The Bip method, based on the induced circular dichroism of a flexible biphenyl probe in terminally protected -Bip-Xaa*-dipeptides, for assignment of the absolute configuration of beta-amino acids

An induced axial chirality of the biphenyl core of the Bip (2′,1′:1,2;1′′,2′′:3,4-dibenzcyclohepta- 1,3-diene-6-amino-6-carboxylic acid) residue in the terminally protected dipeptides Boc-Bip--Xaa*-OMe (-Xaa* ) L-3-HAla, L-3-HVal, L-3-HLeu, L-3-HPro, trans-(1S,2S)-ACHC, trans-(1R,2R)-ACHC, trans- (1S,2S)-ACPC, trans-(1R,2R)-ACPC) resulted in an induced circular dichroism, revealing the usefulness of the Bip method for a reliable and fast assignment of the absolute configuration of chiral -amino acids. Remarkably, the Bip method was also applied to the unique spin-labeled, cyclic, -amino acids cis/trans- -TOAC and trans-POAC. In particular, this study allowed the assignment of the unknown absolute configurations of the enantiomers of the latter compound.