Synthetic Strategies Employed for the Construction of Fostriecin and Related Natural Products

Asymmetric Synthesis of Trisubstituted Vicinal Diols through Copper(I)-Catalyzed Diastereoselective and Enantioselective Allylation of Ketones with Siloxypropadienes

Chiral trisubstituted vicinal diols are a type of important organic compounds, serving as both common structure units in bioactive natural products and chiral auxiliaries in asymmetric synthesis. Herein, by using siloxypropadienes as the precursors of allyl copper(I) species, a copper(I)-catalyzed diastereoselective and enantioselective reductive allylation of ketones was achieved, providing both syn-diols and anti-diols in good to excellent enantioselectivity. DFT calculations show that cis-γ-siloxy-allyl copper species are generated favorably with either 1-TBSO-propadiene or 1-TIPSO-propadiene. Moreover, the steric difference of TBS group and TIPS group distinguishes the face selectivity of acetophenone, leading to syn-selectivity for 1-TBSO-propadiene and anti-selectivity for 1-TIPSO-propadiene. Easy transformations of the products were performed, demonstrating the synthetic utility of the present method. Moreover, one chiral diol prepared in the above transformations was used as a suitable organocatalyst for the catalytic asymmetric reductive self-coupling of aldimines generated in situ with B2(neo)2.

Endophytic fungi as a potential source of anti-cancer drug

Endophytes are considered one of the major sources of bioactive compounds used in different aspects of health care including cancer treatment. When colonized, they either synthesize these bioactive compounds as a part of their secondary metabolite production or augment the host plant machinery in synthesising such bioactive compounds. Hence, the study of endophytes has drawn the attention of the scientific community in the last few decades. Among the endophytes, endophytic fungi constitute a major portion of endophytic microbiota. This review deals with a plethora of anti-cancer compounds derived from endophytic fungi, highlighting alkaloids, lignans, terpenes, polyketides, polyphenols, quinones, xanthenes, tetralones, peptides, and spirobisnaphthalenes. Further, this review emphasizes modern methodologies, particularly omics-based techniques, asymmetric dihydroxylation, and biotic elicitors, showcasing the dynamic and evolving landscape of research in this field and describing the potential of endophytic fungi as a source of anticancer drugs in the future.

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.

Rhodium-Catalyzed Anti-Markovnikov Transfer Hydroiodination of Terminal Alkynes

A rhodium-catalyzed anti-Markovnikov hydroiodination of aromatic and aliphatic terminal alkynes is reported. Depending on the choice of ligand and substrate, either (E)- or (Z)-configured alkenyl iodides are obtained in high to exclusive isomeric purity. The reaction exhibits a broad substrate scope and high functional group tolerance, employing easily accessible or commercially available aliphatic iodides as HI surrogates through a shuttle process. The synthesized vinyl iodides were applied in several C-C and C-heteroatom bond-forming reactions with full retention of the stereoselectivity. The developed method could be used to significantly shorten the total synthesis of a marine cis-fatty acid. Additionally, initial deuterium-labeling experiments and stoichiometric reactions shed some light on the potential reaction mechanism.

Protein Phosphatase 2A Improves Cardiac Functional Response to Ischemia and Sepsis

Reversible protein phosphorylation is a posttranslational modification of regulatory proteins involved in cardiac signaling pathways. Here, we focus on the role of protein phosphatase 2A (PP2A) for cardiac gene expression and stress response using a transgenic mouse model with cardiac myocyte-specific overexpression of the catalytic subunit of PP2A (PP2A-TG). Gene and protein expression were assessed under basal conditions by gene chip analysis and Western blotting. Some cardiac genes related to the cell metabolism and to protein phosphorylation such as kinases and phosphatases were altered in PP2A-TG compared to wild type mice (WT). As cardiac stressors, a lipopolysaccharide (LPS)-induced sepsis in vivo and a global cardiac ischemia in vitro (stop-flow isolated perfused heart model) were examined. Whereas the basal cardiac function was reduced in PP2A-TG as studied by echocardiography or as studied in the isolated work-performing heart, the acute LPS- or ischemia-induced cardiac dysfunction deteriorated less in PP2A-TG compared to WT. From the data, we conclude that increased PP2A activity may influence the acute stress tolerance of cardiac myocytes.

The phosphate ester group in secondary metabolites

Covering: 2000 to mid-2021The phosphate ester is a versatile, widespread functional group involved in a plethora of biological activities. Its presence in secondary metabolites, however, is relatively rare compared to other functionalities and thus is part of a rather unexplored chemical space. Herein, the chemistry of secondary metabolites containing the phosphate ester group is discussed. The text emphasizes their structural diversity, biological and pharmacological profiles, and synthetic approaches employed in the phosphorylation step during total synthesis campaigns, covering the literature from 2000 to mid-2021.

