H); 1.99–2.08 (m, 1H);
2.27–2.38 (m, 1H); 2.51 (dddd, J = 1.6, 5.2, 5.6, 14.6Hz,
1H); 3.27 (s, 1H); 5.14 (s, 2H); 7.29–7.40(m, 5H).
4.8. (1R,2R,4aR,8aR)- and (1S,2S,4aS,8aS)-1-Hydroxy-
methyl-2-hydroxy-5,5,8a-trimethyl-decahydronaphtalene,
(ꢀ)-16 and (+)-16
A solution of (+)-15 (0.34g, 0.90mmol) in anhydrous
ether (10mL) was added dropwise to a cooled (0 ꢁC) sus-
pension of lithium aluminum hydride (100mg,
0.26mmol) in anhydrous ether (25mL). The reaction
mixture was stirred at room temperature for 3h, then
quenched by careful addition of water, filtered, and
the inorganic salts washed with ether. The filtrate was
dried and evaporated to give a residue, which was chro-
matographed (ether–petroleum ether 6:4) to give (ꢀ)-16
(0.15g, 75%) as a white solid. Mp 132–134ꢁC (EtOAc–
pentane) [lit.14 mp 132–134ꢁC (EtOAc–hexane)].
and crystallization from EtOH yielded 13 (0.66g,
20
D
73%). Mp 154–155ꢁC (ethanol 95%). ½aŠ ¼ ꢀ11:9 (c
0.92, CHCl3). IR (KBr) mmax: 1723, 1650cmꢀ1
.
1H
NMR (400MHz, CDCl3) d: 0.73–1.15 (m, 16H); 1.16–
1.82 (m, 17H); 1.82–2.18 (m, 3H); 2.24–2.56 (m, 2H);
3.17 (s, 1H); 4.70(dt, J = 4.4, 10.6Hz, 1H). Anal. Calcd
for C24H40O3 requires C, 76.55; H, 10.71. Found: C,
76.30; H, 10.63.
20
D
24
D
½aŠ ¼ ꢀ26:24 (c 0.94, CHCl3) {lit.14 ½aŠ ¼ ꢀ25:5 (c
0.94, CHCl3)}. IR (KBr) mmax: 3330, 1445, 1021cmꢀ1
.
When the first basic treatment (NaOMe) was performed
in a nonoxygen-free atmosphere, compound 10 gave 12
in variable yield after purification by column chroma-
1H NMR (400MHz, CDCl3) d: 0.91 (s, 3H); 1.04 (dd,
J = 3.6, 12.4Hz, 1H); 1.11–1.28 (m, 8H); 1.33–1.62 (m,
6H); 1.65–2.08 (m, 5H); 3.90 (dd, J = 3.6, 11.0Hz,
1H); 4.06 (dd, J = 7.6, 11.0Hz, 1H); 4.21–4.25 (m,
1H). Anal. Calcd for C14H26O2 requires C, 74.29; H,
11.58. Found: C, 74.22; H, 11.50.
tography (ether–petroleum ether 3:7). Mp 99–100ꢁC
20
(EtOH 95%). ½aŠ ¼ ꢀ79:3 (c 0.75, CH3OH). IR
D
(KBr) mmax: 3380, 1715, 1650, 1260cmꢀ1
.
1H NMR
(200MHz, acetone-d6) d: 0.74–1.81 (m, 30H); 1.81–
2.24 (m, 3H); 4.89 (dt, J = 4.4, 10.8Hz, 1H); 6.31 (s,
1H); 8.57 (s, 1H). Anal. Calcd for C24H36O4 requires
C, 74.19; H, 9.34. Found: C, 74.05; H, 9.28.
Similarly, (ꢀ)-15 was converted to (+)-16, which showed
spectral data identical to (ꢀ)-16. Mp 132–134ꢁC
(EtOAc–pentane) [lit.14 mp 132–134ꢁC (EtOAc–hex-
20
D
ane)].
½aŠ ¼ þ26:8
(c
0.94,
CHCl3)
{lit.14
24
D
The same protocol described for the preparation of 13
from 10 allowed a clean conversion of 11–14. Mp 101–
½aŠ ¼ þ26:1 (c 0.93, CHCl3)}.
