Prasinic acid

Bioassay-guided fractionation of an extract of the lichen Cladonia incrassata against Staphylococcus aureus led to a novel compound, 1,5-dihydroxy-2,4,6-trichloro-7-methylxanthone (1), along with six known compounds: (-)-usnic acid (2), didymic acid (3), condidymic acid (4), squamatic acid (5), thamnolic acid (6), and prasinic acid (7). Didymic, condidymic, and prasinic acids were isolated for the first time from C. incrassata. Didymic, condidymic, and (-)-usnic acids were active against S. aureus (a minimum inhibitory concentration of 7.5 µg/mL).

Six new Micarea species are described from Europe. Phylogenetic analyses, based on three loci, i.e. mtSSU rDNA, Mcm7 and ITS rDNA and ancestral state reconstructions, were used to evaluate infra-group divisions and the role of secondary metabolites and selected morphological characters on the taxonomy in the M. prasina group. Two main lineages were found within the group. The Micarea micrococca clade consists of twelve species, including the long-known M. micrococca and the newly described M. microsorediata, M. nigra and M. pauli. Within this clade, most species produce methoxymicareic acid, with the exceptions of M. levicula and M. viridileprosa producing gyrophoric acid. The M. prasina clade includes the newly described M. azorica closely related to M. prasina s.str., M. aeruginoprasina sp. nov. and M. isidioprasina sp. nov. The species within this clade are characterised by the production of micareic acid, with the exception of M. herbarum which lacks any detectable substances and M. subviridescens that produces prasinic acid. Based on our reconstructions, it was concluded that the ancestor of the M. prasina group probably had a thallus consisting of goniocysts, which were lost several times during evolution, while isidia and soredia evolved independently at multiple times. Our research supported the view that the ancestor of M. prasina group did not produce any secondary substances, but they were gained independently in different lineages, such as methoxymicareic acid which is restricted to M. micrococca and allied species or micareic acid present in the M. prasina clade.

The first total synthesis of prasinic acid is being reported along with its biological evaluation. The ten step synthesis involved readily available and cheap starting materials and can easily be transposed to large scale manufacturing. The crucial steps of the synthesis included the formation of two different aromatic units (7 and 9) and their coupling reaction. The synthetic prasinic acid exhibited moderate antitumor activity (IC(50) 4.3-9.1 μM) in different lines of cancer cells.