Lipase-catalyzed resolution of p-menthan-3-ols monoterpenes: Preparation of the enantiomer-enriched forms of menthol, isopulegol, trans- and cis-piperitol, and cis-isopiperitenol

Article abstract of DOI:10.1016/j.tetasy.2003.08.010

A study on the enzymic resolution of the most common p-menthan-3-ol monoterpene isomers is described. Enantioenriched alcohols 1, 5, 10, 11 and 12 are obtained by means of the lipase-mediated kinetic acetylation of the corresponding racemic materials. The stereochemical aspects of the enzymic process have been investigated. We found that the structural features of the starting p-menthan-3-ol as well as the kind of lipase used, impacted strongly on the enantioselectivity of the resolution. The potentialities of this approach for preparative purposes are discussed.

Full text of DOI:10.1016/j.tetasy.2003.08.010

TETRAHEDRON:
ASYMMETRY
Pergamon
Tetrahedron: Asymmetry 14 (2003) 3313–3319
Lipase-catalyzed resolution of p-menthan-3-ols monoterpenes:
preparation of the enantiomer-enriched forms of menthol,
isopulegol, trans- and cis-piperitol, and cis-isopiperitenol
Stefano Serra,* Elisabetta Brenna, Claudio Fuganti and Francesco Maggioni
CNR, Istituto di Chimica del Riconoscimento Molecolare, Dipartimento di Chimica,
Materiali ed Ingegneria Chimica del Politecnico, Via Mancinelli, 7 I-20131 Milano, Italy
Received 25 June 2003; accepted 12 August 2003
Abstract—A study on the enzymic resolution of the most common p-menthan-3-ol monoterpene isomers is described. Enantioen-
riched alcohols 1, 5, 10, 11 and 12 are obtained by means of the lipase-mediated kinetic acetylation of the corresponding racemic
materials. The stereochemical aspects of the enzymic process have been investigated. We found that the structural features of the
starting p-menthan-3-ol as well as the kind of lipase used, impacted strongly on the enantioselectivity of the resolution. The
potentialities of this approach for preparative purposes are discussed.
© 2003 Elsevier Ltd. All rights reserved.
formations of terpenes are correlated to this class of
compounds because of the high commercial require-
ment of (−)-menthol. All the most common p-men-
1. Introduction
The 3-oxygenated monoterpenes of p-menthane family
are important natural compounds. They are wide-
spread in nature and are used extensively in flavour
and fragrance industries and as pharmaceuticals, cos-
metics, agrochemicals and cooling substances. Many
industrial processes of syntheses, extraction and trans-
than-3-ols isomers 1–13 (Fig. 1) are intermediates in
1–3
the preparation of (−)-menthol
and are used as
starting building blocks in several syntheses of fine
chemicals and natural compounds with biological
activity.
Figure 1. The most common p-menthan-3-ols.
* Corresponding author. Tel.: 39-2-23993076; fax: 39-2-23993080; e-mail: stefano.serra@polimi.it
0957-4166/$ - see front matter © 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2003.08.010

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S. Serra et al. / Tetrahedron: Asymmetry 14 (2003) 3313–3319
It is well known that the enantiomer and diastereoiso-
mer composition of these products determine their
activity features.4 This is the case of the eight saturated
p-menthan-3-ols isomers where only (−)-menthol 1 is
used since it gives the best organoleptic performance.
Moreover, recent studies have showed that while the
mixture of the isopulegol isomers 5–8 has been used
conventionally as a perfume ingredient, its main com-
ponent (−)-isopulegol 5 is odourless5 and can be used as
a cooling agent. Therefore, several new methods of
enantioselective synthesis2,6 and of resolution of
racemic material7–14 have been developed in recent
years.
cedures (Scheme 1). Cyclisation of citronellal under
Lewis acid catalysis (SnCl4)16 afforded a mixture of the
isopulegol isomers 5–8 in the ratio: 66/30/3/1 respec-
tively. These latter were used for the preparation of
cis-pulegol 9 by sequential oxidation, base-catalyzed
isomerization17 and LiAlH4 reduction.18 The latter reac-
tion afforded only the cis-isomer whereas the trans-iso-
mer was not formed because of its great instability.
Piperitol isomers 10 and 11 were prepared from racemic
limonene 16 by a multistep sequence. Regioselective
hydrogenation of the disubstituted double bond by
means of platinum catalysis followed by a photooxida-
tion reaction with oxygen and bengal rose as sensitizer
gave a mixture of hydroperoxides which were reduced
to the related alcohols.19 The latter mixture contained
mostly p-menth-2-en-1-ol that gave piperitone 17 by
oxidation with Jones’ reagent. Treatment of the crude
mixture with the oxidant followed to the chromato-
graphic separation of 17 and LiAlH4 reduction20 of the
latter gave the separable trans-piperitol 10 (51%) and
the cis-piperitol 11 (42%).
