Medium-sized rings versus macrocycles through rhodium-catalyzed ring-expansion reactions of cyclic acetals.

α-Diazo β-ketoesters and diketones react with cyclic acetals under Rh(II) catalysis to yield unprecedented polyoxygenated 8- and 9-membered rings in one pot. The reactions occur under mild conditions with yields up to 90%. A perfect regioselectivity is obtained, which can be rationalized through a mechanistic hypothesis that considers 1) the formation of an oxonium ylide, 2) its transformation into an unsaturated acyclic oxocarbenium electrophilic intermediate, and 3) an intramolecular nucleophilic attack in a direct application of Baldwin's rules.