Resolution of 1-(4-amino-3-chloro-5-cyanophenyl)-2-bromo-1-ethanol by lipase mediated enantioselective alcoholysis, hydrolysis and acylation

Article abstract of DOI:10.1016/S0957-4166(98)00252-3

Resolution of 1-(4-amino-3-chloro-5-cyanophenyl)-2-bromo-1-ethanol has been achieved by ethanol enantioselective lipase-catalysed alcoholysis, hydrolysis and acylation. Although a good enantioselectivity was observed in the three reactions, the best results were obtained by hydrolysis. This α- bromohydrin is an intermediate in the synthesis of a new adrenergic agent.

Full text of DOI:10.1016/S0957-4166(98)00252-3

TETRAHEDRON:
ASYMMETRY
Pergamon
Tetrahedron: Asymmetry 9 (1998) 2229–2232
Resolution of 1-(4-amino-3-chloro-5-cyanophenyl)-2-bromo-
1-ethanol by lipase mediated enantioselective alcoholysis,
hydrolysis and acylation
Santiago Conde,a,∗ Marta Fierros,a María Isabel Rodríguez-Franco a and Carles Puig b
aInstituto de Química Médica (C.S.I.C.), Juan de la Cierva 3, 28006 Madrid, Spain
bLaboratorios ALMIRALL-PRODESFARMA, Centro de Investigación, Cardener 68-79, 08024 Barcelona, Spain
Received 29 May 1998; accepted 22 June 1998
Abstract
Resolution of 1-(4-amino-3-chloro-5-cyanophenyl)-2-bromo-1-ethanol has been achieved by ethanol enantio-
selective lipase-catalysed alcoholysis, hydrolysis and acylation. Although a good enantioselectivity was observed
in the three reactions, the best results were obtained by hydrolysis. This α-bromohydrin is an intermediate in the
synthesis of a new adrenergic agent. © 1998 Elsevier Science Ltd. All rights reserved.
1. Introduction
Many 2-amino-1-arylethanols derived from (R)-adrenaline 1a and (R)-noradrenaline 1b (Fig. 1) as the
parent compounds, are important adrenergic drugs. Studies on these drugs showed that the biological
activity resides mostly in their (R)-enantiomer while the (S)-isomer is usually less active or may even
cause undesired side effects.1 This fact and the current US Food & Drug Administration guidelines2
for the marketing of chiral drugs have stimulated the development of new biocatalytic methods to obtain
pure enantiomers. In this regard, lipases are widely used in organic chemistry in the resolution of racemic
mixtures because of their low cost and high stereoselectivity combined with a high catalytic activity in
organic media under very mild experimental conditions.3 These outstanding catalytic features have also
been used for obtaining enantiomerically pure adrenergic agents by acylation4 of the hydroxy group of the
intermediate halohydrin or by hydrolysis5 or alcoholysis6 of its esters. In this paper we present our studies
aimed at obtaining the enantiomerically pure intermediate bromohydrin 2a in the synthesis of LAS
32.5217 2b, a new molecule whose (R)-enantiomer displays interesting bronchial β2-adrenergic agonist
properties, by enantioselective enzyme-catalysed reactions. Enantiopure 2a was obtained chemically by
∗
Corresponding author. E-mail: iqmcr21@pinar1.csic.es
0957-4166/98/$ - see front matter © 1998 Elsevier Science Ltd. All rights reserved.
PII: S0957-4166(98)00252-3

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S. Conde et al. / Tetrahedron: Asymmetry 9 (1998) 2229–2232
Fig. 1.
asymmetric reduction of the corresponding α-bromoketone 3 with a D-proline-borane agent,8,9 a chemo-
and enantioselective but highly expensive reactant.
Results of the enzymatic reaction were not predictable because, in spite of the above-mentioned
previous use of lipases in this field, to our knowledge, there are no examples of the resolution of these
racemic alcohols when the aromatic ring is trisubstituted or when one substituent, even a unique one, is
an unprotected amino group. A double strategy of enantioselective acylation and alcoholysis has been
developed.
2. Enantioselective hydrolysis of the racemic ester (ꢀ)-4 (Scheme 1)
2.1. Chemoselective reduction of the α-bromoketone 3
This reaction must be carried out under controlled experimental conditions in order to reduce the
keto group while the bromine atom remains untouched. NaBH4 (1 mmol) was slowly added to a stirred
suspension of the α-bromoketone 3 (1 mmol) in ice-cooled anhydrous MeOH (5 mL). The reaction
mixture become clear when the ketone disappeared (TLC, hexane:EtAcO=4:1) in about 15 min. A small
amount of silica gel was then put into the flask and stirred while the solvent was evaporated under
vacuum. The remaining solid was eluted in a silica gel column (hexane:EtAcO=6:1) to yield the racemic
bromoalcohol 2a (68%).
Scheme 1.
The experimental conditions of this reaction are rather critical: insufficient anhydrous MeOH, higher
temperature or a standard extracting–washing process afforded a mixture of countless by-products, with
the predominant one (up to 15%) isolated and identified10 as the α-methoxy derivative 5 (Fig. 2).
2.2. Acetylation of the bromohydrin (ꢀ)-2a
A nearly quantitative yield of (ꢀ)-411 was obtained when a solution of (ꢀ)-2a (300 mg) in pyridine (2
mL) was treated with Ac2O (3 mL) at room temperature for 24 h (TLC-control, hexane:EtAcO=4:1).

