Diastereo- and Enantioselective Synthesis of β-Hydroxy-α-Amino Acids: Application to the Synthesis of a Key Intermediate for Lactacystin

The development of a highly efficient and stereoselective methodology for the preparation of beta-hydroxy-alpha-amino acids is described. Nucleophilic addition of enolates of tricyclic iminolactones 1a and 1b to aldehydes in the presence of 6 equiv of lithium chloride in THF at -78 degrees C leads to aldol adducts in good yield (63-86%) and high diastereoselectivity (up to >25:1 dr); Subsequently, hydrolysis of the aldol adducts under acidic conditions leads to the corresponding P-hydroxy-cc-amino acids in good yields (up to 83%) and excellent enantiomeric excesses (99% ee) with good recovery yields of the chiral auxiliaries 6 and 7. This methodology was applied to the facile synthesis of the key intermediate for lactacystin along with several isomers.