C2-symmetric copper(II) complexes as chiral Lewis acids.: Catalytic enantioselective carbonyl-ene reactions with glyoxylate and pyruvate esters

The C-2-symmetric complexes [Cu(S,S)-tert-butylbis(oxazolinyl)](SbF6)(2) (2a), its bis(aquo) counterpart [Cu(S,S)-tert-butylbis(oxazolinyl)(H2O)(2)] (SbF6)(2) (4), and [Cu(S,S)-phenylbis(oxazolinyl)](OTf)(2) (1c) have been shown to catalyze the enantioselective ene reaction between glyoxylate esters and various olefins. Monosubstituted, disubstituted, and trisubstituted olefins each react with ethyl glyoxylate in the presence of 0.2-10 mol % of catalysts 1, 2, and 4 to afford the ene products in good yield and enantioseiectivity. Complex 2a has also been shown to catalyze the addition of 1,1-disubstituted olefins to methyl pyruvate in good yields (76-95%) and excellent enantioselectivies (greater than or equal to 98% eel. The synthetic utility of the glyoxylate-ene reaction was demonstrated by conversion of the resulting alpha-hydroxy ester into the corresponding methyl ester, free acid, Weinreb amide, and alpha-azido ester, all with no detectable racemization.