19β,28-Epoxy-18α-olean-3β-ol

The title triterpene, C30H50O2, is an 18α-oleanane derivative prepared by the Wagner–Meerwein rearrangement of betulin with Bi(OTf)3.xH2O (OTF is trifluoromethanesulfonate). There are two symmetry-independent molecules in the asymmetric unit that show no significant differences concerning bond lengths and angles. The conformation of the six-membered rings is close to a chair form, while the five-membered epoxide rings adopt envelope conformations. All rings are trans-fused. In the crystal, molecules are held together by O—H⋯O hydrogen bonds. A quantum-mechanical ab initio Roothan Hartree–Fock calculation on the isolated molecule gives values for bond lengths and valency angles close to the experimental values. The calculations also reproduce well the molecular conformation with calculated puckering parameters that match well the observed values.

The title triterpene, C 30 H 50 O 2 , is an 18-oleanane derivative prepared by the Wagner-Meerwein rearrangement of betulin with Bi(OTf) 3 .xH 2 O (OTF is trifluoromethanesulfonate). There are two symmetry-independent molecules in the asymmetric unit that show no significant differences concerning bond lengths and angles. The conformation of the six-membered rings is close to a chair form, while the fivemembered epoxide rings adopt envelope conformations. All rings are trans-fused. In the crystal, molecules are held together by O-HÁ Á ÁO hydrogen bonds. A quantum-mechanical ab initio Roothan Hartree-Fock calculation on the isolated molecule gives values for bond lengths and valency angles close to the experimental values. The calculations also reproduce well the molecular conformation with calculated puckering parameters that match well the observed values.
The molecules are hydrogen bonded involving the hydroxyl and epoxyde groups. The hydroxyl group of molecule A acts as a donor towards a neighbour hydroxyl group of a neighbour A molecule acting as an acceptor forming a chain of hydrogen bonds running along the a axis. The hydroxyl group of molecule B acts as a donor towards the epoxyde group of a neighbour A molecule.
In order to gain some insight on how the crystal packing of (I) might affect the molecular geometry we have performed a quantum chemical calculation on the equilibrium geometry of the free molecule. These calculations were performed with the computer program GAMMESS (Schmidt et al., 1993). A molecular orbital Roothan Hartree-Fock method was used with an extended 6-31 G(d,p) basis set. Tight conditions for convergence of both the self-consistent field cycles and maximum supplementary materials sup-2 density and energy gradient variations were imposed (10 -6 atomic units). The program was run on the Milipeia cluster of UC-LCA (using 16 Opteron cores, 2.2 GHz runing Linux).
The ab-initio calculations reproduce well the observed experimental bond length and valency angles of the molecule. Also, the calculated conformation of the rings are very close to the experimental values, with the exception of ring C for which the calculations gave a conformation closer to the ideal chair than experiment.

Experimental
The synthesis of the 19β,28-epoxy-18α-olean-3β-ol was efficiently acomplished by Wagner-Meerwein rearrangement of commercially available betulin with Bi(OTf) 3 .xH 2 O in CH 2 Cl 2 at reflux (Salvador et al., 2009). The product of this reaction was isolated in 95% yield and identified as the title compound from IR, 1 H and 13 C NMR spectroscopy data (Salvador et al.,2009). Recrystallization from ethanol at room temperature gave colourless single crystals suitable for X-ray diffraction analysis.

Refinement
All hydrogen atoms were refined as riding on their parent atoms using SHELXL97 defaults except for that of the hydroxyl group which had its coordinates freely refined with U iso = 1.5 U eq of the O atoms.
The absolute configuration was known from the synthetic route, but could not be determined from the X-ray data. Thus, Friedel pairs were merged for refinement. Fig. 1. ORTEPII plot of one of the two molecules in the asymmetric unit. Displacement ellipsoids are drawn at the 50% level.

Figures
19β,28-Epoxy-18α-olean-3β-ol  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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq C1A