Sándor Antus

Dihydroisocoumarins from fungi : Isolation, structure elucidation, circular dichroism and biological activity

Five known and three new dihydroisocoumarins were isolated from different fungi. The new isocoumarins are 5-chloro-6-hydroxymellein, 5-chloro-4,6-dihydroxymellein and 5,6-dihydroxymellein. The absolute configuration of these secondary metabolites was confirmed by CD measurements and in two cases by X-ray structure analysis.

New Mono‐ and Dimeric Members of the Secalonic Acid Family: Blennolides A–G Isolated from the Fungus Blennoria sp.

Blennolides A-G (2-8), seven unusual chromanones, were isolated together with secalonic acid B (1) from Blennoria sp., an endophytic fungus from Carpobrotus edulis. This is the first reported isolation of the blennolides 2 and 3 (hemisecalonic acids B and E), the existence of which as the monomeric units of the dimeric secalonic acids had long been postulated. A compound of the proposed structure 4 (beta-diversonolic ester) will need to be revised, as its reported data do not fit those of the established structure of blennolide C (4). Other monomers, the blennolides D-F (5-7) seem to be derived from blennolides A (2) and B (3) by rearrangement of the hydroaromatic ring. The heterodimer 8, composed of the monomeric blennolide A (2) and the rearranged 11-dehydroxy derivative of blennolide E (6), extends the ergochrome family with an ergoxanthin type of skeleton. The structures of the new compounds were elucidated by detailed spectroscopic analysis and further confirmed by an X-ray diffraction study of a single crystal of 2. The absolute configurations were determined by TDDFT calculations of CD spectra, including the solid-state CD/TDDFT approach. Preliminary studies showed strong antifungal and antibacterial activities of these compounds against Microbotryum violaceum and Bacillus megaterium, respectively. They were also active against the alga Chlorella fusca and the bacterium Escherichia coli.

Functional significance of five noncanonical Ca2+‐binding sites of human transglutaminase 2 characterized by site‐directed mutagenesis

The multifunctional tissue transglutaminase 2 (TG2) has a four‐domain structure with several Ca2+‐regulated biochemical activities, including transglutamylation and GTP hydrolysis. The structure of the Ca2+‐binding form of the human enzyme is not known, and its Ca2+‐binding sites have not been fully characterized. By mutagenesis, we have targeted its active site Cys, three sites based on homology to Ca2+‐binding residues of epidermal transglutaminase and factor XIIIa (S1–S3), and two regions with negative surface potentials (S4 and S5). CD spectroscopy, antibody‐binding assay and GTPase activity measurements indicated that the amino acid substitutions did not cause major structural alterations. Calcium‐45 equilibrium dialysis and isothermal calorimetric titration showed that both wild‐type and active site‐deleted enzymes (C277S) bind six Ca2+. Each of the S1–S5 mutants binds fewer than six Ca2+, S1 is a strong Ca2+‐binding site, and mutation of one site resulted in the loss of more than one bound Ca2+, suggesting cooperativity among sites. All mutants were deficient in transglutaminase activity, and GTP inhibited remnant activities. Like those of the wild‐type enzyme, the GTPase activities of the mutants were inhibited by Ca2+, except in the case of the S4 and S5 mutants, which exhibited increased activity. TG2 is the major autoantigen in celiac disease, and testing the reactivity of mutants with autoantibodies from celiac disease patients revealed that S4 strongly determines antigenicity. It can be concluded that five of the Ca2+‐binding sites of TG2 influence its transglutaminase activity, two sites are involved in the regulation of GTPase activity, and one determines antigenicity for autoantibodies in celiac patients.

Bioactive nonanolide derivatives isolated from the endophytic fungus Cytospora sp.

Cytospolides F-Q (6-17) and decytospolides A and B (18 and 19), 14 unusual nonanolide derivatives, were isolated from Cytospora sp., an endophytic fungus from Ilex canariensis. The structures were elucidated by means of detailed spectroscopic analysis, chemical interconversion, and X-ray single crystal diffraction. The solution- and solid-state conformers were compared by the combination of experimental methods (X-ray, NMR) supported by DFT calculations of the conformers. Absolute configurations were assigned using the modified Moshers method and solution- and solid-state TDDFT ECD calculations. In an in vitro cytotoxicity assay toward the tumor cell lines of A549, HCT116, QGY, A375, and U973, the γ-lactone 17 demonstrated a potent growth inhibitory activity toward the cell line A-549, while nonanolide 16 with (2S) configuration showed the strongest activity against cell lines A-549, QGY, and U973. A cell cycle analysis indicated that compound 16 can significantly mediate G1 arrest in A549 tumor cells, confirming the important role of the C-2 methyl in the growth inhibition toward the tumor line. The discovery of an array of new nonanolides demonstrates the productivity of the fungus, and it is an example of chemical diversity, extending the nonanolide family by derivatives formed by ring cleavage, oxidation, esterification, and Michael addition.

