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Controlling axial conformation in 2-arylpyridines and 1-arylisoquinolines: application to the asymmetric synthesis of QUINAP by dynamic thermodynamic resolution.

Unlike related biphenyl compounds, 2-arylpyridines and 1-arylisoquinolines can be induced to adopt preferentially one of two axial conformations by the presence of a sulfinyl substituent adjacent to the Ar-Ar bond. In the case of more substituted biaryls, the compounds are atropisomeric, and thermodynamic selectivities of about 4:1 may be attained on heating. In the case of less hindered compounds, conformer ratios of up to 20:1 may be achieved. Preferred conformations are deduced by comparison of experimental CD spectra with those derived from theory. The conformational preferences induced by the sulfoxides may be exploited in the asymmetric synthesis of atropisomers, including the ligand QUINAP, by dynamic resolution under thermodynamic control.