Efficient synthesis of 5,6,7-trisubstituted 1H-pyrrolizines

Crystalline phosphorus ylides are obtained in excellent yields from the 1:1:1 addition reaction between triphenylphosphine, dialkyl acetylenedicarboxylates and strong NH-acids, such as 2-pyrrolylglyoxalate or N-benzyl-2-pyrrolylglyoxamate. These phosphoranes undergo a smooth intramolecular Wittig reaction in boiling toluene to produce 5,6,7-trisubstituted 1H-pyrrolizine derivatives in quantitative yields. Dynamic NMR effects are observed in the 1H NMR spectra of stabilized ylides 6a and 6d (ΔG≠=67.6 and 69.5 kJ mol−1, respectively) and are attributed to restricted rotation around the carbon–carbon partial double bond resulting from conjugation of the ylide moiety with the adjacent carbonyl group.