Boletus calopus and closely related mushrooms of the Boletus section Calopodes are characterized by their bitter taste, which is caused mainly by O-acetylcyclocalopin A (6a), a member of a series of unique δ-lactone derivatives. Besides the simple lactone calopin (1b) and its O-acetyl derivative la, the more complex cyclocalopins 6-12 have been isolated and their structures elucidated. Cyclocalopin D (10a) is an unprecedented α-glucoside in which the sugar residue is bound to an enolic hydroxy group., The chemical conversion of 6b into la established the same stereochemistry for the calopin and cyclocalopin types of metabolites.