Synthesis of Novel Stilbene-Coumarin Derivatives and Antifungal Screening of Monotes kerstingii-Specialized Metabolites Against Fusarium oxysporum.

Fusarium is one of the most toxigenic phytopathogens causing diseases and reduced agricultural productivity worldwide. Current chemical fungicides exhibit toxicity against non-target organisms, triggering negative environmental impact, and are a danger to consumers. In order to explore the chemical diversity of plants for potential antifungal applications, crude extract and fractions from Monotes kerstingii were screened for their activity against two multi-resistant Fusarium oxysporum strains: Fo32931 and Fo4287. Antifungal activity was evaluated by the determination of minimum inhibitory concentration (MIC) by broth dilution of fermentative yeasts using kinetic OD600 nm reading by a spectrophotometer. The n-butanol fraction showed the best activity against Fo4287. We screened eleven previously reported natural compounds isolated from different fractions, and a stilbene-coumarin 5-[(1E)-2-(4-hydroxyphenyl)ethenyl]-4,7-dimethoxy-3-methyl-2H-1-benzopyran-2-one (1) was the most active compound against both strains. Compound 1 was employed as a nucleophile with a selection of electrophilic derivatizing agents to synthesize five novel stilbene-coumarin analogues. These semisynthetic derivatives showed moderate activity against Fo32931 with only prenylated derivative exhibiting activity comparable to the natural stilbene-coumarin (1), demonstrating the key role of the phenolic group.

Trypanocidal and leishmanicidal activity of six limonoids.

Six limonoids [kotschyienone A and B (1, 2), 7-deacetylgedunin (3), 7-deacetyl-7-oxogedunin (4), andirobin (5) and methyl angolensate (6)] were investigated for their trypanocidal and leishmanicidal activities using bloodstream forms of Trypanosoma brucei and promastigotes of Leishmania major. Whereas all compounds showed anti-trypanosomal activity, only compounds 1-4 displayed anti-leishmanial activity. The 50% growth inhibition (GI50) values for the trypanocidal and leishmanicidal activity of the compounds ranged between 2.5 and 14.9 µM. Kotschyienone A (1) was found to be the most active compound with a minimal inhibition concentration (MIC) value of 10 µM and GI50 values between 2.5 and 2.9 µM. Only compounds 1 and 3 showed moderate cytotoxicity against HL-60 cells with MIC and GI50 values of 100 µM and 31.5-46.2 µM, respectively. Compound 1 was also found to show activity against intracellular amastigotes of L. major with a GI50 value of 1.5 µM. The results suggest that limonoids have potential as drug candidates for the development of new treatments against trypanosomiasis and leishmaniasis.

Cross-Dehydrogenative-Coupling of Alkoxybenzenes with Toluenes: Copper(II) Halide Mediated Tandem Halo/Benzylation of Arenes.

A cross-dehydrogenative-coupling of alkoxybenzenes and toluenes with concomitant halogenation is reported. Conditions employed were the use of stoichiometric copper halide salts and dialkylperoxides to afford a range of bromoalkoxydi- and triarylmethanes. Preliminary mechanistic studies suggest that the in situ production of haloarenes (or dihaloarenes) followed by a copper-mediated coupling of a benzylic radical is operational.

Expedient synthesis of an atypical oxazolidinone compound library.

In order to address the current downturn in the drug discovery pipeline, initiatives are being undertaken to synthesise screening libraries of sp3-rich, low molecular weight compounds. As part of the European Lead Factory initiative, the synthesis and derivatisation of a simple hexahydrooxazolo[5,4-c]pyridin-2(1H)-one bicyclic carbamate has been achieved. The synthetic route employed involved a telescoped hetero-Diels-Alder/[2,3]-sigmatropic rearrangement/cyclisation sequence to deliver the desired core scaffold containing two points for further diversification. When applied, this synthesis was found to be robust and scalable which allowed the production of a 155 compound library.

Versatile C(sp(2) )-C(sp(3) ) Ligand Couplings of Sulfoxides for the Enantioselective Synthesis of Diarylalkanes.

The reaction of chiral (hetero)aryl benzyl sulfoxides with Grignard reagents affords enantiomerically pure diarylalkanes in up to 98 % yield and greater than 99.5 % enantiomeric excess. This ligand coupling reaction is tolerant to multiple substitution patterns and provides access to diverse areas of chemical space in three operationally simple steps from commercially available reagents. This strategy provides orthogonal access to electron-deficient heteroaromatic compounds, which are traditionally synthesized by transition metal catalyzed cross-couplings, and circumvents common issues associated with proto-demetalation and ß-hydride elimination.

Combining two-directional synthesis and tandem reactions. Part 21: Exploitation of a dimeric macrocycle for chain terminus differentiation and synthesis of an sp(3)-rich library.

The application of a tandem condensation/cyclisation/[3+2]-cycloaddition/elimination reaction gives an sp(3)-rich tricyclic pyrazoline scaffold with two ethyl esters in a single step from a simple linear starting material. The successive hydrolysis and cyclisation (with Boc anhydride) of these 3-dimensional architectures, generates unprecedented 16-membered macrocyclic bisanhydrides (characterised by XRD). Selective amidations could then be achieved by ring opening with a primary amine followed by HATU-promoted amide coupling to yield an sp(3)-rich natural product-like library.

