LECTURE 1
Background Carotene, the orange terpeniod pigment in carrots, is used as a source of vitamin A which is essential for sight I do not know what I may appear to the outside world, but to myself I seem to have been like a boy playing on the sea-shore, and diverting myself in now and then finding a smoother pebble or a prettier shell than ordinary, whilst the great ocean of truth lay all undiscovered before me. Isaac Newton Using a simple five carbon building block, nature creates an array of terpenoid chemicals with an infinite variety of structural variation and vast range of biological functions. Such a cornucopia cannot but leave the terpene chemist feeling as Newton did.KEY POINTS The simple isoprene unit is the basis of an enormous range and a variety of chemical structures which we know as terpenoids. In nature, terpenoids serve a variety of purposes including defence, signalling and as key agents in metabolic processes. Terpenoids have been used in perfumery, cosmetics and medicine for thousands of years and are still extracted from natural sources for these uses. 1.1 DEFINITIONS AND CLASSIFICATION Plants and animals produce an amazingly diverse range of chemicals.Most of these are based on carbon and so the chemistry of carbon came to be known as organic chemistry, i.e. the chemistry of living organisms, the chemistry of life. These chemical products of plants and animals can be classified into primary and secondary metabolites. Primary metabolites are those which are common to all species and can be sub-divided into proteins, carbohydrates, lipids and nucleic acids. These four groups of materials are defined according to the chemical structures of their members.
LECTURE 2
Biosynthesis The infinite variety of terpenoids produced in growing plants are made from two key feedstocks and a handful of reaction types Ipraise you for I am fearfully and wonderfully made Psalm 139 When a poet, an artist or a perfumer looks closely at an individual flower, he will find inspiration for his work from the beauty of its shape, colour and the scent it emits. When the natural products chemist delves deeper into the detail, he sees the diversity and intricacy of the multitudinous molecular structures of which the flower is made and the amazing, complex and very elegant processes which nature uses to produce them. I believe that this can only increase the awe with which we view the wonderful universe in which we live.KEY POINTS Isopentenyl pyrophosphate is produced from glucose and can be Head-to-tail coupling of these two gives geranyl pyrophosphate. Further sequential addition of isopentenyl pyrophosphate units to geranyl pyrophosphate then builds chains containing multiples of five carbons in which the individual “isoprene” units can be identified. Using enzyme-mediated carbocation chemistry, the CSn materials can be cyclised and rearranged into an almost infinite variety of skeletal forms. Tail-to-tail coupling is also possible and this leads to, for example, steroids and carotenoids. isomerised to prenyl pyrophosphate. 2.1 INTRODUCTION The process by which nature produces the chemicals it needs, is known as biosynthesis. By studying biosynthesis, we not only learn about nature’s chemistry - but also generate knowledge which is also useful in many ways. For example, by understanding nature’s chemistry and how natural products are constructed, the patterns of nature can be understood and used to assist in identifying the structures of newly isolated materials and in manufacturing new compounds with similar properties. What follows is a very brief summary of biosynthesis. Further information can be found in the books by Bu’Lock, Mann et al., Torssell, Rawn and Matthews, and van Holde, details of which can be found in the Bibliography.
LECTURE 3
Linear and Monoterpenoids Roses rely heavily on terpenoids for their beautiful scent But earthlier happy is the rose distilled than that which withering on the virgin thorn, grows, lives and dies in single blessedness.William Shakespeare Shakespeare was well acquainted with flowers and herbs and makes many references to them in his writings. This quotation from “A Midsummer Night’s Dream” shows that he could see value in chemical technology and its ability, through distillation, to preserve the scent of a rose well beyond its natural life expectancy.KEY POINTS Before spectroscopic methods of structural determination became available, the structure of natural products was determined by degradation of the molecule to give known fragments. The fragments would be pieced together rather like a jigsaw puzzle and the suggested structure would be proved or disproved by synthesis. When a synthesis is undertaken for the purpose of confirmation of structure, the methods used must give unambiguous bond formation. Different criteria apply to synthetic routes which are intended to provide a means of commercial production.Even in the chemistry of these relatively simple terpenoids, we begin to learn about the principles of carbocation chemistry such as rearrangement of carbocations and the trans-anti-periplanar rule. Much of the fundamental work on understanding the basic principles of organic chemistry was carried out on terpenoids.In this and the following five chapters, we will see how work on terpenoid chemistry led to a much deeper understanding of all of organic chemistry. Terpenoid chemistry contributed especially to our understanding of biogenesis, structural determination, carbocation chemistry, stereochemistry and conformational analysis.
