Reaction between proline and γ-oxo α,β-unsaturated esters: new access to polysubstituted pyrrolizidines

The reactions between proline and several γ-oxo α,β-unsaturated esters have been studied in detail. The products isolated are in all cases derived from one molecule of proline and two molecules of the ester. The new polysubstituted pyrrolizidines result from the cycloaddition of an azomethine ylide to a second molecule of the unsaturated ester in a stereoselective manner.