Recent advances in the chemo-biological characterization of decalin natural products and unraveling of the workings of Diels-Alderases

Identification and characterization of a novel decalin derivative with anti-Candida activity from Streptomyces chrestomyceticus strain ADP4

A novel decalin derivative, trans-1-oxo-2,4-diacetylaminodecalin (1) with anti-Candida activity, had been isolated from Streptomyces chrestomyceticus strain ADP4. The structure of the compound was determined from the analysis of spectral data (LCMS/MS, UV, FTIR, 1D- and 2D-NMR). The anti-Candida activity of 1 was specific to Candida albicans and Candida auris. Further, it displayed inhibition of the early-stage biofilm of C. albicans. In-silico analysis of the compound revealed its drug likeness properties without any violations and PAINS alert when investigated for ADME properties. Along with the overall bioavailability, compound 1 did not show any predicted bioaccumulation and mutagenicity in the analysis by TEST software. Non-cytotoxic property was further confirmed by in-vitro assay on the HepG2 cell line.

Elucidation of the stereocontrol mechanisms of the chemical and biosynthetic intramolecular Diels-Alder cycloaddition for the formation of bioactive decalins

The intramolecular Diels-Alder reaction (IMDA) is a powerful method for regioselective and stereoselective construction of functionalised decalin skeletons, and the recent discovery of enzymes that catalyse IMDA cycloaddition in biosynthesis has generated considerable interest. This study focused on the role of the absolute configuration of the C-6 carbon of the substrate polyene in the stereocontrol of the IMDA reaction catalysed by Fsa2 and Phm7, which construct different enantiomeric decalin skeletons. Their enantiomeric precursor polyenes were synthesised and subjected to enzymatic or thermal IMDA reactions to isolate various diastereomeric decalines and determine their absolute configuration. Furthermore, density functional theory calculations were performed to elucidate the stereocontrol mechanism underlying the formation of decalin. The results showed that Fsa2 exhibits the same equisetin-type stereoselectivity for enantiomeric substrates regardless of the 6-methyl group configuration of the substrate, while Phm7 shows two types of stereoselectivity depending on the configuration of the 6-methyl group. We also found a unique stereochemistry-activity relationship in antibacterial activity for decalin diastereomers, including new derivatives. This study provides new insights into the stereoselectivity of DAase, which is important in the synthesis of natural product skeletons.

cis-Decalin-containing tetramic acids as inhibitors of insect steroidogenic glutathione S-transferase Noppera-bo

Decalin-containing tetramic acid is a bioactive scaffold primarily produced by filamentous fungi. The structural diversity of this group of compounds is generated by characteristic enzymes of fungal biosynthetic pathways, including polyketide synthase/nonribosomal peptide synthetase hybrid enzymes and decalin synthase, which are responsible for the construction of a linear polyenoyl tetramic acid structure and stereoselective decalin formation via the intramolecular Diels-Alder reaction, respectively. Compounds that differed only in the decalin configuration were collected from genetically engineered mutants derived from decalin-containing tetramic acid-producing fungi and used for a structure-activity relationship study. Our evaluation of biological activities, such as cytotoxicity against several cancer cell lines and antibacterial, antifungal, antimalarial, and mitochondrial inhibitory activities, demonstrated that the activity for each assay varies depending on the decalin configurations. In addition to these known biological activities, we revealed that the compounds showed inhibitory activity against the insect steroidogenic glutathione S-transferase Noppera-bo. Engineering the decalin configurations would be useful not only to find derivatives with better biological activities but also to discover overlooked biological activities.

Additivity of Diene Substituent Gibbs Free Energy Contributions for Diels-Alder Reactions between Me2C=CMe2 and Substituted Cyclopentadienes

Systematic computational studies of pericyclic Diels-Alder reactions between (H₃C)₂C=C(CH₃)₂, 1, and all permutations of substituted cyclopentadienes c-C₅R¹R²R³R⁴R^5aR^5b (R = H, CH₃, CF₃, F) allowed isolation of substitutional effects on Gibbs free energy barrier heights and reaction Gibbs free energies. "Average Substitution Gibbs Free Energy Correction" ΔG _ASC# ^‡/ΔG _ASC# values for each substituent in each position appeared to be additive. Substituent effects on barriers showed interesting contrasts. Methyl substitution at positions 5a and 5b increased barriers significantly, while substitution at all other positions had essentially no impact. In contrast, fluoro substitution at positions 5a and 5b lowered barriers more than substitution at other positions. Trifluoromethyl substitution mixed these effects, in that substitution at positions 5a and 5b increased barriers, but substitution at other positions lowered them. Despite the variances, ΔG _ASC# ^‡/ΔG _ASC# values allowed reliable prediction of barriers and exergonicities for reactions between 1 and highly substituted cyclopentadienes, and between 1 and cyclopentadienes with random mixtures of CH₃/CF₃/F substituents. ΔG _ASC# ^‡/ΔG _ASC# values were correlated with steric considerations and quantum theory of atoms in molecules (QTAIM) calculations. Overall, the ASC values provide a resource for predicting which Diels-Alder reactions of this type should occur at rapid rates and/or give stable bicyclic products.

Enzymatic cis-Decalin Formation in Natural Product Biosynthesis

Stereoselective synthesis of cis-decalin structures using [4 + 2] cycloaddition is challenging. We explored the biosynthetic pathway of the fungal natural product fischerin (1) to identify a new pericyclase FinI that can catalyze such a reaction. The cocrystal structure of FinI, a predicted O-methyltransferase, with the product and SAM provides insight into cis-decalin formation in nature.