Asymmetric Michael-Aldol Tandem Reaction of 2-Substituted Benzofuran-3-ones and Enones: A Facile Synthesis of Griseofulvin Analogues

Highly Regio- and Diastereoselective Phosphine-Catalyzed [2 + 4] Annulation of Benzofuran-Derived Azadienes with Allyl Carbonates: Access to Spiro[benzofuran-cyclohexanes]

A novel highly regio- and diastereoselective phosphine-catalyzed [2 + 4] annulation of benzofuran-derived azadienes (BDAs) with acidic hydrogen-tethered allyl carbonates has been developed ingeniously. A range of functionalized spiro[benzofuran-cyclohexane] derivatives with two consecutive stereocenters were smoothly obtained in moderate to excellent yields under mild reaction conditions from readily available materials. Moreover, this method is a practical and scalable strategy that creates the core structural motif of the fungistatic drug, griseofulvin.

Synthesis and Antifungal Activity of Derivatives of the Natural Product Griseofulvin against Phytopathogenic Fungi

Natural products are important sources for the discovery of new pesticides. Chemical synthesis and structural modification can lead to pesticides. Despite abundant research in fungicide discovery for crop protection, there is an emerging need for the development of novel antifungal agrochemicals. Herein, 39 diversified griseofulvin derivatives were effectively synthesized from the natural product griseofulvin by diversity-oriented synthesis through the reactions of demethylation, ammonolysis, methylation, nitration, acylation, reduction, and chlorination. Among them, 31 derivatives were novel. All structures were characterized by ¹H NMR, ¹³C NMR, and high-resolution mass spectrometry (HR-MS), and the antifungal activity was investigated against five phytopathogenic fungi. Compounds 5h and 5l had excellent activity against Botrytis cinerea (5h, IC₅₀ = 17.29 ± 0.64 μg/mL) and Alternaria solani (5l, IC₅₀ = 22.52 ± 0.79 μg/mL), respectively. Compound 9 exhibited the more promising activities against three target fungi, especially against Colletotrichum gloeosporioides (IC₅₀ = 7.24 ± 0.66 μg/mL), which is obviously better than positive control hymexazol, thifluzamide, and parent compound griseofulvin. In addition, compound 10 showed significant and extensive activities against four target fungi Cytospora sp. (IC₅₀ = 18.72 ± 0.35 μg/mL), C. gloeosporioides (IC₅₀ = 31.39 ± 1.48 μg/mL), A. solani (IC₅₀ = 40.82 ± 1.04 μg/mL), and Fusarium solani (IC₅₀ = 36.81 ± 0.82 μg/mL). Unexpectedly, 11 and 12, the chlorinated products of compound 9, exhibited the most promising activity against C. gloeosporioides (IC₅₀ = 4.48 ± 0.54 μg/mL for 11, 2.24 ± 0.76 μg/mL for 12). Furthermore, 12 showed remarkable activity against Cytospora sp. (IC₅₀ = 5.85 ± 0.72 μg/mL). Additionally, in vivo antifungal activity against C. gloeosporioides, homology modeling, and docking analysis of 11, 12, and griseofulvin were conducted. All results indicated that 11 and 12 had potency as antifungal agents against C. gloeosporioides, and the modifications of the 2' and 4' positions of griseofulvin should be further explored for higher-activity lead compounds or potential agricultural fungicides.

Regiodivergent catalytic asymmetric dearomative cycloaddition of bicyclic heteroaromatics

The catalytic dearomative cycloaddition of bicyclic heteroaromatics including benzofurans and indoles provides rapid access to functionalized heterocyclic molecules. Because of the inherent stereoelectronic differences, the furan or pyrrole nucleus is more prone to dearomative cycloaddition than the benzene ring. Here, we realized a geometry-based differentiation approach for achieving C6-C7 and C7-C7a regioselectivity. The rotationally restricted σ bond at C7 position respectively placed the C6-C7 and C7-C7a sites of benzofurans or indoles in an optimal spatial orientation toward the axially chiral heterodiene, thus affording two enantioenriched polycyclic compounds from a single racemic heterobiaryl atropisomers. Calculation results of density functional theory interpreted the mechanism of this parallel kinetic resolution. The bioactivity of the dearomatized products was evaluated in cancer cell lines with certain compounds exhibiting interesting biological activities.

