Homochiral (R)- and (S)-1-heteroaryl- and 1-aryl-2-propanols via microbial redox

Article abstract of DOI:10.1016/S0957-4166(98)00215-8

Preparation of various heteroaryl propanols 2a-g and of the corresponding propanones 3a-g as starting materials for microbial redox is described. The kinetic resolution of the racemic propanols 2a-g is obtained via oxidation with Pseudomonas paucimobilis and Bacillus stearothermophilus [(R)-alcohols, ee 74-100%]. Similar results are achieved with 3-(2- hydroxypropyl)trifluoromethylbenzene 7 (44%, ee 100% of the (R)-alcohol 6). The reduction of the propanones 3a-d and 3g with baker's yeast and other fungi gives the (S)-alcohols (ee 100%). The pure (S)-alcohols are also obtained by reduction of 1-[3-(trifluoromethyl)phenyl]-2-propanone 7. 1- [(4,4-Dimethyl)-2-(Δ²)oxazolinyl]-2-propanone 3e and 1[2-(Δ²)- thiazolinyl)-2-propanone 3f are not reduced. The heterocyclic rings of (S)- 5-(2-hydroxypropyl)-3-methylisoxazole 2d and of (S)-2-(2-hydroxypropyl)-4- methylthiazole 2g are deblocked to the homochiral enamino ketone 8 (78%) and to the protected β-hydroxy aldehyde 9 (73%), respectively. The (R)-3-(2- hydroxypropyl)trifluoromethylbenzene 6 is transformed into the homochiral precursor of (S)-fenfluramine 10 (overall yield 65%).

Full text of DOI:10.1016/S0957-4166(98)00215-8

TETRAHEDRON:
ASYMMETRY
Pergamon
Tetrahedron: Asymmetry 9 (1998) 2317–2327
Homochiral (R)- and (S)-1-heteroaryl- and 1-aryl-2-propanols via
microbial redox
Marco Fogagnolo,∗ Pier Paolo Giovannini, Alessandra Guerrini, Alessandro Medici,
Paola Pedrini and Nicola Colombi
Dipartimento di Chimica, Università di Ferrara, Via L. Borsari 46, I-44100 Ferrara, Italy
Received 21 April 1998; accepted 26 May 1998
Abstract
Preparation of various heteroaryl propanols 2a–g and of the corresponding propanones 3a–g as starting materials
for microbial redox is described. The kinetic resolution of the racemic propanols 2a–g is obtained via oxidation
with Pseudomonas paucimobilis and Bacillus stearothermophilus [(R)-alcohols, ee 74–100%]. Similar results are
achieved with 3-(2-hydroxypropyl)trifluoromethylbenzene 7 (44%, ee 100% of the (R)-alcohol 6). The reduction
of the propanones 3a–d and 3g with baker’s yeast and other fungi gives the (S)-alcohols (ee 100%). The pure
(S)-alcohols are also obtained by reduction of 1-[3-(trifluoromethyl)phenyl]-2-propanone 7. 1-[(4,4-Dimethyl)-
2-(∆2)oxazolinyl]-2-propanone 3e and 1[2-(∆2)-thiazolinyl)-2-propanone 3f are not reduced. The heterocyclic
rings of (S)-5-(2-hydroxypropyl)-3-methylisoxazole 2d and of (S)-2-(2-hydroxypropyl)-4-methylthiazole 2g are
deblocked to the homochiral enamino ketone 8 (78%) and to the protected β-hydroxy aldehyde 9 (73%),
respectively. The (R)-3-(2-hydroxypropyl)trifluoromethylbenzene 6 is transfomed into the homochiral precursor
of (S)-fenfluramine 10 (overall yield 65%). © 1998 Elsevier Science Ltd. All rights reserved.
1. Introduction
Enantiomerically pure secondary alcohols are useful chiral auxiliaries in organic chemistry both for
analytical and synthetic applications.1 In particular chiral heteroaryl secondary alcohols, synthesized
with the aim of unmasking a hidden functionality after the desired transformations,2 have been mainly
obtained from the corresponding ketones by catalytic asymmetric hydrogenation,3 by treatment with a
hydride reagent modified with a chiral auxiliary,3 or by microbial reductions with baker’s yeast4 or other
microrganisms.5,6
Compared with the reduction of ketones, however, there are few reports of microbial oxidations
of alcohols7 because in general their oxidations are thermodynamically unfavoured and inhibited by
∗
Corresponding author.
0957-4166/98/$ - see front matter © 1998 Elsevier Science Ltd. All rights reserved.