Convenient preparation of 1,1-diacetates from aromatic aldehydes catalysed by zinc-montmorillonite

1,1-Diacetates were produced from a variety of aromatic aldehydes using Zn-montmorillonite as a catalyst. The procedure generally resulted in good yields (98%) of aromatic aldehydes including those carrying electron donating- or withdrawing substituents. The great advantage of the application of Zn-montmorillonite catalyst is that the reaction takes place under mild conditions. In addition, the zinc cation exchanged form of montmorillonite has a minimal environmental impact.

Absolute Configuration of Two New 6-Alkylatedα-Pyrones (=2H-Pyran-2-ones) fromRavensara crassifolia

The stem bark CH2Cl2 extract of Ravensara crassifolia showed antifungal activity against the phytopathogenic fungus Cladosporium cucumerinum in a bioautographic TLC assay. Activity-guided fractionation afforded two new alpha -pyrones : (6S)-5,6-dihydro-6-[(2R)-2-hydroxy-6-phenylhexyl]-2H-pyran-2-one (1) and (6R)-6-[(4R,6R)-4,6-dihydroxy-10-phenyldec-1-enyl]-5,6-dihydro-2H-pyran-2-one (2). Their structures and absolute configurations were established by NMR spectroscopy, chemical methods, and CD spectroscopy. The antifungal activity against C. cucumerinum was determined for both compounds.

Five flavans from Mariscus psilostachys

Phytochemical investigation of two batches of Mariscus psilostachys led to the isolation and characterization of five flavans and three flavanones. Two flavans are new natural products and their structures have been established as (2S)-4′-hydroxy-5,7,3′-trimethoxyflavan and (±)-5,4′-dihydroxy-7-3′-dimethoxyflavan. Absolute configuration of the optically active compound isolated has been determined on the basis of its CD spectrum. Antimicrobial activity against Candida albicans and Cladosporium cucumerinum were determined for all compounds. The less polar flavans showed to be more active in both TLC assays and dilution assays.

Synthesis of sophorol, violanone, lonchocarpan, claussequinone, philenopteran, leiocalycin, and some other natural isoflavonoids by the oxidative rearrangement of chalcones with thallium(III) nitrate

Isoflavones are conveniently prepared by the oxidative rearrangement of 2′-hydroxy- or 2′-acetoxy-chalcones with thallium(III) nitrate in methanol into 1-(2-hydroxyphenyl)-3,3-dimethoxy-2-phenylpropan-1-ones [e.g.(46)] followed by cyclisation, and the following natural isoflavonoids were synthesised in this way: the isoflavanones sophorol (49) and violanone (68), the isoflavans vestitol (57), duratin (58), mucronulatol (59), laxifloran (60), and Ionchocarpan (61), the isoflavan quinones mucroquinone (66) and claussequinone (67), the pterocarpans philenopteran (69), leiocalycin (54), and 2-hydroxy-3-methoxy-8,9-methylenedioxy-6a,11a-dihydropterocarpan (56).Acid catalysed cyclisation of 1-(2-hydroxy-4-methoxyphenyl)-2-(2-hydroxy-4,5-methylenedioxyphenyl)-3,3-dimethoxypropan-1-one (48) gave no isoflavone but the two tetracyclic compounds, (53)(a derivative of 2-methoxypterocarp-6-ene) and (52)(a benzofuro[2,3-b][1]benzopyran).Claussequinone (67) was rapidly formed by autoxidation of its hydroquinone precursor (64), suggesting that oxidation may also have occurred during the isolation of the metabolite.

New insight into the mechanism of hypervalent iodine oxidation of flavanones

Flavanone (1) on oxidation with iodobenzene diacetate (PIDA) in the presence of sulfuric acid in trimethyl orthoformate (TMOF) undergoes a stereospecific ring contraction by an aryl shift to result in trans methyl 2-aryl-2,3-dihydrobenzo[b]furan-3-carboxylate (4a) as a major product. The mechanism of this transformation has been discussed on the basis of NMR, CD and chiral HPLC evidence.

Biologically active metabolites from fungi. Part 16: New preussomerins J, K and L from an endophytic fungus: Structure elucidation, crystal structure analysis and determination of absolute configuration by CD calculations

Abstract Three known preussomerins, G (1), H (2) and I (3), and three new representatives, J (4), K (5) and L (6), were isolated from an endophytic fungus, a Mycelia sterila, from Atropa belladonna. Their absolute configuration was determined by comparison of calculated and experimental CD spectra.