Determination of the Absolute Configurations of γ-Rubromycin and Related Spiro Compounds by Quantum Chemical CD Calculations

The absolute configurations of the spiro compounds gamma- and beta-rubromycin (2 and 3), natural antibiotics derived from actinomycetes, have been elucidated as (S) by quantum chemical calculations of their chiroptical properties and comparisons with the respective experimental CD spectra. Surprisingly, their spiro centers show an absolute configuration opposite to that of heliquinomycin (1), the only related spiro compound for which the configuration has hitherto been assigned. By further quantum chemical CD calculations, we were likewise able to confirm the absolute stereostructure of heliquinomycin (1), previously assigned by X-ray structural and degradative studies, thus providing independent corroboration of our results for gamma- and beta-rubromycin (2 and 3). Accordingly, it has emerged that diverse actinomycetes can generate different acetal configurations.