6-Hydroxyimino-5α-cholestane

The title compound, C27H47NO, is a steroid derivative composed of a saturated carbon fused-ring framework with an alkyl side chain. Ring bond lengths have normal values with an average of 1.533 (2) Å, while the cholestane side chain shows an average bond length of 1.533 (2) Å. The three cyclohexane rings adopt chair conformations or close to chair conformations while the cyclopentane ring is twisted. The cholesterol side-chain is fully extended with a gauche–trans conformation of the terminal methyl groups. There are eight chiral centres in the molecule; the absolute configuration of these sites was determined from the structure presented. There are two molecules in the asymmetric unit; in one, the alkyl chain is disordered over two sets of sites [occupancy ratios of 0.50:0.50 and 0.67:0.33].

The title compound, C 27 H 47 NO, is a steroid derivative composed of a saturated carbon fused-ring framework with an alkyl side chain. Ring bond lengths have normal values with an average of 1.533 (2) Å , while the cholestane side chain shows an average bond length of 1.533 (2) Å . The three cyclohexane rings adopt chair conformations or close to chair conformations while the cyclopentane ring is twisted. The cholesterol side-chain is fully extended with a gauche-trans conformation of the terminal methyl groups. There are eight chiral centres in the molecule; the absolute configuration of these sites was determined from the structure presented. There are two molecules in the asymmetric unit; in one, the alkyl chain is disordered over two sets of sites [occupancy ratios of 0.50:0.50 and 0.67:0.33].

Related literature
For background on steroidal hormone applications, see: Grover et al. (2007). For background to this study and previous syntheses, see : Shoppee et al. (1955). For related structures, see: Ketuly et al. (2011);Park (2004). For reference bond-length data, see: Allen et al. (1987). For the stability of the temperature controller used for the data collection, see: Cosier & Glazer (1986). In the view of the biological importance of cholesterol derivatives, we report here the crystal structure of the title compound (I). There are two molecules in the asymmetric unit ( Fig. 1), while unit cell contains 4 molecules (Fig. 2). The ring bond lengths have normal values (Allen et al., 1987) with average C(sp3)-C(sp3) of 1.533 (2) Å, excluding the shorter C1-C62 bond of 1.496 (2) Å. The cholestane side-chain shows C(sp3)-C(sp3) bond lengths varying from C25 -C26 [1.525 (2) Å] up to C20-C22 [1.539 (2) Å]. Each of the six-member rings adopts a chair conformation or close to a chair conformation, and the five-member ring has a twisted conformation. There are eight chiral centres in the molecule, the absolute configuration of these sites was determined from the structure presented, these sites exhibit the following chiralities: C2= S, C7 = R, C8 = S, C11 = R, C12 = R, C15 = S, C16 = S and C20 = R.

Refinement
All H atoms were positioned geometrically [C-H = 0.98-1.00 Å] and were refined using a riding model, with U iso (H) = 1.2-1.5U eq (C). H atoms attached to O atom were positioned with idealized geometry and were refined isotropically with U eq (H) set to 1.5 times of U eq (O) using a riding model with O-H = 0.87 Å. In the absence of significant anomalous scattering, Friedel pairs were merged and the absolute structure was assigned arbitrarily. Friedel pairs have been merged using MERG 3, which makes the Flack x parameter meaningless and therefore has been removed from the final cif file.
There are two molecules in the asymmetric unit and only alkyl chain (C23a-C25a) of one molecule is disordered.
C23a and C25a are disordered over two positions with site occupancy factors of 50% while C24a is disordered over two positions with site occupancy factors of 33% and 67%. publCIF (Westrip, 2010).

Figure 1
The molecular structure of (I) showing the atomic numbering and 30% probability displacement ellipsoids.

Figure 2
The unit cell diagram projected down the b axis.

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.