Total Syntheses of (+)-Chloropuupehenone and (+)-Chloropuupehenol and Their Analogues and Evaluation of Their Bioactivities

Siccanin-related drimane meroterpenoids: biological activities and synthesis

Covering: 1962 to 2023Drimane (hydro)quinones biosynthetically arise from the combination of drimane-type terpenoids with phenols or equivalents. Since the isolation of siccanin in 1962 (structure identified in 1967), over 200 natural drimane (hydro)quinones have been reported. They are widespread with remarkably diverse architectures and biological functions, which are achieved by varying either the drimane subunit, hydroquinone segment, or the fusion types of drimane and hydroquinone segment both of them. This type of natural products has drawn increasing attention in the discovery of novel pharmaceutical leads. Enormous efforts have been devoted to developing efficient and divergent synthesis approaches to facilitate the SAR study of drimane (hydro)quinones, aiming for more promising functional leads. This review is arranged mainly in terms of scaffold types of drimane (hydro)quinones and further categorized on the basis of biological functions. The mechanisms of action are also briefly discussed. Synthetic methods are categorized according to the strategies forging the Csp2-Csp3 linker between drimane segments and (hydro)quinone subunits.

Divergent and Stereoselective Synthesis of Ustusal A, (-)-Albrassitriol, and Elegansin D

The first synthesis of ustusal A as well as expeditious access to (-)-albrassitriol is described as featuring a singlet oxygen [4 + 2] cycloaddition, achieving the desired stereoselectivity for the 1,4-cis-hydroxyl groups. Transformation of (+)-sclareolide to III followed by a key Horner-Wadsworth-Emmons (HWE) reaction and stereospecific allylic oxidation facilitated the first synthesis of elegansin D. The biological evaluation of these natural products together with seven elegansin D analogues was performed, among which several elegansin D analogues exhibited potential anticancer activity against liver cancer HepG2 cells (IC50 = 11.99-25.58 μM) with low cytotoxicity on normal liver HL7702 cells (IC50 > 100 μM).

Synthesis of Aminoalkyl Sclareolide Derivatives and Antifungal Activity Studies

Sclareolide was developed as an efficient C-nucleophilic reagent for an asymmetric Mannich addition reaction with a series of N-tert-butylsulfinyl aldimines. The Mannich reaction was carried out under mild conditions, affording the corresponding aminoalkyl sclareolide derivatives with up to 98% yield and 98:2:0:0 diastereoselectivity. Furthermore, the reaction could be performed on a gram scale without any reduction in yield and diastereoselectivity. Additionally, deprotection of the obtained Mannich addition products to give the target sclareolide derivatives bearing a free N-H group was demonstrated. In addition, target compounds 4-6 were subjected to an antifungal assay in vitro, which showed considerable antifungal activity against forest pathogenic fungi.

Advancing the Logic of Chemical Synthesis: C-H Activation as Strategic and Tactical Disconnections for C-C Bond Construction

The design of synthetic routes by retrosynthetic logic is decisively influenced by the transformations available. Transition-metal-catalyzed C-H activation has emerged as a powerful strategy for C-C bond formation, with myriad methods developed for diverse substrates and coupling partners. However, its uptake in total synthesis has been tepid, partially due to their apparent synthetic intractability, as well as a lack of comprehensive guidelines for implementation. This Review addresses these issues and offers a guide to identify retrosynthetic opportunities to generate C-C bonds by C-H activation processes. By comparing total syntheses accomplished using traditional approaches and recent C-H activation methods, this Review demonstrates how C-H activation enabled C-C bond construction has led to more efficient retrosynthetic strategies, as well as the execution of previously unattainable tactical maneuvers. Finally, shortcomings of existing processes are highlighted; this Review illustrates how some highlighted total syntheses can be further economized by adopting next-generation ligand-enabled approaches.