As part of a program of preparation of enantiopure
odorants, we have previously shown that enantiomer-
enriched p-menthan-3,9-diols are easily obtainable by
means of the lipase-mediated acetylation of racemic
materials.15 The latter approach seems to be rather
flexible and accordingly, we extended the acquired
enzymic methodology to the study of the resolution of
p-menthan-3-ols 1–13.
Although many processes for the enzyme-mediated
preparation of (−)-menthol 1 are described in the litera-
ture,7–14 a similar approach to the preparation of the
enantiopure compounds 5–13 is still lacking. In light of
these observations we report here the results of our
studies on the preparation of racemic compounds 5–13
and on their lipase-mediated resolution.
Although various syntheses of isopiperitenol are
described in the literature,3,21–23 few of them are highly
diastereoselective affording a mixture of cis- and trans-
isopiperitenol 12 and 13. Since we were interested in the
study of each single diastereoisomer we looked for a
more selective pathway. We found that the Diels–Alder
reaction of methyl-vinyl ketone 19 and 1-acetoxy-3-
methyl 1,3-butadiene 18 gave 6-(acetyl)-3-methylcyclo-
hex-2-en-1-yl acetate 20 with good diastereoselectivity
(90%) though a similar reaction employing 1-silyloxy-3-
methyl 1,3-butadiene16 gave inferior selectivity (75%).
The following Wittig methylenation of the ketone func-
tionality allowed the construction of the entire p-men-
thane framework. Subsequent KOH hydrolysis of the
acetate moiety afforded cis piperitol 12 without varia-
tion of the diastereoisomeric ratio. In order to obtain
diastereoselectively trans piperitol 13, we tried the iso-
merization reaction of compound 20, however under
both acid and base catalysis the substrate undergoes
elimination with the formation of dienic ketone.
2. Results and discussion
Our study first involved the preparation of the racemic
starting materials. Racemic menthol 1 is commercially
available while the syntheses of racemic 5–12 were
performed by slight modification of some reported pro-
Each of the obtained p-menthan-3-ol was treated with
t
vinyl acetate in BuOMe solution in the presence of
lipases. The reactivity of the substrates towards the
irreversible acetylation reaction was tested by monitor-
ing at regular time intervals the distribution of chemical
species (GC analysis) and by isolation and full charac-
terization of the products. The results of this study,
seen together (Table 1 and Scheme 2), lead to some
interesting conclusions.
Porcine pancreatic lipase does not catalyse the acetyla-
tion reaction for any of the substrates used and only a
trace of acetates was detected in few cases 1 and 10.
Both candida rugosa lipase and lipase PS are good
catalysts for the same reaction although the latter
enzyme seems to be superior in terms of efficiency and
enantioselectivity.
Scheme 1. Reagents and conditions: (i) SnCl4 cat., CH2Cl2; (ii)
CrO3/H2SO4, acetone; (iii) NaOH cat., EtOH; (iv) LiAlH4,
Et2O; (v) H2, PtO2, EtOH; (vi) O2, MeOH, rose bengal, light;
(vii) Na2SO3 aq.; (viii) benzene, reflux; (ix) Ph3PCH2, THF;
(x) KOH, MeOH.

S. Serra et al. / Tetrahedron: Asymmetry 14 (2003) 3313–3319
3315
Table 1. Results of the enzyme-mediated acetylation of p-menthane-3-ol ( )-1, ( )-5–8, ( )-9, ( )-10, ( )-11 and ( )-12
Enzyme
Time (days)
Configuration of the
Ee of
Ee of acetylated Enantiomer
Conversion
reacting enantiomer
recovered
alcohol (%)
product (%)
ratio
b
b
b
b
( )-1
PPL
CRL
PS
PPL
CRL
PS
PPL
CRL
PS
PPL
CRL
PS
PPL
CRL
PS
PPL
CRL
PS
21
14
14
80
60
50
30
30
30
21
21
10
21
21
14
21
28
8
0.01
0.390
1R,3R,4S
60
94
60
1R,3R,4S
78
97
156
0.446
a
a
a
a
a
( )-5–8
( )-9
b
b
a
b
b
a
b
b
b
b
b
a
b
b
1R,3R,4S
95
1R,3R,4S
98
a
a
b
b
b
b
0
8
1R,3R
b
b
( )-10
( )-11
( )-12
0.01
0.396
3S,4S
59
90
35
3S,4S
81
99
501
0.450
a
a
a
a
a
3S,4R
71
90
40
0.441
3S,4R
91
99
638
0.479
a
a
a
a
a
3R,4R
3R,4R
55
92
89
99
298
662
0.382
0.482
a The substrate was not acetylated.
b Data not determined.
