Efficient Synthesis of Dihydrofurans with Sulfide Groups by Ceric(IV) Ammonium Nitrate-Mediated Oxidative Cycloaddition of 1,3-Dicarbonyl Compounds to Vinyl Sulfides. Application to the Synthesis of Benzo[b]naphtho[2,3-d]furan-6,11-dione and First Total Synthesis of Millettocalyxins C and Pongamol M