The structural diversity of plant defensive compounds

As indicated previously, space simply does not permit comprehensive presentation of the chemical structures of the thousands of plant defensive compounds dealt with in this book, although the structures of particular representative compounds or their related parent compounds are shown in the Appendix. Indeed there are clear advantages in attempting to distil molecular complexity down to readily comprehended groupings of covalently linked moieties that can be described by succinct text. Thus, this approach enables common structural patterns of pharmacological interest to become more evident and reduces molecular complexity to a kind of functional Lego that can be appreciated by chemist and nonchemist readers alike. The conventions for the simplified skeletal structural presentations used in this chapter are summarized below. 

Carbon chain length of alkyl groups or the total number of carbons in a molecule is represented as CH, for example, ethane (C2 CH3—CH3). When a C has four different substituents, as for example the a-C of a-amino acids, parentheses are used to define the substituents. Thus, the general structure of an a-amino acid is OOC—CH(R)-NH3+ and the structure of the a-amino acid alanine (R=CH3) is OOC—CH(CH3)—NH3+. 

In some cases, a group cross-links across a ring and hence creates two further rings however, clarity dictates that in this case the cross-link is indicated simply in square brackets. Thus, a compound with a ring cross-linked with a N-methyl group would be denoted X[-CH3-N ], the epoxy analogue as X[—O—] (or X[epoxy]) and the dimethylene cross-link analogue as X[—CH2—CH2—]. 

Further complexity arises when, for example, three rings are all fused with each other (as opposed to the linear and angular arrangements indicated above) and share a common G. 

A simple example is the tricyclic aromatic phenalene, this arrangement being indicated by an asterisk Phe Phe Phe (or G6 C6 G6 in the case of the fully hydrogenated entity). In very few and very complicated structures multiple shared Cs are indicated by and superscripts (or, in the most complicated example to be encountered here, by 3, 3 and 4 superscripts to indicate two Gs each shared by three rings and another G shared by four rings). 

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This first chapter deals with the structural diversity of plant defensive compounds. Chapter 2 provides a succinct but comprehensive summary of the essentials of biochemistry (the chemistry of living things). This biochemical review provides a detailed background for understanding the nature and function of the targets of plant defensive metabolites and proteins. The remainder of the book summarizes (mainly in table form) a wealth of information... 

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