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. The formation of the cyclic products has been studied for numerous substrates and in the presence of a number of bases. The reaction is regioselective and stereospecific and on the basis of the formation of the kinetically more strained diastereoisomer it is possible to conclude that the cycloaddition is a concerted one. The thermodynamically less strained diastereoisomers 6 and 7 are obtained from the compounds 4 and 5 respectively when they are subjected to the action of a base in a protic media. The other epimers of 4, the compounds 8 and 9, have been obtained by the addition of diethylphosphite to the pyrrolines 5 and 7. The pyrrolines 5 have been reduced to the 3-ethoxycarbonylpyrrolidines 12. The isomerization of the pyrrolines 5 with a phenyl substituent in position 5 leads to the formation of the 3-ethoxycarbonyl-2-phenyl-Δ1-pyrrolines 10. The latter spontaneously oxidize with atmospheric oxygen to yield the 3-ethoxycarbonyl-3-hydroxy-2-phenyl-Δ1-pyrrolines 11. [Journal translation]