Chiral amino acid scaffolds yield new antifungal leads against Aspergillus

A team of chemists in Armenia has reported the design and laboratory synthesis of a family of unnatural amino acids built around arylalkylamine fragments, and their tests show that two members of the series can suppress the growth of pathogenic Aspergillus fungi by more than two-thirds at the highest concentration examined. The work, published as an open-access original article in the journal Amino Acids, combines a well-established asymmetric synthesis platform with a structured antifungal screening program against three clinically relevant species: Aspergillus flavus, Aspergillus fumigatus, and Aspergillus candidus.

The synthetic strategy centers on a square-planar Ni(II) complex of a Schiff base derived from dehydroalanine and the chiral auxiliary (S)-BPB. This complex, designated compound 1, serves as a template that holds the substrate in a defined geometry while benzylamine derivatives 2a through 2g add nucleophilically to its carbon-carbon double bond under basic catalysis, with potassium carbonate as the base and acetonitrile as the solvent at 50 degrees Celsius. The reaction is thermodynamically controlled, which favors formation of the main (S,S)-diastereomer of each addition product with high diastereomeric excess, reported at 92 to 95 percent across the series. The structures and absolute configurations of the resulting complexes 3a through 3g were established by physicochemical methods after chromatographic purification, including circular dichroism spectroscopy, with the (S)-configuration of the alpha-carbon assigned by analogy to previously characterized Ni(II) complexes using the positive sign of the optical rotation.

After the addition step, the target amino acids 4a through 4g were liberated by acid hydrolysis of the diastereomeric complexes, followed by ion-exchange demineralization and crystallization from aqueous ethanol. Chiral HPLC analysis revealed enantiomeric excesses of 91 to 98 percent for all compounds except one notable outlier, compound 4b, which reached only 57 percent despite its precursor complex having been obtained with a diastereomeric excess of 84 percent. The authors attribute this loss of optical purity not to racemization during hydrolysis, which they consider unlikely, but to partial epimerization of the (S)-configured alpha-carbon during isolation, including ion-exchange adsorption, desorption, and crystallization. They suggest that a sensitive neighboring imino group may increase the acidity and mobility of the alpha-proton, facilitating this epimerization, a phenomenon previously reported for structurally related systems.

An appealing practical feature of the methodology is the fate of the chiral auxiliary. The (S)-BPB auxiliary was regenerated from the spent complexes with a nearly quantitative yield exceeding 96 percent, without loss of enantiomeric purity, and can be reused. The overall approach furnished the amino acids in crystalline form in moderate yields, ranging from 38 percent for compound 4g to 55 percent for compound 4a, with the individual complexes themselves isolated in yields of 85 to 95 percent. Detailed characterization of every intermediate and final product was provided, including melting points, optical rotations, elemental analyses, proton and carbon NMR spectra, and liquid chromatography-mass spectrometry data.

The biological evaluation focused on fungistatic activity against three fungal test strains obtained from the National Collection of Microorganism Cultures of Armenia: Aspergillus flavus 8454, Aspergillus candidus 10,550, and Aspergillus fumigatus 8344. These species were chosen because they are pathogenic or opportunistically pathogenic to humans and animals; Aspergillus flavus, for example, can affect the paranasal sinuses and, like other members of the genus, can produce aflatoxins with mutagenic, carcinogenic, and allergenic effects. Stock solutions of the compounds were added to molten Czapek agar medium in volumes corresponding to final concentrations of 0.167, 0.333, and 0.500 millimolar. Plates were inoculated with the test organisms, incubated at 28 degrees Celsius for five to seven days, and all experiments were performed in triplicate. Growth on medium without any compound served as the negative control, and the reference antifungal drug fluconazole, tested under identical conditions, served as the positive control. Mycelial growth and sporulation were evaluated visually and with a binocular magnifier, and inhibition was calculated as a percentage relative to the negative control.

All tested compounds, 4a through 4f, showed dose-dependent antifungal effects, with inhibition rising linearly as the concentration increased from 0.167 to 0.500 millimolar, although the magnitude of activity varied substantially with the nature of the arylalkyl substituent. Compound 4d, which bears a 4-fluorophenethyl group, demonstrated the highest and most balanced activity in the series, approaching the efficacy of fluconazole. At the highest tested concentration it inhibited the growth of Aspergillus fumigatus by 67.86 percent, Aspergillus candidus by 64.00 percent, and Aspergillus flavus by 56.67 percent. Compound 4e, carrying a 4-chlorophenethyl group, performed best at the initial and intermediate doses, achieving 56.25 and 62.50 percent inhibition against Aspergillus candidus and 47.06 and 58.82 percent against Aspergillus fumigatus at 0.167 and 0.333 millimolar respectively, and its inhibitory effect on sporulation reached 68.75 percent at the maximum concentration, a level the authors describe as comparable to fluconazole against the same strains.