PII: S0957-4166(98)00215-8

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M. Fogagnolo et al. / Tetrahedron: Asymmetry 9 (1998) 2317–2327
Fig. 1.
the products formed.8 Recently we reported the kinetic resolution of racemic 1-heteroaryl- and 1-
arylethanols via oxidation with whole Bacillus stearothermophilus9 cells and baker’s yeast.10
In this paper we describe the synthesis of homochiral (R)- and/or (S)-1-heteroaryl- and 1-aryl-
2-propanols (Fig. 1) by kinetic resolution of the racemic alcohols via oxidation with Pseudomonas
paucimobilis and Bacillus stearothermophilus or by reduction of the corresponding ketones with baker’s
yeast and other fungi (yeasts and moulds).
2. Results and discussion
The preparation of the starting materials (racemic alcohols or ketones) are achieved by lithiation of the
appropriate heterocycle followed by quenching with the appropriate electrophile (Scheme 1).
Scheme 1.
Lithiation of the 2-H heterocycles 1a,b and quenching with propylene oxide11 afford the racemic
alcohols 2a,b in good yield (86–90%). The corresponding ketones 3a,b are obtained by oxidation with
pyridinium chlorochromate (PCC) (3a, 88%) or Jones’ reagent (3b, 95%).
On the other hand, metallation of the 2-methyl heterocycles 4c–g and treatment with ethylacetate
produce the ketones 3c–g (82–95%). All the ketones, including the commercially available 1-[3-
(trifluoromethyl)-phenyl]2-propanone 7, are quantitatively reduced to the corresponding racemic alco-
hols with NaBH4.
On the basis of results obtained in our laboratory in the kinetic resolution of 1-heteroaryl-
and 1-arylethanols,9,10 the racemic 1-heteroaryl-2-propanols 2a–g and 3-(2-hydroxylpropyl)-

M. Fogagnolo et al. / Tetrahedron: Asymmetry 9 (1998) 2317–2327
2319
trifluoromethylbenzene 6 are oxidized with Bacillus sterarothermophilus ATCC 2027 and with
Pseudomonas paucimobils11 (Scheme 2). The results are summarized in Table 1.
Scheme 2.
The racemic alcohols were added to a growing culture of Bacillus sterothermophilus and Pseudomonas
paucimobilis as a concentrated solution in DMF and the incubation was carried out for 48 h at 39°C and
at 29°C, respectively. All the reactions have been carried out on an analytical scale in various culture
media and monitored by GLC. The best results have been confirmed on a preparative scale.
The oxidation of the racemic alcohols 2a, 2f and 2g with Bacillus stearothermophilus gives the
corresponding ketones 3a, 3f and 3g (52–60%) and the homochiral (R)-alcohols (40–48% yield, ee
95–100%). The same substrates are also resolved by oxidation with Pseudomonas paucimobilis on an
analytical scale and the results are confirmed on a preparative scale (40–55% yield, ee 74–100%). The
oxidation with Pseud. pauc. is also very efficient for the racemic alcohols 2b–e and 6 (40–47% yield, ee
90–100% of the recovered (R)-alcohols), while Bac. sterarotherm., that affords ketone 3b in good yield
(51%) and leaves the (R)-alcohol 2b (49%) with good enantiomeric excess (72%), produces ketones 3c–e
in low yields (10–25%) and do not oxidize alcohol 6. On the other hand the (S)-alcohols are obtained
in good yields and excellent enantiomeric excesses by microbial reduction (Scheme 3). The results are
summarized in Table 2.
The ketones were added to a growing culture of various microrganisms or to a suspension of baker’s
yeast as a concentrated solution in DMF and the incubation was carried out for 48 h at 29°C. All the
reactions have been carried out on an analytical scale in the appropriate culture medium and monitored
by GLC. The reductions with baker’s yeast have been repeated on a preparative scale.
The ketones 3a–d, 3g, and 7 are enantioselectively reduced with baker’s yeast to give the corresponding
(S)-alcohols (80–92%, ee 100%). Also other yeasts and moulds are checked to obtain the enatiomerically
pure alcohols. Yarrowia lipolytica YL10 is very efficient in the reduction of the ketones 3g and 7
(92–95%, ee 100% of the (S)-alcohols) and Mucor spirescens gives almost quantitative yields of the (S)-
alcohols 2a (ee 94%). On the other hand Trichoderma affords the pure (S)-alcohols 2b and 2g (89–96%
yields) while the alcohols 2a, 2c and 2d (89–98% yields) are obtained with poor enantiomeric excesses
(46–52%).
Because one of the most important characteristics of these heteroaryl rings is the possibility of
unmasking various functions after obtaining the chiral side-chain, the isoxazolic ring of the (S)-alcohol
2d was reductively cleaved12 to the enantiomerically pure (S)-enamino ketone 8 using hydrogen with
PtO2 and Ni/Raney to give 78% yield and 100% ee (Scheme 4).