20
102ꢁC (ethanol 95%). ½aŠ ¼ ꢀ58:8 (c 0.83, CHCl3).
4.9. (4aR,8aS)- and (4aS,8aR)-3,4,4a,5,6,7,8,8a-Octa-
hydro-5,5,8a-trimethyl-2(1H)-naphthalenone, (+)-17 and
(ꢀ)-17
D
1
IR (KBr) mmax: 1723, 1650cmꢀ1. H NMR (400MHz,
CDCl3) d: 0.73–1.13 (m, 16H); 1.15–1.25 (m, 6H);
1.25–1.81 (m, 11H); 1.81–2.13 (m, 3H); 2.23–2.54 (m,
2H); 3.15 (s, 1H); 4.73 (dt, J = 4.4, 10.8Hz, 1H). Anal.
Calcd for C24H40O3 requires C, 76.55; H, 10.71. Found:
C, 76.41; H, 10.59.
A suspension of (+)-15 (0.36g, 1.09mmol) and 10% pal-
ladium on charcoal (100mg) in EtOH (10mL) was sha-
ken for 2h at 20psi in a Parr apparatus, filtered on a pad
of Celiteꢂ and the filtrate evaporated. The residue was
purified by chromatography (EtOAc–petroleum ether
4.7. (1S,4aR,8aR)- and (1R,4aS,8aS)-5,5,8a-Trimethyl-
2-oxo-decahydro-naphthalene-1-carboxylic acid benzyl
ester, (+)-15and ( ꢀ)-15
1:40) to give (+)-17 (0.13g, 72%) as white crystals. Mp
20
D
89–91ꢁC [lit.21 mp 88–89ꢁC (hexane)]. ½aŠ ¼ þ86:1 (c
20
D
1
0.40, CHCl3) {lit.21 ½aŠ ¼ þ84:9 (c 1.07, CHCl3)}. IR
A mixture of 13 or 14 (0.50g, 1.33mmol), benzyl alcohol
(0.28mL, 2.65mmol), and 4-dimethylaminopyridine
(16mg, 0.13mmol) in toluene (40mL) was heated at
reflux for 36h and then the solvent evaporated in vacuo
and the residue chromatographed on silica gel (ether–
petroleum ether 1:9) to give (+)-15 or (ꢀ)-15 (0.31g,
72%), respectively.
(KBr) mmax: 1720cmꢀ1. H NMR (200MHz, CDCl3) d:
0.83 (s, 3H); 0.87 (s, 3H); 0.97 (s, 3H); 1.20–1.26 (m,
2H); 1.40–1.48 (m, 4H); 1.60–1.66 (m, 2H); 1.96–1.99
(m, 2H); 2.11–2.15 (m, 1H); 2.23–2.26 (m, 1H); 2.36–
2.40(m, 1H). Anal. Calcd for C 13H22O requires C,
80.35; H, 11.41. Found: C, 80.44; H, 11.60.
Similarly, (ꢀ)-15 was converted to (ꢀ)-17, having spec-
Compound (ꢀ)-15: Mp 80–81ꢁC (ethanol 95%).
tral data identical to (+)-17. Mp 89–91ꢁC [lit.21 mp 88–
20
½aŠ ¼ ꢀ43:2 (c 0.81, CHCl3). Anal. Calcd for
20
89ꢁC (hexane)]. ½aŠ ¼ ꢀ85:1 (c 0.40, CHCl3) {lit.21
D
C21H28O3 requires C, 76.79; H, 8.59. Found: C, 76.59;
D
20
D
½aŠ ¼ ꢀ84:0 (c 1.30, CHCl3)}. Anal. Calcd for
H, 8.51. Compound (+)-15: Mp 76–77ꢁC (ethanol
C13H22O requires C, 80.35; H, 11.41. Found: C, 80.38;
H, 11.55.
