Bisphosphoric Acid - An Intriguing Catalyst Motif

Significance

A Brønsted acid catalyzed three-component asymmetric 1,3-dipolar cycloaddition reaction between aldehydes 1, α-amino esters 2, and maleates 3 is described. The new chiral bisphosphoric acid 4 derived from linked (R)-BINOL was used as catalyst. Substituted pyrrolidines 5 are obtained as endo diastereomers in good to high yields and enantioselectivities. The use of an aliphatic aldehyde (1, R¹ = Cy) leads to somewhat decreased enantioselectivity (er = 88:12). Mechanistically, the reaction proceeds via an azomethine ylide dipole, which is generated from the corresponding azomethine in presence of a bisphosphoric acid (see scheme).