Asymmetric Nitrocyclopropanation of α-Substituted α,β-Enals Catalyzed by Diphenylprolinol Silyl Ether for the Construction of All-Carbon Quaternary ­Stereogenic Centers
The diphenylprolinol silyl ether mediated asymmetric nitrocyclopropanation of α-substituted α,β-unsaturated aldehydes with bromonitromethane, followed by base-promoted isomerization was found to afford trans-nitrocyclopropanecarbaldehydes with all-carbon quaternary stereogenic centers with excellent diastereo- and enantioselectivities. DFT calculations indicated that the s-trans conformer of the iminium ion intermediate is more stable than the s-cis conformer. In addition, nucleophilic attack of the bromonitromethane anion to the iminium ion intermediate was calculated to occur preferentially from the opposite side of the bulky substituents of the pyrrolidine iminium intermediate.