Sideritol, C20H32O3, diterpenoid isolated from Sideritisangustifolia Lag., has been shown to be ent-13-atisen-1β, 16α, 17-triol (1). Sideritol is the first reported oxygenated diterpenoid of the ent-atisane class which does not contain nitrogen. The transformation of sideritol into ent-17-noratisane (11), previously prepared from atisine, rigorously defines the skeleton and absolute stereochemistry of sideritol.