The Total Synthesis of Chalcitrin

Gold-catalyzed cyclization and cycloaddition in natural product synthesis

Covering: 2016 to mid 2023Transition metal catalysis, known for its remarkable capacity to expedite the assembly of molecular complexity from readily available starting materials in a single operation, occupies a central position in contemporary chemical synthesis. Within this landscape, gold-catalyzed reactions present a novel and versatile paradigm, offering robust frameworks for accessing diverse structural motifs. In this review, we highlighted a curated selection of publications in the past 8 years, focusing on the deployment of homogeneous gold catalysis in the ring-forming step for the total synthesis of natural products. These investigations are categorized based on the specific ring formations they engender, accentuating the prevailing gold-catalyzed methodologies applied to surmount intricate challenges in natural products synthesis.

Total Synthesis of Penicibilaenes Enabled by a Tandem Double Conia-ene Type Reaction

The total syntheses of penicibilaenes A and B are described. The key step is the tBuOK/DMSO-mediated tandem 5-exo-dig Conia-ene type reaction and 6-exo-dig Conia-ene type reaction to install the tricyclic [6.3.1.01,5] dodecane core of penicibilaenes from dibutynyl cyclohexanone in a single step, together with a sequence of copper-mediated conjugate addition and Crabtree's hydrogenation to forge the stereogenic centers at C5 and C2, respectively.

Catalytic Asymmetric Polycyclization of Tertiary Enamides with Silyl Enol Ethers: Total Synthesis of (-)-Cephalocyclidin A

A catalytic enantioselective polycyclization of tertiary enamides with terminal silyl enol ethers has been developed by virtue of Cu(OTf)2 catalysis with a novel spiropyrroline-derived oxazole (SPDO) ligand. This tandem reaction offers an effective approach to assemble bicyclic and tricyclic N-heterocycles bearing both aza- and oxa-quaternary stereogenic centers, which are primal subunits in a range of natural alkaloids. Strategic application of this methodology and a late-stage radical cyclization as key steps have been showcased in the concise total synthesis of (-)-cephalocyclidin A.

Total Synthesis of Mollanol A

Mollanol A is the first isolated member of the mollane-type grayanoids which possesses an unprecedented C-nor-D-homograyanane carbon skeleton and an 5,8-epoxide. Due to its transcriptional activation effects on the Xbp1 upstream promoters in different cell types, it has a potential therapeutic effect on inflammatory bowel disease. Here we report the first total synthesis of mollanol A, which constitutes a 15-step synthesis from commercially available materials via a convergent strategy. The synthesis involves an InCl₃-catalyzed Conia-ene cyclization reaction to construct the bicyclo[3.2.1]octane moiety and a vinylogous aldol reaction/intramolecular oxa-Michael addition sequence to rapidly assemble the oxa-bicyclo[3.2.1] core.

An efficient approach to angular tricyclic molecular architecture via Nazarov-like cyclization and double ring-expansion cascade

A modular and efficient method for constructing angular tri-carbocyclic architectures containing quaternary carbon center(s) from 1,3-dicycloalkylidenyl ketones is established, which involves an unconventional synergistic cascade of a Nazarov cyclization and two ring expansions. It features high selectivity, mild conditions and convenient operation, wide scope and easy availability of substrate. Substitution with R¹ and R² at the 4πe-system with electron-donating group favors this reaction, while that with electron-withdrawing group or proton disfavors. The electron-donating group as R¹ directs the initial ring expansion at its own site, while the p-π- or n-π- associated substituent as R² favors selectively the later ring expansion near its location because of the beneficial maintenance of an original conjugated system. The stereoselectivity has proved to be governed by either the steric effect of R³ and R⁴ at the expanded rings, or the migration ability of the migrating atom. Density Functional Theory calculation suggests the initial Nazarov cyclization would be the rate-determining step. A racemic total synthesis of the natural (±)-waihoensene is realized in 18 steps by use of this methodology.

Sequential iodine-mediated diallylsilane rearrangement/asymmetric dihydroxylation: Synthesis and reactions of enantioenriched oxasilacycles

Products from an iodine-mediated diallylsilane rearrangement were taken into an asymmetric dihydroxylation (AD) reaction resulting in the formation of diastereomeric 6-membered oxasilacycles. Removal of the epimeric stereocenter among this mixture of diastereomers by elimination of iodine produced a single enantioenriched cyclic allyl silyl ether. The resulting allyl silane was then successfully engaged in several further transformations, providing an alternative means to prepare useful intermediates for enantioselective synthesis.

Total Synthesis of Kadcoccinic Acid A Trimethyl Ester

The first total synthesis of the trimethyl ester of kadcoccinic acid A is described. The central structural element of our synthesis is a cyclopentenone motif that allows the assembly of the natural product skeleton. A gold(I)-catalyzed cyclization of an enynyl acetate led to efficient construction of the cyclopentenone scaffold. In this step, optimization studies revealed that the stereochemistry of the enynyl acetate dictates regioisomeric cyclopentenone formation. The synthesis further highlights an efficient copper-mediated conjugate addition, merged with a gold(I)-catalyzed Conia-ene reaction to connect the two fragments, thereby forging the D-ring of the natural product. The synthetic strategy reported herein can provide a general platform to access the skeleton of other members of this family of natural products.

