An efficient synthesis of bicyclic β-turn dipeptides via a photochemical key step

Abstract

A novel synthetic route to bicyclic β-turn dipeptides (BTD) is described. It starts with L-β-benzoylalanine 1, which is N-protected and coupled with proline esters yielding the dipeptides 3. Upon irradiation 3a-c undergo a cyclization to the indolizinones 4 in a highly stereoselective manner. 4a and 4c were converted into the N-protected BTD derivatives suited for the peptide synthesis. The absolute configuration of products 4 and 5 were unambiguously elucidated from NOE results.