3β,6α-Diacetoxy-5,9α-dihydroxy-5α-cholest-7-en-11-one

The title compound, C31H48O7, a polyoxygenated steroid, was obtained by chemical oxidation of 7-dehydrocholesteryl acetate. The molecular geometry features trans A/B and C/D junctions at the steroid core with the acetyl groups in the equatorial position and a fully extended conformation for the alkyl side chain. A chair conformation is observed for rings A and C while ring B adopts a half-chair conformation. The five-membered ring D has an envelope conformation, with the C atom bearing the methyl group at the flap. The terminal isopropyl group and one acetyl group are disordered over two sets of sites with 0.774 (8):0.226 (8) and 0.843 (7):0.157 (7) ratios, respectively. An intramolecular S(6) O—H⋯O hydrogen-bonding motif involving a hydroxy donor and acceptor is observed. In the crystal, chains of molecules running along the b axis are formed via O—H⋯O hydrogen bonds between hydroxy donors and carbonyl acceptors of the ordered acetyl group, giving rise to a C(14) motif. The chains are wrapped around the 21 screw axes.

Our group has previously been involved into studies aimed at the isolation of polyoxygenated steroids from marine sources (Notaro et al., 1991(Notaro et al., , 1992, as well as at the synthesis and structural modification of some members of this class (Migliuolo et al., 1992). This has led to the development of new efficient oxidation protocols, mostly based on the use of transition-metal oxo-species (Piccialli et al., 1993;Notaro et al., 1994;Caserta et al., 2005;Piccialli, D′Errico et al., 2013), to selectively introduce oxygenated functions into specific positions of the steroidal nucleus. Recently we have undertaken a study aimed at preparing new polyoxygenated steroids for structure-activity relationship studies. In this frame the title compound, shown in the Scheme, was synthesized from commercially available 7-dehydrocholesteryl acetate ( Fig. 1) according to a previously developed RuO 4 -catalyzed route (Notaro et al., 1994). Its 3β,5α,6α-oxygenation pattern is a motif found in some biologically active steroids isolated from sponges of genus Dysidea and, in particular, its C7-C10 functionalization pattern was seen as a key feature to introduce diversely configurated oxygenated functions at these carbon centres and/or neighbouring positions. The present X-ray diffraction study was undertaken in order to confirm the stereostructure of the title compound.
The molecular structure determined by X-ray analysis ( Fig.2) fully confirms the stereostructure of the synthesized compound and shows an almost planar shape of the molecule. A chair conformation is observed both in A and C rings while the ring B, containing the C7=C8 double bond, adopts a half-chair conformation (twist at C5-C10 bond). The five-membered D ring has an envelope conformation, with C13 at the flap. In the steroid ring core trans junctions at A/B and C/D rings are observed. The two acetyl groups at C3 and C6 occupy equatorial positions of A and B rings. The alkyl side-chain is fully extended and the isopropyl group is disordered over two positions. Also in the acetyl moiety at C3 disorder in two positions is observed. An intramolecular H bonding motif S(6) involving hydroxy O2-H donor and hydroxy O1 acceptor is observed. In the crystal packing ( Fig. 3) molecules are linked into chains running along b, through intermolecular H bonding between hydroxy O1-H donor and carbonyl O7 acceptor, giving rise to a C(14) motif. The chains are generated by the binary screw rotation of the space group. It is a remarkable finding that the isomeric 6β compound crystallizes with three independent molecules in the asymmetric unit (Piccialli, Oliviero et al., 2013).

Experimental
The title compound was prepared according to the recipe given in Notaro et al., 1994. Crystals suitable for X-ray analysis were obtained by slow evaporation of CHCl 3 -MeOH (8:2) solutions of the compound.

Refinement
H atoms of hydroxy groups were located in DIF maps and were refined with U iso = 1.2×U eq of the carrier atom. The positions of the other H atoms were determined stereochemically (C-H = 0.98-1.00 Å) and refined by the riding model with U iso =1.2×U eq of the carrier atom (1.5 for H atoms of methyl group). Two different positions were found for the isopropyl group of the lateral alkyl chain (occupancy factor refined to 0.774 (8) for C25A/C26A/C27A; 0.226 (8) for C25B/C26B/C27B) and for the acetyl group at C3 (occupancy factor refined to 0.843 (7) for C28A/O5A/C29A; 0.157 (7) for C28B/O5B/C29B). SIMU and SAME restraints (Sheldrick, 2008) were applied to keep similar geometry in the disordered parts. Friedel pairs were merged using MERG3 and the absolute structure was assigned by reference to known chiral centers. Mercury (Macrae et al., 2006); software used to prepare material for publication: WinGX (Farrugia, 2012).

Figure 1
Synthesis of the title compound by ruthenium catalyzed oxidation of 7-dehydrocholesteryl acetate.

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.