Total syntheses of dalesconol A and B

Ruthenium-Catalyzed 1,6-Hydroalkylation of para-Quinone Methides with Ketones via the in Situ Activation of C(sp3)-H Bonds

A simple and efficient method for the ruthenium-catalyzed 1,6-hydroalkylation of para-quinone methides (p-QMs) with ketones via the in situ activation of C(sp3)-H bonds has been disclosed. Without the need for preactivation of the substrates and oxidant, a broad range of p-QMs and ketones are well tolerated, producing the expected 1,6-hydroalkylation products with moderate to good yields. Step-by-step control experiments and DFT calculation were conducted systematically to gain insights for the plausible reaction mechanism. This finding may have potential application in the selective diarylmethylation of ketones at the α-C position in organic synthesis.

Conceptual advances in nucleophilic organophosphine-promoted transformations

Catalysis by trivalent nucleophilic organophosphines has emerged as an essential tool in organic synthesis. Several new organic transformations promoted by phosphines substantiate and complement the existing synthetic chemistry tools. Mere design of the substrate and reagent combinations has introduced new modes of reactivity patterns, which are otherwise difficult to achieve. These design considerations have led to the rapid build-up of complex molecular entities and laid a solid foundation to synthesise bioactive natural products and pharmaceuticals. This article presents an overview of some of the conceptual advances, including our contributions to nucleophilic organophosphine chemistry. The scope, limitations, mechanistic insights, and applications of these metal-free transformations are discussed elaborately.

Synthesis of Spirotricyclic Core of Bonnadiene

We herein describe a new approach for the efficient synthesis of the tricyclic core of diterpene bonnadiene. The synthetically challenging and unusual [6-7-5] spirotricyclic skeleton including the all-carbon quaternary stereocenter, was installed diastereoselectively via a type II [5 + 2] cycloaddition, followed by a unique vinylogous semipinacol rearrangement. The described chemistry demonstrates the feasibility of making the [6-7-5] spirotricyclic skeleton of the final product from the strained bridged [7-8-5] ring system.

Divergent cyclodimerizations of styrylnaphthols under aerobic visible-light irradiation and Brønsted acid catalysis

Dimeric cyclization reactions show great potential to rapidly form highly substituted complex cyclic molecules from simple starting materials. However, such an appealing process is often hampered by the lack of selectivity. Herein we report two divergent cyclodimerization reactions of 1-styrylnaphthalen-2-ol derivatives under simple and very mild reaction conditions. A stereoselective visible light-induced oxidative (1 + 1 + 4 + 4) homodimerization gave rise to highly substituted 1,5-dioxocanes in moderate yields. This transformation harnessed singlet oxygen as a safe and mild oxidant under photocatalyst-free reaction conditions. Additionally, we demonstrated that the same substrates undergo a (4 + 2) heterodimerization under Brønsted-acid catalysis to produce chromane derivatives featuring 3 contiguous tertiary stereocenters in good to high yields with excellent diastereoselectivities.

Dearomative logic in natural product total synthesis

Covering: 2011 to 2022The natural world is a prolific source of some of the most interesting, rare, and complex molecules known, harnessing sophisticated biosynthetic machinery evolved over billions of years for their production. Many of these natural products represent high-value targets of total synthesis, either for their desirable biological activities or for their beautiful structures outright; yet, the high sp3-character often present in nature's molecules imparts significant topological complexity that pushes the limits of contemporary synthetic technology. Dearomatization is a foundational strategy for generating such intricacy from simple materials that has undergone considerable maturation in recent years. This review highlights the recent achievements in the field of dearomative methodology, with a focus on natural product total synthesis and retrosynthetic analysis. Disconnection guidelines and a three-phase dearomative logic are described, and a spotlight is given to nature's use of dearomatization in the biosynthesis of various classes of natural products. Synthetic studies from 2011 to 2021 are reviewed, and 425 references are cited.

Total Synthesis of Dalesconol A by Pd(0)/Norbornene-Catalyzed Three-Fold Domino Reaction and Pd(II)-Catalyzed Trihydroxylation

Herein, we describe a concise total synthesis of dalesconol A through a "polycyclization/oxidation" approach. In the polycyclization stage, a Pd(0)/NBE-catalyzed 3-fold domino reaction and a subsequent intramolecular Michael addition have been utilized for the one-step assembly of the heptacyclic molecular skeleton. In the late stage of oxidation state adjustments, a stepwise sequence including site-selective benzylic oxidation, Pd(II)-catalyzed oxime ether directed trihydroxylation, and desaturation has been adopted to introduce the oxygen functionalities and furnish the synthesis of dalesconol A. With the advantage of the late-stage amidation of three C-H bonds in a single step, the amino analogue of dalesconol A has also been obtained with high efficiency.

Rhodium(iii)-catalyzed asymmetric [4+1] spiroannulations of O-pivaloyl oximes with α-diazo compounds

Chiral Rh^III catalysts can catalyze the asymmetric [4+1] spiroannulation of O-pivaloyl oximes with α-diazo homophthalimides under redox-neutral and acid/base-neutral conditions, leading to formation of chiral spirocyclic imines as a result of C-H activation and N-O cleavage. The reaction proceeded with high efficiency and features broad substrate scope, mild reaction conditions, and high to excellent enantioselectivities.