On the other hand the equivalence of the thiazole ring to a formyl group13 permits the preparation
of chiral β-hydroxy aldehyde 9 (73%, ee 100%) from the (S)-2-(2-hydroxypropyl)-4-methylthiazole 2g
(Scheme 5).
A very interesting application is the synthesis of (S)-fenfluramine,14 an anorectic agent, starting
from the (R)-alcohol 6 obtained from kinetic resolution of the racemic mixture with Pseudomonas
paucimobilis (Scheme 6).
Mitsunobu reaction15 with phthalimide affords the compound 10 with inverted configuration. Treat-
ment with hydrazine and successively with acetic anhydride and pyridine gives the N-acetyl derivative

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Table 1
Kinetic resolution via oxidation of the racemic alcohols 2a–g and 6 with Bacillus stearothermophilus
and Pseudomonas paucimobilis
Scheme 3.
11 (overall yield 65%, ee 100%). Reduction with LiAlH4 as reported in a previous work14 affords the
(S)-fenfluramine.
In conclusion 1-heteroaryl- and 1-aryl-2-propanols are obtained in the pure (R)-form by kinetic
resolution of the corresponding racemates via oxidation with Pseudomonas paucimobilis while it is

M. Fogagnolo et al. / Tetrahedron: Asymmetry 9 (1998) 2317–2327
2321
Table 2
Microbial reduction of the ketones 3a–d, 3g, and 7
Scheme 4.
Scheme 5.
possible to achieve the (S)-enantiomers by reduction with baker’s yeast and other yeasts and moulds.
Some applications of these homochiral compounds confirm their synthetic utility.
3. Experimental
3.1. General
1H and 13C NMR spectra (in CDCl3) were obtained with a Varian Gemini 300 spectrometer. Chemical
shifts were given in parts per million from Me4Si as an internal standard. IR spectra were recorded
on a Perkin–Elmer Model 297 grating spectrometer. Gas chromatographic analyses were perfomed on a

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M. Fogagnolo et al. / Tetrahedron: Asymmetry 9 (1998) 2317–2327
Scheme 6.
Carlo Erba HRGC 5160 Mega series chromatograph. Optical rotations were measured on a Perkin–Elmer
Model 241 polarimeter.
Furan 1a, 2-methylthiophene 1b, 2-methylbenzothiazole 4c, 3,5-dimethylisoxazole 4d,
2,4,4-trimethyl-2-oxazoline 4e, 2-methyl-2-thiazoline 4f, 2,4-dimethylthiazole 4g and 1-[3-
(trifluoromethyl)phenyl]-2-propanone 7 are commercially available. Metallation reactions have
been carried out with dried THF (LiAlH4) and under a nitrogen atmosphere.
Yarrowia lypolitica YL10, Saccharomyces cerevisiae SC4 and SC14, Mucor sp., Trichoderma sp., and
Pseudomonas paucimobilis, except Bacillus stearothermophilus ATCC 2027, belong to the Dipartimento
di Chimica collection.
3.2. Enantiomer separation
Enantiomer separation was obtained on a Megadex 1 column (25 m×0.32 mm) containing permethyl-
ated β-cyclodextrin in OV 1701 from Mega s.n.c.: carrier gas helium (0.8 atm). For compound 2a
temp. 70–200°C (2°C/min), retention time in min: S-2a, 9.92; R-2a, 10.33. For compound 2b temp.
115–200°C (2°C/min), retention time in min: S-2a, 9.04; R-2b, 9.36. For compound 2c temp. 150°C
(22 min) and 150–200°C (20°C/min), retention time in min: S-2c, 24.45; R-2c, 24.56. For compound
2d temp. 130–200°C (1°C/min), retention time in min: S-2d, 7.08; R-2d, 7.34. For compound 2e temp.
90–200°C (1.5°C/min), retention time in min: S-2e, 7.21; R-2e, 7.37. For compound 2f temp. 95–200°C
(2°C/min), retention time in min: S-2f, 12.95; R-2f, 13.12. For compound 2g temp. 100–200°C (2°C/min),
retention time in min: S-2g, 10.30; R-2g, 10.62. For compounds 6 and 7 temp. 120–200°C (3°C/min),
retention time in min: 7, 7.61; S-6 (as acetyl derivative), 8.82; R-6 (as acetyl derivative), 9.38. The
absolute configurations are assigned on the basis of the reductions with baker’s yeast that afforded the
(S)-enantiomers on the basis of the Prelog rule.
