Application of Metal-Free Dearomatization Reaction as a Sustainable Strategy to Direct Access Complex Cyclic Compounds

The highly efficient construction of complicated heterocyclic frameworks in an atom- and step-economic manner is still one of the cores of synthetic chemistry. Dearomatization reactions show the unique advantage for the construction of functionalized heterocycles and have attracted widespread attention over the past two decades. The metal-free approach has proved to be a green and sustainable paradigm for the synthesis of spirocyclic, polycyclic and heterocyclic scaffolds, which are widely present in natural products and bioactive molecules. In this review, the advances in the recent six years (2017-2023) in metal-free dearomatization reactions are highlighted. Emphasis is placed on developments in the field of organo-catalyzed dearomatization reactions, oxidative dearomatization reactions, Brønsted acid- or base-promoted dearomatization reactions, photoredox-catalyzed dearomatization reactions, and electrochemical oxidation dearomatization reactions.

Concise Total Synthesis of (+)-Lanceolactone A: Revision of Absolute Stereochemistry

A chiral-pool protecting-group-free five-step total synthesis of tetranorsesquiterpenoide (+)-lanceolactone A and all of its four stereoisomers using (S)-(+)-, and (R)-(-)-linalool (coriandrol) as building blocks is disclosed. The key steps involved in this synthetic route are regioselective ozonolysis, Au(I)-catalyzed cycloisomerization-induced construction of furan from alleneone, and dye-sensitized photo-oxidation (through ¹O₂; singlet oxygen) of hydroxyalkyl-tethered furan to access oxaspirolactone. After a thorough evaluation of electronic circular dichroism (ECD) and optical rotation data of all possible stereoisomers, the absolute configuration of natural lanceolactone A at the C4 and C7 positions has been assigned as (+)-(4S,7S), which is an enantiomer to the initially proposed structure (+)-(4R,7R). Further, these investigations led us to extend Feringa and Gawronski's CD correlation method to [5,5]- and [6,5]-oxaspirolactones.

Collective Synthesis of Illudalane Sesquiterpenes via Cascade Inverse Electron Demand (4 + 2) Cycloadditions of Thiophene S,S-Dioxides

Thiophene S,S-dioxides are underutilized tools for the de novo construction of benzene rings in organic synthesis. We report a collective synthesis of nine illudalane sesquiterpenes using bicyclic thiophene S,S-dioxides as generalized precursors to the indane core of the natural products. Exploiting furans as unusual dienophiles in this inverse electron demand Diels-Alder cascade, this concise and convergent approach enables the synthesis of these targets in as little as five steps. Theoretical studies rationalize the reactivity of thiophene S,S-dioxides with both electron-poor and electron-rich dienophiles and reveal reaction pathways involving either nonpolar pericyclic or bifurcating ambimodal cycloadditions. Overall, this work demonstrates the wider potential of thiophene S,S-dioxides as convenient and flexible precursors to polysubstituted arenes.

Actualities in the phytochemical research on selected terpenes

A short review of our recent research on the essential oil phytochemical composition of Petasites albus (L.) Gaertn. and Petasites hybridus (L.) G. Gaertn., B. Mey. & Scherb. (Asteraceae) as well as on the oils of Globularia cordifolia L., Globularia meridionalis (Podp.) O. Schwarz and Globularia punctata Lapeyr. (Plantaginaceae) is presented. All essential oils contained a variety of oxygenated sesquiterpenes among their major constituents, including a bakkane type sesquiterpene fukinanolid (bakkenolide A). The paper is focused on: i) a short overview of the abundance of major terpenes in the essential oils of Petasites and Globularia species from Croatia; ii) possible biosynthetic pathways of major identified sesquiterpenes; and iii) biological activities (literature data) of major sesquiterpenes from Petasites and Globularia species.

Two New Eudesmane Sesquiterpenoids from the Flowers of Chrysanthemum indicum

The flowers of Chrysanthemum indicum, i.e. Ye-ju-hua recorded in the Chinese Pharmacopoeia, has been widely used in China as an important heat-clearing and detoxifying herb for the treatment of inflammation, headache, and vertigo. A phytochemical investigation of this herb has led to the isolation of two new eudesmane sesquiterpenoids, 7-epi-eudesm-4(15),11(13)-diene-1β,3β-diol (1) and 7-epi-1β-hydroxy-β-eudesmol (2). The molecular structures of these new sesquiterpenoids were established based on the comprehensive spectroscopic analyses, including NMR, MS, and IR, and comparing with the literatures.

