Protecting group-free syntheses of natural products and biologically active compounds

Mechanism of Z-Selective Allylic Functionalization via Thianthrenium Salts

A detailed mechanistic study of the Z-selective allylic functionalization via thianthrenium salts is presented. Kinetic analyses, deuterium labeling experiments, and computational methods are used to rationalize the observed reactivity and selectivity. We find that the reaction proceeds via a rate-determining and stereodetermining allylic deprotonation of an alkenylthianthrenium species. The Z-configuration of the resultant allylic ylide is translated into the Z-allylic amine product through a sequence of subsequent fast and irreversible steps: protonation to form a Z-allylic thianthrenium electrophile and then regioselective substitution by the nucleophile. In the stereodetermining deprotonation step, computational studies identified a series of stabilizing nonbonding interactions in the Z-alkene-forming transition state that contribute to the stereoselectivity.

Design, Synthesis and Biological Activity of Novel Methoxy- and Hydroxy-Substituted N-Benzimidazole-Derived Carboxamides

This work presents the design, synthesis and biological activity of novel N-substituted benzimidazole carboxamides bearing either a variable number of methoxy and/or hydroxy groups. The targeted carboxamides were designed to investigate the influence of the number of methoxy and/or hydroxy groups, the type of substituent placed on the N atom of the benzimidazole core and the type of substituent placed on the benzimidazole core on biological activity. The most promising derivatives with pronounced antiproliferative activity proved to be N-methyl-substituted derivatives with hydroxyl and methoxy groups at the phenyl ring and cyano groups on the benzimidazole nuclei with selective activity against the MCF-7 cell line (IC50 = 3.1 μM). In addition, the cyano-substituted derivatives 10 and 11 showed strong antiproliferative activity against the tested cells (IC50 = 1.2-5.3 μM). Several tested compounds showed significantly improved antioxidative activity in all three methods compared to standard BHT. In addition, the antioxidative activity of 9, 10, 32 and 36 in the cells generally confirmed their antioxidant ability demonstrated in vitro. However, their antiproliferative activity was not related to their ability to inhibit oxidative stress nor to their ability to induce it. Compound 8 with two hydroxy and one methoxy group on the phenyl ring showed the strongest antibacterial activity against the Gram-positive strain E. faecalis (MIC = 8 μM).

An osmium(II) methane complex: Elucidation of the methane coordination mode

The activation of inert C─H bonds by transition metals is of considerable industrial and academic interest, but important gaps remain in our understanding of this reaction. We report the first experimental determination of the structure of the simplest hydrocarbon, methane, when bound as a ligand to a homogenous transition metal species. We find that methane binds to the metal center in this system through a single M···H-C bridge; changes in the ¹J_CH coupling constants indicate clearly that the structure of the methane ligand is significantly perturbed relative to the free molecule. These results are relevant to the development of better C─H functionalization catalysts.

Total Syntheses of Calyciphylline A-Type Alkaloids (-)-10-Deoxydaphnipaxianine A, (+)-Daphlongamine E and (+)-Calyciphylline R via Late-Stage Divinyl Carbinol Rearrangements

Among the famous Daphniphyllum alkaloids family, the calyciphylline A-type subfamily has triggered particular interest from the organic synthesis community in recent years. Here, we report divergent total syntheses of three calyciphylline A-type alkaloids, namely, (-)-10-deoxydaphnipaxianine A, (+)-daphlongamine E, and (+)-calyciphylline R. Our work highlights an efficient, divergent strategy via late-stage divinyl carbinol rearrangements, including an unprecedented oxidative Nazarov electrocyclization using an unfunctionalized tertiary divinyl carbinol and an unusual allylic alcohol rearrangement. A highly efficient "donor-acceptor" platinum catalyst was used for a critical nitrile hydration step. Moreover, the power of selective amide reductions has also been showcased by novel and classic tactics.

Concise Stereoselective Synthesis of β-Hydroxy-γ-lactones: (4R,5R)-4-Hydroxy-γ-decalactone from the Japanese Orange Fly and Enantiomers of Arachnid Harvestmen Isolates

The naturally occurring (4R,5R)-4-hydroxy-γ-decalactone from the Japanese orange fly and the antipode of (4S,5R)-4-hydroxy-γ-dodecalactone from the harvestmen arachnid and their stereoisomers are synthesized from the chiral pool material d-glucono-δ-lactone in a few steps. The one-pot conversion of the latter to γ-vinyl-β-hydroxy-γ-lactone, cross-metathesis with requisite olefin, and hydrogenation enabled the synthesis of syn-lactones in just a two-pot operation. An additional efficient Pd-catalyzed allylic isomerization of γ-vinyl-β-hydroxy-γ-lactone led to the anti-lactones in high yields.

