Application of Transition Metal-Catalyzed Reactions in the Synthesis of N- Heterocyclic Natural Products
Chaterpaul, Stephen Joshua
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Metal-catalyzed carbon-carbon bond forming reactions have become one of the most powerful synthetic reactions of the modern organic chemistry reaction arsenal. These reactions have allowed for easier access to more complex structures, and brought about the development of various catalytic asymmetric and asymmetric transformations. At the core of metal-catalyzed asymmetric synthesis are chiral ligands, which are required for the asymmetric inductions afforded by these reactions. From the very inception of asymmetric reactions, mono- and bidentate phosphorus- based ligands have played a critical role in achieving high enantioselectivity. Although, bidentate ligands have dominated the scene over the past thirty years, monodentate ligands have re-emerged from obscurity to become an essential asset in asymmetric catalysis. The Ojima laboratory has developed a series of novel phosphorus ligands based on axially chiral 5,5`,6,6`-tetramethylbiphenyl-2,2`-diol. These novel ligands have the capacity to be fine-tuned, which is a particularly attractive feature as these ligands can be applied to a variety of reactions. Thus, a library of these axially chiral phosphorus ligands were designed and synthesized. Subsquently these ligands were successfully applied in a palladium-catalyzed bicycloannulation reaction, the key-step in the synthesis of the lycopodium alkaloid huperzine-A. In addition to exploring asymmetric catalysis, we also explored cyclohydrocarbonylation as a means to access several classes of nitrogen-based heterocycles. The cyclohydrocarbonylative process can be described as a hydroformylation reaction, followed by concomintant cyclization resulting from an intramolecular attack by an amide-nitrogen, forming an N-acyliminium ion. We studied the trapping of this species with various carbon nucleophiles, including electron rich aromatics, allyl silanes, and enols. This concomitant process permits facile access to several traditionally difficult to synthesize alkaloid classes. Successful application of cyclohydrocarbonylative trapping is demonstrated.