ABSTRACT
Acknowledgment ...................................................................................................118 References..............................................................................................................119
Quinones constitute a large group of natural pigments, widely distributed in higher plants but also present in fungi, lichens, and some invertebrate animals.1 The common structural feature of quinones is the presence of two ketone groups on a monocyclic or polycyclic aromatic skeleton. The two ketone groups can be located on the same ring or on different rings, but it is necessary to have a conjugation involving π (from greek electrons) electrons of carbon-carbon double bonds. Depending on their structures, quinones can be categorized as benzoquinones, naphthoquinones, anthaquinones, or extended quinones. In addition to ketone groups, some other functional groups such as methyl, hydroxyl, methoxyl, or glycosylated phenol can be found in natural quinones. Some quinones with important biological functions, for example, ubiquinones, have long prenyl side chains. The so-called extended quinones such as aphins are complex structures that represent dimerization products of naphthoquinones or anthaquinones. Figure 2.5.1 represents the general quinone structure and some important natural representatives.