Catalytic Enantioselective Allylation of Acetylenic Aldehydes by Chiral Phosphoric Acid/Transition Metal Cooperative Catalysis: Formal Synthesis of Fostriecin

An enantioselective allylation of silyl-substituted acetylenic aldehydes by chiral phosphoric acid (CPA)/transition metal cooperative catalysis was developed. Enantioenriched homoallylic propargyl alcohols were obtained in good yields with excellent enantioselectivities (>99% ee) under mild conditions. Moreover, the shortest formal synthesis of fostriecin was achieved by the present enantioselective allylation protocol as the key step. The known intermediate of fostriecin reported by McDonald and co-worker was synthesized in only nine steps in 39% total yield.

Stereoselective total synthesis of obolactones and 7′,8′-dihydroobolactones

A concise stereoselective total synthesis of two diastereomeric obolactones and 7′,8′-dihydroobolactones has been achieved using a metal-free catalytic δ-hydroxyalkynone rearrangement and ring-closing metathesis.

Catalyst-free synthesis of oxazol-2(3H)-ones from sulfilimines and diazo compounds through a tandem rearrangement/aziridination/ring-expansion reaction

Oxazol-2(3H)-ones play a significant role in the fields of organic synthesis and drug development.

Total and Formal Syntheses of Fostriecin

Two formal syntheses and one total synthesis of fostriecin (1) have been achieved, as well as, the synthesis of its related congener dihydro-dephospho-fostriecin. All the routes use the Sharpless dihydroxylation to set the absolute stereochemistry at C-8/9 positions and a Leighton allylation to set the C-5 position of the natural product. In the formal syntheses a Noyori transfer hydrogenation of an ynone was used to set the C-11 position while the total synthesis employed a combination of asymmetric dihydroxylation and Pd-π-allyl reduction to set the C-11 position. Finally in the total synthesis, a trans-hydroboration of the C-12/13 alkyne was used in combination with a Suzuki cross coupling to establish the Z,Z,E-triene of fostriecin (1).

Stereoselective Synthesis of Conjugated Polyenes Based on Tethered Olefin Metathesis and Carbonyl Olefination: Application to the Total Synthesis of (+)-Bretonin B

The combination of a highly stereoselective tethered olefin metathesis reaction and a Julia-Kocienski olefination is presented as a strategy for the synthesis of conjugated polyenes with at least one Z-configured C═C bond. The strategy is exemplified by the synthesis of the marine natural product (+)-bretonin B.

Enhanced Heck reaction on flower-like Co(Mg or Ni)Al layered double hydroxide supported ultrafine PdCo alloy nanocluster catalysts: the promotional effect of Co

A series of PdCo alloy nanocluster (NC) catalysts x-PdCo_r/Co(Mg or Ni)Al-LDH (x: Pd loading, r: Co/Pd molar ratio) were synthesized by immobilizing ultrafine PdCo_r-PVP NCs on flower-like layered double hydroxide (LDH) supports. The sizes of PdCo alloy NCs of the catalysts can be elaborately tuned in ∼1.6-3.2 nm by both Co/Pd ratios and Pd loadings, and the PdCo NCs are mainly dispersed on the edge sites of LDH nanosheets upon a flower-like morphology. The PdCo bimetallic catalysts 0.81-PdCo_0.10/MgAl-LDH (2.6 ± 0.6 nm), 0.86-PdCo_0.28/MgAl-LDH (2.3 ± 0.7 nm) and 0.79-PdCo_0.54/MgAl-LDH (3.2 ± 0.9 nm) exhibit enhanced activity compared with the monometallic Pd catalyst for Heck coupling of iodobenzene with styrene. Particularly, 0.86-PdCo_0.28/MgAl-LDH shows the highest activity, which can be attributed to its smallest PdCo_0.28 alloy NCs, and the maximum electron density of the Pd⁰ center resulted from the electron transfer from Co and the strongest PdCo_0.28 NCs - LDH synergistic effect. 0.67-PdCo_0.28/CoAl-LDH shows much better activity than those of 0.64-PdCo_0.28/NiAl-LDH and 0.86-PdCo_0.28/MgAl-LDH. The lowest Pd loading sample 0.01-PdCo_0.28/CoAl-LDH (1.6 ± 0.4 nm) shows an ultrahigh turnover frequency of 163 000 h^-1 (Pd: 1.9 × 10^-5 mol%), which is the highest value obtained so far. Meanwhile, the catalyst shows excellent adaptability for the substrates and can be reused for 12 runs without significant loss of activity. The present work may provide a new idea for the simple and green synthesis of ultrafine Pd-based non-noble bimetallic catalysts for varied catalytic processes.

