Ascospiroketals A and B, Unprecedented Cycloethers from the Marine-Derived Fungus Ascochyta salicorniae

Total Synthesis of Marine Polyketide Plakortone Q

The total synthesis of the natural bicyclo[3.3.0]furanolactone polyketide, plakortone Q, was achieved in 24 steps from (R)-Roche ester. The main feature of this synthetic strategy is the stereoselective construction of a central tetrahydrofuran moiety with four consecutive stereoisomeric centers using the Upjohn dihydroxylation of oxiranyl-substituted alkenes and acid-mediated 5-endo-tet cyclization.

Sharpless Asymmetric Dihydroxylation: An Impressive Gadget for the Synthesis of Natural Products: A Review

Sharpless asymmetric dihydroxylation is an important reaction in the enantioselective synthesis of chiral vicinal diols that involves the treatment of alkene with osmium tetroxide along with optically active quinine ligand. Sharpless introduced this methodology after considering the importance of enantioselectivity in the total synthesis of medicinally important compounds. Vicinal diols, produced as a result of this reaction, act as intermediates in the synthesis of different naturally occurring compounds. Hence, Sharpless asymmetric dihydroxylation plays an important role in synthetic organic chemistry due to its undeniable contribution to the synthesis of biologically active organic compounds. This review emphasizes the significance of Sharpless asymmetric dihydroxylation in the total synthesis of various natural products, published since 2020.

Palladium-Catalyzed Hydrocarbonylative Spirolactonization for Expedite Construction of Oxaspirolactones

A novel palladium-catalyzed cascade hydrocarbonylation of the alkene moiety followed by intramolecular spirolactonization of 2-vinylaryl hydroxyalkyl ketones has been developed, which offers efficient and expedited access to furnishing oxaspirolactones in high yields with high chemoselectivities. This new method is compatible with an array of functional groups and proceeds under mild reaction conditions with commercially available catalyst components.

Biomimetic total synthesis of plakortone Q via acid-mediated tandem cyclization

Plakortone Q and plakdiepoxide are natural polyketides isolated from the marine sponge Plakortis simplex. Bicyclo[3.3.0]furanolactone compounds, including plakortone Q, are expected to exhibit a wide range of pharmacological activities. Therefore, developing a simple and versatile synthetic method to produce these compounds is an important research goal. We have achieved the first total synthesis of plakortone Q and plakdiepoxide through an efficient protecting-group-free strategy. The key transformation was an acid-mediated tandem 5-endo-tet/5-endo-tet cyclization of vicinal diepoxide to build the tetrahydrofuran-γ-lactone motif.

Total Synthesis of Ascospiroketal B

An enantioselective total synthesis of the marine tricyclic polyketide ascospiroketal B, previously isolated from the marine-derived fungus Ascochyta salicorniae, was accomplished in 21 steps by using an improved route. The intriguing 5,5-spiroketal-cis-fused-γ-lactone core was constructed through rearrangement of an epoxide, in conjunction with an acid-mediated spiroketalization.

Total Synthesis of ent-Ascospiroketal B

Ascospiroketal B was isolated from a marine-derived fungus as a structurally unique polyketide possessing a rare tricyclic core including 5,5-spiroketal-γ-lactone. An asymmetric total synthesis of ent-ascospiroketal B was achieved using an original synthetic route. The synthesis included the stereoselective construction of 5,5-spiroketal for ascospiroketal B and stereocontrolled construction of a quaternary asymmetric carbon by rearrangement of a trisubstituted epoxide.

Recent progress in the isolation, bioactivity, biosynthesis, and total synthesis of natural spiroketals

The isolation, bioactivity, biosynthesis, and total synthesis of natural spiroketals from 2011 to July 2017 have been summarized in this review.

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.

