New Class of Pyridine Catalyst Having a Conformation Switch System: Asymmetric Acylation of Various <i>s</i><i>ec</i>-Alcohols

We have developed a new class of pyridine catalyst for asymmetric acylation of sec-alcohols having a conformation switch system in which interconversion between self-complexation and uncomplexation is induced by acylation and deacylation steps, respectively. Kinetic resolution of various sec-alcohols is performed by the asymmetric acylation with isobutyric anhydride using 0.05 to 0.5 mol % catalyst 1a with s values of up to 30. In addition, dl-diols are also resolved in a similar manner in good selectivity. Moreover, asymmetric desymmetrization of meso-1,X-diols (X = 2−6) are achieved in the presence of 0.5−5 mol % catalyst 1a. A working model for the reaction mechanism is proposed on the basis of the 1H NMR measurements, X-ray structural analyses, and AM1 and DFT calculations, where the conformation switch system governed by an intramolecular cation−π interaction between a pyridinium ring and a thiocarbonyl group would play a key role to attain both good selectivity and high catalytic activity.