DCG-1/4 can be used as a pharmacological tool to study the function of group II mGluR (metabotropic glutamate receptor). 3H-DCG-1/4 is useful in establishment of a radioligand binding assay and to study the distribution of group II mGluR in the brain. The mGluR is an exitatory amino acid (EAA) receptor which mediates synaptic excitation in the mammalian central nervous system.

After the independent discovery of the first mGluR by Sladeczeck et al.

(Nature, 317, p. 245 (1985) and by Nicoletti et al. (Jour. Neuro. Chem., 46, p.

40, (1986), the multiplicity of this class has been disclosed by expression cloning studies. Currently, eight mGluRs (and several splice variants) have been isolated and subdivided in three groups according to sequence homology, signal transduction and pharmacology: The first group includes mGluRl and mGluR5 which are coupled to IP3/Ca2+ signal transduction via activation of phospholipase C, whereas the members of group II, mGluR2 and mGluR3, as well as those of group III, mGluR4, mGluR6, mGluR7 and mGluR8, are negatively linked to adenylate cyclase. The conformationally restricted analogue of L-glutamic acid, (2S, 2'R, 3'R)-2- (2,3-dicarboxylcyclopropyl)-glycine (DCG-1/4), has revealed to be a particularly interesting compound, being a potent group II mGluR agonist, also active as an agonist at NMDA receptor site.

The synthesis of DCG-1/4 described in the literature (Ohfune et al., International Patent Application WO 93-08158, Ohfune, et al., Bioorg. Med.

Chem. Lett. 1993,3,15, and Ohfune et al., J. Med. Chem. 1996,39,407) is very long (more than 30 steps) and unefficient (only mg-quantities of the compound can be prepared). In addition, the synthesis of the radiolabeled compound is not possible using the described procedure.

In the present invention it is described a short and efficient method for the synthesis of DCG-1/4 in 8 steps starting from (-)-Feist's acid or in 3 steps starting from rac-tra7ls-2, 3-dicarbomethoxy-1-formyl-cyclopropane. The present descriptions allow the synthesis of DCG-1/4 in g-quantities as well as the introduction of tritium in the position 1'of the molecule.

The object of the present invention is a process for preparation of (2S, 2'R, 3'R)-2- (2,3-dicarboxycyclopropyl)-glycine and this compound per se, which process comprises the following steps a) brominating Feist's acid (1S, 2S)-3-methylene-cyclopropane-1,2- dicarboxylic acid of formula to give (1R, 2R)-3-bromo-3-bromomethyl-cyclopropane-1,2-dicarboxylic acid of formula

and b) forming the lactone (lRS, 5R, 6R)-1-bromo-4-oxo-bicyclo carboxylic acid of formula IV on the basic of the compound of formula III and c) opening the lactone ring and esterifying the resulted compound to give a compound of formula

wherein Ri is lower alkyl, and d) oxidizing a compound of formula V to give (lR, 2R)-3-bromo-1,2-dicarbo- alkyloxy-3-formyl-cvclopropane of formula

wherein Ri is described as above, and e) dehalogenating a compound of formula VI to yield (lR, 2R)-1,2-dicarbo- methoxy-3-formyl-cyclopropane of formula

wherein Ri is described as above and R2 is hydrogen, and, if desired, introducing tritium in position 1'of the cyclopropyl ring to give 3- [3H]-(1R, 2R)-1, 2-dicarbomethoxv-3-formvl-cyclopropane of formula VII, wherein R2 is tritium, and f) reacting a compound of formula VII or the compound (RS)-trans-2,3- dicarbomethoxy-1-formyl-cyclopropane with (R)-a-phenylglycinol and with a cyanide to give (2S, 2'R, 3'R)-N- [(R)--hydroxy-1-phenyl-ethyl]-2-(2', 3'-dicarbo- methoxycyclopropyl)-glvcinonitrile of formula

wherein Ri and R2 are described as above, and g) cleaving off the directing group and hydrolyzing the ester-and nitrile groups to give a compound of formula I.

The above process steps are described in the following in more detail.

Procedure A The synthesis of the enantiomerically pure compounds of formula VII in five steps starting from (-)-Feist's acid is described in this procedure.

