3β-Chlorocholest-5-en-7-one

The title compound, C27H43ClO, is a steroid derivative composed of a saturated carbon fused-ring framework with an alkyl side chain. The A and C rings have chair conformations and the B and D rings assume half-chair conformations. The cholesterol side chain is fully extended with a gauche, trans conformation of the terminal methyl groups. In the crystal structure, the molecules are aligned in an antiparallel fashion, forming alternate layers. These layers are then linked via C—H⋯O hydrogen bonds, forming a three-dimensional network.

The title compound, C 27 H 43 ClO, is a steroid derivative composed of a saturated carbon fused-ring framework with an alkyl side chain. The A and C rings have chair conformations and the B and D rings assume half-chair conformations. The cholesterol side chain is fully extended with a gauche, trans conformation of the terminal methyl groups. In the crystal structure, the molecules are aligned in an antiparallel fashion, forming alternate layers. These layers are then linked via C-HÁ Á ÁO hydrogen bonds, forming a threedimensional network.

Related literature
For related structures, see: Kang et al. (1985); Yun et al. (1989); Ahn & Park (1990); Park & Shin (2002); Park (2004); Park et al. (2005). For the role of cholesterol derivatives in biological systems, see: Abrahamsson et al. (1977). For ring conformations, see: Cremer & Pople (1975). For the stability of the temperature controller used in the data collection, see: Cosier & Glazer (1986 interactions with enzymes, cholesterol-binding proteins and related effectors of gene transcription. A series of crystal structures of the esters and carbonates of cholesterol (Ahn & Park, 1990;Kang et al., 1985;Yun et al., 1989;Park & Shin, 2002;Park, 2004) has been examined in order to obtain structural information relevant to the liquid crystalline phases and the possible modes of association of the cholesterol derivatives themselves, as well as of other substances in biological systems (Abrahamsson et al., 1977). In view of the biological importance of cholesterols, we report here the crystal structure of the title compound (I) -a new cholesterol derivative.
In the asymmetric unit of the title compound ( In the crystal structure, the molecules are aligned in an antiparallel fashion to form alternate layers. These layers are then linked via C-H···O (Table 1)

Refinement
All hydrogen atoms were positioned geometrically [C-H = 0.93-0.98Å] and were refined using a riding model, with U iso (H) = 1.2-1.5 U eq (C). 2745 Friedel pairs were used to determine the absolute configuration. Fig. 1. The molecular structure of (I) showing the atomic numbering and 30% probability displacement ellipsoids.

Special details
Experimental. The crystal was placed in the cold stream of an Oxford Cryosystems Cobra open-flow nitrogen cryostat (Cosier & Glazer, 1986) operating at 100.0 (1) K.
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds 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 > 2sigma(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.