(E)-17β,19-Epoxymethano-17,23,24-tridemethyl-4-nor-5β,18α-olean-3-one oxime

In the pentacyclic triterpenoide skeleton of the title molecule, C27H43NO2 [systematic name: (3E,3aS,5aR,5bR,7aR,11R,11aR,11bR,13aR,13bR)-5a,5b,10,10,13b-pentamethylicosahydro-1H-11,7a-(epoxymethano)cyclopenta[a]chrysen-3-one oxime], the five-membered ring A has an envelope conformation, while the six-membered rings B–E adopt chair conformations. Rings A and B are cis-fused. The hydroximino group has an E configuration. Strong intermolecular O—H⋯O hydrogen bonds link the molecules into helical chains.

The X-ray structure determination of the title compound was carried out in order to confirm its spatial structure that had been proposed on the basis of spectroscopic data by Medvedeva et al. (2004).
The results obtained for the title compound confirm the cis-junction of A/B rings. The corresponding interplanar angle between the least-squares planes of the A/B rings is 71.85 (8)°. The H atom at the C5 asymmetric centre exhibits β-orientation and occupies a pseudo-axial position with respect to the A ring and an equatorial position to the B ring [the angles of the H5-C5 bond vector to the Cremer & Pople A and B ring plane normals are 9.60 (9) and 64.05 (7)°, respectively (Cremer & Pople, 1975;Spek, 2009)]. The torsion angle H5-C5-C10-C25 of 38° reveals a halfway conformation between synperiplanar and synclinal for bonds H5-C5 and C10-C25.
In the molecule the six-membered rings B-E of the pentacyclic ring system are trans-fused as in allobetuline. The dihedral angles between the least-squares planes of these rings are B/C 7.09 (10), C/D 0.80 (10), D/E 14.95 (9)°.
In the title structure, each of the six-membered rings B-E has a differently distorted chair conformation, whereas both five-membered rings, i.e. the carbocyclic ring A and the heterocyclic ring C17\C18\C19\O2\C28 including epoxymethylene group, assume envelope conformations. The respective puckering parameters (Cremer & Pople, 1975)  The molecular packing is stabilized by O1-H···O2 i hydrogen bonds (Tab. 1). The hydroxyl hydrogen is donated to the remote-ring epoxy O atom from the neighbour molecule. These hydrogen bonds link the molecules into helical chains which proceed in the c direction (Fig. 2).

Experimental
The title compound was obtained according to the procedure described by Medvedeva et al. (2004). Single colourless needle-crystals suitable for analysis were grown from ethanol by slow evaporation at room temperature.
supplementary materials sup-2 Refinement All the hydrogens were discernible in the difference electron density map. Except for the hydroxyl H atom that was refined freely the remaining hydrogens were situated into the idealized positions and were refined within the riding model approximation: C methyl -H = 0.96, C methylene -H 0.97, C methine = 0.98 Å. U iso (H) = 1.2 U eq C methylene /C methine ; U iso (H) = 1.5U eq (C methyl ). The methyl group was allowed to rotate during refinement. The absolute configuration of the title structure is known by reference to the known chirality of the enantiopure allobetulin employed as the initial reagent used in the synthesis as well as to the chirality of the other oleanane derivatives [see: CSD, Cambridge; Allen (2002)].  (3E,3aS,5aR,5bR,7aR,11R,11aR, 11bR,13aR,13bR)-5a,5b,10,10,13b-pentamethylicosahydro-1H-11,7a-(epoxymethano)cyclopenta[a]chrysen-3-one oxime

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.
Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating Rfactors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.