Oleanolic acid ethanol monosolvate

Crystals of the title compound (systematic name: 3β-hydroxyolean-12-en-28-oic acid ethanol monosolvate), C30H48O3·C2H5OH, were obtained from unsuccessful co-crystallization trials. The asymmetric unit contains two symmetry-independent oleanolic acid molecules, as well as two ethanol solvent molecules. Intermolecular O—H⋯O hydrogen bonds stabilize the crystal packing. In the oleanolic acid molecules, ring C has a slightly distorted envelope conformation, while rings A, B, D and E adopt chair conformations and rings D and E are cis-fused. Both independent ethanol molecules are orientationally disordered [occupancy ratios of 0.742 (8):0.258 (8) and 0.632 (12):0.368 (12).

Crystals of the title compound (systematic name: 3-hydroxyolean-12-en-28-oic acid ethanol monosolvate), C 30 H 48 O 3 Á-C 2 H 5 OH, were obtained from unsuccessful co-crystallization trials. The asymmetric unit contains two symmetry-independent oleanolic acid molecules, as well as two ethanol solvent molecules. Intermolecular O-HÁ Á ÁO hydrogen bonds stabilize the crystal packing. In the oleanolic acid molecules, ring C has a slightly distorted envelope conformation, while rings A, B, D and E adopt chair conformations and rings D and E are cis-fused. Both independent ethanol molecules are orientationally disordered [occupancy ratios of 0.742 (8):0.258 (8) and 0.632 (12):0.368 (12).
The structure presented in this paper was solved for the crystal obtained during the unsuccessful co-crystallization attempts. Crystallization process was carried out using ethanol with the addition of 12α-bromo-3a-aza-A-homo-18β-olean-28,13β-olide. The impact of the latter one on the crystals size requires further investigations.
Oleanolic acid crystallizes in form of an ethanol solvate. The asymmetric unit contains two symmetry-independent triterpenoid molecules (host) and two ethanol molecules (guest). Both solvent molecules are orientationally disordered. The figure 1 shows that in ethanol molecule A all atoms whereas in molecule B only methylene and methyl groups are split into two alternative positions. In molecule A the non-H atoms assigned to O1AA, C1AA and C2AA positions have the site occupancy factor of 63% whereas in molecule B the carbon atoms assigned to C1BA and C2BA positions have the factor of 74%.
The conformational differences between the symmetry-independent oleanolic acid molecules A and B are significant only in the angular arrangement of the carboxylic group. In molecule A, the carbonyl group C28═O2 adopts conformation halfway between synperiplanar and synclinal (-sp/-sc) with respect to C17-C18 bond [torsional angle C18A-C17A-C28A-O2A: -27.6 (2)°]. In molecule B, the conformation is halfway between anticlinal and antiperiplanar (+ac/+ap)[torsional angle: C18B-C17B-C28B-O2B: 145.84 (16)°]. The angular orientation of the carboxylic group in both independent molecules A and B is stabilized by the intermolecular hydrogen bonds in which this group acts both as a proton donor and acceptor (Fig. 2, Table 1).
In both independent molecules, rings A, B, D and E have chair conformation distorted to a different degree. Ring C assumes an envelope conformation [Cremer & Pople (1975)   The hydroxyl group at C3 in molecules A and B of oleanolic acid is equatorial with respect to ring A and adopts β orientation.
In the crystal lattice, the symmetry-independent molecules A and B of oleanolic acid are connected with hydrogen bonds O1A-H11···O2B and O1B-H12···O2A i (Fig. 2, Table 1) into infinite chains extending along the c axis. The hydrogen bonds involve the C3 hydroxyl group and the carbonyl oxygen atom of the C17 carboxylic group. Both of the mentioned functional groups form also hydrogen bonds with the solvent molecules. The ethanol molecules marked A are hydrogen bonded (O1AA-H1AO···O1A and O3B-H32···O1AA) to the triterpenoid molecules A and B belonging to the same chain, whereas the ethanol molecules B are linked through the O3A-H31···O1BA ii and O1BA-H1BO···O1B hydrogen bonds with the triterpenoide molecules A and B but from the adjacent chains (Fig. 2, Table 1). Therefore, the ethanol molecules B connect the chains of triterpenoid molecules into two-dimensional layers that extend parallel to the bc plane. The layer thickness is about half of the a parameter length.

Experimental
Oleanolic acid was extracted from mistletoe leaves. Equimolar quantities of oleanolic acid and 12α-bromo-3a-aza-A-homo-18β-olean-28,13β-olide as an additive were dissolved together in hot ethanol and the mixture was set aside to crystallize at room temperature.