Enantioselective Total Synthesis of (+)‐Jungermatrobrunin A

Leveraging Nonstrained C-C Bonds for Selective Carboacylation of an Unactivated Alkyne via Transient Dearomatization

Carboacylation of an unsaturated bond represents a powerful transformation. However, only a few examples of carboacylation of alkyne have been reported through C-C bond scission and reconnection. Here, we report a method of carboacylation of an unactivated alkyne by utilizing nonstrained C-C bonds under gold(I) catalysis. The density functional theory computational and experimental studies reveal that the reaction proceeds through a C-to-C formal 1,3-acyl migration via a solvent cage-nested acylium cation.

Catalytic Asymmetric Total Synthesis of (-)-Garryine via an Enantioselective Heck Reaction

The first asymmetric total synthesis of the hexacyclic veatchine-type C20-diterpenoid alkaloid (-)-garryine is presented. Key steps include a Pd-catalyzed enantioselective Heck reaction, a radical cyclization, and a photoinduced C-H activation/oxazolidine formation sequence. Of note, a highly enantioselective Heck reaction developed in this work provides efficient access to 6/6/6 tricyclic compounds, in particular, containing a C19-functionalitiy, which is useful for diverse transformations.

Aromatic Homo-Nazarov-Type Cyclization of Benzonorcaradienes: Stereoselective Synthesis of Hydrochrysenes

Formal homo-Nazarov cyclization of benzonorcaradienes produced by intramolecular hydroarylation of arylated alkynylcyclopropanes promoted by TfOH has been described, providing stereoselective access to highly substituted hydrochrysenes. An unprecedented 1,2-acyl migration occurred for the 2-heteroaroyl substrates, thus giving the same products as their 3-heteroaroyl analogs. Moreover, these products could be readily oxidized by air to fully π-conjugated chrysenes after decarboxylation.

Combining the best of both worlds: radical-based divergent total synthesis

A mature science, combining the art of the total synthesis of complex natural structures and the practicality of delivering highly diverged lead compounds for biological screening, is the constant aim of the organic chemistry community. Delivering natural lead compounds became easier during the last two decades, with the evolution of green chemistry and the concepts of atom economy and protecting-group-free synthesis dominating the field of total synthesis. In this new era, total synthesis is moving towards natural efficacy by utilizing both the biosynthetic knowledge of divergent synthesis and the latest developments in radical chemistry. This contemporary review highlights recent total syntheses that incorporate the best of both worlds.

Total Synthesis of (-)-Antroalbocin A Enabled by a Strain Release-Controlled Photochemical 1,3-Acyl Shift

The first bioinspired, enantioselective, and protecting group free total synthesis of the antibacterial sesquiterpenoid (-)-antroalbocin A (1) has been achieved in 12 steps (5.4% overall yield) from dimedone. An organocatalytic Robinson annulation gave rapid access to the tricyclic enone (19) as starting material for the photochemical domino process of deconjugation and sigmatropic 1,3-acyl shift. Computational data of this process indicate that the 1,3-acyl shift benefits from the highly strained 1,3-enone 8. The transformation of 8 to its bridged isomer 5 is exergonic and, therefore, enables an increased conversion compared to unstrained substrates.

Harnessing Natural Products by a Pharmacophore-Oriented Semisynthesis Approach for the Discovery of Potential Anti-SARS-CoV-2 Agents

Natural products possessing unique scaffolds may have antiviral activity but their complex structures hinder facile synthesis. A pharmacophore-oriented semisynthesis approach was applied to (-)-maoelactone A (1) and oridonin (2) for the discovery of anti-SARS-CoV-2 agents. The Wolff rearrangement/lactonization cascade (WRLC) reaction was developed to construct the unprecedented maoelactone-type scaffold during semisynthesis of 1. Further mechanistic study suggested a concerted mechanism for Wolff rearrangement and a water-assisted stepwise process for lactonization. The WRLC reaction then enabled the creation of a novel family by assembly of the maoelactone-type scaffold and the pharmacophore of 2, whereby one derivative inhibited SARS-CoV-2 replication in HPA EpiC cells with a low EC50 value (19±1 nM) and a high TI value (>1000), both values better than those of remdesivir.

