A new depside and two new diphenyl ether compounds from the lichen Ramalina farinacea (L.) Ach

Abstract

The chemical investigation of the Vietnamese lichen Ramalina farinacea (L.) Ach. led to the isolation and the structure elucidation of eight compounds (1–8), including one new depside, formulated as methyl homosekikate (1) and two new diphenyl ethers, 3-(2-carboxy-5-methoxy-3-propylphenoxy)-2-hydroxy-4-methoxy-6-propylbenzoic acid, namely ramalinic acid A (2), and 3-(2-carboxy-5-methoxy-3-propylphenoxy)-2-hydroxy-4-methoxy-6-pentylbenzoic acid, namely ramalinic acid B (3). Their chemical structures were unambiguously determined by analysis of 1D and 2D NMR and high resolution ESI mass spectroscopic data, as well as by comparison with literature data. We also proposed a possible biosynthetic route for the formation of the two new diphenyl ethers (2) and (3) via an enzymatically induced intramolecular Smiles rearrangement of the depsides sekikaic acid and homosekikaic acid, respectively.