Spirodionic acid (1), a novel microbial metabolite with a spiro[4.5]decene skeleton, the 6-ethyl-2H-pyrone 5, dihydrosarkomycin (6), and other metabolites were isolated from the strain Streptomyces sp. Tü 6077. Structural elucidation was accomplished by NMR spectroscopic and mass-spectrometric studies, and the biosyntheses of compounds 1, 5, and 6 were investigated by feeding experiments with (13)C-labeled precursors. All results indicate a biogenetic sequence with metabolite 5 and sarkomycin (7) as precursors in the formation of spirocyclus 1 through an intermolecular Diels-Alder-type reaction.

To elucidate the structure of the new natural product spirodionic acid, which has three stereogenic centers of hitherto unknown configuration, two 4-ethenylspiro[4.5]dec-6-en-1,8-diones with opposite relative configuration at C4 and C5 were prepared. The first access involved the synthesis of 3-ethenyl-2-formylcyclopentanone (8) using a three-component coupling process. A sequence of Michael addition to penten-3-one (7) and intramolecular aldol condensation then led to the highly selective formation of the 4S*,5S*-configured spirocycle 5. In contrast, a selective spiroannelation of a cyclohexenone ring was accomplished by a novel type of twofold Michael addition to the dialkenyl ketone 11 with subsequent dehydrogenation to furnish the 4S*,5R*-configured spirocycle 25. Diastereoselective methylation and oxidative degradation then completed a highly efficient synthesis of the natural product as prerequisite for the assignment of its absolute configuration.