Preparation of β 2 ‐Amino Acid Derivatives ( β 2 hThr, β 2 hTrp, β 2 hMet, β 2 hPro, β 2 hLys, Pyrrolidine‐3‐carboxylic Acid) by Using DIOZ as Chiral Auxiliary

The title compounds were prepared from valine‐derived N ‐acylated oxazolidin‐2‐ones, 1 – 3, 7, 9 , by highly diastereoselective (≥ 90%) Mannich reaction (→  4 – 6 ; Scheme 1 ) or aldol addition (→  8 and 10 ; Scheme 2 ) of the corresponding Ti‐ or B‐enolates as the key step. The superiority of the ‘5,5‐diphenyl‐4‐isopropyl‐1,3‐oxazolidin‐2‐one’ (DIOZ) was demonstrated, once more, in these reactions and in subsequent transformations leading to various t ‐Bu‐, Boc‐, Fmoc‐, and Cbz‐protected β 2 ‐homoamino acid derivatives 11 – 23 ( Schemes 3–6 ). The use of ω ‐bromo‐acyl‐oxazolidinones 1 – 3 as starting materials turned out to open access to a variety of enantiomerically pure trifunctional and cyclic carboxylic‐acid derivatives.