Synthesis of the Common Core Structure of the Stemofoline Alkaloids

Novel Insecticidal Butenolide-Containing Methylxanthine Derivatives: Synthesis, Crystal Structure, Biological Activity Evaluation, DFT Calculation and Molecular Docking

The design of novel agrochemicals starting from bioactive natural products is one of the most effective ways in the discovery and development of new pesticidal agents. In this paper, a series of novel butenolide-containing methylxanthine derivatives (Ia-Ir) were designed based on natural methylxanthine caffeine and stemofoline, and the derivatized insecticide flupyradifurone of the latter. The structures of the synthesized compounds were confirmed via 1H-NMR, 13C NMR, HRMS and X-ray single crystal diffraction analyses. The biological activities of the compounds were evaluated against a variety of agricultural pests including oriental armyworm, bean aphid, diamondback moth, fall armyworm, cotton bollworm, and corn borer; the results indicated that some of them have favorable insecticidal potentials, particularly toward diamondback moth. Among others, Ic and Iq against diamondback moth possessed LC50 values of 6.187 mg ⋅ L-1 and 3.269 mg ⋅ L-1, respectively, - 2.5- and 4.8-fold of relative insecticidal activity respectively to that of flupyradifurone (LC50=15.743 mg ⋅ L-1). Additionally, both the DFT theoretical calculation and molecular docking with acetylcholine binding protein were conducted for the highly bioactive compound (Ic). Ic and Iq derived from the integration of caffeine (natural methylxanthine) and butenolide motifs can serve as novel leading insecticidal compounds for further optimization.

Strategies for the synthesis of Stemona alkaloids: an update

Covering: 2009 to 2022The Stemona alkaloids, which are found in plant species from the family Stemonaceae, represent a tremendously large and structurally-diverse family of natural products. This review presents and discusses a selection of case studies, grouped by alkaloid class, that showcase the key strategies and overall progress that has been made in the synthesis of Stemona alkaloids and related compounds since 2009. Structural reassignments that have been reported over this period are also identified where necessary.

Enantioselective Total Syntheses of the Proposed and Revised Structures of Methoxystemofoline: A Stereochemical Revision

This article describes the full details of our synthetic efforts toward the enantioselective total synthesis of the complex alkaloid methoxystemofoline. The enantioselective construction of the tetracyclic core features: (1) the Keck allylation at the N-α bridgehead carbon to forge the tetrasubstituted stereocenter; (2) an olefin cross-metathesis reaction for the side-chain elongation that is amenable for the synthesis of congeners and analogues; and (3) a regioselective aldol addition reaction with methyl pyruvate that ensured the subsequent regioselective cyclization reaction to construct the fourth ring. Overman's method was employed to install the 5-(alkoxyalky1idene)-3-methyl-tetronate moiety. In the last step, a nonstereoselective reaction resulted in the formation of both the proposed structure of methoxystemofoline and its E-stereoisomer, the natural product (revised structure), in a 1:1 ratio. We suggest to rename the natural product as isomethoxystemofoline, and report for the first time the complete ¹H NMR data for this natural product.

Strategies for the synthesis of furo-pyranones and their application in the total synthesis of related natural products

The furo-pyranone framework is widely present in the molecular structure of various biologically potent natural products and un-natural small molecules, and it represents a valuable target in synthetic organic chemistry and medicinal chemistry.

Enantioselective total syntheses of (+)-stemofoline and three congeners based on a biogenetic hypothesis

The powerful insecticidal and multi-drug-resistance-reversing activities displayed by the stemofoline group of alkaloids render them promising lead structures for further development as commercial agents in agriculture and medicine. However, concise, enantioselective total syntheses of stemofoline alkaloids remain a formidable challenge due to their structural complexity. We disclose herein the enantioselective total syntheses of four stemofoline alkaloids, including (+)-stemofoline, (+)-isostemofoline, (+)-stemoburkilline, and (+)-(11S,12R)-dihydrostemofoline, in just 19 steps. Our strategy relies on a biogenetic hypothesis, which postulates that stemoburkilline and dihydrostemofolines are biogenetic precursors of stemofoline and isostemofoline. Other highlights of our approach are the use of Horner–Wadsworth–Emmons reaction to connect the two segments of the molecule, an improved protocol allowing gram-scale access to the tetracyclic cage-type core, and a Cu-catalyzed direct and versatile nucleophilic alkylation reaction on an anti-Bredt iminium ion. The synthetic techniques that we developed could also be extended to the preparation of other Stemona alkaloids.

Stemofoline alkaloids are promising lead structures for further development in the fields of agriculture and medicine. Here, the authors report the enantioselective total syntheses of four stemofoline alkaloids in 19 steps based on a biogenetic hypothesis.

A modular and divergent approach to spirocyclic pyrrolidines

An efficient three-step sequence to afford a valuable class of spirocyclic pyrrolidines is reported. A reductive cleavage/Horner-Wadsworth-Emmons cascade facilitates the spirocyclisation of a range of isoxazolines bearing a distal β-ketophosphonate. The spirocyclisation precursors are elaborated in a facile and modular fashion, via a [3 + 2]-cycloaddition followed by the condensation of a phosphonate ester, introducing multiple points of divergence. The synthetic utility of this protocol has been demonstrated in the synthesis of a broad family of 1-azaspiro[4,4]nonanes and in a concise formal synthesis of the natural product (±)-cephalotaxine.

