Enzymatic resolution of benzothiazepine for the preparation of squalene synthetase inhibitors

Article abstract of DOI:10.1016/S0957-4166(00)00458-4

A key intermediate with a 4,1-benzothiazepine skeleton, useful for the synthesis of potent squalene synthetase inhibitors, has been prepared via enzymatic resolution providing excellent yield and enantiomeric purity.

Full text of DOI:10.1016/S0957-4166(00)00458-4

TETRAHEDRON:
ASYMMETRY
Pergamon
Tetrahedron: Asymmetry 11 (2000) 4447–4450
Enzymatic resolution of benzothiazepine for the preparation
of squalene synthetase inhibitors
Xiaojing Yang, Leanne Buzon, Ernie Hamanaka and Kevin K.-C. Liu*
Pfizer Central Research, Eastern Point Road, Groton, CT 06340 USA
Received 30 October 2000; accepted 6 November 2000
Abstract
A key intermediate with a 4,1-benzothiazepine skeleton, useful for the synthesis of potent squalene
synthetase inhibitors, has been prepared via enzymatic resolution providing excellent yield and enan-
tiomeric purity. © 2000 Elsevier Science Ltd. All rights reserved.
Over the last decade, industrial as well as academic investigators have intensified their
research efforts toward the discovery of squalene synthetase (SQS) inhibitors. This focus on SQS
inhibitors derives from their potential as anti-atherosclerotic and hypocholesterolemic agents:1,2
SQS is a key enzyme in the cholesterol biosynthetic pathway that catalyzes the reductive
dimerization of farnesyl diphosphate to squalene, the first compound committed toward sterol
production. The position of SQS in the cascade offers potential advantages over HMG-CoA
reductase inhibitors, such as pravastatin, lovastatin and simvastatin, in that the biosynthesis of
isoprenoid products and isoprenylated protein are not affected.1,2 Therefore, the hope for SQS
inhibitors becoming the next generation of cholesterol-lowering drugs1,2 is high.
CP-340868 10, a 5-(1-naphthyl)-4,1-benzothiazepine derivative, was discovered at Pfizer as a
potent SQS inhibitor;2b however, its enantiomer was an almost four-fold less active SQS
inhibitor (90 nM versus 23 nM, Scheme 1). CP-340868 was derived from the 5-(1-naphthyl)-4,1-
benzothiazepine carboxylic acid 9. Therefore, an efficient preparation of enantiomerically pure
compound 9 was crucial for the synthesis of this series of SQS inhibitors for further evaluation.
Recently hydrolytic enzymes as tools for enantiomeric resolution have been utilized more
frequently as demand for pure enantiomers in pharmaceuticals and agrochemicals3,4 has
increased. In addition, mild and environmentally friendly reaction conditions, as well as
recoverable and inexpensive biocatalysts used in the reactions, have made the enzymatic
resolution an attractive alternative to other traditional chemical resolutions. Here we report the
use of lipases to prepare the enantiomeric pure compound 9 providing excellent yield and
* Corresponding author. Tel: 1-860-4415498; fax: 1-860-7158070; e-mail: kevin k liu@groton.pfizer.com
– –
0957-4166/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0957-4166(00)00458-4

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X. Yang et al. / Tetrahedron: Asymmetry 11 (2000) 4447–4450
enantiomeric excess. This is the first time that a compound with 4,1-benzothiazepine structure
has been resolved successfully through the use of lipases.
Scheme 1.
The racemic key intermediate 8 was prepared as shown in Scheme 2.2b,c The synthesis started
with the Friedel–Crafts hydroxyalkylation of 4-chlorophenyl-2,2-dimethylpropyl amine 1, pre-
pared by reductive amination of p-chloroaniline and trimethylacetaldehyde (NaBH4, HOAc,
96%), with 1-naphthaldehyde 2. The resulting compound 3 was treated with mercaptoacetic acid
to produce the corresponding thiacarboxylic acid 4. Subsequently, carbodiimide (morpho-CDI)
was employed for the intramolecular amidation to provide the 4,1-benzothiazepine derivative 5.
Scheme 2. (a) BCl3, CH2Cl2, benzene, Et3N, 80%; (b) mercaptoacetic acid, 6N HCl, 100°C, 90%; (c) morpho-CDI
(CMC metho-p-toluenesulfonate), CH2Cl2, 75%; (d) LDA, THF, t-butyl bromoacetate, 85%; (e) K2CO3, MeOH,
65°C, 80%; (f) i. CF3COOH, CH2Cl2, 95%. ii. H+, ROH, 80–95%

