1,3-Dipolar Cycloaddition of Diazoalkanes to (<i>S</i>)-(+)-3-[(4-Methylphenyl)sulfinyl]-5,6-dihydropyran-2-one. Synthesis of Optically Pure Cyclopropanes by Denitrogenation of Sulfinyl and Sulfonyl Pyrazolines

The addition of diazomethane and diazoethane to enantiopure (S)-(+)-3-[(4-methylphenyl)sulfinyl]-5,6-dihydropyran-2-one (3) afforded the corresponding pyrazolines 4 and 6-exo in good yields and with almost complete π-facial selectivity. When the reaction is effected in the presence of Yb(OTf)3, the facial selectivity is inverted to give the pyrazolines 5 and 7-exo. The denitrogenation of optically pure sulfinyl pyrazolines 4−7-exo into the corresponding cyclopropanes with Yb(OTf)3 occurred with complete retention of configuration but moderate chemoselectivity and yields. These results were significantly improved starting from sulfonyl pyrazolines, which afforded optically pure 3-oxabicyclo[4.1.0]heptan-2-ones with yields ranging between 65% (17 and ent-17) and ≥95% (16 and ent-16).