3.3. Synthesis of 2-(2-hydroxypropyl)furan 2a and 2-(2-hydroxypropyl)-5-methylthiophene 2b. General
procedure
To a stirred and cooled (0°C) solution of the proper heterocycle 1 (50 mmol) in dry THF (50 ml) a
solution of n-butyllithium 1.6 M in hexane (35 ml, 55 mmol) was added. After 2 h stirring at the same
temperature, a solution of propylene oxide (4.3 g, 75 mmol) in THF (30 ml) was added dropwise and the
temperature was maintained at 0°C for a further 1 h. The reaction mixture was allowed to warm to room

M. Fogagnolo et al. / Tetrahedron: Asymmetry 9 (1998) 2317–2327
2323
temperature, treated with a saturated aqueous solution of NaHCO3 (50 ml) and extracted with diethyl
ether (2×50 ml). The organic layer was washed with a saturated aqueous solution of NaCl (2×50 ml)
and dried over anhydrous Na2SO4. The solvent was removed under vacuum.
2-(2-Hydroxypropyl)furan 2a was purified by distillation (5.67 g, 90%): bp 42°C (0.3 mmHg) [lit.16
bp 47–50°C (0.3 mmHg)]; IR (film) 3500, 1600, 1500, 1150 cm−1; 1H NMR δ 1.20 (d, 3H, J=7 Hz), 1.9
(br, 1H), 2.74 (dd, 1H, J=6.9 and 13.8 Hz), 2.82 (dd, 1H, J=4.2 and 13.8 Hz), 4.20 (m, 1H), 6.10 (d, 1H,
J=3.4 Hz), 6.30 (m, 1H), 7.34 (d, 1H, J=1.3 Hz).
2-(2-Hydroxypropyl)-5-methylthiophene 2b was chromatographed (silica gel, cyclohexane:diethyl
1
ether=6:4) (6.7 g, 86%): oil; IR (film) 3380, 1580, 1540, 1450 cm−1; H NMR δ 1.25 (d, 3H, J=7
Hz), 1.9 (br, 1H), 2.44 (s, 3H), 2.80 (dd, 1H, J=8.2 and 13.8 Hz), 2.92 (dd, 1H, J=4.1 and 13.8 Hz), 4.96
(m, 1H), 6.62 (m, 2H). Anal. calcd for C8H12OS: C, 61.50; H, 7.74. Found: C, 61.58; H, 7.68.
3.4. Synthesis of 1-heteroaryl-2-propanones 3c–g. General procedure
To a stirred and cooled (−78°C) solution of the proper heterocycle 4 (40 mmol) in THF (60 ml) a
solution of n-butyllithium 1.6 M in hexane (28 ml, 44 mmol) was added. After 2 h stirring at the same
temperature, a solution of ethyl acetate (7 ml, 75 mmol) in THF (40 ml) was added dropwise and the
temperature was maintained at −78°C for a further 1 h. The reaction mixture was allowed to warm to
room temperature, treated with a saturated aqueous solution of NaHCO3 (50 ml) and extracted with
diethyl ether (2×50 ml). The organic layer was washed with a saturated aqueous solution of NaCl (2×50
ml) and dried over anhydrous Na2SO4. The solvent was removed under vacuum.
1-(2-Benzothiazolyl)-2-propanone 3c was purified by crystallization from diethyl ether–hexane (7.02
g, 92%): mp 118–120°C [lit.17 mp 112°C]; IR (KBr) 3070, 1600, 1580, 1520 cm−1; 1H NMR δ 2.1 (s,
0.6H, enolic CH3), 2.34 (s, 2.4H, ketonic CH3), 4.25 (s, 1.6H, ketonic CH2), 5.63 (s, 0.2H, enolic _CH),
7.3–8.05 (m, 4H). From NMR spectrum the keto:enolic ratio is 4:1.
1-[(3-Methyl)-5-isoxazolyl]-2-propanone 3d18 was chromatographed (silica gel, diethyl
ether:cyclohexane=8:2) (4.56 g, 82%): bp 85°C (2 mmHg); IR (film) 1720, 1610, 1580, 1420
cm−1; 1H NMR δ 2.22 (s, 3H), 2.27 (s, 3H), 3.82 (s, 2H), 6.05 (s, 1H).
1-[(4,4-Dimethyl)-2-(∆2)-oxazolinyl]-2-propanone 3e19 was chromatographed (silica gel, diethyl
ether:cyclo-hexane=7:3) (5.4 g, 87%): mp 116–119°C [lit.19 mp 125–127°C]; IR (KBr) 3260, 1630,
1550 cm−1; 1H NMR (enamine form) δ 1.40 (s, 6H), 2.02 (s, 3H), 4.07 (s, 2H), 4.87 (s, 1H), 9.2 (br, 1H).
