Resolution of Racemic Mixtures

Session Objectives

• By the end of this session, students will be able to:

• Define racemic modification

• Discuss factors which lead to formation of racemic modification

• Explain various methods for resolving racemic mixtures

Resolution of racemic modifications

• It is a process where by a racemic modification is separated into its two enantiomers

• It is quantitative and sometimes only one isomer will be isolated

• Several methods have been developed and type of separation will be selected depending upon the nature of compound

1) Mechanical separation

• Introduced by Pasteur and also known as spontaneous resolution by crytallisation

• Applicable only for the racemic mixtures where the crystal forms of enantiomers are enantiomorphous – which are separated by hand

• Pasteur separated sodium ammonium tartrate racemate in this way

• He crystallized sodium ammonium tartrate racemate from a concentrated solution at room temperature below 28 ⁰C and separated mechanically

2) Preferential crystallization through inoculation

• Supersaturated solution of the racemic modification is treated with a crystal of one enantiomer, this form is precipitated

• Resolution of glutamic acid by inoculation has been perfectly suitable for industrial use

• But this method was found impractical with amino acids

3) Biochemical separation

• Certain bacteria and moulds when they grow in dilute solution of racemic modification, destroy one enantiomer

• For example, Pencillium glaucum when grown in solution of racemic ammonium tartrate, attacks the (+)-form and leaves (-)-form

Drawbacks

• Dilute solutions must be used, so amounts obtained will be small

• One form is always destroyed and other form is not always obtained in 50%, some may also be destroyed

• To find a microorganism which will attack only one enantiomer

4) Conversion into diastereoisomers

• Best of all methods of resolution

• Racemic modification is treated with optically active substance 

• Diastereoisomers produced are separated by fractional crystallization

• For example, racemic acids can be separated by optically active bases

(D_acid + L_acid) + 2D_{base    à}    (D_acid D_base) + (L_acid D_base)

• Now the two diastereoisomers can be separated by fractional crystallization

• Enantiomers of acids can be regenerated by hydrolysis with inorganic acids or with alkalis

• Further purification can be done by chromatography

• For resolution, compounds used are-

• Acids: optically active bases are alkaloids- brucine, quinine, strychnine, morphine, cinchonine

• Bases: optically active acids used are tartaric acid, camphor-β-sulphonic acid

• Alcohols: converted to acid ester using succinic or phthalic anhydride

• Aldehydes and ketones: by means of optically active hydrazines

• Amino compounds: by means of optically active aldehydes  

5) Chromatography

• Optically active substances may be selectively adsorbed by some optically active adsorbent

• For example, Henderson and Rule (1939) partially resolved p-Phenylene bisimino camphor on lactose as adsorbent

• Bradley and Easty (1951) have found that wool and casein selectively adsorb (+)-mandelic acid from an aqueous solution of (±)-mandelic acid

• Separation of quinine and quinidine, cinchonine and cinchonidine by passing through silica gel

6) Resolution through formation of Molecular Complexes

• Common method of resolution because of weak interactions between substrate and resolving agent

• Other methods involves formation of stable salts or covalent compounds

• Complexes should be easy to form and easy to decompose

• For example, complexation with platinum reagents has been used to resolve trans-cyclooctane 

7) Biochemical methods of resolution using enzymes

• Laboratory reduction of 2-butanone gives racemic product of 2-butanol

• But reduction in the presence of chiral enzyme gives optically pure (R)-2-butanol

• Easy to acetylate racemic alcohol and treat racemic esters with lipase

• One enantiomer is hydrolyzed to alcohol while other remains as ester

• Further separated by chromatography

• Enzyme acyalse 1 (hog kidney acylase) is capable of hydrolyzing amide links (-NHAc) of L-amino acids only (stereospecificity)

8) Resolution via chiral recognition and Inclusion compounds

• Chiral recognition- only one enantiomer fits into chiral host activity, while the other does not

• Generally both diastereoisomers may be formed, one is formed more readily than other 

• For example, aqueous solution of rac-amine salt is mixed with chloroform solution of optically active crown ether, could separate complex of chiral crown ether and (R) amine salt 

• Cyclodextrins (CDs) are made up of six, seven or eight glucose units connected in a large ring termed as α, β and γ CDs

• CDs are of tub shape with primary OH groups projecting from narrow side and secondary OH groups from wide side

• Can be used for resolution of variety of compounds via inclusion complexes

Structure of cyclodextrins

Structures of common crown ethers

Summary

• Racemic modification is separation of its isomers

• Supersaturated solution of the racemic modification is treated with a crystal of one enantiomer, this form is precipitated

• Racemic modification is treated with optically active substance is the best method

• Optically active substances may be selectively adsorbed by some optically active adsorbent