Racemic menthol 1 was converted by CRL and lipase
PS into the (−)-menthyl acetate 21 with good enantiose-
lectivity though the e.e. of the recovered alcohol was
moderate. Hydrolysis of 21 gave (−)-menthol 1 in good
accord with the processes described in the patent
literature.
tiomeric purity. The same reactions performed with
CRL gave analogous results although with inferior
selectivity. Moreover, since natural (−)-(R)-piperitone is
available in moderate e.e. (60–90%) by extraction from
eucalyptus dives, the piperitols resulting from its reduc-
tion are not good chiral building blocks for asymmetric
synthesis. Taking advantage of the above-described
selectivity, we designed a method for the purification of
these easy available materials. Separate treatment of
natural (+)-10 and (−)-11 with lipase PS in our reaction
conditions (see experimental) afforded enatiopure (+)-
10 and (−)-25, respectively. The saponification of this
latter gave enantiopure (−)-11.
The mixture of the isopulegol isomers 5–8 was acetyl-
ated very slowly and both CRL and lipase PS afforded
only the (−)-isopulegol acetate 22 with good to excellent
enantioselectivity and complete diastereoselectivity.
Hydrolysis of the latter acetate gave (−)-isopulegol with
high enantiomeric purity. This fact is noteworthy since
(−)-5 is a relevant product for industrial processes, fine
chemical production or for its cooling activity. The
industrial preparation of (−)-5 is correlated to the syn-
thesis of enantiopure citronellal which is manufactured
mainly by the Takasago Co. in the process of asymmet-
ric isomerization of geranyl and neryldiethylamine.
Since this latter reaction is performed by the propri-
etary technology based on the chiral Rh(I)-BINAP
catalyst, the preparation of enantiopure isopulegol
remains restricted to the owner of this process. Other-
wise, by our resolution method, (−)-5 is obtained
straightforwardly from commercially available racemic
citronellal without troublesome chemical manipulation.
Relating to the resolution of the dienic alcohol 12, we
obtained similar results. Racemic isopiperitenol 12 was
converted by CRL and lipase PS to the acetate (−)-26
with good to excellent enantioselectivity (89 and 99%
e.e., respectively) whereas recovered alcohol (+)-12
showed moderate to good enantioenrichment by CRL
(55% e.e.) and lipase PS (92% e.e.) respectively.
Although the starting material ( )-12 was not a single
diastereoisomer (90% of cis derivative) the isolated
products showed comparable d.e.s confirming that both
lipases are not diastereoselective. Moreover, in order to
assign the absolute stereochemistry of (+)- and (−)-cis
isopiperitenol, we converted (+)-12 in the (+)-isopiper-
itenone 27 by MnO2 oxidation. Since the latter ketone
is known24 to have the (S)-configuration (+)-cis
isopiperitenol and (−)-cis isopiperitenol have (3S,4S)
and (3R,4R) configurations. On the other hand, the
oxidation of (+)-12 (92% e.e.) afforded (+)-27 with
significantly lower e.e. (70%). Judging from these exper-
imental data, it is clear that (3R,4S)-trans isopiperitenol
13 was acetylated by CRL and lipase PS similar to
piperitenol 10 and 11.
Concerning the acetylation of the isomeric monounsat-
urated alcohols 9, 10 and 11 the behaviour was again
different. Racemic 9 was slowly converted in its acetate
but with concomitant isomerisation to tertiary alcohol
23 and its acetate. Isolation gave only alcohol 23 with
very low enantiomeric enrichment (8% e.e. by PS lipase
and 0% e.e. by CRL). Otherwise, trans-piperitol 10 and
cis-piperitol 11 were converted by lipase PS in enan-
tiopure acetate (−)-24 and (−)-25, respectively whereas
the unreacted (+)-10 and (+)-11 show good enan-

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S. Serra et al. / Tetrahedron: Asymmetry 14 (2003) 3313–3319
from the C(3) configuration. The latter selectivity
3. Conclusions
allowed us to prepare the enantiomeric form of trans-
and cis-piperitol either by resolution of the correspond-
ing racemic materials and from the natural enantioen-
riched material by enzyme-based purification. At last,
the dienic cis-isopiperitenol 12 was resolved and the
absolute stereochemistry of the enantiomers assigned by
chemical correlation.
The preparation and the study of the enzyme-mediated
resolution of racemic p-menthan-3-ols 1–13 are
described. Some general results have been achieved.
Porcine pancreatic lipase does not catalyse the acetyla-
tion reaction whereas CRL and lipase PS afforded the
acetates in good and excellent enantioselectivity respec-
tively with the single exception of cis-pulegol 9. The
latter reaction is also diastereoselective for the p-men-
than-3-ol without a double bond in the cyclohexanic
rings 1–4 and 5–8 as confirmed from the patent litera-
ture 1–4 and from the present work 5–8. In the latter
case the enzymic process performed on the racemic
mixture of the eight-isopulegol isomers afforded exclu-
sively the relevant (−)-isopulegol 5 in enantiopure form.
