Enzymatic ammonolysis of ethyl (±)-4-chloro-3-hydroxybutanoate. Chemoenzymatic syntheses of both enantiomers of pyrrolidin-3-ol and 5- (chloromethyl)-1,3-oxazolidin-2-one

Article abstract of DOI:10.1016/S0957-4166(99)00034-8

Lipase B from Candida antarctica efficiently catalysed the kinetic resolution of ethyl (±)-4-chloro-3-hydroxybutanoate through an ammonolysis reaction. Using this methodology, both enantiomers of 4-chloro-3- hydroxybutanamide were prepared and converted into pyrrolidin-3-ol and 5- (chloromethyl)-1,3-oxazolidin-2-one by simple processes consisting of a reduction reaction and a Hofmann rearrangement, respectively.

Full text of DOI:10.1016/S0957-4166(99)00034-8

Pergamon
Tetrahedron: Asymmetry 10 (1999) 721–726
Enzymatic ammonolysis of ethyl (ꢀ)-4-chloro-
-hydroxybutanoate. Chemoenzymatic syntheses of both
enantiomers of pyrrolidin-3-ol and 5-(chloromethyl)-
3
1
,3-oxazolidin-2-one
∗
∗
Eduardo García-Urdiales, Francisca Rebolledo and Vicente Gotor
Departamento de Química Orgánica e Inorgánica, Universidad de Oviedo, 33071 Oviedo, Spain
Received 15 December 1998; accepted 26 January 1999
Abstract
Lipase B from Candida antarctica efficiently catalysed the kinetic resolution of ethyl (ꢀ)-4-chloro-3-
hydroxybutanoate through an ammonolysis reaction. Using this methodology, both enantiomers of 4-chloro-3-
hydroxybutanamide were prepared and converted into pyrrolidin-3-ol and 5-(chloromethyl)-1,3-oxazolidin-2-one
by simple processes consisting of a reduction reaction and a Hofmann rearrangement, respectively. © 1999
Elsevier Science Ltd. All rights reserved.
1. Introduction
Lipase-catalysed kinetic resolution of racemates is one of the most frequently used strategies for the
preparation of optically active compounds. A great variety of carboxylic acids, esters, alcohols and, in
minor extensions, amides and amines have been obtained by esterification, hydrolysis and aminolysis
processes. In recent years, we and other authors have shown the efficiency of the lipase B from Candida
antarctica (CALB) to catalyse ammonolysis reactions. The preparation, under very mild conditions, of
1
2
different N-unsubstituted amides, as well as the resolution and asymmetrisation of esters by means of
this methodology have been some of the contributions to this field.
In connection with these studies, we explore here the potential of CALB to catalyse resolution by
ammonolysis of ethyl (ꢀ)-4-chloro-3-hydroxybutanoate. This substrate has been chosen due to the
further applications of the resulting enantiomerically enriched 4-chloro-3-hydroxybutanamide. Thus
we also report in this work two new chemoenzymatic methods for the synthesis of both enantiomers
of pyrrolidin-3-ol and 5-(chloromethyl)-1,3-oxazolidin-2-one. Enantiomerically enriched pyrrolidin-3-ol
∗
Corresponding author. E-mail: vgs@sauron.quimica.uniovi.es
0957-4166/99/$ - see front matter © 1999 Elsevier Science Ltd. All rights reserved.
PII: S0957-4166(99)00034-8

