Ring Expansion\nvia One-Pot Conversion of Lactone Acetals\nto Cyclic Enones. Synthesis of (±)-1-<i>epi</i>-Xerantholide

Baeyer–Villiger oxidation of α-alkoxy ketones <b>1</b> provides lactone acetals <b>2</b>, which react with\nthe lithium salts of dimethyl(alkyl) phosphonates in the presence\nof LaCl<sub>3</sub>·2LiCl to provide cyclic enones <b>3</b> in good to excellent yields after treatment with dilute aqueous\npotassium carbonate. Thus, five-, six-, and seven-membered lactones\nare converted to five-, six-, and seven-membered cyclic enones. The\nutility of this two-step ring expansion method is demonstrated in\nthe synthesis of (±)-1-<i>epi</i>-xerantholide from\n5-methyl-2-cyclohexen-1-one.