Formal Homoiodo Allylsilane Annulations: Dual Total Syntheses of (±)-Hirsutene and (±)-Capnellene

An Overview of Secondary Metabolites from Soft Corals of the Genus Capnella over the Five Decades: Chemical Structures, Pharmacological Activities, NMR Data, and Chemical Synthesis

There has been no specific review on the secondary metabolites from soft corals of the genus Capnella till now. In this work, all secondary metabolites from different species of the title genus were described. It covered the first work from 1974 to May 2024, spanning five decades. In the viewpoint of the general structural features, these chemical constituents were classified into four groups: sesquiterpenes, diterpenes, steroids, and lipids. Additionally, the 1H and 13C NMR data of these metabolites were provided when available in the literature. Among them, sesquiterpenes were the most abundant chemical compositions from soft corals of the genus Capnella. A variety of pharmacological activities of these compounds were evaluated, such as cytotoxic, antibacterial, antifungal, and anti-inflammatory activities. In addition, the chemical synthesis works of several representative sesquiterpenes were provided. This review aims to provide an up-to-date knowledge of the chemical structures, pharmacological activities, and chemical synthesis of the chemical constituents from soft corals of the genus Capnella.

Iron-Catalyzed Coupling of Alkenes and Enones: Sakurai-Michael-type Conjugate Addition of Catalytic Allyliron Nucleophiles

The iron-catalyzed coupling of alkenes and enones through allylic C(sp3)-H functionalization is reported. This redox-neutral process employs a cyclopentadienyliron(II) dicarbonyl catalyst and simple alkene substrates to generate catalytic allyliron intermediates for 1,4-addition to chalcones and other conjugated enones. The use of 2,4,6-collidine as the base and a combination of triisopropylsilyl triflate and LiNTf2 as Lewis acids was found to facilitate this transformation under mild, functional group-tolerant conditions. Both electronically unactivated alkenes as well as allylbenzene derivatives could be employed as pronucleophilic coupling partners, as could a range of enones bearing electronically varied substituents.

Six-Step Total Synthesis of Isohirsut-4-ene through [5+2+1] Cycloaddition and Transannular Epoxide-Alkene Cyclization

A six-step total synthesis of isohirsut-4-ene with a 5/5/5 tricyclic core has been achieved. The synthesis features a Rh(I)-catalyzed [5+2+1] cycloaddition, a Corey-Chaykovsky reaction, and a transannular epoxide-alkene cyclization that afford the skeleton of the target molecule. This three-step strategy was further utilized to synthesize more 5/5/5 tricyclic analogues with one or two bridgehead quaternary centers.

Crystal Structure of Cucumene Synthase, a Terpenoid Cyclase That Generates a Linear Triquinane Sesquiterpene

Linear triquinanes are sesquiterpene natural products with hydrocarbon skeletons consisting of three fused five-membered rings. Importantly, several of these compounds exhibit useful anticancer, anti-inflammatory, and antibiotic properties. However, linear triquinanes pose significant challenges to organic synthesis because of the structural and stereochemical complexity of their hydrocarbon skeletons. To illuminate nature's solution to the generation of linear triquinanes, we now describe the crystal structure of Streptomyces clavuligerus cucumene synthase. This sesquiterpene cyclase catalyzes the stereospecific cyclization of farnesyl diphosphate to form a linear triquinane product, (5 S,7 S,10 R,11 S)-cucumene. Specifically, we report the structure of the wild-type enzyme at 3.05 Å resolution and the structure of the T181N variant at 1.96 Å resolution, both in the open active site conformations without any bound ligands. The high-resolution structure of T181N cucumene synthase enables inspection of the active site contour, which adopts a three-dimensional shape complementary to a linear triquinane. Several aromatic residues outline the active site contour and are believed to facilitate cation-π interactions that would stabilize carbocation intermediates in catalysis. Thus, aromatic residues in the active site not only define the template for catalysis but also play a role in reducing activation barriers in the multistep cyclization cascade.

Linear Triquinane Sesquiterpenoids: Their Isolation, Structures, Biological Activities, and Chemical Synthesis

Linear triquinane sesquiterpenoids represent an important class of natural products. Most of these compounds were isolated from fungi, sponges, and soft corals, and many of them displayed a wide range of biological activities. On account of their structural diversity and complexity, linear triquinane sesquiterpenoids present new challenges for chemical structure identification and total synthesis. 118 linear triquinane sesquiterpenoids were classified into 8 types, named types I–VIII, based on the carbon skeleton and the position of carbon substituents. Their isolation, structure elucidations, biological activities, and chemical synthesis were reviewed. This paper cited 102 articles from 1947 to 2018.

Intramolecular [2 + 2] Cycloadditions of Alkyl(phenylthio)ketenes: Total Synthesis of (+)-Sphaerodiol

Asymmetric total synthesis of (+)-sphaerodiol (2) has been achieved. A key step is an intramolecular [2 + 2] cycloaddition of alkyl(phenylthio)ketene for rapid assembly of the decalin ring.

Exploration of Ring Rearrangement Metathesis Reaction: A General and Flexible Approach for the Rapid Construction [5,n]-Fused Bicyclic Systems en Route to Linear Triquinanes

Structurally diverse [5,n] bicyclic systems with cis ring junction stereochemistry were accessed readily through RRM (ring rearrangement metathesis) reaction of properly functionalized [2.2.1]norbornene skeletons. Several bicyclic enones, ketones, alcohols, and ethers acted as the substrates and yielded the respective linearly fused [5,n] bicyclic systems stereoselectively after the RRM reaction. Such [5,5]bicyclic enone scaffolds were then synthetically manipulated to core structural analogue of naturally occurring liner triquinane hirsutene having cis-syn-cis stereochemistry.

Total synthesis of (±)-chondrosterin I using a desymmetric aldol reaction

The first total synthesis of racemic chondrosterin I was accomplished featuring a desymmetric intramolecular aldol reaction of the meso diketoester compound.