The present invention relates to the preparation of (+)-2- aminobicyclo[3.1.0]hexane-2, 6-dicarboxylic acid, or a pharmaceutically acceptable salt thereof.

(+) -2-Aminobicyclo[3.1.0]hexane-2, 6-dicarboxylic acid is a novel agonist at negatively coupled cAMP-linked metabotropic glutamate receptors. It is useful for the treatment of neurological and psychiatric disorders linked to excitatory amino acid receptors, for example anxiety disorders, drug tolerance, withdrawal and cessation, and smoking cessation. The compound, its properties and methods for preparing it (including the subject matter claimed in the present application) are described in detail in European Patent Application Publication No. 696,577 and equivalent applications, which were all published after the priority date of the present application. European Patent Application Publication No. 696,577 and the equivalent applications claim priority from United Stateε Patent Application Serial No. 08/289,957, filed August 12, 1994, United States Patent Application Serial No. 08/337,349, filed November 10, 1994, and United States Patent Application Serial No. 08/496,643, filed June 29, 1995. The subject matter claimed in the present application was first described in United States Patent Application Serial No. 08/496,643, filed June 29, 1995, and in the priority application for the present application, also filed June 29, 1995.

According to one aspect, the present invention provides a process for the preparation of (+) -2-aminobicyclo[3.1.0]hexane- 2, 6-dicarboxylic acid, or a pharmaceutically acceptable salt thereof, which comprises hydrolysing (-) -2-spiro-5 ' - hydantoinbicyclo [3.1.0]hexane-6-carboxylic acid, which compound has the formula

or a salt thereof, followed if necessary and/or desired by forming a pharmaceutically acceptable salt.

The hydrolysis is preferably performed in the presence of an acid or base aε catalyst. Suitable acids include hydrochloric and hydrobromic acid. Suitable bases include alkali metal carbonates and hydroxides, and alkaline earth metal hydroxides, such as sodium carbonate, potassium carbonate, sodium hydroxide, potasεium hydroxide, calcium hydroxide and barium hydroxide.

Suitable hydrolyεiε conditionε are further described in G. Kruger, Houben- eyl, Methoden der Organische Chemie; Vol E5,G. Thieme Verlag: Stuttgart, 1985, pp. 534-546 and S. Kubic et al. ; Tetrahedron Letterε, 35, 6635(1994) .

Preferably the hydrolysis is performed in an aqueous solution, most preferably in water.

The temperature at which the hydrolyεiε is performed iε conveniently in the range of from 0 to 130 ^* C, more preferably from 50 to 110'C.

Heating the compound of formula II under reflux in a 2-4 molar solution of sodium hydroxide haε been found to be particularly convenient. (+) -2-Aminobicyclo[3.1.0]hexane-2, 6-dicarboxylic acid may be converted into a pharmaceutically acceptable salt using a conventional technique. It will be appreciated that the

compound may form acid addition salts and salts with bases. The acid addition saltε are generally prepared by reaction of an acid with the compoundε. The εaltε with baεes are generally formed by reaction with a base, for example an alkali metal or alkaline earth metal hydroxide.

Acids commonly employed to form such saltε include inorganic acids εuch as hydrochloric, hydrobromic, hydriodic, sulfuric, and phosphoric acid, as well aε organic acids such as para-tolueneεulfonic, methaneεulfonic, oxalic, para-bromophenylεulfonic, carbonic, εuccinic, citric, benzoic, and acetic acid, and related inorganic and organic acidε. Such pharmaceutically-acceptable εalts thus include sulfate, pyroεulfate, bisulfate, sulfite, bisulfite, phosphate, ammonium, monohydrogenphoεphate, dihydrogenphosphate, meta¬ phosphate, pyrophosphate, chloride, bromide, iodide, acetate, propionate, decanoate, caprylate, acrylate, formate, isobutyrate, caprate, heptanoate, propiolate, oxalate, malonate, succinate, εuberate, εebacate, furmarate, hippurate, maleate, butyne-1, 4-dioate, hexyne- 1,6-dioate, benzoate, chlorobenzoate, methylbenzoate, dinitrobenzoate, hydroxybenzoate, methoxybenzoate, phthalate, sulfonate, xylenesulfonate, phenylacetate, phenylpropionate <, phenylbutyrate, citrate, lactate, α- hydroxybutyrate, glycolate, maleate, tartrate, methanesulfonate, propaneεulfonate, naphthalene-1- sulfonate, napthalene-2-sulfonate, mandelate, magnesium, tetramethylammonium, potasεium, trimethylammonium, εodium, methylammonium, calcium, and the like salts. The compound of formula II and its salts are believed to be novel, and are provided as a further aspect of the inventio .

