Catalytic Asymmetric Spirocyclizing Diels-Alder Reactions of Enones: Stereoselective Total and Formal Syntheses of α-Chamigrene, β-Chamigrene, Laurencenone C, Colletoic Acid, and Omphalic Acid

Synthesis of thiopyran derivatives via [4 + 2] cycloaddition reactions

In this review, we provide a comprehensive overview of the synthesis of thiopyran family compounds via cycloaddition reactions, with examples spanning from the year 2000 to the present. We have categorized the [4 + 2] cycloaddition processes using several criteria, particularly distinguishing between intermolecular and intramolecular types based on the Diels-Alder partners. Additionally, from a mechanism standpoint, we differentiate between concerted and stepwise [4 + 2] processes, offering an analysis of these mechanisms based on the current literature.

Characterization and Engineering of a Bisabolene Synthase Reveal an Unusual Hydride Shift and Key Residues Critical for Mono-, Bi-, and Tricyclic Sesquiterpenes Formation

Sesquiterpene synthases (STSs) catalyze carbocation cascade reactions with various hydrogen shifts and cyclization patterns that generate structurally diverse sesquiterpene skeletons. However, the molecular basis for hydrogen shifts and cyclizations, which determine STS product distributions, remains enigmatic. In this study, an elusive STS SydA was identified in the biosynthesis of sydonol, which synthesized a new bisabolene-type sesquiterpene 6 with a unique saturated terminal pendant isopentane. Extensive evidence from isotope labeling experiments, crystal structures of SydA and its variant, quantum chemical calculations, and mutagenesis experiments reveal a plausible mechanism for the formation of 6 involving an unusual 1,7-hydride shift, which may be a key branchpoint for monocyclic, bicyclic, and tricyclic products. Structure-based engineering resulted in SydA variants that promote different reaction pathways, leading to the production of bicyclic α-cuprenene and (+)-β-chamigrene and tricyclic 7-epi-β-cedrene and β-microbiotene. These findings not only reveal a new bisabolene and its biosynthesis but also provide insights into the molecular basis of the hydride shifts and cyclizations, which pave the way for engineering STSs to produce complex terpenoid products.

Connecting the complexity of stereoselective synthesis to the evolution of predictive tools

Synthetic methods have seemingly progressed to an extent where there is an apparent and increasing need for predictive models to navigate the vast chemical space. Methods for anticipating and optimizing reaction outcomes have evolved from simple qualitative pictures generated from chemical intuition to complex models constructed from quantitative methods like quantum chemistry and machine learning. These toolsets are rooted in physical organic chemistry where fundamental principles of chemical reactivity and molecular interactions guide their development and application. Here, we detail how the evolution of these methods is a successful outcome and a powerful response to the diverse synthetic challenges confronted and the innovative selectivity concepts introduced. In this review, we perform a periodization of organic chemistry focusing on strategies that have been applied to guide the synthesis of chiral organic molecules.

Recent advancements in the chemistry of Diels-Alder reaction for total synthesis of natural products: a comprehensive review (2020-2023)

Despite being discovered nearly a century ago, the Diels-Alder (DA) reaction remains a crucial tool in the total synthesis of natural products. It accommodates a broad range of building blocks with varying complexity and levels of derivatization, allowing the formation of six-membered rings with precise stereochemistry. This, in turn, simplifies the synthesis of core structures found in many natural products. In recent years, modifications to the traditional Diels-Alder reaction have expanded its scope. These modifications include the inverse electron demand Diels-Alder reaction, dehydro Diels-Alder reaction, hetero-Diels-Alder reaction, photoenolization Diels-Alder reaction, asymmetric Diels-Alder reaction, and domino Diels-Alder reaction have been employed to extend the scope of this process in the synthesis of natural products. This review discusses the application of the Diels-Alder reaction in the total synthesis of natural products from 2020 to 2023, along with select methodologies that are inspired by or can be used to synthesize natural products.

