A biomimetic semisynthesis enables structural elucidation of selaginellin U: a tautomeric cyclic alkynylphenol from Selaginella tamariscina

Two new lactone-containing selaginellins T and U (1 and 2) together with eleven known selaginellin derivatives (3 and 7–16) were isolated from the whole plant of Selaginella tamariscina. The structure of tautomeric selaginellin U along with its biogenetic pathway was confirmed and proposed by a cross-validation of the semisynthesis of compound 4 from selaginellin (3) and derivation from 2 to 4. Additionally, compounds 3, 13 and 16 exhibited good inhibitory activities against β-site amyloid precursor protein cleaving enzyme 1 (BACE1) with IC(50) values of 81.17, 51.13 and 48.89 µM, respectively.