Otherwise the alcohol with one unsaturation on C(1)
10–13, are acetylated without diastereoselectivity and
with an enantiodiscrimination depending exclusively
4. Experimental
4.1. General methods
1H NMR spectra were recorded in CDCl3 solution at
room temperature unless otherwise stated, on a Bruker
AC-250 spectrometer (250 MHz 1H). The chemical-shift
is based on internal tetramethylsilane. IR spectra were
recorded on a Perkin–Elmer 2000 FTIR spectrometer.
Mass spectra were measured on a Finnigan-MAT TSQ
70 spectrometer. Melting points were measured on a
Reichert melting-point apparatus, equipped with a
Reichert microscope, and are uncorrected. Optical rota-
tions were determined on a Propol automatic digital
polarimeter. TLC analyses were performed on Merck
Kieselgel 60 F254 plates. All the chromatographic sepa-
rations were carried out on silica gel columns. Lipase
PS from Pseudomonas cepacia (Amano Pharmaceuticals
Co., Japan), Candida rugosa lipase (CRL; Sigma, type
VII), porcine pancreatic lipase (PPL, Sigma, type II)
were employed. GC: DANI-HT-86.10 gas chro-
matograph; enantiomer and diastereoisomer excesses
determined on a Chirasil DEX-CB column (25 m×0.25
mm; Chrompack) with the following temp. program:
Analysis of menthol acetate: 30°–3°/min −180°; tR
21.72, 22.93. Analysis of the isopulegol acetate: 30°–
0.5°/min −90° −20°/min–180°; tR 73.87, 75.97. Analysis
of the piperitol acetate and isopiperitenol acetate: 50°(3
min)–0.5°/min −70°–20°/min–180°; tR (trans piperitol
acetate) 45.10, 45.39; tR (cis piperitol acetate) 44.86,
45.20; tR (cis isopiperitenol acetate) 45.22, 45.79.
4.2. Synthesis of the racemic substrates 5–13
4.2.1. Isopulegol isomers 5–8. To an ice-cooled and
stirred solution of citronellal 14 (60 g, 389 mmol) in dry
CH2Cl2 (500 mL) is added dropwise a solution of SnCl4
(2 mL, 17 mmol) in CH2Cl2 (20 mL) maintaining the
temperature below 5°C. After the addition is complete,
stirring is continued for 1 h and then a solution of
saturated NaHCO3 (200 mL) is added. The organic
solvent was evaporated and the residue was steam
distilled. The distillate was extracted with ether (3×200
mL), and the organic phase was washed with brine,
dried (Na2SO4) and concentrated in vacuo. The residue
was distilled to give a mixture of the isopulegol isomers
5–8 (50 g, 83%), sum of isomers (by GC analysis) 96%;
isopulegol/neoisopulegol/isoisopulegol/isoneoisopulegol
ratio: 66/30/3/1.
4.2.2. cis-Pulegol 9. Jones reagent (2.7 M, 60 mL) was
added dropwise to a stirred solution of the mixture of
the isopulegol isomers 5–8 (25 g, 162 mmol) in acetone
Scheme 2. Reagents and conditions: (i) Lipase, vinyl acetate
t-butylmethylether; (ii) KOH, MeOH; (iii) MnO2, CH2Cl2.

S. Serra et al. / Tetrahedron: Asymmetry 14 (2003) 3313–3319
3317
(100 mL) at 0°C. The reaction was stirred at the same
temperature until all starting material was consumed
and then poured in a mixture of water (100 mL) and
crushed ice (100 g). The resulting mixture was extracted
with diethyl ether (3×100 mL) and the organic phase
was washed with water (100 mL) and brine. The solvent
was eliminated by concentration at reduced pressure
and the residue was dissolved in ethanol (100 ml) and
treated with catalytic NaOH (1 g). The solution was
heated at reflux for 1 h, then the ethanol was eliminated
in vacuo and the residue partitioned between diethyl
ether and water. The organic phase was washed with
5% aq. HCl (50 mL) and brine and then concentrate at
reduced pressure. The distillation of the residue gave
pure pulegone 15 (19.8 g, 80%). A solution of the
obtained pulegone (15 g, 99 mmol) in dry diethyl ether
(50 mL), was added dropwise under N2 to a stirred
suspension of LiAlH4 (3.76 g, 99 mmol) in dry diethyl
ether (150 mL) at 0°C. When the addition was complete
the mixture was stirred at rt until no more starting
ketone was detected by TLC analysis. The reaction was
quenched by dropwise addition of water (10 mL) and a
10% aqueous solution of NaOH (20 mL). The ether
layer was separated, was washed with brine, was dried
(Na2SO4) and the solvent evaporated in vacuo. Distilla-
tion of the residue (bp 68–70°/0.8 mmHg) afforded cis
pulegol 9 (12 g, 79%).
