Synthesis of optically active β'-hydroxy-β-enaminoketones via enzymatic resolution of carbinols derived from 3,5-disubstituted isoxazoles

Article abstract of DOI:10.1016/S0957-4166(00)00225-1

The preparation of several enantiomerically pure β'-hydroxy-β-enaminoketones from the corresponding isoxazolic carbinols, which have been obtained by enzymatic kinetic resolution of the racemic β-hydroxyisoxazoles catalyzed by lipases, is described. The enzymatic transesterification of racemic (±)-5-(2-hydroxypropyl)-3-methylisoxazole 3a, and racemic (±)-5-(2-hydroxy-2-p-tolylethyl)-3-methylisoxazole 3d, has been studied with respect to the influence of experimental variables such as the used enzyme, the acylating agent or the solvent on the enantioselectivity of the reaction. After the reductive cleavage of the isoxazolic ring of the enantiopure carbinols, (R)- and (S)-2-amino-4-oxo-2-hepten-6-ol, (R)- and (S)-5, and (R)-2-amino-6-p-tolyl-4-oxo-2-hexen-6-ol, (R)-7 with an enantiomeric excess >98% were obtained. Copyright (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0957-4166(00)00225-1

Tetrahedron: Asymmetry 11 (2000) 2565±2577
Synthesis of optically active b0-hydroxy-b-enaminoketones via
enzymatic resolution of carbinols derived from
3,5-disubstituted isoxazoles
Jose Antonio Fuentes, Alicia Maestro, Ana Ma Testera and Jose Manuel Banez*
Â
Departamento de Quõmica Organica, E.T.S. Ingenieros Industriales, Universidad de Valladolid,
Â
47071 Valladolid, Spain
Received 2 May 2000; accepted 5 June 2000
Abstract
The preparation of several enantiomerically pure b0-hydroxy-b-enaminoketones from the corresponding
isoxazolic carbinols, which have been obtained by enzymatic kinetic resolution of the racemic b-hydroxy-
isoxazoles catalyzed by lipases, is described. The enzymatic transesteri®cation of racemic (þ)-5-(2-hydroxy-
propyl)-3-methylisoxazole 3a, and racemic (þ)-5-(2-hydroxy-2-p-tolylethyl)-3-methylisoxazole 3d, has been
studied with respect to the in¯uence of experimental variables such as the used enzyme, the acylating agent
or the solvent on the enantioselectivity of the reaction. After the reductive cleavage of the isoxazolic ring of
the enantiopure carbinols, (R)- and (S)-2-amino-4-oxo-2-hepten-6-ol, (R)- and (S)-5, and (R)-2-amino-6-p-
tolyl-4-oxo-2-hexen-6-ol, (R)-7 with an enantiomeric excess >98% were obtained. # 2000 Elsevier Science
Ltd. All rights reserved.
1. Introduction
The enantioselective preparation of highly functionalized compounds is of central importance
in synthetic chemistry. Often, in order to mask a labile functionality, heterocycles are used in
these synthetic routes. The isoxazoles are an important class of heterocycles both as key inter-
mediates in the construction of the framework of many natural products1 and as a masked aldolic
moiety.2 Recently, we reported3 the extension of the chain of the chiral 2,3-O-isopropylidene-d-
glyceraldehyde using several alkenylisoxazoles as key intermediates synthesized from isoxazolyl-
phosphonium salts.4
The b-enaminoketone unit has received considerable attention in organic synthesis. Unsym-
metrical enamino ketones can be used as bidentate ligands in transition metal complexes,5 as
precursor of interesting functionalities, such as g-aminoalcohols or tetrahydro-1,3-oxazines,6 or
* Corresponding author. E-mail: jmbsanz@dali.eis.uva.es
0957-4166/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0957-4166(00)00225-1

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heterocycles such as pyrazoles,2a,c 2-(1H)-pyridones2d and 2-aminopyrimidines.7 Recently, the
preparation of ¯uorinated b-enaminoketones,8 which are useful building blocks for the con-
struction of new compounds of pharmacological interest,9 has been described. Herein we describe
the preparation of various optically active b0-hydroxy-b-enaminoketones from chiral isoxazolic
secondary alcohols. The enantioselective synthesis of these polyfunctionalized compounds
remains scarcely reported. To our knowledge only one report describes the preparation of an
enantiomerically pure enaminoketone.10 The method consists of the microbial reduction of an 5-
isoxazolyl propanone and subsequent reductive cleavage of the isoxazolic ring. We now report
the preparation of several racemic isoxazolyl carbinols and their enzymatic kinetic resolution.
Nowadays the use of enzymes is a common tool in organic chemistry; however, there are only a
few examples of enzymatic resolution of heteroaryl carbinols11 and, in particular, isoxazolic car-
binols.12 Therefore, a systematic study of the transesteri®cation of isoxazolic carbinols catalyzed
by di€erent commercial lipases has also been carried out. The in¯uence of the nature of the sub-
strate, the solvent (polarity and hydrophobicity) and the acylating agent on the enantioselectivity
has been examined.
2. Results and discussion
The starting isoxazolic carbinols were synthesized by nucleophilic addition to aldehydes of the
isoxazolic carbanion (Scheme 1).
Scheme 1.
The lateral lithiation of 3,5-dimethylisoxazole, 1, was carried out using LDA in THF solution
at ^78ꢀC. Subsequent addition of the aldehyde in THF gave the racemic b-hydroxyisoxazoles 3a±i
in the yields indicated in the Experimental section. In this way, we achieved better results than
those obtained by the traditional lithiation with nBuLi.13
The racemic carbinols (þ)-5-(2-hydroxypropyl)-3-methylisoxazole 3a, and (þ)-5-(2-hydroxy-2-
p-tolylethyl)-3-methylisoxazole 3d, were obtained in good chemical yields and moreover they
have a very di€erent substituent at the C5 position of the isoxazole ring, so were chosen out of
synthesized racemic b-hydroxyisoxazoles to be enzymatically resolved.
The study of the enzymatic transesteri®cation of the racemic alcohols (þ)-3a and (þ)-3d was
made using six commercial lipases as biocatalysts: Pseudomonas cepacia lipase (PSL), porcine
pancreatic lipase (PPL), Candida cylindracea lipase (CCL), Candida antarctica lipase (CAL),
lipomod AC and Aspergillus niger lipase (ANL). In every biocatalytic process the solvent is a
variable that must be optimized. Therefore, hydrophobic and apolar solvents such as hexane or
very hydrophilic such as 1,4-dioxane were employed. Finally, the in¯uence of the acylating agent

