CA2019167A1 - Tetrazole excitatory amino acid receptor antagonists - Google Patents

Abstract

Abstract of the Disclosure The present invention provides novel tetrazole derivatives useful as excitatory amino acid receptor antagonists and in treating a variety of associated nervous system disorders.

Description

TEI'RAZOLE EXCITATORY AMINO ACID RECEPTOR ANTAGONISTS

European Patent Application 893~1337.5 teaches a series of 4-L(tetrazol-5-yl)alkyl]-2-piperidinecar-boxylic acids which are capable of blocking excitatoryamino acid receptors in mammals. Various degrees of activity are shown to be possessed by the disclosed compounds. A new compound, related to but not taught by the earlier application, has now been made and has activity superior to that of all the previous compounds.
The present invention provides a tetrazole derivative which is an antagonist of excitatory amino acid receptors. More specifically, the present inven-tion relates to the compound cis-(-)-4-[(1(2)H-tetrazol-5-yl)methyl]-2-piperidinecarboxylic acid, or a pharma-ceutically acceptable salt thereof.
The invention also provides pharmaceutical formulations comprising the compound, associated with one or more pharmaceutically acceptable carriers, diluents or excipients therefor.
Further embodiments of the invention include the use of the compound as a pharmaceutical, especially for blocking one or more excitatory amino acid receptors, as well as methods for treating a variety of disorders which have been linked to the excita-tory amino acid receptors including neurological disorders (for example, epilepsy), stroke, anxiety, cerebral ischaemia, muscular spasms and neurodegenerative disorders such as Alzheimer's Disease and Huntington's Disease.

As polnted out above, this invention includes the pharmaceutically acceptable salts of the compounds defined by Formula I. These salts can exist in conjunc-tion with the acidic or basic portion of the molecule and can exist as acid addition, primary, secondary, tertiary or quaternary arnmonium or alkali metal or alkali earth metal salts. Acids commonly employed to form such salts include inorganic acids such as hydro-chloric, hydrobrornic, hydroiodic, sulfuric and phosphoric acid, as well as organic acids such as para-toluenesul fonic, methanesulfonic, oxalic, para-bromophenylsulfonic, carbonic, succinic, citric, benzoic and acetic acid, and related inorganic and organi.c acids. Such pharma-ceutically acceptable salts thus include sulfate, pyrosulfate, bisulfate, sulfite, bisulfite, phosphate, ammonium, monohydrogenphosphate, dihydrogenphosphate, metaphosphate, pyrophosphate, chloride, lithium, bromide, iodide, acetate, magnesium, propionate, tetramethyl-a~monium, decanoate, caprylate, acrylate, formate, isobutyrate, caprate, heptanoate, potassium, propiolate, oxalate, trimethylammonium, malonate, succinate, suber-ate, sebacate, fumarate, maleate, butyne-1,4-dioate, sodium, hexyne-1,6-dioate, benzoate, chlorobenzoate, methylbenzoate, dinitrobenzoate, hydroxybenzoate, methoxybenzoate, phthalate, sulfonate, methylammonium, xylenesulfonate, phenylacetate, phenylpropionate, phenylbutyrate, citra-te, lactate, calcium, ~-hydroxy-butyrate, glycollate, maleate, tartrate, methanesul-fonate, propanesulfonate, naphthalene-1-sulfonate, naphthalene-2-sulfonate, mandelate and the like salts.

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The compound provided by this invention is prepared by a process which comprises reacting an alkyl cis-(-)-4-cyanomethyl-N-vinyloxycarbonyl-2-piperidine-carboxylate with azidotributylstannane and hydrolyæing the resulting intermediate, and salifying if the com-pound in salt form is desired.
More particularly, the process is illustrated by the following Example.

