Infoscience

Thesis

Access to Chiral Cyclopentane Scaffolds by Enantioselective Nickel-Catalyzed Annulations

The cyclopentane scaffold has attracted much attention due to its ubiquity in numerous bioactive natural products such as prostaglandins and polyquinanes. However, despite a plethora of available procedures to access this motif, a lack of methods that rival the generality of the Diels-Alder reaction for the synthesis of six-membered rings represents an important gap. Thus, the development of protocols providing efficient access to functionalized chiral cyclopentanes from simple and readily accessible starting materials is highly desirable. In this respect, the tremendous advances in the development of nickel-catalyzed carbon-carbon bond-forming reactions, combined with the unique properties of N-heterocyclic carbenes have led to the development of new approaches to five-membered carbocyclic rings. These include nickel-catalyzed reductive couplings and reductive [3+2] cycloadditions. These strategies are attractive, as functionalized five-membered carbocyclic rings can be accessed from two simple pi-components without any changes in atom connectivity. Despite the recent development of numerous chiral N-heterocyclic carbenes and their successful application in various transition metal-catalyzed reactions, applications in asymmetric nickel catalysis remain scarce. This thesis describes the development of efficient enantioselective nickel-catalyzed methodologies for the formation of five-membered carbocycles from readily accessible and inexpensive starting materials, enabled by two novel chiral N-heterocyclic carbene ligands. First, the development of an intermolecular enantioselective nickel-catalyzed reductive [3+2] cycloaddition of unsaturated aromatic esters and internal alkynes for the synthesis of cyclopentenones is described. For this scalable methodology, a bulky chiral C1-symmetric N-heterocyclic carbene ligand was shown to facilitate the efficient asymmetric synthesis of cyclopentenones from a wide range of mesityl enoates and internal alkynes under mild conditions. Unsymmetrically substituted alkynes were incorporated in a regioselective fashion and the cyclopentenones were synthesized in high yields and enantioselectivity. As a second methodology for the construction of chiral cyclopentane scaffolds, an intermolecular diastereoselective and enantioselective nickel-catalyzed reductive three-component coupling of various aryl aldehydes, norbornene derivatives and silanes is described, which gives access to synthetically valuable silyl-protected indan-1-ols via an aromatic C(sp2)-H functionalization. A bulky chiral C2-symmetric N-heterocyclic carbene ligand possessing a 1,2-(dinaphthalen-1-yl)ethylene diamine scaffold provided the silylated indan-1-ols under mild and scalable conditions, as a single diastereoisomer, in high enantioselectivity and good regioselectivity in the case of meta-substituted aryl aldehydes.

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