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Module No. 20: Methods of Resolution Subject Subject Chemistry Paper No and Title 1; Organic Chemistry-I (Nature of bonding and Stereochemistry) Module No and 21: Methods of Resolution Title Module Tag CHE_P1_M21 CHEMISTRY Paper No. 1: Organic Chemistry-I (Nature of bonding and Stereochemistry) Module No. 20: Methods of Resolution TABLE OF CONTENTS 1. Learning Outcomes 2. Introduction 3. Methods of Resolution 3.1 Resolution by mechanical Separation of Crystals 3.2 Resolution by formation of Diastereoisomers 3.3 Resolution by Formation of molecular Complexes 3.4 Resolution by Chromatography 3.5 Resolution Through Equilibrium Asymmetric Transformation 3.6 Resolution Through Asymmetric Transformation 3.7 Resolution by Biochemical Transformation 3.8 Resolution Through inclusion Compounds 3.9 Methods Based on NMR Spectroscopy 3.9.1 Use of Diastereomers 3.9.2 Use of Shift Reagents 3.9.3 Use of Chiral Solvating Agents 4. Summary CHEMISTRY Paper No. 1: Organic Chemistry-I (Nature of bonding and Stereochemistry) Module No. 20: Methods of Resolution 1. Learning Outcomes After studying this module, you shall be able to Know that pure form of isomers can be obtained from a racemic mixture. Understand the process of resolution. Learn about the different methods of resolution. Analyse the efficacy of various methods of resolution. 2. Introduction In the previous modules we have discussed in detail various types of isomers, viz., enantiomers, diastereomers, erythro and threo isomers etc. When an enantiomer is converted into a racemic mixture or a racemic modification (usually through chemical reactions), this process is known as racemisation. Conversely, when a racemic modification is separated into its constituent enantiomers, the process is known as resolution. In this module, we shall focus our attention to various methods of resolution and their efficacy. Preparation of pure stereoisomeric forms or resolution is an indispensable part of many stereochemical investigations. Only after this is achieved, many further steps can be undertaken- the study of the physical properties of stereoisomers and of their chemical reactions. Another route to the preparation of pure stereoisomers is furnished by stereospecific reactions, which involve the formation of the desired stereoisomer free from impurities of the other forms. 3. Methods of resolution Most of the natural products, foodstuffs, drugs, flavouring agents, perfumes and other biologically active materials usually show their desirable or beneficial effects in one enantiomeric form only, e.g., (+)-morphine is a powerful analgesic, on the other hand its (-)- isomer is not. Thus, resolution is a method of considerable practical importance. CHEMISTRY Paper No. 1: Organic Chemistry-I (Nature of bonding and Stereochemistry) Module No. 20: Methods of Resolution Nearly all modern methods of resolution of racemates rely on the classical experiments carried out by Louis Pasteur about a hundred and fifty years ago in 1848. The basic resolution methods are developed in and around the experiments carried out by Pasteur. 1. Mechanical separation, a method also known as spontaneous resolution. It depends on the crystallization of the two forms separately, which are then separated by hand. 2. Chemical resolution based on the formation of diastereomers. 3. Resolution by the biochemical method which makes use of the ability of the microorganisms, their enzymatic systems, to destroy one enantiomer more rapidly than the other. 4. Adsorption methods, in particular chromatographic techniques. A considerable improvement has been made in this area since then. In this module we shall discuss the various methods of resolution in depth. Efficiency of Resolution The efficiency of a resolution method is estimated by optical purity of the pure product obtained and is expressed by p (in per cent). [ ] of product p 100 [ ] of pure enantiomer Numerically the optical purity corresponds to the excess of one enantiomer over the other in per cent: it does not coincide with the fraction of an optical antipode in the mixture. The various methods of resolution and its details are discussed at length below: 3.1 Resolution by Mechanical Separation of Crystals The first resolution ever to be brought about was achieved in this way as stated earlier by Louis Pasteur in 1848. Pasteur prepared the sodium ammonium salt of racemic tartaric acid and allowed it to crystallize in large crystals by slow evaporation of the aqueous solution. He CHEMISTRY Paper No. 1: Organic Chemistry-I (Nature of bonding and Stereochemistry) Module No. 20: Methods of Resolution then picked apart the two kinds of crystals, making use of the fact that they showed dissymmetry in the crystal state. A more useful variation of mechanical separation is the method of