ExamPle 1 cis-(-)-4-[(1(2)H-Tetrazol-5-yl)methyl]-2-piperidinecarboxylic acid A. 4-Hydroxy-2-pyridinecarboxylic acid hydrobromide To a solution of 30.5 g (0.24 mol) of 4-methoxypyridine-N-oxide in 250 ml of methylene chloride was added 30.3 g (0.31 mol, 40.7 ml) of trimethylsilyl cyanide. Approximately five minutes later 32.8 g (0.31 mol, 28.0 ml) of N,N-dimethylcarbamoyl chloride was added in four 7 ml portions over one hour. The resulting mixture was stirred overnight at room tem-perature. To the mixture was carefully added 250 ml of 10% by weight aqueous potassium carbonate. After 15 minutes at room temperature the organic layer was separated and the aqueous layer was extracted twice with methylene chloride and once with diethyl ether. The combined organic extracts were dried over anhydrous magnesium sulfate, filtered and concentrated under 30 vacuum. The residue was dissolved in 150 ml of 48% by J~ g~

X-7264s -4-weight a~ueous hydrobromic acid. The resulting mixture was heated to reflux overnight and cooled to 0C. The crystals that formed were collected by vacuum filtra-tion, washed with diethyl ether, and dried under vacuum at 50C to afford 45.5 g of 4-hydroxy-2-pyridinecarboxylic acid hydrobromide.
B. Ethyl 4-hydroxy-2-pyridinecarboxylate hydrochloride To a 1 1. round bottom flask was added 45.5 g 10 (0.21 mol) of 4-hydroxy-2-pyridinecarboxylic acid hydro-bromide and 500 ml of ethanol saturated with hydro-chloric acid. The mixture was heated to reflux over-night, cooled and concentrated under vacuum to 1/3 of its original volume. After cooling the mixture to about 0C, the resul~ant crystals were collected by vacuum filtration, washed with ethanol and diethyl ether, and dried under vacuum to afford 29.5 g of ethyl 4-hydroxy-2-pyridinecarboxylate hydrochloride.
C. Ethyl c,s-4-hydroxy-N-t-butoxycarbonyl-2-piperidinecarboxylate Ethyl 4-hydroxy-2-pyridinecarboxylate hydro-chloride (27.2 g, 0.13 mol) was hydrogenated in 200 ml of ethanol with 15.5 g of 5% by weight rhodium on alumina at 100C and 1000 p.s.i. for 10 hours. The mixture was cooled, filtered and concentrated under vacuum. To the residue was added 250 ml of methylene chloride, 50 ml of ethanol and 25.2 g (0.20 mol, 34.0 ml) of Hunig's base, followed by the dropwise addition of 28.4 g (0.13 mol, 29.9 ml) of di-t-butyldicarbonate over a period of thirty minutes. After one hour the j i $ ,/ ij !J