Palladium catalysed cross-dehydrogenative-coupling of 1,3,5-trialkoxybenzenes with simple arenes.

Pd-catalysed cross-dehydrogenative coupling of 1,3,5-trialkoxybenzenes with simple aromatic hydrocarbons is reported. The method enables the coupling of two aromatic C-H positions to generate multi-ortho-substituted biaryls.

Ruthenium-catalyzed α-(hetero)arylation of saturated cyclic amines: reaction scope and mechanism.

Transition-metal-catalyzed sp(3) C-H activation has emerged as a powerful approach to functionalize saturated cyclic amines. Our group recently disclosed a direct catalytic arylation reaction of piperidines at the α position to the nitrogen atom. 1-(Pyridin-2-yl)piperidine could be smoothly α-arylated if treated with an arylboronic ester in the presence of a catalytic amount of [Ru3(CO)12] and one equivalent of 3-ethyl-3-pentanol. A systematic study on the substrate and reagent scope of this transformation is disclosed in this paper. The effect of substitution on both the piperidine ring and the arylboronic ester has been investigated. Smaller (pyrrolidine) and larger (azepane) saturated ring systems, as well as benzoannulated derivatives, were found to be compatible substrates with the α-arylation protocol. The successful use of a variety of heteroarylboronic esters as coupling partners further proved the power of this direct functionalization method. Mechanistic studies have allowed for a better understanding of the catalytic cycle of this remarkable transformation featuring an unprecedented direct transmetalation on a Ru(II)-H species.

Palladium-catalyzed arylation of simple arenes with iodonium salts.

The development of an arylation protocol for simple arenes with diaryliodonium salts using the Herrmann-Beller palladacycle catalyst is reported. The reaction takes simple aromatic feedstocks and creates valuable biaryls for use in all sectors of the chemical industry.

The role of the alcohol and carboxylic acid in directed ruthenium-catalyzed C(sp3)-H α-alkylation of cyclic amines.

A general directed Ru-catalyzed C(sp(3))-H α-alkylation protocol for piperidines (less-reactive substrates than the corresponding five-membered cyclic amines) has been developed. The use of a hindered alcohol (2,4-dimethyl-3-pentanol) as the solvent and catalyst activator, and a catalytic amount of trans-1,2-cyclohexanedicarboxylic acid is necessary to achieve a high conversion to product. This protocol was used to effectively synthesize a number of 2-hexyl- and 2,6-dihexyl piperidines, as well as the alkaloid (±)-solenopsin A. Kinetic studies have revealed that the carboxylic acid additive has a significant effect on catalyst initiation, catalyst longevity, and reverses the reaction selectivity compared with the acid-free reaction (promotes alkylation versus competing alkene reduction).

A sequential direct arylation/Suzuki-Miyaura cross-coupling transformation of unprotected 2'-deoxyadenosine affords a novel class of fluorescent analogues.

Novel rigid 8-biaryl-2'-deoxyadenosines with tuneable fluorescent properties can be accessed by an efficient sequential catalytic Pd(0)-coupling approach.

Pd(0)/Cu(I)-mediated direct arylation of 2'-deoxyadenosines: mechanistic role of Cu(I) and reactivity comparisons with related purine nucleosides.

Pd/Cu-mediated direct arylation of 2'-deoxyadenosine with various aryl iodides provides 8-arylated 2'-deoxyadenosine derivatives in good yields. Following significant reaction optimization, it has been determined that a substoichiometric quantity of piperidine (secondary amine) in combination with cesium carbonate is necessary for effective direct arylation. The general synthetic protocol allows lower temperature direct arylations, which minimizes deglycosylation. The origin of the piperidine effect primarily derives from the in situ generation of Pd(OAc)(2)[(CH(2))(5)NH](2). Various copper(I) salts have been evaluated; only CuI provides good yields of the 8-arylated-2'-deoxyadenosines. Copper(I) appears to have a high binding affinity for 2'-deoxyadenosine, which explains the mandatory requirement for stoichiometric amounts of this key component. The conditions are compared with more general direct arylation protocols, e.g., catalytic Pd, ligand, acid additives, which do not employ copper(I). In each case, no detectable arylation of 2'-deoxyadenosine was noted. The conformational preferences of the 8-aryl-2'-deoxyadenosine products have been determined by detailed spectroscopic (NMR) and single crystal X-ray diffraction studies. Almost exclusively, the preferred solution-state conformation was determined to be syn-C2'-endo (ca. 80%). The presence of a 2-pyridyl group at the 8-position further biases the solution-state equilibrium toward this conformer (ca. 88%), due to an additional H-bond between H1' and the pyridyl nitrogen atom. The Pd/Cu catalyst system has been found to be unique for adenosine type substrates, the reactivity of which has been placed into context with the reported direct arylations of related 1H-imidazoles. The reactivity of other purine nucleosides has been assessed, which has revealed that both 2'-deoxyguanosine and guanosine are incompatible with the Pd/Cu-direct arylation conditions. Both substrates appear to hinder catalysis, akin to the established inhibitory effects in Suzuki cross-couplings with arylboronic acids.