United States Patent [191 [11] 3,943,181 Fleischer et al. , [45] Mar. 9, 1976

[54] SEPARATING OPTICALLY PURE d- AND 397,212 8/1933 United Kingdom ...... 260/631 R l-ISOMERS 0F MENTHOL, NEOMENTHOL AND ISOMENTHOL " OTHER PUBLICATIONS [75] Inventors: Jiirgen Fleischer, Cologne; Kurt Gilman Ed., Organic Chemistry, Wiley, N.Y., 1943, p. Bauer; Ruldolf Hopp, both of 254. Holzminden, all of Germany Reed et al_, J. Chem. Soc., pp. 167-173 (1933). [73] Assignee: Haarmann & Reimer Gesellschaft mit beschrankter Haftung Primary Examiner-Be1‘nard l-lel?n Assistant Examiner-James H. Reamer [22] Filed: Sept. 3, 1974 Attorney, Agent, or Firm—Burgess, Dinklage & [21] Appl. No.: 503,004 Sprung Related US. Application Data [63] Continuation of Ser. No. 229,109, Feb. 24, 1972, [57] ABSTRACT ' abandoned. A process for the separation of optically active d— and [30] Foreign Application Priority Data l-isomers of a compound selected from the group con sisting of menthol, neomenthol and isomenthol, which Feb. 27, 1971 Germany ...... 2109456 comprises esterifying a d,l-isomeric mixture of the Feb. 2, 1972 Germany ...... 2204771 compound with an acid selected from the group con sisting of benzoic acid optionally carrying at least one [52] US. Cl...... 260/631 R; 260/473 R; 260/471 R; substituent on the benzene ring and hexahydrobenzoic 260/468 R; 260/476 R acid; forming a supersaturated solution or a super [51] Int. Cl.2 ...... C07C 35/12 cooled melt of the dJ-ester; inoculating the solution or [58] Field of Search ...... 260/631 R, 631 H melt with crystals of the d- or l-form of the ester to effect selective crystallization of one of the two [56] References Cited optical isomers; separating the crystals and subjecting UNITED STATES PATENTS them to hydrolysis to yield the corresponding enantio 2,120,131 6/1938 Harris ...... 260/631 H morph of the starting compound. FOREIGN PATENTS OR APPLICATIONS 5 Claims, 1 Drawing Figure 568,085 1/1933 Germany ...... 260/631 H US. Patent March 9, 1976 3,943,181 3,943,181 1 2 such cases the carbocyclic nucleus may contain as SEPARATING OPTICALLY PURE d- AND substituents: alkoxy groups, preferably lower alkoxy l-ISOMERS OF MENTHOL, NEOMENTHOL AND groups containing up to 4 carbon atoms; nitro groups; ISOMENTHOL halogen atoms such as ?uorine, chlorine, bromine; or This is a continuation of application Ser. No. alkyl groups, preferably lower alkyl groups containing 229,109, ?led Feb. 24, 1972, now abandoned. up to 4 carbon atoms which may be substituted by This application relates to separating optically pure halogen atoms, such as trichloromethyl or tri?uoro d- and l-isomers of menthol, neomenthol and isomen methyl groups. p thol. ‘ The d,l-esters of benzoic acid, hexahydrobenzoic Processes for the synthetic production of menthol, acid, 4-methylbenzoic acid, 3,5-dinitrobenzoic acid neomenthol and isomenthol are known. Unfortunately, 4-methoxy-and 4-ethoxybenzoic acid are preferably the products obtained are mixtures of the d- and l-form used for the process according to the invention. and are inferior in their properties to the optically pure It has proven to be of particular advantage for carry isomers. For example, a mixture of d- and l-menthol is ing out the separation by the process according to the distinctly inferior to the l-menthol which occurs natu invention to use the d,l-ester of benzoic acid,_hexahy rally in peppermint oil because of the dcontent. Ac drobenzoic acid, 4-methylbenzoic acid, 3,5-dinitroben cordingly, there is considerable interest in processes for zoic acid or of 4-ethoxybenzoic acid for menthol, and separating the d- and l-isomers of menthol, neomenthol the d,l-ester of 4-methylbenzoic acid, 3,5-dinitroben and isomenthol. zoic acid or of 4-methoxybenzoic acid for neomenthol. The separation of these optical isomers can be car 20 The separation of isomenthol by the process according ried out for example by fractional crystallization of the to the invention