mixture was concentrated under vacuum, and the residue was dissolved in methylene chloride and washed twice with 10% by weight aqueous sodium bisulfate. The combined aqueous washes were extracted once with methylene chloride and once with diethyl ether. The organic extracts were combined, dried over anhydrous sodium sulfate, filtered and concentrated under vacuum.
High pressure liquid chromatography of the residue provided 21.3 g of ethyl cis-4-hydroxy-N-t-butoxy-carbonyl-2-piperidinecarboxylate as a colorless oil.
D. Ethyl 4-oxo-N-t-butoxycarbonyl-2-piperi-dinecarboxylate To a 1 l. round bottom flask was added 33.6 g (0.16 mol) of pyridinium chlorochromate, 35 g of powdered 4A molecular sieves and 200 ml of methylene chloride.
After stirring the mixture at room temperature for sixty minutes, a solution of 21.3 g (0.078 mol) of ethyl cis-4-hydroxy-N-t-butoxycarbonyl-2-piperidine-carboxylate in 50 ml of methylene chloride was added.
After stirring the mixture for sixty minutes at room temperature, 700 ml of diethyl ether was added. The mixture was filtered through three-fourths inch of Celite and three-fourths inch of silica gel (230-400 mesh) in a 650 ml medium porosity sintered glass funnel.
The solids were washed with 1 l. of diethyl ether and the filtrate was concentrated under vacuum. To the residue was added 200 ml of diethyl ether and the mixture filtered through three-eighths inch of Celite and three-eighths inch of silica gel (230-400 mesh) in a 150 ml medium porosity sintered glass funnel. The X-7264B -~-solids were washed with 500 ml of diethyl ether and the filtrate was concentrated under vacuum. The residue was purified by high pressure liquid chromatography to provide 14.6 g ~f ethyl 4-oxo-N-_-butoxycarbonyl-2-piperidinecarboxylate as a colorless oil.
E. Ethyl 4-cyanomethylidene-N-t-butoxy-carbonyl-2-piperidinecarboxylate To a suspension of 0.75 g (0~019 mol, 60% by weight in oil) of sodium hydride (washed three times with hexanes) in 40 ml of THF was added 3.34 g (0.019 mol) of diethylcyanomethylphosphonate. After stirring the reaction mixture for thirty minutes at room temper-ature, a solution of 4.26 g (0.016 mol) of ethyl 4-oxo-N-t-butoxycarbonyl-2-piperidinecarboxylate in lO ml of THF was added. The mixture was stirred for 30 minutes at room temperature and 90 minutes at the reflux tempera~
ture of the reaction mixture, then cooled to room temperature and quenched with water. The organic layer was separated and the aqueous layer extracted twice with diethyl ether. The organic extracts were combined, dried over anhydrous magnesium sulfate, filtered and concentrated under vacuum. High pressure liquid chroma-tography of the residue afforded 3.58 g of ethyl 4-cyanomethylidene-N-_-butoxycarbonyl-2-piperidinecar-boxylate.
F. Ethyl cis-4-cyanomethyl-N-t-butoxycar-bonyl-2-piperidinecarboxylate Ethyl 4-cyanomethylidene~N-t-butoxycarbonyl-2-piperidinecarboxylate (9.00 g, 0.031 mol) was hydrogen-ated in 140 ml of ethanol with 0.90 g of 5% by weight X-7264B ~7-palladium-on-carbon at room temperature and 60 p.s.i.
for 60 minutes. The mixture was filtered through Celite and concentrated under vacuum. High pressure liquid chromatography of the residue provided 8.20 g of e-thyl cis-4-cyanomethyl-N-t-butoxycarbonyl-2-piperidine-carboxylate.
G. Ethyl cis-(~)-4-cyanomethyl-N-allyl-2-piperidinecarboxylate To a solution of 19.9 g (67.2 mmol) of ethyl 4-cyanomethyl-N-t-butoxycarbonyl-2-piperidinecarboxylate (prepared as shown in step F) in 100 ml of dichloro-methane was added 50 ml of trifluoroacetic acid (CO2 evolution). The mixture was stirred for 3 hr at room temperature and then was concentrated under vacuum. To the residue was added 100 ml of dichloromethane, and the solution was again concentrated under vacuum. The residue was dissolved in 200 ml of dichloromethane, 200 ml of saturated aqueous sodium bicarbonate was added, and the mixture was stirred for 15 minutes at room temperature. The organic layer was separated and washed with 100 ml of saturated aqueous sodium bicarbonate, and the combined aqueous washes were extracted twice with 100 ml each of dichloromethane and once with 50 ml of diethyl ether. The corr~ined organic extracts were dried 25 over Na2 S04, fil-tered and concentrated to afford 12.7 g (96%) of ethyl ~-cyanomethyl-2-piperidinecarboxylate.
GC analysis showed an 85:15 mixture of cis:trans isomers.
To a solution of 11.6 g (59.1 mmol) of the product in 60 ml of dimethylsulfoxide was added 9.9 g (118.2 mmol) 30 of sodiurn bicarbonate and 5.7 ml (7.9 g, 65.0 mmol) of allyl bramide. After l hr at room ~emperature, another 1.1 ml portion of allyl bromide was added, and after another 2 hours at room temperature, the mixture was poured into lO0 ml of water and 100 ml of brine and was extracted 5 times with 50 ml each of dichloromethane and once with 50 ml of diethyl ether. The combined organics were washed with 100 ml of water, then dried cver Na2 S04, filtered and concentrated. The residue was purified by preparative HPLC to af~ord 8.6 g (62%) of ethyl cis-(_)-4-cyanomethyl-N-allyl-2-piperidlne-carboxylate and 1.2 g (9%) of ethyl trans-(_~-4-cyano-methyl-N-allyl-2-piperidinecarboxylate, both of which were >99.9% one isomer by GC.
H. Ethyl cis-(+~-4-cyanomethyl-N-allyl-2-piperidinecarboxylate di-p-toluoyl-D-and L-tartrate salt A mixture of 7.36 g (31.1 mmol) of the racemic product above, 12.0 g (31.1 mmol) of di-p-toluoyl-D-tartrate and 0.56 ml ~0.56 g, 31.1 mmol) of water were dissolved in ethyl acetate with heating.
The solution was filtered and most of the ethyl acetate was removed to give a final volume of about 50 ml~ The mixture was cooled to room temperature, and the crystals that formed were collected and washed with ethyl acetate, diethyl ether and pentane and dried to 25 afford 13.0 g (67%). The material was recrystallized from ethyl acetate to afford 6.4 g (33%) of the desired (+)-salt, m.p. 142-142.2C, [~]D = +108-9 (c = l, methanol). A small portion of the (+)-salt was free based, and 1H NMR of i-t in d6-benzene with one equivalent of R-(-)-2,2,2-trifluoro-l-(9-anthryl)ethanol showed it to be <97% one enantiomer.

