Total synthesis of palau'amine

Photoinduced rearrangement of α-(2-nitrophenyl)ketones

Photoirradiation of α-(2-nitrophenyl)ketones produced cyclic hydroxamates. The reaction proceeded via photoinduced oxygen transfer from the nitro group to the benzylic position, forming an α-hydroxyketone having a nitroso group. Subsequent addition of the nitroso group to the ketone moiety and the concomitant cleavage of the C-C σ bond between the carbonyl group and the benzyl position produced hydroxamic acid, which underwent formation of a hemiacetal to give cyclic hydroxamate.

Photoinduced Intramolecular Cyclization of Acylsilanes Bearing a Boronate Moiety: Construction of a Highly Strained trans-Fused Bicyclo[3.3.0]octane Skeleton

A reliable strategy for the construction of trans-fused bicyclo[n.3.0] skeletons was explored by utilizing photoinduced cyclization of acylsilanes bearing a boronate. The substrates having an acylsilane and a boronate in a 1,2-trans relationship were prepared via hydroboration of cycloalkene derivatives. The desired cyclization was efficiently promoted by photoirradiation to give the trans-fused bicyclo[n.3.0] derivatives as a single diastereomer. It is noteworthy that this methodology enables the efficient construction of a highly strained trans-5-5 ring system.

Recent advances in cascade reactions and their mechanistic insights: a concise strategy to synthesize complex natural products and organic scaffolds

The beauty of cascade reactions to bestow us with cumbersome organic scaffolds has made them a cutting-edge area of research. Although the planning of cascades may require intuition, their results can be highly impactful. The development of cascades to provide specific targeted molecules of an appropriate structural and stereochemical framework poses a significant challenge but can serve as one of the most impressive tools in organic synthesis. This review shares a broad interest in compiling cascade transformations towards the construction of polycyclic frameworks, induction of chirality/asymmetry in the protocol, etc. to solve diverse challenges in organic synthesis pursuits, as cascades enable the rapid and efficient construction of complex architectures from simple molecules. The studies highlighted herein manifest the utilization of a range of cascade reactions under various classifications for generating natural product skeletons such as palau'amine, benzosimuline, arcutinine, and others from simple building blocks, with emphasis on breakthroughs and potential for asymmetric synthesis. The exquisite synthetic designs of recently completed total synthesis of natural products with a focus on strategic concerns are also highlighted in this review.

Efficient construction of the hexacyclic ring core of palau'amine: the pK a concept for proceeding with unfavorable equilibrium reactions

Palau'amine has received a great deal of attention as an attractive synthetic target due to its intriguing molecular architecture and significant immunosuppressive activity, and we achieved its total synthesis in 2015. However, the synthesized palau'amine has not been readily applicable to the mechanistic study of immunosuppressive activity, because it requires 45 longest linear steps from a commercially available compound. Here, we report the short-step construction of the ABCDEF hexacyclic ring core of palau'amine. The construction of the CDE tricyclic ring core in a single step is achieved by our pK_a concept for proceeding with unfavorable equilibrium reactions, and a palau'amine analog without the aminomethyl and chloride groups is synthesized in 20 longest linear steps from the same starting material. The palau'amine analog is confirmed to retain the immunosuppressive activity. The present synthetic approach for a palau'amine analog has the potential for use in the development of palau'amine probes for mechanistic elucidation.

A palau'amine analog (2) was synthesized from 2-cyclopentenone in 20 steps. The construction of the CDE tricyclic ring core in a single step is achieved by our pK_a concept for proceeding with the unfavorable equilibrium reactions.

Synthesis of natural products containing highly strained trans-fused bicyclo[3.3.0]octane: historical overview and future prospects

Owing to high strain energy, molecules with trans-fused bicyclo[3.3.0]octane ring systems are very difficult to synthesize, and there are very few approaches to access them. Recently, a number of natural products with such ring systems have been made by the synthetic community. However, there has been no review in this field before. This review provides a systematic and comprehensive discussion on the synthesis of natural products containing trans-fused bicyclo[3.3.0]octanes and the historical context of this work. The prospects for future research in this field are also discussed. Covering the literature before 2021, this review aims to offer a helpful reference for total synthesis of highly strained natural products containing trans-fused bicyclo[3.3.0]octane ring systems.

