Divergent Total Synthesis of Euphoranginol C, Euphoranginone D, ent ‐Trachyloban‐3β‐ol, ent ‐Trachyloban‐3‐one, Excoecarin E, and ent ‐16α‐Hydroxy‐atisane‐3‐one

Enantioselective total synthesis of atisane diterpenoids: (+)-sapinsigin H, (+)-agallochaol C, and (+)-16α, 17-dihydroxy-atisan-3-one

Enantioselective total synthesis of (+)-sapinsigin H, (+)-agallochaol C, and (+)-16α, 17-dihydroxy-atisan-3-one has been accomplished starting from enantiopure Wieland-Miescher ketone. Key features of the syntheses include a benzannulation step to construct the tricyclic core, an oxidative dearomatization step to generate the diene, and a Diels-Alder reaction with ethylene gas to establish the bicyclo[2.2.2]octane framework. Efficient late-stage functionalisation of the A-ring by aerobic oxidation and Baeyer-Villiger oxidation completed the atisane target molecules.

Asymmetric Synthesis and Biological Evaluation of Platensilin, Platensimycin, Platencin, and Their Analogs via a Bioinspired Skeletal Reconstruction Approach

Platensilin, platensimycin, and platencin are potent inhibitors of β-ketoacyl-acyl carrier protein synthase (FabF) in the bacterial and mammalian fatty acid synthesis system, presenting promising drug leads for both antibacterial and antidiabetic therapies. Herein, a bioinspired skeleton reconstruction approach is reported, which enables the unified synthesis of these three natural FabF inhibitors and their skeletally diverse analogs, all stemming from a common ent-pimarane core. The synthesis features a diastereoselective biocatalytic reduction and an intermolecular Diels-Alder reaction to prepare the common ent-pimarane core. From this intermediate, stereoselective Mn-catalyzed hydrogen atom-transfer hydrogenation and subsequent Cu-catalyzed carbenoid C-H insertion afford platensilin. Furthermore, the intramolecular Diels-Alder reaction succeeded by regioselective ring opening of the newly formed cyclopropane enables the construction of the bicyclo[3.2.1]-octane and bicyclo[2.2.2]-octane ring systems of platensimycin and platencin, respectively. This skeletal reconstruction approach of the ent-pimarane core facilitates the preparation of analogs bearing different polycyclic scaffolds. Among these analogs, the previously unexplored cyclopropyl analog 47 exhibits improved antibacterial activity (MIC80 = 0.0625 μg/mL) against S. aureus compared to platensimycin.

Intramolecular Alkyne-de Mayo Reaction Using the Enol Methylene Acetal Linker: Experimental Implementation and Computational Simulation

The enol-linked intramolecular alkyne-de Mayo reaction is a photochemically triggered cascade reaction suitable for the synthesis of substituted dihydrotropones by two-carbon ring expansion of enol ethers of cyclopentane-1,3-dion. We report on the implementation of the methylene acetal linker and the isolation of the initial (2 + 2) photocycloadduct in substances. We have investigated in depth the modus operandi of the ring-opening of the π-donor-π-acceptor cyclobutene derivatives by computational chemistry.

Asymmetric Divergent Synthesis of ent-Kaurane-, ent-Atisane-, ent-Beyerane-, ent-Trachylobane-, and ent-Gibberellane-type Diterpenoids

A unified strategy toward asymmetric divergent syntheses of nine C8-ethano-bridged diterpenoids A1-A9 (candol A, powerol, sicanadiol, epi-candol A, atisirene, ent-atisan-16α-ol, 4-decarboxy-4-methyl-GA₁₂, trachinol, and ent-beyerane) has been developed based on late-stage transformations of common synthons having ent-kaurane and ent-trachylobane cores. The expeditious assembly of crucial advanced ent-kaurane- and ent-trachylobane-type building blocks is strategically explored through a regioselective and diastereoselective Fe-mediated hydrogen atom transfer (HAT) 6-exo-trig cyclization of the alkene/enone and 3-exo-trig cyclization of the alkene/ketone, showing the multi-reactivity of densely functionalized polycyclic substrates with π_C═C and π_C═O systems in HAT-initiated reactions. Following the rapid construction of five major structural skeletons (ent-kaurane-, ent-atisane-, ent-beyerane-, ent-trachylobane-, and ent-gibberellane-type), nine C8-ethano-bridged diterpenoids A1-A9 could be accessed in the longest linear 8 to 11 steps starting from readily available chiral γ-cyclogeraniol 1 and known chiral γ-substituted cyclohexenone 2, in which enantioselective total syntheses of candol A (A1, 8 steps), powerol (A2, 9 steps), sicanadiol (A3, 10 steps), epi-candol A (A4, 8 steps), ent-atisan-16α-ol (A6, 11 steps), and trachinol (A8, 10 steps) are achieved for the first time.