A Three-Step Process to Facilitate the Enantioselective Assembly of Cis-Fused Octahydrophenanthrenes with a Quaternary Stereocenter

A three-step process for the enantioselective assembly of cis-fused octahydrophenanthrenes with a quaternary stereocenter is reported. This synthetic strategy relies on a regioselective γ-alkylation, a one-pot sequence of asymmetric hydrogenation and oxidation, and an intramolecular enolate arylation to facilitate the rapid and enantioselective construction of cis-fused octahydrophenanthrene scaffolds with an arylated all-carbon quaternary stereocenter concisely and efficiently.

Eugenunilones A–H: rearranged sesquiterpenoids from Eugenia uniflora

Six rearranged sesquiterpenoids (1–6) with four types of new polycyclic caged skeletons were isolated from Eugenia uniflora.

Total Synthesis of Isorosthin L and Isoadenolin I

Total syntheses of two Isodon diterpenes, isorosthin L and isoadenolin I, are reported. The synthetic strategy features a quick assembly of two simple building blocks through a diastereoselective intermolecular aldol reaction and a subsequent radical cyclization for efficient construction of a rather complex advanced intermediate bearing a quaternary stereocenter present in all Isodon diterpenes. Oxidative cleavage of the C-C bond in the cyclopentane enabled the conversion to a lactone moiety which is desired for the construction of the molecular skeleton through reductive coupling with an aldehyde carbonyl group.

Recent advances in the synthesis of ent-kaurane diterpenoids

Covering: 2015 to 2020The ent-kaurane diterpenoids are integral parts of tetracyclic natural products that are widely distributed in terrestrial plants. These compounds have been found to possess interesting bioactivities, ranging from antitumor, antifungal and antibacterial to anti-inflammatory activities. Structurally, the different tetracyclic moieties of ent-kauranes can be seen as the results of intramolecular cyclizations, oxidations, C-C bond cleavages, degradation, or rearrangements, starting from their parent skeleton. During the past decade, great efforts have been made to develop novel strategies for synthesizing these natural products. The purpose of this review is to describe the recent advances in the total synthesis of ent-kaurane diterpenoids covering the period from 2015 to date.

Asymmetric Total Syntheses of 8,9-Seco-ent-kaurane Diterpenoids Enabled by an Electrochemical ODI-[5+2] Cascade

An electrochemical ODI-[5+2] cascade reaction was developed which enables the rapid assembly of diversely functionalized bicyclo[3.2.1]octadienones from sensitive ethynylphenols. By combining a directed retro-aldol/aldol process, a [2,3]-sigmatropic rearrangement, and an Al(O-iPr)₃ -promoted reductive 1,3-transposition, the asymmetric total syntheses of five 8,9-seco-ent-kauranoids-(-)-shikoccin, (-)-O-methylshikoccin, (-)-epoxyshikoccin, (+)-O-methylepoxyshikoccin, and (+)-rabdo-hakusin-have been achieved in a concise and efficient manner.

Tetracyclic Diterpenoid Synthesis Facilitated by ODI-Cascade Approaches to Bicyclo[3.2.1]octane Skeletons

Tetracyclic diterpenoids (C₂₀) mainly refer to the plant terpenoids bearing biogenetically related carbon skeletons derived from copalyl diphosphates (ent-CPP and syn-CPP). This large family contains over 1600 known members that can be categorized into 11 major structural types. Among them, more than three-quarters share a bridged bicyclo[3.2.1]octane subunit, which is also an important branching point in biosynthesis en route to the other types of bicyclic scaffolds, such as bicyclo[2.2.2]-, bicyclo[3.3.0]-, and tricyclo[3.2.1.0]octanes. Combined with the significance of its stereochemical importance in biological activity, the assembly of the bicyclo[3.2.1]octane skeletons is critical to the success of the whole synthesis blueprint toward tetracyclic diterpenoids. Although a number of inspiring methodologies have been disclosed, general approaches by the incorporation of innovative cascade reactions permitting access to diverse structural types of tetracyclic diterpenoids remain limited and in urgent demand.Because of the long-standing interest in the synthesis of bridged diterpenoids, we have recently developed two complementary types of oxidative dearomatization induced (ODI) cascade approaches to the rapid and efficient construction of bicyclo[3.2.1]octane skeletons. In this Account, we summarize our original synthesis design, methodology development, and the application of these two strategies in tetracyclic diterpenoid synthesis during the past few years in our laboratory.First, we detail our preliminary investigation of the ODI-[5 + 2] cycloaddition/pinacol rearrangement cascade reaction, which showed a wide scope of vinylphenol substrates and led to cyclopentane and cyclohexane-fused bicyclo[3.2.1]octanes in good yields with excellent dr values. Next, we describe the utilization of this ODI-[5 + 2] cascade reaction which resulted in the asymmetric total syntheses of four highly oxygenated ent-kauranoids. The strategy concerning accurate stereochemical control in the ODI-[5 + 2] cycloaddition was then successfully transplanted to the total syntheses of three stemaranoids, thus providing a straightforward and diastereoselective route to C9-ethano-bridged tetracyclic diterpenoids. To access more complex diterpenoid rhodomollanol A, we exploited two additional biomimetic rearrangements, namely, the retro-Dieckmann fragmentation/vinylogous Dieckmann cyclization cascade and the photo-Nazarov cyclization/intramolecular cycloetherification cascade. Taken together with the ODI-[5 + 2] cascade, the asymmetric total synthesis of the target molecule was realized, which shed light on the biogenetic pathway of the unprecedented rhodomollane-type carbon framework. Finally, we describe an ODI-Diels-Alder/Beckwith-Dowd cascade approach as a valuable supplement to the ODI-[5 + 2] cascade for the fabrication of cycloheptane-fused bicyclo[3.2.1]octane skeletons. Its versatility was also demonstrated by the total syntheses of two challenging grayanane diterpenoids. In view of the high functional-group compatibility and scalability, we anticipate that the two novel cascade approaches will find further use in the field of complex natural product synthesis.

