A theory of biogenesis of lichen depsides and depsidones
Lichen depsides and depsidones are considered to arise from a common source (XIV) which originates from aldol condensation between a hexose and a biose and elimination of water. Oxidation and reduction lead to various modifications of this C8 unit and increase in the length of the sidechain arises from condensation with simple sugars and reduction. Depsides are formed by the combination of two of these units. β-Orcinol derivatives are obtained by nuclear methylation by means of formaldehyde and this reaction in general takes place prior to depside formation though the other possibility is not altogether excluded as far as the left half is concerned Depsidones come last in the evolution; they are based on depsides and require oxidation or dehydrogenation involving position 5 which is para to the activating hydroxyl. Nuclear oxidation without leading to depsidone formation also occurs. Either the 3-or the 5-position is involved and meta depsides result. Oxidation involving the left half is also possible and is represented by diploschistesic acid. The occurrence of orcinol and psoromic acid is attributed to decarboxylation taking place in the plant.