J. N. Collie, who predicted that fatty acids were made from acetic acid units (see Chapter 3), and one of his students went further and suggested that many other substances were made from acetic acid molecules joined head to tail. His speculations were largely forgotten, but the idea was resurrected by Sir Robert Robinson in the 1940s and extended by A. J. Birch. They showed that many complex natural compounds could be conceived as made of chains of acetate units linked together, first to form a polyketide (Figure 4.1), and then by suitable folding, condensation reactions (elimination of water) and an occasional further oxidation or reduction, to give the structure of the natural compound. Subsequently, using radioactively-labelled acetic acid, these predictions have been proved largely correct (Chapters 1 and 2 in E. Haslam Metabolites and Metabolism see Further Reading). We now recognize that a wide range of naturally occurring substances are derived through these linear polyketides. The polyketides shown in figures here may or may not exist as intermediates, but the structures provide a convenient way to show how the final products are obtained. The final products are called acetogenins because they are derived from acetate units. With a little experience, many compounds can be seen to be acetogenins by a quick examination of their molecular structure. Most of the known acetogenins come from micro-organisms. A very simple example is orsellinic acid, a common metabolite of many micro-organisms (Figure 4.1).

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