SYNTHESIS OF GREVILLINS, NOVEL PYRANDIONE PIGMENTS OF FUNGI - BIOGENETIC INTERRELATIONSHIPS BETWEEN GREVILLINS, PULVINIC ACIDS, TERPHENYLQUINONES AND XYLERYTHRINS

A synthesis of the grevillin group of pyrandione pigments e.g. 3, 23 and 24 present in fungi is described. The synthesis, which is based on a biogenetic model, uses bis-benzylacyloins 9 and their corresponding oxalate derivatives as key intermediates (Scheme 3). Treatment of the grevillins 25-c with sodium ethoxide in ethanol effects their quantitative isomerisation into the corresponding terphenylquinone pigments 4a-c. Perkin-type condensations between the terphenylquinones 4 and arylacetic acids in the presence of sodium acetate-acetic anhydride then produces the xylerythrin pigments 29a-e, whereas rearrangements of 4 in the presence of dimethyl sulphoxide leads to pulvinic acid derivatives, e.g. 31, 32 and 5. These synthetic studies interrelate the biosynthetic origins of the pigment types 3, 4, 5 and 8 together with the related pulvinone 6 and furanone 7 fungal pigments.