Starting from (lR, 2R)-3-bromo-1,2-dicarbomethoxy-3-formyl- cyclopropane (VI) it is possible to prepare (lR, 2R)-1,2-dicarbomethoxy-3- formyl-cyclopropane (VII-1) as well as 3H- (lR, 2R)-1,2-dicarbomethoxy-3- formyl-cyclopropane (VII-2).

Halogenation, preferentially bromination of (lS, 2S)-3-methylene- cyclopropane-1,2-dicarboxylic acid (II) [a) F. Feist, Chem. Ber. 1893,26,747; also commercially available. b) a modified procedure is described in: A. T. Blomquist, D. T. Longone, J. Am. Chem. Soc. 1959,81,2012. c) optical resolution: W. von E. Doering, H. D. Roth, Tetrahedron 1970,26,2825).] using standard methods yielded (lR, 2R)-3-bromo-3-bromomethyl-cyclopropane-1,2- dicarboxylic acid (III), which is then heated in water to give (lRS, 5R, 6R)-1- bromo-4-oxa-bicyclo [3.1.0] hexane-6-carboxylic acid (IV). Opening of the lactone and esterification is performed with acids in alcohols preferentially sulfuric acid in MeOH to give (1R, 2R)-3-bromo-1.2-dicarbomethoxy-3-hydroxymethyl- cyclopropane (V), which is then oxidized by use of standard methods, preferentially PCC (pyridinium chlorochromate), to give (lR, 2R)-3-bromo-1,2- dicarbomethoxv-3-formyl-cvclopropane (VI).

Dehalogenation can be achieved by use of standard methods, preferentially by treatment of (lR, 2R)-3-bromo-1,2-dicarbomethoxy-3-formyl- cyclopropane (VI) with zinc in acetic acid or, alternatively, with tributyltin hydride in diethyl ether or other ethers to give (lR, 2R)-1,2-dicarbomethoxy-3- formyl-cyclopropane (VII-1).

These methods, preferentially the reaction with tributyl tin tritide [H. Andres et al., PCT Int. Appl. WO 91 15,858], can be used for the preparation of 3H- (lR, 2R)-1,2-dicarbomethoxy-3-formyl-cyclopropane (VII-2).

Further reaction of (1R, 2R)-1,2-dicarbomethoxy-3-formyl-cyclopropane (VII-1) and 3H- (lR, 2R)-1, 2-dicarbomethoxy-3-formyl-cyclopropane (VII-2) are performed according to procedure B to give (2S, 2'R, 3'R)-2- (2,3- dicarboxylcyclopropyl)-glycine (I-1) [DCG-1/4] and 3H-(2S, 2'R, 3'R)-2-(2, 3- dicarboxylcyclopropyl)-glycine (1-2) [3H-DCG-1/4].

Procedure B This synthesis starts from (RS)-tra7Ls-2, 3-dicarbomethoxy-1-formyl- cyclopropane (VII-3) which is prepared by the addition of dimethylsulfonium- 3-carboxallylide to dimethyl fumarate and treatment of the formed cyclopropyl derivative with osmiumtetroxide/sodium metaperiodate according to a protocol described in the literature [Nordlander et al., J. Org. Chem. 39 (1974) 3814].

The key step of the synthesis is a diastereoselective Strecker-reaction involving the nucleophilic addition of a cyanide ion to the Schiff base formed by condensation of the racemic aldehyde with optically active a-aminoalcohols, preferentially a-phenylglycinol. Reaction of the aldehyde with (R)-a- phenylglycinol in alcohols, preferentially methanol, at room temperature for 2 hours, followed by treatment of the Schiff base with cyanide, preferentially trimethylsilyl cyanide, for 12 hours at room temperature yielded a mixture of four a-amino nitriles as two major and two minor components in ca. 8: 2 ratio.

Extensive column chromatography of the mixture followed by crystallization yielded (2S, 2'R, 3'R)-N- [(R)-2-hvdroxy-1-phenvl-ethyl]-2-(2', 3- dicarbo-methoxycyclopropyl) glycinonitrile (VIII).

Cleavage of the directing groups can be performed by standard procedures, preferentially oxidative cleavage with lead tetra acetate, acidic hydrolysis (6N HC1) and ion exchange chromatography on Dowex 50WX4 to afford (2S, 2'R, 3'R)-2- (2, 3-dicarboxylcyclopropyl)-glycine (I) [DCG-1/4].