by chromatography eluting with hexane–diethyl ether
(95:5“80:20) to give pure cis-piperitol 11 (6.4 g, 42%),
1H NMR, l, ppm: 5.63 (1H, dm, J=5.5 Hz, H-C(2)),
4.13 (1H, bt, J=3.5 Hz, H-C(3)), 2.02–1.91 (2H, m),
1.80–1.55 (2H, m), 1.69 (3H, s, Me(7)), 1.45–1.10 (2H,
m), 1.00 and 0.96 (6H, d+d, J=6.6 Hz, Me(9) and
Me(10)); m/z (EI): 154 (M+, 4), 139 (34), 121 (3), 112
(9), 111 (10), 93 (16), 91 (12), 84 (100), 83 (42), 77 (14),
69 (10), 55 (20), 41 (33); FT-IR (film) 3380, 2956, 1674,
1474, 1449, 1429, 1384, 1047, 1023, 957, 902, 848, 803
1
cm−1 and trans-piperitol 10 (10.4 g, 51%), H NMR, l,
ppm: 5.39 (1H, s, H-C(2)), 4.01 (1H, bs, H-C(3)),
2.12–1.88 (3H, m), 1.76–1.60 (1H, m), 1.68 (3H, s,
Me(7), 1.57 (1H, d, J=7 Hz, OH), 1.40–1.17 (2H, m),
0.97 and 0.84 (6H, d+d, J=6.9 Hz, Me(9) and Me(10));
m/z (EI): 154 (M+, 6), 139 (34), 121 (3), 112 (8), 111
(12), 93 (17), 91 (13), 84 (100), 83 (43), 77 (14), 69 (11),
55 (24), 41 (36); FT-IR (film) 3325, 2959, 1676, 1467,
1385, 1159, 1049, 1027, 986, 899 cm−1.
4.2.4. Isopiperitenol isomers 12 and 13. A solution of
methyl vinyl ketone 19 (30 mL, 360 mmol) and 1-ace-
toxy-3-methyl 1,3-butadiene 1825 (40 g, 317 mmol) in
benzene (200 mL) was heated at reflux under nitrogen
for 6 h. After cooling at rt the solvent was removed
under reduced pressure and the residue was purified by
chromatography eluting with hexane–ethyl acetate (9:1)
to give pure 6-(acetyl)-3-methylcyclohex-2-en-1-yl ace-
tate 20 (53.2 g, 85%) as a diastereoisomer mixture
4.2.3. Piperitol isomers 10 and 11. Racemic limonene 16
(dipentene, 137 g, 1 mol) in absolute ethanol (500 mL)
was hydrogenated at atmospheric pressure and rt using
PtO2 as catalyst. After 1 mol of hydrogen was
absorbed, the catalyst was removed by filtration and
the solution was diluted with additional ethanol (500
mL), treated with bengal rose (1 g) and irradiated at rt
in a Rayonet photochemical apparatus (120 W high-
pressure Hg lamp) with continuous purging of dry
oxygen. When the photooxidation was complete (10
days) the resulting mixture of peroxides was added
dropwise to a cooled (0°C) and stirred solution of
Na2SO3 (250 g) in water (1 L). The reaction was stirred
at rt overnight and then extracted with ether (3×200
mL). The organic phase was washed with brine, con-
centrated in vacuo and the residue was dissolved in
acetone (300 mL). The obtained solution was cooled to
0°C and treated under stirring with 2.7 M Jones reagent
(250 mL). After stirring for 2 h, the reaction was
diluted with water (600 mL), extracted with ether (3×
200 mL) and the organic phase was concentrated under
reduced pressure. The residue was purified by chro-
matography (hexane–diethyl ether, 95/5) and distilla-
tion (bp 70°C/0.5 mmHg) to give pure piperitone 17 (41
g, 27%).
1
(cis/trans 9:1 by GC), H NMR (cis isomer), l, ppm:
5.64 (2H, m, H-C(1) and H-C(2)), 2.60 (1H, dt, J=
11.3, 3.6 Hz, H-C(6)), 2.30–1.72 (4H, m, H-C(4) and
H-C(5)), 2.18 (3H, s, OAc), 1.99 (3H, s, acetyl), 1.74
(3H, Me-C(3)); m/z (EI): 153 (45), 136 (22), 121 (42),
111 (26), 93 (100), 79 (35), 43 (92); FT-IR (film) 2939,
1732, 1721, 1432, 1372, 1239, 1158, 1065, 1018, 956
cm−1. A sample of the obtained 20 (22 g, 112 mmol),
dissolved in dry THF (60 mL), was added to 1 M
(triphenylphosphonio)methanide (140 mmol) previously
prepared by reaction of (Ph3PMe)I (58.5 g, 145 mmol)
in THF (200 mL) with 10 M BuLi in hexane (14.5 mL).
