Chemoenzymatic synthesis of (R)-(+)-2-methylbutan-1-ol, a chiral synthon for the preparation of optically active pheromones

Article abstract of DOI:10.1016/S0957-4166(97)00613-7

Optically active (R)-(+)-2-methylbutan-1-ol (which is not commercially available) was prepared by a chemoenzymatic synthesis, in which the key step involved a reduction catalyzed by baker's yeast. The synthon was used in the synthesis of (R)-10-methyldodecan-1-yl acetate, the chiral methyl-branched pheromone of Adoxophyes sp.

Full text of DOI:10.1016/S0957-4166(97)00613-7

Pergamon
Tetrahedron: Asymmetry 9 (1998) 89–96
Chemoenzymatic synthesis of (R)-(+)-2-methylbutan-1-ol, a
chiral synthon for the preparation of optically active pheromones
Shimona Geresh,a,∗ Thomas J. Valiyaveettil,a Yair Lavie b and Arnon Shani b
aThe Institute for Applied Research, Ben-Gurion University of the Negev, Beer-Sheva, Israel
bDepartment of Chemistry, Ben-Gurion University of the Negev, Beer-Sheva, Israel
Received 19 October 1997; accepted 26 November 1997
Abstract
Optically active (R)-(+)-2-methylbutan-1-ol (which is not commercially available) was prepared by a chemoen-
zymatic synthesis, in which the key step involved a reduction catalyzed by baker’s yeast. The synthon was used
in the synthesis of (R)-10-methyldodecan-1-yl acetate, the chiral methyl-branched pheromone of Adoxophyes sp.
© 1998 Elsevier Science Ltd. All rights reserved.
1. Introduction
Optically active starting materials are required in the synthesis of biologically active materials such as
chiral pheromones, compounds for which the stereochemistry generally dictates the behavioral response
of the insect. Until recently, natural products, such as amino acids, sugars or terpenoid structures, were
used as the starting materials in the synthesis of chiral pheromones. When the desired stereoisomer
was not available, alternative routes had to be developed. For example, (S)-(−)-2-methylbutan-1-ol was
used as the starting material in the synthesis of (S)-Z-(+)-14-methyl-8-hexadecen-1-ol (the intermediate
in the synthesis of the antipode of trogodermal).1,2 The (R)-Z- stereoisomer could not be prepared
by the same route, because natural (R)-(+)-2-methylbutan-1-ol was not available.3 However, another
route, in which (R)-(+)-citronellol was used as the starting material, was elaborated.4 Recently another
strategy was undertaken for the synthesis of the chiral synthon, (R)-(+)-2-methylbutan-1-ol: reduction
of the enantiomerically pure 2-alkylalkanoic acid obtained by chemical resolution of a mixture of
diastereomeric amides.5 This example is by no means unique: other cases are listed in the literature.6
Incorporation of biocatalysis into modern organic synthesis for the asymmetric synthesis and kinetic
resolution of chiral compounds is already widely practised.7–9 Optically active α-alkyl substituted
alcohols, such as 2-methylbutan-1-ol, can be used as synthons in the synthesis of a series of chiral
∗
Corresponding author.
0957-4166/98/$19.00 © 1998 Elsevier Science Ltd. All rights reserved.
PII: S0957-4166(97)00613-7

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methyl-branched pheromones (Scheme 1). In this study, (R)-2-methylbutan-1-ol was prepared by an
asymmetric synthesis comprising a combination of chemical methods and baker’s yeast reduction. The
synthon was then further used in the synthesis of (R)-10-methyldodecan-1-yl acetate — the component
of the pheromone of Adoxophyes sp.
Scheme 1.
2. Results and discussion
2.1. Preparation of the substrate
The preparation of the chiral synthon was performed by a chemoenzymatic synthesis based on
the well-documented baker’s yeast reduction of β-keto esters, especially those derived from alkyl
acetoacetates.10–13 Ethyl acetoacetate 1 was alkylated using sodium ethoxide as the base, followed by
reaction with methyl iodide.14 The purity of ethyl 2-methyl-3-oxobutyrate 2 (Scheme 2) was ascertained
by GC and NMR analyses.
2.2. Reduction with immobilized and non-immobilized baker’s yeast
Substrate 2 was reduced stereospecifically with baker’s yeast (Scheme 2) and results similar to those
published by other groups12,13 were obtained. A mixture of (2R,3S)- and (2S,3S)-diastereomers was
obtained, the major product being the (2R,3S)-diastereomer. In the biotransformation with baker’s yeast,
a number of parameters were varied with the aim of obtaining the best stereospecificity, the highest yield,
and the most efficient procedure for bio-reduction (Table 1).
From the results in Table 1 it is evident that the stereoselectivity of the asymmetric reduction is very
similar with both types of yeast — immobilized and non-immobilized. A significant reduction in the
amount of yeast used was achieved by immobilization. The main difference between the two techniques
was the ease of manipulation of the reaction mixture at the end of the reaction when immobilized baker’s

