Enhancing structural diversity through chemical engineering of Ambrosia tenuifolia extract for novel anti-glioblastoma compounds.

Natural products are an unsurpassed source of leading structures in drug discovery. The biosynthetic machinery of the producing organism offers an important source for modifying complex natural products, leading to analogs that are unattainable by chemical semisynthesis or total synthesis. In this report, through the combination of natural products chemistry and diversity-oriented synthesis, a diversity-enhanced extracts approach is proposed using chemical reactions that remodel molecular scaffolds directly on extracts of natural resources. This method was applied to subextract enriched in sesquiterpene lactones from Ambrosia tenuifolia (Fam. Asteraceae) using acid media conditions (p-toluenesulfonic acid) to change molecular skeletons. The chemically modified extract was then fractionated by a bioguided approach to obtain the pure compounds responsible for the anti-glioblastoma (GBM) activity in T98G cell cultures. Indeed, with the best candidate, chronobiological experiments were performed to evaluate temporal susceptibility to the treatment on GBM cell cultures to define the best time to apply the therapy. Finally, bioinformatics tools were used to supply qualitative and quantitative information on the physicochemical properties, chemical space, and structural similarity of the compound library obtained. As a result, natural products derivatives containing new molecular skeletons were obtained, with possible applications as chemotherapeutic agents against human GBM T98G cell cultures.

Thiol-free multicomponent synthesis of non-racemic ß-acyloxy thioethers from biocatalytically obtained chiral halohydrins.

A novel multicomponent chemoenzymatic strategy for the preparation of enantioenriched ß-acyloxy thioethers has been developed. This robust methodology employs mild bases, air atmosphere, room temperature and avoids the use of foul-smelling thiols. Instead, potassium thioacetate is employed as a universal sulfur source. This chemoselective strategy tolerates aromatic and aliphatic components and diverse functional groups. The chirality is enzymatically defined by ADH-catalyzed bioreduction of α-haloketones delivering an enantioenriched halohydrin which is one of the three components, and the optical purity remains untouched in the final product. Semipreparative scale multicomponent reaction affords high yield of the products (up to 96%).

Psilostachyins as trypanocidal compounds: Bioguided fractionation of Ambrosia tenuifolia chemically modified extract.

This work focusses on the chemical diversification of an Ambrosia tenuifolia extract and its bioguided fractionation, aiming to unveil the chemical entity responsible for the trypanocidal activity. Besides, a revision of the phytochemical study of this species, based on previous reports of the antiparasitic psilostachyins A and C as main compounds, was conducted. To improve the biological properties of a plant extract through a simple chemical reaction, the oxidative diversification of the dichloromethane extract of this plant species was carried out. A bioguided fractionation of a chemically modified extract was performed by evaluating the inhibitory activity against Trypanosoma cruzi trypomastigotes. This experiment led to the isolation of one of the most active compounds. In general terms, epoxidized metabolites were obtained as a result of the oxidation of the major metabolite of the species. The trypanocidal activity of some tested metabolites overperformed the reference drug, benznidazole, displaying no cytotoxicity at trypanocidal concentrations. Key structure-activity relationships were obtained for designing previously undescribed antiparasitic sesquiterpene lactones.

Room Temperature Ionic Liquids in Asymmetric Hetero-Ene Type Reactions: Improving Organocatalyst Performance at Lower Temperatures.

Room temperature ionic liquids (RTILs) have been widely used as (co)solvents in several catalytic processes modifying, in most of the cases, the catalyst activity and/or the selectivity for the studied reactions. However, there are just a few examples of their use in hydrogen bonding organocatalysis. In this paper, we show the positive effect of a set of imidazole-based ionic liquids ([bmim]BF4 and [hmim]PF6) in the enantioselective addition of formaldehyde tert-butylhydrazone to prochiral α-keto esters catalyzed by a sugar-based chiral thiourea. Reactions performed in the presence of low percentages of RTILs led to an increase of the catalyst activity, thereby making possible to work at lower temperatures. Thus, the chiral tert-butyl azomethyl tertiary alcohols could be obtained with moderate to good conversions and higher enantioselectivities for most of the studied substrates when using up to 30 vol% of [hmim]PF6 as a cosolvent in processes performed in toluene.

Thiol-free chemoenzymatic synthesis of ß-ketosulfides.