J. A. Fuentes et al. / Tetrahedron: Asymmetry 11 (2000) 2565±2577
2567
was also evaluated. All the assays carried out are shown in Tables 1 and 2. The experiments in
which the acylated product was produced are indicated with a +' sign and when the untrans-
formed alcohol was recovered, with a `^' sign. In general, it can be pointed out that the isoxazole
3a is a better recognized substrate by the enzyme than the compound 3d, probably due to steric
reasons. All reactions were carried out at 27ꢀC and shaken at 250 rpm.
Table 1a
Assays carried out with the racemic carbinol (þ)-3a
Table 2a
Assays carried out with the racemic carbinol (þ)-3d
The enzymatic transesteri®cation of the b-hydroxyisoxazole (þ)-3 produced the acylation product
in a wide range of solvents and enzymes (Scheme 2). The speci®c rotations of the O-acetyl derivative
(R)-4, and the remaining unreacted alcohol (S)-3a, were measured (Table 3) and based on these
rotation values, we decided to determine the enantioselectivity, E, for the more promising
experiments (Table 4).
Scheme 2.
The best enantiomeric ratio (E=35) for the process was achieved using PSL and vinyl acetate
as acylating agent in diethyl ether (Table 4, entry 1). Under these reaction conditions the enantio-
pure alcohol (S)-3a could be obtained if the reaction was extended beyond 50% conversion.
When CAL catalyzed the reaction in tert-butyl methyl ether (TBME) (Table 4, entries 4 and 5)

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J. A. Fuentes et al. / Tetrahedron: Asymmetry 11 (2000) 2565±2577
Table 3
Speci®c rotation values of the compounds (R)-4 and (S)-3a
Table 4
Enzymatic transesteri®cation of the b-hydroxyisoxazole (þ)-3a

J. A. Fuentes et al. / Tetrahedron: Asymmetry 11 (2000) 2565±2577
2569
the reaction times substantially decreased. The use of isopropenyl acetate did not result in a sig-
ni®cant improvement on the enantiomeric excesses.
In order to obtain the alcohol (S)-3a in an enantiomerically pure form, we carried out the
acetylation of racemic (þ)-3a in the presence of CAL, in 1,4-dioxane and using vinyl acetate as
acyl donor (Table 4, entry 3). When the reaction was stopped at a conversion of 59% (24 hours)
the desired compound was achieved enantiomerically pure, ee >98% (only one diastereomer
25
1
could be observed in the H and 13C NMR spectra) (‰ꢀŠD =+25.8, c=1.15, CHCl3) while the
acylated product (R)-4 was recovered with 66% ee (‰ꢀŠ2D5=+12.2, c=1.75, CHCl3).
Since it is not possible to obtain the O-acetyl derivative (R)-4 in enantiopure form we decided
to slightly change the strategy. Following the previously described method, the alcohol (S)-3a was
obtained optically pure, therefore we again carried out the enzymatic transesteri®cation of the
enantiomerically enriched (R)-4 (89% ee) (Table 4, entry 4) after hydrolysis on a solution of
potassium carbonate in aqueous methanol (Scheme 3). So the reaction of (R)-3a with vinyl acetate
in the presence of CAL, in tert-butyl methyl ether, yielded the enantiopure (R)-4 (ee >98%,
‰ꢀŠ2D5=+19.6, c=1.64, CHCl3) after 3 hours of reaction. After the hydrolysis of (R)-4, we could
characterize the enantiomerically pure (R)-3a (‰ꢀŠ2D5=^26.3, c=0.95, CHCl3).
Scheme 3.
Our group had already worked on the reductive cleavage of the isoxazolic ring using Raney
nickel as catalyst to produce several b-diketones.3 Therefore, we decided to use another catalyst
in order to obtain the desired enaminoketones. E€ectively, the optically pure isoxazolic carbinol
(S)-3a yielded the b0-hydroxy-b-enaminoketone (S)-5 (‰ꢀŠD25=+48.0, c=0.8, CHCl3, lit.10) without
racemization of the stereogenic center when the hydrogenation was carried out using Pd/C
(Scheme 4). When the same reaction scheme was applied to (R)-3a, after hydrolysis of enantio-
pure (R)-4, the enaminoketone (R)-5 (‰ꢀŠ2D5=^50.6, c=0.57, CHCl3) was also obtained.
Scheme 4.

