The present volume, the 27*^ of this series, is devoted to the chemistry of several exciting
classes of natural products.
The article by Kinghom and Kim reviews the current state of knowledge of sweet-tasting
and sweetness-modifying constituents of plants. Some of these highly sweet natural
products are already marketed as sweetners or flavoring agents in some countries.
Spiteller has presented an overview of chemical responses to plant injury and plant
aging. Interest in sialic acid chemistry is growing, particulariy due to its role in the
regulation of a number of important biological processes. The article by Bianco and
Melchioni on the structure, chemistry and biological activity of neuraminic acid covers this
area comprehensively. Quaternary benzo[c]phenanthridine alkaloids (QBA) are bright
coloured compounds which are interesting both for their chemistry and their biological
activities. The article by Dostal and Slavfk covers some aspects of the chemistry of these
compounds. Sicker and Schuiz have reviewed the field of acetai glycosides of the 2-
hydroxy-2H-1, 4-benzoxazine-3(4H)-one skeleton which occur naturally in a number of
plant families and which can impart resistance in plants towards insects, microbes as well
as pathogenic fungi. Structural identification and bioactivity aspects of simple indolizidine
and quinolizidine alkaloids isolated from amphibians, ants, fungi, plants and marine
sources is covered comprehensively in an article by Lourenco and co-workers. These
substances are responsible for the toxic and teratogenic effects observed in livestock and
their action can be associated with their affinity for nicotinic and muscarinic receptors.