Total Synthesis of Highly Oxygenated Bisabolane Sesquiterpene Isolated from Ligularia lankongensis: Relative and Absolute Configurations of the Natural Product

The relative and absolute configurations of an oxygenated bisabolane natural product, isolated from Ligularia lankongensis, were determined by synthesis. All four possible stereoisomers and their tiglate analogues were synthesized from R-(-)-carvone, and their ¹H and ¹³C NMR spectra were compared to establish the 6R,8S,10S configuration. The stereoselective synthesis of the natural product was also achieved, featuring Brown allylation, vanadium-catalyzed epoxidation, and the Mitsunobu reaction.

Asymmetric Synthesis of Rupestonic Acid and Pechueloic Acid

In this report, the originally proposed rupestonic acid (5) and pechueloic acid (3) were efficiently synthesized. The chiral lactone 13, recycled from the degradation of saponin glycosides, was utilized to prepare the key chiral fragment 11. During the exploration of this convergent assembly strategy, the ring-closing metathesis (RCM), SmI₂-prompted intermolecular addition, and [2,3]-Wittig rearrangement proved to be effective transformations for the synthesis of subunits.

Phytometabolite Dehydroleucodine Induces Cell Cycle Arrest, Apoptosis, and DNA Damage in Human Astrocytoma Cells through p73/p53 Regulation

Accumulating evidence supports the idea that secondary metabolites obtained from medicinal plants (phytometabolites) may be important contributors in the development of new chemotherapeutic agents to reduce the occurrence or recurrence of cancer. Our study focused on Dehydroleucodine (DhL), a sesquiterpene found in the provinces of Loja and Zamora-Chinchipe. In this study, we showed that DhL displayed cytostatic and cytotoxic activities on the human cerebral astrocytoma D384 cell line. With lactone isolated from Gynoxys verrucosa Wedd, a medicinal plant from Ecuador, we found that DhL induced cell death in D384 cells by triggering cell cycle arrest and inducing apoptosis and DNA damage. We further found that the cell death resulted in the increased expression of CDKN1A and BAX proteins. A marked induction of the levels of total TP73 and phosphorylated TP53, TP73, and γ-H2AX proteins was observed in D384 cells exposed to DhL, but no increase in total TP53 levels was detected. Overall these studies demonstrated the marked effect of DhL on the diminished survival of human astrocytoma cells through the induced expression of TP73 and phosphorylation of TP73 and TP53, suggesting their key roles in the tumor cell response to DhL treatment.

Metal-Free Oxidative Spirocyclization of Alkynes with Sulfonylhydrazides Leading to 3-Sulfonated Azaspiro[4,5]trienones

A novel and direct oxidative spirocyclization of arylpropiolamides with sulfonylhydrazides leading to 3-sulfonated azaspiro[4,5]trienones has been developed under metal-free conditions. The reaction is performed in a tandem manner constituted by the sequential sulfonylation of alkynes, ipso-carbocyclization, dearomatization, hydration, and oxidation processes, providing a convenient and efficient approach to various sulfonated azaspiro[4,5] trienones of biological importance.

Penarines A-F, (nor-)sesquiterpene carboxylic acids from Hygrophorus penarius (Basidiomycetes)

Five sesquiterpene carboxylic acids (1-5) and one nor-sesquiterpene carboxylic acid (6) of the very rare ventricosane type, named penarines A-F, were isolated from fruiting bodies of the basidiomycete Hygrophorus penarius (Hygrophoraceae). This is the first report of (nor)-sesquiterpenes isolated from basidiocarps of the family Hygrophoraceae. Their structures were elucidated on the basis of extensive 1D ((1)H, (13)C) and 2D (HSQC, HMBC, COSY, ROESY) NMR spectroscopic analyses as well as high-resolution mass spectrometry studies. Additionally, the only known member of this rare type of sesquiterpenes, ventricos-7(13)-ene (7), could be identified via headspace GC-MS analysis in a fruiting body of H. penarius. Compounds 1-6 were devoid of remarkable antifungal activity against Cladosporium cucumerinum. Additionally, the cytotoxic activities of compounds 1 and 2 were evaluated against the human prostate cancer cell line PC-3 and the colon cancer cell line HT-29 showing no significant cytotoxic activity.

Heterocyclic terpenes: linear furano- and pyrroloterpenoids

This review of furano- and pyrroloterpenoids covers the literature, 180 articles in all, published from January 2006 to December 2013.

Natural sesquiterpenoids

This review covers the isolation, structural determination, synthesis and chemical and microbiological transformations of natural sesquiterpenoids. The literature from January to December 2012 is reviewed, and 471 references are cited.

