Andrea M. Escalante

Evidence for the Mechanism of Action of the Antifungal Phytolaccoside B Isolated from Phytolacca tetramera Hauman

Phytolaccoside B (1), an antifungal monodesmoside triterpenoid glycoside isolated from berries of Phytolacca tetramera Hauman (Phytolaccaceae), alters the morphology of yeasts and molds. The malformations were similar to those produced by enfumafungin, a known inhibitor of (1-->3)-beta-D-glucan synthase, an enzyme that catalyzes the synthesis of (1-->3)-beta-D-glucan, one of the major polymers of the fungal cell wall. However, enzymatic assays revealed that 1 did not inhibit (1-->3)-beta-D-glucan synthase, but it did produce a notable enhancement of the chitin synthase 1 activity and, concomitantly, a rise in chitin, another important polymer of the fungal cell walls. This finding was corroborated by fluorescence microscopy and also by quantification of the chitin. In addition, a 2-fold increase in the thickness of the fungal cell wall was observed with transmission electronic microscopy. On the other hand, 1 neither bound to ergosterol nor caused hemolysis of red blood cells, although some fungal membrane damage was observed at the MIC of 1.

Isolation of antifungal saponins from Phytolacca tetramera, an Argentinean species in critic risk

The methanolic extract of the berries of Phytolacca tetramera, an Argentinean species submitted to a great anthropic impact, and therefore in critic risk of extinction, not previously studied, showed antifungal activity against opportunistic pathogenic fungi. Through fractionation of the extract followed by agar dilution assays, three monodesmosidic triterpenoid saponins have been isolated from the butanolic extract of P. tetramera. The structures were established as phytolaccosides: B [3-O-beta-D-xylopiranosyl-phytolaccagenin], E [3-O-beta-D-glucopyranosyl-(1-->4)-beta-D-xylopiranosyl-phytolaccagenin]. and F [3-O-alpha-L-rhamnopyranosyl-(1-->2)-beta-D-glucopyranosyl-(1-->2)-beta-D-xylopyranosyl-phytolaccagenic acid]. The three saponins belong to the olean-type triterpenoid saponins, with 28,30 dicarboxylic groups and an olefinic double bond on C-12. Phytolaccosides B and E but not phytolaccoside F, showed antifungal activities against a panel of human pathogenic opportunistic fungi. Phytolaccoside B was the most active compound and showed the broadest spectrum of action. The most sensitive fungus was Trichophyton mentagrophytes.

Covalent double level dynamic combinatorial libraries: selectively addressable exchange processes

Hydrazones and disulfides have been combined in one dynamic system: hydrazones were exchanged by acid catalysis in the presence of disulfide and a thiol group without interference; neutralization of the reaction medium turns off the exchange of hydrazones and, at the same time, activates thiolate-disulfide exchange.

Simultaneous and orthogonal covalent exchange processes in dynamic combinatorial libraries.

