Protecting-Group-Free Syntheses of ent-Kaurane Diterpenoids: [3+2+1] Cycloaddition/Cycloalkenylation Approach

Site-specific template generative approach for retrosynthetic planning

Retrosynthesis, the strategy of devising laboratory pathways by working backwards from the target compound, is crucial yet challenging. Enhancing retrosynthetic efficiency requires overcoming the vast complexity of chemical space, the limited known interconversions between molecules, and the challenges posed by limited experimental datasets. This study introduces generative machine learning methods for retrosynthetic planning. The approach features three innovations: generating reaction templates instead of reactants or synthons to create novel chemical transformations, allowing user selection of specific bonds to change for human-influenced synthesis, and employing a conditional kernel-elastic autoencoder (CKAE) to measure the similarity between generated and known reactions for chemical viability insights. These features form a coherent retrosynthetic framework, validated experimentally by designing a 3-step synthetic pathway for a challenging small molecule, demonstrating a significant improvement over previous 5-9 step approaches. This work highlights the utility and robustness of generative machine learning in addressing complex challenges in chemical synthesis.

Unveiling the Potential of Ent-Kaurane Diterpenoids: Multifaceted Natural Products for Drug Discovery

Natural products hold immense potential for drug discovery, yet many remain unexplored in vast libraries and databases. In an attempt to fill this gap and meet the growing demand for effective drugs, this study delves into the promising world of ent-kaurane diterpenoids, a class of natural products with huge therapeutic potential. With a dataset of 570 ent-kaurane diterpenoids obtained from the literature, we conducted an in silico analysis, evaluating their physicochemical, pharmacokinetic, and toxicological properties with a focus on their therapeutic implications. Notably, these natural compounds exhibit drug-like properties, aligning closely with those of FDA-approved drugs, indicating a high potential for drug development. The ranges of the physicochemical parameters were as follows: molecular weights-288.47 to 626.82 g/mol; number of heavy atoms-21 to 44; the number of hydrogen bond donors and acceptors-0 to 8 and 1 to 11, respectively; the number of rotatable bonds-0 to 11; fraction Csp3-0.65 to 1; and TPSA-20.23 to 189.53 Ų. Additionally, the majority of these molecules display favorable safety profiles, with only 0.70%, 1.40%, 0.70%, and 46.49% exhibiting mutagenic, tumorigenic, reproduction-enhancing, and irritant properties, respectively. Importantly, ent-kaurane diterpenoids exhibit promising biopharmaceutical properties. Their average lipophilicity is optimal for drug absorption, while over 99% are water-soluble, facilitating delivery. Further, 96.5% and 28.20% of these molecules exhibited intestinal and brain bioavailability, expanding their therapeutic reach. The predicted pharmacological activities of these compounds encompass a diverse range, including anticancer, immunosuppressant, chemoprotective, anti-hepatic, hepatoprotectant, anti-inflammation, antihyperthyroidism, and anti-hepatitis activities. This multi-targeted profile highlights ent-kaurane diterpenoids as highly promising candidates for further drug discovery endeavors.

Photochemical Deoxygenative Hydroalkylation of Unactivated Alkenes Promoted by a Nucleophilic Organocatalyst

The direct utilization of simple and abundant feedstocks in carbon-carbon bond-forming reactions to embellish sp3 -enriched chemical space is highly desirable. Herein, we report a novel photochemical deoxygenative hydroalkylation of unactivated alkenes with readily available carboxylic acid derivatives. The reaction displays broad functional group tolerance, accommodating carboxylic acid-, alcohol-, ester-, ketone-, amide-, silane-, and boronic ester groups, as well as nitrile-containing substrates. The reaction is operationally simple, mild, and water-tolerant, and can be carried out on multigram-scale, which highlights the utility of the method to prepare value-added compounds in a practical and scalable manner. The synthetic application of the developed method is further exemplified through the synthesis of suberanilic acid, a precursor of vorinostat, a drug used for the treatment of cutaneous T-cell lymphoma. A novel mechanistic approach was identified using thiol as a nucleophilic catalyst, which forms a key intermediate for this transformation. Furthermore, electrochemical studies, quantum yield, and mechanistic experiments were conducted to support a proposed catalytic cycle for the transformation.