1-[2-(∆2)-Thiazolinyl]-2-propanone 3f was chromatographed (silica gel, ethyl acetate:cyclohex-
1
ane=8:2) (5.4 g, 95%): oil; IR 3050, 1635, 1565, 1500 cm−1; H NMR (enolic form) δ 2.02 (s, 3H),
3.20 (t, 2H, J=6.6 Hz), 3.85 (t, 2H, J=6.6 Hz), 5.30 (s, 1H), 10.2 (br, 1H). Anal. calcd for C6H9NOS: C,
50.32; H, 6.33; N, 9.78. Found: C, 50.40; H, 6.26; N, 9.83.
1-[(4-Methyl)-2-thiazolyl]-2-propanone 3g was distilled (5.1 g, 82%): bp 130–131°C (20 mmHg); IR
1
(film) 1710, 1630, 1605, 1520 cm−1; H NMR δ 2.27 (s, 3H), 2.42 (s, 3H), 4.1 (s, 2H), 6.85 (s, 1H).
Anal. calcd for C7H9NOS: C, 54.37; H, 5.82; N, 8.99. Found: C, 54.44; H, 5.80; N, 8.94.
3.5. Oxidation of alcohol 2a
To a stirred solution of 2-(2-hydroxypropyl)furan 2a (0.5 g, 4 mmol) in dichloromethane (30 ml)
pyridinium chlorochromate (2 g, 9.3 mmol) and Al2O3 (1 g) were added. After 24 h stirring the reaction
mixture was filtered on silica gel and treated with a saturated aqueous solution of NaHCO3. The organic
layer was separated and the aqueous layer was extracted with dichloromethane (20 ml). After treatment
with anhydrous Na2SO4 the solvent was removed in vacuo and the residue chromatographed (silica

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M. Fogagnolo et al. / Tetrahedron: Asymmetry 9 (1998) 2317–2327
gel, cyclohexane:diethyl ether=6:4) to give the pure 1-(2-furanyl)-2-propanone 3a20 (0.44 g, 88%): bp
62–64°C (8 mmHg); IR (film) 1720, 1600, 1580, 1350 cm−1; 1H NMR δ 2.18 (s, 3H), 3.70 (s, 2H), 6.20
(d, 1H, J=3.2 Hz), 6.35 (m, 1H), 7.38 (d, 1H, J=1.1 Hz).
3.6. Oxidation of alcohol 2b
To a stirred and cooled (0°C) solution of 2-(2-hydroxypropyl)-5-methylthiophene 2b (0.5 g, 3.2 mmol)
in freshly distilled acetone (10 ml) Jones’ reagent21 (prepared dissolving 10.3 g of CrO3 and 8.7 ml of
98% H2SO4 in 30 ml of H2O) was added dropwise. The oxidation is complete when the reaction colour
does not change from yellow to green by adding the Jones’ reagent. Isopropanol (0.5 ml) was added
and the reaction mixture was filtered through Celite washing the chromium salts with acetone several
times. The solvent was removed in vacuo and the crude reaction mixture was chromatographed (silica
gel, cyclohexane:diethyl ether=6:4) to give 0.47 g (95%) of 1-[(5-methyl)-2-thiophenyl]-2-propanone 3b:
oil; IR (film) 3380, 1640, 1580, 1450 cm−1; 1H NMR δ 2.22 (s, 3H), 2.43 (s, 3H), 3.80 (s, 2H), 6.60 (d,
1H, J=3.2 Hz), 6.65 (d, 1H, J=3.2 Hz). Anal. calcd for C8H10OS: C, 62.30; H, 6.54. Found: C, 62.35; H,
6.51.
3.7. Reduction with NaBH4 of the ketones 3c–g, and 7
To a stirred and cooled (0°C) solution of the appropriate ketone (5 mmol) in diethyl ether (5 ml) and
methanol (5 ml) NaBH4 (0.3 g, 10 mmol) was added portionwise. After 30 min of stirring the reaction
mixture was concentrated, treated with a saturated aqueous solution of NH4Cl (10 ml) and extracted
with diethyl ether (or dichloromethane) (20 ml). The organic layer was dried over anhydrous Na2SO4.
Filtration and evaporation of the solvent under vacuum afforded the pure racemic alcohols 2c–g, 6g and
8 in quantitative yield.
2-(2-Hydroxypropyl)benzothiazole 2c showed the following: mp 94–97°C; IR (Nujol) 3450, 1675
cm−1; 1H NMR δ 1.36 (d, 3H, J=7 Hz), 3.13 (dd, 1H, J=8.3 and 15.2 Hz), 3.26 (dd, 1H, J=3.4 and 15.2
Hz), 3.85 (br, 1H), 4.2 (m, 1H), 7.38 (t, 1H, J=6.3 Hz), 7.49 (t, 1H, J=6.3 Hz), 7.85 (d, 1H, J=6.9 Hz),
7.98 (d, 1H, J=6.9 Hz). Anal. calcd for C10H11NOS: C, 62.15; H, 5.74; N, 7.25. Found: C, 62.21; H,
5.70; N, 7.30.