S. Conde et al. / Tetrahedron: Asymmetry 9 (1998) 2229–2232
2231
Fig. 2.
2.3. Enantioselective lipase-catalysed cleavage of (ꢀ)-4
2.3.1. Alcoholysis
Our search was initially directed towards the alcoholysis. Several commercially available lipases (20
mg/mL) were checked in an iPr2O solution of (ꢀ)-4 (10 mM) and nBuOH (50 mM) in screw-cap 2 mL
vials, sealed, shaken for 24 h at 45°C and then analyzed by TLC (hexane:AcOEt=4:1). A low conversion,
as judged visually, had taken place with the lipase of Candida rugosa and Amano’s Pseudomonas lipases
P and PS while a great transformation appeared in the lipase B of Candida antarctica12 (CAL from
here on) reaction. This reaction was repeated and analysed by chiral HPLC:13 after 4 days and at 53.5%
conversion, (−)-(R)-4, 42%, ee=99%, and (+)-(S)-2a, 50%, ee=86% (E=75) were afforded.14,15
2.3.2. Hydrolysis
Hydrolytic resolution of (ꢀ)-4 was also studied. The four above-mentioned lipases were checked in
three different media: tBuOH:phosphate buffer (pH 7.0)=9:1, buffer-saturated iPr2O and neat buffer
suspension. Very good results were obtained when CAL (20 mg/mL) was added to a solution of (ꢀ)-
4 (20 mM) in buffer-saturated iPr2O in 2 mL screw-cap vials, sealed and stirred at 60°C: after 24 h and
at 49.6% conversion, (−)-(R)-4, 49%, ee=96%, and (+)-(S)-2a, 47%, ee=98% (E≥200) were afforded.
From an economical and easy-to-handle point of view, it is desirable to work at the highest possible
substrate concentration in order to reduce the volume of solvent. Five concentrations (20, 50, 100, 250
and 500 mM) were tested with increasing amounts of enzyme (25, 50 and 125 mg/mL) and, from the
results obtained, the reaction was carried out on a preparative scale: CAL (250 mg) was added to a (ꢀ)-4
(250 mM) iPr2O:dioxane:buffer solution (90:9:1) (2 mL) and stirred for 2 days (conversion about 52%)
at 60°C. The enzyme was filtered off and washed with EtAcO. The combined organic extracts were
evaporated to dryness and the resulting residue eluted (hexane:EtAcO=6:1) in a silica gel column. (−)-
(R)-4 was obtained in 42% yield, ee=99%, [α]D=−68.8 (c=0.5, MeOH). (+)-(S)-2a was also obtained,
41%, ee=96%, [α]D=+23.3 (c=0.5, MeOH). The recovered enzyme displayed a remaining activity of
93% when it was measured with a standard procedure16 of alcoholysis of tributyrin.
3. Enantioselective acylation of the racemic alcohol (ꢀ)-2a (Scheme 2)
Resolution of the racemic α-bromohydrin (ꢀ)-2a via enantioselective lipase-catalysed acylation was
also investigated. The four above-mentioned lipases were checked under acylating conditions. They were
added (20 mg/mL) to solutions of (ꢀ)-2a (20 mM) and trifluoroethyl butyrate or vinyl acetate (50 mM)
in anhydrous iPr2O or in neat vinyl acetate as the acyl donor and organic medium, in screw cap 2 mL
vials, sealed and shaken for 24 h at 40°C. Results appeared promising by TLC when vinyl acetate was
used as the acyl donor and solvent in reactions catalysed by CAL and both lipases PS and P. When these
three reactions were repeated and analysed by chiral HPLC, the best results were obtained in the lipase
P reaction: conversion after 5 days was 53%, 47.1% of (−)-(R)-2a, ee=96%, E=85.

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S. Conde et al. / Tetrahedron: Asymmetry 9 (1998) 2229–2232
Scheme 2.
On a preparative scale, lipase P (400 mg) was added to a solution of the racemic alcohol (ꢀ)-2a (110
mg, 100 mM) in vinyl acetate (4 mL) and the resulting suspension was incubated at 40°C for 5 days
(conversion 55%). The enzyme was filtered off and washed with EtOAc. The combined organic extracts
were evaporated to dryness and the resulting residue was eluted (hexane:EtOAc=6:1) on a silica gel
column. (−)-(R)-2a was obtained in 44% yield, ee=97%, [α]D=−27 (c=0.5, MeOH). (+)-(S)-3 was also
obtained, 46%, ee=90%, [α]D=+69.2 (c=0.5, MeOH). The recovered enzyme retained 89% of its original
activity.
Acknowledgements
We want to thank Novo Nordisk Bioindustrial S.A. for the generous gift of Novozym 435.
References
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