Examples of salts of the compound of formula II include salts formed with baseε, for example alkali metal

saltε, εuch aε the εodium or potasεium εalt, and εaltε formed with organic amines, for example, 1- phenylethylamine.

The compound of formula II may be prepared by resolution of the correεponding racemic hydantoin, for example by forming a cryεtalline salt with an optically active amine, such as (R) -1-phenylethylamine.

The racemic hydantoin may be prepared by reacting the compound of formula

with an alkali metal cyanide such as sodium or potasεium cyanide, and ammonium carbonate. The reaction is conveniently performed in an aqueous εolution, such as aqueous ethanol, at a temperature in the range of from 25 to 50'C.

Alternatively, the compound of formula II may be prepared by reacting the chiral compound of formula

with an alkali metal cyanide and ammonium carbonate, as described hereinafter. The compound of formula Ilia may be obtained from the corresponding racemic ketone by resolution, for example, by

forming a crystalline salt with (S) -1-phenylethylamine. Suitable solvents include aqueous acetone and mixtures of ethanol and ethyl acetate. An alternative resolving agent, which might be used is quinidine. The compound of formula III, and the reεolved compound of formula Ilia are believed to be novel, and are provided as further aspectε of the invention. They may be prepared by hydrolyεing the corresponding ethyl ester. This compound may in turn be prepared by reacting an ylide derived from carboethoxymethyl dimethylsulfonium bromide and a base, such as 1, 8-diazabicyclo[5.4.0]undec-7-ene, with 2-cyclopenten-l-one. The reaction is conveniently performed in an organic εolvent, εuch aε toluene. The temperature iε conveniently in the range of from 25 to 50 ^* C.

The following Exampleε illuεtrate the invention.

Example 1

Carboethoxymethyl Dimethylsulfonium Bromide

A solution of ethyl bromoacetate (265g) and dimethyl sulfide (114g) in acetone (500mL) was stirred at room temperature. After three days, the title compound was isolated by filtration of the reaction mixture. Melting point 88-90°C.

Example 2

( 1SR, 5RS, 6SR) Ethyl 2-Oxobicyclo[3.1.0]hexane-6-carboxylate

A suspension of carboethoxymethyl dimethylsulfonium bromide (45.5g) in toluene (350mL) was treated with 1,8- diazabicyclo[5.4.0]undec-7-ene (30.2g). The resulting mixture was stirred at room temperature. After one hour, the reaction mixture was treated with 2-cyclopenten-l-one (19.57g) . After an additional 18 hours, the reaction mixture was added to a 1 N hydrochloric acid/sodium chloride solution. The reεulting mixture waε extracted with diethyl ether. The combined ether extracts were dried over magnesium sulfate, filtered, and concentrated in vacuo . The residue was purified using silica-gel chromatography, eluting with a linear gradient of 10% ethyl acetate/hexanes to 50% ethyl acetate/hexanes, to give 22.81g of the title compound. Melting point: 36-38"C.

FDMS: m/z = 168 (M+) . Analysiε calculated for C ₉ H ₁₂ O ₃ : C, 64.27; H, 7.19. Found: C, 64.54; H, 7.11.

Example 3

2-Oxobicyclo[3.1.0]hexane-6-carboxylic acid

A mixture of 60 g of ethyl 2-oxobicyclo[3.1.0]hexane-6- carboxylate and 300 ml of 1 N εodium hydroxide waε εtirred at 25-30°C. After 2.5 hourε, concentrated hydrochloric acid was added to adjust the pH to 0.8-1.2. The resulting solution was extracted with ethyl acetate. The extracts were dried over magnesium εulfate, filtered, and concentrated to give 49.1 g (98%) of the crude material. Recryεtallization from 100 ml of ethyl acetate gave the title compound, mp 123.5-128°C.

FDMS: m/z = 140 (M+)

Analyεiε calculated for C7H8O3. c, 60.00; H, 5.75. Found: C, 60.14; H, 5.79.

