The anions of the 2-azaallylphosphonates 1 react with the α, β-unsaturated esters 2 to yield the intermediates 4'. These intermediates form the 2-diethoxyphosphonyl-4- ethoxycarbonylpyrrolidines 4 following a protonation while an elimination of the anion of the diethylphosphonate leads to the 4-ethoxycarbonyl-Δ1-pyrrolines 5. The proportions of these two products vary with the experimental conditions and the bulkiness of the substrates. ...