Synthesis and Formation Mechanism of a Compound with an Unprecedented Skeleton: Dodecahydro-4,10:5,9-diepoxydipyrrolo[3,4-b:3',4'-f][1,5]diazocine

The reaction of N-(2-{[(tert-butyldimethylsilyl)oxy]imino}ethyl)-4-methyl-N-(3-phenylprop-2-yn-1-yl)benzenesulfonamide (6b) with BF3·OEt2 afforded a compound with an unprecedented dodecahydro-4,10 : 5,9-diepoxydipyrrolo[3,4-b:3',4'-f][1,5]diazocine skeleton (7) after aqueous work-up. The formation mechanism of meso-7 appears to involve dimerization of the hydrated forms (6aS)-C and (6aR)-C of the initial racemic product 9 via cation B generated by facile protonation at the C3a position of 9. Extensive computational studies revealed that the driving force of the facile hydration of 9 is probably release of the ring strain of 9, which arises in part from the bent sp2-hybridized C3a carbon.

Bond-Forming and -Breaking Reactions at Sulfur(IV): Sulfoxides, Sulfonium Salts, Sulfur Ylides, and Sulfinate Salts

Organosulfur compounds have long played a vital role in organic chemistry and in the development of novel chemical structures and architectures. Prominent among these organosulfur compounds are those involving a sulfur(IV) center, which have been the subject of countless investigations over more than a hundred years. In addition to a long list of textbook sulfur-based reactions, there has been a sustained interest in the chemistry of organosulfur(IV) compounds in recent years. Of particular interest within organosulfur chemistry is the ease with which the synthetic chemist can effect a wide range of transformations through either bond formation or bond cleavage at sulfur. This review aims to cover the developments of the past decade in the chemistry of organic sulfur(IV) molecules and provide insight into both the wide range of reactions which critically rely on this versatile element and the diverse scaffolds that can thereby be synthesized.

Total Syntheses of Bisdehydroneostemoninine and Bisdehydrostemoninine by Catalytic Carbonylative Spirolactonization

The first total syntheses of the stemona alkaloids bisdehydroneostemoninine and bisdehydrostemoninine in racemic forms have been achieved. The synthetic strategy features a novel palladium-catalyzed carbonylative spirolactonization of a hydroxycyclopropanol to rapidly construct the oxaspirolactone moiety. It also features a Lewis acid promoted tandem Friedel-Crafts cyclization and lactonization to form the 5-7-5 tricyclic core of the target stemona alkaloids.

Direct Synthesis of Polycyclic Tropinones by a Condensation-[4+3]-Cycloaddition Cascade Reaction

A concise method of constructing polycyclic tropinone frameworks was developed. The single-step synthesis of polycyclic tropinone consists of an intramolecular [4+3] cycloaddition reaction of N-nosyl-pyrrole with oxyallyl cation that was generated in situ by an intermolecular condensation reaction of the nucleophilic functional groups on a tethered pyrrole with the aldehyde of 2-(silyloxy)-acrolein. This cascade reaction afforded various polycyclic tropinones including tri-, tetra-, and pentacyclic systems in high yields as single diastereomers.

Rapid synthesis of bicyclic lactones via palladium-catalyzed aminocarbonylative lactonizations

A novel and efficient palladium-catalyzed aminocarbonylative lactonization of amino propargylic alcohols has been developed to provide rapid access to various bicyclic lactones especially dihydropyrrole-fused furanones, which are novel structures and have not been explored in biological and medicinal settings. This method can also be used to access β-lactone products such as 16. Preliminary biological evaluations revealed that compounds 13h and 13s demonstrated promising activity against Clostridium difficile and compounds 13h, 13k, 13s, and 16b showed activity against several important fungal pathogens.

Synthesis of five-membered cyclic nitrones based on the Lewis acid-catalysed [3+2]-annulation reaction of donor–acceptor cyclopropanes with 1,4,2-dioxazoles

A Lewis acid-catalysed reaction of D–A cyclopropanes with 1,4,2-dioxazoles has been developed for the synthesis of five-membered cyclic nitrones.

Functionalizing the γ-Position of α-Diazo-β-ketoesters

Although α-diazo-β-ketoesters are synthetically versatile intermediates, methodology for introducing this functionality into complex molecules is still limited, most frequently involving a carboxylic acid precursor, which is then activated and transformed into a β-ketoester, with the diazo group being subsequently added with a diazo transfer reagent. While introducing this highly functional moiety in a convergent one step process would be ideal, such an objective is limited by the relatively few studies which address functionalization of the α-diazo-β-ketoester at the γ-position. In the present investigation, we evaluate strategies, both new and established, for functionalizing α-diazo-β-ketoesters, particularly with regard to generating compounds prospectively useful in the synthesis of C1-substituted carbapenems. We report the first δ-aldehydo-α-diazo-β-ketoester as well as a method for its oxidation to the corresponding methyl ester, and the formation of a new substituted pyrazole under basic conditions.

Divergent Stereocontrolled Synthesis of the Enantiopure Tetracyclic Cores of Asparagamine A and Stemofoline via an Intramolecular 2-Propylidine-1,3-(bis)silane Bicyclization

A concise and highly diastereoselective synthesis of the polyfused tetracyclic cores of the Stemona alkaloids asparagamine A and stemofoline that relies on a 2-propylidine-1,3-(bis)silane bicyclization onto a enantiodefined pyrrolidine 2,5-di(cation) equivalent derived from l-malic acid is reported. A crucial feature of this divergent synthetic approach involves the solvolysis of a transient and highly labile tertiary-propargylic hydroxylactam trifluoroacetate in the strongly ionizing medium 5 M LiClO4/Et2O. The acyliminium ion generated in this manner undergoes stereospecific interception by the aforementioned (bis)silane nucleophile.