AbstractPrelactones C (3) B (8) were isolated from the concanamycin‐producingStreptomyces sp.(strain Gö 22/15) and bafilomycin‐producingStreptomyces griseus(strain Tü 2599), respectively, by chemical screening methods. The constitution and relative configuration of3and8were established by one‐ and two‐dimensional NMR methods. The absolute configuration of3was determined using theHelmchenmethod and that of8by 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