Total Synthesis and Structural Revision of the Cytotoxin Aruncin B

Synthesis and biological evaluation of γ-alkylidenebutenolides isolated from Melodorum fruticosum: the role of the propylidene-type side chain structure on anti-melanogenic activity

The first structure-activity relationship study on γ-alkylidenebutenolides [(4Z)- and (4E)-6,7-dihydroxyhepta-2,4-dien-4-olide 6-monobenzoate] (4Z, 3 and 4E, 4) and 7-monobenzoate (4Z, 1 and 4E, 2) which are potent melanogenesis inhibitors (IC50 = 0.3-2.9 μM) isolated from the medicinal plant Melodorum fruticosum, was carried out using the related analogous (7-21). The inhibitory activities of the (4Z)- and (4E)-6,7-dideoxylated compounds (7) completely disappeared, while those of the 6-oxo-7-hydroxy-type compounds (4Z, 20 and 4E, 21) were significantly attenuated compared with those of 1-4. By contrast, the 6,7-dihydroxylated compounds (4Z, 8 and 4E, 9; IC50 = 5.8-1.5 μM) showed inhibitory activities similar to those of 1-4, suggesting that the two hydroxyl groups at positions C6 and C7 on the side chain play an essential role in the onset of potent inhibitory activity. The inhibitory activities of the 6,7-diacylated analogues (10-19, acyl group = Ac, Piv, or Bz; IC50 = 0.7-3.3 μM) were also nearly identical to those of the 6- or 7-monobenzoates (1, 3, and 4), regardless of the geometry of the double bond. However, acylation of the strongest inhibitor (4E, 2; IC50 = 0.3 μM) at position C6, reduced the activity by approximately 1/3 to 1/5. This result suggests the importance of the cooperative role of the acyl moiety at position C7 and the hydroxyl group at position C6 in the E-configured double bond. The total synthesis of the natural products melodorinone A (20) and melodorinone B (21) was also achieved during the synthesis of the γ-alkylidenebutenolide analogues.

Triflic Anhydride-Mediated Unprecedented Reactivities of Diacetonide Endoglucofuranose and Ribose Tetracetate: Direct Access to γ-Butenolides and Oxazoles

Herein, we present two unprecedented reactions for the synthesis of γ-butenolides and oxazoles, leveraging Tf2O's promoted reactivity of nitriles with diacetonide endoglucofuranose and 1,2,3,5-tetra-O-acetyl-β-d-ribofuranose. This method is highly efficient and straightforward and employs a one-step, metal-free protocol. It is effective with both aromatic and aliphatic nitriles and demonstrates a broad substrate scope. Our approach provides a versatile and practical pathway for the synthesis of structurally diverse compounds, significantly expanding the utility of Tf2O in synthetic chemistry.

Kallopterolides A-I, a New Subclass of seco-Diterpenes Isolated from the Southwestern Caribbean Sea Plume Antillogorgia kallos

Kallopterolides A-I (1-9), a family of nine diterpenoids possessing either a cleaved pseudopterane or a severed cembrane skeleton, along with several known compounds were isolated from the Caribbean Sea plume Antillogorgia kallos. The structures and relative configurations of 1-9 were characterized by analysis of HR-MS, IR, UV, and NMR spectroscopic data in addition to computational methods and side-by-side comparisons with published NMR data of related congeners. An investigation was conducted as to the potential of the kallopterolides as plausible in vitro anti-inflammatory, antiprotozoal, and antituberculosis agents.

