Asymmetric Phase-Transfer Catalysis as a Powerful Tool in the Synthesis of Biologically Active Chiral Complex Natural Products

Abstract The field of complex (natural) product syntheses has been dominated by (transition-)metal-catalyzed asymmetric reactions for decades. However, besides the very broad range of different transformations that can be achieved by metal catalysis, the use of substoichiometric amounts of small-organic molecule catalysts (organocatalysis) has proven to possess an enormous potential for a variety of reactions. Among the different types of activation modes that are commonly employed when using organocatalysts, the use of chiral cation-based phase-transfer catalysts has obtained an outstanding position. The high potential of asymmetric phase-transfer catalysis can be attributed to several reasons (e.g., mild aqueous reaction conditions, operational simplicity, easily handled catalysts, scalability, etc.), making it a powerful and versatile methodology for a broad scope of different applications. In addition, this methodology allows the activation and control of reagents that are only hardly possible using any other activation modes, thus making it one of the outstanding catalytic principles that are currently available. The intention of this contribution is to provide the reader with an illustrative overview concerning successful applications of asymmetric phase-transfer catalysis in the field of (biologically active) complex (natural) product synthesis. The main focus will be on the phase-transfer-catalyzed key steps for each (multistep) synthesis described, whereas other often particularly innovative transformations will be omitted, as this would be beyond the scope of this chapter.