Optical Activation or Enantiomer Resolution?

[GhostChemist]

Optical Activation or Enantiomer Resolution?​

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There are numerous scientific publications describing the separation of enantiomers of various compounds. But what does the term optical activity mean? Is it the same as enantiomeric separation?​

In chemistry, optical activity refers to the ability of a chiral substance to rotate the plane of polarized light. This property is a direct consequence of molecular chirality, where two enantiomers (mirror-image isomers) rotate light in opposite directions: one is dextrorotatory (rotates light to the right, denoted “+” or “D”), and the other is levorotatory (rotates light to the left, denoted “–” or “L”).​

Optical activity is not the same as enantiomer separation. Instead, it is a measurable physical property used to characterize chiral molecules. Enantiomeric separation, often achieved by chromatographic methods (such as chiral HPLC, capillary electrophoresis, or gas chromatography with chiral stationary phases), aims to physically isolate the two enantiomers from a racemic mixture.​

The measurement of optical activity (using a polarimeter) is frequently applied after separation to determine the optical purity or enantiomeric excess of a sample. Thus, optical activity provides information about chirality and purity, while separation is the actual process of isolating enantiomers.​

In this review, we highlight the most accessible and practical methods for the separation of enantiomers, with a particular focus on techniques that do not require specialized or high-precision instrumentation. Such approaches are especially valuable in laboratories with limited resources, in educational settings, or during preliminary investigations where the use of advanced chromatographic or spectroscopic equipment may not be feasible. The discussion emphasizes methods that are relatively simple to implement, cost-effective, and reliable, while still providing sufficient resolution to demonstrate the principles of enantiomeric separation.​

Examples of Methods​

Many resolution agents are presented in: David Kozma. CRC handbook of optical resolutions via diastereomeric salt formation. CRC Press LLC, 2002. ISBN 0-8493-0019-3.

1. Methamphetamine and Amphetamine​

No direct correlation could be identified between the examined parameters and the overall separation efficiency. However, factors such as the molar ratio, temperature, and pressure, along with the possible formation of a specific combination of neutral and acidic salts of defined composition, appear to play a crucial role in determining the effectiveness of the separation process.​

NOTE I: The sign of optical rotation, although different for the two enantiomers of a chiral molecule, at the same temperature, cannot be used to establish the absolute configuration of an enantiomer!​

NOTE II: It is important to emphasize that the absolute configuration of a stereocenter, denoted as R or S, does not determine whether a compound will rotate plane-polarized light to the right (+) or to the left (–). The direction of optical rotation must always be established experimentally using a polarimeter, as there is no intrinsic correlation between these two descriptors [5].​

This methods can be apply for resolution of racemic Amphetamine.​

Method 1.1. Resolution with using R,R-Tartaric Acid​

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Racemic base (3 g, 0.02 mol) was dissolved in absolute ethanol (8 ml). To this clear solution, a solution of RR-tartaric acid (L-(+)-Tartaric acid) (3 g) in absolute ethanol (15 ml) was added dropwise with continuous stirring. The precipitation of diastereoisomeric salt mixture started immediately. The mixture was stirred at 5℃ for 72 h and the crystalline mass was then filtered off and recrystallized from absolute ethanol, [α]D20 +4.77 (c 5, water), m.p. 163-164℃ for (R)-(-)-N-methylamphetamine (R,R)-tartrate (RMERTA). The mother liquor was concentrated to dryness. The residue was recrystallised from ethanol, until the optical rotation was unchanged [α]D20 +22.1 (c 5, water), m.p. 114-115℃ for (S)-(+)-N-methylamphetamine (R,R)-tartrate (SMERTA).​

Method 1.2. Resolution with using R,R-Tartaric Acid and HCl. Pope-Peachey Method​

Racemic N-methylamphetamine (free base) can be resolved by the Pope-Peachey method using half an equivalent of R,R- tartaric acid (II) and half equivalent of hydrochloric acid (HCl) after 115 hrs for crystallization.

