(+)-Usnic acid (1) is a commercially available lichen metabolite. Its biological activity is diverse: from antibacterial and anticancer to immunomodulating. Its biological properties, as well as broad occurrence in various lichen species and high optical purity of the isolated product make it promising as a base for developing novel pharmaceuticals.
Earlier it was shown that the interaction of (+)-usnic acid 1 with reducing agents occurs along the C ring of the dibenzofuran core. The reaction proceeds either under the action of H2/Pd (C) through the reduction of the C4-C4a double bond with the formation of dihydrousnic acid 2, or under the action of sodium borohydride through the reduction of C1=O and C11=O carbonyl groups to hydroxyl ones with the formation of mixture of diastereomeric compounds 3. We synthesized new dibenzofuran compounds by hydrogenation of (+)-dihydrousnic acid 2 under the action of H2/Pd (C). It was found that the reduction proceeds predominantly at the exocyclic carbonyl group (C11=O) of ring C with the formation of compound 4. Compound 5 (according to NMR data in the enolic tautomeric form) was isolated as a minor product, wherein the endocyclic carbonyl group (C3=O) undergoes reduction to methylene.
The triketone system of the C ring in the new compounds is destroyed, which should contribute to the loss of protonophore properties responsible for the toxicity of the native compound. This feature allow the synthesized compounds to be considered as promising platforms for creating new biologically active compounds with increased safety.
