Enantioselective Michael Addition of 2-Nitropropane to Substituted Chalcones and Chalcone Analogues Catalyzed by Chiral Crown Ethers Incorporating an α-D-Glucose or an α-D-Mannose Unit

The chiral monoaza-15-crown-5 lariat ethers annellated to methyl-4,6-O-benzylidene-α-D-glucopyranoside- (1) or -mannopyranoside (2) used as phase transfer catalysts in the Michael addition of 2-nitropropane to substituted chalcones and chalcone analogues resulted in a significant asymmetric induction. The type of substituent on the chalcone molecule was found to have a significant influence on both the chemical yield and the enantioselectivity of the reaction: 24 novel chiral Michael adducts were prepared in 14-68% ee. These ee values were somewhat lower than that experienced in the case of the unsubstituted chalcone (85% ee). In the series of chalcone analogues, the 1-naphthyl Michael adduct was formed in 87% ee. Using glucose-based crown ether 1, formation of the (+)-enantiomers was preferred, while applying mannose-based 2 as the catalyst, the (-)-enantiomers were in excess.