Activation Mode and Origin of Selectivity in Chiral Phosphoric Acid-Catalyzed Oxacycle Formation by Intramolecular Oxetane Desymmetrizations

Density functional theory methods were used to elucidate the activation mode and origin of stereoselectivity in chiral phosphoric acid-catalyzed intramolecular oxetane desymmetrizations. Computed enantioselectivities are in excellent agreement with experiment. An unexpected, distortion-driven activation mode was observed, instead of the usual “bifunctional activation”. This mode is favored for only some intramolecular oxetane openings, highlighting an exception to known models. Stereoselectivity in these reactions can be explained by the balance of favorable noncovalent interactions of the substrates with both the aryl substituents and phosphoric acid functionality of the catalysts.