Dynamic combinatorial libraries (DCLs) are constituted by members that are reversibly assembled from a set of building blocks. The functional groups selected to generate the reversible connections between building blocks will affect the conditions required for activation as well as the formation kinetics of the library members (i.e., the velocity at which the DCL responds to external perturbations). Some of the most successful reversible covalent reactions used for the preparation of DCLs are hydrazone, disulfide, and thioester exchange. Although most of the DCLs reported to date have been prepared using one reversible exchange process, there are some examples where two exchange reactions are combined in the same system. Depending on the relative functional state of the exchange processes (activated/inactivated), the system can be simultaneous or orthogonal. In simultaneous DCLs, both exchange reactions occur at the same time, as observed in the dynamic system based in thioesters and disulfides reported by Otto et al. In orthogonal DCLs, each reversible reaction used can be activated under different conditions. Such setup allows the manipulation of the reactions in a sequential order as described for covalent dynamic systems wherein hydrazone exchange and disulfide exchange were combined in aqueous or in organic media. When orthogonal exchange processes are employed, each reversible reaction can be used to explore independently its own dimension in structural space opening the possibility for evolutionary approaches. Each of these strategies increase the DCLs potential molecular diversity in a different manner. In a simultaneous approach, all possible molecular combinations resulting from the different exchange processes are produced in a single step. The whole library molecular space is covered simultaneously. Differently, molecular generation in each step of an orthogonal approach covers a reduced molecular space, allowing optimization of the search in the vicinity of known good targets. Thus, the selection of one strategy (simultaneous/ orthogonal) is a trade-off between exploration and optimization: exploration of a vast space can lead to inefficient search, whereas focusing the search in a limited molecular space can lead to an important loss in the explored diversity, trapping the composition in a local optimum. In this work, we report the first example of a mixed strategy involving simultaneous and orthogonal covalent reactions in one single system through the combination of the exchange of disulfides, thioesters, and hydrazones. As a consequence of altering the activation order of the sequentially addressable processes, biased DCLs are obtained where the history of each DCL is manifested as distinct distribution of products. It has been reported that the exchange of hydrazones in chlorinated solvents can be achieved by acid catalysis with trifluoroacetic acid (TFA), whereas disulfide exchange and thioester exchange proceed in the presence of organic bases such as triethylamine (TEA). The compatibility of these three functional groups and their corresponding exchange processes were evaluated with building blocks incorporating either one disulfide bond and one protected aldehyde group (1), one hydrazide group (2 and 3), one thioester (4), and one thiol group (5) (Figure 1).

A simple synthesis of kaurenoic esters and other derivatives and evaluation of their antifungal activity

Representative esters derived from kaurenoic acid were prepared in order to evaluate their antifungal properties. Alkyl and substituted benzyl esters were obtained in good yield under mild conditions by esterification of kaurenoic acid with the corresponding alkyl halide in KOH-acetone. All synthesized compounds were tested for antifungal properties against pathogenic yeasts, hialohyphomycetes and dermatophytes. Kaurenoic acid and derivatives containing a free carboxyl group were moderately active against dermatophytes.

Two-Stage Amplification of Receptors Using a Multilevel Orthogonal/Simultaneous Dynamic Combinatorial Library

Sequential activation of different reversible exchange reactions in a dynamic combinatorial library allows directed exploration of the chemical space: initially a macrocyclic scaffold is selected by the template and finally side chain and conformational constrains are introduced into such a scaffold.

Discovery of a β-glucosidase inhibitor from a chemically engineered extract prepared through sulfonylation

A semisynthetic β-glucosidase inhibitor was identified from a chemically engineered extract prepared by reaction with benzenesulfonyl chloride. The structure includes a natural histamine portion and a benzenesulfonyl portion introduced during the diversification step.

Dithioacetal Exchange: A New Reversible Reaction for Dynamic Combinatorial Chemistry

Reversibility of dithioacetal bond formation is reported under acidic mild conditions. Its utility for dynamic combinatorial chemistry was explored by combining it with orthogonal disulfide exchange. In such a setup, thiols are positioned at the intersection of both chemistries, constituting a connecting node between temporally separated networks.

Host Amplification in a Dithioacetal-Based Dynamic Covalent Library

Molecular amplification in a dithioacetal-based dynamic library is described for the first time. The homatropine induced selection, amplification, and isolation of one cyclophane host demonstrates the utility of dithioacetal exchange for preparing responsive dynamic libraries. Nuclear magnetic resonance and isothermal titration calorimetry analysis suggest that the amplified macrocycle forms a 1:1 complex with the template. This is the first report about a host/guest system involving a dithioacetal cyclophane.

Rewiring Chemical Networks Based on Dynamic Dithioacetal and Disulfide Bonds

The control of the connectivity between nodes of synthetic networks is still largely unexplored. To address this point we take advantage of a simple dynamic chemical system with two exchange levels that are mutually connected and can be activated simultaneously or sequentially. Dithioacetals and disulfides can be exchanged simultaneously under UV light in the presence of a sensitizer. Crossover reactions between both exchange processes produce a fully connected chemical network. On the other hand, the use of acid, base or UV light connects different nodes allowing network rewiring.