Practical and General Alcohol Deoxygenation Protocol

Herein, we describe a practical protocol for the removal of alcohol functional groups through reductive cleavage of their benzoate ester analogs. This transformation requires a strong single electron transfer (SET) reductant and a means to accelerate slow fragmentation following substrate reduction. To accomplish this, we developed a photocatalytic system that generates a potent reductant from formate salts alongside Brønsted or Lewis acids that promote fragmentation of the reduced intermediate. This deoxygenation procedure is effective across structurally and electronically diverse alcohols and enables a variety of difficult net transformations. This protocol requires no precautions to exclude air or moisture and remains efficient on multigram scale. Finally, the system can be adapted to a one-pot benzoylation-deoxygenation sequence to enable direct alcohol deletion. Mechanistic studies validate that the role of acidic additives is to promote the key C(sp3 )-O bond fragmentation step.

Recent progress in alkynylation with hypervalent iodine reagents

Although alkynes are one of the smallest functional groups, they are among the most versatile building blocks for organic chemistry, with applications ranging from biochemistry to material sciences. Alkynylation reactions have traditionally relied on the use of acetylenes as nucleophiles. The discovery and development of ethynyl hypervalent iodine reagents have allowed to greatly expand the transfer of alkynes as electrophilic synthons. In this feature article the progress in the field since 2018 will be presented. After a short introduction on alkynylation reactions and hypervalent iodine reagents, the developments in the synthesis of alkynyl hypervalent iodine reagents will be discussed. Their recent use in base-mediated and transition-metal catalyzed alkynylations will be described. Progress in radical-based alkynylations and atom-economical transformations will then be presented.

Computational Study of Mechanisms and Tether Length Effects of Rh-Catalyzed [3+2] and [3+2+1] Reactions of Ene/Yne-Vinylcyclopropanes

DFT calculations have been applied to study the mechanisms of [3+2] and [3+2+1] reactions of ene/yne-vinylcyclopropanes (shorted as ene/yne-VCPs). The [3+2] reactions of ene/yne-VCPs start from C-C cleavage of cyclopropane (CP cleavage) to form six-membered rhodacycle, followed by alkene/alkyne insertion and reductive elimination. The [3+2+1] reactions have two competing pathways, one is the [3+2+1] pathway (CP cleavage, ene/yne insertion, CO insertion and reductive elimination) and the other is the [3+1+2] pathway (CP cleavage, CO insertion, ene/yne insertion and reductive elimination). The length of tether in substrates affects the ene/yne insertion steps in these cycloadditions, making some reactions fail or changing the reaction pathways. The reasons for these tether length effects are discussed.

Divergent Total Syntheses of Napelline-Type C20-Diterpenoid Alkaloids: (-)-Napelline, (+)-Dehydronapelline, (-)-Songorine, (-)-Songoramine, (-)-Acoapetaldine D, and (-)-Liangshanone

The napelline-type alkaloids possess an azabicyclo[3.2.1]octane moiety and an ent-kaurane-type tetracyclic skeleton (6/6/6/5) along with varied oxidation patterns embedded in the compact hexacyclic framework. Herein, we disclose a divergent entry to napelline-type alkaloids that hinges on convergent assembly of the ent-kaurane core using a diastereoselective intermolecular Cu-mediated conjugate addition and subsequent intramolecular Michael addition reaction as well as rapid construction of the azabicyclo[3.2.1]octane motif via an intramolecular Mannich cyclization. The power of this strategy has been demonstrated through efficient asymmetric total syntheses of eight napelline-type alkaloids, including (-)-napelline, (-)-12-epi-napelline, (+)-dehydronapelline, (+)-12-epi-dehydronapelline, (-)-songorine, (-)-songoramine, (-)-acoapetaldine D, and (-)-liangshanone.

