Highly Stereoselective Diels–Alder Reactions Catalyzed by Diboronate Complexes**

Abstract A highly enantioselective catalytic system for exo ‐Diels–Alder reactions was developed based on the newly discovered bispyrrolidine diboronates (BPDB). Activated by various Lewis or Brønsted acids, BPDB can catalyze highly stereoselective asymmetric exo ‐Diels–Alder reactions of monocarbonyl‐based dienophiles. When 1,2‐dicarbonyl‐based dienophiles are used, the catalyst can sterically distinguish between the two binding sites, which leads to highly regioselective asymmetric Diels–Alder reactions. BPDB can be prepared as crystalline solids on a large scale and are stable under ambient condition. Single‐crystal X‐ray analysis of the structure for acid‐activated BPDB indicated that its activation involves cleavage of a labile B←N bond.