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-dodeca­hydro­phenanthrene-1-carb­oxy­lic acid], C20H30O2, is a pimarane-type diterpene extracted from Croton oblongifolius. There are two independent mol­ecules 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 mol­ecules 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