Xialenons, new pentalenons from Streptomyces

New pentalenons, named xialenon A to E (1 to 5), were discovered by a chemi cal screening of the culture broth of the Streptomyces sp. (strain GT 06116 9). The chemical structures of these secondary metabolites were determined by detailed spectroscopic investigation as well as chemical derivatization reactions. The absolute stereochemistry of 1 was determined by esterificati on with chiral acids via Helmchen's method. A common structural element of the xialenons is an alpha,beta-unsaturated ketone in one of the two fused 5 -membered rings (reduced double bond in 1), that is substituted with both, a hydroxyl group in a-position, and an additional allyl side chain in beta- position.