Example 4

2-Oxobicyclo[3.1.0]hexane-6-carboxylic acid εalt with (S)-l- phenylethylamine

A εolution of 14 g of the compound prepared in Example 3 in 140 ml of 25% ethanol in ethyl acetate waε combined with (S)-l- phenylethylamine (1 eq.) . After stirring overnight, the precipitated salt was isolated by filtration and dried to give 11.87 g (45.4%) of the desired salt. Converεion of the salt to the partially resolved 2-oxobicyclo [3.1.0]hexane-6-carboxylie acid by the method of Example 3 and analyεiε indicated that the εalt was 68% ee. The enantiomeric excess was determined by conversion to the methyl ester with diazomethane followed by chiral HPLC on a Chiralpak AS column at 40°C eluted with 10% isopropanol/90% hexane at 1 ml/min with detection at 210 nm.

Example 5

(+) -2-Oxobicyclo[3.1.0]hexane-6-carboxylie acid

A mixture of 1.31 g of the product of Example 4 and 10 ml of IN hydrochloric acid waε stirred for 5 minutes and extracted with ethyl acetate. The extracts were dried over sodium sulfate, filtered, and concentrated to give 0.61 g of the title compound, mp 110-115°C. The product was determined to be 68% ee by chiral HPLC (method of Example 4) .

FDMS: m/z = 141 (M+H) Optical Rotation: (XD = 49.85°

Example 6

(-) -2-Spiro-5 ' -hydantoinbicyclo[3.1.0]hexane-6-carboxylic acid

A solution of the compound prepared as described in Example 5 (68% ee, 1 eq.), potaεsium cyanide (1.25 eq.), and ammonium carbonate (2.5 eq) were combined and εtirred in ethanol/water at 25°C for 40 hourε. The mixture was acidified with 6N hydrochloric acid, concentrated, diluted with water, and filtered to give a 79% yield of a 90:10 mixture of diastereomers, mp 286-290°C. The diastereomeric mixture was recrystallized from isopropanol/water to give in 48% yield the title compound in 100% diaεtereomeric and 100% enantiomeric purity (enantiomeric ratio determined by chiral HPLC on a 4.6 x 150 mm Chiralcel OD-H column, eluted with 15 % iεopropanol/85% hexane at 1 ml/min at 40°C with detection at 220 nm; diastereomeric ration determined by HPLC on a Zorbax SB-phenyl column at 40°C with elution with 90:10 buffer/acetonitrile eluted at 2 ml/min with detection at 220 nm (buffer = 0.1 M dibasic sodium phosphate monohydrate adjusted to pH 2.1 with phosphoric acid) .

FDMS: m/z = 211 (M + H)

Optical Rotation: o = - 25.98°

Analysis calculated for C9H10N2O4: C, 51.43; H, 4.79; N, 13.33. Found: C, 51.38; H, 4.80; N, 13.26.

Example 7

Ethyl 2-spiro-5 ' -hydantoinbicyclo[3.1.0]hexane-6-carboxylate

A mixture of 5.05 g of ethyl 2-oxobicyclo [3.1.0]hexane-6- carboxylate, 2.15 g of potaεεium cyanide, 5.77 g of ammonium carbonate, 30 ml of 2B-3 ethanol, and 12 ml of water waε stirred at 35°C until the reaction was complete by HPLC. After 15 hourε, the reaction mixture was cooled to 0°C and 33 ml of water waε added to the mixture. After 2 hourε at 0°C, the precipitate waε iεolated by filtration and dried to give 5.23 g (73%) of the title compound, mp 217-220°C.

FDMS: m/z = 238.1 (M+)

Analytical calculated for C11H14N2O4: C, 55.46; H, 5.92; N, 11.76. Found: C, 55.74; H, 5.88; N, 11.50.

Example 8

2-Spiro-5 ' -hydantoinbicyclo[3.1.0]hexane-6-carboxylie acid

A mixture of 16.32 g of the product of Example 7 and 137 ml of 2N NaOH waε εtirred at 25°C. After 1 hour, concentrated hydrochloric acid waε added to adjuεt the pH to 1.0. The reεulting precipitate was isolated by filtration and dried to give 13.70 g (95%) of the title compound, mp 277-279°C.