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J. A. Fuentes et al. / Tetrahedron: Asymmetry 11 (2000) 2565±2577
The absolute con®guration of (S)-5 was assigned by comparison of it speci®c rotation with the
described value in the literature,10 which must also be the con®guration of the initial alcohol (S)-
3a. Based on these experimental results, we can say that the (R)-enantiomer reacts faster than its
counterpart when the enzymatic acylation was carried out in the presence of lipases PSL, CAL,
PPL and ANL. This fact is in accordance with Kazlauskas' rule15 for the resolution of secondary
alcohols. On the contrary, and despite the speci®c rotation values indicating a low enantio-
selectivity for the reaction, it is noteworthy that the lipases CCL and Lipomod AC showed an
opposite enantiopreference towards the substrate (þ)-3a (Table 3, entries 15±22).
The enzymatic transesteri®cation of racemic b-hydroxyisoxazole (þ)-3d was positive on ®ve out
of 34 experiments (Scheme 5). The obtained results are collected in Table 5.
Scheme 5.
Table 5
Enzymatic transesteri®cation of the b-hydroxyisoxazole (þ)-3d
From the data shown in Table 5 it can be seen that CCL and Lipomod AC seem to be the most
suitable lipases for the enzymatic resolution of (þ)-3d. When the reaction was carried out in the
presence of Lipomod AC (Table 5, entry 6) enantiopure (R)-6 was obtained, ee >98% (only one
diastereomer could be observed in the 1H and 13C NMR spectra) (‰ꢀŠD25=+44.9, c=0.52, CHCl3).
Although the enantioselectivity for the reaction, E=110, indicates that it is possible to produce
enantiomerically pure remaining substrate (S)-3d, this reaction is not interesting for practical
purposes due to its low conversion, only 10% after 6 days.
Following a similar procedure to that employed with the substrate (þ)-3a, CCL catalyzed the
acetylation of enantiomerically enriched (R)-6, 91% ee (Table 5, entry 3) in hexane, using vinyl
acetate as acyl donor (Scheme 6). This led to the O-acetyl derivative (R)-6 with an enantiomeric
1
excess >98% (only one diastereomer could be observed in the H and 13C NMR spectra)
(‰ꢀŠ2D5=+49.5, c=2.0, CHCl3).

J. A. Fuentes et al. / Tetrahedron: Asymmetry 11 (2000) 2565±2577
2571
Scheme 6.
As shown in Scheme 6, the reductive cleavage of the isoxazolic ring of carbinol (R)-3d
(‰ꢀŠ2D5=+21.1, c=1.4, CHCl3) gave the enantiopure enaminoketone (R)-7 (‰ꢀŠ2D5=44.4, c=0.5,
CHCl3). The absolute con®guration of the stereogenic carbon atom of the (R)-7 was assigned by
X-ray analysis (Fig. 1; absolute structure parameter 0.0(15)).
Figure 1. X-Ray structure of enaminoketone (R)-7
In summary, we have reported a systematic study of the transesteri®cation of isoxazolic carbinols
catalyzed by di€erent commercial lipases. The enzymatic resolution of the isoxazolic alcohols
constitutes an easy and ecient method to prepare b0-hydroxy-b-enaminoketones in enantiopure
form. These are highly interesting compounds in organic synthesis and their synthesis is dicult
by other methods.
3. Experimental
3.1. General
All reagents were of commercial quality. Solvents were distilled over an adequate dessicant and
stored under argon. Precoated TLC plates of silica gel 60 F254 from Merck were used while for
column chromatography, Merck silica gel 60/230±400 mesh was applied. Mps were determined
with a Gallenkamp MFB-595 apparatus and are uncorrected. Optical rotations were measured at
1
room temperature with a 141 Perkin±Elmer polarimeter. H and 13C NMR spectra for solutions
in CDCl3 were measured using a Bruker AC-300 (1H 300 MHz and 13C 75 MHz). Chemical-shift
values are expressed in ppm (ꢁ), relative to TMS as internal reference: J values are given in hertz.
IR spectra were measured using a Nicolet FTIR-20-SX spectrometer. Mass spectra were recorded

