L-methyl prolinate and various α,β-unsaturated ketones were reacted by a tandem sequence of conjugate addition and subsequent autocatalytic intramolecular cyclization to yield 1,3-dicarbonyl-pyrrolizidine ring systems in modest yield. This methodology represents a tandem and green preparative reaction sequence where L-proline is self-generated as an organocatalyst subsequent to the first step and prior to the second step of the process. This sequence of reactions is a one-pot, tandem, and green preparative reaction for the synthesis of various pyrrolizidine derivatives which can be performed in a general and simple way under solventless and room temperature conditions. This methodology is modest yielding and environmentally friendly, factors that represent relevant advantages from a synthetic point of view. Additionally, the catalyst utilized for the reaction has the advantage of being chiral, readily available, inexpensive, non-toxic, and easily removed. The reaction is partially diastereoselective which contributes to the remarkable advantages for this synthetic methodology.
β-monosubstituted enones react in a tandem sequence of conjugate addition - cyclization whereas β-disubstituted enones react by a homodimerization process. The cyclization reaction is only partially accomplished for α-substituted unsaturated enones. The hindrance provided by the group in the α-position determines the extent of completion of the cyclization reaction to afford the final product.