Enhancing Efficiency of Natural Product Structure Revision: Leveraging CASE and DFT over Total Synthesis

Natural products remain one of the major sources of coveted, biologically active compounds. Each isolated compound undergoes biological testing, and its structure is usually established using a set of spectroscopic techniques (NMR, MS, UV-IR, ECD, VCD, etc.). However, the number of erroneously determined structures remains noticeable. Structure revisions are very costly, as they usually require extensive use of spectroscopic data, computational chemistry, and total synthesis. The cost is particularly high when a biologically active compound is resynthesized and the product is inactive because its structure is wrong and remains unknown. In this paper, we propose using Computer-Assisted Structure Elucidation (CASE) and Density Functional Theory (DFT) methods as tools for preventive verification of the originally proposed structure, and elucidation of the correct structure if the original structure is deemed to be incorrect. We examined twelve real cases in which structure revisions of natural products were performed using total synthesis, and we showed that in each of these cases, time-consuming total synthesis could have been avoided if CASE and DFT had been applied. In all described cases, the correct structures were established within minutes of using the originally published NMR and MS data, which were sometimes incomplete or had typos.

Synthetic studies towards naturally occurring γ-(Z)/(E)-alkylidenebutenolides through bimetallic cascade cyclization and an adventitious photoisomerization method

A general and flexible visible light-induced photoisomerization method of γ-(Z)-alkylidenebutenolides to their corresponding E-components was reported in this article. Initially, a series of naturally occurring enantiopure γ-(Z)-alkylidenebutenolides was synthesized by employing a "Pd-Cu" bimetallic cascade cyclization protocol. In the later part, the synthesized γ-(Z)-alkylidenebutenolides were photoisomerized in the presence of a triplet photosensitizer to γ-(E)-alkylidenebutenolides in reasonably acceptable yields. Total synthesis of goniobutenolides, hygrophorones, ramariolide D, melodorinols/acetyl-melodorinols, versicolactones, and phomopsolidones was achieved by employing the developed methods.

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.

Total Synthesis of γ-Alkylidenebutenolides, Potent Melanogenesis Inhibitors from Thai Medicinal Plant Melodorum fruticosum

A hitherto unreported member of γ-alkylidenebutenolides in Melodorum fruticosum (Annonaceae), (4 E)-6-benzoyloxy-7-hydroxy-2,4-heptadiene-4-olide, named as isofruticosinol (4) was isolated from the methanol extract of flowers, along with the known related butenolides, namely, the (4 Z)-isomer (3) of 4, melodrinol (1), and its (4 E)-isomer (2). To unambiguously determine the absolute configuration at the C-6 position in these butenolides, the first total syntheses of both enantiomers of 2-4 were achieved over 6-7 steps from commercially available D- or L-ribose (D- and L-5). Using the same protocol, both enantiomers of 1 were also synthesized. Based on chiral HPLC analysis of all synthetic compounds ( S- and R-1-4), all naturally occurring butenolides were assigned as partial racemic mixtures with respect to the chiral center at C-6 (enantiomeric ratio, 6 S/6 R = ∼83/17). Furthermore, the melanogenesis inhibitory activities of S- and R-1-4 were evaluated, with all shown to be potent inhibitors with IC₅₀ values in the range 0.29-2.9 μM, regardless of differences in the stereochemistry at C-6. In particular, S-4 (IC₅₀ = 0.29 μM) and R-4 (0.39 μM) showed potent inhibitory activities compared with that of reference standard arbutin (174 μM).

Unequivocal determination of complex molecular structures using anisotropic NMR measurements

Assignment of complex molecular structures from nuclear magnetic resonance (NMR) data can be prone to interpretational mistakes. Residual dipolar couplings and residual chemical shift anisotropy provide a spatial view of the relative orientations between bonds and chemical shielding tensors, respectively, regardless of separation. Consequently, these data constitute a reliable reporter of global structural validity. Anisotropic NMR parameters can be used to evaluate investigators' structure proposals or structures generated by computer-assisted structure elucidation. Application of the method to several complex structure assignment problems shows promising results that signal a potential paradigm shift from conventional NMR data interpretation, which may be of particular utility for compounds not amenable to x-ray crystallography.