Peroxycarbenium-Mediated Asymmetric Synthesis of 1,2-Dioxanes and 1,2-Dioxolanes

The endoperoxide scaffold is found in numerous natural products and synthetic substances of pharmaceutical interest. The main challenge to their synthetic access remains the preparation of chiral compounds due to the weakness of the peroxide bond, which limits the scope of available or applicable methods. Here, we demonstrate how peroxycarbenium species can be trapped by silylated nucleophiles with high enantioselectivities and diastereoselectivities when applicable, using a chiral imidophosphorimidate (IDPi) as a catalyst. The scope of the methodology is broad, encompassing a large variety of enoxysilanes and yielding 1,2-dioxanes or 1,2-dioxolanes. Peroxides can be converted into alcohols or trans-epoxides, and the methodology was applied in a key step of the total synthesis of ethyl plakortide Z, enhancing the selectivity compared to a conventional Lewis acid-catalyzed transformation. Kinetic studies have shown that the reaction necessitates an induction period, indicating the formation of a silylium species that behaves as a true catalyst.

Efficient genome editing using CRISPR/Cas9 technology and its application for identifying Sesquiterpene synthases involved in the biosynthesis of Steperoxides in Steccherinum ochraceum

CRISPR technology has been widely used for gene editing in various species，but the genetic manipulation in basidiomycete mushrooms is still notoriously difficult for unknown endogenous promoters and inefficient DNA delivery. Steccherinum ochraceum is a white rot basidiomycete fungus with abundant secondary metabolites and plays an important ecological role worldwide. To facilitate the study of gene function in S. ochraceum, an effective CRISPR/Cas9 system was successfully developed by identifying highly efficient endogenous promoters, and utilizing the Agrobacterium-transformation method. Two efficient endogenous RNA polymerase II promoters (Psogpd and Psotef1) and one efficient RNA polymerase III promoter (Pu6-d) were identified and characterized, with an editing efficiency of 61.5 % at the ura3 locus. Using this optimized system, the sesquiterpene gene A0064, which could produce 10 possible sesquiterpenes in the heterologous expression system of A. oryzae, was knocked out to obtain A0064 knockout strain S. ochraceum (∆A0064). Steperoxide A could not be detected in S. ochraceum (∆A0064), demonstrating that A0064 was the only enzyme responsible for the biosynthesis of β-chamigrene (the sesquiterpene skeleton of steperoxide A) in S. ochraceum. This efficient system will enable precise targeting and multiplex editing of S. ochraceum genes, facilitating functional studies of genes involved in lignin degradation and natural product biosynthesis in S. ochraceum, and providing some valuable guidance for gene editing in tens of thousands of macrofungi.

Catalytic Asymmetric Reactions of Ketimines and Alkenes via [2 + 2] Cycloaddition: Chemical Reactivity Controlled by Switching a Heteroatom

Azetidine units are commonly found in natural products and biologically active drugs. The [2 + 2] cycloaddition of imines and alkenes has been extensively used in the synthesis of such structures, while enantioselective approaches remain elusive. Herein, an efficient B(C6F5)3/chiral phosphoric acid-catalyzed asymmetric [2 + 2] cycloaddition of ketimines and aryl vinyl selenides was presented, delivering valuable chiral azetidines with excellent stereoselectivities (>20:1 dr and up to 96:4 er). What's even more interesting was that when a "Se" atom was switched to an "S" atom, the reaction proceeded through a [2 + 2] cycloaddition/ring-opening cascade process, affording a range of chiral thioacetals with high enantioselectivities (up to 98:2 er), which were also important organic sulfur compounds. Mechanistic experiments, coupled with density functional theory (DFT) calculations, shed light on a mechanism involving stepwise [2 + 2] cycloaddition and ring-opening processes, with the initial alkenylation step identified as crucial for achieving stereoselective control.

Catalytic asymmetric fragmentation of cyclopropanes

The stereoselective activation of alkanes constitutes a long-standing and grand challenge for chemistry. Although metal-containing enzymes oxidize alkanes with remarkable ease and selectivity, chemical approaches have largely been limited to transition metal-based catalytic carbon-hydrogen functionalizations. Alkanes can be protonated to form pentacoordinated carbonium ions and fragmented into smaller hydrocarbons in the presence of strong Brønsted acids. However, catalytic stereocontrol over such reactions has not previously been accomplished. We show here that strong and confined acids catalyze highly enantioselective fragmentations of a variety of cyclopropanes into the corresponding alkenes, expanding the boundaries of catalytic selective alkane activation. Computational studies suggest the involvement of the long-debated cycloproponium ions.

