Novel rearrangements of oxonium ylides and chain extension of beta-ketoesters

A series of ketal-containing diazocarbonyl substrates was prepared. Exposure of these substrates to catalytic Rh(II) and Cu(II) resulted in intramolecular oxonium ylide formation followed by selective rearrangements. The isolated products suggested that bicyclic oxonium ylide rearrangement is controlled by ring strain and ketal substituents. Ylides prepared from larger ring size ketals efficiently generated ring fused products by 1,2-shift of the ketal carbon. Ylides which were prepared from dioxolanes underwent $\beta$-elimination reactions competitively with 1,2-shift of the ketal carbon. When stabilizing groups were present at dioxolane 4 and 5 positions, a 1,2-shift of the exocyclic, stabilized carbon was observed. This novel rearrangement was successfully applied to the synthesis of the 2,8-dioxabicyclo (3.2.1) octane core of zaragozic acid. A one-pot procedure for the conversion of $\beta$-ketoesters to $\gamma$-ketoesters with zinc carbenoids was identified