according to the invention , ( 2r , 3s )- β - phenylisoserine , optionally in salt form , may be obtained by the action of ammonia solution on ( 2r , 3r )- β - phenylglycidic acid , preferably in the form of an alkali metal or alkaline earth metal ( sodium , potassium , calcium ) salt , of an ammonium salt or of a salt with a nitrogenous base ( α - methylbenzylamine , pyridine ). generally , the process is carried out in water , optionally mixed with an organic solvent such as methanol . it is preferable to work in water . to carry out the process , it is necessary to use an excess of ammonia relative to ( 2r , 3r )- β - phenylglycidic acid . generally , from 10 to 100 moles of ammonia , and preferably from 50 to 80 moles , are used per mole of ( 2r , 3r )- β - phenylglycidic acid . the ammonia is preferably used in the form of a concentrated aqueous solution such as a solution whose concentration is between 20 and 32 % ( w / w ) at a temperature in the region of 25 ° c . the process being carried out at a temperature of between 0 ° and 100 ° c ., and preferably between 40 ° and 60 ° c . generally , it is performed at atmospheric pressure or alternatively under an autogenous pressure which is in the region of 2 . 5 bars at 60 ° c . in order to speed up the reaction , it is especially advantageous to work in the presence of an ammonium salt such as ammonium chloride or ammonium hydrogen carbonate . it is preferable to use ammonium hydrogen carbonate which enables the reaction rate to be increased while retaining the selectivity . generally , a stoichiometric amount of ammonium salt is used relative to the β - phenylglycidic acid employed . generally , the process is carried out using the salt of ( 2r , 3r )- β - phenylglycidic acid with α - methylbenzylamine . however , it is also possible to use an alkali metal ( sodium , potassium ) salt which is obtained by the action of a base ( sodium hydroxide , potassium hydroxide ) in a stoichiometric amount on the salt of ( 2r , 3r )- β - phenylglycidic acid with α - methylbenzylamine , or the ammonium salt which is obtained by displacement of the salt of ( 2r , 3r )- β - phenylglycidic acid with α - methylbenzylamine by an excess of ammonia solution . in the latter case , it is possible to favor the displacement by continuous or semi - continuous extraction of the α - methylbenzylamine by means of a suitable organic solvent such as toluene . it is especially advantageous to use the ammonium salt of ( 2r , 3r )- β - phenylglycidic acid , which enables the ring - opening by means of ammonia to be rendered both regioselective and stereoselective . irrespective of the manner in which the action of ammonia on ( 2r , 3r )- β - phenylglycidic acid is carried out , the ( 2r , 3s )- β - phenylisoserine may be isolated according to one of the following methods : 1 ) the excess ammonia may be removed under reduced pressure so as to obtain the ammonium salt of ( 2r , 3s )- β - phenylglycidic acid in aqueous solution . after the addition of a strong mineral acid , the ( 2r , 3s )- β - phenylisoserine precipitates and is separated by filtration , or alternatively 2 ) before , during or after the removal of the ammonia under reduced pressure , it is possible to add an alkali metal base ( sodium hydroxide , potassium hydroxide ) or alkaline earth metal base ( quicklime or slaked lime ); the salt formed precipitates after the addition , where appropriate , of an organic solvent such as acetone . the alkali metal or alkaline earth metal salt thereby obtained is separated by filtration . in order to facilitate salting - out of the salt , especially of the sodium salt , of ( 2r , 3s )- β - phenylisoserine , and to improve the yield , it may be advantageous to saturate the water present in the reaction mixture by adding sodium chloride . ( 2r , 3r )- β - phenylglycidic acid may be prepared under the conditions described by j - n . denis et al ., j . org . chem ., 51 , 46 - 50 ( 1986 ). in denis , cis - cinnamyl alcohol , easily obtained from phenylacetylene in 88 % yield by hydroxymethylation followed by lindlar reduction , was subjected to the titanium - catalyzed asymmetric epoxidation process , resulting in a 61 - 65 % yield of the desired ( 2s , 3r )-(-)- phenoxyranemethanol . the phenoxyranemethanol was oxidized by using ruthenium trichloride - sodium periodate . because of the observed relative instability of the resultant free ( 2r , 3r )-(+)- phenyloxiranecarboxylic add , the reaction product , without isolation , was routinely converted to the methyl ester with etheral diazomethane . the ( 2r , 3s )- β - phenylisoserine obtained by carrying out the process according to the invention is especially useful for performing the synthesis of therapeutically active products such as the taxane derivatives of general formula : ## str3 ## in which r represents a hydrogen atom or an acetyl radical and r 1 represents a phenyl or t - butoxy radical . by the action of a benzoylating agent ( benzoyl chloride ) or of a t - butoxycarbonylating