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Chemoenzymatic asymmetric total synthesis of (S)-Rivastigmine

using x-transaminaseswzMichael Fuchs,a Dominik Koszelewski,b Katharina Tauber,a Wolfgang Kroutila and

Kurt Faber*a

Received 25th March 2010, Accepted 26th April 2010

First published as an Advance Article on the web 11th May 2010

DOI: 10.1039/c0cc00585a

A straightforward, high-yielding, chemoenzymatic total synthesis

of enantiopure (S)-Rivastigmine was developed using various

x-transaminases for the asymmetric amination of appropriate

acetophenone precursors. Optimisation of the biotransformation

allowed scale-up and the total synthesis of (S)-Rivastigmine.

(S)-Rivastigmine {(S)-3-[1-(dimethylamino)-ethyl]phenyl ethyl-

(methyl)carbamate (1)} represents one of the most potent

drugs for the treatment of Alzheimer’s disease at early stages1,2

and it also exerts a beneficial influence on dementia of

Parkinson patients.3 Trials proved that the (S)-enantiomer

exhibits the desired cholinesterase inhibition, which requires

the drug in enantiomerically pure form.1

To date, enantiomerically pure (S)-Rivastigmine is obtained

via racemate resolution using various chiral acids,4 through

transition metal catalysis5,6 or via lipase-catalyzed kinetic

resolution of hydroxy-precursors.7 All of these methods have

certain drawbacks, such as complex sequential operations,

trace impurities of metals, or multiple crystallisation steps

involving diastereomeric salts.

Alternatively, newly developed biocatalytic processes based

on o-transaminases have demonstrated to be an efficient tool

for the preparation of enantiopure amines.8 In this process,

transamination of ketones gives direct access to the corres-

ponding a-chiral primary amines, thereby breaking the limits

of 50% maximum yield of racemate resolution. The challenge

in enzymatic transamination is to shift the unfavourable

equilibrium, which is on the substrate side, i.e. the ketone

substrate and the amine donor alanine.

Various approaches have proven to be successful, mainly by

pulling the equilibrium via removal of the formed coproduct

pyruvate. Enzyme cascade reactions using either pyruvate

dehydrogenase9,10 or pyruvate decarboxylase have been

employed,11 and the recycling of alanine using an amino acid

dehydrogenase in the presence of ammonium to directly

regenerate the amine donor alanine has been also demon-

strated as a formal reductive amination protocol.10,12

Due to the facile substrate-inhibition of transaminases,13

acetophenone derivatives are generally poor substrates, but we

envisaged that a combination of the appropriate type of

substituent(s), optimization of the reaction conditions and a

suitable enzyme might give access to the enantiopure amine

precusor 3 and the (S)-enantiomer of Rivastigmine (1).

For an initial screening of unprotected 3-hydroxyaceto-

phenone (2a) seven different transaminases were used:

o-transaminases from Bacillus megaterium SC6394 (BM-oTA),14

from Chromobacterium violaceum DSM 30191 (CV-oTA),15

and from Arthrobacter species CNB05-01 (ArS-oTA)16 as

well as 4 commercial enzymes [Vibrio fluvialis o-transaminase

(Vf-oTA), ATA-113, ATA-114, and ATA-117] obtained

from Codexis. In order to shift the equilibrium towards the

amine product by removal of the co-product pyruvate,

we chose lactate dehydrogenase combined with the glucose

dehydrogenase/glucose recycling system to regain the nicotin-

amide adenine dinucleotide cofactor (NADH, see Scheme 1).

Initial results showed that the stereoselectivities for

m-hydroxyacetophenone (2a) were high, but the conversions

were low. Only Vf-oTA gave amine 3a in 64% (Table 1,

entries 1–4). In order to improve the reaction rates, we focused

on the employment of cosolvents to increase the solubility of

the substrates (all substrates in this study were insoluble in the

reaction buffer) and substrate-engineering as the two major

strategies. Since dimethylsulfoxide (DMSO) exerted a negative

effect (Table 1, entry 5), we focused on substrate engineering

Scheme 1 Transamination of acetophenone derivatives using

o-transaminases.

