Enantioselective Synthesis of the Pyrrolidine Core of Endothelin Antagonist ABT‐627 (Atrasentan) via 1,2‐Oxazines

Diastereoselective syntheses of the pyrrolidine core 6a of the endothelin antagonist ABT‐627 (Atrasentan) either as a racemic mixture or as an enantiopure compound are presented. The crucial steps of these syntheses utilized the highly diastereoselective conjugate addition of 1,3‐benzodioxol‐5‐yllithium to racemic 6 H ‐1,2‐oxazine 3 or enantiopure 6 H ‐1,2‐oxazines 7 or 8 , followed by trapping with ethyl cyanoformate (Mander′s reagent). The resulting 5,6‐dihydro‐4 H ‐1,2‐oxazines were transformed into the 2,3,4‐trisubstituted pyrrolidine 6a . (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)