The marine environment is distinguished by unique groups of organisms being the source of a wide array of fascinating structures. The enormous biodiversity of marine habitats is mirrored by the molecular diversity of secondary metabolites found in marine animals, plants and microbes. The recognition that many marine invertebrates contain endo- and epibiotic microorganisms and that some invertebrate-derived natural products are structurally related to bacterial metabolites suggests a microbial origin for some of these compounds. Other marine natural products, however, are clearly located in invertebrate tissue and microbial involvement in the biosynthetic process seems unlikely. The complexity of associations in marine organisms, especially in sponges, bryozoans and tunicates, makes it extremely difficult to definitively state the biosynthetic source of many marine natural products or to deduce their ecological significance. Whereas many symbiotic marine microorganisms cannot be isolated and cultured, numerous epi- and endobiotic marine fungi produce novel secondary metabolites in laboratory cultures. The potent biological activity of many marine natural products is of relevance for their ecological function but is also the basis of their biomedical importance.

Chemical investigation of the marine-derived fungus Ascochyta salicorniae led to the isolation of two novel natural products, ascospiroketals A (1) and B (2). From a biosynthetic standpoint, the compounds possess new ring systems. [structure: see text].

Chemical investigation of the marine fungus Ascochyta salicorniae led to the isolation of two new epimeric compounds, ascolactones A (1) and B (2), in addition to the structurally-related polyketides hyalopyrone (3), ascochitine (4), ascochital (5) and ascosalipyrone (6). The absolute configurations of the epimeric compounds 1 and 2 were assigned as (1R,9R) and (1S,9R), respectively, through simulation of the chiroptical properties using quantum-chemical CD calculations, and chiral GC-MS subsequent to oxidative cleavage (Baeyer-Villiger oxidation) of the side chain. In silico screening using the PASS software identified some of the A. salicorniae compounds (1-6) as potential inhibitors of protein phosphatases. Compound was found to inhibit the enzymatic activity of MPtpB with an IC(50) value of 11.5 microM.

Two stereoisomers of ascolactone (A, B), natural products with two asymmetric carbon atoms, are isolated from the marine-derived fungus Ascochyta salicorniae. Although these compounds show virtually opposite CD spectra and [alpha]D, 1H- and 13C-NMR data exclude the presence of enantiomers and suggest ascolactone A and B to be epimeric lactones. By comparing the experimental CD spectra with those calculated employing time-dependent density functional theory (TDDFT), we elucidate the configuration at one of the asymmetric carbon atoms.

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