Introduction to Natural Products Chemistry | Natural Products Chemistry
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In this video, we will present introduction to Natural Products Chemistry.
Cells of organisms – plants, fungi, bacteria, lichens, 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.
Crude aqueous extracts of certain plants (and animals) provided pigments, such as indigo and alizarin.
Other examples of natural products:
Ephedrine from Ephedra sinica
(Respiratory ailments) tetrahydrocannabinol
(Marijuana)
Geraniol (rose oil)
Cinnamaldehyde (cinnamon)
Diallyl disulfide (garlic)
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.
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
In the late 1700’s, chemists moved from myth and mystery to basics of modern scientific methods to begin to uncover the true properties of natural extracts from biological systems.
They discovered that natural extracts had more complex compositions and properties than salts and minerals.
This lead Berzelius, in 1807, to distinguish between “inorganic” and “organic”;(“Organic” substances were believed to be obtainable only from organs of living systems and could not be man-made because only living systems have the vital (“life”) force. “Inorganic” materials were from non living, e.g. mineral, sources.)
In the 1800’s, organic chemistry was exclusively the study of natural products.
Natural extracts were subjected to separation into component compounds, which were then purified and analyzed.
In the late 1800’s, synthetic methods were being developed for some of these natural compounds. Some examples of natural products and when they where discovered are as follows:
Morphine (narcotic analgesic) 1817
Strychnine (poison) 1818
Cocaine (narcotic stimulant) 1859
Nicotine (toxic) 1828
General isolation strategy of natural products:
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.
Modern chromatographic methods have been greatly developed to isolate and purify a large number of different compounds in very small quantities: column, GC, TLC, HPLC, paper, electrophoresis, ion exchange, etc.
Natural products are usually given names that are derived from the species name of the plant or animal, or from the biological action, or property, of the compound.
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)
Elemental analysis
Reactivity (leading to new reactions)
Stereochemistry
Synthesis of the smaller fragments (A, B, C) and the entire molecule (A-B-C)
Classification of the compound into a biogenetic family of compounds
More modern structural elucidation and characterization by spectroscopy:
1930’s UV (ultraviolet) light (cf. Woodward’s Rules, 1941)
1940’s IR (infrared) spectroscopy (note: penicillin structure problem in W.W.II)
1950’s NMR (nuclear magnetic resonance) spectroscopy
1960’s MS (mass spectrometry)
ESR (electron spin resonance) spectroscopy and by other methods:
ORD (Optical Rotatory Dispersion)
CD (Circular Dichroism)
Acidity and basicity measurements (pK)
Advanced synthetic and biosynthetic technology
X-ray crystallography
Modern methods reduces the necessity of chemical degradation methods, so much less material is required.
Why synthesize natural products?
Structure determination
Challenge
Develop new synthetic methods
Practical and commercial interests
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