7.84 g (0.0525 mol) racemic N-methylamphetamine and 9.73 g (0.0525 mol) racemic N-methylamphetamine hydrochloride were dissolved in 70 ml of abs. ethanol and 7.91 g (0.0525 mol) R,R-tartaric acid in 56 ml of abs. ethanol. Both stock solutions were divided into fifteen portions by volume by a burette and reacted at 22±1℃. The mixtures were standing undisturbed for 15, 90 minutes and 5, 24, 115 hours, than the precipitates were filtered and dried. From 0.4 g of each salt the base was liberated by ccNaOH and extracted from the aqueous phase by DCM.​

All experiments were performed at the same work-bench at 22℃ room temperature. Stock solutions of the racemate and the resolving agent were used to avoid the error of weight measurements. All the flasks used had identical size and shape​

The weights of precipitated salts and their tartrate content were approximately the same in each experiment, indicating that nearly all tartrates precipitated immediately. The optical purity of the precipitated salt was quite low in the beginning, at longer times the optical purity of the precipitates definitely increased, proving that there is an enantiomer exchange between the precipitated salt and the hydrochloride salt. The R-I enantiomer accumulated in the solid phase, as R-I.II salt, while in the mother liquor the S-I.HCI accumulated. Optical purity not more 50%​

Method 1.3. Resolution of N-Methylamphetamine Via Diastereoisomeric Salt Formation With 2R,3R-O,O*-Di-p-Toluoyltartaric Acid​

General Procedure for Optical Resolution of N-methylamphetamine by O,O-Di-p-toluoyltartaric Acid 3.0 g (0.02 mol) N-methylamphetamine was dissolved in half of the solvent and mixed at room temperature with given amount of O,O-di-p-toluoyltartaric acid dissolved in the other half of the solvent. (In the third experiment ccHCl was added to the cooled solution of N-methylamphetamine, before adding the solution of the resolving agent.) The mixture was stirred at room temperature for 30 min while the precipitation started. The mixture was stored at 10°C for 2 h without stirring and then was filtered. The salt was dried in air; the mother liquor was evaporated to dryness in vacuum. Both fractions were reacted by 15 ml 2N NaOH and extracted three times with 25 ml of dichloromethane. After drying over MgSO4, the solvent was evaporated and the specific rotation was measured. The specific rotations of the precipitates range from [α]D20 –15.5 to –11.5, while those of the mother liquor fall within +8.0 to +11.6.​

Method 1.4. Optical Resolution of N-Methyl-Amphetamine by O,O'-Dibenzoyl-R,R-Tartaric Acid in Dichloroethane-Water-Methanol Solvent System​

15.0g (0.1 mol) racemic base was dissolved in the mixture of 20 ml of dichloroethane and 15 ml of water A solution of 9.4 g (0.025 mol) O,O'-dibenzoyl-2R,3R-tartaric acid in 40 ml of dichloroethane and given amount of methanol was added to the two phase solution of the racemic base in 30 minutes at room temperature. From the stirred solution the crystallization starts in 10-15 minutes. The resulted suspension was stirred at 5 "C overnight, then filtered. The precipitate was washed on the filter three times with 5 ml dichloroethane 5 "C and dried under infra lamp. The precipitated salts were reacted by 30 ml 2N NaOH and extracted tree times with 25 ml of dichloromethane. After drying over MgS04, the solvent was evaporated and the specific rotation was measured. The specific rotation of the optically pure R-N-methylamphetamine is [α]D20= -18.90 (c 0.1; 1N HCI). The results are summarised in Table 1.​

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Method 1.5. Resolution of DL-Amphetamine​