A Review of the Synthetic Strategies toward Dihydropyrrolo[1,2-a]Pyrazinones

Dihydropyrrolo[1,2-a]pyrazinone rings are a class of heterocycles present in a wide range of bioactive natural products and analogues thereof. As a direct result of their bioactivity, the synthesis of this privileged class of compounds has been extensively studied. This review provides an overview of these synthetic pathways. The literature is covered up until 2020 and is organized according to the specific strategies used to construct the scaffold: fusing a pyrazinone to an existing pyrrole, employing a pyrazinone-first strategy, an array of multicomponent reactions and some miscellaneous reactions.

Antimalarial Activity of Sea Sponge Extract of Stylissa massa originating from waters of Rote Island

Research on the isolation, toxicity test, antimalarial test, and identification of the active compound from the ethyl acetate fraction of Stylissa massa sponge from Oenggae waters, Rote Island, has been conducted. This study aimed to investigate the antimalarial activity of the ethyl acetate fraction of the Stylissa massa sponge. Isolation was carried out by the extraction method using a mixed solvent of methanol: dichloromethane of 3: 2 (v/v), then the extract was partitioned in a solvent mixture of ethyl acetate: water of 1: 2 (v/v). The ethyl acetate extract obtained was separated by column chromatography using the gradient polarity system method. The toxicity test of each fraction was carried out by the Brine Shrimp Lethality Test (BSLT) method, and the antimalarial test was carried out by the haematin polymerization inhibition method. Identification of compounds from the active fraction in the antimalarial test was carried out using Liquid Chromatography-Mass Spectrometry (LC-MS). The extraction yield was 1.14 g (0.23%) of the ethyl acetate extract in the form of a dark brownish-yellow oily solid. Separation by column chromatography resulted in 15 fractions. Toxicity test results showed the four most active fractions with LC50 values, which are very promising for new drug discovery. The IC50 value in the antimalarial activity test of the four fractions indicated that the Stylissa massa sponge ethyl acetate extract was more active than the standard chloroquine compound (115 μg/mL). The LC-MS analysis indicates that fraction 11 contains two compounds that have been reported, and 1 compound is unknown. In contrast, fraction 14 indicates that it contains three compounds that have been reported and one unknown compound.

The chemistry and biology of guanidine secondary metabolites

Covering: 2017-2019Guanidine natural products isolated from microorganisms, marine invertebrates and terrestrial plants, amphibians and spiders, represented by non-ribosomal peptides, guanidine-bearing polyketides, alkaloids, terpenoids and shikimic acid derived, are the subject of this review. The topics include the discovery of new metabolites, total synthesis of natural guanidine compounds, biological activity and mechanism-of-action, biosynthesis and ecological functions.

Quick construction of a C–N bond from arylsulfonyl hydrazides and Csp2–X compounds promoted by DMAP at room temperature

A mild C–N coupling reaction with arylsulfonyl hydrazides and 2(5H)-furanones shows good yields, excellent reaction regioselectivity and functional group tolerance.

Intramolecular Diels-Alder Reaction of a Silyl-Substituted Vinylimidazole en Route to the Fully Substituted Cyclopentane Core of Oroidin Dimers

An intramolecular Diels-Alder reaction of a silyl-substituted vinylimidazole delivers a diastereomeric mixture of C4-silyl functionalized dihydrobenzimidazoles. Subsequent diastereoselective reduction and elaboration of the lactone gives rise to a polysubstituted tetrahydrobenzimidazole, which, upon oxidative rearrangement, affords a single spirofused imidazolone containing all of the relevant functionality for an approach to the oroidin dimers axinellamine, massadine, and palau'amine.

Versatile One-Pot Synthesis of Polysubstituted Cyclopent-2-enimines from α,β-Unsaturated Amides: Imino-Nazarov Reaction

The imino-Nazarov cyclization of the polysubstituted pentan-1,4-diene-3-imines was realized. To this aim, a one-pot procedure involving reductive alkenyliminylation of α,β-unsaturated secondary amides with potassium organotrifluoroborates, followed by acid-catalyzed imino-Nazarov cyclization of the polysubstituted pentan-1,4-diene-3-imine intermediates, was studied systematically. This mild, operationally simple, flexible, and high-yielding protocol efficiently affords polysubstituted pentan-1,4-diene-3-imines, cyclopentenimines, and α-amino cyclopentenones, which are useful scaffolds in organic synthesis. The substituent effect at the C2 position of the polysubstituted pentan-1,4-diene-3-imines was studied by means of density-functional theory calculations. Results suggested that the electron-donating group facilitates the imino-Nazarov cyclization process.