The resulting mixture was heated under reflux for 2 h,
cooled to rt and then poured into cool (0°C) H2O (300
mL). The quenched mixture was extracted with diethyl
ether (2×200 mL), and the organic phase was succes-
sively washed with sat. NH4Cl solution and brine, dried
(Na2SO4) and concentrated in vacuo. The residue was
dissolved in hexane/Et2O 3:1, and the triphenylphos-
phine oxide was eliminated by crystallization (ice-bath
cooling). The liquid phase was evaporated and treated
with methanolic KOH (70 mL of 20% solution) stirring
at rt until no more starting acetate was detected by
TLC analysis. The mixture was diluted with water (200
mL) and extracted with diethyl ether (3×100 mL). The
organic phase was washed with brine, dried (Na2SO4)
and concentrated in vacuo. The residue was purified by
chromatography and bulb-to-bulb distillation to give
pure isopiperitenol (11.8 g, 70%) as a diastereoisomer
A solution of the obtained ketone 17 (20 g, 132 mmol)
in dry diethyl ether (60 mL), was added dropwise under
N2 to a stirred suspension of LiAlH4 (5 g, 132 mmol) in
dry diethyl ether (200 mL) at 0°C. When the reduction
was complete (2 h), the reaction was quenched by
dropwise addition of water (15 mL) and a 10% aqueous
solution of NaOH (25 mL). The ether layer was sepa-
rated, washed with brine, dried (Na2SO4) and the sol-
vent was evaporated in vacuo. The residue was purified
1
mixture of 12 and 13 (cis/trans 9:1 by GC), H NMR
(cis isomer), l, ppm: 5.68 (1H, dq, J=5, 1.5 Hz,
H-C(2)), 5.00 (1H, s, H-C(9)), 4.81 (1H, s, H-C(9)), 4.13
(1H, bt, J=4 Hz, H-C(3)), 2.18–1.98 (3H, m), 1.89–1.67
(2H, m), 1.83 (3H, s, Me(10)), 1.72 (3H, s, Me(7)),

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S. Serra et al. / Tetrahedron: Asymmetry 14 (2003) 3313–3319
1.62–1.49 (1H, m, OH); m/z (EI): 152 (M+, 5), 134 (7),
121 (12), 119 (9), 109 (14), 91 (13), 84 (100), 83 (54), 69
(12), 56 (12), 41 (15); FT-IR (film) 3422, 2930, 1672,
1648, 1449, 1376, 1245, 1154, 1068, 957, 885, 815 cm−1
acetate, the isomerized alcohol 23 and its acetate. All
the attempt to the separation of the mixture by chro-
matography or distillation gave alcohol 23 (98% GC),
1H NMR, l, ppm: 5.70 (1H, m, H-C(3)), 2.13–1.92 (3H,
m), 1.81–1.48 (4H, m), 1.32–1.08 (1H, m), 1.31 and 1.30
(6H, s+s, Me(9) and Me(10), 0.95 (3H, d, J=6.1 Hz,
Me(7)); m/z (EI): 154 (M+, 13), 139 (100), 121 (29), 107
(7), 95 (15), 81 (22), 67 (6), 59 (18), 43 (44); FT-IR
(film) 3358, 2924, 1457, 1373, 1153, 949, 899, 843; and
a inseparable mixture of its acetate and some elimina-
tion side products. When lipase PS was used as catalyst,
the obtained alcohol 23 show [h]2D0=−7.2 (c 2, CHCl3),
lit.27 [h]D20=+92.9 (CHCl3), e.e.=8%, while CRL gave
racemic 23.
4.3. Lipase-mediated resolution of the racemic
substrates
4.3.1. General procedure. A mixture of the suitable
racemic substrate (5 g), lipase (3 g), vinyl acetate (5 mL)
t
and BuOMe (30 mL) was stirred at rt and the forma-
tion of the acetate monitored by TLC analysis. The
reaction was stopped at about 50% of conversion by
filtration of the enzyme and evaporation of the solvent
at reduced pressure. The residue was purified by chro-
matography eluting with hexane–diethyl ether (95:5“
80:20) to give alcohol acetate and unreacted alcohol.
The acetate was then dissolved in methanol (10 mL)
and treated with KOH (2 g) in methanol (15 mL)
stirring at rt until no more starting material was
detected by TLC analysis. The mixture was diluted with
water (70 mL) and extracted with diethyl ether (3×30
mL). The organic phase was washed with brine, dried
(Na2SO4) and concentrated in vacuo. The residue was
purified by chromatography and bulb-to-bulb distilla-
tion to give the enantiomeric form of the unreacted
alcohol.
