Strategies for the Construction of Benzobicyclo[3.2.1]octane in Natural Product Synthesis

Benzobicyclo[3.2.1]octane is a cage-like unique motif containing a bicyclo[3.2.1]octane structure fused with at least one benzene ring. It is found in various natural products that exhibit structural complexities and important biological activities. The total synthesis of natural products possessing this challenging structure has received considerable attention, and great advances have been made in this field during the past 15 years. This review summarizes thus far achieved chemical syntheses and synthetic studies of natural compounds featuring the benzobicyclo[3.2.1]octane core. It focuses on strategic approaches constructing the bridged structure, aiming to provide a useful reference for inspiring further advancements in strategies and total syntheses of natural products with such a framework.

Synthesis and evaluation of butylphthalide-scutellarein hybrids as multifunctional agents for the treatment of Alzheimer's disease

A series of butylphthalide and scutellarein hybrids 3-(alkyl/alkenyl) hydroxyphthalide derivatives were designed, synthesized and evaluated as multifunctional agents against Alzheimer's disease. In vitro bioactivity assays indicated that most of the compounds displayed excellent antioxidant activity and moderate to good inhibition activities of self-induced Aβ1-42 aggregation. Among them, compound 7c was demonstrated as a potential and balanced multifunctional candidate displaying the best inhibitory effects on self- and Cu2+-induced Aβ1-42 aggregation (90.2 % and 35.4 %, respectively) and moderate activity for disaggregation of Aβ1-42 aggregation (42.5 %). In addition, 7c also displayed excellent antioxidant (2.42 Trolox equivalents), metal ions chelating, oxidative stress alleviation, neuroprotective and anti-neuroinflammatory activities. Furthermore, in vivo study demonstrated that 7c could ameliorate the learning and memory impairment induced by sodium nitrite and Aβ1-42 in the step-down passive avoidance test. These balanced multifunctional profiles supporting compound 7c as a novel potential candidate for the treatment of AD.

New Insights on Biological Activities, Chemical Compositions, and Classifications of Marine Actinomycetes Antifouling Agents

Biofouling is the assemblage of undesirable biological materials and macro-organisms (barnacles, mussels, etc.) on submerged surfaces, which has unfavorable impacts on the economy and maritime environments. Recently, research efforts have focused on isolating natural, eco-friendly antifouling agents to counteract the toxicities of synthetic antifouling agents. Marine actinomycetes produce a multitude of active metabolites, some of which acquire antifouling properties. These antifouling compounds have chemical structures that fall under the terpenoids, polyketides, furanones, and alkaloids chemical groups. These compounds demonstrate eminent antimicrobial vigor associated with antiquorum sensing and antibiofilm potentialities against both Gram-positive and -negative bacteria. They have also constrained larval settlements and the acetylcholinesterase enzyme, suggesting a strong anti-macrofouling activity. Despite their promising in vitro and in vivo biological activities, scaled-up production of natural antifouling agents retrieved from marine actinomycetes remains inapplicable and challenging. This might be attributed to their relatively low yield, the unreliability of in vitro tests, and the need for optimization before scaled-up manufacturing. This review will focus on some of the most recent marine actinomycete-derived antifouling agents, featuring their biological activities and chemical varieties after providing a quick overview of the disadvantages of fouling and commercially available synthetic antifouling agents. It will also offer different prospects of optimizations and analysis to scale up their industrial manufacturing for potential usage as antifouling coatings and antimicrobial and therapeutic agents.

Naturally Occurring Organohalogen Compounds-A Comprehensive Review

The present volume is the third in a trilogy that documents naturally occurring organohalogen compounds, bringing the total number-from fewer than 25 in 1968-to approximately 8000 compounds to date. Nearly all of these natural products contain chlorine or bromine, with a few containing iodine and, fewer still, fluorine. Produced by ubiquitous marine (algae, sponges, corals, bryozoa, nudibranchs, fungi, bacteria) and terrestrial organisms (plants, fungi, bacteria, insects, higher animals) and universal abiotic processes (volcanos, forest fires, geothermal events), organohalogens pervade the global ecosystem. Newly identified extraterrestrial sources are also documented. In addition to chemical structures, biological activity, biohalogenation, biodegradation, natural function, and future outlook are presented.

Cephaloliverols A and B, two sterol-hybrid meroterpenoids from Cephalotaxus oliveri

Cephaloliverols A (1) and B (2), two meroterpenoids based on a sterol and an abietane diterpene possessing a dioxane ring, were isolated from the twigs and leaves of Cephalotaxus oliveri. Their structures were established by spectroscopic analysis and quantum chemical calculation. 1 and 2 represent the first sterol-hybrid meroditerpenoids. The two compounds and their precursors decreased NO production in a dose-dependent manner in LPS-stimulated RAW 264.7 macrophages.

Tandem 1,6-addition/cyclopropanation/rearrangement reaction of vinylogous para-quinone methides with 3-chlorooxindoles: construction of vicinal quaternary carbon centers

A novel tandem 1,6-addition/cyclopropanation/rearrangement reaction of vinylogous para-quinone methides with 3-chlorooxindoles has been developed, providing dispirooxindole–cyclopentane–cyclohexadienones with vicinal quaternary carbon centers.

