Induced axial chirality in the biphenyl core of the proatropoisomeric, Cα-tetrasubstituted α-amino acid residue Bip in peptides

An induced axial chirality in the biphenyl core of the 2',1':1,2;",2":3,4-dibenzcyclohepta-1,3-diene-6-amino-6-carboxylic acid (Bip) residue, a conformationally labile, atropoisomeric, C-alpha-tetrasubstituted alpha-amino acid, was observed by CD and H-1 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, -(alpha Me)Val and -(alpha Me)Leu. Chiral induction was significantly lower in the isomeric dipeptides Boc-Xaa*-Bip-OMc, 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.