De novo synthesis of high-value plant sesquiterpenoids in yeast

Terpenoids comprise a structurally diverse group of natural products. Despite various and important uses of terpenoids (e.g., flavors, drugs, and nutraceuticals), most of them are, however, still extracted from plant sources, which suffer from high cost and low yield. Alternatively, terpenoids can be produced in microbes using their biosynthetic genes. With the explosion of sequence data, many genes for terpenoid metabolism can be characterized by biochemical approaches and used for the microbial production of terpenoids. However, substrates for in vitro studies of terpene synthases are costly, and the enzymatic synthesis of terpenoids in vitro using recombinant enzymes is insufficient to meet the chemical characterization need. Here, we describe the use of engineered yeast (EPY300) to evaluate in vivo production of sesquiterpenoids. Two sesquiterpene synthase genes (for valencene and 5-epi-aristolochene synthases) were expressed in EPY300 in native and N-terminal thioredoxin fusion forms. By using the thioredoxin fusion, valencene biosynthesis was slightly decreased; however, the production of 5-epi-aristolochene was increased by 10-fold, producing 420 μg mL(-1) of 5-epi-aristolochene. Accordingly, the thioredoxin-fused 5-epi-aristolochene was coexpressed with 5-epi-aristolochene dihydroxylase (cytochrome P450 monooxygenase) and its reductase in EPY300. This combinatorial expression yielded hydroxylated sesquiterpene, capsidiol, at ~250 μg mL(-1). Detailed experimental procedures and other considerations for this work are given.

Diketopiperazines from marine organisms

Diketopiperazines (DKPs), which are cyclic dipeptides, have been detected in a variety of natural resources. Recently, the interest in these compounds increased significantly because of their remarkable bioactivity. This review deals with the chemical structures, biosynthetic pathways, and biological activities of DKPs from marine microorganisms, sponges, sea stars, tunicates (ascidians), and red algae. The literature has been covered up to December 2008, and a total 124 DKPs from 104 publications have been discussed and reviewed. Some of these compounds have been found to possess various bioactivities including cytotoxicity, and antibacterial, antifungal, antifouling, plant-growth regulatory, and other activities.

Ribosomal peptide natural products: bridging the ribosomal and nonribosomal worlds

Ribosomally synthesized bacterial natural products rival the nonribosomal peptides in their structural and functional diversity. The last decade has seen substantial progress in the identification and characterization of biosynthetic pathways leading to ribosomal peptide natural products with new and unusual structural motifs. In some of these cases, the motifs are similar to those found in nonribosomal peptides, and many are constructed by convergent or even paralogous enzymes. Here, we summarize the major structural and biosynthetic categories of ribosomally synthesized bacterial natural products and, where applicable, compare them to their homologs from nonribosomal biosynthesis.

Catalytic enantioselective approach to the eudesmane sesquiterpenoids: total synthesis of (+)-carissone

A catalytic enantioselective approach to the eudesmane sesquiterpenoids is reported. The strategic use of a palladium-catalyzed enantioselective alkylation of vinylogous ester substrates forged the C(10) all-carbon quaternary center. This key transformation enabled a diastereoselective olefin hydrogenation to create the syn stereochemistry at C(7). The devised synthetic strategy allowed for the preparation of the antibacterial agent (+)-carissone and a formal synthesis of the P/Q-type calcium channel blocker (-)-alpha-eudesmol.

Marine natural products

This review covers the literature published in 2007 for marine natural products, with 948 citations(627 for the period January to December 2007) referring to compounds isolated from marine microorganisms and phytoplankton, green algae, brown algae, red algae, sponges, cnidarians,bryozoans, molluscs, tunicates, echinoderms and true mangrove plants. The emphasis is on new compounds (961 for 2007), together with the relevant biological activities, source organisms and country of origin. Biosynthetic studies, first syntheses, and syntheses that lead to the revision of structures or stereochemistries, have been included.1 Introduction, 2 Reviews, 3 Marine microorganisms and phytoplankton, 4 Green algae, 5 Brown algae, 6 Red algae, 7 Sponges, 8 Cnidarians, 9 Bryozoans, 10 Molluscs, 11 Tunicates (ascidians),12 Echinoderms, 13 Miscellaneous, 14 Conclusion, 15 References.

Natural sesquiterpenoids

This review covers the isolation, structural determination, synthesis and chemical and microbiological transformations of natural sesquiterpenoids. 423 references are cited.

Reaction of Furanoeremophilans with Pyridoxal

Furanoeremophilan-10β-ol, a major component of Ligularia cymbulifera growing in northwestern Yunnan Province of China, reacted with pyridoxal or its 5′-phosphate at room temperature in aqueous solution to afford condensation products. It is plausible that the reaction also occurs in nature.