A preparation of ß-ketosulfides avoiding the use of thiols is described. The combination of a multicomponent reaction and a lipase-catalysed hydrolysis has been developed in order to obtain high chemical diversity employing a single sulfur donor. This methodology for the selective synthesis of a set of ß-ketosulfides is performed under mild conditions and can be set up in one-pot two-step and on a gram-scale.

Biotransformation of dehydro-epi-androsterone by Aspergillus parasiticus: Metabolic evidences of BVMO activity.

The research on the synthesis of steroids and its derivatives is of high interest due to their clinical applications. A particular focus is given to molecules bearing a D-ring lactone like testolactone because of its bioactivity. The Aspergillus genus has been used to perform steroid biotransformations since it offers a toolbox of redox enzymes. In this work, the use of growing cells of Aspergillus parasiticus to study the bioconversion of dehydro-epi-androsterone (DHEA) is described, emphasizing the metabolic steps leading to D-ring lactonization products. It was observed that A. parasiticus is not only capable of transforming bicyclo[3.2.0]hept-2-en-6-one, the standard Baeyer-Villiger monooxygenase (BVMO) substrate, but also yielded testololactone and the homo-lactone 3ß-hydroxy-17a-oxa-D-homoandrost-5-en-17-one from DHEA. Moreover, the biocatalyst degraded the lateral chain of cortisone by an oxidative route suggesting the action of a BVMO, thus providing enough metabolic evidences denoting the presence of BVMO activity in A. parasiticus. Furthermore, since excellent biotransformation rates were observed, A. parasiticus is a promising candidate for the production of bioactive lactone-based compounds of steroidal origin in larger scales.

Stereoselective one-pot synthesis of ß-alkylsulfide enol esters. Base-triggered rearrangement under mild conditions.

A stereoselective one-pot procedure was developed to prepare S-substituted (Z)-enol esters through a base-triggered rearrangement. This transition metal-free multicomponent approach can be performed under an air atmosphere at room temperature, tolerates a wide set of chemical functionalities and generally affords high isolated yields. The (Z)-selectivity arises from the [1,4]-S- to O-acyl migration.

Coupling biocatalysis and click chemistry: one-pot two-step convergent synthesis of enantioenriched 1,2,3-triazole-derived diols.

A fully convergent one-pot two-step synthesis of different chiral 1,2,3-triazole-derived diols in high yields and excellent enantio- and diastereoselectivities has been achieved under very mild conditions in aqueous medium by combining a single alcohol dehydrogenase (ADH) with a Cu-catalysed 'click' reaction.

Coumarin metabolic routes in Aspergillus spp.

Coumarin metabolism by several Aspergillus strains was studied. Aspergillus ochraceus and Aspergillus niger carried out the reduction of the C3-C4 double bond to yield dihydrocoumarin in 24h. Meanwhile, the first strain did not transform dihydrocoumarin after 7d, A. niger demonstrated to have two divergent catabolic pathways: (a) the lactone moiety opening and further reduction of the carboxylic acid furnishing the primary alcohol 2-(3-hydroxypropyl)phenol and, (b) the hydroxylation of the aromatic ring of dihydrocoumarin at a specific position to give 6-hydroxy-3,4-dihydrochromen-2-one. Aspergillus flavus did not perform double bond reductions, and only produced oxygenated metabolites, mainly 5-hydroxycoumarin. Enzyme-specific inhibitors and a coumarin analogous were useful to confirm the A. niger catabolic route.

Ketone-alcohol hydrogen-transfer equilibria: is the biooxidation of halohydrins blocked?

To ensure the quasi-irreversibility of the oxidation of alcohols coupled with the reduction of ketones in a hydrogen-transfer (HT) fashion, stoichiometric amounts of α-halo carbonyl compounds have been employed as hydrogen acceptors. The reason that these substrates lead to quasi-quantitative conversions has been tacitly attributed to both thermodynamic and kinetic effects. To provide a clear rationale for this behavior, we investigate herein the redox equilibrium of a selected series of ketones and 2-propanol by undertaking a study that combines experimental and theoretical approaches. First, the activity of the (R)-specific alcohol dehydrogenase from Lactobacillus brevis (LBADH) with these substrates was studied. The docking of acetophenone/(R)-1-phenyethanol and α-chloroacetophenone/(S)-2-chloro-1-phenylethanol in the active site of the enzyme confirms that there seems to be no structural reason for the lack of reactivity of halohydrins. This assumption is confirmed by the fact that the corresponding aluminum-catalyzed Meerwein-Ponndorf-Verley-Oppenauer (MPVO) reactions afford similar conversions to those obtained with LBADH, showing that the observed reactivity is independent of the catalyst employed. While the initial rates of the enzymatic reductions and the IR ν(C=O) values contradict the general belief that electron-withdrawing groups increase the electrophilicity of the carbonyl group, the calculated ΔG values of the isodesmic redox transformations of these series of ketones/alcohols with 2-propanol/acetone support the thermodynamic control of the reaction. As a result, a general method to predict the degree of conversion obtained in the HT-reduction process of a given ketone based on the IR absorption band of the carbonyl group is proposed, and a strategy to achieve the HT oxidation of halohydrins is also shown.