X. Yang et al. / Tetrahedron: Asymmetry 11 (2000) 4447–4450
4449
Compound 5 was then reacted with t-butyl bromoacetate to yield the alkylated product, 6. The
pure trans-isomer, 7, was obtained after recrystallization. To prepare the enantiomerically pure
9, compound 7 was transesterified to a variety of esters, 8, to investigate their enzymatic
resolution.
The asymmetric hydrolysis of the various esters by a variety of lipases and esterases was
investigated at three different temperatures (25, 37 and 45°C), with or without organic
co-solvents (THF, acetone, dioxane and DMF). Due to the poor solubility and high crystallinity
of compound 9, elevated temperatures, adequate stirring and an organic co-solvent were
required in order for the enzymatic reaction to proceed with a reasonable rate and yield. Among
the different reaction conditions that were tried, the optimal condition for enzyme-catalyzed
hydrolysis was at 37°C in 10% DMF as the co-solvent. In general, lipases produced better results
than esterases; therefore, we directed our efforts toward lipase-catalyzed hydrolysis. Because of
the slow lipase-catalyzed hydrolysis that we observed with the methyl ester, we investigated
esters such as chloromethyl and trifluoromethyl esters, which are more susceptible to hydrolysis
but stable in an aqueous solution. Two lipases, rhizopus arrhizus and FAP-15, provided
promising results when the chloromethyl ester was used as the substrate (Table 1). These two
reactions were then scaled up to gram scale: 45% yield (90% theoretical yield) and 98% e.e.5 of
9 was obtained when lipase FAP-15 was used; 36% yield (72% theoretical yield) and 99.3% e.e.
of 9 was obtained with the lipase from rhizopus arrhizus. Because of the reproducible results
from this large scale reaction, compound 9 was prepared efficiently through the lipase FAP-156
catalyzed resolution and was used for making other SQS inhibitors by modifying the carboxylic
acid functionality for further structure–activity relationship studies.
Table 1
Enzymatic resolution of benzothiazepine
Enzyme
Conversion (%)
E.e.p (%)
E.e.s (%)
ORꢀOCH2CH2Cl
Lipase from rhizopus arrhizus
Lipase FAP-15
36
99
98
50
72
45
ORꢀOMe
Lipase MAP-10
Lipase (mucor javanicus)
30
40
85
90
20
60
ORꢀObu
Lipase (rhizopus arrhizus)
36
90
55
ORꢀOCH2CF3
20
30
Lipase N
Lipase (rhizopus arrhizus)
92
90
31
30
*E.e.=enantiomeric excess (see reference 5 for details).

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X. Yang et al. / Tetrahedron: Asymmetry 11 (2000) 4447–4450
In summary, we have found an efficient enzymatic resolution for the preparation of a key
intermediate with the 4,1-benzothiazepine skeleton providing excellent yield and enantiomeric
excess. We believe that this mild enzyme-catalyzed ester hydrolysis can be applied to other
compounds with similar structures for enantiomeric resolution purposes.
References
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5. (a) The absolute configuration of 9 is determined according to reference 2c; (b) Chiralpak AD (4.6×250) HPLC
column was used to analyze the e.e. of 9. Mobil phase: 950 ml of hexane, 50 ml of 2-propanol and 1 ml of
trifluoroacetic acid. Flow rate: 1.0 ml/min; rt; wavelength: 220 nm. The retention time for 8b is 20 min and its
enantiomer is 15.9 min.
6. Lipases were purchased from Amano Co Japan.
.