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I. Ethyl cis-(+)-4-cyanomethyl-N-allyl-2-piperidinecarboxylate To a flask were added 6.0 g (9.7 mmol) of the (+)-salt prepared above~ lO0 ml of dichloromethane and lO0 ml of saturated aqueous sodium bicarbonate. The mixture was stirred for lO min at room temperature, the organic layer w~s separated, and the aqueous layer was extracted thrice with 100 ml each of dichlorome-thane and once with 75 ml of diethyl ether. The combinecl organic extracts were dried over Na2SO4, filtered and concentrated. The residue was purified on lO0 g of silica gel, eluting with l/1 ethyl acetate/hexane to afford 2.0 g (89%) of ethyl cis-(+)-4-cyanomethyl-N-allyl-2-piperidinecarboxylate, [~]D = +72.3 (c = l, dichloromethane).
J. Ethyl cis-(-)-4-cyanomethyl-N-vinyloxy-carbonyl-2-piperidinecarboxylate A solution of 2.0 g of the product from step I above, 1.8 g (16.5 mmol~ of vinyl chloroformate and 3.5 g ~16.5 mmol) of 1,8-bis-dimethylaminonaphthalene in 40 ml of dichloromethane was heated to reflux for 6 hr. The mixture was then cooled to ambient temperature and concentrated under vacuum. The residue was dissolved in diethyl ether and was washed twice with 10% aqueous sodium hydrogen sulfate and once with saturated aqueous sodium bicarbonate. The organic layer was dried over magnesium sulfate, filtered and concentrated under vacuum.
Preparative HPLC afforded 1.8 g (79%) of the desired intermediate, ~(Y]D = -24.8D (c = l, dichloromethane).

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K. cis~ 4-[(1(2)H-Tetrazol-5-yl)methyl]-2-piperidinecarboxylic acid A rnixture of 1.6 g (6.2 mmol) of the product from step D and 4.0 g (12.4 mmol) of azidotributylstannane were heated at 60C for 44 hours. The mixture was cooled to ambient temperature. Fifty ml of 6N hydro-chloric acid was added and the mixture was heated for 1.5 hours at 80~C and then at 105C for 3 hours. The mixture was cooled, extracted thrice with diethyl ether, and the aqueous layer was concentrated under vacuum.
The residue was lyophilized and purified by ion exehange chromatography. The purified solid was refluxed in acetone for 1 hr. The solid was washed with aeetone and diethyl ether and dried under vaeuum at 80C to obtain l.Og of the desired product, [~]D = -18.7~ (c =
1, N HCL). m.p. 162-167C (foams). 1H NMR(D2O):
~3.57(dd, J=13.0, 3.1 Hz, lH), 3.44 (bd, J=ll.1 Hz, lH), 2.96 (m, 3H), 2.21 (m, 2H), 1.82(d, J=14.2Hz, lH), 1.40(m, 2~).
As noted above, the compound of this inven tion is an excitatory amino acid antagonist. Therefore, another embodirnent of the present invention is a method of blocking one or more excitatory amino acid receptors in mammals which comprises administering to a mammal requiring decreased excitatory amino acid neurotrans-mission a pharrnaceutically effective amount of a eom-pound of the invention.
The term "pharmaceutically effeetive amount", as used herein, represents an amount of the compound of the invention which is capable of blockin~ one or more excitatory amino acid receptors. The particular dose of compound administered according to this invention will of course be determined by the particular circumstances S surrounding the case, including the compound admin-istered, the route of administration, the particular condition being -treated, and similar considerations.
The compound can be administered by a variety of routes including -the oral, rectal, transdermal, subcutaneous, intravenous, intramuscular or intranasal routes. A
typical daily dose will contain from about 0.01 mg/kg to about 20 mg/kg of the active compound of this invention.
Preferred daily doses will be about 0.05 to about 10 mg/kg, ideally about 0.1 to abou~ 5 mg/kg.
A variety of physiologic functions have been shown to be subject to influence by excessive stimula tion of excitatory amino acid neurotransmission. As such, the compound of the present inventior is believed to have the ability to treat a variety of disorders in mammals associated with this condition which include neurological disorders such as convulsive disorders for example, epilepsy; stroke; anxiety; cerebral ischaemia;
muscular spasms; and neurodegenerative disorders such as Alzheimer's Disease and Huntington's Disease.
Therefore, the present invention also provides methods of treating the above disorders at rates set forth above for excitatory amino acid receptors in mammals.
The following experiment was conductecl to demonstrate the superior ability of the compound of -the present invention to inhibit responses due to excitatory amino acid agonists. A typical receptor substance is characterized by N-me-thyl-D-aspartic acid (NMDA).

