3β,6β-Diacetoxy-5,9α-dihydroxy-5α-cholest-7-en-11-one acetic acid 0.04-solvate

The title compound, C31H48O7·0.04CH3COOH, is a polyoxygenated steroid obtained by selective chemical oxidation of 7-dehydrocholesteryl acetate. The asymmetric unit comprises three molecules of the steroid (Z′ = 3) and a molecule of acetic acid which has occupancy factor 0.131 (5). The geometric parameters of the independent molecules do not reveal significant differences. In one molecule, the terminal isopropyl group is disordered over two sets of sites with occupancy ratio 0.869 (5):0.131 (5). The three molecules reveal different hydrogen-bonding patterns. Each of them is involved in an intramolecular S(6) hydrogen-bonding motif, involving hydroxy groups as donor and acceptor. In the crystal, two independent molecules form dimers through hydrogen bonding between an OH donor and an acetate carbonyl acceptor, giving rise to R 2 2(16) ring patterns. A single hydrogen bond between the OH group and a ketone carbonyl group is observed between two symmetry-independent molecules.

The title compound, C 31 H 48 O 7 Á0.04CH 3 COOH, is a polyoxygenated steroid obtained by selective chemical oxidation of 7dehydrocholesteryl acetate. The asymmetric unit comprises three molecules of the steroid (Z 0 = 3) and a molecule of acetic acid which has occupancy factor 0.131 (5). The geometric parameters of the independent molecules do not reveal significant differences. In one molecule, the terminal isopropyl group is disordered over two sets of sites with occupancy ratio 0.869 (5):0.131 (5). The three molecules reveal different hydrogen-bonding patterns. Each of them is involved in an intramolecular S(6) hydrogen-bonding motif, involving hydroxy groups as donor and acceptor. In the crystal, two independent molecules form dimers through hydrogen bonding between an OH donor and an acetate carbonyl acceptor, giving rise to R 2 2 (16) ring patterns. A single hydrogen bond between the OH group and a ketone carbonyl group is observed between two symmetry-independent molecules.
Data collection: COLLECT (Nonius, 1999); cell refinement: DIRAX/LSQ (Duisenberg et al., 2000); data reduction: EVALCCD (Duisenberg et al., 2003); program(s) used to solve structure: SIR97 (Altomare et al., 1999); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012) and Mercury (Macrae et al., 2006); software used to prepare material for publication: WinGX (Farrugia, 2012). oxygenation protocols developed in our laboratories (Piccialli et al., 2013;Notaro et al., 1994). As a continuation of our interest in this field, we have now undertaken a study aimed at preparing a new collection of polyoxygenated steroids for structure-activity relationship studies and in particular at the synthesis of new 9,11-secosteroids. Many representatives of this sub-class of steroids have been shown to possess in vitro cytotoxic activity against various human cancer cell lines (Chen et al., 2011). Due to its functionalization pattern, the title compound, shown in the Scheme, is a good starting product to further oxygenate the steroid nucleus at C and D rings, as well as to obtain the 9,11-secosteroid motif. For now, the title compound has been synthesized starting from commercially available 7-dehydrocholesteryl acetate (Fig. 1).
The X-ray analysis of the title compound confirms the structure and stereochemistry of the synthesized compound. The asymmetric unit contains three independent molecules that have similar geometric parameters (Fig. 2). Rings A and C of the steroid skeleton adopt a chair conformation. Ring B, which contains a double bond at C7=C8, adopts a half-chair conformation with twist at C5-C10 bond. The five-membered D ring is in envelope conformation, with C13 at the flap.
In the steroid core trans junctions at A/B and C/D rings are observed. The acetyl group at C3 (ring A) is equatorial while acetyl group at C6 (ring B) is axial. The lateral alkyl chain is fully extended. In one molecule, two different positions were found for the isopropyl group of the lateral alkylic chain (occupancy factor refined to 0.869 (5) for C25B/C26B/C27B; 0.131 (5) for C25Z/C26Z/C27Z). In the crystal, acetic acid solvent of crystallization was found with occupancy factor refined to 0.131 (5).
The molecule of the title compound contains several H-bonding donor and acceptor groups (Allen et al., 1999;Steiner, 2002) that are responsible for the formation of H bonds (Centore et al., 2013a(Centore et al., , 2013bCentore, Fusco, Jazbinsek et al. 2013;Centore, Fusco, Capobianco et al. 2013). The three independent molecules A, B and C are involved in intra and intermolecular H bonding patterns (Fig. 3). In each of the three independent molecules there is an intramolecular H bonding motif S(6) between hydroxy O2 donor and hydroxy O1 acceptor of the same molecule. Molecules B and C create dimers through intermolecular H bonding between OH donors (O1B and O1C) and acetate carbonyl acceptors (O5C and O5B), giving rise to ring patterns R 2 2 (16). Molecules C and A are involved in a single intermolecular H bond between OH donor (O1A) and ketone carbonyl acceptor (O3C).

Experimental
The title compound was synthesized starting from commercially available 7-dehydrocholesteryl acetate according to a procedure already described (Notaro et al., 1994). Crystals suitable for X-ray analysis were obtained from acetic acid solution by slow evaporation of the solvent.

Refinement
The H atom of the hydroxy groups were located in difference map with exception of carboxy H atom of the acetic acid solvent molecule that was not assigned. Other H atoms were generated stereochemically. All H atoms were refined by the riding model with U iso =1.2×U eq of the carrier atom (1.5 for H atoms of the methyl groups). Some reflections with theta below 3° were removed from the final refinement (OMIT instruction) because their intensities were strongly affected by the beamstop. Some constraints were introduced in the refinement to handle the disorder of the terminal isopropyl group of molecule B. In the absence of strong anomalous scatterer the Flack parameter is not meaningful. Data were merged using MERG 3 instruction and the absolute configuration was assigned on the basis of the known chirality of the precursor used in the synthesis.    H-bonding pattern joining the three independent molecules. The H-bonds are shown as dashed lines.

Computing details
3β,6β-Diacetoxy-5,9α-dihydroxy-5α-cholest-7-en-11-one acetic acid 0.04-solvate Special details Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s 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 R-factors(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.