Generation of Fused Seven-Membered Polycyclic Systems via Ring Expansion and Application to the Total Synthesis of Sesquiterpenoids

Seven-membered polycyclic architectures, widely present in natural products and molecular drugs, are challenging synthetic targets. However, methods for synthesizing fused medium-sized bicyclo[m.n.0] ring systems, including the benzo-cycloheptane systems, are still urgent. Herein we describe a base-induced ring expansion as a general strategy to construct a wide range of fused seven-membered ring systems. The application of this method was demonstrated by the efficient total syntheses of two sesquiterpenoids, plecarpenene and plecarpenone, both bearing a fused bicyclo[5.3.0]decane skeleton.

Cationic cyclization reactions with alkyne terminating groups: a useful tool in biomimetic synthesis

Cyclization reactions through cationic intermediates have become a highly valuable tool in organic synthesis. The use of alkynes as the terminating group in this type of cationic process offers wide synthetic possibilities because this group can serve as a precursor of different functionalities. This article shows relevant examples of cationic cyclization reactions with alkynes as terminating groups with the intention of demonstrating the potential of this type of process, particularly in the context of biomimetic synthesis of natural products.

Total Synthesis and Late-Stage C-H Oxidations of ent-Trachylobane Natural Products

Herein, we present our studies to construct seven ent-trachylobane diterpenoids by employing a bioinspired two-phase synthetic strategy. The first phase provided enantioselective and scalable access to five ent-trachylobanes, of which methyl ent-trachyloban-19-oate was produced on a 300 mg scale. During the second phase, chemical C-H oxidation methods were employed to enable selective conversion to two naturally occurring higher functionalized ent-trachylobanes. The formation of regioisomeric analogs, which are currently inaccessible via enzymatic methods, reveals the potential as well as limitations of established chemical C-H oxidation protocols for complex molecule synthesis.

Total Synthesis and Late-Stage C-H Oxidations of ent-Trachylobane Natural Products

Herein, we present our studies to construct seven ent-trachylobane diterpenoids by employing a bioinspired two-phase synthetic strategy. The first phase provided enantioselective and scalable access to five ent-trachylobanes, of which methyl ent-trachyloban-19-oate was produced on a 300 mg scale. During the second phase, chemical C-H oxidation methods were employed to enable selective conversion to two naturally occurring higher functionalized ent-trachylobanes. The formation of regioisomeric analogs, which are currently inaccessible via enzymatic methods, reveals the potential as well as limitations of established chemical C-H oxidation protocols for complex molecule synthesis.

A Vinylogous Norrish Reaction as a Strategy for Light-Mediated Ring Expansion

The reactions of bicyclic divinyl ketones display wavelength-dependent changes in product formation. UV irradiation results in the formation of competitive [6,3,5] and [7,3,5] tricyclic unsaturated ketones that subsequently undergo ring...

Dynamic Kinetic Sensitization of β-Dicarbonyl Compounds-Access to Medium-Sized Rings by De Mayo-Type Ring Expansion

Herein, we present a photocatalyzed two-carbon ring expansion of β-dicarbonyl compounds with unactivated olefins that provides facile access to medium-sized rings. Selective sensitization of the substoichiometric enol tautomer enables reactivity of substrates incompatible with the classical De Mayo reaction conditions. Key to success is the identification of the metal-based sensitizer fac-[Ir(CF₃ -pmb)₃ ], which can be excited using common near-visible LEDs, and possesses a high triplet excited state energy of 73.3 kcal mol^-1 . This exactly falls in the range between the triplet energies of the enol and keto tautomer, thereby enabling a dynamic kinetic sensitization. Demonstrating the applicability of fac-[Ir(CF₃ -pmb)₃ ] as a photocatalyst in organic synthesis for the first time, we describe a two-step photocycloaddition-ring-opening cascade with β-ketoesters, -diketones, and -ketoamides. The mechanism has been corroborated by time-resolved spectroscopy, as well as further experimental and computational studies.

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.

Electrochemical ODI-[5+2] Cascade for the Syntheses of Diversely Functionalized Bicyclo[3.2.1]octane Frameworks

A metal- and hypervalent iodine reagent-free electrochemical oxidative dearomatization-induced [5+2] cycloaddition/pinacol rearrangement cascade reaction was described. The electrosynthetic method showed strong tolerance for vinylphenols, ethynylphenols, and allenylphenols, which thus enabled the rapid assembly of diversely functionalized bicyclo[3.2.1]octanes in 41-95% yields and up to >20:1 dr. This protocol could be scaled up to gram amounts and should find wide application in complex natural product synthesis.

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.