A convenient resolution of racemic lavandulol through lipase-catalyzed acylation with succinic anhydride: Simple preparation of enantiomerically pure (R)-lavandulol

Article abstract of DOI:10.1016/j.tetasy.2005.12.021

(R)- and (S)-lavandulol and their esters are important compounds in perfumery and have recently become significant in pheromone research. (R)-Lavandulol and its esters, as well as the esters of (S)-lavandulol have been identified as sex and aggregation pheromones in two mealybugs, in thrips and in weevils. We report a convenient resolution of racemic lavandulol through lipase-catalyzed acylation with succinic anhydride. This method does not require tedious chromatographic separation and is particularly suitable for the preparation of enantiomerically pure (R)-lavandulol with 98% ee in one resolution cycle. The (S)-lavandulol with 90% ee can be obtained by a second resolution cycle.

Full text of DOI:10.1016/j.tetasy.2005.12.021

Tetrahedron:
Asymmetry
Tetrahedron: Asymmetry 17 (2006) 230–233
A convenient resolution of racemic lavandulol through
lipase-catalyzed acylation with succinic anhydride:
simple preparation of enantiomerically pure (R)-lavandulol
Anat Zada* and Ezra Dunkelblum
Department of Entomology, Institute of Plant Protection, The Volcani Center, ARO, Bet Dagan 50250, Israel
Received 9 November 2005; accepted 21 December 2005
Abstract—(R)- and (S)-lavandulol and their esters are important compounds in perfumery and have recently become significant in
pheromone research. (R)-Lavandulol and its esters, as well as the esters of (S)-lavandulol have been identified as sex and aggregation
pheromones in two mealybugs, in thrips and in weevils. We report a convenient resolution of racemic lavandulol through lipase-
catalyzed acylation with succinic anhydride. This method does not require tedious chromatographic separation and is particularly
suitable for the preparation of enantiomerically pure (R)-lavandulol with 98% ee in one resolution cycle. The (S)-lavandulol with
90% ee can be obtained by a second resolution cycle.
ꢀ 2006 Elsevier Ltd. All rights reserved.
1. Introduction
acetate. Due to our continuing interest in the lavandulol
enantiomers and the recent discovery of pheromones
comprising of (R)-lavandulol and its esters in different
insects, we herein report a simplified enzymatic resolution
of ( )-lavandulol, which avoids chromatography
through the use of succinic anhydride as the acylating
reagent, particularly for preparation of the (R)-enantio-
mer. In addition, easy access to the two lavandulol
enantiomers is also of importance in perfumery because
they may display different pharmacological activities
and certainly different fragrances7 and odor thresholds.
Lavandulol and its simple esters, mainly the acetate, are
minor but important constituents of essential oils, par-
ticularly that of lavender. They are common ingredients
in the cosmetic industry. Recently, (R)-lavandulol and
the different esters of this enantiomer have been identi-
fied as sex or aggregation pheromone components in
several insects. (R)-Lavandulyl (S)-methylbutanoate is
a component of the female sex pheromone of the hibis-
cus mealybug,1 (R)-lavandulyl acetate is a component of
the male produced sex pheromone of the western flower
thrips2 and (R)-lavandulol is a component of the aggre-
gation pheromone of the strawberry blossom weevil.3
The (S)-enantiomer, as the senecioate and isovalerate
esters, has been identified in the female sex pheromone
of the vine mealybug.4,5
2. Results and discussion
The enzymatic transesterification of alcohols is a wide-
spread method for the separation of racemic alcohols.
The acylating agents may vary, but enol-esters are usu-
ally the first choice. Several years ago, succinic anhy-
dride was introduced as an acylating agent,8,9 but
since then has not been used extensively. Thus, acylation
converts one enantiomer of the alcohol into the succinic
half acid, which can be separated from the unreacted
alcohol by extraction with aqueous sodium carbonate,
avoiding tedious chromatography.
We have recently developed a two stage enzymatic sepa-
ration of commercial ( )-lavandulol using vinyl acetate
as the acylating reagent to provide the two enantiomers
in good enantiomeric excess.6 The drawback of this
method is the need of lengthy column chromatography in
order to separate the unreacted alcohol from the formed
*
Application of this method to the resolution of ( )-lav-
Corresponding author. Tel.: +972 3 9683760; fax: +972 3 9683781;
e-mail: anatzada@volcani.agri.gov.il
andulol 1 gave directly, in one step, (R)-lavandulol 2
0957-4166/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2005.12.021