is best carried out through its esters salts of optically active amines such as cinchonine, with 4-methoxybenzoic acid or 3,5-dinitrobenzoic acid. brucine, strychnine, ephedrine and l~( a-naphthyl) The d,l-isomer mixturev can be esteri?ed by any ethyl amine, with d,l-menthyl hydrogen phthalate or known method, for example in accordance with the hydrogen succinate or the corresponding d,l-neoment 25 description in Beilstein’s Handbuch der Organischen hyl derivatives. The separation process as a whole in Chemie, 4th Edition, Vol. 9 (1926) on page 1 15 for the cludes formation of a salt with the optically active preparation of benzoic acid-l-menthyl ester or ibid., amine; separation of the two diastereomeric semi ester Vol. 9 (Suppl. 3) (1970) on page 408 for the prepara salts by crystallization; splitting the d- and l-semi ester tion of benzoic acid-d-neomenthyl ester. salt into the optically active esters and ?nally hydrolyz 30 The process according to the invention can be car ing these into the d- and l-forms. In addition to the fact ried out by initially preparing a saturated solution of that the described process is a multi-stage process the d,l-ester at temperatures below the melting point of which uses optically active bases, which are themselves the racemate, preferably at temperatures of from 0° to extremely difficult to prepare, the physiologically 20°C. Examples of suitable solvents include hydrocar harmful bases used during splitting must be very care 35 bons, such as light petrol, cleaning spirit, hexane, cy fully removed in view of the use of l-menthol for exam clohexane, or toluene; ethe‘rs such as diethyl ether, ple in foodstuffs, confectionary and body-care prepara diisopropyl ether, dioxane or tetrahydrofuran; ketones, tions. such as acetone, methylethyl ketone, diethyl ketone, Because of the dif?culties mentioned above, natu methylpropyl ketone, methylbutyl ketone, di-n-propyl rally occurring starting materials must always be used 40 ketone or methylisopropyl ketone; or alcohols, such as in the largescale production of optically pure isomers methanol, ethanol, n-propanol, isopropanol, butanol, of menthol, neomenthol and isomenthol, for example linear or branched-chain amyl alcohols or hexanols, in'the recovery of l-menthol or neomenthol, pepper benzyl alcohol or cyclohexanol and mixtures of the mint oil which contains the l-menthol or d-neomenthol aforementioned solvents. Preferred solvents are petrol, itself, or optically active citronella recovered from oil 45 methanol, ethanol or acetone. The saturated solution of citronella may be used. Unfortunately, these natural separated from the sediment is heated by about 2° C., starting materials ?uctuate both in quality and in com as a result of which any crystal seeds still present disap position, depending upon their origin. Accordingly, it pear, and is divided into two, equal parts. The solutions would constitute a major technical stepforward if a are then adjusted to a slightlysupersaturated state in synthetically obtained isomer mixture could be used as 50 which there is still no spontaneous seed formation, by starting material in the large-scale production of the cooling to about 1° to 3° C. below the saturation tem aforementioned compounds. perature. If one batch is then inoculated with crystals of It has now been found that the optically active d- and the l-form and the other with crystals of the d-form only l-isomers of menthol, neomenthol and of isomenthol that menthyl ester corresponding to the form of the can be recovered by esterifying the d,l-isomer mixture 55 optically active seed crystals crystallizes out of the of the corresponding compound with benzoic acid supersaturated solution. The crystallizates of both which optionally contains one or more substituents on batches can be separated from the mother liquor in the the benzene ring, or with hexahydrobenzoic acid; inoc usual ways, for example by ?ltration or by