Enantioselective Michael reaction of β-keto esters organocatalyzed by recoverable Cinchona-derived dimeric ammonium salts

Dimeric anthracenyldimethyl-derived Cinchona ammonium salts are used as chiral organocatalysts in 1–10 mol % for the enantioselective conjugate addition of 2-alkoxycarbonyl-1-indanones to β-unsubstituted Michael acceptors. The corresponding adducts bearing a new all-carbon quaternary center were usually obtained in high yield and with up to 94% ee when using ammonium salts derived from quinidine and its pseudoenantiomer quinine as organocatalysts. These catalysts can be almost recovered quantitatively by precipitation in ether and reused.

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α,α′-Bis(quininium)-9,10-dimethylanthracene dichlorideC56H60Cl2N4O4[α]D25=-444 (c 0.5, CHCl3)Source of chirality: (−)-quinine

α,α′-Bis(quininium)-9,10-dimethylanthracene ditetrafluoroborateC56H60B2F8N4O4[α]D25=-112 (c 0.1, CHCl3)Source of chirality: (−)-quinine

α,α′-Bis(quininium)-9,10-dimethylanthracene dihexafluorophosphateC56H60F12N4O4P2[α]D25=-35 (c 0.1, CHCl3)Source of chirality: (−)-quinine

α,α′-Bis(quinidinium)-9,10-dimethylanthracene dichlorideC56H60Cl2N4O4[α]D25=+339 (c 0.5, CHCl3)Source of chirality: (+)-quinidine