Scheme 3. Diastereoselective ring expansion of 2-phenyl oxetane and 2-phenyl thietane and investigations on other oxygen-containing heterocycles: (a) substrate scope of aryldiazoacetates; (b) multi-carbene transfer reactions for the synthesis of bis-tetrahydrofurans; (c) ring expansion reactions with chiral diazoesters; (d) reactions of thietanes; (e) de novo photochemical synthesis of tetrahydrofurans. ^aTreatment with aq. KMnO₄ after completion of the reaction. ^bTreatment with Br₂ after completion of the reaction.