Advances in catalytic and protecting-group-free total synthesis of natural products: a recent update

Catalytic processes in protecting-group-free syntheses of natural products are fast emerging towards achieving the goal of efficiency and economy in total synthesis. Present day sustainable development in synthesis of natural products does not permit the luxury of using stoichiometric reagents and protecting groups. Catalysis and step-economy can contribute significantly toward economy and efficiency of synthesis. This feature article details the ingenious efforts by many researchers in the last couple of years toward concise total syntheses, based on catalytic steps and protecting-group-free-strategies. These would again serve as guidelines in future development of reagents and catalysts aimed at achieving higher efficiency and chemoselectivity to the point that catalysis and protecting-group-free synthesis will be an accepted common practice.

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.

Total synthesis of sesterterpenoids

Total syntheses of biologically and structurally fascinating sesterterpenoids published between Jan. 2012 and Jan. 2018 are summarized and discussed here.

Mechanistic Insight into Asymmetric Hetero-Michael Addition of α,β-Unsaturated Carboxylic Acids Catalyzed by Multifunctional Thioureas

Carboxylic acids and their corresponding carboxylate anions are generally utilized as Brønsted acids/bases and oxygen nucleophiles in organic synthesis. However, a few asymmetric reactions have used carboxylic acids as electrophiles. Although chiral thioureas bearing both arylboronic acid and tertiary amine were found to promote the aza-Michael addition of BnONH₂ to α,β-unsaturated carboxylic acids with moderate to good enantioselectivities, the reaction mechanism remains to be clarified. Detailed investigation of the reaction using spectroscopic analysis and kinetic studies identified tetrahedral borate complexes, comprising two carboxylate anions, as reaction intermediates. We realized a dramatic improvement in product enantioselectivity with the addition of 1 equiv of benzoic acid. In this aza-Michael reaction, the boronic acid not only activates the carboxylate ligand as a Lewis acid, together with the thiourea NH-protons, but also functions as a Brønsted base through a benzoyloxy anion to activate the nucleophile. Moreover, molecular sieves were found to play an important role in generating the ternary borate complexes, which were crucial for obtaining high enantioselectivity as demonstrated by DFT calculations. We also designed a new thiourea catalyst for the intramolecular oxa-Michael addition to suppress another catalytic pathway via a binary borate complex using steric hindrance between the catalyst and substrate. Finally, to demonstrate the synthetic versatility of both hetero-Michael additions, we used them to accomplish the asymmetric synthesis of key intermediates in pharmaceutically important molecules, including sitagliptin and α-tocopherol.

Synthesis and Study of the Antimalarial Cardamom Peroxide

A full account of our previously disclosed synthesis of the monoterpene dimer cardamom peroxide is reported. Inspired by hypotheses regarding the potential biosynthetic origins of this natural product, several unproductive routes are also reported. The chemical reactivity of this structurally unique metabolite in the presence of iron(II) sources is also reported as is its antimalarial activity against Plasmodium falciparum clinical isolates from several Cambodian provinces.

Catalytic Enantioselective Carbonyl Allylation and Propargylation via Alcohol-Mediated Hydrogen Transfer: Merging the Chemistry of Grignard and Sabatier

Merging the characteristics of transfer hydrogenation and carbonyl addition, we have developed a new class of catalytic enantioselective C-C bond formations. In these processes, hydrogen transfer between alcohols and π-unsaturated reactants generates carbonyl-organometal pairs that combine to deliver products of addition. On the basis of this mechanistic paradigm, lower alcohols are converted directly to higher alcohols in the absence of premetalated reagents or discrete alcohol-to-carbonyl redox reactions. In certain cases, due to a pronounced kinetic preference for primary versus secondary alcohol dehydrogenation, diols and higher polyols are found to engage in catalytic stereo- and site-selective C-C bond formation-a capability that further enhances efficiency by enabling skeletal construction events without extraneous manipulations devoted to the installation and removal of protecting groups. While this Account focuses on redox-neutral couplings of alcohols, corresponding aldehyde reductive couplings mediated by 2-propanol were developed in parallel for most of the catalytic transformations reported herein. Mechanistically, two distinct classes of alcohol C-H functionalizations have emerged, which are distinguished by the mode of pronucleophile activation, specifically, processes wherein alcohol oxidation is balanced by (a) π-bond hydrometalation or (b) C-X bond reductive cleavage. Each pathway offers access to allylmetal or allenylmetal intermediates and, therefrom, enantiomerically enriched homoallylic or homopropargylic alcohol products, respectively. In the broadest terms, carbonyl addition mediated by premetalated reagents has played a central role in synthetic organic chemistry for well over a century, but the requisite organometallic reagents pose issues of safety, require multistep syntheses, and generate stoichiometric quantities of metallic byproducts. The concepts and catalytic processes described in this Account, conceived and developed wholly within the author's laboratory, signal a departure from the use of stoichiometric organometallic reagents in carbonyl addition. Rather, they reimagine carbonyl addition as a hydrogen autotransfer process or cross-coupling in which alcohol reactants, by virtue of their native reducing ability, drive the generation of transient organometallic nucleophiles and, in doing so, serve dually as carbonyl proelectrophiles. The catalytic allylative and propargylative transformations developed to date display capabilities far beyond their classical counterparts, and their application to the total synthesis of type-I polyketide natural products have evoked a step-change in efficiency. More importantly, the present data suggest that diverse transformations traditionally reliant on premetalated reagents may now be conducted catalytically without stoichiometric metals. This Account provides the reader and potential practitioner with a catalog of enantioselective alcohol-mediated carbonyl additions-a user's guide, 10-year retrospective, and foundation for future work in this emerging area of catalytic C-C bond formation.

