An NmrA-like enzyme-catalysed redox-mediated Diels–Alder cycloaddition with anti -selectivity

The Diels–Alder cycloaddition is one of the most powerful approaches in organic synthesis and is often used in the synthesis of important pharmaceuticals. Yet, strictly controlling the stereoselectivity of the Diels–Alder reactions is challenging, and great efforts are needed to construct complex molecules with desired chirality via organocatalysis or transition-metal strategies. Nature has evolved different types of enzymes to exquisitely control cyclization stereochemistry; however, most of the reported Diels–Alderases have been shown to only facilitate the energetically favourable diastereoselective cycloadditions. Here we report the discovery and characterization of CtdP, a member of a new class of bifunctional oxidoreductase/Diels–Alderase, which was previously annotated as an NmrA-like transcriptional regulator. We demonstrate that CtdP catalyses the inherently disfavoured cycloaddition to form the bicyclo[2.2.2]diazaoctane scaffold with a strict α- anti -selectivity. Guided by computational studies, we reveal a NADP + /NADPH-dependent redox mechanism for the CtdP-catalysed inverse electron demand Diels–Alder cycloaddition, which serves as the first example of a bifunctional Diels–Alderase that utilizes this mechanism.