S. Geresh et al. / Tetrahedron: Asymmetry 9 (1998) 89–96
91
Scheme 2.
Table 1
Reduction of CH3COCH(CH3)COOCH2CH3 2 with baker’s yeast
yeast was used. The work-up was straightforward with the immobilized yeast: there was no need to use
aggregation agents or relatively large volumes of solvents to separate the yeast biomass from the reaction
products. When glucose was not added to the reaction mixture, only with non-immobilized yeast was an
asymmetric reduction observed.
1
The diastereomeric ratio of the products was determined by GC and H NMR analyses. Research
to improve the diastereomeric excess with various additives13,15 was not attempted. The (2R,3S)-
diastereomer was obtained by chromatography on a silica gel column.

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S. Geresh et al. / Tetrahedron: Asymmetry 9 (1998) 89–96
2.3. Conversion to the chiral synthon
Since the ratio of diastereomers was about 9:1 in favor of the desired diastereomer, tosylation or
mesylation was performed mainly with the diastereomeric mixture as previously reported,16,17 and the
products were purified by column chromatography. Due to possible diastereomer enrichment during
purification, the specific rotation of the tosylate, for example, was measured before {[α]D22=+9.87 (c
1.08, CHCl3)} and after crystallization {[α]D22=+10.60 (c 1.01, CHCl3)}. The reduction of the tosyl or
mesyl groups was not straightforward. The reagents conventionally used for this type of reduction, such
as LiAlH418 or NaI/Zn,19 produced not only the desired synthon 5 but also significant quantities of olefin
(Scheme 2). The only successful route involved the use of NaBH4 in DMSO20 at 85–90°C (Scheme 3),
but the yield was relatively low (27% after purification by silica gel column chromatography). The
specific rotation of the product (R)-2-methylbutan-1-ol obtained as described above was [α]D22=+5.89 (c
1.08, CHCl3) while that of the commercial (S)-enantiomer was {[α]D22=−6.67 (c 2.4, CHCl3} (optical
purity of about 88%). The e.e. of the alcohol was estimated by GC using a Chiraldex G-TA chiral column
at 31°C. The retention times of (S)- and (R)-2-methylbutan-1-ol were found to be 14.37 min and 14.86
min, respectively. The enantiomeric excess of (R)-2-methylbutan-1-ol was 82%.
Scheme 3.
2.4. Preparation of the pheromone
After obtaining the chiral synthon, (R)-2-methylbutan-1-ol 5, the pheromone of Adoxophyes sp. was
synthesized as previously described (Scheme 4).5 (R)-2-Methylbutan-1-ol 5 was converted into the
mesylate 6 and coupled with Li2CuCl4 as catalyst to a Grignard reagent prepared from the tetrahy-
dropyrane (THP) derivative 9 of 8-bromooctan-1-ol, 8. The latter compound was prepared separately by
bromination of 1,8-octane diol 7 with HBr. After removing the THP-protecting group from 10 with p-
TsOH, the resulting alcohol 10-methyldodecan-1-ol 11 was acetylated with acetic anhydride and pyridine
to give (R)-10-methyldodecan-1-yl acetate 12 (75% yield). The specific rotation was [α]D24=−4.80 (c
1.2, CHCl3); {lit.5 [α]D23=−5.84 (c 2.21, CHCl3)}.
In summary, the optically active pheromone of Adoxophyes sp. was prepared by a route based on
the building block (R)-(+)-2-methylbutan-1-ol, which was in turn synthesized by an immobilized yeast-
mediated reduction as the key-step in the asymmetric synthesis.
3. Experimental
3.1. General
Melting points are uncorrected and were determined in open-ended capillaries on a Capillary Melting
1
Point Apparatus. H NMR spectra were recorded on a Bruker 200 MHz instrument. The solvent in all
cases was deuterochloroform, with tetramethylsilane added as the internal standard. Chemical shifts are
given in δ, ppm. Optical rotation was measured on a Perkin–Elmer 141 polarimeter, either neat or as a

