Application of 1,3-azomethine ylides derived from alfa-dicarbonyl compounds and L-proline to the synthesis of pyrrolizidines

The reaction of 2,3-butanedione, ethyl pyruvate and phenylglyoxal with β-nitrostyrene and L-proline in isopropanol at room temperature gives substituted pyrrolizidines, as a result of one-pot three component reaction.On the contrary, a spiropyrrolizidine is formed from 1,2-cyclohexanedioneonly when the reaction is carried out in refluxing isopropanol, whereas at room temperature, incorporation of the amine component into the products is not observed and bicyclo[3.2.1]octanones are formed, as a result of atandem Michael-Henry reaction. In this latter case L-proline acts as an organocatalyst, although with modest enantioselectivity. Thestereochemistry of the products is given and the mechanism of formation of products is postulated, on the basis of stereochemical arguments.