Organocatalytic asymmetric synthesis of quaternary α-isoxazole-α-alkynyl amino acid derivatives.

An enantioselective addition of 5-amino-isoxazoles with β,γ-alkynyl-α-ketimino esters catalyzed by a chiral phosphoric acid has been developed. This procedure allowed the formation of quaternary α-isoxazole-α-alkynyl amino acid derivatives with high yields (up to 99%) and good to excellent enantioselectivities (up to 97%), and the corresponding products enabled many further elaborations. The control experiment revealed that the hydrogen-bonding interaction of 5-amino-isoxazole with the chiral phosphoric acid played a vital role in the enantioselectivity, and the transition state of the reaction was proposed.