centrifuging. ulating a supersaturated solution or a supercooled melt After they have been combined and reheated, the of the resulting d,l-ester with crystals of the d- or l-form 60 mother liquors are used to dissolve more racemate and of the ester and, in this way, selectively crystallizing the thus resaturated solution may again be used for the one of the two optical enantiomorphs, and hydrolyzing process according to the invention. the resulting ester of the d- or l-form by methods known Separation can of course, also be carried out by inoc per se. ulating a melt supercooled by about 1 to 3° C. with Apart from the esters of benzoic acid or of hexahy 6.5. crystals of l- or d-menthyl ester. In general, however, drobenzoic acid themselves, the esters of monosubsti separation is carried out using a solvent. tuted or polysubstituted benzoic acids are also prefera The separation of d,l-methyl esters can be carried out bly used for the process according to the invention. In either continuously or in batches. For example, contin 3,943,181 3 4 uous separation can be carried out as shown in the accompanying drawing, which is a schematic ?ow sheet EXAMPLE 1 of a suitable apparatus. 220 liters of petrol are cooled to +3°C in a vessel A saturated solution of the d,l-menthyl ester is pre equipped with stirring mechanism and cooling coils. pared in a 500 liter vessel 1 equipped with a stirring Solid d,l-methyl benzoate is added with stirring in a mechanism and cooling coils 2. This solution is ?ltered suf?cient quantity (about 350kg) to leave behind a off under suction of two metering pumps 4 and 5 small undissolved residue. The solution thus saturated at +3°C is ?ltered off from the undissolved menthyl through a wire basket 3 which retains most of the benzoate and heated to approximately +5“ C. so that solids. Both pumps are adjusted to have the same out any crystal seeds still present disappear. The solution is put and force the streams of solution through candle then introduced in equal parts into two vessels which ?lters 6 and 7, which separate off any crystals still are equipped with stirring mechanisms and cooling present, and into a heat exchanger 8, in which the coils, and cooled to 0° C. One of the two containers is solutions are heated by about 2° C. The solutions are inoculated ‘with approximately 5 kg of l-menthyl-ben now slightly undersaturated, so that any crystal seeds 15 zoate crystals and the other with an equivalent quantity still present after passage through the ?lters disappear. of d-menthylbenzoate crystals. Crystallization is com In a second heat exchanger 9, the two streams of solu plete after 1 hour stirring. The crystals are separated tion are cooled to about 1° to 3° C. below the saturation from the mother liquor by centrifuging and washed temperature and then ?ow into two 250 liter capacity with cold petrol. Approximately 19 kg of each of d- and vessels 11 and 12 which are equipped with stirring 20 l-menthylbenzoate crystals are thus obtained. 5 kg mechanisms and cooling coils l0 and 13 and which are batches of each are retained for inoculating the next at substantially the same temperature as the in?owing batch. Optically pure d- and l-menthol is obtained from solutions. The l- or d-form is selectively crystallized the main quantity by hydrolysis. The mother liquors from the supersaturated solutions in these vessels by from both crystallizations are combined and returned addition of seed crystals of the optically pure l- or d 25 together with 28 kg of additional solid menthylbenzo menthyl esters. The solutions are ?ltered off from the ate to the dissolution vessel in which they are resatu crystallizing vessels 11 and 12 under suction of two rated at 3° C. metering pumps 16 and 17, which are adjusted to have EXAMPLE 2 the same output, through wire baskets l4 and 15 which 30 A continuous separation process which can be car retain the crystals. The output of the metering