New Strategies in the Efficient Total Syntheses of Polycyclic Natural Products

Polycyclic natural products are an inexhaustible source of medicinal agents, and their complex molecular architecture renders challenging synthetic targets where innovative and effective approaches for their rapid construction are urgently required. The total synthesis of polycyclic natural products has witnessed exponential progression along with the emergence of new synthetic strategies and concepts, such as sequential C-H functionalizations, radical-based transformations, and functional group pairing strategies. Our group exerts continued interest in the construction of bioactive and structurally complex natural products as well as evaluation of the mode of action of these molecules. In this Account, we will showcase how these new synthetic strategies are employed and guide our total synthesis endeavors.During the last two decades, a series of remarkable advances in C-H functionalization have led to the emergence of many new approaches to directly functionalize C-H bonds into useful functional groups. These selective transformations have provided a great opportunity for the step- and atom-economical construction of key fragments in complex molecule synthesis. We recently furnished the total syntheses for polycyclic natural products: incarviatone A, chrysomycin A, polycarcin V, and gilvocarcin V by employing a multiple C-H bond functionalization strategy. The polysubstituted benzene or naphthalene skeleton was constructed through sequential and site-selective C-H functionalizations from readily available simple starting materials, which reduced the number of steps and streamlined synthesis.Recently, we have also completed the total syntheses for a number of skeletally diverse tetracyclic Isodon diterpenoids inspired by their biogenesis and radical-based retrosynthetic disconnections. Radical transformations are strategically and tactically utilized in our syntheses, and radical-based reactions, including organo-SOMO catalysis, Birch reduction, regioselective 1,6-dienyne reductive cyclization, visible-light-mediated Schenck ene reaction, and photoradical-mediated late-stage skeletal rearrangement, play significant roles in our synthetic endeavors. Protecting-group-free and scalable syntheses are also built into our work to achieve the "ideal" synthesis. Furthermore, our synthetic work reveals that late-stage skeletal rearrangement through a photo radical process is possible in a biological setting in complement with nature's carbocation chemistry in complex natural product biosynthesis.Lycopodium alkaloids are a large family of structurally unique polycyclic natural products with impressive biological activities. Owing to their fascinating polycyclic architectures and diverse biological activities, these alkaloids have continued to serve as targets as well as inspirations for the synthetic community for decades. To access these bioactive natural products or natural product-like molecules for biological exploration and drug discovery, we applied a novel functional group pairing strategy to furnish the total syntheses for several Lycopodium alkaloids and obtained numerous skeletally diverse compounds with structural complexity comparable to natural products.

Syntheses of Skeletally Diverse Tetracyclic Isodon Diterpenoid Scaffolds Guided by Dienyne Radical Cyclization Logic

We report herein the diversity-oriented synthesis of various tetracyclic Isodon diterpenoid scaffolds guided by radical cyclization logic. Our substrate-based dienyne radical cyclization approach is distinctive from reagent-based rearrangement approaches that are generally applied in biosynthesis or previous synthetic studies. An unprecedented cyclization at C₁₄ via 1,5-radical translocation/5-exo-trig cyclization is observed, which enriches our radical cyclization pattern. Furthermore, biological evaluations revealed that several new natural product-like compounds showed promising anticancer activities against various cancer cell lines.

