Synthetic and Biocatalytic Strategies for Natural Product Synthesis via ortho-Quinone Methide Intermediates
Organisms across all domains of life have developed elegant strategies to produce specialized metabolites, also referred to as natural products, for defense, structure, and communication. Natural products provide novel inspiration for pharmaceutical development given their potent biological activities and structural diversity. Recent advancements in DNA sequencing technology and the development of predictive bioinformatics tools have unveiled a vast collection of biosynthetic machinery responsible for generating this structural diversity that may be harnessed by synthetic chemists. Many of the enzymes encoded by biosynthetic genes catalyze chemical reactions that are difficult to replicate with the same selectivity by traditional synthetic methods. An example of this discrepancy is illustrated by the current enzymatic and chemical methods known to generate a highly reactive ortho-quinone methide (o-QM) intermediate. The utility of o-QMs in total synthesis has been hampered by complications that surround the preparation of their precursors, the harsh generation methods, and poor chemo-, regio-, and stereoselective control. In contrast, multiple enzymes have been reported to catalyze o-QM formation under mild conditions with remarkable selectivity. Chapter 1 of this dissertation examines several different enzymatic strategies developed to generate o-QMs. Chapter 2 describes progress towards the biomimetic total synthesis of (–)-chlorizidine A that aims to synthetically replicate the final biosynthetic intramolecular cyclization via an o-QM intermediate. Chapter 3 reports the discovery of tetrachlorizine, a novel tetrachlorinated marine natural product, and the identification of its associated biosynthetic gene cluster. Biochemical characterization of a pivotal flavin-dependent oxidase revealed a dehydrogenation reaction that proceeds via an unprecedented o-QM mechanism. This oxidase was then repurposed to generate an isolable o-QM, an extremely rare chemical motif. Chapter 4 investigates the structure-function relationships of two homologous flavin-dependent oxidases with divergent reaction selectivities and highlights their favorable biocatalytic properties. This chapter also discloses the first reported bacterial enzymes capable of performing oxidative cyclization reactions to produce cannabinoids.