Densely Substituted l‑Proline Esters as Catalysts for Asymmetric Michael Additions of Ketones to Nitroalkenes

Homochiral methyl 4-aminopyrrolidine-2-carboxylates are readily obtained by means of asymmetric (3 + 2) cycloadditions between azomethine ylides and nitroalkenes, followed by catalytic hydrogenation of the intermediate 4-nitro cycloadducts. These 4-aminopyrrolidine-2-carboxylate esters belong to the l-series of natural amino acids and catalyze asymmetric Michael additions of ketones to nitroalkenes. However, the enantioselectivity observed with these novel unnatural organocatalysts is opposite to that obtained with l-proline. Since both 4-nitro and 4-amino l-proline esters are efficient organocatalysts of aldol reactions, these results permit to modulate asymmetric quimioselective aldol and conjugate addition reactions.