5-(2-Hydroxypropyl)-3-methylisoxazole 2d: oil; IR (film) 3400, 1610, 1605, 1420 cm−1; 1H NMR δ
1.22 (d, 3H, J=6.9 Hz), 2.22 (d, 3H, J=0.7 Hz), 2.7 (br, 1H), 2.82 (m, 2H), 4.13 (m, 1H), 5.9 (s, 1H).
Anal. calcd for C7H11NO2; C, 59.56; H, 7.85; N, 9.92. Found: C, 59.50; H, 7.82; N, 9.87.
4,4-Dimethyl-2-(2-hydroxypropyl)-∆2-oxazoline 2e: oil; IR (film) 3350, 2960, 1655 cm−1; 1H NMR
δ 1.22 (d, 3H, J=7 Hz), 1.25 (s, 6H), 2.25 (dd, 1H, J=7.7 and 14.7 Hz), 2.35 (dd, 1H, J=2.8 and 14.7 Hz)
3.88 (s, 2H), 4.10 (m, 1H), 4.3 (br, 1H). Anal. calcd for C8H15NO2: C, 61.12; H, 9.62; N, 8.91. Found:
C, 61.16; H, 9.55; N, 8.88.
2-(2-Hydroxypropyl)-∆2 -thiazoline 2f: oil; IR (film) 3350, 2980, 1620 cm−1; 1H NMR δ 1.21 (d, 3H,
J=6.8 Hz), 2.47 (dd, 1H, J=9 and 16.5 Hz), 2.58 (dd, 1H, J=1.3 and 16.5 Hz), 3.26 (pseudot, 2H, J=7.6
Hz), 4.18 (br, 1H), 4.21 (m, 4H). Anal calcd for C6H11NOS: C, 49.62; H, 7.64; N, 9.65. Found: C, 49.55;
H, 7.71; N, 9.60.
2-(2-Hydroxypropyl)-4-methylthiazole 2g: oil; IR (film) 3400, 1610, 1520 cm−1; 1H NMR δ 11.3 (d,
3H, J=6.9 Hz), 2.42 (s, 3H), 2.96 (dd, 1H, J=7.6 and 14.5 Hz), 3.10 (dd, 1H, J=2.8 and 14.5 Hz), 3.70
(br, 1H), 4.22 (m, 1H), 6.72 (s, 1H). Anal. calcd for C7H10NOS: C, 53.82; H, 6.45, N, 8.97. Found: C,
53.90; H, 6.48; N, 9.02.

M. Fogagnolo et al. / Tetrahedron: Asymmetry 9 (1998) 2317–2327
2325
1
3-(2-Hydroxypropyl)trifluoromethylbenzene 6:14 oil; IR (film) 3380, 1580, 1490 cm-1; H NMR δ
1.22 (d, 3H, J=7 Hz), 1.71 (br, 1H), 2.75 (dd, 1H, J=7.7 and 14.5 Hz), 2.84 (dd, 1H, J=4.8 and 14.5 Hz),
4.05 (m, 1H), 7.45 (m, 4H).
3.8. Culture media
Culture medium A for Bacillus sterothermophilus: bactotryptone (20 g), yeast extract (10 g), sucrose
(20 g), K2SO4 (2.6 g) and Na2HPO4 (6.4 g) in H2O (1 l).
Culture medium B for Pseudomonas paucimobilis: bactotryptone (10 g), yeast extract (5 g), glucose
(2 g), and K2HPO4 (0.4 g) in H2O (1 l).
Culture medium C: bactotryptone (20 g), yeast extract (10 g) and sodium ossalate (3 g) in H2O (1 l).
Culture medium D for moulds and yeasts: glucose (50 g), (NH4)2PO4 (5 g), K2HPO4 (2 g), CaCl2
(0.25 g), MgSO4·7H2O (0.25 g), inositol (25 mg), H3BO3 (1 mg), ZnSO4 (1 mg), MnCl2 (1 mg), FeCl2
(0.5 mg), CuSO4 (0.1 mg), KI (0.1 mg), tiamine (0.3 mg), biotine (0.025 mg, calcium pantothenate (0.3
mg), pyridoxine (0.3 mg), and nicotinic acid (0.3 mg) in H2O (1 l).
Culture medium E for baker’s yeast: glucose (31 g) in H2O (1 l).
