Controlling the position of the nucleophilic ring-opening of 2-EWG-substituted azetidinium salts with fluoride by the N-1-(1-naphthyl)ethyl substituent and BINAM-derived bis-urea organocatalyst

Abstract

BINAM (1,1′-binaphthyl-2,2′-diamine)-derived bis-urea-catalyzed nucleophilic ring-opening of optically active and diastereomerically pure 2-EWG (electron-withdrawing group)-substituted N-(1-(1-naphthyl)ethyl)azetidinium salts with cesium fluoride in dichloromethane at room temperature proceeds at the less sterically hindered 4-position over the electronically deficient 2-position. Selective ring-opening at the 4-position is achieved by the combination of chiral stereocenters, as in N-(1-(1-naphthyl)ethyl)azetidinium salt, and steric bulk of the chiral bis-urea catalysts. The reaction affords the corresponding γ-fluoro-α-aminobutyric acid derivatives in diastereomerically pure. This protocol is applicable to the synthesis of enantiomerically enriched (98 % ee) γ-fluoro-α-amino acid derivatives starting from the commercially available chiral 1-(1-naphthyl)ethylamine.