aDepartment of Chemistry, Organic & Bioorganic Chemistry,University of Graz, Heinrichstrasse 28, A-8010 Graz, Austria.E-mail: Kurt.Faber@Uni-Graz.at; Fax: +43 316-380-9840;Tel: +43 316-380-5332

bAustrian Centre of Industrial Biotechnology, Heinrichstrasse 28,A-8010 Graz, Austria

w This article is part of the ‘Enzymes and Proteins’ web-theme issue forChemComm.z Electronic supplementary information (ESI) available: Detailedresults from screening experiments, experimental data, NMR spectraand selected chromatograms. See DOI: 10.1039/c0cc00585a

5500 | Chem. Commun., 2010, 46, 5500–5502 This journal is �c The Royal Society of Chemistry 2010

COMMUNICATION www.rsc.org/chemcomm | ChemComm

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with respect to the protective group masking the phenolic

OH-moiety.17 Thus, various protective groups were tested,

including the trimethylsilyl- (TMS, 2b) and the methyl-

group (2c).

Substrate 2b was not accepted by all o-transaminases

(Table 1, entries 6 and 7, detailed data are given in the ESIz).On the other hand, O-methyl-protected substrates gave good

to excellent conversions and moderate to excellent enantio-

selectivities (Table 1, entries 8–11). Again, DMSO did

not have any beneficial effects on the reaction rate (Table 1,

entries 12 and 13), although it considerably increased the

enantiomeric excess (ee) from 66 to 99% in the case of

Vf-oTA (Table 1, entry 12). On the other hand, ATA-117

showed slightly reduced enantioselectivity under these condi-

tions (Table 1, entry 13). Changes in the enantioselectivity of

biocatalysts induced by cosolvents have been frequently

observed for various enzymes, such as nitrile- and epoxide-

hydrolases18 and lipases.19

Next, we investigated the substitution pattern on the aryl

moiety by using p-methoxy- (2d) and o-methoxyacetophenone

(2e). All o-transaminases showed significantly reduced

activities for the para-substituted substrate 2d (Table 1,

entries 14 and 15). On the other hand, the o-substrate 2e gave

full conversion with excellent (S)-selectivity using Vf-oTA,

whereas ATA-114 showed again a significant drop in activity

(Table 1, entries 16 and 17). This strong impact of subtle

changes in the substitution pattern of the aryl moiety is

presumably due to steric effects: the less sterically demanding

substrate 2c gave good conversions and high enantio-

selectivities with ATA-114 and Vf-oTA in the presence of

DMSO. The stereo-complementary (R)-selective ATA-117

gave the mirror image product in 93% ee (Table 1, entry 11).

Encouraged by these results, we envisaged protective groups

that can be cleaved under milder conditions than the methyl-

group.5–7 In this context, the methoxymethyl ether (MOM)

group proved to be perfectly suitable (Table 1, entries 18–20),

giving higher conversions and perfect enantioselectivities in

case of Vf-oTA (Table 1, entries 9–18). The origin of this

selectivity-enhancement is most probably due to the increased

steric requirements of the MOM-group. Again, (S)-3f was

predominantly formed, only ATA-117 gave the (R)-enantiomer

(detailed data are given in the ESIz). With the optimized

conditions in hand, the reaction was performed at 50 mmol

scale using (S)-selective Vf-oTA and (R)-selective ATA-117 in

order to access both enantiomers (Table 1, entries 21 and 22).

In both cases conversion as well as enantioselectivity were

similar to those from screening experiments, while isolated

yields were acceptable.

To complete synthesis of (S)-1, precursor (S)-3f (499% ee)

was N-methylated in the presence of formaldehyde and

sodium triacetoxyborohydride in 95% yield (Scheme 2).7

O-Deprotection was achieved quantitatively using acidic condi-

tions (aqueous hydrochloric acid, 5 M) at room temperature.

Table 1 Stereo-complementary enzymatic amination of aceto-phenones 2a–f yielding (R)- or (S)-a-methylbenzylamines 3a–fa

Entry Enzyme Substrate R = Productb Conv.c (%) eed (%)