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J. A. Fuentes et al. / Tetrahedron: Asymmetry 11 (2000) 2565±2577
on a Hewlett±Packard 5988/A spectrometer. Elemental analyses were determined with a Perkin±
Elmer Elemental Analyzer 2400 CHN and HPLC analyses were carried out on a Shimadzu LC
liquid chromatograph. 3,5-Dimethylisoxazole 1 and 3-methyl-5-phenylisoxazole 2 were prepared
following methods described in the literature.16,17
3.2. Preparation of racemic ꢂ-hydroxyisoxazoles 3a±i. General procedure
In a three-neck ¯ask THF (30 mL) was placed at ^40ꢀ C under an argon atmosphere. Then
were added, in turn, BuLi (1.6 M, 40 mmol) and diisopropylamine (DIA) (4.45 g, 6.28 mL, 44
mmol). The mixture was slowly allowed to reach rt and stirred for 15 min. To the solution of
LDA in THF, the isoxazole (40 mmol) in THF was added dropwise at ^78ꢀC. The reaction mixture
was stirred for 1 h and the aldehyde (40 mmol) was then slowly added. The mixture was stirred at
the same temperature and after that it was hydrolyzed with water and extracted with dichloro-
methane. The combined organic layers were dried and evaporated to dryness. The residue was
chromatographed to give the desired compounds.
3.2.1. (þ)-5-(2-Hydroxypropyl)-3-methylisoxazole 3a
The reaction of 3,5-dimethylisoxazole 1 (0.97 g, 10.0 mmol) with acetaldehyde (0.57 mL, 10.0
mmol) following the previously described method gave the racemic b-hydroxyisoxazole 3a in a
yield of 78%. The product was puri®ed by ¯ash chromatography using ethyl acetate:hexane (1:1)
as eluent. It was a yellow syrup.10 1H NMR (300 MHz, CDCl3): 1.26 (d, 3H, J=6.2 Hz, CH3),
2.26 (s, 3H, CH3-isx), 2.86 (d, 2H, J=6.2 Hz, CH2), 4.14±4.20 (m, 1H, CH-OH), 5.95 (s, 1H, H-isx).
13C NMR (75 MHz, CDCl3): 11.2 (CH3), 22.9 (CH3), 36.2 (CH2), 65.7 (CH), 103.0 (C-arm),
159.7 (C-arm), 170.1 (C-arm). MS (EI) m/z: 141 (0.49%, M+), 126 (1.22, M+^15), 97 (100,
C5H7NO+), 82 (12.83, C4H4NO+), 54 (20.19, C3H4N+), 45 (22.08, C2H5O). IR (thin layer): 3400,
2990, 1610, 1420 cm^1.
3.2.2. (þ)-5-(2-Hydroxy-3-methylbutyl)-3-methylisoxazole 3b
The reaction of 3,5-dimethylisoxazole 1 (0.97 g, 10.0 mmol) with isobutyraldehyde (0.91 mL,
10.0 mmol)) following the previously described method gave the racemic b-hydroxyisoxazole 3b
in a yield of 24%. The product was puri®ed by ¯ash chromatography using diethyl ether:hexane
1
(3:1) as eluent. It was a yellow syrup. H NMR (300 MHz, CDCl3): 0.98 (d, 6H, J=6.8 Hz,
2CH3), 1.69±1.76 (m, 1H, CH-OH), 2.09 (d, 1H, J=4.5 Hz, OH), 2.26 (s, 3H, CH3-isx), 2.81 (dd,
1H, J=15.0 Hz, J=8.5 Hz, H-CH2), 2.90 (dd, 1H, J=15.0 Hz, J=3.9 Hz, H-CH2), 3.71±3.77 (m,
1H, CH-OH), 5.96 (s, 1H, H-isx). 13C NMR (75 MHz, CDCl3): 11.2 (CH3), 17.1 and 18.5 (CH3),
31.7 (CH2), 33.2 (CH), 74.2 (CH), 102.9 (C-arm), 159.7 (C-arm), 170.8 (C-arm). MS (EI) m/z: 170
(2.31%, M++1), 97 (100, C5H7NO+), 82 (36.05, C4H4NO+), 73 (20.98, C4H9O+), 54 (32.68,
C3H4N+). IR (thin layer): 3425, 2950, 1610, 1420 cm^1.
3.2.3. (þ)-5-(2-Hydroxy-2-phenylethyl)-3-methylisoxazole 3c
The reaction of 3,5-dimethylisoxazole 1 (0.97 g, 10.0 mmol) with benzaldehyde (1.02 mL, 10.0
mmol) following the previously described method gave the racemic b-hydroxyisoxazole 3c in a
yield of 77%. The product was puri®ed by ¯ash chromatography using ethyl acetate:hexane (1:2)
ꢀ
1
as eluent. It was a white solid, mp 66±67 C (hexane±isopropanol) (lit.18). H NMR (300 MHz,
CDCl3): 2.16 (s, 3H, CH3-isx), 2.75 (br s, 1H, OH), 3.02 (dd, 1H, J=15.2 Hz, J=5.0 Hz, H-CH2),
3.12 (dd, 1H, J=15.2 Hz, J=8.2 Hz, H-CH2), 4.99 (dd, 1H, J=8.2 Hz, J=5.0 Hz, CH-OH), 5.82

