Resolution

The analysis and Separation of Enanatiomers.

1. Prof. Amol . S. Dighe Department of pharmaceutical Chemistry Pravara Rural College of Pharmacy, Loni RESOLUTION OR  THE ANALYSIS AND SEPARATION OF ENANTIOMERS

2. The process of separation of racemic form into individual enantiomers is called resolution. Separating enantiomers is called resolution. A pair of enantiomers can be separated in several ways, of which conversion to diastereomers and separation of these by fractional crystallization is the most often used. In this method and in some of the others, both isomers can be recovered, but in some methods it is necessary to destroy one.

3. Methods of resolution The following are various methods used for resolution of racemic mixtures: Conversion to Diastereomers . Differential Absorption Chiral Recognition. Biochemical Processes Mechanical Separation Kinetic Resolution Deracemization

4. Conversion to Diastereomers To separate components of a racemate (reversibly) we make a derivative of each with a chiral substance that is free of its enantiomer (resolving agent). This gives diastereomers that are separated by theirdifferential solubility. The resolving agent is then removed. Usually, fractional crystallizations must be used and the process is long and tedious. Fortunately, naturally occurring optically active bases (mostly alkaloids) are readily available. Among the most commonly used are brucine , ephedrine, strychnine, and morphine. Once the two diastereomers have been separated, it is easy to convert the salts back to the free acids and the recovered base can be used again. Most resolution is done on carboxylic acids and often, when a molecule does not contain a carboxyl group, it is converted to a carboxylic acid before resolution is attempted. However, the principle of conversion to diastereomers is not confined to carboxylic acids, and other functional groups may be coupled to an optically active reagent. Racemic bases can be converted to diastereomeric salts with active acids. Alcohols can be converted to diastereomeric esters, aldehydes to diastereomeric hydrazones, and so on.

5. Using an Achiral amine doesn’t change the relationship of the products Still can’t separate the Enantiomeric Salts.

6. Using a Chiral amine changes the relationship of the products Now we can separate the Diastereomeric Salts.

7. 2) Differential Absorption/Chromatographic Resolution of Enantiomers When a racemic mixture is placed on a chromatographic column, if the column consists of chiral substances, then in principle the enantiomers should move along the column at different rates and should be separable without having to be converted to diastereomers . This has been successfully accomplished with paper, column, thinlayer , and gas and liquid chromatography. For example, racemic mandelic acid has been almost completely resolved by column chromatography on starch. Columns packed with chiral materials are now commercially available and are capable of separating the enantiomers of certain types of compounds.

8. Chiral Recognition In this host forms an inclusion compound with one enantiomer of a racemic guest, but not the other. This is called chiral recognition. One enantiomer fits into the chiral host cavity, the other does not. More often, both diastereomers are formed, but one forms more rapidly than the other, so that if the guest is removed it is already partially resolved. An example is use of the chiral crown ether partially to resolve the racemic amine salt. When an aqueous solution of 59 was mixed with a solution of optically active 58 in chloroform, and the layers separated, the chloroform layer contained about twice as much of the complex between 58 and (R)-59 as of the diastereomeric complex. Many other chiral crown ethers and cryptands have been used, as have been cyclodextrins , cholic acid, and other kinds of hosts.

9. Biochemical Processes Biological molecules may react at different rates with the two enantiomers. For example, a certain bacterium may digest one enantiomer , but not the other. Pig liver esterase has been used for the selective cleavage of one enantiomeric ester. This method is limited, since it is necessary to find the proper organism and since one of the enantiomers is destroyed in the process. However, when the proper organism is found, the method leads to a high extent of resolution since biological processes are usually very stereoselective .

10. Mechanical Separation This is the method by which Pasteur proved that racemic acid was actually a mixture of (+)- and (-)-tartaric acids. In the case of racemic sodium ammonium tartrate , the enantiomers crystallize separately: all the (+) molecules going into one crystal and all the (-) into another. Since the crystals too are non- superimposable , their appearance is not identical and a trained crystallographer can separate them with tweezers. However, this is seldom a practical method, since few compounds crystallize in this manner. Even sodium ammonium tartrate does so only when it is crystallized <270C. A more useful variation of the method, although still not very common, is the seeding of a racemic solution with something that will cause only one enantiomer to crystallize.

11. Kinetic Resolution Since enantiomers react with chiral compounds at different rates, it is sometimes possible to effect a partial separation by stopping the reaction before completion. A method has been developed to evaluate the enantiomeric ratio of kinetic resolution using only the extent of substrate conversion. When a racemic substrate reacts with a homochiral reagent two reactions occur, one with each enantiomer of the substrate.  Each process has a transition state which incorporates the same enantiomer of the reagent, but opposite enantiomers of the substrate.  The transition states for the two processes have a  diastereoisomeric  relationship, and the respective activation energies are different.  Therefore the reactions proceed at  different rates .  If the rates of the two reactions are sufficiently different, the reaction can be used for separating the enantiomeric reactants.

12. For a kinetic resolution to be  100% efficient , the faster reaction would have to be finished before the slower reaction starts. 

13. Deracemization In this type of process, one enantiomer is converted to the other, so that a racemic mixture is converted to a pure enantiomer , or to a mixture enriched in one enantiomer . This is not quite the same as the methods of resolution previously mentioned, although an outside optically active substance is required. To effect the deracemization two conditions are necessary: (1) the enantiomers must complex differently with the optically active substance; they must interconvert under the conditions of the experiment. When racemic thioesters were placed in solution with a specific optically active amide for 28 days, the solution contained 89% of one enantiomer and 11% of the other. In this case, the presence of a base ( EtN ) was necessary for the interconversion to take place. Biocatalytic deracemization processes induce deracemization of chiral secondary alcohols.

14. Thank you