Synthesis of pyrrolidin-3-ones from dihydropyran precursors via spiro-N,O-acetals

2,2-Disubstituted pyrrolidin-3-ones are prepared in three steps from simple dihydropyran derivatives; the key spiro-N,O-acetal intermediate is a useful N-sulfonylketoiminium ion precursor. This route represents a formal synthesis of the indolizidine alkaloid core, with potential application to pyrrolizidines and quinolizidines.