Catalytic Carbonylative Spirolactonization of Hydroxycyclopropanols

A palladium-catalyzed cascade carbonylative spirolactonization of hydroxycyclopropanols has been developed to efficiently synthesize oxaspirolactones common to many complex natural products of important therapeutic value. The mild reaction conditions, high atom economy, broad substrate scope, and scalability of this new method were highlighted in expedient total syntheses of the Turkish tobacco natural products α-levantanolide and α-levantenolide in two and four steps, respectively. The hydroxycyclopropanol substrates are readily available in one step via a Kulinkovich reaction of the corresponding lactones. Mechanistic studies utilizing high-resolution electrospray ionization mass spectrometry (ESI-MS) identified several key intermediates in the catalytic cycle, as well as those related to catalyst decomposition and competitive pathways.

Asymmetric Total Syntheses of ent-Ascospiroketals A and B

A new hypothetic biosynthesis of the tricyclic spiroketal core of ascospiroketals A and B is proposed, which guided the development of a novel synthetic strategy for the asymmetric total synthesis of ent-ascospiroketals A and B. The synthesis features an efficient ring contraction rearrangement of the 10-membered lactone to the tricyclic spiroketal cis-fused γ-lactone core, which served as the common intermediate for the synthesis of both ent-ascospiroketals A and B through the Stille coupling reaction at the final step. In addition, seven diastereomers were prepared to conclusively confirm the structure of ent-ascospiroketal B.

Synthesis of (−) cephalosporolide E and (+) cephalosporolide F utilizing the vinylogous Mukaiyama type reaction with an α-chloro sulfide

An asymmetric synthesis of (−)-cephalosporolide E and (+)-cephalosporolide F employing vinylogous Mukaiyama type reaction with an α-chloro sulfide, Noyori transfer hydrogenation and oxidative cyclization as the key steps is described.

Total Synthesis and Configurational Assignment of Ascospiroketal A

The total synthesis of the marine fungus-derived natural product ascospiroketal is described. This concise synthesis relies on a unique Ag(I) -promoted tandem cascade cyclization that provides direct access to the correctly configured tricyclic core of the natural product from a linear precursor. The synthesis of candidate stereostructures of ascospiroketal A allowed for the confident assignment of both the relative and absolute stereochemistry of this unusual octaketide.

A concise protecting-group-free synthesis of cephalosporolides E and F

A concise protecting-group-free synthesis of cephalosporolides E and F has been accomplished from l-mannonic-γ-lactone by one-pot conversion to γ-vinyl-β-hydroxy-γ-lactone, cross-metathesis and Wacker-type oxidative spiroketalization.

Total synthesis of ascospiroketal a through a Ag(I) -promoted cyclization cascade

The total synthesis of four candidate stereostructures for the marine octaketide ascospiroketal A have been achieved. These concise and highly stereocontrolled syntheses feature a unique Ag(I) -promoted cyclization cascade involving an oxetanyl ketochlorohydrin to access the entire tricyclic core of the natural product in one step. These syntheses also establish the full stereochemistry for the ascospiroketal natural products.

An assessment of natural product discovery from marine (sensu strictu) and marine-derived fungi

The natural products community has been investigating secondary metabolites from marine fungi for several decades, but when one attempts to search for validated reports of new natural products from marine fungi, one encounters a literature saturated with reports from ‘marine-derived’ fungi. Of the 1000+ metabolites that have been characterized to date, only approximately 80 of these have been isolated from species from exclusively marine lineages. These metabolites are summarized here along with the lifestyle and habitats of their producing organisms. Furthermore, we address some of the reasons for the apparent disconnect between the stated objectives of discovering new chemistry from marine organisms and the apparent neglect of the truly exceptional obligate marine fungi. We also offer suggestions on how to reinvigorate enthusiasm for marine natural products discovery from fungi from exclusive marine lineages and highlight the need for critically assessing the role of apparently terrestrial fungi in the marine environment.

