Reticulatin, a novel C-43-spiroterpenoid from the lichen Parmotrema reticulatum growing in Vietnam

A novel C-43-spiroterpenoid, reticulatin (1), was isolated from the lichen Parmotrema reticulatum (Taylor) M. Choisy (Parmeliaceae). Five previously-reported compounds were also isolated: zeorin (2), leucotylin (3), lupeol (4), betulinic acid (5), and dihydroreynosin (6). The structures were elucidated by 1D, 2D NMR, and HRESIMS spectroscopy and comparison with the literature. We propose that reticulatin is a biosynthetic product of fusicoccadiene and vinapraesorediosic acid A via Diels-Alder addition. Reticulatin is the first C-43-spiroterpenoid identified from lichen metabolites. All compounds were evaluated for inhibition of alpha-glucosidase. Compound 1 showed the most potent inhibition, with an IC50 value of 3.90 mu M, much lower than that of the acarbose positive control (IC50 165 mu M).