Sesquiterpenoids. Part XXI. Conformations of the episantonins: crystal structures of 2-bromo-6-epi-α-santonin and 2-bromo-6-epi-β-santonin

Abstract

The conformations and absolute stereochemistries of 2-bromo-6-epi-α-santonin (7) and 2-bromo-6-epi-β-santonin (8) have been determined by X-ray diffraction. In each molecule ring A adopts a very flattened boat conformation while ring B, because of cis-fusion to the γ-lactone, deviates notably from ideal chair geometry. The puckering of the γ-lactone results in the C(13)-methyl group at C(11) being placed in a quasi-axial orientation in (7) and quasi-equatorial in (8); the differences in torsion angles between the isomers are largest in the region of C(11) and indicate that torsional strain is greater in (7). Crystals of (7) are monoclinic, space group P2₁, with a= 5.997(4), b= 14.230(7), c= 9.081(4)Å, β= 111.22(5)°, and Z= 2; crystals of (8) are orthorhombic, space group P2₁2₁2₁, with a= 12.210(10), b= 10.161(5), c= 11.822(8)Å, and Z= 4. The structures were solved by the heavy-atom method from diffractometer data, and refined by least-squares calculations to R 6.5 (7; 1 059 reflections) and 7.6%(8; 907 reflections).