FDMS: m/z = 210.1 (M+) Analysis Calculated for C9H10N2O4: C, 51.43; H, 4.79; N, 13.33. Found: C, 51.70; H, 4.93; N, 13.43.

Example 9

2-Spiro-5'-hydantoinbicyclo[3.1.0]hexane-6-carboxylic acid, (R)- 1-phenylethylamine salt

A mixture of 1.05 g of the product of Example 8 and 16.6 ml of a 1.6 : 1 solution of acetone : water was stirred at 25°C while adding 1.53 g of R-(+) -1-phenylethylamine. The mixture was stirred for 2 hours at room temperature. The crystals were filtered, rinsed with acetone, and dried to give 0.74 g (45%) of the title compound, mp 205-212°C.

Optical Rotation: oc D = -31. 88° ( c = 1 , methanol )

Example 10

(-)-2-Spiro-5' -hydantoinbicyclo[3.1.0]hexane-6-carboxylie acid

A mixture of 0.74 g of the product of Example 9 and 10 ml of water was εtirred at 25°C while the pH was adjusted from 6.81 to 1.0 using IN HCl. The reaction mixture was stirred for 1 hour and the product was collected by filtration and dried to give 0.35 g (75%) of the title compound, mp 310°C (decomp).

FDMS: 210.1 (M+)

Optical Rotation: α D = -24.22° (c = 1, methanol)

Analysis calculated for C9H10N2O4: C, 51.43; H, 4.80; N,13.33.

Found: C, 51.67; H, 4.87; N, 13.61.

Example 11

(+) -2-Aminobicyclo[3.1.0]hexane-2, 6-dicarboxylic Acid

A solution of 184 g of (-) -2-spiro-5 ' -hydantoinbicyclo[3.1.0] - hexane-6-carboxylic acid and 1750 ml of 3N NaOH waε heated at reflux until the reaction waε complete by HPLC. After 28 hours, the εolution was cooled to room temperature and filtered through glasε paper to remove trace amounts of insoluble material. The pH of the solution was adjusted to 3.0 uεing concentrated HCl. The reaction mixture was stirred 1 hour at room temperature and two hourε at 0°C. The precipitated product waε collected by filtration, waεhed with 170 ml of cold water and dried to give 152.5 gramε (86%) of the title compound.

FDMS: m/z = 186.1 (M+l)

Optical rotation : CC D = 23 . 18° ( c = 1 , IN HCl )

Example 12

2-Oxobicyclo[3.1.0]hexane-6-carboxylic acid εalt with (S)-l- phenylethylamine

A εuεpension of 1.0 g of the product of Example 4 and 10 ml of acetone waε heated to reflux and combined with 1 ml of water to give a clear εolution. On cooling to 5°C, the reεulting εlurry waε filtered, and the collected εolid washed with 5 ml of cold 10% water in acetone and dried to give 0.42 g of the title compound. Conversion of a sample of the salt to (+)-2- oxobicyclo[3.1.0]hexane-6-carboxylic acid by the method of Example 5 and analysis indicated that the chiral purity was 90.2% ee. The enantiomeric excess was determined using the method described in Example 4.

Example 13

2-Oxobicyclo[3.1.0]hexane-6-carboxylic acid salt with (S)-l- phenylet ylamine

To 0.7 g of the compound prepared in Example 3 was added at room temperature 7 ml of a εolution of 10% water in acetone. The reεulting εolution waε combined at room temperature with (S) -1-phenylethylamine (1 eq.) and 0.5 ml of 10% water in acetone. The reaction mixture waε stirred overnight at room temperature, seeded with a few crystals of the salt prepared in Example 12 and stirred an additional 6 hours. The resulting slurry was cooled to 5°C, filtered, and the collected solid was washed with 1.5 ml of 10% water in acetone and dried to give

0.35 g (26.7%) of the title salt. Conversion of a sample of the salt to (+) -2-oxobicyclo[3.1.0]hexane-6-carboxylic acid by the method of Example 5 and analysis indicated that the chiral purity was 92% ee. The enantiomeric excess was determined using the method described in Example 4.

Other solvent combinations can also be uεed to prepare the (+) -2-oxobicyclo[3.1.0]hexane-6-carboxylic acid εalt with (S)-l- phenylethylamine. For example, εubεtitution of 30% ethanol in ethyl acetate for 10% water in acetone gave the title εalt in 29% yield and 82% ee.