Ascotoxin (Decumbin), a Metabolite of Ascochyta imperfecta PECK

Abstract

From the mycelium of Ascochyta imperfecta decumbin, C₁₆H₂₄O₄, mp203°C, [α]²⁵_D+91.7° was obtained in one percent yield.
The absolute structure of decumbin was presented as [II] by the following evidences: The configuration about C₄ was determined as (S) by the benzoate rule on the tetrahy-dromonoketone (21). The hydroxyl at C₇ is α, because tetrahydrodecumbin (23) showed no intramolecular hydrogen bond, while its C₇ epimer (24) did. Ring juncture was de termined by ORD of a five membered ketone (16). Two double bonds were found to be trans from IR data. The stereochemistry of decumbin monoepoxide (7), tetrahydropyrans (12 and 13) was also studied. Plant tests of the twenty derivatives of decumbin on lucerne and rape revealed that the growth inhibition activity has close relation with the presence of double bond in the thirteen membered lactone ring.