The crystal structure of (2R,6′R)-2′,7-dichloro-4,6-dimethoxy-6′-methyl-3H-spiro[benzofuran-2,1′-cyclohexan]-2′-ene-3,4′-dione, C16H14Cl2O5

C16H14Cl2O5, orthorhombic, P212121 (no. 19), a = 23.2297(5) Å, b = 16.3509(3) Å, c = 8.7300(2) Å, V = 3315.89(12) Å3, Z = 8, Rgt (F) = 0.0472, wRref (F 2) = 0.1268, T = 293 K.

Total syntheses of (+)-adunctins C and D: assignment of their absolute configurations

The first total synthesis of (+)-adunctin C (ent-1) and (+)-adunctin D (2), two monoterpene-substitued dihydrochalcones isolated from Piper aduncum (Piperaceae), was achieved. A regioselective oxidative [3 + 2] cycloaddition of acylphloroglucinol with (-)-β-phellandrene was developed to construct their unique spirobenzofuran skeleton. The absolute configurations of natural adunctins 1 and 2 were thus assigned through these endeavors.

Synthesis, structure and in vitro cytotoxicity testing of some 2-aroylbenzofuran-3-ols

Five 2-aroyl-5-bromobenzo[b]furan-3-ol compounds (two of which are new) and four new 2-aroyl-5-iodobenzo[b]furan-3-ol compounds were synthesized starting from salicylic acid. The compounds were characterized by mass spectrometry and ¹H NMR and ¹³C NMR spectroscopy. Single-crystal X-ray diffraction studies of four compounds, namely, (5-bromo-3-hydroxybenzofuran-2-yl)(4-fluorophenyl)methanone, C₁₅H₈BrFO₃, (5-bromo-3-hydroxybenzofuran-2-yl)(4-chlorophenyl)methanone, C₁₅H₈BrClO₃, (5-bromo-3-hydroxybenzofuran-2-yl)(4-bromophenyl)methanone, C₁₅H₈Br₂O₃, and (4-bromophenyl)(3-hydroxy-5-iodobenzofuran-2-yl)methanone, C₁₅H₈BrIO₃, were also carried out. The compounds were tested for their in vitro cytotoxicity on the four human cancer cell lines KB, Hep-G2, Lu-1 and MCF7. Six compounds show good inhibiting abilities on Hep-G2 cells, with IC₅₀ values of 1.39-8.03 µM.

Antifungal Activity of Griseofulvin Derivatives against Phytopathogenic Fungi in Vitro and in Vivo and Three-Dimensional Quantitative Structure-Activity Relationship Analysis

With environmental pollution, residual hazards accumulate and severe drug resistance and many other problems appear; some highly toxic drugs have been banned, and antifungal agents are far from satisfactory. Natural products play an important role in the discovery and development of new pesticides. The natural product griseofulvin (1) has been known as an antifungal agent in the treatment of dermatomycoses for decades. In this study, a series of new griseofulvin derivatives were synthesized with good yields. Their structures were characterized by ¹H and ¹³C nuclear magnetic resonance and high-resolution mass spectrometry (electrospray ionization). The antifungal activities of griseofulvin analogues were first evaluated against five phytopathogenic fungi ( Cytospora sp., Colletotrichum gloeosporioides, Botrytis cinerea, Alternaria solani, and Fusarium solani) in vitro. Of significance is that most of them showed excellent antifungal activities against C. gloeosporioides. The antifungal activities of the four best compounds (6a, 6c, 6e, and 6f) against C. gloeosporioides were further investigated in vivo using infected apples. The results suggested that compounds 6c, 6e, and 6f [half-maximal inhibitory concentration (IC₅₀) = 47.25 ± 1.46, 49.44 ± 1.50, and 53.63 ± 1.74 μg/mL, respectively] were better than thiophanate-methyl (IC₅₀ = 69.66 ± 6.07 μg/mL). Furthermore, comparative molecular field analysis was performed on the basis of the antifungal activity results of all 22 of the compounds against C. gloeosporioides in vitro. The three-dimensional coefficient contour plots revealed that the suitable bulky and electronegative acyl-substituted groups seem to be more favorable for increasing activity at the 4' position of griseofulvin. The structure-activity relationships were also discussed. Griseofulvin derivatives can be used for the development of highly effective and safe agricultural fungicides.

