Electrophilic Amination of Silyl Ketene Acetals

Absolute configurations and enantiomeric excesses of the products were checked by [a]D comparison, by HPLC and capillary GC using chiral columns. 

In summary, a-hydrazino acids and natural and unnatural a-amino acids could be obtained in both the R and S configurations by this practical method. 

The diastereomeric excesses were determined by HPLC and H NMR on the crude product. The major diastereomers 34a were then purified by flash chromatography or crystallization and obtained in virtually 100 % de. Their (25) absolute configuration was assigned by transformation to the corresponding (25)-a-amino acids. 

The (25) absolute configuration and enantiomeric purities (up to 99 % ee) of the crude amino acids 37 were readily determined by GC comparison (using a chiral capillary column) of their (jV-trifluoroacetyl)-n-propyl esters with those of racemic and enantiomerically pure authentic samples and were further supported by chiroptic comparison. 

In summary, this methodology is a predictable enantioselective entry to (2S)-a-amino acids. (2/ )-a-Amino acids could be obtained in the same manner, starting from the commercially available antipode of the alcohol 32. 

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