Absolute structure of (3a,5,7a,7b,9a,10,12a,12b)-7b-hy-droxy-4,4,7a,9a,12a-penta-methyl-10-[(2')-6-methyl-heptan-2-yl]-2,8,9-trioxo-octa-deca-hydro-benzo[]indeno-[4,5-]azepin-5-yl acetate from 62-year-old crystals.

The structure of the title compound, CHNO, was determined from 62-year-old crystals at room temperature and refined with 100 K data in a monoclinic (2) space group. This compound with a triterpenoid structure, now confirmed by this study, played an important role in the determination of the structure of lanosterol. The mol-ecules pack in linear O-H⋯O hydrogen-bonded chains along the short axis (), while parallel chains display weak van der Waals inter-actions that explain the needle-shaped crystal morphology. The structure exhibits disorder of the flexible methyl-heptane chain at one end of the main mol-ecule with a small void around it. Crystals of the compounds were resistant to data collection for decades with the available cameras and Mo α radiation single-crystal diffractometer in our laboratory until a new instrument with Cu α radiation operating at 100 K allowed the structure to be solved and refined.