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.