Ring Expansion via One-Pot Conversion of Lactone Acetals to Cyclic Enones. Synthesis of (±)-1-epi-Xerantholide.

Baeyer-Villiger oxidation of α-alkoxy ketones 1 provides lactone acetals 2, which react with the lithium salts of dimethyl(alkyl) phosphonates in the presence of LaCl3·2LiCl to provide cyclic enones 3 in good to excellent yields after treatment with dilute aqueous potassium carbonate. Thus, five-, six-, and seven-membered lactones are converted to five-, six-, and seven-membered cyclic enones. The utility of this two-step ring expansion method is demonstrated in the synthesis of (±)-1-epi-xerantholide from 5-methyl-2-cyclohexen-1-one.