Thursday, April 29, 2021 - 4:00pm

Abstract: This dissertation describes the development and applications of a radical bicyclization reaction that is initiated via hydrogen-atom-transfer (HAT) to 1,1-disubstituted alkenes. In the first chapter, biomimetic radical polyene cyclizations and HAT-mediated C-C bond forming reactions are reviewed. Mechanistic and strategic aspects pertaining to radical-polar crossover are emphasized throughout, highlighting unique possibilities offered by oxidative or reductive termination.

The second chapter details the development of a novel cobalt(II)‐catalyzed bicyclization. The process is presumably initiated by metal‐catalyzed hydrogen‐atom transfer (MHAT) to 1,1‐disubstituted or monosubstituted alkenes. Notably, electron‐rich methyl or electron‐deficient nitrile substituted internal alkenes are tolerated. Electron‐rich aromatic terminators are required in both cases. Terpenoid scaffolds with different substitution patterns are obtained with excellent diastereoselectivities, and the bioactive C20‐oxidized abietane diterpenoid carnosaldehyde was made to showcase the utility of the nitrile‐bearing products. Also provided are the results of several mechanistic experiments that suggest the process features an MHAT‐induced radical bicyclization with late‐stage oxidation to regenerate the aromatic terminator.

The third chapter details our studies in applying cobalt(II)‐catalyzed bicyclizations to the synthesis of oxygenated terpenoid scaffolds. A systematic evaluation of substrates bearing different oxygenation patterns was conducted to assess the practicality of using preoxidized bicyclization precursors. The degree of stereoselectivity was often high, but varied with oxygenation locus, with free hydroxy groups performing well in most contexts. The stereochemical outcomes of cyclizations of dioxygenated polyenes were dictated by aliphatic chain oxygenation closest to the initiating 1,1-disubstituted alkene. Simple analyses of nonbonding interactions in the putative cyclization transition states were sufficient to rationalize stereochemical outcomes in most cases. Intramolecular hydrogen bonding directed regioselectivity was observed in one substrate class. To further validate the use of preoxidized polycyclization precursors in bioactive diterpenoid synthesis, we completed the first total syntheses of (+)-2-O-deacteyl plebedipene A, (±)-plebedipene B, and (+)-2-O-deacetyl plebedipene C. Lastly, experiments aimed at expanding the scope of cobalt(II)‐catalyzed polycyclizations are proposed.  


Darius Vrubliauskas


Vanderwal Group