Scopranones with Two Atypical Scooplike Moieties Produced by Streptomyces sp. BYK-11038

The Carbene Chemistry of N-Sulfonyl Hydrazones: The Past, Present, and Future

N-Sulfonyl hydrazones have been extensively used as operationally safe carbene precursors in modern organic synthesis due to their ready availability, facile functionalization, and environmental benignity. Over the past two decades, there has been tremendous progress in the carbene chemistry of N-sulfonyl hydrazones in the presence of transition metal catalysts, under metal-free conditions, or using photocatalysts under photoirradiation conditions. Many carbene transfer reactions of N-sulfonyl hydrazones are unique and cannot be achieved by any alternative methods. The discovery of novel N-sulfonyl hydrazones and the development of highly enantioselective new reactions and skeletal editing reactions represent the notable recent achievements in the carbene chemistry of N-sulfonyl hydrazones. This review describes the overall progress made in the carbene chemistry of N-sulfonyl hydrazones, organized based on reaction types, spotlighting the current state-of-the-art and remaining challenges to be addressed in the future. Special emphasis is devoted to identifying, describing, and comparing the scope and limitations of current methodologies, key mechanistic scenarios, and potential applications in the synthesis of complex molecules.

Synthesis of 5-Amino-3(2H)-furanones via S-Methylation/Intramolecular Cyclization of γ-Sulfanylamides

The S-methylation/intramolecular cyclization of γ-sulfanylamide is depicted. Different methylating reagents were successfully employed for S-methylation, depending on the substituent pattern of the amide in the starting γ-sulfanylamides; trimethyloxonium tetrafluoroborate was used for N-aryl substituted γ-sulfanylamides, and the combination of methyl iodide and silver(I) tetrafluoroborate was used for N-alkyl substituted γ-sulfanylamides. When the resulting sulfonium salt was treated with DBU, it smoothly underwent intramolecular cyclization to produce a series of N-aryl, N-alkyl, N,N-dialkyl or N-alkyl-N-aryl substituted 5-amino-3(2H)-furanones in 55%-quantitative yields.

Palladium-Catalyzed Oxidative Cyclization of α-Allenols in the Presence of TBN: Access to 3(2H)-Furanones

A new palladium-catalyzed oxidative cyclization of α-allenols is described. The readily accessible α-allenols participate in intra-molecular oxidative cyclization in the presence of TBN to grant access to multisubstituted 3(2H)-furanones, which are common motifs in several biologically important natural products and pharmaceuticals.

Iodocycloisomerization/Nucleophile Addition Cascade Transformations of 1,2-Alkynediones

A general electrophilic iodocyclization/nucleophile addition cascade transformation for 1,2-alkynediones for the synthesis of various oxygen heterocycles and access to regioselective alkyne hydroxylation is reported. Furan-tethered ynediones resulted in the construction of exo-enol ethers via carbonyl-alkyne cyclization-initiated heteroarene dearomatization, whereas other (hetero)arene-, alkenyl-, and alkyl-tethered ynediones resulted in the formation of highly functionalized 3(2H)-furanones. Importantly, the developed domino protocols involve the construction of important heterocyclic scaffolds and installation of two functional groups in a single operation. Moreover, the use of water as a nucleophile resulted in regioselective alkyne hydroxylation via furanone ring opening. The developed protocols are characterized by a wide substrate scope, and their utility has been demonstrated by a number of postsynthetic transformations.

[Comprehensive Studies on the Synthetic Organic Chemistry of Unique Bioactive Natural Products; Total Synthesis, Drug Discovery, and Development of New Reactions]

Research on natural product chemistry via organic chemistry ranges from isolation and structural elucidation to total synthesis, drug discovery, and chemical biology. Discoveries in organic chemistry, such as novel reactions and synthetic strategies, are enabled by the studies of total synthesis. Thus, organic (synthetic) chemistry and natural product chemistry are correlated. We conducted comprehensive studies, including structure-activity relationship, drug discovery, and total synthesis studies, on the synthetic organic chemistry of natural products with unique biological activities and the development of novel reactions discovered through these products. This review describes the total synthesis of simpotentin, a novel potentiator of amphotericin B, and the development of the novel lactonization reactions of homopropargyl alcohols via intramolecular ketene trapping.

Gold(I)-Catalyzed Cycloisomerization-Indole Addition Cascade: Synthesis of 3(2H)-Furanone-Incorporated Unsymmetrical 3,3'- Bis(indolyl)methanes

An unprecedented Au(I)-catalyzed domino intramolecular carbonyl-alkyne cyclization/indole addition strategy has been disclosed here. This generalized strategy enables the synthesis of 3(2H)-furanone-incorporated unsymmetrical bis(indolyl)methanes with generation of a stereocenter at the furanone junction from easily accessible indole-tethered ynediones. In addition, this present protocol could also be extended for the synthesis of a number of indolyl-(hetero)arylmethanes by employing a variety of (hetero)arenes as a nucleophile coupling partner.

Discovery of prescopranone, a key intermediate in scopranone biosynthesis

A key intermediate in scopranone biosynthesis, prescopranone, accumulated in the mycelium of Streptomyces avermitilis SUKA carrying the biosynthetic gene cluster for scopranone lacking the sprT encoding the monooxygenase. The structure of prescopranone was elucidated by NMR and other spectral data. Prescopranone consists of a 2-pyranone ring with two atypical scoop-like moieties (1-ethyl-1-propenyl and 2-ethylbutyl groups), which was deduced as a product of the modular polyketide syntheses encoded by sprA, sprB, and sprC. Prescopranone inhibited bone morphogenetic protein (BMP)-induced alkaline phosphatase activity in a BMP receptor mutant cell line.