The following reaction scheme illustrate the process steps for the preparation of the compound of formula I in more detail.

Reaction scheme Br Ber HO HO o Br O p'O 0 HO A 0 HO 0 OH/A Br Br 'O , R O O'A O RO A VI Vl w R (RS)-trans-2, 3-dicarboalkyloxy-1- p formyl-cyclopropane, Vl1-3 (1 R, 2R)-1. 2-dicarboalkylory- (1 R>2R)-1. 2-dicarboalkylOxy-preparation in accordance with 3-formyl-cyclopropane Org. Chem., 39,3814, (1974) Vit H 2 N,,, OH _ HO 0 S0RU VIII HA R, O UII

A-pocedure B-procedure

Procedure A Starting from (lS, 2S)-3-methylene-cyclopropane-1,2-dicarboxylic acid [ (-)-Feist's acid] (II) a) (1R, 2R)-3-bromo-3-bromomethyl-cyclopropane-1,2-dicarboxylic acid.

To a cooled (0°) and stirred solution of (lS, 2S)-3-methylene-cyclopropane- 1,2-dicarboxylic acid [ (-)-Feist's acid] (4.0 g, 28.2 mmol) in diethyl ether (250 ml) was added bromine (2 ml), and stirring was continued over a period of 16 h at room temperature. Filtration, evaporation of the solvent and crystallization of the crude product from dichloromethane/hexane yielded (lR, 2R)-3-bromo-3- bromomethyl-cvclopropane-1,2-dicarboxylic acid (5.45 g, 64%) as a pale brown solid. mp 233° C (dec.) ; [a] 20D = +80° (c = 0.25 in MeOH); 1H NMR (DMSO- d6): d 2.58 (d, J = 6.5 Hz, 1 H), 2.63 (d, J = 6. 5 Hz, 1 H), 4. 07 (d, J = 11 Hz, 1 H), 4.17 (d, J = 11 Hz, 1 H). MS (FAB) m/z 299,301,303 [M-H+]. b) (lRS, 5R, 6R)-l-bromo-4-oxa-bicyclo [3.1.0] hexane-6-carboxylic acid.

A solution of (1R, 2R)-3-bromo-3-bromomethyl-cyclopropane-1,2- dicarboxylic acid (5.4 g, 17.9 mmol) in water (100 ml) was boiled under reflux conditions over a period of 4 h. Filtration, evaporation of the solvent and column chromatography of the crude product (dichloromethane/methanol 9: 1) gave 2.75 g of a solid. Further crystallization from dichloro-methane/hexane yielded (lRS, 5R, 6R)-l-bromo-4-oxa-bicyclo [3.1.0] hexane-6-carboxylic acid (2.41 g/61%) as a light yellow solid. mp 196-1980 C; [a] 20D =-360 (c = 0.25 in MeOH); 1H NMR (DMSO-d6): d 2.64 (d, J = 3 Hz, 1 H), 2.93 (d, J = 3 Hz, 1 H), <BR> <BR> <BR> 4.56 (d, J = 10 Hz, 1 H), 4.71 (d, J = 10 Hz, 1 H). MS (EI) m/z 220,222 [M+], 202,204 (48) [M+-H20], 123 (100) [M+-H20,-Br], 97 (98). c) (lR, 2R)-3-bromo-1, 2-dicarbomethoxy-3-hydroxymethyl- cyclopropane.

To a stirred solution of (lRS, 5R, 6R)-l-bromo-4-oxa-bicyclo [3.1.0] hexane- 6-carboxylic acid (2.41 g, 10.9 mmol) in methanol (25 ml) was added sulfuric acid (conc., 2.5 ml) and stirring was continued over a period of 2 h. The reaction mixture was poured into ice/water (100 ml) and extracted with two 100 ml portions of ethyl acetate. The combined organic layers were washed with water (50 ml) and two 50 ml portions of saturated sodium hydrogen carbonate solution, dried (MgS04) and evaporated to yield (1R, 2R)-3-bromo-

1,2-dicarbomethoxy-3-hydroxymethyl-cyclopropane (2.85 g, 97%) as a light yellow oil. [a] 20D = +98.4° (c = 0.25 in MeOH); 1H NMR (CDC13): d 2.66 (dd, J = 6,8 Hz, 1 H), 2.70 (d, J = 6.5 Hz, 1 H), 2.87 (d, J = 6.5 Hz, 1 H), 3.77 (s, 3 H), 3.80 (s, 3 H), 3.99 (dd, J = 8,12.5 Hz, 1 H), 4.16 (dd, J = 6,12.5 Hz, 1 H).

