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.