J. A. Fuentes et al. / Tetrahedron: Asymmetry 11 (2000) 2565±2577
2573
(s, 1H, H-isx), 7.25±7.33 (m, 5H, H-arm). 13C NMR (75 MHz, CDCl3): 11.2 (CH3), 36.5 (CH2),
71.8 (CH), 103.3 (CH), 125.6, 127.8 and 128.4 (CH-arm), 143.0 (C-arm), 159.7 (C-arm), 169.6
(C-arm). MS (EI) m/z: 204 (19.16%, M++1), 97 (100, C5H7NO+), 77 (96.91, C6H5 ), 107 (28.61,
+
C7H7O). IR (KBr): 3390, 2925, 1607, 1497, 1420, 1055 cm^1. C12H13NO2 calcd: C, 70.92%; H,
6.45; N, 6.89; found: C, 71.01%; H, 6.57; N, 6.86.
3.2.4. (þ)-5-(2-Hydroxy-2-p-tolylethyl)-3-methylisoxazole 3d
The reaction of 3,5-dimethylisoxazole 1 (0.97 g, 10.0 mmol) with p-toluylaldehyde (1.18 mL,
10.0 mmol) following the previously described method gave the racemic b-hydroxyisoxazole 3d in
a yield of 88%. The product was puri®ed by ¯ash chromatography using ethyl acetate:hexane
ꢀ
1
(1:2) as eluent. It was a white solid, mp 51±52 C (hexane±isopropanol) (lit.18). H NMR (300
MHz, CDCl3): 2.20 (s, 3H, CH3-isx), 2.33 (s, 3H, CH3-Ph), 3.03 (dd, 1H, J=15.2 Hz, J=5.0 Hz,
H-CH2), 3.13 (dd, 1H, J=15.2 Hz, J=8.3 Hz, H-CH2), 4.98 (dd, 1H, J=8.3 Hz, J=5.0 Hz, CH-
OH), 5.85 (s, 1H, H-isx), 7.13±7.24 (m, 4H, H-arm). 13C NMR (75 MHz, CDCl3): 11.1 (CH3),
20.9 (CH3-Ph), 36.3 (CH2), 71.5 (CH), 103.1 (CH), 125.5 and 129.0 (CH-arm), 137.3 and 140.0
(C-arm), 159.6 (C-arm), 169.7 (C-arm). MS (EI) m/z: 217 (1.52%, M+), 121 (65.55, C8H9O+), 97
+
+
(100, C5H7NO+), 91 (33.96, C7H7 ), 65 (7.42, C5H5 ), 54 (5.13, C3H4N+). IR (KBr): 3375, 2950,
1607, 1515, 1420, 780 cm^1. C13H15NO2 calcd: C, 71.86%; H, 6.96; N, 6.45; found: C, 71.90%; H,
7.11; N, 6.17.
3.2.5. (þ)-5-(2-Hydroxy-2-p-methoxyphenylethyl)-3-methylisoxazole 3e
The reaction of 3,5-dimethylisoxazole 1 (0.97 g, 10.0 mmol) with p-methoxybenzaldehyde (1.21
mL, 10.0 mmol) following the previously described method gave the racemic b-hydroxyisoxazole
3e in a yield of 79%. The product was puri®ed by ¯ash chromatography using ethyl acetate:hexane
(1:2) as eluent. It was a white solid, mp 42±43ꢀC (hexane±isopropanol) (lit.18). H NMR (300
1
MHz, CDCl3): 2.16 (s, 3H, CH3-isx), 2.75 (br s, 1H, OH), 3.00 (dd, 1H, J=15.2 Hz, J=5.3 Hz,
H-CH2), 3.12 (dd, 1H, J=15.2 Hz, J=8.1 Hz, H-CH2), 3.76 (s, 3H, OCH3), 4.94 (dd, 1H, J=8.1
Hz, J=5.3 Hz, CH-OH), 5.81 (s, 1H, H-isx), 6.82±7.24 (m, 4H, H-arm). 13C NMR (75 MHz,
CDCl3): 11.0 (CH3), 36.2 (CH2), 54.9 (OCH3), 71.2 (CH), 103.0 (CH), 113.5 and 126.8 (CH-arm),
135.1 and 158.8 (C-arm), 159.5 (C-arm), 169.6 (C-arm). MS (EI) m/z: 233 (0.77%, M+), 137
+
+
(91.80, C8H9O2 ), 109 (55.02, C7H9O+), 97 (100, C5H7NO+), 77 (57.69, C6H5 ). IR (thin layer):
3400, 2950, 1610, 1515, 1250, 1420, 1035 cm^1. C13H15NO3 calcd: C, 66.94%; H, 6.48; N, 6.00;
found: C, 67.10%; H, 6.46; N, 5.69.
3.2.6. (þ)-5-(2-Hydroxy-2-p-nitrophenylethyl)-3-methylisoxazole 3f
The reaction of 3,5-dimethylisoxazole 1 (0.97 g, 10.0 mmol) with p-nitrobenzaldehyde (1.52 g,
10.0 mmol) following the previously described method gave the racemic b-hydroxyisoxazole 3f in
a yield of 5%. The product was puri®ed by ¯ash chromatography using diethyl ether:hexane (3:1)
as eluent. It was a yellow solid, mp 75±76ꢀC (hexane±isopropanol). 1H NMR (300 MHz, CDCl3):
2.26 (s, 3H, CH3), 2.63 (d, 1H, J=3.7 Hz, OH), 3.08±3.21 (m, 2H, CH2), 5.19±5.24 (m, 1H, CH-
OH), 5.89 (s, 1H, H-isx), 7.50±7.57 (m, 2H, H-arm), 8.19±8.24 (m, 2H, H-arm). 13C NMR (75
MHz, CDCl3): 11.2 (CH3), 36.5 (CH2), 70.9 (CH), 103.8 (CH), 123.7 and 126.5 (CH-arm), 147.3
and 150.2 (C-arm), 159.9 (C-arm), 168.6 (C-arm). MS (EI) m/z: 249 (0.52%, M++1), 152 (15.74,
+
+
C7H6NO3), 97 (100, C5H7NO+), 77 (13.72, C6H5 ), 54 (15.77, C3H4N+). IR (KBr): 3375, 2950,
1610, 1525, 1420, 1350 cm^1. C12H12N2O4 calcd: C, 58.07%; H, 4.87; N, 11.28; found: C, 58.54%;
H, 4.98; N, 11.27.

