Intramolecular Oxonium Ylide Formation–[2,3] Sigmatropic Rearrangement of Diazocarbonyl-Substituted Cyclic Unsaturated Acetals: A Formal Synthesis of Hyperolactone C

Rh­(II)-catalyzed oxonium ylide formation–[2,3]
sigmatropic
rearrangement of α-diazo-β-ketoesters possessing γ-cyclic
unsaturated acetal substitution, followed by acid-catalyzed elimination–lactonization,
provides a concise approach to 1,7-dioxaspiro­[4.4]­non-2-ene-4,6-diones.
The process creates adjacent quaternary stereocenters with full control
of the relative stereochemistry. An unsymmetrical monomethylated cyclic
unsaturated acetal leads to hyperolactone C, where ylide formation–rearrangement
proceeds with high selectivity between subtly nonequivalent acetal
oxygen atoms.