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3
4
has been employed as a precursor of alkaloids, β-proline derivatives and other compounds of medicinal
5
interest, and, more specifically, its (S)-isomer is a constituent of compounds with activity as kappa-
6
receptor agonists. In addition, enantiopure 5-(chloromethyl)-1,3-oxazolidin-2-one has been used as a
precursor of building blocks for hydroxyethylene isosters.7
2. Results and discussion
Since the solvent can have a remarkable effect on the enantiomeric ratio and the catalytic activity of
8
enzymes, we decided to test the effect of different solvents on the ammonolysis of ethyl (ꢀ)-4-chloro-3-
hydroxybutanoate (ꢀ)-1. All the reactions were carried out at room temperature using a freshly prepared
solution of ammonia in the appropriate solvent. In order to avoid the competitive enzymatic hydrolysis of
(
ꢀ)-1, the ammonia was previously dried using BaO as desiccant. As shown in Table 1, CALB retained
its chiral preference in all the solvents used, (S)-4-chloro-3-hydroxybutanamide, (S)-2, being obtained
as the major isomer. However, the reaction rate and enantiomeric ratio (E) varied markedly. Reaction
9
rate changes can be explained in terms of the different solubility of ammonia in each solvent. Thus,
good reaction rates were attained with hydrogen-bond acceptor solvents (Table 1, entries 1, 2 and 5),
especially with tert-butyl alcohol (TBA). Unfortunately, in this medium the lipase showed the poorest
E-value (E=17). The most suitable solvent was 1,4-dioxane (entry 5), which allowed us to achieve both
amide (S)-2 and the remaining ester (R)-1 with high enantiomeric excesses (ee) and yields. By controlling
the extent of conversion the ee of (S)-2 could be slightly improved. Thus, at 34% conversion (S)-2 was
obtained with 93% ee (entry 6). In addition, when the reaction was carried out at 55% conversion (entry
7), the remaining (R)-1 was obtained with 98% ee.
Table 1
Ammonolysis of (ꢀ)-1
Amide 2 was assigned the S-configuration according to the configuration of the remaining ester 1, for
which the R-configuration was established by comparison of its specific rotation with the reported data
10
for (S)-(−)-1. Enantiomeric excesses for (R)-1 and (S)-2 were determined by chiral HPLC analysis of
their O-benzoyl and O-tert-butyldiphenylsilyl derivatives, respectively (see experimental section).
With the aim of obtaining the amide (R)-2, we tried the conventional ammonolysis (NH /MeOH, 4°C)
3
of the enantiomerically enriched ester (R)-1 (98% ee), but after several days of reaction amide (R)-2 was
not detected, (R)-1 being transformed into a mixture of compounds which were not identified. For this
reason, we decided to use CALB to perform the ammonolysis of (R)-1 (Scheme 1). From the results

E. García-Urdiales et al. / Tetrahedron: Asymmetry 10 (1999) 721–726
723
collected in Table 1 it is deduced that, for this purpose, the best solvent is tert-butyl alcohol, because in
this medium the enzyme showed very high activity and low E-value. As expected, the ammonolysis of
(R)-1 was slower than that of the racemic substrate, but, after three days of reaction, amide (R)-2 was
obtained with very high yield and without racemisation as determined by chiral HPLC.
t
Scheme 1. Reagents and conditions: (i) NH
CH Cl , H
3
, CALB, Bu OH, rt; (ii) BH3·THF, reflux; (iii) 6 N HCl; (iv) CbzCl, K
2 3
CO ,
2
2
2
O; (v) TMEDA, MeOH; (vi) PIDA, THF, MeOH
Transformation of (R)-2 into (R)-pyrrolidin-3-ol (Scheme 1) was accomplished by reduction of the
amide group with BH ·THF and subsequent intramolecular nucleophilic subtitution of the chlorine
3
by the resulting amino group. In order to achieve the intermediate 4-amino-1-chlorobutan-2-ol, the
crude material obtained from the acid hydrolysis of the reduction process was treated with benzyl
†
chloroformate, thus obtaining the highly functionalised benzyl carbamate (R)-3. In addition, when
the residue of the acid hydrolysis step was treated with TMEDA, (R)-4-amino-1-chlorobutan-2-ol was
converted into the desired cyclic (R)-pyrrolidin-3-ol, which was isolated as its hydrochloride (R)-4.
Similarly, compound (S)-2 was converted into the corresponding (S)-3 and (S)-4.
Both enantiomers of 5-(chloromethyl)-1,3-oxazolidin-2-one (R)-5 and (S)-5 were obtained from (R)-2
and (S)-2, respectively, by means of a Hofmann rearrangement. Reaction of (R)-2 with Hg(OAc) and
2
NBS in DMF led to (R)-5 in a moderate yield (63%). Nevertheless, a remarkable improvement in the
yield of (R)-5 was reached when the rearrangement was accomplished with iodosobenzene diacetate
11
(
PIDA). In a similar manner, treatment of (S)-2 with PIDA yielded (S)-5.
Optical purities of compounds 4 and 5 were established by comparison of their specific rotations with
the reported data for these compounds (see experimental), showing that no racemisation had taken place.
As compound 4 was obtained without racemisation, compound 3 was assumed to have the same ee as its
precursor compound 2.
In conclusion, we have demonstrated the utility of lipase B from Candida antarctica in the preparation
of (R)- and (S)-4-chloro-3-hydroxybutanamide 2 from (ꢀ)-1. Starting from (R)- and (S)-2, both enan-
tiomers of pyrrolidin-3-ol and 5-(chloromethyl)-1,3-oxazolidin-2-one have effectively been obtained.
The chemoenzymatic syntheses described here constitute an interesting alternative to other multistep
12
syntheses of these compounds.
3. Experimental
Lipase B from Candida antarctica (CALB), Novozym 435, was a gift from Novo Nordisk Co.
All reagents were of commercial quality and were purchased from Aldrich Chemie. Hydroxy ester
†
In this process a small amount (5%) of N-benzyloxycarbonylpyrrolidin-3-ol was also obtained.