3.9. Screening of microbial oxidation of the racemic alcohols 2a–g, and 6. General procedure
A sterilized nutrient broth (A or C for Bac. stearotherm. and B or C for Pseud. pauc.) (10 ml) was
inoculated with a loopful of the proper microrganism. The mixture was incubated on a reciprocatory
shaker for 48 h at 39°C for Bac. stearotherm and at 29°C for Pseud. pauc., respectively. To the resulting
suspension of grown cells the appropriate racemic alcohols (10 mg) in DMF (0.1 ml) were added. After
48 h of further incubation, the reaction mixture was extracted with diethyl ether and dried over anhydrous
Na2SO4. After filtration the solvent was evaporated and the crude reaction mixture analyzed by GLC. The
best results are reported in Table 1.
3.10. Oxidation with Pseudomonas paucimobilis on a preparative scale. General procedure
The reactions were repeated on a preparative scale inoculating with a loopful of Pseudomonas
paucimobilis 400 ml of the proper nutrient broth (see Table 1) and adding, after 48 h of incubation,
the racemic alcohols (0.5 g) in DMF (1 ml). After 48 h under vigorous stirring at 29°C, the reaction
mixtures were extracted with diethyl ether (200 ml) with a continous liquid–liquid extractor, dried
over anhydrous Na2SO4 and concentrated under reduced pressure. The products were purified by
column chromatography (silica gel, diethyl ether:cyclohexane=6:4) to give the ketones and the unreacted
alcohols. The enantiomeric excesses of the recovered alcohols are determined by GLC (see Table 1).
3.11. Screening of microbial reduction of ketones 3a–g and 7. General procedure
A sterilized culture medium D (10 ml) was inoculated with a spore suspension (yeast or mould) and
grown for 48 h at 29°C. Each substrate is tested for Yarrowia lypolitica YL10, Saccharomyces cerevisiae
SC4 and SC14, Mucor sp. and Trichoderma sp. To the resulting suspension of grown cells the appropriate
ketones (10 mg) in DMF (0.1 ml) were added. After 48 h of further incubation, the reaction mixture was
extracted with diethyl ether and dried over anhydrous Na2SO4. After filtration the solvent was evaporated
and the crude reaction mixture analyzed by GLC.

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M. Fogagnolo et al. / Tetrahedron: Asymmetry 9 (1998) 2317–2327
The reaction with baker’s yeast was carried out as above starting from 2 g of yeast in 10 ml of
unsterilized culture medium E. The best results are reported in Table 2.
3.12. Reductions with baker’s yeast on a preparative scale. General procedure
To a vigorously stirred culture medium E (400 ml) baker’s yeast (80 g) was added and stirring
continued for 1 h at 29°C. To the stirred suspension, the appropriate ketone (0.5 g) in DMF (1 ml)
was added and incubation was continued for a further 48 h at the same temperature. The reaction
mixture was extracted with diethyl ether (200 ml) with a continuous liquid–liquid extractor, dried over
anhydrous Na2SO4 and concentrated under reduced pressure. The products were purified by column
chromatography (silica gel, diethyl ether:cyclohexane=6:4) to give the (S)-alcohols. The enantiomeric
excesses are determined by GLC (see Table 2).
3.13. Synthesis of (S)-2-amino-4-oxo-2-hepten-6-ol 8
A solution of the (S)-alcohol 2d (0.3 g, 2 mmol) in methanol (20 ml) was reduced at atmospheric
pressure over a PtO2 catalyst (0.04 g) pre-reduced by adding a small amount of Raney nickel in methanol
(4 ml). After hydrogenation was complete (4 h), the mixture was filtered through Celite and concentrated
under reduced pressure. Chromatography of the residue (silica, diethyl ether) gives the enamino-ketone
8 (0.11 g, 78%): mp 82–85°C; [α]D20=26.0 (c 6.0, CHCl3); IR (Nujol) 3300, 1590, 1525, 1300 cm−1
;
1H NMR δ 1.18 (d, 3H, J=7 Hz), 1.93 (s, 3H), 2.33 (dd, 1H, J=9.1 and 15.5 Hz), 2.46 (dd, 1H, J=3 and
15.5 Hz), 4.15 (m, 1H), 4.98 (s, 1H), 5.3 (br, 1H), 9.7 (br, 2H).
3.14. Synthesis of (S)-3-dimethyl-t-butylsilyloxy-butanal 9
To a stirred solution of the alcohol S-2g (0.4 g, 3 mmol) and imidazole (0.4 g, 6 mmol) in DMF
(20 ml) dimethyl-t-butylsilyl chloride (0.8 g, 6 mmol) was added. After 12 h stirring the solvent was
removed in vacuo, the residue was treated with a saturated aqueous solution of NaHCO3 (20 ml) and
extracted with diethyl ether (20 ml). The organic layer was dried over anhydrous Na2SO4 and the solvent
removed under vacuum. The crude O-dimethyl-t-butylsilyl derivative was diluted with acetonitrile (10
ml) and after adding methyl iodide (2 ml, 30 mmol) refluxed for 12 h. The reaction mixture was allowed
to cool to room temperature and the solvent was removed in vacuo. The crude N-methyl derivative
was dissolved in methanol (20 ml) and NaBH4 (0.12 g, 3 mmol) was added portionwise at 0°C.