inoculation, originally discovered by Gernez. If a saturated solution of a racemic mixture is carefully inoculated with a pure crystal of one of the enantiomers, the crystal will grow and an appreciable amount of the active isomer shall be separated from the racemic mixture. For example, (+)- sodium ammonium tartarate can be crystallized from a solution of the racemic modification not only by inoculation with a crystal of the (+) salt but also by inoculation with (-)- asparagine, H2NCOCH2CH(NH2)CO2H Crystallization methods by themselves are rarely practical methods of resolution, but there are often used in a practical methods of resolution, but they are often used in practical way in conjunction with other methods. For example, phenylmethylcarbinyl hydrogen phthalate, After bring resolved to the extent of about 95% or so by means of brucine is dissolved in carbon disulphide and seeded with a small crystal of the racemic compound. In this particular case, the racemic compound is less soluble than enantiomers and so most of the excess of it left the resolved phthalate crystallizes out. The mother liquor is decanted and problem ether added to it, whereupon the enantiomeric phthalate crystallizes in turn, in other cases, where a racemic mixture is formed or where the racemic compound is more soluble than the enantiomers, the active from may be purifies by crystallization, any residual racemic material remaining in the mother liquor. 3.2 Resolution by formation of diastereoisomers When a racemic modification is allowed to interact with an optically active material to give a derivative (such as a salt), in actual fact two diastereoisomeric derivatives result. For example, in a reaction of a racemic acid (+)-A with an active base, (-)-B, the individual molecules of the acid are either (+) or (-), and therefore, the individual molecules of the salt formed are either (+) or (-), and, therefore, the individual molecules of the salt formed are either (+)-A•(-)-B or (-)-A•(-)-B. These two types of salt molecules are evidently no longer enantiomers, but diasteroisomers. Therefore, they have different properties and may in general be separated on the basis of difference in properties. CHEMISTRY Paper No. 1: Organic Chemistry-I (Nature of bonding and Stereochemistry) Module No. 20: Methods of Resolution Several conditions should be fulfilled by a good resolving agent. First, the compound between the resolving agent and the substance to be resolved should be easily formed and should also be easily broken up, for once one of the diastereoisomers, e.g.,(-)-A•(-)B, is obtained in the pure state, it must be decomposed chemically so that pure (-)-A may be recovered. This condition is generally met by salts, which are usually formed readily by mixing the organic acid base in a solvents and may be decomposed, following resolution, by treatment with mineral acid (if the organic acid is to be recovered) or mineral base (if the organic base is desired). 3.3 Resolution Through the Formation of Molecular Complexes Instead of forming stable salts or covalent compounds with the substrates and the resolving reagents, it is possible, in a few cases to have molecular which form easily and thus are ideally suited for resolution. The first observation was made by Pasteur in the formation of molecular compounds between amides of (-)-malic acid and of tartaric acid. Digitonin (XXI) as shown in the figure below, which is steroidal saponin forms addition complexes with various alcohols, e.g., α-terpineol, isocarvomenthol, and phenolic compounds which can be preferentially crystallized from appropriate solvents and then decomposed to give enantiomerically pure alcohols and phenols. Figure 1: Some chiral complexing agents CHEMISTRY Paper No. 1: Organic Chemistry-I (Nature of bonding and Stereochemistry) Module No. 20: Methods of Resolution Compound Resolution: complex formation (+)-2-Naphthylcamphylamine (XXII) Used to resolve N-s-butylpicramide Α-(2,4,5,7-tetranitro-9- Forms charge transfer complexes (π-complexes) fluorenylideneaminooxy)propionic acid with many aromatic hydrocarbons (TAPA) (XXIII) Pt (IV) reagent containing R-1- Through complexation phenylethylamino moiety (XXIV) Chiral complexing agent (XXV) Through enantioselective portioning between an aqueous phase and a solvent containing a chiral complexing agent (like chiral crown ether) Table 1: Complex formation by some resolution agents 3.4 Resolution by chromatography The chromatographic method of resolution of racemic mixtures is carried out generally under four different conditions: (i) Formation of diastereomeric mixture by derivatisation with optically active reagents and separation by classical chiral using achiral adsorbents based on the different adsorption coefficients of the diastereomer; (ii) Direct resolution
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