Catalytic Asymmetric Benzoin Condensation for the Synthesis of Natural Products and Their Analogs

Catalytic enantioselective benzoin condensation conditions that are potentially useful for the total synthesis of cycloimidamicins and structure-activity relationship studies of these natural products were developed. A triazole-based NHC catalyst promoted the transformation to afford the cyclization products. To make the reactions applicable to substrates with complex structural features, a Lewis acidic additive, B(OMe)Et₂, was used, but fine-tuning of the base and solvent conditions was required to attain a good chemical yield and excellent enantioselectivity. These conditions showed a broad substrate scope and were not limited to substrates useful for the synthesis of cycloimidamicins and their related compounds. This study also clearly demonstrates the utility of N-heterocyclic carbene organocatalysts that have not been reported to exhibit perfect enantioselectivity in intramolecular benzoin condensation.