Depsides, hydrolysable tannins, lignans, lignin and humic acids

Publisher Summary Depsides, depsidones, and depsones that occur in lichens are described in this chapter. Depsides and depsidones are present in lichens in varying concentrations up to 5% dry weight. Spectroscopic methods applied to the structural elucidation of depsides are reviewed. Mass spectroscopy continues to rank highly as a means of determining the structure of depsides. Fragmentation occurs to give inter alia three extremely informative daughter ions. Two ions differing by one mass unit characterize the S-portion and the third ion the A-portion. Proton nuclear magnetic resonance ( 1 H NMR) is used routinely for structural work and for determining subtle structural effects. The synthesis of depsides follows traditional methods. The benzyl group is now the group of choice for protecting acids as illustrated by the synthesis of 2-O-methylsekikaic acid. When a carbonyl group is present in the natural product, the usual methods of synthesis lead to complex mixtures. The chapter presents the synthetic routes to depsidones. The major difficulty is the formation of the diphenyl ether linkage, particularly due to steric effects and reactive functional groups. The Ullmann reaction is extensively employed and except in the synthesis of 4-O-methylhypoprotocetraric acid , it is usually undertaken before the lactone linkage is made.