Broad-spectrum antifungal activities and mechanism of drimane sesquiterpenoids

Eight drimane sesquiterpenoids including (-)-drimenol and (+)-albicanol were synthesized from (+)-sclareolide and evaluated for their antifungal activities. Three compounds, (-)-drimenol, (+)-albicanol, and (1R,2R,4aS,8aS)-2-hydroxy-2,5,5,8a-tetramethyl-decahydronaphthalene-1-carbaldehyde (4) showed strong activity against C. albicans. (-)-Drimenol, the strongest inhibitor of the three, (at concentrations of 8 – 64 µg/ml, causing 100% death of various fungi), acts not only against C. albicans in a fungicidal manner, but also inhibits other fungi such as Aspergillus, Cryptococcus, Pneumocystis, Blastomyces, Saksenaea and fluconazole resistant strains of C. albicans, C. glabrata, C. krusei, C. parapsilosis and C. auris. These observations suggest that drimenol is a broad-spectrum antifungal agent. At a high concentration (100 μg/ml) drimenol caused rupture of the fungal cell wall/membrane. In a nematode model of C. albicans infection, drimenol rescued the worms from C. albicans-mediated death, indicating drimenol is tolerable and bioactive in metazoans. Genome-wide fitness profiling assays of both S. cerevisiae (nonessential homozygous and essential heterozygous) and C. albicans (Tn-insertion mutants) collections revealed putative genes and pathways affected by drimenol. Using a C. albicans mutant spot assay, the Crk1 kinase associated gene products, Ret2, Cdc37, and orf19.759, orf19.1672, and orf19.4382 were revealed to be involved in drimenol's mechanism of action. The three orfs identified in this study are novel and appear to be linked with Crk1 function. Further, computational modeling results suggest possible modifications of the structure of drimenol, including the A ring, for improving the antifungal activity.

Total syntheses of (−)-15-oxopuupehenol and (+)-puupehenone and formal syntheses of (−)-puupehenol and (+)-puupehedione

A concise collective synthesis of (−)-15-oxopuupehenol, (+)-puupehenone, (−)-puupehenol and (+)-puupehedione has been accomplished.

Aliphatic C-H Oxidations for Late-Stage Functionalization

The atomistic change of C( sp3)-H to C( sp3)-O can have a profound impact on the physical and biological properties of small molecules. Traditionally, chemical synthesis has relied on pre-existing functionality to install new functionality, and directed approaches to C-H oxidation are an extension of this logic. The impact of developing undirected C-H oxidation reactions with controlled site-selectivity is that scientists gain the ability to diversify complex structures at sites remote from existing functionality, without having to carry out individual de novo syntheses. This Perspective offers a historical view of why, as recently as 2007, it was thought that the differences between aliphatic C-H bonds of the same bond type (for example, 2° aliphatic) were not large enough to distinguish them preparatively with small-molecule catalysis in the absence of directing groups or molecular recognition elements. We give an account of the discovery of Fe(PDP)-catalyzed non-directed aliphatic C-H hydroxylations and how the electronic, steric, and stereoelectronic rules for predicting site-selectivity that emerged have affected a shift in how the chemical community views the reactivity among these bonds. The discovery that site-selectivity could be altered by tuning the catalyst [i.e., Fe(CF3-PDP)] with no changes to the substrate or reaction now gives scientists the ability to exert control on the site of oxidation on a range of functionally and topologically diverse compounds. Collectively, these findings have made possible the emerging area of late-stage C-H functionalizations for streamlining synthesis and derivatizing complex molecules.

Pleiotropic Role of Puupehenones in Biomedical Research

Marine sponges represent a vast source of metabolites with very interesting potential biomedical applications. Puupehenones are sesquiterpene quinones isolated from sponges of the orders Verongida and Dictyoceratida. This family of chemical compounds is composed of a high number of metabolites, including puupehenone, the most characteristic compound of the family. Chemical synthesis of puupehenone has been reached by different routes, and the special chemical reactivity of this molecule has allowed the synthesis of many puupehenone-derived compounds. The biological activities of puupehenones are very diverse, including antiangiogenic, antitumoral, antioxidant, antimicrobial, immunomodulatory and antiatherosclerotic effects. Despite the very important roles described for puupehenones concerning different pathologies, the exact mechanism of action of these compounds and the putative therapeutic effects in vivo remain to be elucidated. This review offers an updated and global view about the biology of puupehenones and their therapeutic possibilities in human diseases such as cancer.

