Some Recent Aspects in the Structure Elucidation of Natural Products
Abstract Although in the past 20 yr a large number of new synthetic approaches have been developed methods of isolation and structural elucidation have changed more dramatically in the last two decades than have those of synthesis. For the purification of natural products more and more different chromatographic techniques like paper, thin-layer, column, and gas-liquid chromatography are being used. Characterizations of natural products by elemental analysis, melting points, optical rotation values, and degradation and derivatization reactions are being increasingly supported by parameters of spectroscopic tools like proton nuclear magnetic resonance, carbon nuclear magnetic resonance, phosphorus nuclear magnetic resonance, fluorine nuclear magnetic resonance, infrared spectroscopy, Raman spectroscopy, absorption spectroscopy, optical rotatory dispersion, circular dichroism, magnetic circular dichroism and computer analysis. The principle of one of the most efficient methods for the structural elucidation of natural products, pulse Fourier transform 13C NMR spectroscopy, is discussed in detail. The usefulness of this method is demonstrated using examples from the peptide, carbohydrate and terpenoid field. The structural elucidations by the application of different physicochemical tools for the three following most recently isolated natural products are given: Hypothalamus-releasing hormones, terpenoids from Melia azadirachta Linn., and substances, isolated from urines of sick children.