6β-Acetamido-5α-hydroxycholestan-3β-yl acetate

The title steroid, C31H53NO4, was prepared from the corresponding 5α,6α-epoxycholestane. The conformation of the six-membered rings is close to a chair form, while the five-membered ring adopts a twist conformation. The hydroxyl and acetamide groups are in axial positions. The nucleophilic species bound to the steroid nucleus at position 6 by the β-face, whereas the hydroxyl group at position 5 has α-orientation. All rings are trans-fused. The crystal packing shows that the molecules related by twofold symmetry exist as O—H⋯O hydrogen-bonded dimers.

The title steroid, C 31 H 53 NO 4 , was prepared from the corresponding 5,6-epoxycholestane. The conformation of the six-membered rings is close to a chair form, while the fivemembered ring adopts a twist conformation. The hydroxyl and acetamide groups are in axial positions. The nucleophilic species bound to the steroid nucleus at position 6 by theface, whereas the hydroxyl group at position 5 hasorientation. All rings are trans-fused. The crystal packing shows that the molecules related by twofold symmetry exist as O-HÁ Á ÁO hydrogen-bonded dimers.
Data collection: SMART (Bruker, 2003); cell refinement: SAINT (Bruker, 2003); 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, 2003); software used to prepare material for publication: SHELXL97. title compound has been obtained from the corresponding 5α,6α-epoxycholestane derivative, under similar reaction conditions, in 90% yield (Pinto et al., 2006). The present communication unequivocally demonstrated the trans-diaxial nature of the 5α,6α-epoxide ring-opening by the bismuth(III) salt-catalyzed Ritter reaction. Related X-ray diffraction studies on 5α-hydroxy-6β-substituted steroids have been recently published by our group (Pinto et al., 2007b(Pinto et al., , 2008b. The conformations of the six-membered rings are close to a chair form, as shown by the Cremer & Pople (1975)  The molecules are hydrogen-bonded through the hydroxyl group at C5, acting as donor towards the carbonyl O atom of the amide moiety. Remarkably, the H atom attached to the amide N atom is not involved in any hydrogen bond. This may arise from steric hindrance caused by the C19-methyl group. The following short intramolecular distances are also spotted in the structure: H1B···O5 2.42 Å, H9···O5 2.42 Å and H6···O6 2.39 Å.
The anisotropic displacement tensor of the terminal atoms of the 3β-acetoxy group are strongly anisotropic, suggesting a large amplitude of vibration of these atoms perpendicular to the mean plane of this group.

Experimental
The synthesis of 6β-acetamido-5α-hydroxycholestan-3β-yl acetate was efficiently accomplished by nucleophilic ring-opening of the corresponding 5α,6α-epoxycholestane catalyzed by BiBr 3 in acetonitrile (Pinto et al., 2006). The product of this reaction was isolated in 90% yield and identified as the title compound from IR, 1 H and 13 C NMR spectroscopy data (Pinto et al., 2006). Recrystallization from acetonitrile at room temperature gave colourless single crystals suitable for X-ray diffraction analysis.
supplementary materials sup-2 Refinement H atoms were fixed geometrically (O-H = 0.82 Å, N-H = 0.86 Å and C-H = 0.96-0.98 Å) and treated as riding with U iso (H) = 1.2U eq (C) and 1.5U eq (O and methyl C). In the absence of significant anomalous scattering, Friedel pairs were merged prior to the final refinement. Though the absolute configuration was not determined from the X-ray data but was known from the synthetic route.   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 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.