α-Alkylation of (S)-Asparagine with Self-Regeneration of the Stereogenic Center:  Enantioselective Synthesis of α-Substituted Aspartic Acids^1,2

A convenient method for the α-alkylation of (S)-asparagine with “self-regeneration of the stereogenic center” is described. The synthetic protocol involves stereoselective conversion of (S)-asparagine into enantiopure imino ether (2S,6S)-9, which is then alkylated with complete diastereoselectivity to give trans products 10−13 in good yields. Hydrolysis of these alkylated heterocycles is accomplished under mild acidic conditions to give the desired, enantiopure α-alkyl aspartic acids in excellent yields.