Novel Formal [3+3] Cycloaddition of Silyl Nitronates with Activated Cyclopropanes and Its Application in the Synthesis of Pyrroline-N-oxides

1,1-Cyclopropane diesters with donor substituents undergo formal [3+3] cycloaddition with various silyl nitronates under Yb(OTf)(3) catalysis in the presence of a proton scavenger to give target N-silyloxy tetrahydro-1,2-oxazines in 56-98% yield. The reasons determining the reaction diastereoselectivity are discussed. The resulting tetrahydro-1,2-oxazines are transformed into pyrroline-N-oxides via a novel TfOH-promoted silanol elimination reaction.