Aruncin B: Synthetic Studies, Structural Reassignment and Biological Evaluation

A ring-closing alkene metathesis (RCM)/ oxyselenation-selenoxide elimination sequence was established to the sodium salts E- and Z-25 of the originally proposed structure for the recently isolated cytotoxin aruncin B (1), as well as to the sodium salt Z-34 of a related ethyl ether regioisomer; however, none of their corresponding free acids could be obtained. Their acid sensitivity, together with detailed analysis of the spectroscopic data indicated that profound structural revision was necessary. This led to reassignment of aruncin B as a Z-γ-alkylidenebutenolide Z-36. Although a related RCM/ oxyselenation-selenoxide elimination sequence was used to confirm the γ-alkylidenebutenolide motif, a β-iodo Morita-Baylis-Hillman reaction/ Sonogashira cross-coupling-5-exo-dig lactonisation sequence was subsequently developed, due to its brevity and flexibility for diversification. Aruncin B (36), together with 14 γ-alkylidenebutenolide analogues, were generated for biological evaluation.

Kinetic Benchmarking Reveals the Competence of Prenyl Groups in Ring-Closing Metathesis

A series of prenyl-containing malonates are kinetically benchmarked against the standard allyl-containing congeners using a ruthenium benzylidene precatalyst for ring-closing metatheses. The prenyl grouping is found to be a superior acceptor olefin compared to an allyl group in RCM processes with ruthenium alkylidenes derived from terminal alkenes. The prenyl group is also found to be a highly competent acceptor for a ruthenium alkylidene derived from a 1,1-disubstituted olefin in a RCM process.

Evaluating Nitrogen-Containing Biosynthetic Products Produced by Saltwater Culturing of Several California Littoral Zone Gram-Negative Bacteria

The biosynthetic potential of marine-sediment-derived Gram-negative bacteria is poorly understood. Sampling of California near-shore marine environments afforded isolation of numerous Gram-negative bacteria in the Proteobacteria and Bacteriodetes phyla, which were grown in the laboratory to provide extracts whose metabolites were identified by comparative analyses of LC-mass spectrometry and MSn data. Overall, we developed an assemblage of seven bacterial strains grown in five different media types designed to coax out unique secondary metabolite production as a function of varying culture conditions. The changes in metabolite production patterns were tracked using the GNPS MS2 fragmentation pattern analysis tool. A variety of nitrogen-rich metabolites were visualized from the different strains grown in different media, and strikingly, all of the strains examined produced the same new, proton-atom-deficient compound, 1-methyl-4-methylthio-β-carboline (1), C13H12N2S. Scale-up liquid culture of Achromobacter spanius (order: Burkholderiales; class: Betaproteobacteria) provided material for the final structure elucidation. The methods successfully combined in this work should stimulate future studies of molecules from marine-derived Gram-negative bacteria.

Monoterpenoids from the traditional North Italian vegetable Aruncus dioicus (Walter) Fernald var. vulgaris (Maxim.) H.Hara (Rosaceae)

Investigations of young shoots of Aruncus dioicus (Walter) Fernald var. vulgaris (Maxim.) H.Hara (Rosaceae), collected from the wild and used as vegetables in alpine provinces of Italy, yielded eight monoterpenoids. Besides known compounds, aruncin A, aruncide A, and cimicifugolide, five previously undescribed substances, aruncins C, D, and E, and aruncides D and E, were identified. Based on results from the full synthesis of aruncin B, structures of aruncin A and aruncide A were revised. Structures were established by HR mass spectrometry and extensive 1D and 2D NMR spectroscopy and based on data from synthetic aruncin B. An HPLC-DAD-ESI-MS method was developed to investigate the distribution of the monoterpenoids in different organs of Aruncus dioicus var. vulgaris and in aerial parts of A. dioicus var. aethusifolius (H.Lév.) H.Hara [Syn.: Aruncus aethusifolius (H.Lév.) Nakai]. Preliminary bioactivity studies moreover indicated weak cytotoxicity for some of the compounds against human prostrate adenocarcinoma cells.

Hot off the press

A personal selection of 32 recent papers is presented covering various aspects of current developments in bioorganic chemistry and novel natural products such as kanamienamide from the marine cyanobacterium Moorea bouillonii.