6‐Methylenebicyclo[3.2.1]oct‐1‐en‐3‐one: A Twisted Olefin as Diels–Alder Dienophile for Expedited Syntheses of Four Kaurane Diterpenoids

Natural Product-Inspired Molecules for Covalent Inhibition of SHP2 Tyrosine Phosphatase

Natural products have been playing indispensable roles in the development of lifesaving drug molecules. They are also valuable sources for covalent protein modifiers. However, they often are scarce in nature and have complex chemical structures, which are limiting their further biomedical development. Thus, natural product-inspired small molecules which still contain the essence of the parent natural products but are readily available and amenable for structural modification, are important and desirable in searching for lead compounds for various disease treatment. Inspired by the complex and diverse ent-kaurene diterpenoids with significant biological activities, we have created a synthetically accessible and focused covalent library by incorporating the bicyclo[3.2.1]octane α-methylene ketone, which is considered as the pharmacophore of ent-kaurene diterpenoids, as half of the structure, and replacing the other half with much less complex but more druglike scaffolds. From this library, we have identified and characterized selective covalent inhibitors of protein tyrosine phosphatase SHP2, an important anti-cancer therapeutic target. The success of this study demonstrated the importance of creating and evaluating natural product-inspired library as well as their application in targeting challenging disease targets.

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.

Enantioselective Total Syntheses of Grayanane Diterpenoids: (-)-Grayanotoxin III, (+)-Principinol E, and (-)-Rhodomollein XX

Enantioselective total syntheses of (-)-grayanotoxin III, (+)-principinol E, and (-)-rhodomollein XX were accomplished based on a convergent strategy. The left- and right-wing fragments were assembled via the diastereoselective Mukaiyama aldol reaction catalyzed by a chiral hydrogen bond donor. The unique 7-endo-trig cyclization based on a bridgehead carbocation forged the 5/7/6/5 tetracyclic skeleton that underwent redox manipulations and 1,2-migration to access different grayanane diterpenoids.

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.

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.

Decarboxylative Halogenation of Organic Compounds

Decarboxylative halogenation, or halodecarboxylation, represents one of the fundamental key methods for the synthesis of ubiquitous organic halides. The method is based on conversion of carboxylic acids to the corresponding organic halides via selective cleavage of a carbon-carbon bond between the skeleton of the molecule and the carboxylic group and the liberation of carbon dioxide. In this review, we discuss and analyze major approaches for the conversion of alkanoic, alkenoic, acetylenic, and (hetero)aromatic acids to the corresponding alkyl, alkenyl, alkynyl, and (hetero)aryl halides. These methods include the preparation of families of valuable organic iodides, bromides, chlorides, and fluorides. The historic and modern methods for halodecarboxylation reactions are broadly discussed, including analysis of their advantages and drawbacks. We critically address the features, reaction selectivity, substrate scopes, and limitations of the approaches. In the available cases, mechanistic details of the reactions are presented, and the generality and uniqueness of the different mechanistic pathways are highlighted. The challenges, opportunities, and future directions in the field of decarboxylative halogenation are provided.

Enantioselective Total Synthesis of Andrographolide and 14-Hydroxy-Colladonin: Carbonyl Reductive Coupling and trans-Decalin Formation by Hydrogen Transfer

An enantioselective total synthesis of the labdane diterpene andrographolide, the bitter principle of the herb Andrographis paniculata (known as "King of Bitters"), was accomplished in 14 steps (LLS). Key transformations include iridium-catalyzed carbonyl reductive coupling to form the quaternary C4 stereocenter, diastereoselective alkene reduction to establish the trans-decalin ring, and carbonylative lactonization to install the α-alkylidene-β-hydroxy-γ-butyrolactone.

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.

Divergent Synthesis of Bicyclo[3.2.1]octenes and Cyclohexenes via Catalytic Annulations of Nazarov Reagent and Vinyl 1,2-Diketones

Bicyclo[3.2.1]octanes and related structures are unique units that widely exist in natural products, but the rapid and stereoselective construction of this skeleton is a challenging issue. We report the stereodivergent synthesis of bicyclo[3.2.1]octenes using Nazarov reagents and alkenyl 1,2-diketones with Brønsted base catalysis under mild conditions. Both stereoisomers of the bridged products can be obtained by tuning the reaction conditions, and cyclohexene product can also be selectively formed.

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.