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Diastereo- and enantioselective synthesis of beta-hydroxy-alpha-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 beta-hydroxy-alpha-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.