A solid noncovalent organic double-helix framework catalyzes asymmetric [6 + 4] cycloaddition

Whereas [4 + 2] cycloadditions are among the most powerful tools in the chemist's synthetic arsenal, controlling reactivity and selectivity of [6 + 4] cycloadditions has proven to be extremely challenging. Such transformations, especially if compatible with simple hydrocarbon-based substrates, could ultimately provide a general approach to highly valuable and otherwise difficult to access 10-membered rings. We report here that highly acidic and confined imidodiphosphorimidate catalysts do not catalyze this reaction under homogeneous conditions. Notably, however, they can spontaneously precipitate an insoluble and double helix-shaped noncovalent organic framework, which acts as a distinctively reactive and stereoselective catalyst of [6 + 4] cycloadditions of simple dienes with tropone.

Asymmetric Carbene Transfer: Enhancing Chemical Diversity for Drug Discovery

The quest to explore chemical space is vital for identifying novel disease targets, impacting both the effectiveness and safety profile of therapeutic agents. The tangible chemical space, currently estimated at a conservative 108 synthesized compounds, pales in comparison to the theoretically conceivable diversity of 1060 molecules. To bridge this vast gap, organic chemists are spearheading innovative methodologies that promise to broaden this limited chemical diversity. A beacon of this progressive wave is Asymmetric Carbene Transfer (ACT), a burgeoning strategy that significantly boosts molecular diversity with efficient bond-formation and precise chiral control. This review focuses on the capabilities of ACT in creating pharmaceutically significant molecules, encompassing an array of natural products and bioactive compounds. Through the lens of ACT, we discern its substantial influence on drug discovery, paving the way for novel therapeutic avenues by expanding the boundaries of molecular diversity. This review will shed light on prospective methodological developments of ACT and articulate their conceivable contributions to the medicinal chemistry arena.

Enantioselective and Regiodivergent Gold and Chiral Brønsted Acid Catalyzed Cycloisomerization/Diels-Alder Reaction of 1,10-Dien-4-yn-3-yl Acetates: Synthesis of Norbornene-Embedded Tricarbocycles

A synthetic method for the enantioselective and regiodivergent synthesis of hexahydro-2H-2,4a-methanonaphthalen-4-yl and octahydro-2,4-methanoazulen-1-yl esters that relies on the gold(I)- and chiral Brønsted acid-catalyzed cycloisomerization/Diels-Alder (CDA) reaction of (E)-1,10-dien-4-yn-3-yl acetates is described.

Enantioselective Construction of Quaternary Stereocenters via Cooperative Photoredox/Fe/Chiral Primary Amine Triple Catalysis

The catalytic and enantioselective construction of quaternary (all-carbon substituents) stereocenters poses a formidable challenge in organic synthesis due to the hindrance caused by steric factors. One conceptually viable and potentially versatile approach is the coupling of a C-C bond through an outer-sphere mechanism, accompanied by the realization of enantiocontrol through cooperative catalysis; however, examples of such processes are yet to be identified. Herein, we present such a method for creating different compounds with quaternary stereocenters by photoredox/Fe/chiral primary amine triple catalysis. This approach facilitates the connection of an unactivated alkyl source with a tertiary alkyl moiety, which is also rare. The scalable process exhibits mild conditions, does not necessitate the use of a base, and possesses a good functional-group tolerance. Preliminary investigations into the underlying mechanisms have provided valuable insights into the reaction pathway.