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J. A. Fuentes et al. / Tetrahedron: Asymmetry 11 (2000) 2565±2577
3.2.7. (þ)-3-(2-Hydroxy-2-phenylethyl)-5-phenylisoxazole 3g
The reaction of 3-methyl-5-phenylisoxazole 2 (1.59 g, 10.0 mmol) with benzaldehyde (1.02 mL,
10.0 mmol) following the previously described method gave the racemic b-hydroxyisoxazole 3g in
a yield of 47%. The product was puri®ed by ¯ash chromatography using ethyl acetate:hexane
(1:4) as eluent. It was a white solid, mp 103±104ꢀC (hexane±isopropanol) (lit.19). H NMR (300
1
MHz, CDCl3): 3.00 (d, 1H, J=3.4 Hz, OH), 3.06±3.17 (m, 2H, CH2), 5.05±5.10 (m, 1H, CH-
OH), 6.35 (s, 1H, H-isx), 7.24±7.46 (m, 8H, H-arm), 7.68±7.73 (m, 2H, H-arm). 13C NMR (75
MHz, CDCl3): 36.2 (CH2), 75.5 (CH), 100.1 (CH), 125.7, 127.9, 128.5, 128.9 and 130.1 (CH-arm),
127.3 and 143.1 (C-arm), 161.8 (C-arm), 169.7 (C-arm). MS (EI) m/z: 265 (3.88%, M+), 188 (1.10,
M+^C6H5 ), 159 (100, C10H9NO+), 77 (88.90, C6H5 ), 51 (21.22, C4H3 ). IR (KBr): 3425, 2954,
1608, 1510, 1420, 1059 cm^1. C17H15NO2 calcd: C, 76.96%; H, 5.57; N, 5.28; found: C, 76.82%;
H, 5.82; N, 5.60.
+
+
+
3.2.8. (þ)-3-(2-Hydroxy-2-p-tolylethyl)-5-phenylisoxazole 3h
The reaction of 3-methyl-5-phenylisoxazole 2 (1.59 g, 10.0 mmol) with p-toluylaldehyde (1.18
mL, 10.0 mmol) following the previously described method gave the racemic b-hydroxyisoxazole
3h in a yield of 61%. The product was puri®ed by ¯ashꢀ chromatography using ethyl acet-
1
ate:hexane (1:2) as eluent. It was a white solid, mp 115±116 C (hexane±isopropanol). H NMR
(300 MHz, CDCl3): 2.24 (s, 3H, CH3), 2.94 (br s, 1H, OH), 2.94±3.13 (m, 2H, CH2), 4.95 (dd, 1H,
J=7.5 Hz, J=5.3 Hz, CH-OH), 6.29 (s, 1H, H-isx), 7.05±7.37 (m, 7H, H-arm), 7.61±7.64 (m, 2H,
H-arm). 13C NMR (75 MHz, CDCl3): 21.1 (CH3-Ph), 36.0 (CH2), 72.4 (CH), 100.1 (CH), 125.7,
128.8, 129.1 and 130.0 (CH-arm), 127.3, 137.5 and 140.1 (C-arm), 161.9 (C-arm), 169.5 (C-arm).
+
MS (EI) m/z: 279 (0.65%, M+), 159 (72.49, C10H9NO+), 121 (23.95, C8H9O+), 91 (49.21, C7H7 ),
+
+
77 (100, C6H5 ), 51 (41.21, C4H3 ). IR (KBr): 3419, 2912, 1572, 1498, 1420, 1060 cm^1.
C18H17NO2 calcd: C, 77.40%; H, 6.13; N, 5.01; found: C, 77.19%; H, 6.19; N, 4.63.
3.2.9. (þ)-3-(2-Hydroxy-2-p-methoxyphenyl)-5-phenylisoxazole 3i
The reaction of 3-methyl-5-phenylisoxazole 2 (1.59 g, 10.0 mmol) with p-methoxybenzaldehyde
(1.21 mL, 10.0 mmol) following the previously described method gave the racemic b-hydroxy-
isoxazole 3i in a yield of 43%. The product was puri®ed by ¯ash chromatography using ethyl
ꢀ
1
acetate:hexane (1:1) as eluent. It was a white solid, mp 120±122 C (hexane±isopropanol). H
NMR (300 MHz, CDCl3): 2.63 (d, 1H, J=3.1 Hz, OH), 3.06-3.24 (m, 2H, CH2), 3.81 (s, 3H, O-
CH3), 5.04±5.10 (m, 1H, CH-OH), 6.38 (s, 1H, H-isx), 6.88±6.93 (m, 2H, H-arm), 7.26±7.49 (m,
5H, H-arm), 7.73±7.77 (m, 2H, H-arm). 13C NMR (75 MHz, CDCl3): 36.1 (CH2), 55.2 (OCH3),
72.2 (CH), 100.1 (CH), 113.9, 125.7, 127.0, 128.9 and 130.1 (CH-arm), 127.4, 135.3 and 159.2
(C-arm), 161.9 (C-arm), 169.6 (C-arm). MS (EI) m/z: 295 (2.53%, M+), 159 (100, C10H9NO+), 137
+
+
+
(52.73, C8H9O2 ), 77 (40.01, C6H5 ), 51 (10.39, C4H3 ). IR (KBr): 3433, 2951, 1612, 1512, 1420,
1255, 1009 cm^1. C18H17NO3 calcd: C, 73.21%; H, 5.80; N, 4.74; found: C, 73.09%; H, 5.82; N,
4.61.
3.3. General procedure for the enzymatic resolution of the racemic isoxazolic carbinols (þ)-3a and
(þ)-3d
Carbinol (þ)-3a or (þ)-3d (1 mmol) and vinyl or isopropenyl acetate (10 mmol) were added to a
suspension of the enzyme (360 mg when ANL, CCL, LIPOMOD AC, PPL or PSL were used or
60 mg if CAL was used) in a suitable solvent (9 mL) under an argon atmosphere. The mixture

