RESUMEN

The effectiveness and limitations of density functional theory (DFT) calculations in the structural determination of complexed and conformationally flexible natural products were demonstrated using the cyclohelminthols CP-1 (1) CP-2 (2), CP-3 (3), and CP-4 (4) newly isolated from Helminthosporium velutinum yone96. Prior to DFT calculations, the structures were tentatively assigned using conventional spectroscopic analyses. The structures were verified with reference to DFT-derived 13C and 1H NMR chemical shifts, 3JHH and nJCH values, and electronic circular dichroism (ECD) spectra. The 13C chemical shift calculations were very effective for verifying the ring-structure moieties but less effective for verifying the geometry of the side chain in which the juncture asymmetric carbon (C-16) was apart from the ring-structure moiety. However, 1H chemical shift calculations compensated for the imperfection of the latter. ECD spectral calculations were used to determine the absolute configurations. Calculations for virtual simple model molecules enabled us to evaluate the reliability of the ECD spectral calculation and derive the chiral torsion responsible for the characteristic Cotton effects.

RESUMEN

Oxygenated cyclopentene systems are unique structural motifs found in fungal polyketides such as terrein, cyclohelminthols, and palmaenones. Here we report the identification of the biosynthetic gene clusters for cyclohelminthols and palmaenones and the functional characterization of the polyketide synthases and halogenases involved in the construction of 6-hydroxymellein derivatives. Heterologous expression in Aspergillus oryzae demonstrated that 6-hydroxymellein is a common biosynthetic intermediate and that chlorination occurs in the early stages of its products' biosynthesis. This was further confirmed by in vitro enzymatic reactions conducted in the presence of recombinant proteins. Plausible means of biogenesis of fungal polyketides from 6-hydroxymellein derivatives, additionally supported by the reported labeling patterns of terrein and structurally related fungal polyketides, are also discussed. This study sets the stage for elucidation of the biosynthetic machinery of fungal polyketides of this type.

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RESUMEN

Cyclohelminthols Y1-Y4 (1-4) were isolated from the culture broth of Helminthosporium velutinum yone96. These compounds are diastereomers to each other featuring 3-azabicyclo[3.1.0]hexane-6-spirocyclopentane linked with a cyclopentanespirocyclopropane framework. Their planar structures were established via the comparison of their spectra with the simpler analogue cyclohelminthol X as well as analysis of their HMBC spectra. Although the proton-deficient core frameworks of 1-4 prevented us from obtaining configurational information via conventional NMR analysis, their total structures involving the relative and absolute configurations were established using density functional theory (DFT)-based molecular modeling calculations. The present study demonstrates the effectiveness of the comparison between the theoretical and experimental δ13C values for stereochemical analysis by focusing on the carbons that show relatively large δ13C deviations among the isomers. The G-ring of these molecules most likely originates from the cyclopropanation of the C6C7 double bond with the carbene equivalent 6 derived from cyclohelminthol IV (7), which was isolated from the same producer fungus. Preliminary biological experiments revealed the potent cytotoxicity of the (6 S)-isomers against COLO201 cells, whereas the (6 R)-isomers exhibited weak activity. The antifungal assay with Cochiobolus miyabeanus showed a slightly different profile.

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Helminthosporium velutinum yone96 produces cyclohelminthol X (1), a unique hexa-substituted spirocyclopropane. Although its molecular formula and NMR spectral data resemble those of AD0157, being isolated from marine fungus Paraconiothyrium sp. HL-78-gCHSP3-B005, our detailed analyses disclosed a totally different structure. Chemical shift calculations and electronic circular dichroism spectral calculations were quite helpful to establish the structure, when those were performed based on density functional theory. The carbon framework of cyclohelminthols I-IV is found at the C1-C8 propenylcyclopentene substructure of 1. Thus, 1 is assumed to be biosynthesized by cyclopropanation between an oxidized form of cyclohelminthol IV and a succinic anhydride derivative 4. Cytotoxicity for two cancer cell lines and proteasome inhibition efficiency are measured.

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Several 1-(alpha-tert-butylcinnamoyl)imidazoles were prepared to examine their fungicidal activity. The (Z)-4-chlorocinnamoyl derivative was prepared from (anti)-2-tert-butyl-3-(4-chlorophenyl)-3-hydroxypropanoic acid by treating with 1,1'-carbonyldiimidazole and a subsequent beta-elimination reaction at an elevated temperature. The (Z)-isomer of the 4-chlorocinnamoyl derivative showed good fungicidal activity against Erysiphe graminis and Botrytis cinerea in pot tests, whereas the corresponding (E)-isomer derived from the (Z)-isomer through photoisomerization was much less active.

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