Antibacterial Meroterpenoids, Merochlorins G-J from the Marine Bacterium Streptomyces sp

Four new chlorinated meroterpenoids, merochlorins G-J (1-4), and 10, a dihydronaphthalenedione precursor, along with known merochlorins A (5) and C-F (6-9), were obtained from cultivation of the bacterium strain Streptomyces sp. CNH-189, which was isolated from marine sediment. The planar structures of compounds 1-4 and 10 were elucidated by interpretation of MS, UV, and NMR spectroscopic data. The relative configurations of compounds 1-4 were determined via analysis of nuclear Overhauser effect (NOE) spectroscopic data, after which their absolute configurations were established by comparing the experimental electronic circular dichroism (ECD) spectra of compounds 1-4 to those of previously reported possible enantiomer models and DP4 calculations. Compound 3 displayed strong antibacterial activities against Bacillus subtilis, Kocuria rhizophila, and Staphylococcus aureus, with MIC values of 1, 2, and 2 μg/mL, respectively, whereas compound 1 exhibited weak antibacterial effects on these three strains, with a 16-32 μg/mL MIC value range.

Biomimetic Synthesis of Natural Products: A Journey To Learn, To Mimic, and To Be Better

Total synthesis of natural products has been one of the most exciting and dynamic areas in synthetic organic chemistry. Nowadays, the major challenge in this field is not whether a given target of interest can be synthesized but how to make it with commendable efficiency and practicality. To meet this grand challenge, a wise way is to learn from Mother Nature who is recognized for her superb capability of forging complicated and sometimes beyond-imagination molecules in her own delicate way. Indeed, since Sir Robert Robinson published his groundbreaking synthesis of tropinone in 1917, biomimetic synthesis of natural products, a process of imitating nature's way to make molecules, has evolved into one of the most popular research directions in organic synthesis.Our group has been engaging in biomimetic synthesis of natural products in the past decade. During this time, we have come to realize that the successful implementation of a biomimetic synthesis entails the orchestrated combination of bioinspiration and rational design. On the one hand, we prefer to utilize some elegant bioinspired transformations (e.g., Diels-Alder dimerization, 6π-electrocyclization, and [2 + 2]-photocycloaddition) as the key steps of our synthesis, which enable rapid construction of the core skeletons of the chased targets with high efficiency; on the other hand, various powerful reactions (e.g., dyotropic rearrangement of β-lactone, tandem aldol condensation/Grob fragmentation reaction, and organocatalytic asymmetric Mukaiyama-Michael addition) are rationally designed by us, which allow for facile access to the requisite precursors for attempting biomimetic transformations. In some cases, the proposed biomimetic transformation may fail to give a satisfactory result in practice, and thus we opt to develop creative tactics (e.g., hydrogen atom transfer-triggered vinyl cyclobutane ring opening/oxygen insertion/cyclization cascade) that can meet the challenge. Guided by this synthesis concept, we have achieved the total syntheses of multiple families of natural products of great importance in both chemistry and biology, representatives of which include xanthanolides, cytochalasans, and plakortin-type polyketides. Of note, most of these targets could be accessed in a concise, efficient, and scalable manner, which paves the way for further exploration of their biological functions and medicinal potential. Moreover, owing to their biomimetic nature, our syntheses provide valuable information for deciphering the underlying biosynthetic pathways of the chased targets, which could not be attained by other synthetic modes.

Biomimetic Dearomatization Strategies in the Total Synthesis of Meroterpenoid Natural Products

Natural products are biosynthesized from a limited pool of starting materials via pathways that obey the same chemical logic as textbook organic reactions. Given the structure of a natural product, it is therefore often possible to predict its likely biosynthesis. Although biosynthesis mainly occurs in the highly specific chemical environments of enzymes, the field of biomimetic total synthesis attempts to replicate predisposed pathways using chemical reagents.We have followed several guidelines in our biomimetic approach to total synthesis. The overarching aim is to construct the same skeletal C-C and C-heteroatom bonds and in the same order as our biosynthetic hypothesis. In order to explore the innate reactivity of (bio)synthetic intermediates, the use of protecting groups is avoided or at least minimized. The key step, which is usually a cascade reaction, should be predisposed to selectively generate molecular complexity under substrate control (e.g., cycloadditions, radical cyclizations, carbocation rearrangements). In general, simple reagents and mild conditions are used; many of the total syntheses presented in this Account could be achieved using pre-1980s methodology. We have focused almost exclusively on the synthesis of meroterpenoids, that is, natural products of mixed terpene and aromatic polyketide origin, using commercially available terpenes and electron-rich aromatic compounds as starting materials. Finally, all of the syntheses in this Account involve a dearomatization step as a means to trigger a cascade reaction or to construct stereochemical complexity from a planar, aromatic intermediate.A biomimetic strategy can offer several advantages to a total synthesis project. Most obviously, successful biomimetic syntheses are usually concise and efficient, naturally adhering to the atom, step, and redox economies of synthesis. For example, in this Account, we describe a four-step synthesis of garcibracteatone and a three-step synthesis of nyingchinoid A. It is difficult to imagine shorter, non-biomimetic syntheses of these intricate molecules. Furthermore, biomimetic synthesis gives insight into biosynthesis by revealing the chemical relationships between biosynthetic intermediates. Access to these natural substrates allows collaboration with biochemists to help uncover the function of newly discovered enzymes and elucidate biosynthetic pathways, as demonstrated in our work on the napyradiomycin family. Third, by making biosynthetic connections between natural products, we can sometimes highlight incorrect structural assignments, and herein we discuss structure revisions of siphonodictyal B, rasumatranin D, and furoerioaustralasine. Last, biomimetic synthesis motivates the prediction of "undiscovered natural products" (i.e., missing links in biosynthesis), which inspired the isolation of prenylbruceol A and isobruceol.

