r-Alkylation of (S)-Asparagine with Self-Regeneration of the Stereogenic Center: Enantioselective Synthesis of r-Substituted Aspartic Acids

A convenient method for the R-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 R-alkyl aspartic acids in excellent yields.