Total Synthesis of Swinholide A: An Exposition in Hydrogen-Mediated C-C Bond Formation

Diverse hydrogen-mediated C-C couplings enable construction of the actin-binding marine polyketide swinholide A in only 15 steps (longest linear sequence), roughly half the steps required in two prior total syntheses. The redox-economy, chemo- and stereoselectivity embodied by this new class of C-C couplings are shown to evoke a step-change in efficiency.

Modular Terpenoid Construction via Catalytic Enantioselective Formation of All-Carbon Quaternary Centers: Total Synthesis of Oridamycin A, Triptoquinones B and C, and Isoiresin

Total syntheses of oridamycin A, triptoquinones B and C, and isoiresin are accomplished from a common intermediate prepared via iridium-catalyzed alcohol C-H tert-(hydroxy)prenylation - a byproduct-free process that forms an all-carbon quaternary stereocenter with excellent control of diastereo- and enantioselectivity.

Total Synthesis of (+)-SCH 351448: Efficiency via Chemoselectivity and Redox-Economy Powered by Metal Catalysis

The polyketide natural product (+)-SCH 351448, a macrodiolide ionophore bearing 14 stereogenic centers, is prepared in 14 steps (LLS). In eight prior syntheses, 22-32 steps were required. Multiple chemoselective and redox-economic functional group interconversions collectively contribute to a step-change in efficiency.

Enantioselective Alcohol C-H Functionalization for Polyketide Construction: Unlocking Redox-Economy and Site-Selectivity for Ideal Chemical Synthesis

The development and application of stereoselective and site-selective catalytic methods that directly convert lower alcohols to higher alcohols are described. These processes merge the characteristics of transfer hydrogenation and carbonyl addition, exploiting alcohols and π-unsaturated reactants as redox pairs, which upon hydrogen transfer generate transient carbonyl-organometal pairs en route to products of C-C coupling. Unlike classical carbonyl additions, stoichiometric organometallic reagents and discrete alcohol-to-carbonyl redox reactions are not required. Additionally, due to a kinetic preference for primary alcohol dehydrogenation, the site-selective modification of glycols and higher polyols is possible, streamlining or eliminating use of protecting groups. The total syntheses of several iconic type I polyketide natural products were undertaken using these methods. In each case, the target compounds were prepared in significantly fewer steps than previously achieved.

Scalable Total Syntheses of (-)-Hapalindole U and (+)-Ambiguine H

The Stigonemataceae family of cyanobacteria produces a class of biogenetically related indole natural products that include hapalindoles and ambiguines. In this full account, a practical route to the tetracyclic hapalindole family is presented by way of an eight-step, enantiospecific, protecting-group-free total synthesis of (-)-hapalindole U that features an oxidative indole-enolate coupling. With gram-scale access to hapalindole U, the first total synthesis of an ambiguine alkaloid, (+)-ambiguine H, was completed via an isonitrile-assisted prenylation of an indole followed by a photofragmentation cascade.

Short, enantioselective total synthesis of chatancin

An enantioselective total synthesis of the polycyclic diterpene (+)-chatancin, a potent PAF antagonist, is reported. Proceeding in seven steps from dihydrofarnesal, this synthetic route was designed to circumvent macrocyclization-based strategies to complex, cyclized cembranoids. The described synthesis requires only six chromatographic purifications, is high yielding, and avoids protecting-group manipulations. An X-ray crystal structure of this fragile marine natural product was obtained.