J. A. Fuentes et al. / Tetrahedron: Asymmetry 11 (2000) 2565±2577
2575
was shaken at 27ꢀC and 250 rpm for the times indicated in Tables 4 and 5. Then the biocatalyst
was ®ltered o€ and washed with dichloromethane (2Â10) and the organic solvents evaporated.
The crude was subjected to column chromatography using ethyl acetate:hexane (1:2) as eluent in
the case of carbinol (þ)-3a, or diethyl ether:hexane (1:3) for the carbinol (þ)-3d.
3.3.1. (R)-5-(2-Acetylpropyl)-3-methylisoxazole (R)-4
The enzymatic resolution of (þ)-3a in the presence of CAL and using vinyl acetate as acyl
donor and TBME as solvent yielded (R)-4 as a colorless oil. Then this compound was hydrolyzed
using a mixture of K2CO3 in aqueous methanol and the enriched alcohol was acylated in the
presence of CAL. In this manner the compound (R)-4 showed ee >98%. The determination of
1
the enantiomeric excess for the O-acylated product was made by H NMR spectroscopy in the
presence of 3.0 mol equiv. of Eu(tfc)3 (on the spectrum only one signal was observed).
25
1
‰ꢀŠD =+19.6 (c=1.64, CHCl3). H NMR (300 MHz, CDCl3): 1.25 (d, 3H, J=6.4 Hz, CH3), 2.00
(s, 3H, CH3-COO), 2.24 (s, 3H, CH3-isx), 2.91 (dd, 1H, J=15.0 Hz, J=5.8 Hz, H-CH2), 2.98 (dd,
1H, J=15.0 Hz, J=6.5 Hz, H-CH2), 5.09±5.20 (m, 1H, CH-OAc), 5.87 (s, 1H, H-isx). 13C NMR
(75 MHz, CDCl3): 11.4 (CH3), 19.6 (CH3), 21.2 (CH3-COO), 33.0 (CH2), 68.4 (CH), 103.1 (CH-
arm), 159.8 (C-arm), 168.9 (OCO), 170.4 (C-arm). MS (EI) m/z: 184 (0.16%, M++1), 97 (17.48,
C5H7NO+), 82 (5.98, C4H4NO+), 54 (6.75, C3H4N+), 43 (100, C2H3O+). IR (thin layer): 2950,
1740, 1610, 1420, 1245 cm^1.
3.3.2. (S)-5-(2-Hydroxypropyl)-3-methylisoxazole (S)-3a
The enantiopure carbinol (S)-3a was obtained as a yellow oil; ee >98%, ‰ꢀŠ2D5=+25.8, c=1.15,
CHCl3. The determination of the enantiomeric excess was made by 1H NMR spectroscopy in the
presence of 3.0 mol equiv. of Eu(tfc)3, determined from its O-acetyl derivative and on the spectrum
only one signal was observed.
3.3.3. (R)-5-(2-Acetyl-2-p-tolylethyl)-3-methylisoxazole (R)-6
The double enzymatic resolution of (þ)-3d in the presence of CCL and using vinyl acetate as
acyl donor and hexane as solvent yielded (R)-6 as a colorless oil. This compound showed ee
>98%. The determination of the enantiomeric excess was made by 1H NMR spectroscopy in the
presence of 4.0 mol equiv. of Eu(tfc)3 and on the spectrum only one signal was observed.
‰ꢀŠ2D5=+49.5 (c=2.0, CHCl3). 1H NMR (300 MHz, CDCl3): 2.02 (s, 3H, CH3-COO), 2.20 (s, 3H,
CH3-isx), 2.30 (s, 3H, CH3-Ph), 3.15 (dd, 1H, J=15.4 Hz, J=5.6 Hz, H-CH2), 3.30 (dd, 1H,
J=15.4 Hz, J=8.2 Hz, H-CH2), 5.78 (s, 1H, H-isx), 6.02 (dd, 1H, J=8.2 Hz, J=5.6 Hz, CH-
OAc), 7.11±7.22 (m, 4H, H-arm). 13C NMR (75 MHz, CDCl3): 11.1 (CH3), 20.8 (CH3-COO),
20.9 (CH3-Ph), 33.4 (CH2), 72.8 (CH), 102.8 (CH-arm), 126.1 and 129.0 (CH-arm), 135.7 and
137.9 (C-arm), 159.4 (C-arm), 168.2 (OCO), 169.6 (C-arm). MS (EI) m/z: 259 (0.20%, M+), 216
(0.12, M+^C2H3O+), 121 (22.74, C8H9O+), 97 (100, C5H7NO+), 65 (7.17, C5H+5), 43 (100, C2H3O+).
IR (thin layer): 2950, 1745, 1610, 1520, 1420 cm^1.
3.4. General procedure for the catalytic hydrogenation of the enantiopure isoxazolic carbinols
A solution of isoxazolic carbinol (1 mmol) in ethanol (10 mL) was reduced in the presence of
10% Pd on carbon under H2 at atmospheric pressure. The mixture was stirred until the starting
product disappeared and then was ®ltered through Celite and concentrated under reduced pressure.