Are Organozirconium Reagents Applicable in Current Organic Synthesis?

The search for mild, user-friendly, easily accessible, and robust organometallic reagents is an important feature of organometallic chemistry. Ideally, new methodologies employing organometallics should be developed with respect to practical applications in syntheses of target compounds. In this short review, we investigate if organozirconium reagents can fulfill these criteria. Organozirconium compounds are typically generated via in situ hydrozirconation of alkenes or alkynes with the Schwartz reagent. Alkyl and alkenylzirconium reagents have proven to be convenient in conjugate additions, allylic substitutions, cross-coupling reactions, and additions to carbonyls or imines. Furthermore, the Schwartz reagent itself is a useful reducing agent for polar functional groups.

1 Introduction

2 Synthesis and Generation of the Schwartz Reagent

3 Structure and Properties of Cp2Zr(H)Cl

4 Reactivity of Organozirconium Reagents

4.1 Asymmetric Conjugate Addition

4.2 Asymmetric Allylic Alkylations

4.3 Desymmetrization Reactions

4.4 Cross-Coupling Reactions

4.5 1,2-Additions

5 Conclusions

Late-Stage Diversification of Natural Products

Late-stage diversification of natural products is an efficient way to generate natural product derivatives for drug discovery and chemical biology. Benefiting from the development of site-selective synthetic methodologies, late-stage diversification of natural products has achieved notable success. This outlook will outline selected examples of novel methodologies for site-selective transformations of reactive functional groups and inert C-H bonds that enable late-stage diversification of complex natural products. Accordingly, late-stage diversification provides an opportunity to rapidly access various derivatives for modifying lead compounds, identifying cellular targets, probing protein-protein interactions, and elucidating natural product biosynthetic relationships.

Crokonoids A-C, A Highly Rearranged and Dual-Bridged Spiro Diterpenoid and Two Other Diterpenoids from Croton kongensis

Crokonoid A (1), a highly rearranged diterpenoid featuring a dual-bridged tricyclo[4.4.1.1^1,4]dodecane-2,11-dione ring system and its two possible ent-kaurene diterpenoid precursors (2 and 3), was isolated and structurally characterized by solid data from Croton kongensis. Compound 1 exhibited significant cytotoxicity against HL-60 and A-549 cell lines with IC₅₀ values of 1.24 ± 0.56 and 1.92 ± 0.60 μM, respectively.

Retooling Asymmetric Conjugate Additions for Sterically Demanding Substrates with an Iterative Data-Driven Approach

The development of catalytic enantioselective methods is routinely carried out using easily accessible and prototypical substrates. This approach to reaction development often yields asymmetric methods that perform poorly using substrates that are sterically or electronically dissimilar to those used during the reaction optimization campaign. Consequently, expanding the scope of previously optimized catalytic asymmetric reactions to include more challenging substrates is decidedly nontrivial. Here, we address this challenge through the development of a systematic workflow to broaden the applicability and reliability of asymmetric conjugate additions to substrates conventionally regarded as sterically and electronically demanding. The copper-catalyzed asymmetric conjugate addition of alkylzirconium nucleophiles to form tertiary centers, although successful for linear alkyl chains, fails for more sterically demanding linear α,β-unsaturated ketones. Key to adapting this method to obtain high enantioselectivity was the synthesis of modified phosphoramidite ligands, designed using quantitative structure–selectivity relationships (QSSRs). Iterative rounds of model construction and ligand synthesis were executed in parallel to evaluate the performance of 20 chiral ligands. The copper-catalyzed asymmetric addition is now more broadly applicable, even tolerating linear enones bearing tert-butyl β-substituents. The presence of common functional groups is tolerated in both nucleophiles and electrophiles, giving up to 99% yield and 95% ee across 20 examples.

Expanstines A–D: four unusual isoprenoid epoxycyclohexenones generated byPenicillium expansumYJ-15 fermentation and photopromotion

Expanstines A–D fromP. expansumYJ-15 were derived under UV and visible light from 4′-oxomacrophoin A (5) and its acetate (6), which plausibly underwent a rare intramolecular photoene reaction and a [2 + 2] Paternò–Büchi photoaddition respectively.