A. Zada, E. Dunkelblum / Tetrahedron: Asymmetry 17 (2006) 230–233
231
O
Lipase
R
S
+
+
O
Ether
1
(-)
CH2OC
CH2OH
2
CH2OH
4
O
CO2H
O
Racemic Lavandulol
1) Separation with
1 M Na2CO3
S
CH2N2
2) Basic hydrolysis
with 1M NaOH
CH2OC
CO2CH3
O
(+)
S
5
CH2OH
(+)
3
Scheme 1. Enzymatic resolution of ( )-lavandulol with succinic anhydride mediated by Hog pancreas lipase in ether.
with an enantiomeric excess of 96–98%. The second
enantiomer, (S)-lavandulol 3 could be recovered after
hydrolysis of lavandulyl succinate half acid 4 with aque-
ous sodium hydroxide (Scheme 1). The enantiomeric
excess of this enantiomer was only 54–74%, depending
on the reaction time. Three solvents were tested, tert-
butylmethyl ether, diethyl ether, and hexane and two
similar lipases were used, Porcine pancreas from Sigma
or Hog pancreas from Fluka. These specific enzymes
were chosen on the basis of our previous screening of
different lipases for the resolution of ( )-lavandulol.6
The conversion rate using the Hog pancreas was higher
than that with the Porcine pancreas and the enantio-
selectivity in diethyl ether was higher than that in
tert-butylmethyl ether (Table 1). In a comparative
experiment using the two enzymes in ether, a conversion
rate of 50% was obtained after 24 h with the Hog
pancreas as compared with a 42% conversion rate after
53 h with the Porcine pancreas. The reaction in hexane
was extremely slow, probably due to the low solubility
of succinic anhydride in this solvent. Accordingly, all
other experiments were conducted in diethyl ether with
the Hog pancreas lipase.
hol 1. At 50% conversion, ratio A should be 1.65, which
is the molecular weight of 4 divided by the molecular
weight of 1. Reactions were terminated after 24–48 h,
when the ratio was about 2, in order to obtain the
(R)-enantiomer
2 in a high enantiomeric excess.
Attempts to obtain the (S)-enantiomer with an enantio-
meric excess of more than 74% in one step, in short reac-
tion times or by using an excess of ( )-lavandulol, did
not succeed (Table 1).
The main advantages of the presented method is the
convenient separation between the unreacted (R)-
enantiomer 2 and succinic half acid 4, without tedious
column chromatography, and the very high enantio-
meric excess (96–98%) of the resolved (R)-lavandulol.
(S)-Lavandulol with a moderate enantiomeric excess
was recycled by the same resolution method to yield
(S)-lavandulol with a high enantiomeric excess of 90%.
The chemical purity of the commercial ( )-lavandulol
was 95%; the residual 5% consisted of several additional
small peaks shown in the GC analysis. All the impurities
concentrated into the (R)-enantiomer, after the resolu-
tion, yielding this enantiomer in 90% chemical purity.
One sample of the (R)-enantiomer was purified by flash
chromatography providing 2 in a chemical purity of
98% and enantiomeric excess of 98%. All samples of
the S-enantiomer were chemically pure (ꢀ99%), due to
the isolation as sodium salt of half acid 4. Both purified
enantiomers displayed a very high specific rotation of R
Table 1. Hog pancreas lipase-catalyzed acylation of ( )-lavandulol
with succinic anhydride in organic solvents
Solvent
Time
(h)
Conversion
(%)
ee (%)a
of (R)
ee (%)a
of (S)
27
27
with ½aꢁD ¼ ꢂ11:1 and S with ½aꢁD ¼ þ10:7.
tert-Butylmethyl
ether
24
48
74.0
54.0
Diethyl ether
Diethyl ether
Diethyl ether
5
24
48
25
55
85
64.0
95.0
96.4
74.0
59.0
54.0
3. Conclusion
a As determined by GC on a chiral capillary column.
The enzymatic transesterification of ( )-lavandulol with
succinic anhydride as the acylating agent mediated by
Hog pancreas lipase provides an efficient and convenient
method for the resolution of the racemate, and particu-
larly for the preparation of enantiomerically pure (R)-
lavandulol. The (R)-enantiomer was obtained in one
step with an enantiomeric excess purity of 96–98%.
The remaining (S)-enantiomer only had a moderate
enantiomeric excess of 75–88%, however, it could be
submitted to a second cycle of resolution to provide this
enantiomer in 90% enantiomeric excess.
The resolution of 1 was monitored via the formation of
succinic half acid 4 by GC on the conventional Rtx-
5SILMS column. Succinic half acid 4 was isolated in
one case and characterized as its methyl ester 5 by
NMR and MS analyses. The (S)-enantiomer 3 reacted
much faster than (R)-enantiomer 2. The progress of
the reaction was determined by measuring the ratio A
by GC of the formed half acid 4 to the remaining alco-