Resolution of 10.5 g. (77 mmoles) of racemic amphetamine, accumulated during three separate runs, was achieved by the cyclic crystallization of the bitartrate diastereomers. A portion of the racemic deuterio-amphetamine (5.56 g.; 41 mmoles) was dissolved in 70 ml of 91% isopropanol. The solution was heated to 80℃, 6.38 g. (42.5 mmoles) of L-tartaric acid was added, and the temperature was maintained at 80℃ until the acid dissolved. The solution was allowed to cool slowly (ca. 1 h) to 60℃ and maintained at that temperature for 24 h, whereupon crystals formed. After decantation of the mother liquor, the crystalline amphetamine-L-bitartrate was filtered and washed with 13 ml. of 91% isopropanol at 60℃. Optical purity of the crystalline L-amphetamine-L-bitartrate (3.84 g) was approximately 90%. After one recrystallization at 60℃ from 91% isopropanol, 2.14 g of the diastereomer was obtained with an optical purity of 98% (equivalent to 37% of the L-isomer present in 5.56 g. of racemic amphetamine). The mother liquor, which was rich in d-amphetamine, and all rinses were combined and the solvent was removed in vacuo. The salt was dissolved in 15 ml of water, made strongly alkaline with 25% sodium hydroxide, and extracted with 300 ml. of ether. Approximately 5.0 g of D-tartaric acid was dissolved in a 91 isopropanol solution of the D-rich amphetamine base which had been recovered from the d-rich amphetamine-L-bitartrate salt. Crystallization of d-amphetamine-d-bitartrate was effected at 60℃ in a manner identical to that used for the L-bitartrate diastereomer. A yield of 3.1 g. of d-amphetamine-d-bitartrate was obtained with an optical purity of 98%. This crystallization cycle was repeated three times, adding the remainder of racemic amphetamine (4.94 g) after the third crystallization. The combined L-amphetamine-L-bitartrate fractions yielded 4.94 g of the diastereomer, [α]D20=-29.5 (2%, H2O), equivalent to 45% of the L-isomer present in 10.5 g. of racemic amphetamine base. The yield of D-amphetamine-D-bitartrate was 5.91 g. (54%), [α]D20 +29.8 (2%, H2O). The synthesized isomers possess an optical purity of at least 98% when compared to authentic samples of each diastereomer. Specific rotation of the standard samples was -30.3 and +30.5 for the L- and D- forms, respectively. The diastereomer salt of each isomer was dissolved in 15 ml of water, made strongly alkaline with 25% sodium hydroxide, and extracted with 300 ml of ether. The solvent was removed in vacuo. The sulfate salt of each isomer was prepared by the addition of an equivalent amount of 50% sulfuric acid to a solution of the respective deuterio-amphetamine base in 91% isopropanol. The crystals of each isomer were filtered with suction and dried over P2O5.​

Method 1.6. Resolution of Phenylisopropylamines​

1st. Phenylisopropylamines and phenylisopropylmethylamines and various substituted amines were resolved with 0.5 mole tartaric acid in benzene-water containing 0.5 mole sodium hydroxide or potassium hydroxide by selective extraction of either enantiomer.​

A mixture of 0.1 mole (13.52 g.) phenylisopropylamine (or 14.92 g. methamphetamine base) in 60 ml benzene, 0.05 mole d-tartaric acid (7.50 g.) in 30 ml water, and 2 g sodium hydroxide (reagent grade or titrated equivalent) in 3 ml water was kept 4 hours with intermittent shaking, and the organic phase evaporated to give 98% L-phenylisopropylamine. The aqueous phase was extracted with benzene at pH 13 and evaporated to give 96% D-enantiomer.​

2nd. Phenylisopropylmethylamine was resolved by treatment with 0.4-6 moles of dextro tartaric acid in water or aqueous ethanol containing 0.4-6 moles hydrogen chloride. A mixture of phenylisopropylmethylamine 150 g, d-tartaric acid 82.5 g, and H2O 330 g was treated with HCl to pH 4 to deposit 120 g L-phenylisopropylmethylamine-d-tartrate salt, which gave 88 g L-phenylisopropylmethylamine. The D-enantiomer (58 g as the HCl salt) was isolated from the filtrate.​