After 30 min stirring, the solvent was evaporated and the mixture treated with a saturated solution of
NaHCO3 (20 ml) and diethyl ether (30 ml). The organic layer was dried over anhydrous Na2SO4 and
the solvent removed under vacuum. The residue was dissolved in a mixture of acetonitrile:water (4:1,
5 ml) and HgCl2 (0.81 g, 3 mmol) was added slowly with stirring. A white precipitate was formed.
After 15 min the reaction was concentrated in vacuo, diethyl ether (15 ml) was added and the mixture
was filtered over Celite. The filtrate was treated with saturated NaCl (10 ml), the organic layer was
dried over anhydrous Na2SO4 and the solvent removed under vacuum. Chromatography of the residue
(silica, diethyl ether:cyclohexane=3:7) afforded the protected β-hydroxy aldehyde 9 (0.4 g, 73%): oil;
1
[α]D20=12.1 (c 4.6, CHCl3); IR (film) 2710, 1725, 1460 cm−1; H NMR δ 0.04 (s, 3H), 0.06 (s, 3H),
0.87 (s, 9H), 1.23 (d, 3H, J=6.4 Hz), 2.43 (ddd, 1H, J=2, 5 and 16 Hz), 2.56 (ddd, 1H, J=2.8, 7 and 16
Hz), 4.34 (m, 1H), 9.80 (m, 1H).

M. Fogagnolo et al. / Tetrahedron: Asymmetry 9 (1998) 2317–2327
2327
3.15. Synthesis of (S)-3-(N-acetyl-2-aminopropyl)trifluoromethylbenzene 10
To a stirred solution of the (R)-alcohol 6 (0.4 g, 2 mmol), phthalimide (0.29 g, 2 mmol and triph-
enylphosphine (0.52 g, 2 mmol) in THF (10 ml) diethylazodicarboxylate (0.35 g, 2 mmol) was added
dropwise at room temperature. After 12 h stirring the solvent was evaporated in vacuo, the residue was
treated with diethyl ether (30 ml) and the mixture was filtered. The solvent was removed under vacuum
and the residue was chromatographed (silica, cyclohexane:diethyl ether=7:3) to give the N-phthalimide
derivative (0.52 g, 79%) that was used without further purification in the next step. The 1H NMR spectrum
of the intermediate was the following: δ 1.55 (d, 3H, J=7 Hz), 3.15 (dd, 1H, J=7 and 14 Hz), 3.40 (dd,
1H, J=9.5 and 14 Hz), 4.64 (m, 1H), 7.40 (m, 4H), 7.67 (m, 2H), 7.76 (m, 2H).
A solution of the intermediate (0.5 g, 1.5 mmol) and hydrazine hydrate (200 mg) in ethanol (5 ml) was
refluxed for 2 h. The reaction mixture was treated with 10% HCl and filtered. The filtrate was extracted
with diethyl ether (20 ml). The aqueous layer was basified with 1 N NaOH and extracted with diethyl
ether (2×20 ml). The organic layers were combined, dried over anhydrous Na2SO4 and the solvent
removed under vacuum. The residue was treated wth acetic anhydride (0.3 ml) and pyridine (1 ml). After
1 h the reaction mixture was diluted with diethyl ether (10 ml) and treated with 5% HCl. The organic
layer was washed with H2O (5 ml) and then with a saturated solution of NaHCO3 (5 ml) and dried over
anhydrous Na2SO4. The removal of the solvent gave the N-acetyl derivative 10 (0.32 g, overall yield
68%) with 100% enantiomeric excess determined by GLC on a chiral column.
The N-acetyl derivative showed the following: oil; [α]D20=−19.1 (c 1.5, CHCl3); IR (film) 3290, 1730,
1650, 1550 cm−1; 1H NMR δ 1.1 (d, 3H, J=6.8 Hz), 1.92 (s, 3H), 2.71 (dd, 1H, J=6.8 and 13 Hz), 2.9
(dd, 1H, J=7.5 and 13.6 Hz), 4.20 (m, 1H), 7.38 (m, 4H).
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