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-hy-droxy-5,5,7a,10,12,13b-hexa-methyl-14-methyl-ene-3,9,11-trioxohexa-deca-hydro-8,12-methano-cyclo-octa-[3,4]benzo[1,2-c]oxepine-8-carboxyl-ate], C26H36O7, 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 mol-ecule consists of a fused four-ring arrangement. The seven-membered oxepan-2-one ring has a chair conformation, as do the central cyclo-hexane rings, while the outer cyclo-hexa-1,3-dione ring has a boat conformation. In the crystal, mol-ecules are linked via O-H⋯O hydrogen bonds, forming helical chains propagating along [100].