Late-Stage Terpene Cyclization by an Integral Membrane Cyclase in the Biosynthesis of Isoprenoid Epoxycyclohexenone Natural Products

Macrophorins are representative examples of isoprenoid epoxycyclohexenones containing cyclized drimane moieties. We located and characterized the biosynthetic gene cluster of macrophorin from Penicillium terrestris. MacJ encoded by this cluster was characterized to be the first example of a membrane-bound type-II terpene cyclase catalyzing the cyclization of meroterpenoids via direct protonation of the terminal olefinic bond in acyclic yanuthones. The late-stage functionalization and substrate promiscuity of MacJ make it a potential biocatalyst for the synthesis of macrophorin analogues.

Silver-catalyzed carbon–selenium cross-coupling using N-(phenylseleno)phthalimide: an alternate approach to the synthesis of organoselenides

Silver(I) catalyzed phenylselenylation of terminal alkynes and organoboronic acids has been demonstrated using N-(phenylseleno)phthalimide as an electrophilic SePh donor. A wide variety of terminal alkynes and organoboronic acids are selenylated efficiently to produce the corresponding alkynyl and diaryl selenides, respectively, in good yields. Silver(I) acts as a Lewis acid in this process.

Asymmetric synthesis of chromans via the Friedel–Crafts alkylation–hemiketalization catalysed by an N,N′-dioxide scandium(iii) complex

The direct and sequential Friedel–Crafts alkylation/hemiketalization procedures for the synthesis of chiral chromans resulted in excellent yields, high enantioselectivities and high diastereoselectivities for a broad range of substrates.

Synthesis of the putative structure of 15-oxopuupehenoic acid

Synthesis of the putative structure of the marine natural 15-oxopuupehenoic acid has been achieved starting from commercial (-)-sclareol. Key steps of the synthetic sequence are the Robinson annulation of a β-ketoester and methyl vinyl ketone and an unprecedented cyclization of the resulting α,β-enone, which is mediated by tin(IV) chloride in the presence of N-phenylselenophthalimide. The physical properties of the synthetic compound are somewhat different from those reported for the natural product.

Synthesis and cytotoxic properties of novel (E)-3-benzylidene-7-methoxychroman-4-one derivatives

Background and the purpose of the study

There has been increscent interest in the field of cancer chemotherapy by discovery and development of novel agents with high efficacy, low toxicity, and minimum side effects. In order to find new anticancer agents, we replaced the pyrazolone part of well-known cytotoxic agent SJ-172550 with 7-methoxychroman-4-one. Thus, a novel series of 3-benzylidene-4-chromanones were synthesized and tested in vitro against human cancer cell lines.

Methods

The title compounds were prepared by condensation of 7-methoxychroman-4-one with suitable aldehydes in appropriate alcohol in the presence of gaseous HCl. The antiproliferative activity of target compounds were evaluated against MDA-MB-231 (breast cancer), KB (nasopharyngeal epidermoid carcinoma) and SK-N-MC (human neuroblastoma) cell lines using MTT assay.

Results

Although the direct analog of SJ-172550 (compound 5d) did not show any cytotoxic activity against tested cell lines, but 2-(2-chloro-6-methoxyphenoxy)acetic acid methyl ester analog 5c showed some activity against MDA-MB-231 and SK-N-MC cells. Further modification of compound 5c resulted in the 3-chloro-4,5-dimethoxybenzylidene derivative 5b which demonstrated better cytotoxic profile against all tested cell lines (IC₅₀ values = 7.56–25.04 μg/ml).

Conclusion

The results demonstrated that the cytotoxic activity of compound 5b against MDA-MB-231 and SK-N-MC cells is more than etoposide. Therefore, compound 5b prototype could be considered as novel cytotoxic agent for further developing new anticancer chemotherapeutics.