MS (EI) m/z 267,269 [M+H+], 249,251 (3) [M+-OH], 235, 237 (14) [M+- OMe], 207,209 (94), 175,177 (82), 169 (83), 155 (46), 113 (100), 59 (76). d) (lR, 2R)-3-bromo-1,2-dicarbomethoxy-3-formyl-cyclopropane.

To a stirred solution of (1R, 2R)-3-bromo-1,2-dicarbomethoxy-3- hydroxymethyl-cyclopropane (2.8 g, 10.5 mmol) in dichloromethane (120 ml) was added pyridinium chlorochromate (3.35 g, 15.7 mmol) and stirring was continued over a period of 16 h. Diethyl ether (120 ml) was added to the reaction mixture, which was then filtered with the aid of a Whatman glass microfibre filter and evaporated. Column chromatography of the crude product (diethyl ether/hexane 1: 1) yielded (lR, 2R)-3-bromo-1,2-dicarbomethoxy-3- formyl-cyclopropane (1.93 g, 69%) as a white solid. mp 52° C; [ce] 20D = +108° <BR> <BR> <BR> (c = 0.25 in MeOH); 1H NMR (CDC13): d 3.16 (d, J = 6.5 Hz, 1 H), 3.27 (d, J =<BR> <BR> <BR> <BR> <BR> <BR> 6.5 Hz, 1 H), 3.76 (s, 3 H), 3.83 (s, 3 H), 9.26 (s, 1 H). MS (EI) 17L/t 265,267 [M+H+], 233,235 (24) [M+-OMe], 204, 206 (24), 176,178 (44), 153 (100), 125 (95), 59 (69). e) (lR, 2R)-1.2-dicarbomethoxy-3-formyl-cyclopropane.

A mixture of (lR, 2R)-3-bromo-1. 2-dicarbomethoxy-3-formvl-cyclopropane (428 mg, 1.6 mmmol), zinc powder (230 mg, 3.5 mmol) and acetic acid (2 ml) was stirred at room temperature over a period of 4 h. Filtration, evaporation and column chromatography yielded (lR, 2R)-1,2-dicarbomethoxy-3-formyl- cyclopropane (265 mg, 88%) as a colorless oil. [a] 20D =-48° (c = 1 in MeOH); 1H NMR (CDC13): d 2.53 (ddd, J = 5,6,8 Hz, 1 H), 2.71 (dd, J = 6,8 Hz, 1 H), 3.01 (t, J = 6 Hz, 1 H), 3.76 (s, 6 H), 9.42 (d, J = 5 Hz, 1 H); MS (FAB) m/e 187 [M+H+]. f) (2S, 2'R, 3'R)-N-[(R)-2-hydroxy-1-phenyl-ethyl]-2-(2', 3'-dicarbo- methoxy-cyclopropyl)-glycinonitrile.

To a solution of (lR, 2R)-1,2-dicarbomethoxy-3-formyl-cyclopropane (256 mg, 1.38 mmol) in methanol (15 ml) was added (R)-a-phenylglycinol (189 mg, 1.38 mmol), and the resulting solution was stirred at room temperature for 5 h. After cooling to 0° C, TMSCN (0.34 ml, 2.75 mmol) was added, and the

resulting mixture was stirred for 16 h at room temperature. Evaporation of the solvent gave a yellow oil, which was then purified by column chromatography (ethyl acetate/hexane 1: 1) to yield (2S, 2'R, 3'R)-N- [ (R)-2- hydroxy-1-phenyl-ethyl]-2-(2',(2', 3'-dicarbomethoxy-cyclopropyl)-glycinonitrile (387 mg, 84%) as a colorless oil.

Procedure B Starting from (RS)-trans-2,3-dicarbomethoxy-1-formyl-cyclopropane (VII-3) a) (2S, 2'R, 3'R)-N-[(R)-2-hydroxy-1-phenyl-ethyl]-2-(2', 3'-dicarbo- methoxycyclopropyl)-glycinonitrile.

