Introduction to the different classes of natural products

Plants produce an enormous variety of natural products with highly diverse structures. These products are commonly termed secondary metabolites in contrast to the primary metabolites which are essential for plant growth and development. Secondary metabolites were formerly regarded as waste products without physiological function for the plant. With the emergence of the field of chemical ecology about 30 years ago, it became evident, however, that these natural products fulfill important functions in the interaction between plants and their biotic and abiotic environment. They can serve, for example, as defense compounds against herbivores and pathogens, as flower pigments that attract pollinators, or as hormones or signal molecules. In addition to their physiological function in plants, natural products also have a strong impact on human culture and have been used throughout human history as condiments, pigments, and pharma-ceuticals. This chapter provides an overview about the diversity of secondary metabolites in plants, their multiple biological functions and multi-faceted cultural history. The compounds are classified into four different groups according to their biosynthetic origin: alkaloids, phenylpropanoids, polyketides, and terpenoids. Since more than 200,000 structures of natural products from plants are known, only selected groups and compounds are presented.