The remaining compounds showed weaker or more limited activity. Compounds 4a and 4f exhibited moderate fungistatic effects, with maximum inhibition between 35 and 52 percent at 0.500 millimolar; compound 4a was completely ineffective against Aspergillus fumigatus at the lowest dose, with zero percent inhibition, but its activity rose markedly with concentration. Compounds 4b and 4c proved the least promising: compound 4b showed a plateau at 26.32 percent inhibition against Aspergillus flavus regardless of concentration, and neither compound exceeded 37.5 percent maximum inhibition, well below the leading derivatives and the standard drug. The authors conclude that structural modifications within the series exert a decisive influence on the fungistatic properties of these molecules.

Fluconazole itself showed a strikingly species-dependent profile. It was most active against Aspergillus flavus, producing 60.71 percent inhibition already at 0.167 millimolar and 85.71 percent at 0.500 millimolar, but its activity against Aspergillus fumigatus and Aspergillus candidus was weaker, reaching only 53.85 and 46.15 percent respectively at the highest concentration. This opened a window for the synthetic compounds: estimated IC50 values, calculated by linear interpolation between consecutive concentrations bracketing 50 percent inhibition, showed that compounds 4e and 4d were more active than fluconazole against Aspergillus fumigatus, with estimated values of 0.209 and 0.292 millimolar compared with 0.444 millimolar for the drug. Against Aspergillus candidus, compound 4e was the most active derivative with an estimated IC50 below 0.167 millimolar, followed by compound 4d at 0.271 millimolar, while fluconazole and compound 4f did not reach 50 percent inhibition against that strain within the tested range. Against Aspergillus flavus, fluconazole remained the most active agent with an IC50 below 0.167 millimolar, ahead of compounds 4d, 4f, and 4e. The authors note that these are estimated values within the tested concentration range and recommend further dose-response studies using a broader concentration range and nonlinear regression analysis.

The design rationale behind the series connects two strands of medicinal chemistry. Unnatural alpha-amino acids allow precise control over stereochemistry, conformational rigidity, lipophilicity, and metabolic stability, and have attracted growing attention as platforms for antimicrobial and antifungal drug discovery. Arylalkylamine motifs, including benzylamine fragments, contribute favorable lipophilic and electronic properties and are capable of interacting with essential fungal targets such as lanosterol 14-alpha-demethylase, a key enzyme in ergosterol biosynthesis, although the present study did not test this mechanism directly. The authors frame their work against the backdrop of increasing antifungal resistance, particularly to azole-based therapies, and the limited number of available drug classes, which together underscore the need for new antifungal chemotypes.

Several caveats temper the findings. The activity was measured as growth and sporulation inhibition on agar plates over a narrow concentration window, with only three concentrations tested in triplicate, and the compounds were evaluated against a single strain of each species. The effects observed are fungistatic rather than fungicidal, and no mechanistic, toxicity, or in vivo data are reported in this study. Nevertheless, the authors identify compounds 4d and 4e as promising lead structures, noting that they ensure stable inhibition of pathogenic Aspergillus growth at the level of 56 to 68 percent, and they propose the work as a basis for further structural optimization, mechanistic investigation, and in vivo studies aimed at developing new therapeutics for Aspergillus-associated infections.

Subject of Research: Asymmetric synthesis of arylalkylamine-functionalized alpha-alanine analogues and evaluation of their antifungal activity against Aspergillus species

Article Title: Asymmetric synthesis and antifungal evaluation of arylalkylamine-functionalized β-substituted (s)-α-alanine analogues

Article References: Stepanyan, L. A., Gevorgyan, S., Ghazaryan, K. R., Minasyan, E. V., Tsaturyan, A. H., Mkrtchyan, A. F., & Saghyan, A. S. (2026). Asymmetric synthesis and antifungal evaluation of arylalkylamine-functionalized β-substituted (s)-α-alanine analogues. Amino Acids, 58(1), Article 58. https://doi.org/10.1007/s00726-026-03558-6

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DOI: 10.1007/s00726-026-03558-6

Keywords: asymmetric synthesis, alpha-amino acids, antifungal activity, Aspergillus, Ni(II) complex, dehydroalanine, fluconazole, stereoselectivity, enantiomeric excess, fungistatic, Czapek agar, medicinal chemistry