To a solution of (RS)-tra7ns-2,3-dicarbomethoxy-1-formyl-cyclopropane (3.11 g, 16.7 mmol) in methanol (110 ml) was added (R)-a-phenylglycinol (2.29 g, 16.7 mmol), and the resulting solution was stirred at room temperature for 2 h. After cooling to 0° C, TMSCN (4.2 ml, 33.4 mmol) was added, and the resulting mixture was stirred for 16 h at room temperature. Evaporation of the solvent gave a yellow oil, which was then purified as follows: (1) column chromatography (ethyl acetate/hexane 2: 1) yielded 1.32 g (24%) of a colorless oil, 2.15 g (39%) of a light yellow oil (mixture) and 1.51 g (27%) of a colorless oil. (2) column chromatography (ethyl acetate/hexane 2: 1) of the mixture (2.15 g) gave 0.98 g of a colorless oil and 1.08 g of a colorless oil.

Further separation of the mixtures was performed by crystallization from diethylether/hexane to yield (2S, 2'R, 3'R)-N-[(R)-2-hydroxy-1-phenyl-ethyl]-2- (2', 3'-dicarbo-methoxycyclopropyl)-glycinonitrile (1.43 g, 26 %) as a light yellow oil. IR (KBr) v 1070 (OH), 1190 (ester), 1720 (ester), 2227 (CN). MS <BR> <BR> (FAB) m/ 333 (M+H+). [U] 20D =-90. 40 (c = 0.25 in MeOH). NMR (CDCl3): d 1.61 (br, 1 H), 1.89 (br, 1 H), 2.19 (m, 2 H), 2.42 (dd, J = 8.5,9 Hz, 1 H), 3.61 (m, 1 H), 3.64 (s, 3 H), 3.74 (m, 1 H), 3.75 (s, 3 H), 3.80 (dd, J = 4,9 Hz, 1 H), 4.09 (dd, J= 4,8 Hz, 1 H), 7.36 (m, 5 H). b) (2S, 2'R, 3'R)-2-(2, 3-dicarboxylcyclopropyl)-glycine (DCG-1/4).

Lead (IV) acetate (2.10 g, 4.73 mmol) was added to a cold (0° C), stirred solution of (2S, 2'R, 3'R)-N- [(R)-2-hydroxy-1-phenyl-ethyl]-2-(2', 3'-dicarbo- methoxy-cyclopropyl)-glycinonitrile (1.43 g, 4.30 mmol) in anhydrous methanol-dichloromethane (36 ml, 1: 1). After 15 min, water (36 ml) was added and the resulting mixture was filtered with the aid of Celite. After evaporation of the solvent, the residue was refluxed in 6 N HC1 (100 ml) for 12 h. The

reaction mixture was washed twice with dichloromethane (30 ml each) and evaporated to dryness. The residue was submitted to ion exchange resin chromatography (Dowex 50WX4) to give (2S, 2'R, 3'R)-2- (2, 3-dicarboxylcyclo- propyl)-glycine (0. 55 g, 54%) as a white, hygroscopic foam (diammonium salt). mp 171-1730 C (dec.). [a] 20D =-43. 6° (c = 0.25 in H20). NMR (D20): d 1.93 (ddd, J = 6,9.5,10 Hz, 1 H), 2.07 (dd, J = 5.5,6 Hz), 2.17 (dd, J = 5.5,9.5 Hz), 4.03 (d, J = 10 Hz). MS (FAB) Z7z/~ 204 (M+H+).

Synthesis of 3H- (2S, 2'R, 3'R)-2- (2, 3-dicarboxylcyclopropyl)-glycine (3H-DCG- 1/4) (1-2) starting from (lR, 2R)-3-bromo-1,2-dicarbomethoxy-3-formyl- cyclopropane (VI). a) 3H-(lR, 2R)-1, 2-dicarbomethoxy-3-formyl-cyclopropane.

Starting with 250 ul of 1.39 M n-butvl lithium (0.347 mmoles) a solution of 0.78 ml of lithium triethylborane tritide was prepared according to the method of Andres et al. [H. Andres et al., PCT Int. Appl. WO 91 15,858]: 0.94 ml of tri-n-butyltinchloride was added and the solution was stirred for 15 min. The reaction mixture was lyophilized, the vacuum was relieved with dry nitrogen and the two-necked flask was removed from the tritiation apparatus.