724
E. García-Urdiales et al. / Tetrahedron: Asymmetry 10 (1999) 721–726
(
ꢀ)-1 was obtained by reduction (NaBH , EtOH) of the commercially available ethyl 4-chloro-3-
4
oxobutanoate. Solvents were distilled over an adequate desiccant and stored under nitrogen. For column
chromatography, Merck silica gel 60/230–400 mesh was used. Melting points were taken using a
Gallenkamp apparatus and are uncorrected. Optical rotations were measured using a Perkin–Elmer 241
polarimeter. IR spectra were recorded on a Perkin–Elmer 1720-X FT infrared spectrometer. H and
NMR spectra were obtained on a Bruker AC-300 ( H 300 MHz and C 75.5 MHz) and a Bruker AC-
1
13
C
1
13
1
13
2
00 ( H 200 MHz and C 50.3 MHz) spectrometer, using TMS as internal standard. Mass spectra were
recorded on a Hewlett–Packard 5987 A spectrometer. Microanalyses were performed on a Perkin–Elmer
240B elemental analyser.
3.1. General procedure for the ammonolysis of ethyl (ꢀ)-4-chloro-3-hydroxybutanoate [(ꢀ)-1]
Dry ammonia was bubbled through the appropriate solvent (22 mL) for 10 min at room temperature.
Then, ester (ꢀ)-1 (10 mmol) and CALB (1 g) were added and the resulting mixture was shaken at
rt and 250 r.p.m. during the time shown in Table 1. Afterwards, the enzyme was filtered, washed
with dichloromethane and the organic solvents were evaporated. Column chromatography (ethyl acetate
as eluent) of the residue yielded (R)-1 and (S)-2. In all cases (R)-1 was recovered with >95% yield
(calculated taking into account the conversion value).
3
.1.1. (S)-4-Chloro-3-hydroxybutanamide (S)-2
21
Yield, 85% (reaction with 1,4-dioxane at conv.=48%); mp 66–67°C; [α]
−18.7 (c 0.6, CH OH,
3
D
−1
1
9
3
6
3% ee); IR (KBr) 1615, 1659, 3400 cm ; H NMR (CD OD) δ (ppm) 2.54–2.76 (m, 2H, CH ),
3
2
13
.69–3.85 (m, 2H, CH Cl), 4.38 (m, 1H, CH); C NMR (CD OD) δ (ppm) 41.3 (CH ), 49.9 (CH ),
2
3
2
2
+
+
9.8 (CH), 176.3 (C_O); MS (CI, CH ) m/z 138 [(M+1) , 1.6], 140 [(M+3) , 0.5], 88 (100). Anal. calcd
4
for C H ClNO : C, 34.92; H, 5.86; N, 10.18. Found: C, 34.60; H, 5.62; N, 9.79.
4
8
2
3.2. (R)-4-Chloro-3-hydroxybutanamide (R)-2
Compound (R)-2 was obtained from (R)-1 (98% ee) following the procedure described for (ꢀ)-1, using
as solvent tert-butyl alcohol. Reaction time, 72 h; yield, 90%; [α]D23 +20.4 (c 0.8, CH OH, 98% ee).
3
3.3. Benzyl (S)-N-(4-chloro-3-hydroxybutyl)carbamate (S)-3
BH ·THF complex (1.0 M solution in THF) (1.0 mL) was added dropwise under nitrogen to a solution
3
of amide (S)-2 (0.69 mmol) in THF (19 mL) at rt. After being refluxed for 14 h, 6 N HCl (0.15 mL) was
added at rt. The reaction mixture was stirred for 4 h and the solvents were evaporated. The residue was
dissolved in water (3.0 mL) and K CO (1.9 mmol) was added. Immediately afterwards, dichloromethane
2
3
(3.0 mL) and benzyl chloroformate (1.0 mmol) were added at 4°C, and the resulting reaction mixture was
stirred at rt for 14 h. Then, the organic layer was separated and the aqueous layer was extracted again with
dichloromethane. The combined organic extracts were dried and evaporated. Column chromatography of
2
0
the residue (hexane:ethyl acetate 2:1) gave 0.45 mmol (65%) of (S)-3. Oil; [α]
−17.2 (c 1.0, MeOH);
D
−1 1
IR (KBr) 3416, 3346, 1691 cm ; H NMR (CDCl ) δ (ppm) 1.54–1.66 (m, 1H, CHH), 1.69–1.79 (m,
H, CHH), 3.20–3.30 (m, 1H, CHH), 3.38–3.57 (m, 3H, CHH, CH ), 3.67 (bs, 1H, OH), 3.87 (m, 1H,
3
1
2
13
CH), 5.08 (s, 2H, CH ), 5.45 (m, 1H, NH), 7.33 (m, 5H, Ph); C NMR (CDCl ) δ (ppm) 33.9 (CH ),
2
3
2
3
7.5 (CH ), 49.3 (CH ), 66.6 (CH ), 69.0 (CH), 127.8 (CH), 127.9 (CH), 128.3 (CH), 136.2 (C), 156.9
2 2 2
+
+
(C_O); MS (70 eV) m/z 257 (M , 4), 259 [(M+2) , 1.5], 108 (82), 91 (100).