pumps ried out in the apparatus shown in the drawing will now 16 and 17 is lower than that of the pumps 4 and 5 by be described. A 500 liter vessel 1 equipped with stirring such an extent that the solution volume in the crystal mechanism and cooling coils 2 contains a solution satu lizing vessels increases at the same rate as the quantity ‘ rated at +3° C. of d,l-menthylbenzoate in petrol and an of suspended crystals and the concentration of the 35 excess of solid d,l-methylbenzoate which is continu solution is thus maintained constant. The streams ?ow ously or periodically replenished so that the solution ing out of the metering pumps 16 and 17 are combined, always remain saturated. The solution is '?ltered off restored to the temperature of the dissolution vessel 1 under suction by two metering pumps 4 and 5 through in a heat exchanger 18 and returned to the dissolution a wire basket 3 which retains most of the solids. Both ‘ vessel, in which they are resaturated with the d,l-ment 40 the metering pumps are adjusted to have the same hyl ester which is present as sediment and which is output of 250 liters per hour and force the respective continuously or periodically replenished. Crystal sus solutions through candle ?lters 6 and 7, in which crys pension is removed from the bottom of the crystallizing tal seeds are ?ltered off, and into a heat exchanger 8, in vessels 1 1 and 12 by means of the measuring vessels 19 which the streams are heated from +3° C. to +5° C. so and 20 or continuously by means of slurry pumps and 45 that any crystal seeds which may have passed through the crystals separated off, for example by centrifuging. the ?lter are dissolved into the now undersaturated The mother liquors are also returned to the dissolution solution. In a second heat exchanger 9, the two streams vessel 1 after having been combined and heated to the of solution are cooled to 0° C. and ?ow at this tempera temperature of that vessel. The crystallizates of l- and ture into two 250 liter capacity vessels 11 and 12 which d-menthol ester obtained by a continuous process or a are equipped with stirring mechanisms, kept at 0° C. by cooling coils l0 and 13 and contain seed crystals of batch process according to the invention and the l- and l-menthylbenzoate and d-menthylbenzoate, respec d-menthol obtained from them by hydrolysis are opti tively. The quantity of seed crystals should be fairly cally pure. high to enable crystallization to take place as quickly as It must be regarded as extremely surprising that opti 55 possible (approximately 200 to 300 g/liter of solvent cally pure d- and l-isomers can be obtained by the pro volume). Two metering pumps 16 and 17 remove the cess according to the invention because, d,l-men‘thol solutions under suction from these crystallizing vessels for example cannot be separated into its optical iso through wire baskets l4 and 15 which retain the crys mers by selective crystallization despite the use of a tals. ‘The metering pumps 16 and 17 are adjusted to a variety of different solvents and a variety of di?‘erent lower output (approximately 190 liters per hour) than working temperatures and concentrations. Thus cer the metering pumps 4 and 5 so that the solution volume tain carboxylic acid esters of d,l-menthol, especially in both crystallizing vessels increases at the same rate those of benzoic acid and derivates thereof, had not as the quantity of suspended crystals and their quantity been expected to lend themselves to separation by per liter thus remains constant. The streams ?owing out selective crystallization. 