Total Synthesis of Clovan-2,9-dione via [3 + 2 + 1] Cycloaddition and Hydroformylation/Aldol Reaction

Here we report the total synthesis of clovan-2,9-dione via Rh-catalyzed [3 + 2 + 1] cycloaddition/hydroformylation/aldol reaction. The [3 + 2 + 1] reaction of 1-yne-vinylcyclopropane and CO was used for the generation of a 5/6 bicyclic skeleton with a bridgehead vinyl group. The hydroformylation reaction converted the congested olefin of the [3 + 2 + 1] cycloadduct to a one-carbon elongated aldehyde, which underwent in situ aldol reaction, with the carbonyl group in the [3 + 2 + 1] cycloadduct, to generate the tricyclic bridged-ring skeleton of the target molecule.

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 Synthesis of Hetidine-Type C20-Diterpenoid Alkaloids: (+)-Talassimidine and (+)-Talassamine

Here, we report the first asymmetric total synthesis of (+)-talassimidine and (+)-talassamine, two hetidine-type C₂₀-diterpenoid alkaloids. A highly regio- and diastereoselective 1,3-dipolar cycloaddition of an azomethine ylide yielded a chiral tetracyclic intermediate in high enantiopurity, thus providing the structural basis for asymmetric assembly of the hexacyclic hetidine skeleton. In this key step, the introduction of a single chiral center induces four new continuous chiral centers. Another key transformation is the dearomative cyclopropanation of the benzene ring and subsequent S_N2-like ring opening of the resultant cyclopropane ring with water as a nucleophile, which not only establishes the B ring but also precisely installs the difficult-to-achieve equatorial C7-OH group.

In Pursuit of Synthetic Efficiency: Convergent Approaches

The field of total synthesis has reached a stage in which emphasis has been increasingly focused on synthetic efficiency rather than merely achieving the synthesis of a target molecule. The pursuit of synthetic efficiency, typically represented by step count and overall yield, is a rich source of inspiration and motivation for synthetic chemists to invent innovative strategies and methods. Among them, convergent strategy has been well recognized as an effective approach to improve efficiency. This strategy generally involves coupling of fragments with similar complexity to furnish the target molecule via subsequent cyclization or late-stage functionalization. Thus, methodologies that enable effective connection of fragments are critical to devising a convergent plan. In our laboratory, convergent strategy has served as a long-standing principle for pursuing efficient synthesis during the course of planning and implementing synthetic projects. In this Account, we summarize our endeavors in the convergent synthesis of natural products over the last ten years. We show how we identify reasonable bond disconnections and employ enabling synthetic methodologies to maximize convergency, leading to the efficient syntheses of over two-dozen highly complex molecules from eight disparate families.In detail, we categorize our work into three parts based on the diverse reaction types for fragment assembly. First, we demonstrate the application of a powerful single-electron reducing agent, SmI₂, in a late-stage cyclization step, forging the polycyclic skeletons of structurally fascinating Galbulimima alkaloids and Leucosceptrum sesterterpenoids. Next, we showcase how three different types of cycloaddition reactions can simultaneously construct two challenging C-C bonds in a single step, providing concise entries to three distinct families, namely, spiroquinazoline alkaloids, gracilamine, and kaurane diterpenoids. In the third part, we describe convergent assembly of ent-kaurane diterpenoids, gelsedine-type alkaloids, and several drug molecules via employing some bifunctional synthons. To access highly oxidized ent-kaurane diterpenoids, we introduce the hallmark bicyclo[3.2.1]octane ring system at an early stage, and then execute coupling and cyclization by means of a Hoppe's homoaldol reaction and a Mukaiyama-Michael-type addition, respectively. Furthermore, we showcase how the orchestrated combination of an asymmetric Michael addition, a tandem oxidation-aldol reaction and a pinacol rearrangement can dramatically improve the efficiency in synthesizing gelsedine-type alkaloids, with nary a protecting group. Finally, to address the supply issue of several drugs, including anti-influenza drug zanamivir and antitumor agent Et-743, we exploit scalable and practical approaches to provide advantages over current routes in terms of cost, ease of execution, and efficiency.

Nine sesquiterpenoid dimers with four unprecedented types of carbon skeleton from Chloranthus henryi var. hupehensis

Chlorahupetones A–I (1–9), nine sesquiterpenoid dimers, were isolated from Chloranthus henryi var. hupehensis.

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