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Abstract:
Heterocycles constitute cardinal pharmacophores in a plethora of biologically active substances ranging from synthetic drugs to agrochemicals. Partcularly, enantiomerically pure α-stereogenic heterocycles are versatile building blocks in this field and they often show high bioglogical activity themselves. The direct synthesis of these motifs involves the enantiodiscrimination of the corresponding cyclic, aliphatic heteroatom-stabilized carbocations. These highly electrophilic species lack steric and aromatic bias, which diminishes possible interactions to catalysts. This renders the enantiodifferentiation of the nucleophilic addition especially challenging. Within this thesis, the field of IDPi catalysis was successfully extended toward the enantiocontrol of highly reactive cyclic, aliphatic N-acyliminium ions. These intermediates were generated in situ from the readily available corresponding hemiaminal ethers and controlled in silylium Lewis acid-catalysed addition reactions with a broad scope of silicon nucleophiles. Several of the thereof obtained nitrogen containing heterocycles are direct synthetic precursors of the pyrrolidine and sedum alkaloid natural product families. Furthermore, cyclic, aliphatic oxocarbenium ions were efficiently enantiodifferentiated under IDPi Brønsted acid catalysis in addition reactions with alcohols as nucleophiles. This led to the development of the first catalytic asymmetric THP protection. Taken together, this thesis expands the catiom with N-acyliminium ions toward C–C bond forming reactions with enol silanes and silyl ketene acetals and with the tetrahydropyranium ion toward C–O bond forming reactions with alcohols under asymmetric counteranion-directed catalysis.