65 of the metering pumps 16 and 17 are combined, re The invention is further described in the following heated at +3° C. in a heat exchanger 18 and ?ow back illustrative examples wherein all parts are by weight into the dissolution vessel 1 where they are resaturated unless otherwise expressed. with d,l-methylbenzoate. At approximately hourly in 3,943,181 tervals, crystal suspension is removed‘ from the ‘crystal kg of d-menthyl-4-methylbenzoate and the other with 5 lizing vessels in quantities‘ of 60 liters 1'1 and-l2 by. kg of 1-menthyl-4-methylbenzoate. After stirring for 1 means of the 60 liter capacity measuring vessels 19 and hour, the crystals obtained are separated off from the 20, and the crystals separated voff from‘ the ‘mother mother‘ ‘liquorlb'y "centrifuging and washed with cold liquor, for example by centrifuging. The mother liquors .rrne'thanol ‘to yield approximately‘7 kg of d: and l-ester. ?ow back into the dissolution vessel 1 ‘after ‘being Optically ‘pure d- and l-nienthol is obtained by hydro heated to +3° C. in the heat exchanger 18. The crystal lyzing the d- and l‘m‘enthy‘l esters, respectively. line esters thus obtained 'are' washed- with cold'petroll ’ Approximately 15 kg each of d- and'il-menthylbenzoate . v - EXAMPLE 6 , are obtained per hour. Removal of the crystal suspen ‘ ‘Two 80 kg batches of d,l-menthyl-4-ethoxybenzoate sion from the crystallizing vessels 11 and>l2 can also be aredissolved in 120 kg of toluene in two vessels which carried out continuously, for example by means of are equipped with stirring mechanisms and the solu slurry pumps. Optically pure d- and l-menthol can be tions cooled to -2° C. One of these vessels is inoculated obtained by hydrolyzing the d- and :l-menthyla esters. with 5 kg of d-ment'hyl;4-ethoxy‘benzoate crystals‘v and the other with 5 kg of l-menthyl-4-ethoxybenzoate I EXAMPLE ‘ crystals. After stirring 'for 1.5 hours, the crystals are 'Two 100 kg batches :of d,l-menthylbehzoate are fused separated'off from the mother liquor by centrifuging. by heating to 30° C. in two containers, ‘which are Washing with cold methanol gives approximately 7.0 equipped with stirring mechanisms, and subsequently‘ kg, eachv ‘ of ' d-and ll-menthyl-4-ethoxybenzoate .which cooled “to 25° C. 5 kg of d-menthylbenzoatecrystals and 20 are hydrolyzed into optically pure d-' and l-menthol. 5 kg of l-menthylbenzoate crystals are stirred into the‘ melts. After stirring for 1 hour, crystals are’separated ' ' .EXAMPLE7 _ g ' _. off from the melt by centrifuging ‘and washed with ‘cold Two 21.4 kg batches of d,l-menthyl-3,5~dinitroben methanolnApproximately 6.5- kg each of-d- and ,lV-men-f zoate ,are'dissolved in‘ 178.6 kg of acetone at 25° C. in thylbenzoate crystals are obtained in this way and opti 1 two vessels which ‘are equipped witlr'ilstirring mecha cally pure d- and l-menthol is obtained therefrom by nisms and 'the solutions‘ cooled to’ l-‘5° C.‘ One ofithe hydrolysis. The melt centrifuged off from thel-men vessels is then inoculated’With‘S kg "of'd-menthyl-3,5 thylbenzoate crystals contains an excess ‘of d-inenthyl dinitrihenzoate crystals‘and'the" other with 5 kg of‘ l benzoate (and vice versa). Accordingly, -it isadvisable, menthyl-3 ,5-dinitrobenzoate crystalsk After‘- stirring: for in continuing the process, to heat thismelt to 30° C., ‘1.5 ,hours, the crystals'are "separated off from ‘the replace the quantity which has crystallized out with mother liquor by centrifugingand‘washed with cold fresh menthylbenzoate, and inoculate with, d-menthyl methanol, to give approximately 7.0 kg of d-m'enthyl benzoate crystals (the reverse procedure beingadopted "and 7.0 kg‘ of l'menthyl-3,S-dini'trbbenzoate- which are fortheothe'rmelt); " .1; ' w. . hydrolyzed into'optically pure 6- andsl-menthol'.‘ 35 ‘ "EXAMPLE T42 . , 1 , ..Ex1AMBI./E;8"f-, " } ff" 1 ,_.~ _. 300 liters of petrol cooled’,to.-FL,O° Jinla vessel 50 parts by weight of d,l‘f‘neomenthyl-4-methylbenzo which is equipped with‘astirring'lmechahism and cool ate are dissolved in 50,-parjts by weight of acetone in a ing coils. 'Solid d,l-menthylhexahydrebenzoate is' added ‘_ vessel equipped with stirring mechanism and a cooling with stirring. in suf?ciently'large‘quantity (approxi 40 ‘system, and cooled to 1'4.7°_C. Thesolutiomislinocu mately 520’ kg) to leave behind a small uhdissolyed latediwith 25 parts by weight of d-neom'enthyl-4 .residue. The solution thus saturated at‘fl-I‘WJC,‘ is'?l methylbenzoate.