Forming Quaternary Carbons Using Photoredox Catalysis and Applications in Total Synthesis
- Author(s): Lackner, Gregory
- Advisor(s): Overman, Larry E
- et al.
In Chapter 1, the development of tert-alkyl N-phthalimidoyl oxalates as precursors for generating tertiary carbon radicals and forming quaternary carbons is discussed. The coupling of these radical precursors with conjugate acceptors is determined to be farily general with respect to both oxalate and alkene coupling partners. Studies that elucidate the mechanism of the coupling reaction are also presented. The mechanism of radical termination is found to be directly influenced by the stoichiometric reductant additive included in the reaction.
In Chapter 2, a bimolecular radical fragment coupling is employed as a key step to complete the total syntheses of several trans-clerodane diterpenoid natural products as well as one ent-halimane diterpenoid natural product. In this context, the tertiary radical is optimally generated from an (N-acyloxy)phthalimide substrate rather than the corresponding tert-alkyl N-phthalimidoyl oxalate.
In Chapter 3, a photoredox-catalyzed radical coupling is used to stereoselectively construct the central C8–C14 bond of the rearranged spongian diterpene macfarlandin C. An allylic phosphate displacement and intramolecular carbonyl-ene cyclization enable a robust enantioselective synthesis of the decalin fragment. The unexpected intramolecular cyclization of an alkoxycarbonyl radical derived from the (8S)-oxalate prompts a revised synthetic approach targeting the (8R)-oxalate radical precursor, which couples efficiently with (S)-5-methoxyfuran-2-one. Two synthetic routes to the bicyclic lactone moiety of macfarlandin C are investigated on a simple model substrate. These attempts are ultimately foiled by the difficulty in forming a cis-alpha, beta-disubstituted butyrolactone that bears a quaternary substituent at the beta-position.