Acanthoic Acid.

THE TITLE COMPOUND [SYSTEMATIC NAME: (1R,4aR,7S,8aS,10aS)-1,4a,7-trimethyl-7-vinyl-1,2,3,4,4a,6,7,8,8a,9,10,10a-dodeca-hydro-phenanthrene-1-carb-oxy-lic acid], C(20)H(30)O(2), is a pimarane-type diterpene extracted from Croton oblongifolius. There are two independent mol-ecules in the asymmetric unit. In both of these, the six-membered rings A, B and C adopt chair, boat and half-chair conformations, respectively. Rings A and B are trans-fused. The two mol-ecules in the asymmetric unit form O-H⋯O hydrogen-bonded R(2) (2)(8) dimers. The absolute configuration was assigned on the basis of the published literature on analogous structures.