~ It‘is» then-cooled to‘ 13.8.‘? 'Cmand tered offgand heated to approximately'flTy'C; 'so?that ‘ stirred for a further'ZO minutes. The‘crystals are then any crystalv seeds still present are dissolvedf’Ifhe-s'olu ‘separated off from the mother liquor ‘by centrifuging. tion is then introduced into'ar'iother'_ vesselequipped 45 -Washing with cold- methanol gives approximately’ 7.5 with a stirring mechanism and with cooling‘ coils. After parts" by weight‘v of- d-neomenthyl-4¢methylbenzoate. the solution has been ‘cooled to’9° C.‘,‘i't' isiinoculated *Optically‘ pure d-neomenthol is obtained by hydrolyz with 5 ‘kg of l-menthylhexahydrobenzoate‘. Crystalliza ing‘ the ester. Optically pure l-ne'omenthol is‘similarly tion is complete after stirrin'gifor about 2'hours and'the obtained by1 inoculating the mother liquorwith l-neo 50 crystals are separated off ‘from 'the‘mother'liquor by .menthyl-4-m'ethylbenzoate. ‘ I r centrifuging and washe'd‘with cold petrol. 'After drying, ' approximately 22 kg'arefobtained, (stamens ‘kg are ‘ ExAMélsEg'ffg. Y . 7 retained‘ for 'i'noculat'ing‘the‘ nextlbatch. The tempera 80.2 kg of an almost saturated ‘solution consisting of ture of the mother liquor‘ adjusted to 9° in another, 13 parts by weight of d,l-neomenthyl-4-methoxybenzo vessel“ equipped with stirring mechanism and similarly ate and 87 parts‘ by weight‘ of methanol; are‘cooled to crystallized by inoculation with 5kg offd-menthylhex +2.5“- C. inia vessel which is equipped with Y'stirring ahydrobenzoate. The optically pure d- and l-menthol is "mechanism and afcooling system, and 300g‘ of'd-neo subsequently recovered from the crystallizates ob menthyl-4-methoxybenzoate are then added. tained by hydrolyzing the d- and l-menthyl esters. The The mixture is stirred for approximately 50 minutes mother liquor is resaturated with d,l-menthylhexahyi .60 during which its temperature is reduced to —3.5° C. drobenzoate at 10° C. and the process repeated. The crystals which accumulate are ?ltered off under suction to give 1.23 kg of d-neomenthyl-4-methoxyben EXAMPLE 5 zoate. Optically pure d-neomenthol is obtained by hy Two 87 kg batches of d,l-menthyl-4-menthylbenzoate drolyzing the ester. (an oily liquid which is almost impossible to crystallize)‘ 65 l-neomenthol can be similarly obtained by adding are each mixed with 13 kg of methanol in two vessels l-neomenthyl-4-methoxybenzoate to the d,l-neoment which are equipped with stirring mechanisms and hyl-4-methoxybenzoate solution and by hydrolyzing cooled to —7° C. One of the vessels is inoculated with 5 the obtained ester. ‘ ‘ 3,943,181 7 8 solution is inoculated with 0.6 g d-neomenthylbenzo EXAMPLE l0 ate. It is then cooled to —14° C. and stirred for a further 5 g of d,l-neomenthyl-3,S-dinitrobenzoate are dis 35 minutes. The crystals which accumulate are ?ltered solved in 33 g of acetone in a vessel which is equipped off under suction to give 5.6 g d-neomenthylbenzoate. with stirring mechanism‘ and a cooling system and 0.5 g Optically pure d-neomenthol is obtained by hydrolyz of d-neomenthyl-3,S-dinitrobenzoate are then added. ing the ester. 7 . The solution is then cooled to 169° C- and stirred for Optically pure l-neomenthol is similarly obtained by 30 minutes. The crystals formed are ?ltered off under inoculating the mother liquor resp. to initial solution suction, to give 1.145 g of d-neomenthyl-3,5-dini with l-neomenthylbenzoate followed by hydrolysis. trobenzoate. l-neomenthol-3,S-dinitrobenzoate is simi larly obtained by adding 0.5 g of l-neomenthyl-3,5-dini EXAMPLE 14 trobenzoate to the solution. ' 31 g of d,l-neomenthylhexahydrobenzoate are dis Optically pure d- and l-neomenthol are obtained by solved in 69 g of methanol in a vessel equipped with hydrolyzing the corresponding ester. stirring mechanism and, a