Recent Advances in the Total Synthesis of Natural Products Containing Eight-Membered Carbocycles (2009-2019).

Natural products containing eight-membered carbocycles constitute a class of structurally intriguing and biologically important molecules such as the famous diterpenes taxol and vinigrol. Such natural products are being increasingly investigated because of their fascinating architectural features and potent medicinal properties. However, synthesis of natural products with cyclooctane moieties has proved to be highly challenging. This review highlights the recently completed total syntheses of natural products with eight-membered carbocycles with a focus on strategic considerations. A collection of 27 representative studies from the literature covering the decade from 2009 to 2019 is described in chronological order with relevant studies grouped together, including syntheses of the same natural product by different research groups using different strategies. Finally, a summary and outlook including a discussion of the major features of each strategy used in the syntheses are presented. This review illustrates the diversity and creativity in the elegant synthetic designs of eight-membered carbocycles. We hope this review will provide timely illumination and beneficial guidance for future synthetic efforts for organic chemists who are interested in this area.

Synthesis, antiproliferative activity in cancer cells and DNA interaction studies of [Pt(cis-1,3-diaminocycloalkane)Cl2] analogs.

The search for more effective platinum anticancer drugs has led to the design, synthesis, and preclinical testing of hundreds of new platinum complexes. This search resulted in the recognition and subsequent FDA approval of the third-generation Pt(II) anticancer drug, [Pt(1,2-diaminocyclohexane)(oxalate)], oxaliplatin, as an effective agent in treating colorectal and gastrointestinal cancers. Another promising example of the class of anticancer platinum(II) complexes incorporating the Pt(1,n-diaminocycloalkane) moiety is kiteplatin ([Pt(cis-1,4-DACH)Cl2], DACH = diaminocyclohexane). We report here our progress in evaluating the role of the cycloalkyl moiety in these complexes focusing on the synthesis, characterization, evaluation of the antiproliferative activity in tumor cells and studies of the mechanism of action of new [Pt(cis-1,3-diaminocycloalkane)Cl2] complexes wherein the cis-1,3-diaminocycloalkane group contains the cyclobutyl, cyclopentyl, and cyclohexyl moieties. We demonstrate that [Pt(cis-1,3-DACH)Cl2] destroys cancer cells with greater efficacy than the other two investigated 1,3-diamminocycloalkane derivatives, or cisplatin. Moreover, the investigated [Pt(cis-1,3-diaminocycloalkane)Cl2] complexes show selectivity toward tumor cells relative to non-tumorigenic normal cells. We also performed several mechanistic studies in cell-free media focused on understanding some early steps in the mechanism of antitumor activity of bifunctional platinum(II) complexes. Our data indicate that reactivities of the investigated [Pt(cis-1,3-diaminocycloalkane)Cl2] complexes and cisplatin with glutathione and DNA binding do not correlate with antiproliferative activity of these platinum(II) complexes in cancer cells. In contrast, we show that the higher antiproliferative activity in cancer cells of [Pt(cis-1,3-DACH)Cl2] originates from its highest hydrophobicity and most efficient cellular uptake.

Annulation reaction enables the identification of an exocyclic amide tricyclic chemotype as retinoic acid Receptor-Related orphan receptor gamma (RORγ/RORc) inverse agonists.

RORγt is the master regulator of the IL-23/IL-17 axis, a pathway that is clinically validated for the treatment of various immunological disorders. Over the last few years, our group has reported different chemotypes that potently act as inverse agonists of RORγt. One of them, the tricyclic pyrrolidine chemotype, has demonstrated biologic-like preclinical efficacy and has led to our clinical candidate BMS-986251. In this letter, we discuss the invention of an annulation reaction which enabled the synthesis of a tricyclic exocyclic amide chemotype and the identification of compounds with RORγt inverse agonist activity. Preliminary structure activity relationships are disclosed.

A Case for Bond-Network Analysis in the Synthesis of Bridged Polycyclic Complex Molecules: Hetidine and Hetisine Diterpenoid Alkaloids.