20
95%). ½aŠ ¼ þ45:3 (c 0.91, CHCl3). Anal. Calcd for
D

G. P. Pollini et al. / Tetrahedron: Asymmetry 15 (2004) 3223–3231
3229
4.10. (10S,40aR,80aR)-50,50,80a-Trimethyl-octahydro-
spiro{[1,3]dioxolane-2,20-naphthalene}-10-carboxylic
acid (ꢀ)-menthyl ester, 18
J = 1.1Hz, 1H). Anal. Calcd for C14H22O requires C,
81.50; H, 10.75. Found: C, 81.27; H, 10.71.
4.13. (1R,4aR,8aR)-5,5,8a-Trimethyl-2-methylene-
decahydro-naphthalene-1-carboxylic acid (ꢀ)-menthyl
ester, 21
To a cooled (ꢀ78ꢁC) solution of trimethylsilyl triflate
(30lL, 0.15mmol) in anhydrous methylene chloride
(1mL) was added 1,2-bis(trimethylsilyloxy)ethane
(0.42mL, 1.7mmol). A solution of the ketoester 13
(0.58g, 1.55mmol) in anhydrous methylene chloride
(5mL) was added and the mixture warmed to 0ꢁC. After
the disappearance of the starting materials, the reaction
was quenched with dry pyridine (0.35mL), poured into
saturated sodium bicarbonate solution (10mL), and
extracted with methylene chloride (2 · 20mL). The
extracts were washed with brine, dried, and evaporated
to give an oily residue. Column chromatography on sil-
Sodium amide (0.25g, 6.32mmol) was added to a sus-
pension of triphenylphosphonium bromide (2.40g,
6.65mmol) in toluene (25mL) and the resultant mixture
heated at reflux for 6h. After being cooled at room tem-
perature, the decanted yellow solution was added to a
cooled (0ꢁC) solution of 13 (0.50g, 1.33mmol) in tolu-
ene (25mL). The reaction mixture was stirred at rt for
2h, the solvent evaporated and the residue chromato-
graphed on silica gel (ether–petroleum ether 1:60) to
20
D
ica gel (ether–petroleum ether 0.6:9.4) gave 18 (0.59g,
give 21 (0.45g, 90%). ½aŠ ¼ ꢀ41:1 (c 0.79, CHCl3). IR
20
D
1
91%) as an oil. ½aŠ ¼ ꢀ62:7 (c 0.88, CHCl3). IR (film)
(film) mmax
:
1725, 1450, 1380cmꢀ1
.
1H NMR
m
max: 1720, 1540cmꢀ1. H NMR (400MHz, CDCl3) d:
(200MHz, CDCl3) d: 0.71–1.80 (m, 34H); 1.80–2.12
(m, 3H); 2.36–2.48 (m, 1H); 2.74 (s, 1H); 4.55–4.76 (m,
2H); 4.81 (d, J = 1.2Hz, 1H). Anal. Calcd for
C25H42O2 requires C, 80.16; H, 11.30. Found: C,
79.93; H, 11.15.
