Chiral Brønsted Acid-Catalyzed Direct Mannich Reactions via Electrophilic Activation

It was found that the phosphoric acid derivatives of general structure 1 serve as highly effective catalysts for the direct addition of acetyl acetone to N-Boc-protected arylimines. The beneficial effects of the 3,3'-bisaryl substituents of the catalysts on the enantioselectivity are greatly appreciated, and thus 1d functions as an excellent catalyst. The Brønsted acid-catalyzed direct Mannich reactions presented herein provide an attractive way to construct beta-aminoketones under extremely mild conditions. The stereochemical course of this reaction was established through the synthesis of Boc-(S)-phenylglycine methylester. The transformation thus demonstrated is applicable to a useful method for the synthesis of various phenylglycine derivatives.