Abstract
Prelactones C (3) B (8) were isolated from the concanamycin‐producing Streptomyces sp. (strain Gö 22/15) and bafilomycin‐producing Streptomyces griseus (strain Tü 2599), respectively, by chemical screening methods. The constitution and relative configuration of 3 and 8 were established by one‐ and two‐dimensional NMR methods. The absolute configuration of 3 was determined using the Helmchen method and that of 8 by CD spectra. The structural properties and absolute configuration of these new δ‐lactones reveal strong similarities to the hemiacetal portion of the corresponding macrolide antibiotics. Their possible role in the early polyketide formation of unusual macrolactones is discussed.

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