Ringöffnungen von 1,2‐Didehydroprolinen, I Darstellung von 4‐Hydroxyornithin und geschützten 4‐Amino‐3‐hydroxybutyronitrilen

Ring‐Opening Reactions of 1,2‐Didehydroprolines, I. — Synthesis of 4‐Hydroxyornithine and Protected 4‐Amino‐3‐hydroxybutyronitriles The cyclic azomethines 6a , b , d and f easily undergo ringopening reaction with hydroxylamine and its O ‐alkyl derivatives leading to the oximino compounds 3a , b , d and f – i . Dissolved alkali metals in amines (Birch conditions) give satisfactory yields of reduction products with a stereochemistry depending on the substitution pattern in 3b , d , f – i . In case of 3d , the (2 R ,4 R ) stereoisomer 9b is the sole product, which can subsequently be cleaved to give 8b . Compounds 3g – i give predominantly the (2 S ,4 R ) product 8a . Acylation under alkaline conditions decarboxylates the oximino compounds 3b and d to the corresponding protected butyronitriles 14a – h . Trifluoroacetic anhydride transforms 3b into the nitrile 14i together with the cyclization product 15 .