Study of the Rearrangements of Oxonium Ylides Generated from Ketals

Intramolecular exposure of cyclic ketals to metal carbenoids generates a proposed oxonium ylide intermediate that subsequently rearranges to one of three general products. The product resulting from a 1,2-shift to the ketal carbon is favored by larger ketals that lack radical stabilizing groups. A bridged bicyclic structure is formed by competitive 1,2-shift to the exocyclic carbon of the ketal and is favored by smaller ketal ring sizes that possess radical-stabilizing groups. An alternative β-elimination pathway can also operate when neither of the 1,2-shift pathways are favored. The enol ether that is formed in this latter pathway rearranges easily to an isomeric dioxene.