0.70 (d, J = 6.8Hz, 3H); 0.77–1.31 (m, 21H); 1.31–1.74
(m, 11H); 1.80–1.84 (m, 1H); 1.89–2.08 (m, 2H); 2.45
(s, 1H); 3.68–4.09 (m, 4H); 4.57 (dt, J = 4.4, 10.8Hz,
1H). Anal. Calcd for C26H44O4 requires C, 74.24; H,
10.54. Found: C, 74.10; H, 10.34.
4.14. (1R,4aR,8aR)-5,5,8a-Trimethyl-2-methylene-
decahydro-naphthalen-1-yl-methanol, (ꢀ)-22
4.11. (10R,40aR,80aR)-50,50,80a-Trimethyl-octahydro-
spiro{[1,3]dioxolane-2,20-naphthalen}-10-yl-methanol, 19
A solution of 21 (0.45g, 1.20mmol) in THF (30mL) was
added dropwise to a stirred slurry of lithium aluminum
hydride (90mg, 2.40mmol) in THF (20mL) and the
reaction mixture stirred at rt for 6h. Saturated aqueous
ammonium chloride solution (5mL) was added and the
mixture extracted with EtOAc (3 · 10mL). The com-
bined organic phases were dried and evaporated. The
residue was purified by column chromatography on
silica gel (ether–petroleum ether 1.5:8.5) to give (ꢀ)-
A solution of 18 (0.38g, 0.90mmol) in anhydrous ether
(10mL) was added dropwise to a cooled (0 ꢁC) suspen-
sion of lithium aluminum hydride (100mg, 0.26mmol)
in anhydrous ether (25mL). The reaction mixture was
stirred at room temperature for 3h, then quenched by
careful addition of water, filtered, and the inorganic
salts washed with ether. The filtrate was dried and evap-
orated to give a residue, which was chromatographed on
silica gel (ether–petroleum ether 6:4) to give 19 (0.19g,
albicanol (ꢀ)-22 (0.20g, 75%) as a solid. Mp 69–70ꢁC
20
D
20
77%). Mp 117–118ꢁC (pentane). ½aŠ ¼ þ7:9 (c 0.83,
[lit.24 mp 69–70ꢁC (MeOH)]. ½aŠ ¼ ꢀ12:8 (c 1.00,
20
D
20
D
CHCl3). IR (KBr) mmax: 3551, 1030cmꢀ1
.
1H NMR
CHCl3) {lit.24 ½aŠ ¼ ꢀ14:0 (c 1.00, CHCl3) and lit.25
(200MHz, CDCl3) d: 0.74–1.01 (m, 10H); 1.01–1.72
(m, 9H); 1.72–2.08 (m, 2H); 2.98 (br s, 1H); 3.58–
3.62 (m, 1H); 3.76–4.19 (m, 5H). Anal. Calcd for
C16H28O3 requires C, 71.60; H, 10.52. Found: C,
71.38; H, 10.78.
½aŠ ¼ ꢀ11:5 (c 0.80, CHCl3)}. IR (film) mmax
:
D
3450cmꢀ1
.
1H NMR (400MHz, CDCl3) d: 0.71 (s,
3H); 0.80 (s, 3H); 0.87 (s, 3H); 1.10–1.80 (m, 9H);
1.94–2.05 (m, 2H); 2.38–2.43 (m, 1H); 3.75 (dd,
J = 9.5, 10.8Hz, 1H); 3.83 (dd, J = 3.8, 10.8Hz, 1H);
4.63 (d, J = 1.4Hz, 1H); 4.93 (d, J = 1.4Hz, 1H). Anal.
Calcd for C15H26O requires C, 81.02; H, 11.79. Found:
C, 81.12; H, 11.82.
4.12. (4aR,8aR)-1-Methylene-2-oxo-5,5,8a-trimethyl-
decahydronaphtalene, (+)-20
A solution of 19 (0.40g, 1.49mmol) in acetone (10mL)
containing 10% HCl (4mL) was stirred for 12h at rt.
Most of the solvent was removed in vacuo and the res-
idue neutralized with saturated sodium bicarbonate
solution (10mL) and extracted with ether (2 · 20mL).