4.3.5. Resolution of racemic trans-piperitol. Racemic 10
was resolved according to the general procedure. The
use of lipase PS as catalyst afforded (−)-trans-piperitol
acetate 24 (98% GC), [h]2D0=−144.6 (c 2, CHCl3), lit.28
[h]2D0=+119 (c 1.16, CHCl3), chiral GC (tR 45.10) e.e.=
1
99%, H NMR, l, ppm: 5.33 (1H, m, H-C(2)), 5.27
(1H, m, H-C(3)), 2.05 (3H, s, OAc), 2.02–1.92 (2H, m),
1.80–1.60 (2H, m), 1.69 (3H, MeC(7)), 1.56–1.31 (2H,
m), 0.95 and 0.84 (6H, d+d, J=6.9 Hz, Me(9) and
Me(10)); m/z (EI): 196 (M+, 1), 154 (12), 136 (32), 121
(100), 111 (7), 93 (62), 84 (68), 77 (22), 69 (14), 55 (13),
43 (57); FT-IR (film) 2960, 1732, 1438, 1370, 1240,
1159, 1020, 970, 904 cm−1 and (+)-trans-piperitol 10
(98% GC), [h]D20=+24 (c 2, EtOH), lit.20 [h]1D7=+28 (c 2,
EtOH), chiral GC (tR 45.39) e.e.=81%. The use of
CRL as catalyst gave (−)-trans-piperitol acetate (98%
GC), [h]2D0=−130 (c 2, CHCl3), chiral GC e.e.=90%,
and (+)-trans-piperitol (98% GC), [h]2D0=+17.5 (c 2,
EtOH); chiral GC e.e.=59%. Saponification of the
acetate samples gave pure (−)-trans-piperitol [h]2D0=
−30.4 (c 2, EtOH) and [h]2D0=−27.3 (c 2, EtOH),
respectively.
4.3.2. Resolution of racemic menthol 1. Racemic 1 was
resolved according to the general procedure. The use of
lipase PS as catalyst gave (−)-menthyl acetate 21 (99%
GC), [h]2D0=−80.5 (c 2, CHCl3), chiral GC (tR 21.72)
e.e.=97%, and (+)-menthol 1 (99% GC), [h]2D0=+36.9
(c 2, EtOH); chiral GC (tR 22.93) e.e.=78%. The use of
CRL as catalyst gave (−)-menthyl acetate (99% GC),
[h]2D0=−78.5 (c 2, CHCl3), chiral GC e.e.=94%, and
(+)-menthol (99% GC), [h]2D0=+29.5 (c 2, EtOH); chiral
GC e.e.=60%. Saponification of the acetate samples
gave pure (−)-menthol, [h]2D0=−48.9 (c 2, EtOH) and
[h]2D0=−47.1 (c 2, EtOH) respectively.
4.3.6. Resolution of racemic cis-piperitol. Racemic 11
was resolved according to the general procedure. The
use of lipase PS as catalyst gave (−)-cis-piperitol acetate
25 (98% GC), [h]2D0=−376 (c 2, CHCl3), lit.28 [h]D20=
−284 (c 1.11, CHCl3), chiral GC (tR 44.86) e.e. =99%,
1H NMR, l, ppm: 5.64 (1H, m, H-C(2)), 5.23 (1H, m,
H-C(3)), 2.12–1.87 (1H, m), 2.02 (3H, s, OAc), 1.86–
1.24 (4H, m), 1.70 (3H, s, Me(7)), 1.21–1.07 (1H, m),
0.95 and 0.89 (6H, d+d, J=6.7 Hz, Me(9) and Me(10));
m/z (EI): 196 (M+, 1), 154 (14), 136 (37), 121 (100), 111
(10), 93 (92), 84 (78), 77 (25), 69 (20), 55 (13), 43 (73);
FT-IR (film) 2960, 1736, 1675, 1445, 1370, 1243, 1017,
951, 901 cm−1 and (+)-cis-piperitol 11 (98% GC),
[h]D20=+184 (c 2, EtOH), lit.20 [h]2D0=−246 (c 2, EtOH),
chiral GC (tR 45.20) e.e.=91%. The use of CRL as
catalyst gave (−)-cis-piperitol acetate (98% GC), [h]2D0=
−339 (c 2, CHCl3), chiral GC e.e.=90%, and (+)-cis-
piperitol (98% GC), [h]2D0=+126 (c 2, CHCl3); chiral
GC e.e.=71%. Saponification of the acetate samples
gave pure (−)-cis-piperitol, mp 32°C, [h]2D0=−203 (c 2,
EtOH) and [h]2D0=−182 (c 2, EtOH), respectively.
4.3.3. Preparation of enantiopure (+)-isopulegol 5 from
racemic isopulegol isomers 5–8. A mixture of the four
racemic isopulegol isomers 5–8 (15 g, 97 mmol) was
used as substrate in the general procedure of resolution.