A Nitrone Dipolar Cycloaddition Strategy toward an Enantioselective Synthesis of Massadine

An enantioselective route to the C,D-bicycle of massadine is reported. Enantiopure intermediates were generated by a single stereoselective reduction using the Corey-Bakshi-Shibata reagent. This initial stereoinduction was translated into the five contiguous stereocenters of the massadine D-ring by a synthetic route that features a diastereoselective and stereospecific Ireland-Claisen rearrangement of a trianionic enolate followed by a diastereoselective nitrone dipolar cycloaddition of a highly electron-poor oxime.

A Photorearrangement To Construct the ABDE Tetracyclic Core of Palau'amine

A synthesis of the ABDE tetracyclic carbon core of palau'amine was achieved in 9 steps from commercial materials. The core's most notable feature, a highly strained trans cyclopenta[ c]pyrrolidine, was obtained in high yield using a ring contraction strategy starting from a much less strained trans bicyclic lactam derivative that is accessible in only 7 steps.

Rapid access to the core skeleton of the [3 + 2]-type dimeric pyrrole-imidazole alkaloids by triplet ketone-mediated C-H functionalization

The ability of triplet ketones to abstract a hydrogen atom from hydrocarbons is reminiscent of that of the high-spin metal-oxo complexes in C-H oxidation enzymes. In practice, the reactivity of triplet ketones is easier to control and applicable to promoting a wider range of reactions. We demonstrate herein the synthetic utility of triplet ketone-mediated C-addition of methanol to cyclopentenone derivatives with an expedient synthesis of the core skeleton of the [3+2]-type dimeric pyrrole-imidazole alkaloids. Remarkably, this photochemical C-H functionalization reaction is highly regioselective and can tolerate a good range of functional groups.

Two-Step Synthesis of 3,4-Dihydropyrrolopyrazinones from Ketones and Piperazin-2-ones

An expedient two-step synthesis of 3,4-dihydropyrrolopyrazinones has been achieved via a Vilsmeier-Haack reaction of ketones, followed by an annulation of the corresponding chloroaldehydes with commercially available piperazin-2-ones. A variety of cyclic and acyclic ketones and piperazin-2-ones participated in this two-step chemistry, affording the desired 3,4-dihydropyrrolopyrazinones in up to 78% yield.

Natural products as inspiration for the development of new synthetic methods

Natural products have played an important role in shaping modern synthetic organic chemistry. In particular, their complex molecular skeletons have stimulated the development of many new synthetic methods. We highlight in this article some recent examples of synthetic design inspired by the biosynthesis of natural products.

Marine natural products

Covering: 2015. Previous review: Nat. Prod. Rep., 2016, 33, 382-431This review covers the literature published in 2015 for marine natural products (MNPs), with 1220 citations (792 for the period January to December 2015) referring to compounds isolated from marine microorganisms and phytoplankton, green, brown and red algae, sponges, cnidarians, bryozoans, molluscs, tunicates, echinoderms, mangroves and other intertidal plants and microorganisms. The emphasis is on new compounds (1340 in 429 papers for 2015), together with the relevant biological activities, source organisms and country of origin. Reviews, biosynthetic studies, first syntheses, and syntheses that lead to the revision of structures or stereochemistries, have been included.

Stereocontrolled Formation of a [4.4]Heterospiro Ring System with Unexpected Inversion of Configuration at the Spirocenter

Stereoselective construction of the 1,3-diazaspiro[4.4]nonane core skeleton of massadine and related dimeric pyrrole-imidazole alkaloids is a synthetic challenge. We describe herein the synthesis of all C13/14 diastereomers of this spiro molecule through controlled oxidation and epimerization of the C13 spirocenter under mild acidic conditions.

Asymmetric Synthesis of Axinellamines A and B

Axinellamines A and B are broad-spectrum antibacterial pyrrole-imidazole alkaloids that have a complex polycyclic skeleton. A new asymmetric synthesis of these marine sponge metabolites is described herein, featuring an oxidative rearrangement and an anchimeric chlorination reaction.