The extracts were washed with brine, dried, and evapo-
rated. The residue was chromatographed on silica gel
(ether–petroleum ether 1:1) to give (+)-20 (0.23g, 75%)
4.15. (1R,2S,4aS,8aR)-2-Hydroxy-5,5,8a-trimethyl-
decahydro-naphthalene-1-carboxylic acid (ꢀ)-menthyl
ester, 23
A suspension of 10 (1.33g, 3.20mmol) and 5% palla-
dium on charcoal (100mg) in EtOH (25mL) was shaken
for 4h at 20psi in a Parr apparatus, filtered on a pad of
Celiteꢂ and the filtrate evaporated. The crude residue
was dissolved in MeOH (5mL) containing NaOMe
(0.22g, 0.41mmol) and stirred for 4h at 0ꢁC and then
10% phosphoric acid added until pH = 6. After evapora-
tion of most of the methanol, the residue was extracted
with ether (3 · 25mL), the extracts washed with satu-
rated sodium bicarbonate solution, dried, and evapo-
rated to give 23 (0.99g, 82%). Mp 130–131ꢁC
as a white solid. Mp 55ꢁC (hexane) [lit.12 mp 55–56ꢁC
20
(pentane)]. ½aŠ ¼ þ71:6 (c 1.00, CHCl3) {lit.12
D
20
½aŠ ¼ þ71:9 (c 0.69, CHCl3)}. IR (KBr) mmax: 1697,
D
1
1611, 1461, 1376cmꢀ1. H NMR (200MHz, CDCl3) d:
0.92 (s, 3H); 0.96 (s, 3H); 1.01 (s, 3H); 1.11–2.07 (m,
9H); 2.22–2.46 (m, 1H); 2.67 (ddd, J = 1.9, 5.5,
19.2Hz, 1H); 5.01 (d, J = 1.1Hz, 1H), 5.52 (d,

3230
G. P. Pollini et al. / Tetrahedron: Asymmetry 15 (2004) 3223–3231
20
D
(acetone). ½aŠ ¼ ꢀ40:1 (c 1.83, CHCl3). IR (KBr) mmax
:
Compound 23, C24H42O3; orthorhombic, space group
b = 11.2047(3),
3518, 1704, 1450cmꢀ1. H NMR (400MHz, CDCl3) d:
0.75 (d, J = 7.2Hz, 3H); 0.80–1.28 (m, 20H); 1.28–2.06
(m, 14H); 2.25–2.36 (m, 2H); 3.92–4.10(m, 2H);
4.77 (dt, J = 4.4, 11.2Hz, 1H). Anal. Calcd for
C24H42O3 requires C, 76.14; H, 11.18. Found: C,
75.92; H, 11.02.
P212121,
a = 8.6561(3),
c =
1
3
˚
˚
47.8280(14)A,
V = 4638.8(2)A ,
Z = 8,
Dc =
1.084gcmꢀ3. Intensity data collected with h = 23.5ꢁ;
6600 independent reflections measured; 5365 reflections
observed [I > 2r(I)]. Final R = 0.042 (observed reflec-
tions) and Rw = 0.110 (all reflections). The molecules
form an intramolecular hydrogen bond: O3–Hꢁ ꢁ ꢁO1
˚
4.16. (1S,2S,4aS,8aR)-2-Hydroxy-1-hydroxymethyl-
5,5,8a-trimethyl-decahydronaphtalene, (+)-24
[O3ꢁ ꢁ ꢁO1 = 2.751(3) and 2.798(3)A for the two inde-
pendent molecules of the asymmetric unit].
To a cooled (0ꢁC) suspension of lithium aluminum hy-
dride (0.13g, 3.30mmol) in dry ether (20mL) was added
dropwise a solution of 23 (0.50g, 1.32mmol) in dry ether
(25mL). After the reaction was stirred at room temper-
ature for 8h, water was added, and the precipitate fil-
tered through a pad of Celiteꢂ, after which the filtrate
was concentrated in vacuo. Purification by silica gel
column chromatography (ether–petroleum ether 6:4)
Compound (+)-24, C14H26O2; monoclinic, space group
˚
P21, a = 8.4051(4), b = 11.1196(2), c = 15.3901(7)A,
3
˚
b = 103.070(2)ꢁ,
V = 1401.1(1)A ,
Z = 4,
Dc =
1.073gcmꢀ3. Intensity data collected with h = 27.5ꢁ,
6154 independent reflections measured; 5228 reflections
observed [I > 2r(I)]. Final R = 0.045 (observed reflec-
tions), Rw = 0.115 (all reflections). The asymmetric unit
is built up by two independent molecules which, in the
gave the corresponding diol (+)-24 (0.23g, 77%) as white
crystal, are linked by a network of hydrogen bonds:
˚
20
D
crystals. Mp 133–135ꢁC (EtOH). ½aŠ ¼ þ15:3 (c 0.74,
O2A–Hꢁ ꢁ ꢁO2B(x,y,z)
[O2Aꢁ ꢁ ꢁO2B = 2.725(3)A];
˚
MeOH). IR (KBr) mmax: 3328, 1450, 1023cmꢀ1
.
