Induced Axial Chirality in the Biphenyl Core of the Proatropoisomeric, C α ‐Tetrasubstituted α‐Amino Acid Residue Bip in Peptides
Résumé
Abstract An induced axial chirality in the biphenyl core of the 2′,1′:1,2;1′′,2′′:3,4‐dibenzcyclohepta‐1,3‐diene‐6‐amino‐6‐carboxylic acid (Bip) residue, a conformationally labile, atropoisomeric, C α ‐tetrasubstituted α‐amino acid, was observed by CD and 1 H NMR spectroscopic techniques in the linear dipeptides Boc‐Bip‐Xaa*‐OMe where Boc= tert ‐butoxycarbonyl, OMe=methoxy, and Xaa*= D ‐ and/or L ‐ Ala, ‐Val, ‐Leu, ‐Phe, ‐(αMe)Val and ‐(αMe)Leu. Chiral induction was significantly lower in the isomeric dipeptides Boc‐Xaa*‐Bip‐OMe, with the Xaa* residue located at the N‐terminus of Bip, as well as in the cyclic dipeptide cyclo‐[Bip‐ L ‐ Ala]. The results obtained in solution were confirmed by X‐ray diffraction analysis of a crystalline sample of Boc‐( R )‐Bip‐ D ‐Ala‐OMe.