cooling system, and cooled to +14° C. The solution is inoculated with 0.35 g d EXAMPLE 1 l neomenthylhexahydrobenzoate. It is then cooled to 31 parts by weight of an almost saturated solution +13° C. and stirred for a further 25 minutes. The crys consisting of 78.4 percent by weight of d,l-isomenthyl tals which accumulate are ?ltered off under suction to 4-methoxybenzoate and 26 percent by weight of meth give 2.9 g d-neomenthylhexahydrobenzoate. Optically anol, are heated to +50° C. in a vessel which is 20 pure d-neomenthol is obtained by hydrolyzing the es equipped with stirring mechanism and 0.05 part by ter. weight of d-isomenthyl-4-methoxybenzoate are then Optically pure l-neomenthol is similarly obtained by added. inoculating the mother liquor resp. the initial solution The mixture is stirred for approximately 60 minutes with l-neomenthylhexahydrobenzoate followed by hy ' during which its. temperature is reduced to +45° C. The 25/ drolyzis. crystals which accumulate are ?ltered off under suc What is claimed is: tion. Washing with cold methanol gives 0.65 part by l. A process for the separation of optically active d weight of d-isomenthyl-4-methoxybenzoate. Optically and l-isomers of a compound selected from the group pure d-isomenthol is obtained by hydrolyzing the ester. 30 consisting of menthol, neomenthol and isomenthol, Optically pure l-isomenthol is similarly obtained by which comprises forming a super-saturated solution or inoculating the mother liquor with l-isomenthyl-4 a supercooled melt consisting essentially of a d,l-iso methoxybenzoate. , I meric mixture of an ester of the compound with an acid It will be appreciated that the instant speci?cation - selected from the group consisting of benzoic acid, and examples are set forth by way of illustration and 35 hexahydrobenzoic acid and benzoic acid substituted by not limitation, and that various modi?cations and up to 2 lower alkyl orv alkoxy, ?uorine, chlorine, bro changes may be made without departing from the spirit mine or nitro groups inoculating the solution or melt and scope of the present invention. with crystals of the d- or l-form of the ester to effect EXAMPLE 12 selective crystallization of one of the two optical iso 43.5 g of d,l-isomenthyl-3,5-dinitrobenzoate are dis mers; separating the crystals and subjecting them to solved in 6.5 g of hot toluene in a heatable vessel which hydrolysis to yield the corresponding enantiomorph of is equipped with stirring mechanism and re?ux con the starting compound. . denser. The solution is heated to +105° C. and then 2. A process as claimed in claim 1, wherein the acid inoculated with 0.5 g of d-isomenthyl-3,S-dinitrobenzo is benzoic acid, hexahydrobenzoic acid, 4-methylben ate. The mixture is stirred for approximately 30 min 45 zoic acid, 3,5~dinitrobenzoic acid, 4~methoxybenzoic utes during which its temperature is reduced to 103° C. acid or 4-ethoxybenzoic acid. _ The crystals formed are filtered off under suction, to 3. The process of claim 1, wherein the compound is give 5.6 g d-isomenthyl-3,S-dinitrobenzoate. Optically menthol‘ and the acid is benzoic acid, hexahydroben pure d-isomenthyl is obtained by hydrolyzing the ester. zoic acid, 4-methylbenzoic acid, 3,5-dinitrobenzoic Optically pure l-isomenthol is similarly obtained by 50 acid or 4-ethoxybenzoic acid. ‘ inoculating the mother liquor resp. the initial solution 4. The process of claim 1, wherein the compound is with l-isomenthyl-3,S-dinitrobenzoate. neomenthol and the acid is 4~methylbenzoic acid, 3,5 dinitrobenzoic acid or 4-methoxybenzoic acid. EXAMPLE 1'3 5. The process of claim 1, wherein the compound is 55 g of d,l-neomenthylbenzoate are dissolved in 45 g 55 isomenthol and the acid of 4-methoxyb‘enzoic acid or of methanol in a vessel equipped with stirring mecha 3,5-dinitrobenzoic acid. ‘ nism and a cooling system, and cooled to —-l 3.5° C. The * * Ill * *

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