A Homodimer of Withaferin A Formed by Base-Promoted Elimination of Acetic Acid from 27-O-Acetylwithaferin A Followed by a Diels-Alder Reaction

Treatment of 27-O-acetylwithaferin A (2) with the non-nucleophilic base, 1,8-diazabicyclo[5,4,0]undec-7-ene (DBU), afforded 5β,6β-epoxy-4β-hydroxy-1-oxo-witha-2(3),23(24),25(27)-trienolide (3) and 4, a homodimer of withaferin A resulting from a Diels-Alder [4 + 2] type cycloaddition of the intermediate α,β-dimethylene-δ-lactone (9). Structures of 3 and 4 were elucidated using HRMS and 1D and 2D NMR spectroscopic data. The structure of 4 was also confirmed by single crystal X-ray crystallographic analysis of its bis-4-O-p-nitrobenzoate (8). Formation of withaferin A homodimer (4) as the major product suggests regio- and stereoselectivity of the Diels-Alder [4 + 2] cycloaddition reaction of 9. Acetylation of 2-4 afforded their acetyl derivatives 5-7, respectively. Compounds 2-4 and 6-8 were evaluated for their cytotoxic activities against four prostate cancer (PC) cell lines (LNCaP, 22Rv1, DU-145, and PC-3) and normal human foreskin fibroblast (HFF) cells. Significantly, 4 exhibited improved activity compared to the other compounds for most of the tested cell lines.

Marine natural products

Covering: January to the end of December 2022This review covers the literature published in 2022 for marine natural products (MNPs), with 645 citations (633 for the period January to December 2022) referring to compounds isolated from marine microorganisms and phytoplankton, green, brown and red algae, sponges, cnidarians, bryozoans, molluscs, tunicates, echinoderms, the submerged parts of mangroves and other intertidal plants. The emphasis is on new compounds (1417 in 384 papers for 2022), together with the relevant biological activities, source organisms and country of origin. Pertinent reviews, biosynthetic studies, first syntheses, and syntheses that led to the revision of structures or stereochemistries, have been included. An analysis of NP structure class diversity in relation to biota source and biome is discussed.

Desymmetrization and Parallel Kinetic Resolution of 1-Ethynylcyclobutanols via Asymmetric Cooperative Gold Catalysis

Enantioselective gold catalysis remains a challenging area of research. By harnessing gold-ligand cooperation in the presence of a chiral bifunctional phosphine ligand featuring a novel 3'-phosphine oxide moiety, highly enantioselective desymmetrization of 1-ethynylcyclobutanols is achieved, permitting access to chiral α-methylenecyclopentanones featuring a diverse array of chiral quaternary and tertiary centers. This cooperative gold catalysis also enables parallel kinetic resolution in gold catalysis, delivering cyclopentanone regioisomers with excellent enantiomeric excesses. DFT calculations of the transition states support the distinct mechanism of asymmetric induction via controlling the conformation of the bound substrate and hence dictating the ring bond undergoing migration.

Stereospecific Conversion of Boronic Esters into Enones using Methoxyallene: Application in the Total Synthesis of 10-Deoxymethynolide

Enones are widely utilized linchpin functional groups in chemical synthesis and molecular biology. We herein report the direct conversion of boronic esters into enones using commercially available methoxyallene as a three-carbon building block. Following boronate complex formation by reaction of the boronic ester with lithiated-methoxyallene, protonation triggers a stereospecific 1,2-migration before oxidation generates the enone. The protocol shows broad substrate scope and complete enantiospecificity is observed with chiral migrating groups. In addition, various electrophiles could be used to induce 1,2-migration and give a much broader range of α-functionalized enones. Finally, the methodology was applied to a 14-step synthesis of the enone-containing polyketide 10-deoxymethynolide.

Merging Manganese and Iminium Catalysis: Selective Hydroalkenylation of Unsaturated Aldehydes and Ketones

The use of synergistic catalytic strategy can usually circumvent the intrinsic limitations of one catalytic system. In this communication, we disclose a cooperative catalysis strategy of manganese and iminium catalysis to realize selective hydroalkenylation of unsaturated aldehydes and ketones. Its success stems from the LUMO activation of unsaturated carbonyl compounds with secondary amines as the organocatalyst and the synergistic HOMO activation of alkenylboronic acids with Mn2 (CO)8 Br2 . This protocol exhibits several synthetic advances, e.g., simple operation, good functional group compatibility and good regioselectivity. The theoretical calculation indicates the migratory insertion followed by demetallation-isomerization process is kinetically more favorable than Michael-like nucleophilic addition. The use of proline-derived organocatalyst can deliver the desired products in moderate enantioselectivity.

