Enantioselective Preparation of 2‐Aminomethyl Carboxylic Acid Derivatives: Solving the β²‐Amino Acid Problem with the Chiral Auxiliary 4‐Isopropyl‐5,5‐diphenyloxazolidin‐2‐one (DIOZ). Preliminary Communication

Multigram amounts of suitably protected β 2 ‐amino acids with 17 of the 20 proteinogenic side chains are prepared by diastereoselective reactions of Li, B, or Ti enolates of the corresponding 3‐acyl‐4‐isopropyl‐5,5‐diphenyloxazolidin‐2‐ones (acyl‐DIOZ; 1 ) with appropriate electrophiles (amidomethylation, hydroxyalkylation, (benzyloxycarbonyl)methylation) in yields of 55–90% and with diastereoselectivities of 80 to >97% ( Scheme ). The primary products 2 – 8 thus obtained are converted to protected β 2 ‐amino acids by standard procedures ( Table 1 ). Many of the DIOZ derivatives are highly crystalline compounds (31 X‐ray crystal structures in Table 2 ). The chiral auxiliary DIOZ, readily prepared in either enantiomeric form, is recovered with high yield.