agent ( t - butyl dicarbonate ) and then of an agent for protection of the hydroxyl function , β - phenylisoserine yields the product of general formula : ## str4 ## in which r 1 represents a phenyl or t - butoxy radical and z 1 represents a group of the hydroxyl function ( 1 - ethoxyethyl ). by condensation of the acid of general formula ( iii ) with baccatine iii or 10 - deacetylbaccatine iii in which the hydroxyl functions at the 7 - position and , where appropriate , at the 10 - position are protected by protective groups ( silyl , 2 , 2 , 2 - trichloroethoxycarbonyl radicals ), followed by replacement of the protective groups with hydrogen atoms , the product of general formula ( ii ) is obtained . the condensation of the acid of general formula ( iii ) with protected baccatine iii or protected 10 - deacetylbaccatine iii , as well as the replacement of the protective groups with hydrogen atoms , may be carried out under the conditions described in european patent ep - 0 , 336 , 840 or ep - 0 , 336 , 841 . the example which follows , given without implied limitation , shows how the invention may be put into practice . 3 kg of α - methylbenzylamine ( 2r , 3r )- β - phenylglycidate , assaying at 98 % and the enantiomeric excess of which is greater than 98 . 5 %, and 15 liters of 32 % ( w / w ) ammonia solution are introduced into a column . at the bottom of the column , toluene is introduced by means of a metering pump at a flow rate of 3 to 5 liters / hour . the toluene solution which separates out at the column head is removed by overflowing . after 18 liters of toluene have been introduced , α - methylbenzylamine is no longer detected in the toluene extract . the ammonium β - phenylglycidate solution obtained above and 30 liters of 32 % ( w / w ) ammonia solution ( 30 liters ) are introduced into a 150 - liter autoclave . the autoclave is closed and then heated in the course of 1 hour to 60 ° c . with stirring . the pressure is in the region of 2 . 5 bars . stirring is continued at 60 ° c . for 5 hours and the autoclave is then allowed to cool to 18 ° c . the ammonia is removed by distillation under reduced pressure ( 100 - 700 mm of mercury ; 13 . 3 - 93 kpa ) at 24 ° c . after the addition of 9 kg of sodium chloride and 0 . 42 kg of sodium hydroxide pellets in 2 . 5 liters of water . when the pressure in the apparatus reaches 45 mm of mercury ( 6 kpa ), the reaction mixture is heated to 48 ° c . in order to dissolve the salts and is then cooled to a temperature of between - 5 ° and - 8 ° c . for 3 hours . the white crystals obtained are separated by filtration and then dried at 40 ° c . under reduced pressure ( 1 mm of mercury ; 0 . 13 kpa ). ( 2r , 3s )- β - phenylisoserine sodium salt ( 1932 g ), melting point 218 ° c ., is thereby obtained . the 13 c nuclear magnetic resonance spectrum of the ( 2r , 3s )- β - phenylisoserine sodium salt , determined in deuterated water at 90 mhz , is characterized by the following chemical shifts ( δ ): 60 . 2 ( 1 jch = 140 hz ); 80 . 0 ( 1 jch = 147 hz ; 2 j = 2 . 6 hz ); 129 . 6 ; 130 . 2 ; 131 . 5 ; 144 . 4 and 181 . 6 ppm . 10 kg of α - methylbenzylamine ( 2r , 3r )- β - phenylglycidate ( 35 . 08 mol ), 15 liters of water and 20 liters of toluene are introduced into a 250 - liter reactor , and 10 liters of 4n sodium hydroxide are then added in the course of 10 minutes at a temperature in the region of 20 ° c . the mixture is stirred for 1 hour . the aqueous phase is separated after settling has taken place . the toluene phase , which contains α - methylbenzylamine , is retained . the aqueous phase is washed with 2 times 10 liters of toluene to remove all of the α - methylbenzylamine . from the combined toluene phases , the α - methylbenzylamine may be isolated . 1 . 860 kg of ammonium chloride and 162 liters of 20 % ( w / v ) ammonia solution are added to the aqueous phase placed in a 250 - liter reactor . the mixture is heated to 50 ° c . and then kept stirring for 17 hours . after cooling to 35 ° c ., 35 kg of sodium chloride are added and the mixture is then maintained at this temperature for 30 minutes . it is allowed to cool slowly ( 2 hours ) to a temperature of between 0 ° and 5 ° c . and is then maintained for 1 hour at this temperature . the precipitate is separated by filtration and then dried under reduced pressure at 50 ° c . 7 kg of dry product are thereby obtained , which product contains approximately 25 % of sodium chloride and approximately 5 . 300 kg of pure ( 2r , 3s )- β - phenylisoserine sodium salt . the product thereby obtained may be used in the subsequent synthesis operations without further treatment . although the invention has been described in conjunction with specific embodiments , it is evident that many alternatives and variations will be apparent to those skilled in the art in light of the foregoing description . accordingly , the invention is intended to embrace all of the alternatives and variations that fall within the spirit and scope of the appended claims .