E. García-Urdiales et al. / Tetrahedron: Asymmetry 10 (1999) 721–726
725
3.4. Benzyl (R)-N-(4-chloro-3-hydroxybutyl)carbamate (R)-3
Compound (R)-3 was prepared from (R)-2 as described for (S)-3. [α]D20 +18.8 (c 1.0, MeOH).
.5. (S)-Pyrrolidin-3-ol hydrochloride (S)-4
3
BH ·THF complex (1.0 M solution in THF) (2.3 mL) was added dropwise under nitrogen to a solution
3
of (S)-2 (1.5 mmol) in THF (41 mL) at rt. After being refluxed for 14 h, 6 N HCl (0.30 mL) was added
at rt. The reaction mixture was stirred for 4 h and the solvent was evaporated. The residue was dissolved
in methanol (2.0 mL) and TMEDA (1.7 mmol) was added. After stirring at rt for 18 h the solvents
were evaporated. Column chromatography of the crude product (dichloromethane:methanol:30% aq.
ammonia 2:2:0.1) gave 1.33 mmol (87%) of pyrrolidin-3-ol, which was dissolved in methanol (2.0 mL)
and subsequently treated with 6 N HCl (0.4 mL). After stirring the acid solution at rt for 30 min, the
2
6
solvent was evaporated yielding (S)-4 as a hygroscopic solid. [α]
+8.3 (c 1.9, CH OH); IR (KBr)
3
D
−1 1
3
412, 3005, 1628 cm ; H NMR (CD OD) δ (ppm) 2.21–2.38 (m, 2H, CH ), 3.37–3.45 (m, 2H, CH ),
.56–3.70 (m, 2H, CH ), 4.74 (m, 1H, CH); C NMR (CD OD) δ (ppm) 34.4 (CH ), 45.0 (CH ), 54.3
3
2
2
13
3
2
3
2
2
+
(CH ), 70.8 (CH); MS (NBA matrix) m/z 88 [(M+1) 100], 70 (9).
2
3.6. (R)-Pyrrolidin-3-ol hydrochloride (R)-4
26
12a
Compound (R)-4 was prepared from (R)-2 as described for (S)-4. [α]
−8.5 (c 1.5, CH OH); lit.
3
D
20
[
α]D −7.6 (c 3.45, CH OH, 100% ee).
3
3.7. General procedure for the Hofmann rearrangement of (R)- and (S)-4-chloro-3-hydroxybutanamide
2
Iodosobenzene diacetate (0.67 mmol) was added to a solution of amide 2 (0.51 mmol) in THF:MeOH
2.5:1 (5 mL) at rt. After stirring for 4 h the temperature was raised to 60°C and the reaction was stirred
for 15 min. Immediately afterwards the solvent was evaporated. Column chromatography of the residue
hexane:ethyl acetate 1:1) gave the corresponding product 5.
(
3
.7.1. (S)-5-(Chloromethyl)-1,3-oxazolidin-2-one (S)-5
Obtained from (S)-2. Yield, 92%; [α]D +16.9 (c 0.8, CH Cl ); lit. [α]D25 +19.1 (c 3.3, CH Cl ,
2
5
12b
2
2
2
2
12b
1
1
00% ee). The spectral data for (S)-5 were in accordance with literature values: H NMR (CDCl ) δ
3
13
(
(
[
ppm) 3.50–3.81 (m, 4H, 2CH ), 4.86 (m, 1H, CH), 6.45 (bs, 1H, NH); C NMR (CDCl ) δ (ppm) 43.5
2
3
+
+
CH ), 44.4 (CH ), 74.6 (CH), 159,2 (C_O); MS (CI, CH ) m/z 135 (M , 22), 136 [(M+1) , 8], 137
2
2
4
+
+
(M+2) , 8], 138 [(M+3) , 3], 86 (100).
3
.7.2. (R)-5-(Chloromethyl)-1,3-oxazolidin-2-one (R)-5
Obtained from (R)-2. [α]D −18.1 (c 0.7, CH Cl ); lit. [α]D25 −18.7 (c 3.2, CH Cl , 100% ee).
2
5
12b
2
2
2
2
3.8. Determination of the ee of 1
To a solution of 1 (0.36 mmol) in THF (1.5 mL), pyridine (2.0 mmol) and benzoyl chloride (0.5 mmol)
were added at 0°C. After 12 h at room temperature, the solvent was evaporated and CH Cl was added to
2
2
the residue. The organic solution was successively washed with 2 N HCl and water, and then evaporated