Stereodivergent Synthesis of 3-Aminooxindole Derivatives Containing Vicinal Tetrasubstituted Stereocenters via the Mannich Reaction

A highly enantioselective stereodivergent synthesis of 3-aminooxindole derivatives was accomplished via asymmetric Mannich reaction between 2-substituted benzofuran-3-one and isatin-derived ketimines. Both anti and syn-selective chiral 3,3-disubstituted amino oxindoles bearing two adjacent tetrasubstituted stereogenic centers with high yield and excellent enantioselectivities were obtained using readily available cinchona-alkaloid derived organocatalysts. The control experiment revealed that oxygen atom present in the benzofuran ring played an important role in switching diastereodivergence. The obtained Mannich product was further transformed into a biologically important 2,3-dihydrobenzofuran derivative having three contiguous stereocenters with no loss of enantioselectivity.

Enantioselective Access to Robinson Annulation Products and Michael Adducts as Precursors

The Robinson annulation is a reaction that has been useful for numerous syntheses since its discovery in 1935, especially in the field of steroid synthesis. The products are usually obtained after three consecutive steps: the formation of an enolate (or derivative), a conjugate addition, and an aldol reaction. Over the years, several methodological improvements have been made for each individual step or alternative routes have been devised to access the Robinson annulation products. The first part of this Review outlines the most relevant developments towards the formation of monocarbonyl-derived Robinson annulation products (MRA products, MRAPs) and activated monocarbonyl-derived Robinson annulation products (AMRA products, AMRAPs). The following sections are then devoted to the diastereoselective and enantioselective synthesis of these products, while the last section describes the enantiomeric resolution of racemic mixtures.

Asymmetric conjugate additions of 2-substituted benzofuran-3(2H)-ones to α,β-unsaturated ketones catalyzed by chiral copper complexes

A highly enantioselective conjugate addition of 2-substituted benzofuran-3(2H)-ones to α,β-unsaturated ketones promoted by chiral copper complexes has been developed.

A Serendipitous Synthesis of Bis-Heterocyclic Spiro 3(2H)-Furanones

(Z) Enol triflates 6, 11b-d, (E) enol triflate 11e, and phenol triflate 11a, derived from β-keto esters or 2-carboalkoxy phenols, respectively, react with N-Boc 2-lithiopyrrolidine (5a), N-Boc N-methylaminomethyllithium (5b), or 2-lithio-1,3-dithiane (14) to afford 3(2H)-furanones in modest to good yields (38-81%). Product and carbanion reagent studies suggest that the 3(2H)-furanone is formed in a cascade of reactions involving nucleophilic acyl substitution, enolate formation, trifluoromethyl transfer, iminium or sulfenium ion formation, and subsequent ring closure to form the 3(2H)-furanone. The use of 2-lithio-1,3-dithiane affords a cyclic α-keto-S,S,O-orthoester in which the functionality can be selectively manipulated for synthetic applications.

Highly enantioselective Michael addition reactions of 2-substituted benzofuran-3(2H)-ones to nitroolefins

A highly enantioselective Michael addition reaction of 2-substituted benzofuran-3(2H)-ones to nitroolefins was promoted by a bifunctional squaramide catalyst. As a result, a number of chiral 2,2'-substituted benzofuran-3-one derivatives, bearing adjacent quaternary-tertiary stereocenters, were efficiently synthesized with excellent enantioselectivities.