The use of lipase PS as catalyst gave (−)-isopulegol
acetate 22 (3.6 g, 18 mmol), (98% GC), [h]2D0=−17.9 (c
1, CHCl3), lit.26 [h]D20=+17.3 (c 2, CHCl3), chiral GC
(tR 73.87) e.e.=98% and a mixture of the unreacted
isopulegol isomers 5–8 (11.2 g, 72 mmol). Saponifica-
tion of the acetate gave pure (−)-isopulegol 5 (2.3 g, 15
mmol) (99% GC), [h]D20=−13.6 (c 1, CHCl3). The use of
candida rugosa lipase as catalyst gave (−)-isopulegol
acetate (2.9 g, 15 mmol), (97% GC), [h]2D0=−17.6 (c 1,
CHCl3), chiral GC e.e.=95% and a mixture of the
unreacted isopulegol isomers 5–8 (11.4 g, 74 mmol).
Saponification of the acetate gave pure (−)-isopulegol (2
g, 13 mmol) (99% GC), [h]2D0=−13.4 (c 1, CHCl3).
4.3.4. Enzymic acetylation of racemic cis pulegol.
Racemic 9 was acetylated according to the general
procedure. Either the use of lipase PS or CRL as
catalyst afforded a mixture consisting in unreacted 9, its
4.3.7. Preparation of enantiopure (+)-trans- and (−)-cis-
piperitol from natural (−)-piperitone. A sample of natu-
ral (−)-piperitone 17 (from eucalyptus dives) 59% e.e.

S. Serra et al. / Tetrahedron: Asymmetry 14 (2003) 3313–3319
3319
{[h]2D0=−11 (c 3.5, CHCl3), lit.29 [h]2D0=+18.6 (c 3.45,
References
CHCl3), was reduced as described above to give (−)-cis-
piperitol 11 {[h]2D0=−107.5 (c 2, EtOH)} and (+)-trans-
piperitol 10 {[h]2D0=+19.1 (c 2, EtOH)}. The two alcohols
were submitted to the general procedure of acetylation
using lipase PS as catalyst and the reaction was stopped
at about 70 and 30% of conversion, respectively. The two
mixtures were purified by chromatography to give enan-
tiopure (+)-trans piperitol {[h]2D0=+30.5 (c 2, EtOH) 98%
e.e.} and (−)-cis-piperitol acetate {[h]2D0=−378.2 (c 2,
CHCl3) 99% e.e.}. The latter acetate give after saponifi-
cation (−)-cis-piperitol {[h]2D0=−206 (c 2, EtOH)}.
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4.3.8. Resolution of racemic cis-isopiperitenol. A mixture
of racemic 12 and 13 (cis/trans 9:1) was used as substrate
in the general procedure of resolution. The use of lipase
PS as catalyst gave (−)-cis-isopiperitenol acetate 26 (98%
GC), [h]2D0=−378 (c 2, CHCl3), chiral GC (tR 45.22)
e.e.=99%, d.e. 80%, 1H NMR (cis-isomer), l, ppm: 5.60
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(27), 134 (84), 126 (13), 119 (45), 105 (18), 91 (35), 84
(100), 77 (16), 43 (26); FT-IR (film) 2935, 1732, 1674,
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(+)-cis-isopiperitenol 12 (98% GC), [h]2D0=+213 (c 2,
CHCl3), chiral GC (tR 45.79) e.e.=92%, d.e. 80%. The
use of CRL as catalyst gave (−)-cis-isopiperitenol acetate
(98% GC), [h]2D0=−297 (c 2, CHCl3), chiral GC e.e.=
89%, d.e. 86% and (+)-isopiperitenol (98% GC), [h]2D0=
+79 (c 2, CHCl3); chiral GC e.e.=55%, d.e. 76%.
Saponification of the acetate samples gave pure (−)-cis-
isopiperitenol [h]D20=−228 (c 2, EtOH) and [h]2D0=−210
(c 2, EtOH), respectively.
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CHCl3), e.e.=92%, d.e. 80%} in CH2Cl2 (30 mL) was
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bulb distillation (oven temp. 80–85°/0.6 mmHg) to afford
pure (98% GC) (S)-(+)-isopiperitenone 27 (1.3 g, 8.7
mmol, 87%, [h]D20=+30.1 (c 2, CHCl3)), lit.24 [h]D20=+49
(c 0.96, CHCl3); 1H NMR, l, ppm: 5.90 (1H, bq, J=1.5
Hz, H-C(2)), 4.95 (1H, m, H-C(9)), 4.76 (1H, m, H-C(9)),
2.95 (1H, dd, J=5.3, 10.3 Hz, H-C(4)), 2.40–2.30 (2H,
m), 2.20–1.99 (2H, m), 1.96 (3H, s, Me(7)), 1.75 (3H, bq,
J=0.7 Hz, Me(10)); m/z (EI): 150 (M+, 19), 135 (28), 122
(6), 117 (1), 107 (7), 91 (4), 82 (100), 67 (4), 54 (13); FT-IR
(film) 3075, 2936, 1671, 1648, 1438, 1380, 1319, 1201,
1025, 891 cm−1.
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Acknowledgements
The financial support of COFIN-MURST is
acknowledged.