1H
O1A–Hꢁ ꢁ ꢁO1B(x,y ꢀ 1,z) [O1Aꢁ ꢁ ꢁO1B = 2.820(2)A];
NMR (400MHz, acetone-d6) d: 0.88–1.10 (m, 5H);
1.10–1.26 (m, 7H); 1.26–1.87 (m, 8H); 2.02–2.17 (m,
1H); 2.91 (br s, 2H); 3.96–4.03 (m, 2H); 4.05–4.12 (m,
1H). Anal. Calcd for C14H26O2 requires C, 74.29; H,
11.58. Found: C, 74.33; H, 11.60.
O1B–Hꢁ ꢁ ꢁO2A(ꢀx,y + 1/2,z) [O1Bꢁ ꢁ ꢁO2A = 2.740(3)
˚
A];
O2B–Hꢁ ꢁ ꢁO1A(ꢀx,y + 1/2,z)
[O2Bꢁ ꢁ ꢁO1A =
˚
2.771(3)A].
Complete crystallographic data (excluding structural
factors) for the structures herein have been deposited
at the Cambridge Crystallographic Data Centre and
allocated deposition numbers CCDC 243501–243504.
Copies of the data can be obtained, free of charge, via
www.ccdc.cam.ac.uk./conts/retrieving.html or on appli-
cation to CCDC, 12 Union Road, Cambridge CB2
1EZ, UK [fax: +44(0)-1223-336033, e-mail: deposit@
ccdc.cam.ac.uk].
4.17. X-ray structure determinations
X-ray diffraction data for compound 10 were collected
on a Nonius CAD-4 diffractometer and for compounds
12, 23, and (+)-24 on a Nonius Kappa CCD diffractom-
eter, at room temperature (T = 295K), with graphite
˚
monochromated Mo Ka radiation (k = 0.7107A). The
structures were solved by direct methods (SIR97)26
and refined (SHELXL-97)27 by full matrix least squares
with anisoptropic non-H and calculated H atoms except
for O–H hydrogens, which were refined isotropically.
ORTEP28 views of the molecules are shown in Figures
1–4.
Acknowledgements
`
We gratefully acknowledge Ministero dellÕUniversita e
della Ricerca Scientifica (MURST) (60%) for financial
support.
References
Crystal data: Compound 10, C26H40O4; monoclinic,
space group P21, a = 7.180(1), b = 21.314(2), c =
3
˚
˚
1. Devon, T. K.; Scott, A. I. In Handbook of Naturally
Occurring Compounds; Academic: New York and London,
1972; Vol. 2.
2. Romann, E.; Frey, A. J.; Stadler, P. A.; Eschenmoser, A.
Helv. Chim. Acta 1957, 40, 1900–1917.
16.454(2)A, b = 90.33(1)ꢁ, V = 2518.0(5)A , Z = 4,
Dc = 1.099gcmꢀ3. Intensity data collected with h 6
27.9ꢁ; 6222 independent reflections measured; 3831
reflections observed [I > 2r(I)]. Final R (observed reflec-
tions) = 0.055 and Rw (all reflections) = 0.148.
3. White, J. D.; Skeean, R. W.; Trammel, G. L. J. Org.
Chem. 1985, 50, 1939–1948.
Compound 12, C24H36O4; orthorhombic, space group
P212121,
32.4195(7)A,
4. Nair, M. S.; Anilkumar, A. T. Tetrahedron: Asymmetry
1996, 7, 511–514.
5. Anilkumar, A. T.; Sudhir, U.; Joly, S.; Nair, M. S.
Tetrahedron 2000, 56, 1899–1903.
6. Tanimoto, H.; Oritani, T. Tetrahedron: Asymmetry 1996,
7, 1695–1704.
7. Akita, H.; Nozawa, M.; Futagami, Y.; Miyamoto, M.;
Saotome, C. Chem. Pharm. Bull. 1997, 45, 824–831.