Dimerization of confined Brønsted acids in enantioselective organocatalytic reactions

The formation of Brønsted acid aggregates in the course of asymmetric organocatalytic reactions is often overlooked in mechanistic studies, even though it might have a deep impact on the stereo-controlling factors of the transformations. In this work, we shed light on the influence of the catalyst structure and reaction conditions on the spontaneity of the aggregation process for popular chiral organocatalysts derived from phosphoric acids using high-level quantum mechanical calculations. Our study encompasses small and sterically unhindered chiral phosphoric acids as well as large and "confined" imidodiphosphates and imidodiphosphorimidates. These systems have recently proven particularly effective in promoting a large number of highly relevant asymmetric transformations. While cooperative catalytic effects of sterically less hindered chiral phosphoric acid catalysts are well appreciated in literature, it is found that the formation of catalyst dimers in solution is possible for both standard and confined catalysts. The spontaneity of the aggregation process depends on reaction conditions like solvent polarity, polarizability, temperature, the nature of the interaction with the substrate, as well as the catalyst architecture. Finally, it is shown that, at low temperatures (153 K), the aggregation process can profoundly influence the reaction kinetics and selectivity.

Higher-order [8+2]-cycloadditions of tropothione with levoglucosenone (LGO) and structurally similar exo-cyclic enones derived from cyrene

Levoglucosenone (LGO) and structurally similar exo-cyclic enones derived from cyrene (dihydrolevoglucosenone) react with tropothione following the higher-order [8 + 2]-cycloaddition pathway. Reactions were performed at room temperature in CH₂Cl₂ solutions in absence of any activating reagent. Whereas reaction of tropothione with LGO occurred with complete stereoselectivity, leading to a single, sterically favored exo cycloadduct, identified as polycylic thiophene derivative, reactions performed with exo-cyclic enones yielded in some instances mixtures of two isomeric exo and endo cycloadducts, derived from spiro-tetrahydrothiophene as major and minor components, respectively, of the studied reaction mixtures. Exo and endo [8 + 2] cycloadducts differ in absolute configuration at the newly created chiral centers. Structures of exo and endo cycloadducts were confirmed by single crystal X-ray diffraction analysis.

A Catalytic Asymmetric Hydrolactonization

Despite recent advancements in the development of catalytic asymmetric electrophile induced lactonization reactions of olefinic carboxylic acids, the archetypical hydrolactonization has long remained an unsolved and well-recognized challenge. Here, we report the realization of a catalytic asymmetric hydrolactonization using a confined imidodiphosphorimidate (IDPi) Brønsted acid catalyst. The method is operationally simple, scalable, and compatible with a wide variety of substrates. Its potential is showcased with concise syntheses of the sesquiterpenes (-)-boivinianin A and (+)-gossonorol. Through in-depth physicochemical and DFT analyses, we derive a nuanced picture of the mechanism and enantioselectivity of this reaction.

Recent Progress in the Synthesis of Sesquiterpenoid Involving Spirocyclic Carbon Framework

Spirocarbocyclic natural products have been attracting considerable attention from synthetic organic chemists. This review focused on total syntheses of sesquiterpenoids involving spiro[4.5]decane and spiro[5.5]undecane scaffolds, compiling syntheses of colletoic acid, cubebol, axenol, vetispirene, hinesol, agarospirol, axenol, gleenol, exiguamide, exigurin, erythrodiene, spirojatamol, antroalbocin A, omphalic acid, dactylone, and aplydactonee since 2015.

Insights into the Diels-Alder Reaction of Furan with Maleic Anhydride from Its Prereactive Intermediate

The prereactive intermediate in the furan-maleic anhydride cycloaddition, a classical Diels-Alder reaction, has been captured and characterized in pulsed jets by Fourier transform microwave spectroscopy for the first time. The observed species is stabilized by the π-π* interaction between the two moieties, which connects to the endo channel of the cycloaddition. The secondary interactions between the C=C and C=O in the observed isomer are accountable for its lower energy with respect to the one with the exo channel. The present study tries to fill the significant void of the experimental information on prereactive intermediates as the first stage of Diels-Alder cycloadditions, by outlining the stability of the prereactive intermediate and its accurate molecular structure and by emphasizing the role of the π-π* interaction in governing the stereochemical outcome of the reaction.