Synthesis of (S)‐2‐methylproline: A general method for the preparation of α‐branched amino acids intermediate: 20.0 g (0.109 mol) of (2R,5S)‐2‐tert‐butyl‐1‐aza‐3‐oxabicyclo[3.3.0]octan‐4‐one intermediate: 20.1 g (0.102 mol) of (2R,5S)‐2‐tert‐butyl‐5‐methyl‐1‐aza‐3‐oxabicyclo[3.3.0]octan‐4‐one product: (S)‐2‐methylproline product: (R)‐2‐methylproline

513-523.l'ur rac-4 wurde im Knstall die ,,invertierte" Konformation der Ethanobrucke mit pseudo-aquatorialem He,,,,, (pro-S-H) gefunden (Abb. 221). Die Annahme dieser Konformation der Ethanobriicke fiihrtbei quantitativ naturlich etwas abweichenden Abstandsdateneindeutig zum selben Ergebnis.

structure 2 manifests diamagnetic ring current effects: the outer protons are deshielded and the inner shielded. According to the energetic geometrical, and magnetic criteria, the [1,7] H shift transition structure has pronounced aromatic character.

While there are numerous methods of preparing threonine analogs by aldol additions of chiral glycine derivatives to aldehydes, only few general routes leading to the epimers with C,C-bond formation are presently available. This paper describes the acylation of the title compound 1 with acyl chlorides (+ trans products 2-9), the subsequent reduction with LiBHEt3 in THF (4 hydroxyalkylated BMI derivatives 10-14), and acidic hydrolysis to the free allo-threonines 18-22. The first two steps are totally stereoselective (by Einleitung -Von der thueo-selektiven Aldoladdition zur

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