Inhibition of Acyl-CoA: cholesterol acyltransferase (ACAT), overexpression of cholesterol transporter gene, and protection of amyloid β (Aβ) oligomers-induced neuronal cell death by tricyclic pyrone molecules

A major effort in Alzheimer's disease therapeutic development has targeted Aβ and downstream events. We have synthesized a small library of tricyclic pyrone compounds. Their protective action in MC65 cells and inhibition of ACAT along with the upregulation of cholesterol transporter gene were investigated. Five active compounds exhibited potencies in the nanomolar ranges. The multiple effects of the compounds on Aβ and cellular cholesterol pathways could be potential mechanisms underlying the protective effects in vivo.

Redox potentials, laccase oxidation, and antilarval activities of substituted phenols

Laccases are copper-containing oxidases that are involved in sclerotization of the cuticle of mosquitoes and other insects. Oxidation of exogenous compounds by insect laccases may have the potential to produce reactive species toxic to insects. We investigated two classes of substituted phenolic compounds, halogenated di- and trihydroxybenzenes and substituted di-tert-butylphenols, on redox potential, oxidation by laccase and effects on mosquito larval growth. An inverse correlation between the oxidation potentials and laccase activity of halogenated hydroxybenzenes was found. Substituted di-tert-butylphenols however were found to impact mosquito larval growth and survival. In particular, 2,4-di-tert-butyl-6-(3-methyl-2-butenyl)phenol (15) caused greater than 98% mortality of Anophelesgambiae larvae in a concentration of 180nM, whereas 2-(3,5-di-tert-butyl-4-hydroxyphenyl)-2-methylpropanal oxime (13) and 6,8-di-tert-butyl-2,2-dimethyl-3,4-dihydro-2H-chromene (33) caused 93% and 92% mortalities in concentrations of 3.4 and 3.7μM, respectively. Larvae treated with di-tert-butylphenolic compounds died just before pupation.

HL005--a new selective PPARγ antagonist specifically inhibits the proliferation of MCF-7

Peroxisome proliferator-activated receptor-γ (PPARγ) is a nuclear transcription factor which is involved in many diseases, such as diabetes, inflammation, dyslipidemia, hypertension, and cancer. Recently, there are many reports showing that PPARγ agonists have preclinical and clinical anticancer activity, with relatively few reports on anticancer effects of PPARγ antagonists. From our compound library, a novel 3-thiazolinone-modified benzoic acid derivative HL005 is found as PPARγ selective ligand through SPR analysis (K(D)=0.21 μM), yeast two-hybrid results suggest that HL005 antagonize the potent PPARγ agonist rosiglitazone-induced recruitment of the coactivator for PPARγ (IC(50)=7.97 μM). Different from the most reported PPARγ antagonist, HL005 can inhibit the proliferation of MCF-7 cell line in a concentration-dependent manner and induce cell cycle arrest at G2/M phase, other than interference with cell adhesion. In order to study the binding mode of this compound, three derivatives are synthesized to get more detail about the structure-activity relationship, molecular docking and the NMR spectra indicate that similar to most PPARγ ligand, the carboxylic acid group is an important moiety for HL005 and contributes strong interaction with PPARγ.

Design and synthesis of carbon-11-labeled dual aromatase-steroid sulfatase inhibitors as new potential PET agents for imaging of aromatase and steroid sulfatase expression in breast cancer