Male Charles River CFl mice held in the laboratory for a minimum of three days were housed, 12 per cage, on sawdust bedding in clear plastic boxes with wire mesh lids. Animals were allowed full access to feed and water prior to testing.
Unless otherwise specified, the test compounds were formulated in dimethylsulfoxide (DMSO) and diluted to a 5% DMSO/sterile water solution by volurne. Dosing began at 160 mg/kg. If any significant activity was detected, the test drug dose was divided in half until no more activity was detected. The test compounds were administered using the intraperitoneal injection ~i.p.) route at a volume of 0.01 cc/gm.
Five mice were taken from the plastic cages, dosed with the test compound and placed individually in clear plastic observation cages. After ~ 30 minute drug absorption period, the mice were injected intra-peritoneally with 200 mg/kg of NMDA. This dose of NMDA produces death in more than 95% of control-treated animals. Twenty minutes after the NMDA injection the animals were scored as dead or alive. Data are reported as the minimum effective dose (MED) to block NMDA-induced lethality. Protection from lethality is met by the survival of at least three of the five animals. The data is set forth in Table I below.

Table I
In Vivo NMDA Induced Letha~y Example No. MED
of Compound Tested (mq/kg) In contrast, the MED of the prior art racemic compound, cis-(~)-4-[(1(2)H-tetrazol-5-yl)methyl]-2-piperidinecarboxylic acid, was 10 mg/kg. The superior effect of the presently invented compound is clear.
The compound of the present invention is preferably formulated prior to administration. There-fore, yet another embodiment of the present invention is a pharmaceutical formulation comprising a compound of the invention and a pharmaceutically acceptable carrier, diluent or excipient therefor.
The present pharmaceutical formulations are prepared by known procedures using well known and readily available ingredients. In making the composi-tions of the present invention, the active ingredient will usually be mixed with a carrier, or diluted by a carrier, or enclosed within a carrier which may be in the form of a capsule, sachet, paper or other container.
When the carrier serves as a diluent, it may be a solid, semisolid or liquid material which acts as a vehicle, excipient or medium for the active ingredient. Thus, the compositions can be in the form of tablets, pills, powders, lozenges, sachets, cachets, elixirs, suspen-sions, emulsions, solutions, syrups, aerosol (as a solid or in a liquid medium), ointments containing, for example, up to 10% by weight of the active compound, soft and hard gelatin capsules, suppositories, sterile injectable solutions and sterile packaged powders.
Some examples of suitable carriers, excipi-ents, and diluents include lactose, dextrose, sucrose, sorbitol, mannitol, starches, gum acacia, calcium phosphate, alginates, tragacanth, gelatin, calcium silicate, microcrystalline cellulose, polyvinylpyrroli-done, cellulose, water syrup, methyl cellulose, methyl-and propylhydroxybenzoates, talc, magnesium stearate and mineral oil. The formulations can additionally include lubricating agents, wetting agents, emulsifying and suspending agents, preserving agents, sweetening agents or flavoring agents. The compositions of the invention may be formulated so as to provide quick, sustained or delayed release of the active ingredient after adrninis-tration to the pa-tient by employing procedures well known in the art.
The compositions are preferably formulated in a unit dosage form, each dosage containing from about 5 to about 500 mg, more usually about 25 to about 300 mg, of the active ingredient. The term "unit dosage form"
refers to physically discrete units suitable as unitary dosages for human subjects and other mammals, each unit containing a predetermined quantity of active material calculated to produce the desired therapeutic effect, in association with a suitable pharmaceutical carrier.
The following formulation examples are illus-trative only and are not intended to limit the scope of the invention in any way.
~ormulation 1 Hard gelatin capsules are prepared using the following ingredients:
Quantity ~mg/capsule) Example 1 250 30 starch, dried 200 magnesium stearate 10 Total ~60 mg The above ingredients are mixed and filled into hard gelatin capsules in 460 mg quantities.

