Importance and Difficulties in the Use of Chiroptical Methods to Assign the Absolute Configuration of Natural Products: The Case of Phytotoxic Pyrones and Furanones Produced by <i>Diplodia corticola</i>

α-Pyrones and furanones are metabolites produced by <i>Diplodia corticola</i>, a pathogen of cork oak. Previously, the absolute configuration (AC) of diplopyrone was defined by chiroptical methods and Mosher’s method. Using X-ray and chiroptical methods, the AC of sapinofuranone C was assigned, while that of the (4<i>S,</i>5<i>S</i>)-enantiomer of sapinofuranone B was established by enantioselective total synthesis. Diplofuranone A and diplobifuranylones A–C ACs are still unassigned. Here electronic and vibrational circular dichroism (ECD and VCD) and optical rotatory dispersion (ORD) spectra are reported and compared with density functional theory computations. The AC of the (4<i>S</i>,5<i>S</i>)-enantiomer of sapinofuranone B and sapinofuranone C is checked for completeness. The AC of diplobifuranylones A–C is assigned as (2<i>S</i>,2′<i>S</i>,5′<i>S</i>,6′<i>S</i>), (2<i>S</i>,2′<i>R</i>,5′<i>S</i>,6′<i>R</i>), and (2<i>S</i>,2′<i>S</i>,5′<i>R</i>,6′<i>R</i>), respectively, with the Mosher’s method applied to define the absolute configuration of the carbinol stereogenic carbon. The AC assignment of sapinofuranones is problematic: while diplofuranone A is (4<i>S</i>,9<i>R</i>), sapinofuranones B and C are (4<i>S</i>,5<i>S</i>) according to ORD and VCD, but not to ECD. To eliminate these ambiguities, ECD and VCD spectra of a di-<i>p</i>-bromobenzoate derivative of sapinofuranone C are measured and calculated. For phytotoxicity studies, it is relevant that all six compounds share the <i>S</i> configuration for the stereogenic carbon atom of the lactone moiety

Phytotoxic Metabolites Produced by <i>Diaporthella cryptica</i>, the Causal Agent of Hazelnut Branch Canker

From the culture filtrates of <i>Diaporthella cryptica</i>, an emerging hazelnut pathogen, 2-hydroxy-3-phenylpropanoate methyl ester and its 3-(4-hydroxyphenyl) and 3-(1<i>H</i>-indol-3-yl) analogues, named crypticins A–C, were isolated together with the well-known tyrosol. Crypticins A–C were identified by spectroscopic (essentially nuclear magnetic resonance and high-resolution electrospray ionization mass spectrometry) methods. The <i>R</i> absolute configuration (AC) of crypticin A was determined by comparing its optical rotation and electronic circular dichroism (ECD) spectrum with those of papuline, the methyl ester of (−)­(<i>S</i>)-phenyllactic acid isolated as the main phytotoxin of <i>Pseudomonas syringae</i> pv. <i>papulans</i>, responsible for apple blister spot. The ACs of crypticins B and C were determined by time-dependent density functional theory calculations of their ECD spectra. Papuline and the new metabolites herein isolated, except tyrosol, were tested at 1 mg/mL on cork oak, grapevine, hazelnut, and holm oak leaves using the leaf puncture assay. They were also tested on tomato cuttings at 0.5 and 0.05 mg/mL. In the leaf puncture assay, none of the compounds was found to be active. Crypticin C and papuline were active in the tomato cutting assay. Additionally, crypticin C displayed moderate inhibitory effect against <i>Phytophthora cambivora</i>