The first catalytic asymmetric total synthesis of ent-hyperforin

Different routes for the construction of biologically active diversely functionalized bicyclo[3.3.1]nonanes: an exploration of new perspectives for anticancer chemotherapeutics

Cancer is the second most high-morbidity disease throughout the world. From ancient days, natural products have been known to possess several biological activities, and research on natural products is one of the most enticing areas where scientists are engrossed in the extraction of valuable compounds from various plants to isolate many life-saving medicines, along with their other applications. It has been noticed that the bicyclo[3.3.1]nonane moiety is predominant in most biologically active natural products owing to its exceptional characteristics compared to others. Many derivatives of bicyclo[3.3.1]nonane are attractive to researchers for use in asymmetric catalysis or as potent anticancer entities along with their successful applications as ion receptors, metallocycles, and molecular tweezers. Therefore, this review article discusses several miscellaneous synthetic routes for the construction of bicyclo[3.3.1]nonanes and their heteroanalogues in association with the delineation of their anticancer activities with few selective compounds.

Total Synthesis of a PPAP, Nemorosonol, Using a Tandem Michael Addition-Intramolecular Aldol Reaction

A strategy for constructing a tricyclo[4.3.1.0^3,7]decane skeleton, which is common to many polycyclic polyprenylated acylphloroglucinols, has been established. The key step was a tandem Michael addition-intramolecular aldol reaction with 3-ethoxy-1-phenyl-2-proyn-1-one, which affords a tricyclo[4.3.1.0^3,7]decane skeleton having a benzoyl group at the C8 position and an appropriate oxygen functional group at the C9 position. This synthetic strategy led to the total synthesis of nemorosonol, which was accomplished in 12 steps from 2-methyl-2-cyclopenten-1-one.

Enantioselective Total Synthesis of (+)-Garsubellin A

Garsubellin A is a meroterpene capable of enhancing the enzyme choline acetyltransferase whose decreased level is believed to play a central role in the symptoms of Alzheimer's disease. Due to the potentially useful biological activity together with the novel bridged and fused cyclic molecular architecture, garsubellin A has garnered substantial synthetic interest, but its absolute stereostructure has been undetermined. We report here the first enantioselective total synthesis of (+)-garsubellin A. Our synthesis relies on stereoselective fashioning of a cyclohexanone framework and double conjugate addition of 1,2-ethanedithiol that promotes aldol cyclization to build the bicyclic [3.3.1] skeleton. The twelve-step, protecting group-free synthetic route has enabled the syntheses of both the natural (-)-garsubellin A and its unnatural (+)-antipode for biological evaluations.

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.

Programmable meroterpene synthesis

The bicyclo[3.3.1]nonane architecture is a privileged structural motif found in over 1000 natural products with relevance to neurodegenerative disease, bacterial and parasitic infection, and cancer among others. Despite disparate biosynthetic machinery, alkaloid, terpene, and polyketide-producing organisms have all evolved pathways to incorporate this carbocyclic ring system. Natural products of mixed polyketide/terpenoid origins (meroterpenes) are a particularly rich and important source of biologically active bicyclo[3.3.1]nonane-containing molecules. Herein we detail a fully synthetic strategy toward this broad family of targets based on an abiotic annulation/rearrangement strategy resulting in a 10-step total synthesis of garsubellin A, an enhancer of choline acetyltransferase and member of the large family of polycyclic polyprenylated acylphloroglucinols. This work solidifies a strategy for making multiple, diverse meroterpene chemotypes in a programmable assembly process involving a minimal number of chemical transformations.

