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APPROCH TO NATURAL PRODUCTS

APPROCH TO NATURAL PRODUCTS. Prof. K. M. Lokanatha Rai Department of Chemistry University of Mysore Manasagangotri, Mysore 570 006. Cells of organisms - plants, fungi, bacteria, insects, animals - produce a large variety of organic compounds.

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APPROCH TO NATURAL PRODUCTS

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  1. APPROCH TO NATURAL PRODUCTS Prof. K. M. Lokanatha Rai Department of Chemistry University of Mysore Manasagangotri, Mysore 570 006

  2. Cells of organisms - plants, fungi, bacteria, insects, animals - produce a large variety of organic compounds. Many substances were obtained anciently, e.g. foodstuffs, building materials, dyes, medicinals, and other extracts from nature.

  3. Oils & Fats, Terpenoids Prostaglandins, Alkaloids, Vitamins, Flavanoids, Steroids, Carbohydrates, Lignins, Lignans, Proteins, Nucleic acid, Antibiotics, Pigments, Mycotoxins, Pheromones, etc

  4. Mild heating of certain plants afforded perfumed distillates. Plants and animals have provided substances used for their biological activity, to heal or to kill, and form the foundation for folk medicine.  Most natural products have usually come from plants and microorganisms due to practical difficulties in extracting them from animals.

  5. Plants are particularly interesting because: They have the broadest spectrum of biosynthetic capability, and produce a wide variety of compounds. They use simple starting materials:  water, carbon dioxide, nitrogen (elemental and in salts), phosphorus compounds, and salts. Their biosynthetic paths are known

  6. Crude aqueous extracts of certain plants (and animals) provided pigments, such as indigo and alizarin.

  7. Other examples of natural products: Ephedrine from Ephedra sinica (respiratory ailments) tetrahydrocannabinol  (marijuana)geraniol (rose oil) Cinnamaldehyde cinnamon)diallyl disulfide (garlic)

  8. General isolation strategy of natural products: • Simple extraction 2. Steam distillation 3. Continuous extraction

  9. Extract the dried and ground plant material with a suitable solvent. Concentrate the extract. Separate and purify each component. Since the concentrate contains an enormous variety of compounds, early isolations involved selective crystallization of the most dominant component in the mixture.  Liquid natural products were distilled.  Natural organic acids were isolated by aqueous basic extraction and natural organic bases (alkaloids) were isolated by aqueous acidic extraction

  10. Steam distillation

  11. Soxhelet extraction Solvents: Hexane Benzene Chloroform Ethanol Water

  12. Purification steps 1. Crystallization 2. Fractional Crystllization 3. Sublimation 4. Distillation 5. Fractional distillation 6. Distillation under reduced pressure 7. Chromatographic separation

  13. Fractional distillation

  14. Distillation under reduced pressure

  15. THIN LAYER CHROMATOGRAPHY • Thin layer chromatography (TLC) is an important technique for identification and separation of mixtures of organic compounds. It is useful in: • Identification of components of a mixture (using appropriate standards) • following the course of a reaction, • analyzing fractions collected during purification, • analyzing the purity of a compound.

  16. In TLC, components of the mixture are partitioned between an adsorbent (the stationary phase, usually silica gel, SiO2) and a solvent ( the mobile phase) which flows through the adsorbent. • In TLC, a plastic, glass or aluminum sheet is coated with a thin layer of silica gel.

  17. A very small amount of a solution of the substance to be analyzed is applied in a small spot with a capillary tube, ~1cm from the bottom of the • TLC plate • The TLC is developed in a chamber which contains the developing solvent (the mobile phase).A truncated filter paper placed in the chamber serves to saturate the chamber with mobile phase.

  18. As the mobile phase rises up the TLC plate by capillary action, the components dissolve in the solvent and move up the TLC plate. • Individual components move up at different rates, depending on intermolecular forces between the component and the silica gel stationary phase and the component and the mobile phase. The stationary phase is SiO2 and is very “polar”. It is capable of strong dipole-dipole and H-bond donating and accepting interactions with the “analytes” (the components being analyzed). More polar analytes interact more strongly with the stationary phase in move very slowly up the TLC plate. By comparison, the mobile phase is relatively nonpolar and is capable of interacting with analytes by stronger London forces, as well as by dipole-dipole and H-bonding. More nonpolar analytes interact less strongly with the polar silica gel and more strongly with the less polar mobile phase and move higher up the TLC plate.

  19. Once the solvent is within ~1-2 cm of the top of the TLC sheet, the TLC is removed from the developing chamber and the farthest extent of the solvent (the solvent front) is marked with a pencil. The solvent is allowed to evaporate from the TLC sheet in the hood. The spots are visualized using a UV lamp. A fluorescent compound, usually Manganese-activated Zinc Silicate, is added to the adsorbent that allows the visualization of spots under a blacklight (UV254). The adsorbent layer will fluoresce light green by itself, but spots of analyte quench this fluorescence and appear as a dark spot

  20. As the chemicals being separated may be colorless, several methods exist to visualize the spots: • Visualization of spots under a UV254 lamp. The adsorbent layer will thus fluoresce light green by itself, but spots of analyte quench this fluorescence. • Iodine vapors are a general unspecific color. • Specific color reagents exist into which the TLC plate is dipped or which are sprayed onto the plate. • Once visible, the Rf value of each spot can be determined Chromatogram of 10 essential oils, Stained with vanillin reagent.

  21. The Rf is defined as the distance the center of the spot moved divided by the distance the solvent front moved (both measured from the origin)

  22. Column chromatography

  23. Structural elucidation Physical and chemical methods

  24. Detection of elements Carbon & Hydrogen: by combustion Nitrogen : via sodium fusion extract Halogens : via sodium fusion extract Sulphur : via sodium fusion extract Posphorous : Conversion to phosphoric acid

  25. Elemental analysis Carbon & Hydrogen : By combustion % of Carbon: Amount of CO2 evolved = x gm i.e from “w” gm of the copound. 44 gm of CO2 = 12 gm of carbon

  26. % of halogens by Carrey’s method

  27. Determination of molecular weight Depression in the freezing method:- Rast method Elavation in the boiling point:- Macoys method, Victor Mayer method

  28. Titremtric method Mass spectra Molecular formula

  29. Calculation of hydrogen index:- For C9H10O C9H20 hydrogen index = 20-10/2 = 5 => 5 double bond 0 ring 4 double bond 1 ring 3 double bond 2 ring 2 double bond 3 ring 1 double bond 4 ring 0 double bond 5 ring

  30. For C6H7N C6H14 hydrogen index = 14-(7-1)/2 = 4 For C6H5Cl C6H14 hydrogen index = 14-(5+1)/2 = 4 For C6H4Br2 C6H14 hydrogen index = 14-(4+2)/2 = 4

  31. Classical structural elucidation is done by: Determination of functional groups Determination of the carbon skeleton and the location of the functional groups Degradation to smaller fragments (A-B-C ------>  A   +   B   +   C)

  32. Elemental analysis Reactivity (leading to new reactions) Stereochemistry Synthesis of the smaller fragments (A, B, C) and the entire molecule (A-B-C)

  33. More modern structural elucidation and characterization by spectroscopy: 1930's UV (ultraviolet) light (cf. Woodward's Rules, 1941) 1940's IR (infrared) spectroscopy 1950's NMR (nuclear magnetic resonance) spectroscopy

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