Cephalosporolide B serving as a versatile synthetic precursor: asymmetric biomimetic total syntheses of cephalosporolides C, E, F, G, and (4-OMe-)G

Cephalosporolide B (Ces-B) was efficiently synthesized and exploited for the first time as a versatile biomimetic synthetic precursor for the chemical syntheses of not only cephalosporolides C, G, and (4-OMe-) G via a challenging diastereoselective oxa-Michael addition but also the structurally unprecedented cephalosporolides E and F via a novel biomimetic ring-contraction rearrangement. These findings provide the first direct chemical evidence that Ces-B may be the true biosynthetic precursor of cephalosporolides.

Anionic cascade reactions. One-pot assembly of (Z)-chloro-exo-methylenetetrahydrofurans from β-hydroxyketones

The assembly of (Z)-chloro-exo-methylenetetrahydrofurans by an original and connective anionic cascade sequence is reported. Base-catalysed condensation of β-hydroxyketones with 1,1-dichloroethylene generates, in moderate to good yields, the corresponding (Z)-chloro-exo-methylenetetrahydrofurans. Acidic treatment of this motif leads to several unexpected dimers, possessing unique structural features.

Tricycloalternarenes F-H: three new mixed terpenoids produced by an endolichenic fungus Ulocladium sp. using OSMAC method

Three new mixed terpenoids, tricycloalternarenes (TCAs) F-H (1-3), together with ten known tricycloalternarenes (4-13), were isolated from the Czapek's culture of an endophytic fungus Ulocladium sp. Their structures were identified by extensive spectroscopic experiments (NMR and MS) and comparison with literature data. TCA 1b (5) showed weak activity against the Bacille Calmette-Guerin strain with the MIC of 125μg/mL. TCA 9b (10) exhibited strong cytotoxic activity against Hela cell line with IC50 of 8.58μM.

Marine natural products

This review covers the literature published in 2007 for marine natural products, with 948 citations(627 for the period January to December 2007) referring to compounds isolated from marine microorganisms and phytoplankton, green algae, brown algae, red algae, sponges, cnidarians,bryozoans, molluscs, tunicates, echinoderms and true mangrove plants. The emphasis is on new compounds (961 for 2007), together with the relevant biological activities, source organisms and country of origin. Biosynthetic studies, first syntheses, and syntheses that lead to the revision of structures or stereochemistries, have been included.1 Introduction, 2 Reviews, 3 Marine microorganisms and phytoplankton, 4 Green algae, 5 Brown algae, 6 Red algae, 7 Sponges, 8 Cnidarians, 9 Bryozoans, 10 Molluscs, 11 Tunicates (ascidians),12 Echinoderms, 13 Miscellaneous, 14 Conclusion, 15 References.

Ten-membered lactones from the marine-derived fungus Curvularia sp

Investigation of the secondary metabolites of the marine-derived fungus Curvularia sp. yielded four new 10-membered lactones (1-4), along with the known modiolide A (5). The structures of 1-4 were characterized on the basis of spectroscopic and MS data and resemble known 10-membered lactones, but feature modified oxidation patterns around their macrocycles.

Cephalosporolides H and I, two novel lactones from a marine-derived fungus, penicillium sp

Two novel lactones, Cephalosporolides H (1) and 1 (2), were isolated from the lyophilized culture broth of the marine-derived fungus, Penicillium sp. The structures were elucidated on the basis of extensive 1D- and 2D-NMR, as well as HRESI-MS spectroscopic analyses. The relative stereochemistries of the compounds were assessed by comparison of the NOESY analysis and spectral data with those in the literature.

Penisporolides A and B, Two New Spiral Lactones from the Marine-derived Fungus Penicillium sp

In our continued screening on bioactive constituents from marine-derived fungi, two novel compounds containing a rare spiral-lactone skeleton were isolated from lyophilized culture broth of the marine-derived fungus Penicillium sp. The structures of penisporolides A and B were elucidated on the basis of extensive 1D and 2D NMR as well as HRESI-MS spectroscopic data analysis. The relative stereochemistries of the compounds were assessed by analysis of NOESY data together with the comparison with data from previous literatures.