organic papers Acta Cryst. (2005). E61, o3905–o3907 doi:10.1107/S1600536805034446 Kumar et al.  C 21 H 26 O 3 o3905 Acta Crystallographica Section E Structure Reports Online ISSN 1600-5368 Neridienone A, a pregnane from the roots of Nerium oleander Rishi Kumar, a Ram Sagar, b Arun K. Shaw b and Prakas R. Maulik a * a Molecular and Structural Biology Division, Central Drug Research Institute, Lucknow 226 001, India, and b Medicinal and Process Chemistry Division, Central Drug Research Institute, Lucknow 226 001, India Correspondence e-mail: maulik_prakas@yahoo.com Key indicators Single-crystal X-ray study T = 293 K Mean (C–C) = 0.005 A ˚ R factor = 0.043 wR factor = 0.097 Data-to-parameter ratio = 7.7 For details of how these key indicators were automatically derived from the article, see http://journals.iucr.org/e. # 2005 International Union of Crystallography Printed in Great Britain – all rights reserved In the title compound, 12-hydroxypregna-4,6,16-triene-3,20- dienone, C 21 H 26 O 3 , rings A and B are in almost half-chair conformations, ring C is in a chair conformation and ring D is in a 14-envelope conformation. The A/B ring junction is quasi-trans, whereas the B/C and C/D ring junctions both approach trans character. The molecule as a whole is slightly convex towards the  side. Intra- and intermolecular hydrogen bonding, together with van der Waals interactions, plays a major role in the crystal structure. Comment Neridienone A is a pregnane compound and was initially isolated from the root bark of Nerium odorum (Yamauchi et al., 1974; Abe & Yamauchi, 1976). Later, it was also isolated from the roots of Nerium oleander (Huq et al. 1999). This class of compounds has been found to possess various biological activities. 12-Hydroxypregn-4-ene-3,20-dienone, the dihydro derivative of neridienone A, and several other C 21 steroids, have been found to possess effects against fish as the defence mechanism of water beetles (Chada et al. 1970). Neridienone A appeared to have a similar toxic effect towards goldfish as that reported for the cortexone and cybisterone derivatives (Abe & Yamauchi, 1976). This prompted us to undertake the present X-ray crystallographic study of neridienone A, (I). In the scheme, the asymmetric C atoms are indicated by asterisks. The conformation of compound (I) and the atom- numbering scheme are shown in Fig. 1. The molecule consists of a fused four-ring system (A, B, C and D), to which an acetyl group is attached at the C17 position, two -oriented methyl groups are attached at C13 and C10, respectively, and a - oriented hydroxy group is attached at C12. In the molecule, rings A and B have almost half-chair conformations: the deviations of atoms C1 and C2 are 0.44 (7) and 0.23 (7) A ˚ , respectively, from the least-squares plane through atoms C3, C4, C5 and C10 for ring A, while the deviations of atoms C9 and C10 are 0.43 (6) and 0.31 (6) A ˚ , respectively, from the Received 19 September 2005 Accepted 24 October 2005 Online 31 October 2005 CDRI communication No. 6849.