No abstract

A thermally induced irreversible conformational transition of amylose tris(3,5-dimethylphenylcarbamate) (i.e., Chiralpak AD) chiral stationary phase (CSP) in the enantioseparation of dihydropyrimidinone (DHP) acid racemate was studied for the first time by quasi-equilibrated liquid chromatography with cyclic van't Hoff and step temperature programs and solid-state ((13)C CPMAS and (19)F MAS) NMR using ethanol and trifluoroacetic acid (TFA)-modified n-hexane as the mobile phase. The conformational transition was controlled by a single kinetically driven process, as evidenced by the chromatographic studies. Solid-state NMR was used to study the effect of the temperature on the conformational change of the solvated phase (with or without the DHP acid enantiomers and TFA) and provided some viable structural information about the CSP and the enantiomers.

No abstract

An investigation into the cause of substrate specific hydrogenation performance variability was conducted. A significant and unexpected correlation was observed between apparent pH of a solution of the substrate and rate of conversion and enantioselectivity. This observation led to the examination of low and variable levels of native ammonium chloride in different lots of substrate. The presence of ammonium chloride was found to have a positive effect on reaction rate and enantioselectivity when controlled within a relatively narrow range. Optimal performance was achieved with a mole ratio of 1:1 ammonium chloride to catalyst.

No abstract