Late-Stage Bromination Enables the Synthesis of Rubrolides B, I, K, and O

Natural Rubrolides and Their Synthetic Congeners as Inhibitors of the Photosynthetic Electron Transport Chain

Rubrolides are a family of naturally occurring 5-benzylidenebutenolides, which generally contain brominated phenol groups, and nearly half of them also present a chlorine attached to the butenolide core. Seven natural rubrolides were previously synthesized. When these compounds were tested against the model plant Raphanus sativus, six were found to exert a slight inhibition on plant growth. Aiming to exploit their scaffold as a model for the synthesis of new compounds targeting photosynthesis, nine new rubrolide analogues were prepared. The synthesis was accomplished in 2-4 steps with a 10-39% overall yield from 3,4-dichlorofuran-2(5H)-one. All compounds were evaluated for their ability to inhibit the whole Hill reaction or excluding photosystem I (PSI). Several natural rubrolides and their analogues displayed good inhibitory potential (IC50 = 2-8 μM). Molecular docking studies on the photosystem II-light harvesting complex II (PSII-LHCII supercomplex) binding site were also performed. Overall, data support the use of rubrolides as a model for the development of new active principles targeting the photosynthetic electron transport chain to be used as herbicides.

Ir(I)-Catalyzed Synthesis of Furanones from Vinyl Sulfoxonium Ylides

A method for the synthesis of allyl substituted γ-butenolides via carbonyl ylide rearrangement of vinyl sulfoxonium ylide-derived carbenes has been developed. At rt, the mechanism involves a carbonyl ylide generation/allyloxy furan formation/[3,3]-sigmatropic rearrangement/isomerization sequence for the generation of 3-allyl butenolides. At 70 °C, instead of the final isomerization step, the resulting [3,3]-sigmatropic rearrangement product undergoes further [3,3]-sigmatropic rearrangement to produce 5-allyl butenolide. In the absence of the catalyst, the reaction affords a diene via [2,3]-sigmatropic rearrangement.

Naturally Occurring Organohalogen Compounds-A Comprehensive Review

The present volume is the third in a trilogy that documents naturally occurring organohalogen compounds, bringing the total number-from fewer than 25 in 1968-to approximately 8000 compounds to date. Nearly all of these natural products contain chlorine or bromine, with a few containing iodine and, fewer still, fluorine. Produced by ubiquitous marine (algae, sponges, corals, bryozoa, nudibranchs, fungi, bacteria) and terrestrial organisms (plants, fungi, bacteria, insects, higher animals) and universal abiotic processes (volcanos, forest fires, geothermal events), organohalogens pervade the global ecosystem. Newly identified extraterrestrial sources are also documented. In addition to chemical structures, biological activity, biohalogenation, biodegradation, natural function, and future outlook are presented.

Recent reports on the synthesis of γ-butenolide, γ-alkylidenebutenolide frameworks, and related natural products

γ-Butenolides are fundamental frameworks found in many naturally occurring compounds, and they exhibit tremendous biological activities. γ-Butenolides also have proven their potential as useful synthetic intermediates in the total synthesis of natural compounds. Over the years, many γ-butenolide natural products have been isolated, having exocyclic γ-δ unsaturation in their structure. These natural products are collectively referred to as γ-alkylidenebutenolides. Considering the different biological profiles and wide-ranging structural diversity of the optically active γ-butenolide, the development of synthetic strategies for assembling such challenging scaffolds has attracted significant attention from synthetic chemists in recent times. In this report, a brief discussion will be provided to address isolation, biogenesis, and current state-of-the-art synthetic protocols for such molecules. This report aims to focus on synthetic strategies for γ-butenolides from 2010-2020 with a particular emphasis on γ-alkylidenebutenolides and related molecules. Metal-mediated catalytic transformation and organocatalysis are the two main reaction types that have been widely used to access such molecules. Mechanistic considerations, enantioselective synthesis, and practical applications of the reported procedures are also taken into consideration.

New Developments of the Principle of Vinylogy as Applied to π-Extended Enolate-Type Donor Systems

The principle of vinylogy states that the electronic effects of a functional group in a molecule are possibly transmitted to a distal position through interposed conjugated multiple bonds. As an emblematic case, the nucleophilic character of a π-extended enolate-type chain system may be relayed from the legitimate α-site to the vinylogous γ, ε, ..., ω remote carbon sites along the chain, provided that suitable HOMO-raising strategies are adopted to transform the unsaturated pronucleophilic precursors into the reactive polyenolate species. On the other hand, when "unnatural" carbonyl ipso-sites are activated as nucleophiles (umpolung), vinylogation extends the nucleophilic character to "unnatural" β, δ, ... remote sites. Merging the principle of vinylogy with activation modalities and concepts such as iminium ion/enamine organocatalysis, NHC-organocatalysis, cooperative organo/metal catalysis, bifunctional organocatalysis, dicyanoalkylidene activation, and organocascade reactions represents an impressive step forward for all vinylogous transformations. This review article celebrates this evolutionary progress, by collecting, comparing, and critically describing the achievements made over the nine year period 2010-2018, in the generation of vinylogous enolate-type donor substrates and their use in chemical synthesis.

Synthesis of new tetronamides displaying inhibitory activity against bloom-forming cyanobacteria

BACKGROUND

The increasing frequency and intensity of cyanobacterial blooms pose a serious threat to aquatic ecosystems. These blooms produce potent toxins that can contaminate drinking water and endanger the life of wild and domestic animals as well as humans. Consequently, the development of effective methods for their control is a matter of high priority. We have previously shown that some γ-benzylidenebutenolides, related to the rubrolide family of natural products, are capable of inhibiting the photosynthetic electron transport chain (Hill reaction), a target of commercial herbicides. Here we report the synthesis and biological properties of a new class of rubrolide-inspired molecules featuring a tetronamide motif.

RESULTS

A total of 47 N-aryl tetronamides, including 38 aldol adducts, were prepared bearing phenyl, biphenyl, naphthyl, aliphatic and heteroaromatic groups. Some of the aldol adducts were dehydrated to the corresponding γ-benzylidenetetronamides, although satisfactory yields were obtained in only three cases (52-97%). None of the synthesized compounds were capable of blocking the Hill reaction. This notwithstanding, several aldol adducts equipped with a biphenyl substituent displayed excellent inhibitory activity against Synechococcus elongatus and other cyanobacterial strains (IC₅₀  = 1-5 μM). Further, these tetronamides were found to be essentially inactive against eukaryotic microorganisms.

CONCLUSION

Several newly synthesized biphenyl-containing tetronamides were shown to display potent and selective inhibitory activity against cyanobacteria. These compounds appear to exert their biological effects without interfering with the Hill reaction. As such, they represent novel leads in the search of environmentally benign agents for controlling cyanobacterial blooms. © 2019 Society of Chemical Industry.