Brønsted Acid‐Promoted Tandem Cyclization/Addition of Enaminones with H‐Phosphine Oxides for Synthesis of C2‐Phosphorylated 2,3‐2<i>H</i>‐Chromones

A facile and efficient protocol to synthesize C2‐phosphorylated 2,3‐2 H ‐chromones is discovered via Brønsted acid‐promoted tandem addition/cyclization of enaminones with H‐phosphine oxides. The reaction exhibits a broad scope, good functional group tolerance, and good yields, and features ability of a gram‐scaled synthesis. Results of mechanistic study reveal that the reaction initially undergoes intramolecular cyclization to form chromone 5 , which can subsequently be reacted with 2a to yield C2‐phosphorylated 2,3‐2 H ‐chromones through intermolecular addition.