Inhibitory effects of sesquiterpene lactones from the Indonesian marine sponge Lamellodysidea cf. herbacea on bone morphogenetic protein-induced osteoblastic differentiation

A new unique sesquiterpene lactone, bicyclolamellolactone A (1), was isolated together with two known monocyclofarnesol-type sesquiterpenes, lamellolactones A (2) and B (3), from the Indonesian marine sponge Lamellodysidea sp. (cf. L. herbacea). The planar structure of 1 was assigned based on its spectroscopic data (1D and 2D NMR, HRESIMS, UV, and IR spectra). The relative and absolute configuration of 1 was determined by comparison of its calculated and experimental electronic circular dichroism spectra in combination with NOESY correlations. Compounds 1-3 inhibited bone morphogenic protein (BMP)-induced alkaline phosphatase activity in mutant BMP receptor-carrying C2C12 cells with IC₅₀ values of 51, 4.6, and 20 μM, respectively.

Screening for Small Molecule Inhibitors of BMP-Induced Osteoblastic Differentiation from Indonesian Marine Invertebrates

Fibrodysplasia ossificans progressiva (FOP) is a rare congenital disorder with heterotopic ossification (HO) in soft tissues. The abnormal activation of bone morphogenetic protein (BMP) signaling by a mutant activin receptor-like kinase-2 (ALK2) leads to the development of HO in FOP patients, and, thus, BMP signaling inhibitors are promising therapeutic applications for FOP. In the present study, we screened extracts of 188 Indonesian marine invertebrates for small molecular inhibitors of BMP-induced alkaline phosphatase (ALP) activity, a marker of osteoblastic differentiation in a C2C12 cell line stably expressing ALK2(R206H) (C2C12(R206H) cells), and identified five marine sponges with potent ALP inhibitory activities. The activity-guided purification of an EtOH extract of marine sponge Dysidea sp. (No. 256) resulted in the isolation of dysidenin (1), herbasterol (2), and stellettasterol (3) as active components. Compounds 1–3 inhibited ALP activity in C2C12(R206H) cells with IC₅₀ values of 2.3, 4.3, and 4.2 µM, respectively, without any cytotoxicity, even at 18.4–21.4 µM. The direct effects of BMP signaling examined using the Id1WT4F-luciferase reporter assay showed that compounds 1–3 did not decrease the reporter activity, suggesting that they inhibit the downstream of the Smad transcriptional step in BMP signaling.

Penicillic Acid Congener, a New Inhibitor of BMP-Induced Alkaline Phosphatase Activity in Myoblasts, Produced by the Fungus Penicillium sp. BF-0343

During our screening for microbial regulators of bone metabolism, a new compound, 6-ethoxy-5,6-dihydropenillic acid (1), was isolated together with a known and structurally related penicillic acid (2) from the culture broth of the soil-derived fungus Penicillium sp. BF-0343. The structure of 1 was elucidated by various spectroscopic data including nuclear magnetic resonance experiments. Compounds 1 and 2 dose-dependently inhibited bone morphogenetic protein–induced alkaline phosphatase activity in myoblasts with half-maximal inhibitory concentration values of 19.8 and 2.1 μM, respectively.

An Unusual Extender Unit Is Incorporated into the Modular Polyketide Synthase of Scopranones Biosynthesis

Scopranones, produced by Streptomyces sp. BYK-11038, are the novel bone morphogenetic protein inhibitors characterized by atypical two scoop-like moieties and a 3-furanone moiety. Two scoop-like moieties connected to a 3-furanone have not previously been reported in natural products, and their biosynthesis must occur via a unique pathway. Feeding experiments using ¹³C-labeled precursors indicated that scopranones were synthesized from three acetates and three butyrates in polyketide-type biosynthesis. Genome mining of Streptomyces sp. BYK-11038 revealed that the candidate biosynthetic gene cluster contains 21 open reading frames (ORFs), including three modular polyketide synthases (PKSs; SprA, SprB, and SprC), which were composed of 4 modules with one loading module and 18 additional ORFs (SprD to SprU) spanning a distance of 55 kbp. The characterization of in-frame deletion mutants and feeding experiments with the predicted extender units indicated that two genes, sprP and sprR, encoding discrete 3-oxoacyl-ACP synthases, and a gene, sprO, encoding crotonyl-CoA reductase, were involved in assembling an unusual C₈ branched extender unit, 2-(2-ethylbutyl)malonyl-CoA. Additionally, three ORFs, sprM, sprN, and sprT, encoding cytochrome P450s and a monooxygenase, are important tailoring enzymes in post-PKS modification. SprT is an essential enzyme for decarboxylative ring contraction via oxidation, which converts the 2-pyranone to a 3-furanone.

Recent Topics in Fibrodysplasia Ossificans Progressiva

Fibrodysplasia ossificans progressiva (FOP) is a rare genetic disease that is characterized by the formation of heterotopic bone tissues in soft tissues, such as skeletal muscle, ligament, and tendon. It is difficult to remove such heterotopic bones via internal medicine or invasive procedures. The identification of activin A receptor, type I (ACVR1)/ALK2 gene mutations associated with FOP has allowed the genetic diagnosis of FOP. The ACVR1/ALK2 gene encodes the ALK2 protein, which is a transmembrane kinase receptor in the transforming growth factor-β family. The relevant mutations activate intracellular signaling in vitro and induce heterotopic bone formation in vivo. Activin A is a potential ligand that activates mutant ALK2 but not wild-type ALK2. Various types of small chemical and biological inhibitors of ALK2 signaling have been developed to establish treatments for FOP. Some of these are in clinical trials in patients with FOP.

Hot off the press

A personal selection of 32 recent papers is presented covering various aspects of current developments in bioorganic chemistry and novel natural products such as illisimonin A from Illicium simonsii.