Bond-Forming and -Breaking Reactions at Sulfur(IV): Sulfoxides, Sulfonium Salts, Sulfur Ylides, and Sulfinate Salts

Organosulfur compounds have long played a vital role in organic chemistry and in the development of novel chemical structures and architectures. Prominent among these organosulfur compounds are those involving a sulfur(IV) center, which have been the subject of countless investigations over more than a hundred years. In addition to a long list of textbook sulfur-based reactions, there has been a sustained interest in the chemistry of organosulfur(IV) compounds in recent years. Of particular interest within organosulfur chemistry is the ease with which the synthetic chemist can effect a wide range of transformations through either bond formation or bond cleavage at sulfur. This review aims to cover the developments of the past decade in the chemistry of organic sulfur(IV) molecules and provide insight into both the wide range of reactions which critically rely on this versatile element and the diverse scaffolds that can thereby be synthesized.

Modular Total Syntheses of Hyperforin, Papuaforins A, B, and C via Gold(I)-Catalyzed Carbocyclization

The remarkable biological activities of polyprenylated polycyclic acylphloroglucinols (PPAPs) combined with their highly oxygenated and densely functionalized frameworks have stimulated the interest of synthetic organic chemists over the past decade. Herein, we report the concise total syntheses of four natural products PPAPs, of which some have antibacterial properties, notably hyperforin and papuaforin A. The salient features of this strategy are the short and gram-scalable synthesis of densely substituted PPAPs scaffolds via a Au(I)-catalyzed carbocyclization and the late-stage functionalization for a unified access to a wide variety of PPAPs.

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.

Conversion of cycloalk-2-enones into 2-methylcycloalkane-1,3-diones--assessment of various Tamao-Fleming procedures and mechanistic insight into the use of the Me3SiMe2Si unit

Conjugate addition of Me3SiMe2SiLi to cycloalk-2-en-1-ones, ketalization, Tamao-Fleming oxidation (Bu4NF, then H2O2, KHCO3), TPAP oxidation and acid hydrolysis generates 2-methyl cycloalkane-1,3-diones. Ketalization is needed in order to prevent addition of Me3Si(-) to the carbonyl. The pentamethyldisilanyl group has advantages over other silicon units that are used in Tamao-Fleming procedures.

Total syntheses of hyperforin and papuaforins A-C, and formal synthesis of nemorosone through a gold(I)-catalyzed carbocyclization

The remarkable biological activities of polyprenylated polycyclic acylphloroglucinols (PPAPs) combined with their highly decorated bicyclo[3.3.1]nonane-2,4,9-trione frameworks have inspired synthetic organic chemists over the last decade. The concise total syntheses of four natural products PPAPs; hyperforin and papuaforins A-C, and the formal synthesis of nemorosone are reported. Key to the realization of this strategy is the short and scalable synthesis of densely substituted PPAP scaffolds through a gold(I)-catalyzed 6-endo-dig carbocyclization of cyclic enol ethers for late-stage functionalization.

Gold(I)-catalyzed formation of bridged and fused carbocycles

For many years, despite a rich coordination chemistry, gold (Au) was judged as being catalytically inactive for the formation of carbon–carbon bonds. In mid-1970, few reports demonstrated that Au salts could be very useful reagents to catalyze organic transformations. In recent years, homogeneous catalysis by Au has received considerable attention by the scientific community. It was shown that Au(I) or (III) catalysts are specific and more reactive than most of the other soft Lewis acids such as Hg(II), Cu(II), Pt(II), and Pd(II). Taking advantage of the affinity of cationic phosphine Au complexes to triple bonds, we conceived a Au(I)-catalyzed 6-endo-dig cyclization of cyclic enol ether to prepare bridged and fused bicyclic ketone. Keeping in mind that 5-exo-dig cyclizations can be a competitive process, we surveyed various Au(I) complexes.

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.

Organoiodine(V) reagents in organic synthesis

Organohypervalent iodine reagents have attracted significant recent interest as versatile and environmentally benign oxidants with numerous applications in organic synthesis. This Perspective summarizes synthetic applications of hypervalent iodine(V) reagents: 2-iodoxybenzoic acid (IBX), Dess-Martin periodinane (DMP), pseudocyclic iodylarenes, and their recyclable polymer-supported analogues. Recent advances in the development of new catalytic systems based on the generation of hypervalent iodine species in situ are also overviewed.