Diterpenoids

Throughout history, natural products have produced a plethora of biologically active compounds that have established applications in medicine, biology, and pharmacy. The exploration for improved cytotoxic agents has continued to be a crucial path in natural products drug discovery. The focal point of this thesis sheds light on the biosynthetic relationship between the two distinct classes of briarane diterpenoids, the γ-lactone briarane and the briareolate esters. Additionally, this study elaborates on the discovery and elucidation of structurally unique secondary metabolites from the gorgonian coral Briareum asbestinum.
The first chapter of this thesis provides a review of the development and discovery of diverse
secondary metabolites. In addition, this chapter describes the role of natural products in drug discovery and summarizes the research progress in marine natural product chemistry in conjunction with a detailed overview of the current marine-derived pharmaceuticals.

Natural products play a historical role in the discovery of medicine but present unique challenges for chemical isolation, identification and production. In this work we describe the identification of twenty novel diterpenoids. These were isolated by use of chromatography, and the structures determined by spectroscopic methods, primarily 1D and 2D NMR. Six of these possess unprecedented diterpenoid skeletons and two of them show significant growth inhibitory effects on cancer cell lines in vitro (GI50 < 10 μM). The biomimetic semisynthesis of diterpendoids and analogues is also presented.
Access to the bielschowskyane carbon skeleton by dearomatization of a furanocembranoid precursor is described. Highlights include a stereoselective alkene epoxidation, a novel kinetic furan dearomatization method, and an efficient [2+2] photochemical cycloaddition. The role of conformational steering was studied spectroscopically using VT 1H-NMR and NOESY as well as quantum chemical calculations at the DFT level of theory. We also disclose a biomimetic synthesis of providencin using a photochemical Norrish-Yang cyclization. This provided the absolute configuration by chemical correlation with the precursor bipinnatin E, the latter determined by x-ray diffraction. An unexpected, regioisomeric byproduct was observed and a possible mechanism is proposed. A biomimetic synthesis of the diterpene alkaloid aceropterine is also described, using an epoxidation-rearrangement cascade. This work led to a revised structure of aceropterine, formulated by spectroscopic methods. Finally, the isolation and structure elucidation of a novel, cyclic lipopeptide from Pseudomonas sp. is described. The compound was obtained using a unique antibiotic crowd sourcing approach and the structure determined by spectroscopic methods and advanced Marfey’s analysis.