Three- and Four-Carbon Elongating Ring Expansion of Cyclic Acetals to Medium-Sized Dioxacycloalkenones. Use of the Intramolecular Formation of Oxonium Ylides.

The Rh(II)-catalyzed reaction of 2-(3'-diazo-2'-oxopropyl)-2-methyldioxolane (1) in the presence of a protic nucleophile (NuH) such as AcOH resulted in effective ring enlargement to give the 8-membered 3-acetoxydioxocanone 4a (41%) and dioxocan-2-en-1-one 3 (46%). Similar treatment of 2-(4'-diazo-3'-oxobutyl)-2-methyldioxolane (9) with AcOH gave 4-acetoxydioxonanone 10 (67%), which was readily hydrolyzed on silica gel to a tautomeric mixture of hydrolysis products 16a and 16b (total yield 46%). In contrast, similar treatment of 2-(5'-diazo-4'-oxopentyl)-2-methyldioxolane (19) gave 2,5-dioxa-1-methyldecalin-7-one (20, 24%), and the yield increased to 61% in the absence of AcOH, by Stevens rearrangement. The reaction of 1,3-dioxane homologues 26 and 31 gave similar results. All of these reactions can be explained in terms of the intermediacy of bicyclooxonium ylides, which undergo either a Stevens rearrangement or, after protonation by a NuH, ring enlargement through release of the strain of the bicyclic ylides. Evidence of the reversible formation of oxonium ylides is also provided.