Androstane-3β,5α,6β,17β-tetrol trihydrate

The title hydrated tetrol, C19H32O4·3H2O, was synthesized by stereoselective reduction of the compound 3β,5α,6β-trihydroxyandrostan-17-one. All rings are fused trans. The organic molecules are connected head-to-tail along the c axis via O—H⋯O hydrogen bonds. Layers of water molecules in the ab plane interconnect these chains. A quantum chemical ab initio Roothan Hartree–Fock calculation of the isolated molecule gives values for the molecular geometry close to experimentally determined ones, apart from the C—O bond lengths, whose calculated values are significantly smaller than the measured ones, probably a consequence of the involvement of the C—OH groups in the hydrogen-bonding network.

The title hydrated tetrol, C 19 H 32 O 4 Á3H 2 O, was synthesized by stereoselective reduction of the compound 3,5,6-trihydroxyandrostan-17-one. All rings are fused trans. The organic molecules are connected head-to-tail along the c axis via O-HÁ Á ÁO hydrogen bonds. Layers of water molecules in the ab plane interconnect these chains. A quantum chemical ab initio Roothan Hartree-Fock calculation of the isolated molecule gives values for the molecular geometry close to experimentally determined ones, apart from the C-O bond lengths, whose calculated values are significantly smaller than the measured ones, probably a consequence of the involvement of the C-OH groups in the hydrogen-bonding network.

Experimental
Data collection: APEX2 (Bruker, 2006); cell refinement: SAINT (Bruker, 2006); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: PLATON (Spek, 2009); software used to prepare material for publication: SHELXL97.  (Carvalho, Silva, Moreira et al., 2010), we were able to synthesize the title compound, (I), by stereoselective reduction of compound 3β,5α,6β-trihydroxyandrostan-17-one (Andrade et al., 2011). Evaluation of the cytotoxicity of compound (I) towards HT-29 cancer cells (Carvalho, Silva, Moreira et al., 2010) indicates no relevant values (IC 50 >50µM), in contrast to other 3β,5α,6β-trihydroxy steroids, namely cholestane-3β,5α,6βtriol. Such result points to the importance of a C-17 cholesteryl-type side chain for cytoxicity. Determination of the threedimensional structure of compound (I) by X-ray crystallography will contribute to correlate the importance of this side chain influence on the overall steroid geometry with such biological effect. There is an extensive hydrogen bonding network in the crystal sructure. The steroid molecules are linked head to tail via the O17 and O3 atoms, through a direct H bond where the O3 atom acts as a donor and through two additional H bonds mediated by a water molecule. The chains, aligned along the c axis, are further linked together via the two remaining water molecules. Interestingly the three water molecules are located in layers in the ab plane.
Ab-initio Roothan Hartree-Fock calculations of the free steroid molecule were performed using the computer code GAMESS (Schmidt et al., 1993) in order to access the influence in the molecular geometry of the crystalline field, in particular of the solvent water molecules involved in H-bonding. These calculations gave values of the bond lengths,valency and torsion angles very close to those observed in the crystalline environment, except for the C-O bond lengths of the C-O-H groups, whose calculated values were significantely smaller than the measured ones, an effect that can be attributed to the influence of the hydrogen bonds (C17-O17 calc.

Experimental
Synthesis of the title compound was performed using Luche conditions (NaBH 4 /CeCl 3 ) (Luche et al., 1978). Reduction of the carbonyl in position C17 revealed to be stereoselective rendering the 17β-OH in good yield . Crystallization from ethanol at room temperature afforded colourless crystals suitable for X-ray analysis.
Analytical data of the title compound is in accordance with the literature

Refinement
All hydrogen atoms were refined as riding on their parent atoms using SHELXL97 defaults except for those of the water molecules whose coordinates were refined from the starting coordinates obtained from a difference Fourier synthesis with U eq (H)=1.5U eq (O) using a DFIX restraint for the O-H bond of 0.82 Å and those of the C-OH groups which were positioned and refined with a SELXL97 HFIX 147 instruction.
The absolute configuration was not determined from the X-ray data, as the molecule lacks any strong anomalous scatterer atom at the Mo Kα wavelength, but was known from the synthetic route. Friedel pairs were merged before refinement. Fig. 1. ORTEPII plot of the title compound. Displacement ellipsoids are drawn at the 50% level. Fig. 2 supplementary materials sup-9