Copper-Catalyzed Asymmetric Synthesis of Bicyclo[3.n.1]alkenones

A series of highly strained bicyclo[3.n.1]alkenones have been successfully constructed in good-to-excellent enantioselectivities and moderate-to-good yields via copper-catalyzed formal [3+3] cycloaddition. The versatile chiral cycloadducts could be selectively converted into various valuable bridge systems, which hold considerable potential for the construction of natural and bioactive compounds containing a [3.n.1] moiety.

The Total Synthesis of Diquinane-Containing Natural Products

Diquinane or bicyclo[3.3.0]octane is a conspicuous structural unit existing in the carbo-frameworks of a wide range of natural products such as alkaloids and terpenoids. These diquinane-containing molecules not merely exhibit intriguing architectures, but also showcase a broad spectrum of significant bioactivities, which draw widespread attention from the global synthetic community. During the past decade, with an aim to accomplish the total syntheses of such specified cornucopias of natural products, a variety of elegant strategies for construction of the diquinane ring system have been disclosed. In this Minireview, the achievements on this subject in the timeline from 2010 to June 2020 are demonstrated and it is discussed how the diquinane unit is strategically forged in the context of the specific target structure. In addition, impacts of the selected works to the field of natural product total synthesis is highlighted and the particular outlook of diquinane-containing natural product synthesis is provided.

Meroterpenoid natural products from Streptomyces bacteria - the evolution of chemoenzymatic syntheses

Covering: Up to January 2020Meroterpenoids derived from the polyketide 1,3,6,8-tetrahydroxynaphthalene (THN) are complex natural products produced exclusively by Streptomyces bacteria. These antibacterial compounds include the napyradiomycins, merochlorins, marinones, and furaquinocins and have inspired many attempts at their chemical synthesis. In this review, we highlight the role played by biosynthetic studies in the stimulation of biomimetic and, ultimately, chemoenzymatic total syntheses of these natural products. In particular, the application of genome mining techniques to marine Streptomyces bacteria led to the discovery of unique prenyltransferase and vanadium-dependent haloperoxidase enzymes that can be used as highly selective biocatalysts in fully enzymatic total syntheses, thus overcoming the limitations of purely chemical reagents.

Rapid Generation of Molecular Complexity by Chemical Synthesis: Highly Efficient Total Synthesis of Hexacyclic Alkaloid (-)-Chaetominine and Its Biosynthetic Implications

The efficiency becomes a key issue in today's natural product total synthesis. While biomimetic synthesis is one of the most elegant strategies to achieve synthetic efficiency and thus to approach the ideal synthesis, most biogenetic pathways are unknown or unconfirmed. In this account, we demonstrate, through the shortest and also the most efficient asymmetric total syntheses of the hexacyclic alkaloid (-)-chaetominine to date, that on the basis of biogenetic thinking, one can develop quite efficient bio-inspired total synthesis, which in turn serves to suggest and chemically validate plausible biosynthetic routes for the natural product. The synthetic strategy thus developed is also inspiring for the development of other synthetic methods and efficient total synthesis of other natural products.

Total Synthesis Establishes the Biosynthetic Pathway to the Naphterpin and Marinone Natural Products

The naphterpins and marinones are naphthoquinone meroterpenoids with an unusual aromatic oxidation pattern that is biosynthesized from 1,3,6,8-tetrahydroxynaphthalene (THN). We propose that cryptic halogenation of THN derivatives by vanadium-dependent chloroperoxidase (VCPO) enzymes is key to this biosynthetic pathway, despite the absence of chlorine in these natural products. This speculation inspired a total synthesis to mimic the naphterpin/marinone biosynthetic pathway. In validation of this biogenetic hypothesis, two VCPOs were discovered that interconvert several of the proposed biosynthetic intermediates.

A catalytic allylic cation-induced intermolecular allylation-semipinacol rearrangement

A catalytic intermolecular semipinacol rearrangement induced by allylic carbocations has been realized. This tandem reaction is highly efficient under the catalysis of ZnBr₂, generating a wide range of α-homoallyl substituted ketones which contain all-carbon quaternary centres in good to excellent yields (up to 98%) with moderate to high diastereoselectivities (up to >20 : 1). Synthetic application of this novel methodology in the construction of core structures of natural products is also reported.