Stereospecific Synthesis of Cyclobutanones and Spirohexanones via Formal [3 + 1] Cycloaddition of Cyclopropanones with Sulfur Ylides

A concise synthetic route to enantioenriched cyclobutanones is reported via ring expansion of cyclopropanone surrogates with unstabilized sulfoxonium ylides. The reaction is shown to proceed with complete regio- and stereospecificity with chiral substrates, leading to optically active 2,3-disubstituted cyclobutanones where reversible enamine formation allowed for controlled equilibration to the thermodynamic trans diastereomer. Alternatively, employing Trost's cyclopropylsulfonium reagent provided the first synthesis of enantioenriched spiro[2.3]hexan-4-ones via a unique semipinacol rearrangement of a dicyclopropyl betaine intermediate.