A key challenge in the synthesis of diterpenoid alkaloids lies in identifying strategies that rapidly construct their multiply bridged polycyclic skeletons. Existing approaches to these structurally intricate secondary metabolites are discussed in the context of a "bond-network analysis" of molecular frameworks, which was originally devised by Corey some 40 years ago. The retrosynthesis plans that emerge from a topological analysis of the highly bridged frameworks of the diterpenoid alkaloids are discussed in the context of eight recent syntheses of hetidine and hetisine natural products and their derivatives. This Minireview highlights the extent to which network analyses of the type described here sufficed for designing synthesis plans, as well as areas where they had to be amalgamated with functional group oriented synthetic planning considerations.

Nonconjugated Hydrocarbons as Rigid-Linear Motifs: Isosteres for Material Sciences and Bioorganic and Medicinal Chemistry.

Nonconjugated hydrocarbons, like bicyclo[1.1.1]pentane, bicyclo[2.2.2]octane, triptycene, and cubane are a unique class of rigid linkers. Due to their similarity in size and shape they are useful mimics of classic benzene moieties in drugs, so-called bioisosteres. Moreover, they also fulfill an important role in material sciences as linear linkers, in order to arrange various functionalities in a defined spatial manner. In this Review article, recent developments and usages of these special, rectilinear systems are discussed. Furthermore, we focus on covalently linked, nonconjugated linear arrangements and discuss the physical and chemical properties and differences of individual linkers, as well as their application in material and medicinal sciences.

Dibenzocyclooctynes: Effect of Aryl Substitution on Their Reactivity toward Strain-Promoted Alkyne-Azide Cycloaddition.

Five new dibenzocyclooctynes bearing different substituents on their aryl moieties were synthesized and evaluated for their reactivity toward strain-promoted alkyne-azide cycloaddition (SPAAC). The dinaphthylcyclooctynes proved to be poorly reactive with azides, and the formation of triazole required many days compared to a few hours for the other cyclooctynes. Fluoride atoms and methoxy groups were also introduced to the aryl rings, leading to more active compounds. Oxidation of the alcohol on the cyclooctyne ring also increased the reaction rates by 3.5- to 6-fold. 3,9-Difluoro-4,8-dimethoxy-dibenzocyclooctyne-1-one thus displayed a SPAAC kinetic rate of 3.5 M-1 s-1, which is one of the highest rates ever described. Furthermore, the dibenzocyclooctyn-1-one displayed fluorescence properties that have allowed their detection in the protozoan parasites Plasmodium falciparum and Trypanosoma brucei by microscopy imaging, proving that they can cross cell membranes and that they are stable enough in biological media.

A Metallaphotoredox Strategy for the Cross-Electrophile Coupling of α-Chloro Carbonyls with Aryl Halides.

Here, we demonstrate that a metallaphotoredox-catalyzed cross-electrophile coupling mechanism provides a unified method for the α-arylation of diverse activated alkyl chlorides, including α-chloroketones, α-chloroesters, α-chloroamides, α-chlorocarboxylic acids, and benzylic chlorides. This strategy, which is effective for a wide variety of aryl bromide coupling partners, is predicated upon a halogen atom abstraction/nickel radical-capture mechanism that is generically successful across an extensive range of carbonyl substrates. The construction and use of arylacetic acid products have further enabled two-step protocols for the delivery of valuable building blocks for medicinal chemistry, such as aryldifluoromethyl and diarylmethane motifs.

Diastereodivergent Reductive Cross Coupling of Alkynes through Tandem Catalysis: Z- and E-Selective Hydroarylation of Terminal Alkynes.

