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 β-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-β-Xaa*-OMe (β-Xaa* = L-β3-HAla, L-β3-HVal, L-β3-HLeu, L-β3-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 β-amino acids. Remarkably, the Bip method was also applied to the unique spin-labeled, cyclic, β-amino acids cis/trans-β-TOAC and trans-POAC. In particular, this study allowed the assignment of the unknown absolute configurations of the enantiomers of the latter compound.