1 CV-oTAj m-OH (2a) 3a 29 4992 Vf-oTA m-OH (2a) 3a 64 983 ATA-114 m-OH (2a) 3a 15 4994e ATA-117 m-OH (2a) 3a 33 985f Vf-oTA m-OH (2a) 3a 24 986 Vf-oTA m-OTMS (2b) 3b n.c. n.d.7 ATA-114 m-OTMS (2b) 3b n.c. n.d.8 CV-oTAj m-OMe (2c) (S)-3c 60 4999 Vf-oTA m-OMe (2c) (S)-3c 86 6610 ATA-114 m-OMe (2c) (S)-3c 87 49911e ATA-117 m-OMe (2c) (R)-3c 58 9312g Vf-oTA m-OMe (2c) (S)-3c 89 49913e,g ATA-117 m-OMe (2c) (R)-3c 45 8614 Vf-oTA p-OMe (2d) 3d 20 49915 ATA-114 p-OMe (2d) 3d 10 49916 Vf-oTA o-OMe (2e) 3e 499 49917 ATA-114 o-OMe (2e) 3e 13 49918 Vf-xTA m-OMOM (2f) (S)-3f 96 499

19 ATA-114 m-OMOM (2f) (S)-3f 81 49920e ATA-117 m-OMOM (2f) (R)-3f 63 9421h Vf-xTA m-OMOM (2f) (S)-3f 99 (80)i 499

22e,h ATA-117 m-OMOM (2f) (R)-3f 76 (56)i 98

a Reaction conditions: substrates 2a–f (50 mM), L-alanine (250 mM),

crude o-transaminase (10 mg), LDH mix (30 mg) containing LDH,

GDH, glucose and NAD+, phosphate buffer (100 mM, pH 7.0, 1 mM

pyridoxal 50-phosphate), shaking at 30 1C for 24 h. b Absolute

configuration is only given for compounds with determined [a]20Dvalue. c Determined via GC-analysis; n.c. = no conversion. d Deter-

mined via GC-analysis using a chiral stationary phase. n.d. = not

determined. e D-Alanine was employed. f DMSO (10 v/v%) was used

as cosolvent. g DMSO (5 v/v%) was used as cosolvent. h Experiment

was upscaled to 100 mg scale. i Isolated yields are given in parentheses.j Whole cell system (30 mg) was used.

Scheme 2 Chemoenzymatic asymmetric synthesis of (S)-Rivastigmine (1).

This journal is �c The Royal Society of Chemistry 2010 Chem. Commun., 2010, 46, 5500–5502 | 5501

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Since the latter deprotection step could be easily incorporated

into the work-up of the N-methylation step (see ESIz), thereaction sequence was shortened to furnish (S)-5 in 92% yield

over 2 steps. O-Carbamoylation of the latter was achieved by

treatment with N-ethyl-N-methylcarbamoyl chloride (7),

which was generated from triphosgene (6) and N-ethyl-N-

methylamine, giving (S)-Rivastigmine (1) in 97% yield and

enantiopure form (ee 4 99%).7 It is noteworthy that purifica-

tion of all intermediates was achieved by simple extraction and

washing operations without the necessity for chromatography.

In summary, a highly stereoselective and short chemo-

enzymatic synthesis of (S)-Rivastigmine (1) was developed

with an overall isolated yield of 71% via a four step procedure.

The key building block (S)-3b was derived via enzyme-

catalyzed asymmetric transamination of a structurally tuned

ketone using o-transaminase from V. fluvialis, whereas the

mirror-image product (R)-3b was accessed using the stereo-

complementary enzyme ATA-117. The presented method

towards (S)-Rivastigmine (1) represents the shortest route

published to date.

We would like to acknowledge Klaus Zangger and his

working group for recording the NMR spectra, Barbara

Grischek who supplied us with the freeze dried whole cell

systems and Francesco Mutti for fruitful discussions during

the work.

Notes and references

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H. Stepankova, J. Hajicek, and S. Simek, PCT, WO 2004/037771A1, 2004; H. Stepankova, J. Hajicek and S. Simek, Chem. Abstr.,2004, 142, 6315; D. W. Ma, Q. B. Pan, and S. Pan, PCT,WO 2007/025481, 2007; D. Ma, Q. Pan and S. Pan, Chem. Abstr., 2007, 146,295621; M. J. V. Garrido, A. M. Montserrat, and M. J. Juarez,PCT, WO 2007/014973, 2007; M. Garrido, J. Vicente,A. M. Montserrat and M. J. Juarez, Chem. Abstr., 2007, 146,206113.

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9 D. Koszelewski, I. Lavandera, D. Clay, D. Rozzell andW. Kroutil,Adv. Synth. Catal., 2008, 350, 2761–2766; J.-S. Shin andB.-G. Kim, Biotechnol. Bioeng., 1999, 65, 206–211.

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5502 | Chem. Commun., 2010, 46, 5500–5502 This journal is �c The Royal Society of Chemistry 2010

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