A diastereodivergent hydroarylation of terminal alkynes is accomplished using tandem catalysis. The hydroarylation allows highly selective synthesis of both E and Z diastereoisomers of aryl alkenes, from the same set of starting materials, using the same combination of palladium and copper catalysts. The selectivity is controlled by simple changes in the stoichiometry of the alcohol additive. The hydroarylation has excellent substrate scope and can be accomplished in the presence of various classes of compounds, including esters, nitriles, alkyl halides, epoxides, carbamates, acetals, ethers, silyl ethers, and thioethers. The Z-selective hydroarylation is accomplished using a new approach based on tandem Sonogashira coupling and catalytic semireduction. The E-selective hydroarylation involves an additional catalytic isomerization of the Z-alkene. Our explorations of the reaction mechanism explain the role of individual reaction components and how the subtle changes in the reaction conditions influence the rates of specific steps of the hydroarylation. Our studies also show that, although the Z- and E-selective hydroarylation reactions are mechanistically closely related, the roles of the palladium and copper catalysts in the two reactions are different.

Recent progress of C-glycosylation methods in the total synthesis of natural products and pharmaceuticals.

Chemical C-glycosylation has been well developed to improve stereoselectivity in recent years. Due to its high efficiency to build C-glycosides or O-cyclic compounds, C-glycosylation has found widespread use in the synthesis of biologically active molecules. This review highlights the C-glycosylation methods that have been practised in the total synthesis of natural products and pharmaceuticals in the past decade.

Antifouling activity of portimine, select semisynthetic analogues, and other microalga-derived spirocyclic imines.

A range of natural products from marine invertebrates, bacteria and fungi have been assessed as leads for nature-inspired antifouling (AF) biocides, but little attention has been paid to microalgal-derived compounds. This study assessed the AF activity of the spirocyclic imine portimine (1), which is produced by the benthic mat-forming dinoflagellate Vulcanodinium rugosum. Portimine displayed potent AF activity in a panel of four macrofouling bioassays (EC50 0.06-62.5 ng ml-1), and this activity was distinct from that of the related compounds gymnodimine-A (2), 13-desmethyl spirolide C (3), and pinnatoxin-F (4). The proposed mechanism of action for portimine is induction of apoptosis, based on the observation that portimine inhibited macrofouling organisms at developmental stages known to involve apoptotic processes. Semisynthetic modification of select portions of the portimine molecule was subsequently undertaken. Observed changes in bioactivity of the resulting semisynthetic analogues of portimine were consistent with portimine's unprecedented 5-membered imine ring structure playing a central role in its AF activity.

Mild electrophilic trifluoromethylthiolation of ketones with trifluoromethanesulfanamide.

A straightforward and convenient approach for trifluoromethylthiolation of various acyclic and cyclic ketones with PhNHSCF3 is described. The reaction proceeds smoothly in the presence of acetyl chloride at room temperature and affords α-trifluoromethylthiolated ketones in fair to good yields.

Development of Novel Cyclizations via Rhodacycle Intermediate and Its Application to Synthetic Organic Chemistry.

Novel Rh(I)-catalyzed cyclizations through a different type of rhodacycle intermediate which is formed by hydroacylation of 4,6-dienal or oxidative addition of diene and alkene are described. Hydroacylation of 4,6-dienal afforded various 7-membered rings in good to high yields, while cycloisomerization of diene and alkene provided 5- or 6-membered rings in good yields. On the basis of these studies, we have also succeeded in developing the sequential reaction of hydroacylation followed by cycloisomerization to produce bicyclic compounds in a stereoselective manner and thus this reaction was expanded to the synthesis of epiglobulol. Furthermore, both Rh(I)-catalyzed hydroacylation and cycloisomerization using ionic liquid (IL) as a solvent were investigated and it was found that the IL recovered after the reaction, which contains the Rh(I) catalyst, could be recycled several times without loss of catalytic activity.

Copper-catalyzed intermolecular trifluoromethylarylation of alkenes: mutual activation of arylboronic acid and CF3+ reagent.

A novel copper-catalyzed intermolecular trifluoromethylarylation of alkenes is developed using less active ether-type Togni's reagent under mild reaction conditions. Various alkenes and diverse arylboronic acids are compatible with these conditions. Preliminary mechanistic studies reveal that a mutual activation process between arylboronic acid and CF3(+) reagent is essential. In addition, the reaction might involve a rate-determining transmetalation, and the final aryl C-C bond is derived from reductive elimination of the aryl(alkyl)Cu(III) intermediate.