Recent advances in total syntheses of natural products containing the benzocycloheptane motif

Covering: 2010 to 2020Benzocycloheptane is a fundamental and unique structural motif found in pharmaceuticals and natural products. The total syntheses of natural products bearing the benzocycloheptane subunit are challenging and there are only a few efficient approaches to access benzocycloheptane. Thus, new methods and innovative strategies for preparing such natural products need to be developed. In this review, recent progress in the total syntheses of natural products bearing the benzocycloheptane motif is presented, and key transformations for the construction of benzocycloheptane are highlighted. This review provides a useful guide for those engaged in the syntheses of natural products containing the benzocycloheptane motif.

Enantioselective formation of quaternary carbon stereocenters in natural product synthesis: a recent update

Covering: 2013-2018 Natural products bearing quaternary carbon stereocenters have attracted tremendous interest from the synthetic community due to their diverse biological activities and fascinating molecular architectures. However, the construction of these molecules in an enantioselective fashion remains a long-standing challenge because of the lack of efficient asymmetric catalytic methods for installing these motifs. The rapid progress in the development of new-generation efficient chiral catalysts has opened the door for several asymmetric reactions, such as Michael addition, dearomative cyclization, polyene cyclization, α-arylation, cycloaddition, allylation, for the construction of quaternary carbon stereocenters in a highly enantioselective fashion. These asymmetric catalytic methods have greatly facilitated the synthesis of complex natural products with improved output and overall efficiency. In this concise review, we highlight the progress in the last six years in complex natural product synthesis, in which at least one quaternary carbon stereocenter has been constructed via asymmetric catalytic technologies, with particular emphasis on the analysis of the stereochemical model of each enantioselective transformation.

Cu-Catalyzed radical-triggered spirotricyclization of enediynes and enyne-nitriles for the synthesis of pentacyclic spiroindenes

A new copper-catalyzed radical-triggered fluoromethylation-spirotricyclization of enediyne- and enyne-nitrile-containing para-quinone methides (p-QMs) was reported for the first time, and used to produce a series of hitherto unreported pentacyclic spiroindenes.

Chrysomycin A Derivatives for the Treatment of Multi-Drug-Resistant Tuberculosis

Tuberculosis (TB) is a life-threatening disease resulting in an estimated 10 million new infections and 1.8 million deaths annually, primarily in underdeveloped countries. The economic burden of TB has been estimated as approximately 12 billion USD annually in direct and indirect costs. Additionally, multi-drug-resistant (MDR) and extreme-drug-resistant (XTR) TB strains resulting in about 250 000 deaths annually are now widespread, increasing pressure on the identification of new anti-TB agents that operate by a novel mechanism of action. Chrysomycin A is a rare C-aryl glycoside first discovered over 60 years ago. In a recent high-throughput screen, we found that chrysomycin A has potent anti-TB activity, with minimum inhibitory concentration (MIC) = 0.4 μg/mL against MDR-TB strains. However, chrysomycin A is obtained in low yields from fermentation of Streptomyces, and the mechanism of action of this compound is unknown. To facilitate the mechanism of action and preclinical studies of chrysomycin A, we developed a 10-step, scalable synthesis of the isolate and its two natural congeners polycarcin V and gilvocarcin V. The synthetic sequence was enabled by the implementation of two sequential C-H functionalization steps as well as a late-stage C-glycosylation. In addition, >10 g of the advanced synthetic intermediate has been prepared, which greatly facilitated the synthesis of 33 new analogues to date. The structure-activity relationship was subsequently delineated, leading to the identification of derivatives with superior potency against MDR-TB (MIC = 0.08 μg/mL). The more potent derivatives contained a modified carbohydrate residue which suggests that further optimization is additionally possible. The chemistry we report here establishes a platform for the development of a novel class of anti-TB agents active against drug-resistant pathogens.

Palladium-catalyzed asymmetric dearomative cyclization in natural product synthesis

Asymmetric catalysis is a rapidly growing field in modern organic chemistry and has been indispensable for the synthesis of enantioenriched materials to meet demands from the academies to pharmaceutical industries. Asymmetric dearomative cyclization catalyzed by transition metals has been a hot research area in the last decade. Fascinated by its ability to construct sterically hindered quaternary stereogenic center(s) through dearomatization and simultaneously forging new ring structure(s) through cyclization, palladium-catalyzed asymmetric dearomative cyclization has been applied to the synthesis of structurally complicated natural products and it is increasingly prevalent in the literature. In particular, the resultant product from dearomative cyclization, which usually carries one or more unsaturated C-C bond(s), allows further functional group transformations. Previously reported applications of palladium-catalyzed asymmetric dearomative cyclization in natural product synthesis are presented here and discussed in depth.