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E. García-Urdiales et al. / Tetrahedron: Asymmetry 10 (1999) 721–726
to yield O-benzoyl-1, which was purified by column chromatography (CH Cl :hexane 1:1 as eluent).
2
2
i
Chiral HPLC analysis (Chiralcel-OD, hexane:Pr OH 95:5, 0.4 mL/min): (R)-O-benzoyl-1, t 20.2 min;
R
(
ꢀ)-O-benzoyl-1, two peaks, t 18.1 and 20.2 min; Rs 2.5.
R
3.9. Determination of the ee of 2
A solution of 2 (0.36 mmol) in THF (1.5 mL) was treated with TBDPSCl (0.73 mmol) and imidazole
(1.46 mmol). When the reaction was finished (12 h, rt), the solvent was evaporated and AcOEt was added
to the residue. The organic solution was washed with 2 N HCl and water, dried and then evaporated to
yield O-TBDPS-2, which was purified by column chromatography (AcOEt:hexane 1:1 as eluent). Chiral
i
HPLC analysis (Chiralcel-OD, hexane:Pr OH 95:5, 0.8 mL/min): (S)-O-TBDPS-2, t 28.5 min; (ꢀ)-O-
R
TBDPS-2, two peaks, t 24.8 and 28.2 min; Rs 2.1.
R
Acknowledgements
We are grateful to the CICYT (Project BIO 98-0770) for financial support, to Novo Nordisk Co. for
the generous gift of the CA lipase, and E. G.-U. thanks the Ministerio de Educación y Ciencia for a
predoctoral fellowship.
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