Biocatalysed concurrent production of enantioenriched compounds through parallel interconnected kinetic asymmetric transformations.

Parallel interconnected kinetic asymmetric transformations were performed in order to obtain enantioenriched derivatives starting from a set of racemic or prochiral compounds. Thus, in a one-pot reaction using two redox biocatalysts (a BVMO and an ADH) and a catalytic amount of cofactor that acts as a mediator, enantioenriched ketones, sulfoxides, and sec-alcohols were concurrently obtained in a strict parallel way, minimising the quantity of reagents employed. By selecting the appropriate biocatalysts, this methodology represents a potential tool for performing stereodivergent transformations.

Tandem concurrent processes: one-pot single-catalyst biohydrogen transfer for the simultaneous preparation of enantiopure secondary alcohols.

A novel one-pot tandem biohydrogen transfer process to concurrently obtain two enantiopure sec-alcohols is presented; thus, using a suitable single enzyme and a catalytic amount of cofactor, several interesting building blocks could be easily achieved in an enantiocomplementary fashion, minimizing dramatically the quantity of reagents usually employed in the "coupled-substrate" approach.

Oligopeptides and copeptides of homochiral sequence, via beta-sheets, from mixtures of racemic alpha-amino acids, in a one-pot reaction in water; relevance to biochirogenesis.

As part of our studies on the biochirogenesis of peptides of homochiral sequence during early evolution, the formation of oligopeptides composed of 14-24 residues of the same handedness in the polymerization of dl-leucine (Leu), dl-phenylalanine (Phe), and dl-valine (Val) in aqueous solutions, by activation with N, N'-carbonyldiimidazole and then initiation with a primary amine, in a one-pot reaction, was demonstrated by MALDI-TOF MS using deuterium enantio-labeled alpha-amino acids. The formation of long isotactic peptides is rationalized by the following steps occurring in tandem: (i) creation of a library of short diasteroisomeric oligopeptides containing isotactic peptides in excess in comparison to a binomial kinetics, as a result of an asymmetric induction exerted by the N-terminal residue of a given handedness; (ii) precipitation of the less soluble racemic isotactic penta- and hexapeptides in the form of beta-sheets that are delineated by homochiral rims; (iii) regio-enantiospecific chain elongation occurring heterogeneously at the beta-sheets/solution interface. Polymerization of l-Leu with l-isoleucine (Ile) or l-Phe with l- (1) N-Me-histidine yielded mixtures of copeptides containing both residues. In contrast, in the polymerization of the corresponding mixtures of l- + d-alpha-amino acids, the long oligopeptides were composed mainly from oligo- l-Leu and oligo- d-Ile in the first system and oligo- d-Phe in the second. Furthermore, in the polymerization of mixtures of hydrophobic racemic alpha-amino acids dl-Leu, dl-Val, and dl-Phe and with added racemic dl-alanine and dl-tyrosine, copeptides of homochiral sequences are most dominantly represented. Possible routes for a spontaneous "mirror-symmetry breaking" process of the racemic mixtures of homochiral peptides are presented.

Atypical regioselective biohydrolysis on steroidal oxiranes by Aspergillus niger whole cells: some stereochemical features.

5,6-Epoxycholestan-3beta-ol derivatives were hydrolyzed in a diastereoconvergent manner by growing and resting cells of several strains of Aspergillus niger, particularly A. niger ATCC 11394. These strains displayed opposite regioselectivity toward each isomer in an alpha and beta epoxide mixture, thus, the nucleophilic attack took place at the less substituted and the most substituted carbon atom on each diasteromer, respectively. These biocatalysts opened trisubstituted oxiranes but were unable to hydrolyze the disubstituted oxiranes in the tested sterol derivatives. These findings suggest that A. niger strains possess another hydrolytic ability different from the commercial A. niger epoxide hydrolase (EH) that did not accept this kind of steroidal oxiranes as substrates.