Aromatase and steroid sulfatase (STS) are particularly attractive targets in the treatment of estrogen-receptor-positive breast cancer and the development of enzyme-based cancer imaging agents for the biomedical imaging technique positron emission tomography (PET). New carbon-11-labeled sulfamate derivatives were first designed and synthesized as potential PET dual aromatase-steroid sulfatase inhibitor (DASSI) radiotracers for imaging of aromatase and STS expression in breast cancer. The target tracers 5-(((4-cyanophenyl)(4H-1,2,4-triazol-4-yl)amino)methyl)-2-[(11)C]methoxyphenyl sulfamate ([(11)C]8a) and 4-(((4-cyanophenyl)(4H-1,2,4-triazol-4-yl)amino)methyl)-2-[(11)C]methoxyphenyl sulfamate ([(11)C]8b) were prepared from their corresponding precursors 5-(((4-cyanophenyl)(4H-1,2,4-triazol-4-yl)amino)methyl)-2-hydroxyphenyl sulfamate (16) and 4-(((4-cyanophenyl)(4H-1,2,4-triazol-4-yl)amino)methyl)-2-hydroxyphenyl sulfamate (21) with [(11)C]CH(3)OTf under basic conditions through the O-[(11)C]methylation and isolated by the reversed-phase high pressure liquid chromatography (HPLC) method in 30-45% radiochemical yields based on [(11)C]CO(2) and decay corrected to end of bombardment (EOB). The specific activity at end of synthesis (EOS) was 111-185GBq/micromol.

Substrate specificity of the adenylation enzyme SgcC1 involved in the biosynthesis of the enediyne antitumor antibiotic C-1027

C-1027 is an enediyne antitumor antibiotic composed of a chromophore with four distinct chemical moieties, including an (S)-3-chloro-4,5-dihydroxy-beta-phenylalanine moiety that is derived from l-alpha-tyrosine. SgcC4, a novel aminomutase requiring no added co-factor that catalyzes the formation of the first intermediate (S)-beta-tyrosine and subsequently SgcC1 homologous to adenylation domains of nonribosomal peptide synthetases, was identified as specific for the SgcC4 product and did not recognize any alpha-amino acids. To definitively establish the substrate for SgcC1, a full kinetic characterization of the enzyme was performed using amino acid-dependent ATP-[(32)P]PP(i) exchange assay to monitor amino acid activation and electrospray ionization-Fourier transform mass spectroscopy to follow the loading of the activated beta-amino acid substrate to the peptidyl carrier protein SgcC2. The data establish (S)-beta-tyrosine as the preferred substrate, although SgcC1 shows promiscuous activity toward aromatic beta-amino acids such as beta-phenylalanine, 3-chloro-beta-tyrosine, and 3-hydroxy-beta-tyrosine, but all were <50-fold efficient. A putative active site mutant P571A adjacent to the invariant aspartic acid residue of all alpha-amino acid-specific adenylation domains known to date was prepared as a preliminary attempt to probe the substrate specificity of SgcC1; however the mutation resulted in a loss of activity with all substrates except (S)-beta-tyrosine, which was 142-fold less efficient relative to the wild-type enzyme. In total, SgcC1 is now confirmed to catalyze the second step in the biosynthesis of the (S)-3-chloro-4,5-dihydroxy-beta-phenylalanine moiety of C-1027, presenting downstream enzymes with an (S)-beta-tyrosyl-S-SgcC2 thioester substrate, and represents the first beta-amino acid-specific adenylation enzyme characterized biochemically.

An Unusual Cationic [2 + 2] Cycloaddition in a Divergent Total Synthesis of Hongoquercin A and Rhododaurichromanic Acid A

An unusual polyene cyclization pathway that led to a divergent total synthesis of hongoquercin A and rhododaurichromanic acid A is described here. This work uncovered a unique cationic cyclobutane formation that could be relevant to the biosynthetic pathway for the formation of cyclobutane containing terpenoids in addition to rhododaurichromanic acids.

Marine natural products

This review covers the literature published in 2004 for marine natural products, with 693 citations (491 for the period January to December 2004) referring to compounds isolated from marine microorganisms and phytoplankton, green algae, brown algae, red algae, sponges, coelenterates, bryozoans, molluscs, tunicates and echinoderms. The emphasis is on new compounds (716 for 2004), together with their relevant biological activities, source organisms and country of origin. Biosynthetic studies (8), and syntheses (80), including those that lead to the revision of structures or stereochemistries, have been included.