Asymmetric synthesis of aryl-substituted pyrrolidines by using CFAM ligand-AgOAc chiral system via 1,3-dipolar cycloaddition reaction

We have prepared a ligand library based on a ferrocenyl aziridinyl methanol core unit (simply called FAM) having a phenyl group, a cyclohexyl group, and a naphthyl group to be used in 1,3-dipolar cycloaddition (1,3-DC) reactions for the synthesis of chiral pyrrolidines. These chiral ligands were used with AgOAc in 1,3-DC reactions taking place between the aryl-substituted azomethine ylides and N-methylmaleimide as the dipolarophile. In each case, the expected aryl-substituted pyrrolidines were obtained in good to excellent yields with acceptable enantioselectivities favoring only the endo product. The chiral catalyst system CFAM4-AgOAc was also used in 1,3-DC reaction with different dipolarophiles such as dimethyl maleate, tert-butyl acrylate, methyl acrylate, trans-chalcone, and vinyl sulfone. In each case, the cycloadducts were obtained in acceptable yields albeit with low ee. Fortunately, it was possible to increase the ee up to >99% upon crystallization.