Alternative spiroketalization methods toward purpuromycin: a diketone approach to prevent benzofuran formation.

The central portion of purpuromycin has been assembled via a classical spiroketalization reaction. Key to promoting this reaction mode versus benzofuran formation was the oxidation state of the spiroketal core. With a higher oxidation state, even the electron-deficient isocoumarin found in purpuromycin could be employed directly in the spiroketalization. The two halves of the spiroketalization precursor were joined via a nitrile oxide/styrene 1,3-dipolar cycloaddition. A very mild selenium dioxide oxidation was used to introduce the required oxidation state of the spiroketal core.