The residue was suspended in 2 ml of n-heptane and the suspension was applied onto a 0.5 g silicagel cartridge (Chromabond, Machery Nagel, #730073). The cartridge was rinsed with three 1 ml-aliquots of n-heptane. The total 3H-activity of the n-heptane eluate was 5.03 Ci. The eluate was lyophilized and the residue of pure tri-n-butyltintritide was dissolved in 1 ml of cyclohexane-tetrahydrofuran 1: 1 under argon. 48 mg (0.181 mmoles of (lR, 2R)-3-bromo-1,2-dicarbomethoxy-3-formylcyclopropane and 1.2 mg of 2,2'- azobisisobutyronitrile (AIBN) was added and the reaction mixture was stirred for 18 h at 50°C. Then 1 ml of saturated potassium fluoride solution was added and stirring was continued for 1 h at r. t. Partitioning between diethylether and ice/water, washing the organic layers with saturated sodium chloride solution and drying over anhydrous sodium sulfate yielded crude product with total 3H-activity of 4.84 Ci. Column chromatography on 9 g LichroprepSi60 15- 25 um (Merch #1.09336) with n-hexane-diethylether 1: 1 afforded 2.18 Ci2) of product. The radiochemical purity was 96% according to TLC. The loss of 3H- activity during chromatography was probably due to hydrogen exchange catalyzed by the slightly acidic silica gel. It is therefore recommended to omit chromatography at this stage of the synthesis.

b) 3H-(2S, 2'R, 3'R)-N-[(R)-2-hydroxy-1-phenyl-ethyl] 2-(2', 3'- dicarbomethoxy-cyclopropyl)-glycinonitrile.

1.3 Ci of 3-[3H]-(lR, 2R)-1, 2-dicarbomethoxy-3-formyl-cyclopropane and 6.8 mg (0.05 mmoles) of D-phenylglycinol in 0.45 ml dry methanol was stirred for 6 h at r. t. 12 ul (0.096 mmoles) of trimethylsilylcyanide was added at 0°C and stirring was continued for 15 h at r. t. Column chromatography on 7 g LichroprepSi60 15.25 um with n-hexane-ethyl acetate 1: 1 afforded 456 mCi 3H-activity of product (2S, 2'R, 3'R)-N- [(R)-2-hydroxy-1-phenyl-ethyl]-2-(1'- [3H], 2', 3'-dicarbomethoxy-cyclopropyl)-glycinonitrile. c) 3H-(2S, 2'R, 3'R)-2-(2, 3-dicarboxylcyclopropyl)-glycine (DCG-1/4).

To a solution of 4.2 mg (0.0126 mmoles) of 1 (total 3H-activity 220 mCi) in 0.5 ml of dichloromethane-methanol 1: 1 was added 6.7 mg (0.015 mmoles) lead tetraacetate at 0°C under argon and the mixture was stirred for 15 min. at this temperature. The solvents were evaporated, 1.5 ml of 6N hydrochloric acid was added and the reaction mixture was stirred at 100°C for 15 h.

The crude product was partitioned between dichloromethane and water and the aqueous phase was lyophilized. The residue was dissolved in 5 ml of water and this solution was applied onto a small cation exchange column (5x50 mm, 0.6 g Dowex 50Wx4 H+-form). After rinsing with 15 ml of water the DCG-1/4 was eluted with 20 ml of 2N ammonium hydroxide solution. HPLC- purification of 35 mCi of this sample afforded 14.2 mCi of product. HPLC- conditions: column: LiChrocart Superspher RP-18e 5 um 4x250 mm Merck #16858 mobile phase: 20 mM orthophosphoric acid, flow rate: 0.5 ml/min, LTV- detection at 205 nm. To get rid of the orthophosphoric acid the solution of 3H- DCG-1/4 was applied onto a SP-Sephadex cation exchange column (H+-form, 10x100 mm). After washing with two 10 ml aliquots of water the 3H-DCG-1/4 was eluted with 2N ammonium hydroxide solution using a fraction collector.

Total 3H-activity of product 11.7 mCi. The radiochemical purity was 98.3% according to TLC (silica gel 60, n-butanol-acetic acid-water 3: 1: 1). The specific activity determined HPLC was 17.5 Ci/mmole.