232
A. Zada, E. Dunkelblum / Tetrahedron: Asymmetry 17 (2006) 230–233
4. Experimental
by GC on an Rtx-5SILMS column in order to monitor
the progress of the transesterification. The Rt of ( )-lav-
andulol 1 was 6.26 min and of half acid 4 was 15.90 min.
The reaction was terminated after 48 h by filtration of
the enzyme and dilution with 20 ml ether. The ethereal
mixture was stirred with 1 M Na2CO3 solution (60 ml)
for 30 min. The basic aqueous layer was extracted with
additional 20 ml of ether. The combined organic frac-
tions were extracted with more 1 M Na2CO3 solution
(20 ml), dried over MgSO4 and the solvent removed to
give 0.68 g of the (R)-enantiomer 2 with an enantiomeric
excess of 98% and chemical purity of 90%. The com-
bined basic aqueous fractions were hydrolyzed by stir-
ring with 1 M NaOH solution (50 ml) for 5 h and then
the product was extracted with ether (3 · 60 ml), dried
over MgSO4 and the solvent removed to give 0.80 g of
the (S)-enantiomer 3 with an enantiomeric excess of
54% and chemical purity of 99%. The combined yield
of both enantiomers was 64%. The reaction was
repeated a number of times and the S-enantiomer 3 with
higher optical purity, up to 74%, could be obtained by
terminating the reaction after 5 h.
4.1. Chemicals and enzymes
( )-Lavandulol of 95% purity (Fluka), hexane (Merck),
anhydrous tert-butylmethyl ether (t-BuOMe) (Aldrich)
and succinic anhydride, 99% pure (Aldrich) were used
without further purification. Diethyl ether (Biolab) was
dried by reflux over sodium and benzophenone. Lipase
from Porcine pancreas (41 U/mg) and lipase from Hog
pancreas (23.9 U/mg) were purchased from Sigma and
Fluka, respectively. Diazomethane was prepared in ether
from N-methyl-N0-nitro-N-nitrosoguanidine and 40%
aqueous KOH according to Fieser and Fieser10 and
was used without distillation. Flash chromatography
˚
was carried out on Silica gel 70–230 mesh, 60 A (Aldrich)
with hexane and ether. (R)-Lavandulol, as standard,
isolated from lavandin oil was available from previous
work.5
4.2. Analytical methods
Analysis of chemical purity and monitoring of the
reactions were conducted on a Carlo Erba, Mega gas
chromatograph equipped with a flame ionization detec-
tor and equipped with a Rtx-5SILMS (30 m · 0.32 mm ·
0.5 lm df, Resteck, Bellefonte, PA, USA) column was
kept at 80 ꢁC for 2 min and then programmed to
190 ꢁC at a rate of 20 ꢁC/min. The chiral analysis was
performed on a HP6890 gas chromatograph equipped
with a flame ionization detector and equipped with a
cyclodextrin Rt-bDEXsm (30 m · 0.25 mm ID, 0.25 lm
df, Resteck, Bellefonte, PA, USA) column kept at
100 ꢁC for 5 min and then programmed to 180 ꢁC at a
rate of 5 ꢁC/min. Helium was used as a carrier gas and
the temperature of the injector and detector was kept
at 220 ꢁC on both machines. Analysis on the chiral col-
umn was conducted in the split mode at a ratio of 50:1
and the analysis on the Rtx-5 column was conducted in
the splitless mode, the purge valve was opened after
1 min. Optical rotations were determined in methanol
on a Jasco P-1010 polarimeter, at a wave length of
589 nm and temperature of 27 ꢁC, in a 1 ml cell of 0.5
decimeter length. Proton NMR was recorded on a Bruc-
ker 600 MHz spectrometer in CDCl3 with TMS as inter-
nal standard. The electron impact (EI) mass spectrum
was recorded on an Agilent 6890N GC–MS instrument
equipped with a HP5MS 30 m by 0.25 column. The
high-resolution mass spectrum for the determination of
the molecular weight of compounds 2, 3 and 5 were
recorded by electron spray ionization (ESI) on a
Waters-Micromass Q-TOF Premier Mass Spectrometer
(Milford, MA, USA). The CH analyses were carried
out in the elemental analysis laboratory of the Hebrew
University in Jerusalem.
4.4. Chemical purification of (R)-lavandulol
(R)-Lavandulol (200 mg), 90% pure (from Section 4.3)
was submitted to flash chromatography on silica gel
(15 cm · 1.5 cm). Elution with hexane + 10% ether gave
134 mg of (R)-lavandulol 98% chemically pure and
25 mg 85% pure. The pure (R)-lavandulol (98%)
27
(ee = 98%, by GC) displayed ½aꢁD ¼ ꢂ11:1 (c 2.28,
20
MeOH) (lit.11 ½aꢁD ¼ ꢂ10:7) whereas the 90% (R)-lav-
andulol displayed
a
lower specific rotation of
27
½aꢁD ¼ ꢂ9:4 (c 2.20, MeOH). The exact molecular weight
was determined by ESI as C10H19O (MH)+, found
155.1450 calculated 155.1436; in addition the ion of
C10H17 corresponding to (MHꢂH2O)+ was found to be
137.1336 calculated 137.1331. CH analysis: Found: C,
77.41; H, 11.93. Calcd for C10H18O: C, 77.92; H, 11.69%.
4.5. Second cycle of lipase-mediated transesterification
of partially resolved (S)-lavandulol
Reaction of partially resolved (S)-lavandulol (462 mg),
containing 80% S, with succinic anhydride (600 mg,
6 mmol) and lipase (462 mg, Fluka) in 15 ml was termi-
nated after 18 h and worked-up as described in Section
4.3. Hydrolysis of the corresponding half acid 4 gave
130 mg (S)-lavandulol, 99% pure with a ratio of 95% S
27
and 5% R (ee = 90%, by GC) and ½aꢁD ¼ þ10:7 (c 2.28,
20
MeOH) (lit.11 ½aꢁD ¼ þ10:1). The exact molecular weight
was determined by ESI as C10H19O (MH)+, found
155.1441 calculated 155.1436; in addition the ion of
C10H17 corresponding to (MHꢂH2O)+ was found to be
137.1321 calculated 137.1331. CH analysis: Found: C,
77.01; H, 11.83. Calcd for C10H18O: C, 77.92; H, 11.69%.
4.3. Lipase-mediated resolution of ( )-lavandulol
4.6. Lipase-mediated resolution of ( )-lavandulol using an
excess of lavandulol; isolation of half acid 4 as the methyl
ester 5
A mixture of ( )-lavandulol (2.31 g, 15 mmol), succinic
anhydride (3 g, 30 mmol) and lipase (1.115 g, Fluka,
1777 U/mmol of ( )-lavandulol) was stirred in 50 ml
ether at room temperature. Stirring was stopped period-
ically and aliquots of the supernatant solution analyzed
The reaction of a mixture of ( )-lavandulol (2.77 g,
18 mmol), succinic anhydride (0.6 g, 6 mmol) and