Syn-Dihydroxylation of Alkenes Using a Sterically Demanding Cyclic Diacyl Peroxide

The syn-dihydroxylation of alkenes is a highly valuable reaction in organic synthesis. Cyclic acyl peroxides (CAPs) have emerged recently as promising candidates to replace the commonly employed toxic metals for this purpose. Here, we demonstrate that the structurally demanding cyclic peroxide spiro[bicyclo[2.2.1]heptane-2,4'-[1,2]dioxolane]-3',5'-dione (P4) can be effectively used for the syn-dihydroxylation of alkenes. Reagent P4 also shows an improved selectivity for dihydroxylation of alkenes bearing β-hydrogens as compared to other CAPs, where both diol and allyl alcohol products compete with each other. Furthermore, the use of enantiopure P4 (labeled P4') demonstrates the potential of P4' for a metal-free asymmetric syn-dihydroxylation of alkenes.

Total Synthesis and Structural Validation of Phosdiecin A via Asymmetric Alcohol-Mediated Carbonyl Reductive Coupling

The first total synthesis and structural validation of phosdiecin A was accomplished in 13 steps through asymmetric iridium-catalyzed alcohol-mediated carbonyl reductive coupling. The present route is the shortest among >30 total and formal syntheses of fostriecin family members.

Unveiling the Takai Olefination Reagent via Tris( tert-butoxy)siloxy Variants

The elusive Takai olefination reagent, namely, the iodo-methylidene Cr(III) complex [Cr₂Cl₄(CHI)(thf)₄], has been isolated by careful handling of the reaction between CrCl₂ and CHI₃ in THF at -35 °C. Alternatively, treatment of [Cr(OSi(O tBu)₃)₂] with CHI₃ gave the mixed-valent dihalido-methanide complex [Cr^II/III₂I₂(OSi(O tBu)₃)₂(CHI₂)], featuring a Cr(III)-CHI₂ moiety. In the presence of TMEDA nucleophilic attack at CHI₂ occurred generating the zwitterionic species [Cr^III(OSi(O tBu)₃)₂(tmeda-CHI)][I]. Complexes [Cr₂Cl₄(CHI)(thf)₄] and [Cr^II/III₂I₂(OSi(O tBu)₃)₂(CHI₂)] were screened for their ability to induce monohalido olefination of benzaldehyde. Remarkably, both complexes promote olefination, with [Cr₂Cl₄(CHI)(thf)₄] accomplishing the same E selectivity as Takai 's original mixture. Complex [Cr^II/III₂I₂(OSi(O tBu)₃)₂(CHI₂)], however, appeared to give preferentially Z isomer, corroborating the monoiodo-methylidene species Cr(III)-CHI-Cr(III) as the active olefination component of the original in situ generated Takai reagent mixture.

Lactomycins A-C, Dephosphorylated Phoslactomycin Derivatives that Inhibit Cathepsin B, from the Marine-derived Streptomyces sp. ACT232

Three new polyketides, lactomycins A (1)–C (3), were isolated from the culture broth of a marine-derived Streptomyces sp. ACT232 as cathepsin B inhibitors. Their structures were determined by a combination of NMR and MS data analyses to be the dephosphorylated derivatives of a phoslactomycin class of metabolites. Lactomycins exhibited cathepsin B inhibitory activity (IC₅₀ 0.8 to 4.5 μg/mL). Even though the biosynthetic gene clusters found in the genome of the current strain have high similarity to those of phoslactomycin, neither phoslactomycins nor leustroducsins were detected by LC-MS analyses of the crude extract.

The Difference a Single Atom Can Make: Synthesis and Design at the Chemistry-Biology Interface

A Perspective of work in our laboratory on the examination of biologically active compounds, especially natural products, is presented. In the context of individual programs and along with a summary of our work, selected cases are presented that illustrate the impact single atom changes can have on the biological properties of the compounds. The examples were chosen to highlight single heavy atom changes that improve activity, rather than those that involve informative alterations that reduce or abolish activity. The examples were also chosen to illustrate that the impact of such single-atom changes can originate from steric, electronic, conformational, or H-bonding effects, from changes in functional reactivity, from fundamental intermolecular interactions with a biological target, from introduction of a new or altered functionalization site, or from features as simple as improvements in stability or physical properties. Nearly all the examples highlighted represent not only unusual instances of productive deep-seated natural product modifications and were introduced through total synthesis but are also remarkable in that they are derived from only a single heavy atom change in the structure.