2. Ephedrine​

Method 2.1. Ephedrine Oxalate​

Crude base as obtained when treated with oxalic acid gave a clearcut separation of the two alkaloidal salts. Ephedrine oxalate being only very slightly soluble in cold water and pseudoephedrine oxalate being exceedingly soluble, the soluble and insoluble products represented a separation of the two alkaloids.​

Ephedrine Oxalate 2C10H15ON×C2H2O4 - Prismatic needles from water; m.p. 245°C with decomposition; neutral to litmus; only very slightly soluble in cold water.​

Pseudoephedrine Oxalate. 2C10H15ON*C2H2O4 - Needles; m.p. 218℃ with decomposition; difficultly soluble in alcohol; very soluble in cold H2O; neutral to litmus.​

Method 2.2. Resolution of DL-Ephedrine with L-(-)-Dibenzoyltartaric Acid​

Preparation of L- ephedrine-L-(-)- dibenzoyltartaric Dissolve DL-ephedrine(16.5 g, 0.1 mol) in mixed solvent of dichloroethane (20ml) and distilled water (15 ml), stir 5 minutes. Dropwise add the solution of dichloroethane (40ml) and carbinol (10 ml) which dissolved with L-(-)-dibenzoyltartaric acid (9.4 g, 25 mol) to the two-phase solution within 30 minutes in room temperature. And then stir 10-15 minutes. Leave the suspension 24h in 5℃, use filter to get crystal, recycle the filtrate for the next experiment. Wash and dry the obtained crystal, then we can get L-ephedrine-L-(-)-dibenzoyltartaric (13.5 g).​

Add D-ephedrine-L-dibenzoyltartaric (13.2 g) obtained in the last step to hydrochloric acid (50 ml), stir until completely dissolved in room temperature, extracte with diethyl ether(100 ml) in twice, separate out the level of diethyl ether for recycling resolving agent; reduce the press of water-course to remove water, recrystal the solid with absolute ethyl alcohol, filter out crystal, keep the filtrate for recycling DL- ephedrine. Wash the crystal with acetone, dry to get D-(-)-ephedrine hydrochloride (6.51 g), measure the optical rotation of it, as the following: [α]D20=-33.8° (5.1%， water), it is thus clear that optical purity of the product is very high. In addition, the absolute value of it is close to D-(-)- ephedrine hydrochloride, which prove the resolution is successful.​

Method 2.3. Resolution of Ephedrine·HCl with Half-Equivalent of Resolving Agent​

The resolution of ephedrine·HCl was attempted in several ways. Racemic ephedrine hydrochloride is resolved with half an equivalent of (2R,3R)-DBTA·Na. The result of the resolution is often influenced by the solvent, so the resolutions were also performed in acetone and water. The diastereomeric salt was allowed to crystallize for 2 h, and the pure crystalline enantiomer was isolated in 92.5% yield. In each case, the yields of the pure enantiomers were compared to half of the weight of the racemic compound.​

To a mixture of 0.20 g (1 mmol) of racemic ephedrine·HCl and 0.04 g (0.5 mmol) of NaOH, 0.18 g (0.5 mmol) of (R,R)-(+)-DBTA were added to 1.5 mL of water, which was heated until dissolved. After cooling and standing for 120 min, the crystalline precipitate was filtered off. The diastereomeric salt thus obtained (0.26 g) was suspended in 0.5 mL of water, and 0.2 mL of NH4OH was added to the mixture. The weight of (1S,2R)-(+)-ephedrine obtained was 0.074 g, [α]D20 = +43.2 (c = 1, MeOH), yield 92.5%.​

All methodologies reviewed herein will be subjected to experimental validation within our laboratories. Established protocols will undergo systematic optimization, while novel strategies for isomer separation will also be developed and presented. Comprehensive reports will be provided for each study, encompassing assessments of method purity, reliability and practical applicability.​

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Bibliography​

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