Formulation 2 A tablet is prepared using the ingredients below:
Quantity (mq/tablet) 10 Example 1 250 cellulose, microcrystalline400 silicon dioxide, fumed 10 stearic acid 5 Total 665 mg The components are blended and compressed to form tablets each weighing 665 mg.

Formulation 3 An aerosol solution is prepared containing the following components:
Weiqht %
Example 1 0.25 25 ethanol 29.75 Propellant 22 (cnlorodifluoromethane)70.00 Total 100.00 The active compound is mixed with ethanol and the mixture added to a portion of the Propellant 22, cooled to -30C. and transferred -to a filling device.
The required amount is then fed to a stainless steel container and diluted with the remainder of the propel-lant. The valve units are then fitted to the container.

For_ulation 4 Tablets each containing 60 mg of active ingredient are made as follows-Example 1 60 mg starch 45 mg microcrystalline cellulose 35 mg 15 polyvinylpyrrolidone (as 10% solution in water) 4 mg sodium carboxymethyl starch 4.5 mg magnesium stearate 0.5 mg talc 1 mg 20 Total 150 mg The active ingredient, starch and cellulose are passed through a No. 45 mesh U.S. sieve and mixed thoroughly. The solution of polyvinylpyrrolidone is mixed with -the resultant powders which are then passed through a No. 14 mesh U.S. sieve. The granules so pro-duced are dried at 50C and passed through a No. 18 mesh U.S. sieve. The sodium carboxymethyl starch/ magnesium stearate and talc, previously passed through a No. 60 mesh U.S. sieve, are -then added to the granules which, after mixing, are compressed on a tablet machine -to yield table-ts each weighing 150 mg.

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X-7264~ -17-Formulation 5 Capsules each containing 80 mg of medicament are made as follows:
Example 1 80 mg starch 59 mg microcrystalline cellulose 59 mg magnesium stearate 2 mg Total 200 mg The active ingredient, cellulose, starch and magnesium stearate are blended, passed through a No. 45 mesh U.S. sieve, and filled into hard gelatin capsules in 200 mg quantities.
Formulation 6 Suppositories each containing 225 mg of active ingredient may be made as follows:
20 Example 1 225 mg saturated fatty acid glycerides 2,000 mg Total 2,225 mg The active ingredient is passed through a No. 60 mesh U.S. sieve and suspended in the saturated fatty acid glycerides previously melted using the minimum heat necessary. The mixture is then poured into a suppository mold of nominal 2 g capacity and allowed to cool.

Formulation 7 Suspensions each containing 50 mg of medica-ment per 5 ml dose are made as follows:
Example 1 50 rng sodium carboxymethyl cellulose 50 mg syrup 1.25 ml benzoic acid solution 0.10 ml flavor q.v.
10 color q.v.
purified water to total 5 ml The medicament is passed through a No. 45 mesh U.S. sieve and mixed with the sodium carboxymethyl cellulose and syrup to form a smooth paste. The benzoic acid solution, flavor and color are diluted with some of the water and added, with stirring. Sufficient water is then added to produce the required volume.

Formulation 8 An intravenous formulation may be prepared as follows:
Example 1 100 mg isotonic saline 1000 ml The solution of the above ingredients is administered intravenously at a rate of 1 ml per mimlte to a subject in need of txeatment.

Claims (4)

1. cis-(-)-4[(1(2)H-tetrazol-5-yl)methyl]-2 piperidinecarboxylic acid or a pharmaceutically accept-able salt thereof.

2. A pharmaceutical formulation comprising as an active ingredient the compound as claimed in claim 1, or a pharmaceutically acceptable salt thereof, associated with one or more pharmaceutically acceptable carriers, excipients or diluents therefor.

3. The compound claimed in claim 1 for use as a pharmaceutical.

4. A process for preparing the compound as claimed in claim 1, or a pharmaceutically acceptable salt thereof, which comprises reacting an alkyl cis-(-)-4-cyanomethyl-N-vinyloxycarbonyl-2-piperidinecarboxylate with azidotributylstannane and hydrolyzing the resulting intermediate, and salifying if the compound in salt form is desired.