Crystal structure and absolute configuration of preaustinoid A1

The absolute structure of the title compound preaustinoid A1 [systematic name: (5aR,7aS,8R,10S,12R,13aR,13bS)-methyl 10-hy­droxy-5,5,7a,10,12,13b-hexa­methyl-14-methyl­ene-3,9,11-trioxohexa­deca­hydro-8,12-methano­cyclo­octa­[3,4]benzo[1,2-c]oxepine-8-carboxyl­ate], C(26)H(36)O(7), has been determined by resonant scattering using Cu Kα radiation [Flack parameter = 0.07 (15)]. The structure is consistent with that reported previously [Stierle et al. (2011). J. Nat. Prod. 74, 2272–2277], determined by detailed analysis of MS and NMR data. The mol­ecule consists of a fused four-ring arrangement. The seven-membered oxepan-2-one ring has a chair conformation, as do the central cyclo­hexane rings, while the outer cyclo­hexa-1,3-dione ring has a boat conformation. In the crystal, mol­ecules are linked via O—H⋯O hydrogen bonds, forming helical chains propagating along [100].