Methyl 3-[(chloromethoxy)carbonyloxy]-7-hydroxycholan-24-oate

The title compound, C27H43ClO6, is a derivative of ursodeoxycholic acid, in which the OH group at the 3-position is substituted by a chloromethoxycarbonyloxy substituent and the carboxylic acid group at the 24-position is methylated. The A and B rings are cis-fused, while all other rings are trans-fused. In the crystal, two adjacent molecules located along the b-axis direction are interlocked head-to-tail due to weak C—H⋯O hydrogen bonds. Therefore each molecule is linked to four neighbouring molecules by four C—H⋯O hydrogen bonds, with the OH group at the 7-position and the carbonyl O atom of the ester group acting as the acceptor sites.

The title compound, C 27 H 43 ClO 6 , is a derivative of ursodeoxycholic acid, in which the OH group at the 3-position is substituted by a chloromethoxycarbonyloxy substituent and the carboxylic acid group at the 24-position is methylated. The A and B rings are cis-fused, while all other rings are transfused. In the crystal, two adjacent molecules located along the b-axis direction are interlocked head-to-tail due to weak C-HÁ Á ÁO hydrogen bonds. Therefore each molecule is linked to four neighbouring molecules by four C-HÁ Á ÁO hydrogen bonds, with the OH group at the 7-position and the carbonyl O atom of the ester group acting as the acceptor sites.   Table 1 Hydrogen-bond geometry (Å , ). famous analogues of bile acid. Here we report the crystal structure of a UDCA derivative.

D-HÁ
In the crystal structure, rings A and B are cis fused while rings B/C/D are trans fused. The dihedral angles of A/B, B/C, C/D are 62.89 (8)°, 2.70 (14)° and 5.05 (17)°, respectively. So the skeleton of the title compound exhibits a V shape with the 3-α and 17-β side chains stretched as the arms. In the crystal packing (Fig. 2), two adjacent molecules located along the b axis are interlocked head to tail due to weak hydrogen bondings (C11-H11B···O2 and C27-H27B···O5, Table 1).
Therefore each molecule of the title compound is linked to four neighboring molecules by four C-H···O hydrogen bonds.

Experimental
Chloromethyl-chloroformate was slowly added to a solution of methyl 3,7-dihydroxycholan-24-oate and anhydrous pyridine in anhydrous CH 2 Cl 2 under nitrogen at 273 K. The resulting mixture was allowed to warm to room temperature, was then stirred for 7 h and then extracted with CH 2 Cl 2 . The organic extract was dried over Na 2 SO 4 , filtered and evaporated under reduced pressure. The residue was washed with diethyl ether to afford the title compound as a white solid (>95% yield). No further purification was necessary. Crystals appropriate for X-ray diffraction data collection were obtained by slow evaporation of a saturated DMF/H 2 O solution at room temperature.

Refinement
The H atom of the OH-group in 7-position was located from the difference Fourier map and restraind to ride on its parent O atom. All other H atoms were placed in geometically idealized positions and constrained to ride on their parent atoms with C-H distances of 0.93 Å (0.96 for methyl group) and U iso (H) = 1.2 (1.5 for CH 3 )U eq (C) for CH.  Molecular structure of (I) showing the atom-labelling scheme. Displacement ellipsoids are drawn at the 30% probability level.

Figure 2
The packing of the title compound, viewed down the a axis. Dashed lines indicate the C-H···O hydrogen bonds.

Data collection
Bruker APEXII CCD diffractometer Radiation source: fine-focus sealed tube Graphite monochromator φ and ω scans 15292 measured reflections 5411 independent reflections 3191 reflections with I > 2σ(I)  (9) 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.