Galewone, an Anti-Fibrotic Polyketide from Daldinia eschscholzii with an Undescribed Carbon Skeleton

A novel polyphenolic natural product, galewone, with undescribed carbon skeleton, was isolated as a racemate from the culture of Daldinia eschscholzii IFB-TL01, a fungus obtained from the mantis (Tenodera aridifolia) gut. The galewone structure was elucidated by a combination of MS and NMR spectra, and substantiated by X-ray crystallographic diffraction. The absolute stereochemistry of each galewone enantiomers was determined by the CD spectrum. In compliance of the structural similarities, galewone might be the shunt products of the dalesconol biosynthetic pathway. Both (−)- and ( + )-galewones were evaluated to be anti-fibrotic against activated hepatic stellate cell line, CFSC-8B, with the IC₅₀ values being 3.73 ± 0.21 and 10.10 ± 0.41 μM, respectively. Thus, galewone may serve as a starting molecule for the discovery of new anti-fibrotic drug.

Efficient Enantioselective Syntheses of Chiral Natural Products Facilitated by Ligand Design

The employment of enantioselective transition-metal-catalyzed transformations as key steps in asymmetric natural product syntheses have attracted considerable attention in recent years owing to their versatile synthetic utilities, mild conditions and high efficiency in chirality generation. The chiral catalysts or supporting ligands are believed to be crucial for the requisite reactivity and enantioselectivity. Therefore, the rational design of chiral ligands is at the heart of developing new asymmetric transition-metal catalyzed reactions and provides an avenue to the asymmetric synthesis of natural products. Our group has been engaged in the development of transition-metal-catalyzed enantioselective cross-coupling, cyclization and other related reactions and the application of these methodologies to natural product syntheses. In this account, we summarized our recent synthetic efforts towards the efficient total syntheses of several different types of natural products including terpenes, alkaloids and polyketides facilitated by the design of a series of versatile P-chiral phosphorous ligands.

Metal- and Hydride-Free Pentannulative Reductive Aldol Reaction

Traditionally, the reductive aldol reaction is a metal-catalyzed and hydride-promoted coupling between enones and aldehydes. We present a phosphine-mediated diastereoselective intramolecular reductive aldol reaction of α-substituted dienones and aldehydes, which is metal-free and hydride-free. The synthetic utility of the reductive aldol adducts is demonstrated by elaborating them in one step to indeno[1,2- b]furanones, indeno[1,2- b]pyrans, and dibenzo[ a, h]azulen-8-ones.

Synthesis and Biological Evaluation of Tripartin, a Putative KDM4 Natural Product Inhibitor, and 1-Dichloromethylinden-1-ol Analogues

The natural product tripartin has been reported to inhibit the N-methyl-lysine histone demethylase KDM4A. A synthesis of tripartin starting from 3,5-dimethoxyphenylacrylic acid was developed, and the enantiomers were separated by chiral HPLC. We observed that both tripartin enantiomers manifested an apparent increase in H3K9me3 levels when dosed in cells, as measured by western blot analysis. Thus, there is no enantiomeric discrimination toward this natural product in terms of its effects on cellular histone methylation status. Interestingly, tripartin did not inhibit isolated KDM4A-E under our assay conditions (IC50 >100 μm). Tripartin analogues with a dichloromethylcarbinol group derived from the indanone scaffold were synthesized and found to be inactive against isolated recombinant KDM4 enzymes and in cell-based assays. Although the precise cellular mode of action of tripartin is unclear, our evidence suggests that it may affect histone methylation status via a mechanism other than direct inhibition of the KDM4 histone demethylases.

Pd(ii)-Catalyzed [3 + 2] spiroannulation of α-aryl-β-naphthols with alkynes via a C–H activation/dearomatization approach

A Pd(ii)-catalyzed [3 + 2] spiroannulation of α-aryl-β-naphthols with internal alkynes has been developed by relying on a C–H activation/arene dearomatization approach.

Enantioselective formation of cyclopropane spiroindenes from benzofulvenes by phase transfer catalysis

The enantio- and diastereoselective formation of indenes spirofused to highly substituted cyclopropanes is described.

Muta-mycosynthesis of naphthalene analogs

A mutasynthetic strategy is introduced for the mycosynthesis of naphthalene-based molecules (mutadalesols A-F) with directed substitution patterns and new frameworks by generating and using the ΔpksTL mutant strain of Daldinia eschscholzii. (±)-Mutadalesol A and its (+)-enantiomer are cytotoxic, and its (-)-enantiomer inhibits Toll-like receptor 5 (TLR5). The in-culture reactability of fungal oligoketide intermediates with 5-aminonaphthalen-1-ol (ANL) is demonstrated, shedding light on bioorthogonal accesses to unnatural molecule libraries valuable in drug discovery pipelines.

Pd(0)-catalyzed chemoselective construction of spirocarbocycles via an alkyne insertion/β-naphthol dearomatization cascade

A highly chemoselective route to obtain spiro[indene-1,1′-naphthalen]-2′-ones via a microwave-assisted Pd(0)-catalyzed alkyne migratory insertion/β-naphthol dearomatization cascade is described.