A. Zada, E. Dunkelblum / Tetrahedron: Asymmetry 17 (2006) 230–233
233
lipase (432 mg, Fluka) in 25 ml was terminated after
Acknowledgements
48 h and worked-up as described in Section 4.3.
The ethereal phase gave 1.7 g lavandulol. The aqueous
Na2CO3 fraction was cooled in an ice bath, stirred and
acidified with 1 M HCl. Half acid 4 was extracted with
2 · 30 ml ether, dried over MgSO4 to yield 1.03 g 4 as
oil. GC analysis indicated 98% of 4 with 2% of lavandu-
lol. A sample of 4 (0.85 g) was dissolved in 10 ml ether
and was methylated by a slow addition of ethereal diazo-
methane, until the yellow color persisted. The reaction
mixture was dried over MgSO4 and the solvent removed
to give 0.87 g of crude lavandulyl methyl succinate 5
containing a number of impurities, including traces of
4 and a few percent of lavandulol. Purification by
column chromatography on silica gel (18 cm · 1.5 cm)
and elution with hexane + 5% ether gave 0.56 g of pure
5 (98.5% by GC; Rt of 14.47 min) and additional 0.1 g
of material with 1% of lavandulol. 1H NMR
(600 MHz, CDCl3): d 5.06 (tt, 1H), 4.82 (dq, 1H), 4.73
(m, 1H), 4.07 (d, 2H), 3.69 (s, 3H), 2.62 (s, 4H), 2.39
(q, 1H), 2.15 (dt, 1H), 2.07 (dt, 1H), 1.69 (m, 6H),
1.60 (s, 3H). The exact molecular weight was determined
by ESI as C15H24O4Na(MNa)+, found 291.1570 calcu-
lated 291.1572; EI-MS: main ions at m/z (relative abun-
dance) were 268 (M+, trace), 136 (M+-methyl succinate,
29), 115 (71), 93 (100), 69 (86). CH analysis: Found: C,
67.05; H, 9.19. Calcd for C15H24O4: C, 67.16; H, 8.96%.
We would like to thank Hugo Gotlieb, Department of
Chemistry, Bar Ilan University, Ramat Gan for the
NMR measurement; Arie Tishbee, Weizman Institute
of Science, Rehovot for the ESI molecular analysis
and Neta Avny, Department of Chemistry, Tel Aviv
University for the optical rotation measurements.
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4.7. Hydrolysis of methyl ester 5 to (S)-lavandulol
Ester 5 (89 mg) was hydrolyzed with 1 M KOH/MeOH
(2 ml) for 4 h at room temperature. Dilution with 20 ml
of water and extraction with ether (2 · 20 ml) gave, after
drying and solvent removal, 44 mg of (S)-lavandulol
with 68% enantiomeric excess by GC.