4′-Bromobutyl ent-16-oxobeyeran-19-oate

The title compound, C24H37BrO3, is a tetracyclic diterpenoid with a beyerane skeleton, synthesized by esterification of isosteviol. It comprises a fused four-ring system A/B/C/D. Rings A and B have a chair conformation, whereas ring C is an unsymmetrical distorted chair; the remaining five-membered ring D adopts an envelope conformation. The stereochemistry of the A/B and B/C ring junctions are trans, while the C/D junction is cis.

The title compound, C 24 H 37 BrO 3 , is a tetracyclic diterpenoid with a beyerane skeleton, synthesized by esterification of isosteviol. It comprises a fused four-ring system A/B/C/D. Rings A and B have a chair conformation, whereas ring C is an unsymmetrical distorted chair; the remaining five-membered ring D adopts an envelope conformation. The stereochemistry of the A/B and B/C ring junctions are trans, while the C/D junction is cis.

Experimental
Isosteviol was obtained by hydrolysis of stevioside with 10% sulfuric acid at 95 °C for 7 h and recrystallization from ethanol gave colorless crystals of isosteviol in 80% yield. A mixture of 1,4-dibromobutane (2.4 ml, 20 mmol), K 2 CO 3 (2.8 g, 20 mmol) and acetonitrile (20 ml) was heated to reflux. Isosteviol (3.2 g, 10 mmol) in 30 ml acetonitrile was added dropwise over 10 min, and the resulting mixture was stirred for 2 h further. The mixture was cooled to room temperature, and then distilled to one third volume under reduced pressure. The residue was poured into ice water, and the aqueous layer was extracted with CH 2 Cl 2 (3 × 50 ml). The combined CH 2 Cl 2 extracts were washed with water (1 × 50 ml) and brine (1 × 50 ml) respectively, and then dried with anhydrous Na 2 SO 4 . After the solvent was evaporated, the residue was purified by column chromatography on silica (petroleum ether/ethyl acetate = 18:1, v/v) to give the title compound (2.7 g, 60%).

Refinement
All H atoms were placed in geometrical positions and constrained to ride on their parent atoms with C-H distances in the range 0.96-0.98 Å, and included in the final cycles of refinement using a riding model, with U iso (H) = 1.5U eq (C) for methyl H and 1.2U eq (C) for other H atoms.

Special details
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 > σ(F 2 ) is used only for calculating Rfactors(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.