according to a preferred embodiment of the present invention , in the first step , to dl - carnitine is added dibenzoyl - l (+) tartaric acid and a lower alcohol , the mixture is dissolved under heat to form dibenzoyl - l (+) tartarates of d - carnitine and l - carnitine , and subsequently the tartarates are cooled to fractionally crystallize dibenzoyl - l (+) tartarate of l - carnitine . dibenzoyl - l (+) tartaric acid used as a resolving agent is inexpensive , since it can be synthesized from naturally occurring l (+) tartaric acid . in contrast , dibenzoyl - d (-) tartaric acid disclosed in east german patent no . 93347 specification is expensive resolving agent , since d (-) tartaric acid per se does not exist in nature and its dibenzoyl compound can be only synthetically prepared . further , dibenzoyl - l (+) tartaric acid can achieve the effect of giving higher purity in the crystallizing l - carnitine dibenzoyl - l (+) tartarate , as compared with dibenzoyl - d (-) tartaric acid . the amount of dibenzoyl - l (+) tartaric acid used as the resolving agent is 0 . 7 - 1 . 5 times , preferably equal to that of dl - carnitine . the lower alcohol used for fractional crystallization of l - carnitine dibenzoyl - l (+) tartarate from dibenzoyl - l (+) tartarates of d - carnitine and l - carnitine includes methanol , ethanol and the mixture thereof , and preferably methanol . fractional crystallization is conducted at a temperature of from 5 ° c . to - 20 ° c . further , fractional crystallization conducted at a temperature of below - 10 ° c . can achieve more improved crystallization of l - carnitine dibenzoyl - l (+) tartarate . in the second step of the present invention , dibenzoyl - l (+) tartarate of l - carnitine obtained by fractional crystallization is recrystallized with use of a polar solvent . the polar solvent includes methanol , ethanol , isopropanol or the mixtures thereof , and methanol and ethanol are preferred . in the third step of the present invention , dibenzoyl - l (+) tartarate of l - carnitine is decomposed in the usual way , e . g ., with water , preferably acidic water to obtain the desired l - carnitine . if necessary , l - carnitine may be converted in the usual way to its salt . the following examples typify the manner by which the present invention can be practiced and represent in one aspect the best mode for carrying out the invention . as such , they should be construed merely as illustrative and not as limitative upon the overall scope thereof . a solution of 35 . 0 g ( 0 . 177 mole ) of dl - carnitine hydrochloride in 300 ml of water is passed through oh type strongly basic resin ( dowex - 1x8 ) and the solution is concentrated to give 28 . 0 g ( 0 . 173 mole ) of dl - carnitine . to dl - carnitine are added 61 . 9 g ( 0 . 173 mole ) of dibenzoyl - l (+) tartaric acid and 90 ml of methanol . the mixture is dissolved under heat to form dibenzoyl - l (+) tartarates of d - carnitine and l - carnitine . the tartarates are cooled from 60 ° c . to 5 ° c . over a period of 24 hours to fractionally crystallize l - carnitine dibenzoyl - l (+) tartarate . the tartarate is filtered and washed to give l - carnitine dibenzoyl - l (+) tartarate ( yield 39 . 0 g , purity 92 . 7 %). the tartarate obtained is dissolved under heat in 190 ml of ethanol , and cooled from 60 ° c . to 25 ° c . over a period of 24 hours . this recrystallization gives 21 . 5 g of crystals of l - carnitine dibenzoyl - l (+) tartarate . [ α ] d 25 =- 96 . 2 ° ( c = 1 , meoh ), m . p . 154 ° c . ( dec . ), purity 99 . 7 %. to the crystals are added 30 ml of water and 100 ml of ethyl acetate , and the mixture is vigorously stirred to decompose l - carnitine dibenzoyl - l (+) tartarate . the organic layer containing the recovered resolving agent is removed therefrom , and the aqueous layer containing l - carnitine , after treatment with the strongly basic resin , is charged with 3 . 5 ml of concentrated hydrochloric acid and then concentrated under reduced pressure . recrystallization of the residue from ethanol gives 8 . 6 g ( 0 . 044 mole ) of l - carnitine hydrochloride as white crystal . [ α ] d 25 =- 23 . 7 ° ( c = 2 , h 2 o ), m . p . 142 ° c . in 700 ml of methanol are dissolved under heat 35 . 0 g ( 0 . 177 mole ) of dl - carnitine hydrochloride , and the solution is passed through oh type strongly basic resin ( dowex - 1x8 ). the methanol solution is concentrated to obtain 28 . 3 g ( 0 . 175 mole ) of dl - carnitine . to dl - carnitine are added 62 . 6 g ( 0 . 175 mole ) of dibenzoyl - l (+) tartaric acid and 93 ml of methanol , and the mixture is dissolved under heat to form dibenzoyl - l ( +) tartarates of d - carnitine and l - carnitine . the tartarates are cooled at a temperature of below - 10 ° c . with stirring to fractionally crystallize l - carnitine dibenzoyl - l (+) tartarate . the tartarare is filtered and washed to give l - carnitine dibenzoyl - l (+) tartarate ( yield 30 . 5 g , purity 93 . 0 %). recrystallization of the tartarate from 100 ml of methanol gives 23 . 9 g of l - carnitine dibenzoyl - l (+) tartarate as white crystal . [ α ] d 25 =- 96 . 0 ° ( c = 1 , meoh ), m . p . 154 ° c . ( dec . ), purity 99 . 5 %. the crystal is suspended in 60 ml of acetone and , after passing about 1 . 7 g of hydrogen chloride gas therethrough , the suspension is stirred for 1 hour at room temperature , and the reaction liquid is filtered to obtain 8 . 8 g ( 0 . 045 mole ) of l - carnitine hydrochloride as white crystal . [ α ] d 25 =- 23 . 30 °, ( c = 2 , h 2 o ), m . p . 142 ° c .
l - carnitine and salts thereof are resolved from dl - carnitine by the use as a resolving agent of dibenzoyl - ltartaric acid . l - carnitine is known as vitamin b t and is useful as a medicine .