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.