Concise and Stereoselective Total Syntheses of Annotinolides C, D, and E

The annotinolides are one of the most recent additions to the Lycopodium family of alkaloids, with its members possessing challenging, caged structures that include a [3.2.1]-bicyclic core bearing six contiguous stereocenters, including four that are fully substituted. Herein, we document a concise and stereoselective route that achieves the first total syntheses of three of its members: annotinolides C, D, and E. Key operations include a gold(I)-catalyzed Conia-ene reaction that fashions much of the main core in a single operation, as well as a number of other challenging and chemoselective transformations to generate the remaining elements. Moreover, efforts utilizing the natural products themselves, seeking adjustments in their oxidation states and the rearrangement of individual ring systems, shed light on their potential biogenesis with some outcomes counter to those originally proposed. Finally, formal enantioenriched syntheses of the target molecules are also presented.

Chemoselective Cleavage of Si-C(sp3) Bonds in Unactivated Tetraalkylsilanes Using Iodine Tris(trifluoroacetate)

Organosilanes are synthetically useful reagents and precursors in organic chemistry. However, the typical inertness of unactivated Si-C(sp³) bonds under conventional reaction conditions has hampered the application of simple tetraalkylsilanes in organic synthesis. Herein we report the chemoselective cleavage of Si-C(sp³) bonds of unactivated tetraalkylsilanes using iodine tris(trifluoroacetate). The reaction proceeds smoothly under mild conditions (-50 °C to room temperature) and tolerates various polar functional groups, thus enabling subsequent Tamao-Fleming oxidation to provide the corresponding alcohols. NMR experiments and density functional theory calculations on the reaction indicate that the transfer of alkyl groups from Si to the I(III) center and the formation of the Si-O bond proceed concertedly to afford an alkyl-λ³-iodane and silyl trifluoroacetate. The developed method enables the use of unactivated tetraalkylsilanes as highly stable synthetic precursors.

Synthesis of N-2(5H)-furanonyl sulfonyl hydrazone derivatives and their biological evaluation in vitro and in vivo activity against MCF-7 breast cancer cells

A series of (E)-N-2(5H)-furanonyl sulfonyl hydrazone derivatives have been rationally designed and efficiently synthesized by one-pot reaction with good yields for the first time. This green approach with wide substrate range and good selectivity can be achieved at room temperature in a short time in the presence of metal-free catalyst. The cytotoxic activities against three human cancer cell lines of all newly obtained compounds have been evaluated by MTT assay. Among them, compound 5 k exhibits high cytotoxic activity against MCF-7 human breast cancer cells with an IC₅₀ value of 14.35 μM. The cytotoxic mechanism may involve G2/M phase arrest pathway, which is probably caused by activating DNA damage. Comet test and immunofluorescence results show that compound 5 k can induce DNA damage in time- and dose-dependent manner. Importantly, 5 k also can effectively inhibit the proliferation of MCF-7 cells and angiogenesis in the zebrafish xenograft model. It is potential to further develop N-2(5H)-furanonyl sulfonyl hydrazone derivatives as potent drugs for breast cancer treatment with higher cytotoxic activity by modifying the structure of the compound.

Pot-Economical Total Synthesis of Clinprost

The pot-economical synthesis of clinprost is reported, in which the core bicyclo[3.3.0]octenone structure was synthesized by two key steps: an asymmetric domino Michael/Michael reaction catalyzed by diphenylprolinol silyl ether and an intramolecular Horner-Wadsworth-Emmons reaction. The trisubstituted endocyclic alkene was selectively introduced by 1,4-reduction followed by trapping of the generated enolate with Tf₂NPh and subsequent utilization of the Suzuki-Miyaura coupling reaction. Chiral, nonracemic clinprost was synthesized in seven pots with a 17% total yield and excellent enantioselectivity.

In(OTf)3-Catalyzed One-Pot Tandem Mannich and Conia-Ene Cyclization Reaction of N-Propargyl Amido Alcohols with 1,3-Dicarbonyl Compounds: An Approach To Construct Tetrahydro-1H-pyrrolo[2,1-a]isoindolone-1,1-dicarboxylate and Its Application

A one-pot tandem reaction has been developed for the synthesis of substituted tetrahydropyrroloisoindolone via Mannich reaction of N-propargyl amido alcohols with 1,3-dicarbonyl compounds followed by Conia-ene cyclization reaction in moderate to good yields catalyzed by indium(III)triflate [In(OTf)₃]. The reaction is highly regioselective with an exo-cyclic double bond in the pyrrolidine ring. The substituted tetrahydropyrroloisoindolone can be converted to 5H-pyrrolo[2,1-a]isoindol-5-one via decarboxylative aromatization reaction.