Synthesis of bicyclo[3.1.0]hexanones via 1,3-dipolar cycloaddition of diazoalkanes to homochiral α-sulfinyl-2-cyclopentenones

The reactions of diazomethane, diazoethane, and (trimethylsilyl)diazomethane with (S)-2-p-tolylsulfinylcyclopent-2-en-1-one have been studied. The sulfinyl group increases the reactivity and controls the π-facial and endo/exo selectivities. The π-facial selectivity can be inverted in the presence of Yb(OTf)3, which makes possible the stereodivergent synthesis of both diastereoisomeric pyrazolines. Completely stereoselective denitrogenation of optically pure pyrazolines into cyclopropanes was achieved under substoichiometric Yb(OTf)3 catalysis.