8. Akita, H.; Nozawa, M.; Shimizu, H. Tetrahedron: Asym-
metry 1998, 9, 1789–1799.
a = 11.7988(3),
b = 11.9152(1),
c =
Dc =
3
˚
˚
V = 4557.7(2)A ,
Z = 8,
1.132gcmꢀ3. Intensity data collected with h = 27.5ꢁ,
9750independent reflections measured; 6875 reflections
observed [I > 2r(I)]. Final R = 0.050 (observed reflec-
tions), Rw = 0.119 (all reflections), S = 1.031. The asym-
metric unit is built up by two independent molecules
which, in the crystal, are linked in dimers by means of
the following hydrogen bonds: O3A–Hꢁ ꢁ ꢁO4B
9. Hata, T.; Tanaka, K.; Katsumura, S. Tetrahedron Lett.
1999, 40, 1731–1734.
10. Furuichi, N.; Kato, M.; Katsumura, S. Chem. Lett. 1999,
1247–1248.
˚
(2 ꢀ x,1/2 + y,3/2 ꢀ z)
˚
2.925(2)A].
[O3Aꢁ ꢁ ꢁO4B = 2.779(2)A];
O3B–Hꢁ ꢁ ꢁO4A(2 ꢀ x,y ꢀ 1/2,3/2 ꢀ z) [O3Bꢁ ꢁ ꢁO4A =

G. P. Pollini et al. / Tetrahedron: Asymmetry 15 (2004) 3223–3231
3231
11. Furuichi, N.; Hata, T.; Soetjipto, H.; Kato, M.; Katsu-
mura, S. Tetrahedron 2001, 57, 8425–8442.
12. Akita, H.; Amano, Y.; Kato, K.; Kinoshita, M. Tetrahe-
dron: Asymmetry 2004, 15, 725–732.
21. Gautier, A.; Vial, C.; Morel, C.; Lander, M.; Na¨f, F. Helv.
Chim. Acta 1987, 70, 2039–2044.
22. Brenna, E.; Fuganti, C.; Serra, S. Tetrahedron: Asymmetry
2003, 14, 1–42.
13. Mori, K.; Komatsu, M. Bull. Soc. Chim. Belg. 1986, 95,
771–781.
23. Kutney, J. P.; Cirera, C. Can. J. Chem. 1997, 75, 1136–
1150.
14. Toshima, H.; Oikawa, H.; Toyomasu, T.; Sassa, T.
Tetrahedron 2000, 56, 8443–8450.
24. Laube, T.; Schro¨der, J.; Stehle, R.; Seifert, K. Tetrahedron
2002, 58, 4299–4309.
15. Liapis, M.; Ragoussis, V.; Ragoussis, N. J. Chem. Soc.,
Perkin Trans. 1 1987, 987–992.
16. Schro¨der, J.; Magg, C.; Seifert, K. Tetrahedron Lett. 2000,
41, 5469–5473.
17. Barco, A.; Benetti, S.; Bianchi, A.; Casolari, A.; Guarneri,
M.; Pollini, G. P. Tetrahedron 1995, 51, 8333–8338.
18. Katsumura, S.; Kimura, A.; Isoe, S. Tetrahedron 1989, 45,
1337–1346.
19. Gilbert, J. C.; Kelly, T. A. J. Org. Chem. 1988, 53, 449–
450.
25. Akita, H.; Nozawa, M.; Mitsuda, A.; Ohsawa, H. Tetra-
hedron: Asymmetry 2000, 11, 1375–1388.
26. Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G.
L.; Giacovazzo, C.; Guagliardi, A.; Moliterni, A. G. G.;
Polidori, G.; Spagna, R. J. Appl. Crystallogr. 1999, 32,
115–116.
27. Sheldrick, G. M. SHELXL-97, Program for Crystal
Structures Refinement, University of Go¨ttingen, Germany,
1997.
28. Burnett, M. N., Johnson, C. K. ORTEPIII, Report
ORNL-6895, Oak Ridge National Laboratory, Oak
Ridge, TN, 1996.
20. Kato, M.; Vogler, B.; Yoshikoshi, A. J. Chem. Res. (S)
1991, 114–115.