Synthesis and Application of New Chiral Phosphoric Acids Based on oxa-Spirocyclic Diphenol

Chiral phosphoric acids (CPAs) are a highly versatile class of Brønsted acid catalysts. They have been employed in various challenging organic transformations. However, previously reported chiral phosphoric acids are still limited in variety. In this work, a new type of CPAs with different substitutions (27 examples) derived from the O-SPINOL scaffold was synthesized through 8 steps. The new type of CPAs were applied to the electrophilic amination of indoles bearing bulky substituents and exhibited high catalytic efficiency. Preliminary comparative experiments revealed that the chiral phosphoric acid derived from O-SPINOL gave better catalytic performance in terms of enantioselectivity than that derived from SPINOL in most examples. This new type of chiral phosphoric acids adds to and complements the existing pool of chiral phosphoric acid catalysts.