S. Geresh et al. / Tetrahedron: Asymmetry 9 (1998) 89–96
93
Scheme 4.
chloroform solution. Analytical GLC work was carried out on a Hewlett–Packard 417 gas chromatograph
with a FID detector, on a column of 5% DEGS/Chromosorb W (60–80 mesh), 4.5 m, i.d. 1/8 in., with
argon as the carrier gas. The enantiomeric excess was estimated on an HP 5890 gas chromatograph by
means of a Chiraldex G-TA chiral column (30 m×0.25 mm) with helium as the carrier gas. Fresh baker’s
yeast was purchased from a local market.
3.2. Ethyl 2-methyl acetoacetate 2
Ethyl acetoacetate 1 (48.75 g, 0.375 mol) was methylated with methyl iodide (63.84 g, 0.45 mol)
1
according to known procedures,14 yielding 2 (48% after distillation). H NMR: 1.25 (d+t, 6H), 2.25 (s,
3H), 3.5 (q, 1H), 4.22 (q, 2H).
3.3. Ethyl 3-hydroxy-2-methylbutyrate 3
3.3.1. Reduction with non-immobilized baker’s yeast
Into a 1 L round-bottomed flask was added 250 ml of water, 200 g of baker’s yeast, 1.44 g (10 mmol)
of 2 and glucose (30 g, in 5 g portions every 6 hours). After 48 h with stirring at room temperature, Celite
was added to the suspension which was further stirred for an additional hour. After centrifugation, the
aqueous solution was saturated with NaCl and extracted with ethyl acetate (4 times). The organic phase
was washed with water, dried with anhydrous MgSO4, filtered, and evaporated to yield 800 mg product
(54% yield). From the GC at 130°C, two peaks were obtained at 5.96 and 6.37 min, at a ratio of about
10:1. The optical rotation of this mixture was [α]D22=+8.8 (c 1.14, CHCl3). IR (neat) cm−1: 3400 (OH)
and 1740 (C_O). 1H NMR: 1.18 (2d, 6H), 1.27 (t, 3H), 2.54 (dq, 1H), 2.83 (b, 1H), 4.1 (m, 1H), 4.22
(dq, 2H).
3.3.2. Reduction with immobilized baker’s yeast in alginate
First, 10 g of baker’s yeast suspended in 40 ml of Tris buffer 0.05 M, pH 8, were mixed with 40 ml
of sodium alginate solution (5% w/v) and extruded into a 0.5 M CaCl2 solution. The beads were left
to stiffen for 16 h, decanted, washed with water and filtered off. The immobilized yeast was immersed
in a 500 ml round-bottomed flask together with 200 ml of water, 10 g of glucose and 750 mg of 2. An
additional 50 g of glucose were added in portions of 10 g every 6 hours. After 72 h, the mixture was

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S. Geresh et al. / Tetrahedron: Asymmetry 9 (1998) 89–96
filtered. The solution was saturated with NaCl and extracted as described above. The crude product (690
mg) was chromatographed on a silica gel column with a mixture of petroleum ether:ethyl acetate (4:1)
to yield 425 mg (60% yield). From the GC at 130°C, two peaks were obtained at 6.08 and 6.51 min, in
a ratio of about 9:1. The mixture was chromatographed again on a silica gel column with a mixture of
petroleum ether:ethyl acetate (6:1) to obtain an almost pure (2R,3S)-diastereomer (97.4% by GC).
3.3.3. Chemical reduction of 2
A sample of 2 was reduced with NaBH4 in MeOH to prepare a mixture of diastereomers 3 as a standard.
From the GC at 130°C, two peaks were obtained at 5.81 and 6.29 min, in a ratio of about 1:1.
3.4. Preparation of the tosylate 4
Tosylation of 3 was performed as described in the literature.15 Starting from 1.2 g (8.2 mmol) of
3, a crude product (2.86 g) was obtained with [α]D22=+9.87 (c 1.08, CHCl3). It was purified on a
column of silica gel using a mixture of petroleum ether:ethyl acetate (96:4). The yield was 69%. The
tosylate was recrystallized from chloroform:petroleum ether (m.p. 80–81°C). The specific rotation of 4
was [α]D22=+10.6 (c 1.01, CHCl3).
3.5. Preparation of the mesylate 4a
The mesylate of 3 was obtained by known procedures. A crude product (10.7 g) was obtained from
6.73 g (46.1 mmol) of 3. After purification with petroleum ether:ethyl acetate (9:1) on a column of silica
gel, 8.4 g of 4a was obtained (81% yield). 1H NMR: 1.28 (d+t, 6H), 1.47 (dd, 3H), 2.65 (m, 1H), 3.02 (s,
3H), 4.08 (q, 2H), 5.1 (m, 1H). The optical rotation of 4a was [α]D22=+19.96 (c 2.31, CHCl3).
3.6. Reaction of 4 with NaBH4
Into a triple-necked round-bottomed flask fitted with a thermometer and condenser was added 4 (1.2
g, 4 mmol), DMSO (8 ml), and NaBH4 (800 mg, 22 mmol). The mixture was maintained at 85–90°C
with magnetic stirring for 20 hours. The mixture was then cooled in an ice bath, and HCl (5%) was
added carefully up to pH 7.2 to dissolve the salts. The obtained solution was extracted with CH2Cl2
(×4). The organic phase was washed with a saturated solution of NaCl, and then with water, dried on
anhydrous MgSO4, filtered and evaporated to yield 470 mg of crude material. This material was applied
to a small column of silica gel and eluted with ether. The yield was 95 mg (27%). GC analysis at a column
temperature of 60°C revealed a peak at 2.44 min, which is the same as that of the injected racemic 2-
methylbutan-1-ol. Separation on a Chiraldex G-TA derivatized column at 31°C gave an enantiomeric
1
excess of 82%. H NMR: 0.87 (d+t, 6H), 1,15 (m, 2H), 1.45 (q, 1H), 3.45 (m, 2H), 3.65 (b, 1H). The
optical rotation of the product was [α]D22=+5.89 (c 1.08, CHCl3). The specific rotation for the (S)-
enantiomer was [α]D20=−5.87 (neat). The same reduction procedure of the mesylate 4a produced 81 mg
of end product (23% yield): [α]D23=+6.1 (c 1.4, CHCl3).
3.7. Preparation of the bromohydrin of 1,8-octane diol 8
Into a triple-necked flask connected to a Dean–Stark trap and condenser was placed toluene (160 ml),
1,8-octane diol 7 (11.68 g, 80 mmol) and 48% HBr (14.84 g, 87.9 mmol). After refluxing for 13 h and
measuring the volume of water that was collected, the reaction mixture was cooled to room temperature,

