| Abstract
| - A general and nonempirical approach to determine the absolute configuration (AC) of 2-substitutedchiral carboxylic acids by circular dichroism (CD) spectroscopy has been developed. In this protocol, thechiral acids are converted to the corresponding biphenyl amides, in which a flexible biphenyl probe givesrise to a Cotton effect at 250 nm (A band) in the CD spectrum, the sign of which is related to the acid AC.Two different mechanisms of transfer of chirality from the acid stereogenic center to the biphenyl moietyare operative in amides derived from 2-alkyl- and 2-aryl-substituted acids, respectively. For both classesof compounds, a model has been defined which allows one to predict, for a given acid AC, the preferredtwist of the biphenyl moiety and thus the sign of the A band in the CD spectrum, related to the biphenyltorsion. Interestingly, while in alkyl-substituted substrates the preferred biphenyl twist is determined onlyby steric interactions, in the aryl-substituted ones the structure of the prevalent conformer and thus thebiphenyl twist are dictated by arene−arene edge-to-face stabilizing interactions. Following this protocol,the AC of a 2-substituted chiral acid can be established simply by preparing its biphenyl amides, recordingthe CD spectrum, and looking at the sign of the A band. From the sign of such a band, the torsion of thebiphenyl can be deduced and then the acid AC. Substrates having different structures and functionalitieshave been investigated, always obtaining reliable AC assignments by this simple protocol.
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