Switchable stereocontrolled divergent synthesis induced by aza-Michael addition of deactivated primary amines under acid catalysis.

Switchable tandem intramolecular aza-Michael/Michael and double aza-Michael reactions allow the oriented synthesis of highly functionalised cyclic skeletons. Conjugate addition of deactivated anilines triggers chemo- and stereo-divergent ring-closure reaction pathways with a striking selectivity depending on reaction conditions.

Sulfinate derivatives: dual and versatile partners in organic synthesis.

Sulfinic acids and their salts have recently emerged as versatile coupling partners to efficiently access a wide variety of hetero- and carbocyclic compounds, under relatively mild conditions. Their growing importance is attributable to their dual capacity for acting as nucleophilic or electrophilic reagents. This report summarizes recent advances in the preparation and use of sulfinates in organic synthesis.

Copper-catalyzed intermolecular trifluoromethylazidation of alkenes: convenient access to CF3 -containing alkyl azides.

A novel copper-catalyzed intermolecular trifluoromethylazidation of alkenes has been developed under mild reaction conditions. A variety of CF3 -containing organoazides were directly synthesized from a wide range of olefins, including activated and unactivated alkenes, and the resulting products can be easily transformed into the corresponding CF3 -containing amine derivatives.

Vinylcyclopropane derivatives in transition-metal-catalyzed cycloadditions for the synthesis of carbocyclic compounds.

Vinylcyclopropane (VCP) derivatives participate in a variety of transition-metal-catalyzed multicomponent cycloadditions to produce five- to eight-membered carbocycles. Various cycloaddition modes provide novel approaches to mono-, bi-, and polycyclic molecules. In this Synopsis, recent advances in transition-metal-catalyzed VCP cycloadditions are discussed, with a particular emphasis on the influence of VCP substitution pattern on cycloaddition modes. A tabular summary of applications of the VCP cycloadditions in natural product synthesis is also presented.

Asymmetric synthesis of carbocycles: use of intramolecular conjugate displacement.

Intramolecular conjugate displacement (ICD), the process illustrated in , has been applied to the Morita-Baylis-Hillman adducts formed from (5S)-5-(l-menthyloxy)-2(5H)-furanone and aldehydes that are substituted in the γ- or δ-position by geminal phenylthio groups. When the initial Morita-Baylis-Hillman alcohols are acetylated and oxidized to geminal sulfones, deprotonation causes ring closure by ICD (2.5→2.6). Hydrogenation, DIBAL-H reduction and desulfonylation releases an optically pure carbocycle.

Diastereoselective [2+2] photocycloaddition of chiral cyclic enones with olefins in aqueous media using surfactants.

We conducted diastereodifferentiating [2+2] photocycloadditions of cyclo-hexenones modified with a chiral 8-(p-methoxy phenyl)menthyl auxiliary with olefins in water. Although the photoreaction didn't proceed at all in pure water owing to very low solubility, the use of surfactants [sodium dodecyl sulfate (SDS) or dodecylamine hydrochloride (DAH)] and additive (organic solvent) enabled the reactions to progress with moderate to high conversions and yields. Furthermore, we synthesized a new menthol derivative substrate containing a (p-octyloxy)phenyl group for enhancing hydrophobicity, and elucidated that this new substrate was found to be a suitable chiral auxiliary in this asymmetric photoreaction in aqueous system. The additive effect of organic molecules on the yield and diastereoselectivity of the photo-adducts is also discussed.

A palladium/chiral amine co-catalyzed enantioselective dynamic cascade reaction: synthesis of polysubstituted carbocycles with a quaternary carbon stereocenter.

Polysubstituted 5- and 6-membered carbocycles were synthesized by the title reaction. The one-pot dynamic relay process generates four new stereocenters, including a quaternary carbon center, in a highly enantioselective fashion (99.5:0.5→99:0.5 e.r.) by using a simple combination of palladium and chiral amine co-catalysts.