S. Geresh et al. / Tetrahedron: Asymmetry 9 (1998) 89–96
95
and washed with a saturated solution of NaHCO3, and then with a saturated solution of sodium chloride.
The organic phase was dried, filtered off, and evaporated. The residue was distilled (bp 104/0.3 mmHg).
The yield of 8 was 12.54 g (75%). IR (neat): 3400 cm−1 (OH); 1H NMR: 1.35–1.95 (m, 12H), 2.43 (b,
1H), 3.39 (t, 2H), 3.61 (t, 2H).
3.8. Preparation of THP of bromohydrin 9
The THP derivative of bromohydrin 8 was prepared according to known procedures. Starting from
3.14 g (15 mmol) of 8, a product of 4.57 g was obtained (100% yield). IR (neat): no hydroxyl present;
1H NMR 1.20–1.70 (m, 12H), 1.90 (m, 6H), 3.45 (t, 2H), 3.76 (m, 2H), 3.91 (m, 2H), 4.56 (t, 1H).
3.9. Preparation of 9–11
The Grignard reagent of 9 (0.59 g, 2 mmol) was prepared in 1.5 ml of THF with Mg (51 mg, 2.1 mmol).
This reagent was added dropwise into a precooled THF solution containing the mesylate 6 (obtained by
the usual procedures) and 0.1 M of Li2CuCl4. Stirring at 0°C was continued for another 3 h. Then the
mixture was left to stand at room temperature for another 17 h. Thereafter, HCl (5%) was added carefully
to bring the solution to pH 4–5. After separation of the phases, the aqueous phase was extracted with ether
(×4). The combined organic phase was washed first with a saturated bicarbonate solution and then with a
saturated NaCl solution, dried, filtered and evaporated to yield an oil, 10. This oil was mixed with 10 ml
of methanol, which contained a catalytic amount of para-toluene sulfonic acid, and the mixture was left
to stand at 40°C for 4 h. The solvent was evaporated off. The material obtained after the above-described
work-up was chromatographed on a silica gel column with petroleum ether:ethyl acetate (96:4) to yield
11 (45 mg, 22% yield).
3.10. Preparation of the acetate 12
Compound 11 (45 mg, 0.25 mmol) was mixed at room temperature with acetic anhydride (0.7 ml) and
pyridine (3 ml). The mixture was poured into cold water, and ether was used to extract compound 12.
The end product was chromatographed on a preparative silica gel plate with petroleum ether to yield 30
mg of product. 1H NMR: 0.85 (t, 6H), 1.00 (bm, 18H), 1.56 (m, 1H), 2.05 (s, 3H), 4.05 (t, 2H).
Acknowledgements
This work was supported by the Moriah Fund.
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