Synthetic studies toward the PPAP natural products, prolifenones A and B and hyperforin: an Effenberger cyclization approach

The Total Synthesis of Hyperfirin via a Cyclooctadiene Strategy

Polycyclic polyprenylated acylphloroglucinols (PPAPs) combine compelling structural complexity with effective biological activity. The total synthesis of Hyperfirin is reported as one linear sequence. Key to this novel modular strategy is to access the bicyclo[3.3.1]nonane-2,4,9-trione framework via transannular acylation of a decorated eight-membered ring, followed by late stage bridgehead substitution. The described route adds flexibility to PPAP construction and broadens the scope of eight-membered ring chemistry.

Annulative Methods in the Synthesis of Complex Meroterpene Natural Products

From the venerable Robinson annulation to the irreplaceable Diels-Alder cycloaddition, annulation reactions have fueled the progression of the field of natural product synthesis throughout the past century. In broader terms, the ability to form a cyclic molecule directly from two or more simpler fragments has transformed virtually every aspect of the chemical sciences from the synthesis of organic materials to bioconjugation chemistry and drug discovery. In this Account, we describe the evolution of our meroterpene synthetic program over the past five years, enabled largely by the development of a tailored anionic annulation process for the synthesis of hydroxylated 1,3-cyclohexanediones from lithium enolates and the reactive β-lactone-containing feedstock chemical diketene.First, we provide details on short total syntheses of the prototypical polycyclic polyprenylated acylphloroglucinol (PPAP) natural products hyperforin and garsubellin A, which possess complex bicyclo[3.3.1]nonane architectures. Notably, these molecules have served as compelling synthetic targets for several decades and induce a number of biological effects of relevance to neuroscience and medicine. By merging our diketene annulation process with a hypervalent iodine-mediated oxidative ring expansion, bicyclo[3.3.1]nonane architectures can be easily prepared from simple 5,6-fused bicyclic diketones in only two chemical operations. Leveraging these two key chemical reactions in combination with various other stereoselective transformations allowed for these biologically active targets to be prepared in racemic form in only 10 steps.Next, we extend this strategy to the synthesis of complex fungal-derived meroterpenes generated biosynthetically from the coupling of 3,5-dimethylorsellinic acid (DMOA) and farnesyl pyrophosphate. A Ti(III)-mediated radical cyclization of a terminal epoxide was used to rapidly prepare a 6,6,5-fused tricyclic ketone which served as an input for our annulation/rearrangement process, ultimately enabling a total synthesis of protoaustinoid A, an important biosynthetic intermediate in DMOA-derived meroterpene synthesis, and its oxidation product berkeleyone A. Through a radical-based, abiotic rearrangement process, the bicyclo[3.3.1]nonane cores of these natural products could again be isomerized, resulting in the 6,5-fused ring systems of the andrastin family and ultimately delivering a total synthesis of andrastin D and preterrenoid. Notably, these isomerization transformations proved challenging when employing classic, acid-induced conditions for carbocation generation, thus highlighting the power of radical biomimicry in total synthesis. Finally, further oxidation and rearrangement allowed for access to terrenoid and the lactone-containing metabolite terretonin L.Overall, the merger of annulative diketene methodology with an oxidative rearrangement transformation has proven to be a broadly applicable strategy to synthesize bicyclo[3.3.1]nonane-containing natural products, a class of small molecules with over 1000 known members.

Research Progress of Polycyclic Polyprenylated Acylphloroglucinols

Polycyclic polyprenylated acylphloroglucinols (PPAPs) are a class of hybrid natural products sharing the mevalonate/methylerythritol phosphate and polyketide biosynthetic pathways and showing considerable structure and bioactivity diversity. This review discusses the progress of research into the chemistry and biological activity of 421 natural PPAPs in the past 11 years as well as in-depth studies of biological activities and total synthesis of some PPAPs isolated before 2006. We created an online database of all PPAPs known to date at http://www.chem.uky.edu/research/grossman/PPAPs . Two subclasses of biosynthetically related metabolites, spirocyclic PPAPs with octahydrospiro[cyclohexan-1,5'-indene]-2,4,6-trione core and complicated PPAPs produced by intramolecular [4 + 2] cycloadditions of MPAPs, are brought into the PPAP family. Some PPAPs' relative or absolute configurations are reassigned or critically discussed, and the confusing trivial names in PPAPs investigations are clarified. Pharmacologic studies have revealed a new molecular mechanism whereby hyperforin and its derivatives regulate neurotransmitter levels by activating TRPC6 as well as the antitumor mechanism of garcinol and its analogues. The antineoplastic potential of some type B PPAPs such as oblongifolin C and guttiferone K has increased significantly. As a result of the recent appearances of innovative synthetic methods and strategies, the total syntheses of 22 natural PPAPs including hyperforin, garcinol, and plukenetione A have been accomplished.

The enantioselective total synthesis of (+)-clusianone

(+)-Clusianone, an exo-type B PPAP with reported anti-HIV and chemoprotective activities, was synthesized in eleven steps with 97% ee starting from acetylacetone. An enantioselective decarboxylative Tsuji-Trost-allylation and a Ru-catalyzed ring-closing metathesis-decarboxylative allylation were used to control both diastereo- and enantioselectivity.

Stereoselective total synthesis of nemorosone

The highly stereoselective total synthesis of nemorosone via a new approach to the bicyclo[3.3.1]nonane-2,4,9-trione core which features intramolecular cyclopropanation of an α-diazo ketone, stereoselective alkylation at the C8 position, and regioselective ring-opening of cyclopropane is described. The total synthesis of nemorosone includes chemo- and stereoselective hydrogenation directed by the internal alkene.

The chemistry of the polycyclic polyprenylated acylphloroglucinols

With their fascinating biological profiles and stunningly complex molecular architectures, the polycyclic polyprenylated acylphloroglucinols (PPAPs) have long provided a fertile playing field for synthetic organic chemists. In particular, the recent advent of innovative synthetic methods and strategies together with C-C bond-forming reactions and asymmetric catalysis have revitalized this field tremendously. Consequently, PPAP targets which once seemed beyond reach have now been synthesized. This Review aims to highlight the recent achievements in the total synthesis of PPAPs, as well as notable methods developed for the construction of the bicyclo[3.3.1] core of these chemically and biologically intriguing molecules.