2576
J. A. Fuentes et al. / Tetrahedron: Asymmetry 11 (2000) 2565±2577
The ¯ash chromatography of the crude product (diethyl ether) achieved the desired b0-hydroxy-b-
enaminoketones.
3.4.1. 2-Amino-4-oxo-2-hepten-6-ol (R)-5
The previously described procedure led to the compound (R)-5 in a yield of 88% as a white
25
ꢀ
1
solid, mp 82±85 C (hexane±isopropanol). ‰ꢀŠD =^50.6 (c=0.57, CHCl3). H NMR (300 MHz,
CDCl3): 1.19 (d, 3H, J=6.4 Hz, CH3), 1.94 (s, 3H, CH3), 2.33 (dd, 1H, J=15.7 Hz, J=9.1 Hz,
H-CH2), 2.46 (dd, 1H, J=15.7 Hz, J=2.9 Hz, H-CH2), 4.17 (m, 1H, CH-OH), 4.32 (br s, 1H),
4.99 (s, 1H, CH), 5.35 (br s, 1H), 9.75 (br s, 1H). 13C NMR (75 MHz, CDCl3): 22.2 (CH3), 22.6
(CH3), 48.5 (CH2), 65.4 (CH), 95.4 (CH), 162.7 (CNH2), 198.3 (CO). MS (EI) m/z: 143 (12.97%,
+
M+), 128 (1.32, C6H10NO2 ), 84 (100, C4H6NO+), 42 (17.17, C2H2O+). IR (KBr): 3405, 2959,
2919, 1620, 1534, 1414, 1295, 1076, 937, 639 cm^1.
3.4.2. 2-Amino-6-p-tolyl-4-oxo-2-hexen-6-ol (R)-7
The previously described procedure led to the compound (R)-7 in a yield of 82% as a white
25
solid, mp 138±139ꢀC (hexane±isopropanol) (‰ꢀŠD =+44.4 (c=0.5, CHCl3). H NMR (300 MHz,
CDCl3): 1.93 (s, 3H, CH3), 2.33 (s, 3H, CH3-Ph), 2.63 (d, 2H, J=6.3 Hz, CH2), 4.65 (br s, 1H),
5.00 (s, 1H, CH), 5.07 (dd, 1H, J=6.3 Hz, CH-OH), 5.26 (br s, 1H), 7.13±7.29 (m, 4H, H-arm),
9.78 (br s, 1H). 13C NMR (75 MHz, CDCl3): 21.1 (CH3-Ph), 22.3 (CH3), 49.3 (CH2), 71.3 (CH),
95.6 (CH), 125.6 and 128.9 (CH-arm), 136.8 and 140.7 (C-arm), 162.7 (CNH2), 197.9 (CO). MS
(EI) m/z: 219 (18.88%, M+), 134 (3.26, C9H10O+), 99 (12.27, C5H9NO+), 84 (100, C4H6NO+), 42
(28.11, C2H2O+). IR (KBr): 3362, 2952, 2919, 1613, 1534, 1388, 1136, 997, 811, 566 cm^1.
1
3.5. Crystal structure determination for compound (R)-7
Crystals suitable for X-ray study were grown from hexane±isopropanol solutions. A crystal of
dimensions 0.1Â0.2Â0.4 mm3 was attached to a glass ®ber and transferred to a Bruker AXS
SMART 1000 di€ractometer with graphite monochromatized Mo-Ka X-radiation and a CCD
area detector. A hemisphere of the reciprocal space was collected up to 2ꢃ=46.6ꢀ. Raw frame
data were integrated with the SAINT program.20 The structure was solved by direct methods
with SHELXTL.21 An empirical absorption correction was applied with the SADABS program.22
All non-hydrogen atoms were re®ned anisotropically. Hydrogen atoms were set in calculated
positions and re®ned as riding atoms, with a common thermal parameter. All calculations were
made with SHELXTL. Final R values were R1=0.0324 for 1342 re¯ections with I>2ꢄ(I), and
wR2=0.0806 (for all 1695 data). Additional material is available from the Cambridge Crystallo-
graphic Data Centre, Cambridge, UK, including atomic coordinates, thermal parameters and a
full list of bond lengths and angles.
Acknowledgements
We are grateful to Professor Vicente Gotor for the generous gift of lipases used in this investi-
gative work. Alicia Maestro would also like to thank Professor Gotor for accepting her into his
investigation group during the postdoctoral period. We also thank Daniel Miguel San Jose
for the crystal X-ray analysis, and ®nancial support from the JCYL, Spain (Project VA08/
00B).

J. A. Fuentes et al. / Tetrahedron: Asymmetry 11 (2000) 2565±2577
2577
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