The crystal and molecular structure of 2-bromolumisantonin

Abstract

The molecular structure of lumisantonin, C₁₅H₁₈O₃, a photoirradiation product of α-santonin, has been determined from a three-dimensional X-ray diffraction crystal structure analysis of its 2-bromo-derivative. The crystals are orthorhombic, space group P2₁2₁2₁, with four molecules in a unit cell, a 11·60 ± 0·02, b 16·043 ± 0·008, c, 7·876 ± 0·005 Å. After the bromine atomic co-ordinates had been postulated from Patterson projection syntheses, the structure was solved semi-automatically by the heavy-atom method using an electron density map scanning computer programme. The final atomic parameters were obtained from a full-matrix least-squares refinement with 911 photographically recorded and visually measured data. The final R was 0·136. The structure consists of four fused substituted rings; a cyclopentenone ring and a distorted cyclohexane ring are both cis-fused to a doubly methyl-substituted cyclopropane ring, and a methyl-substituted γ-lactone ring is trans-fused to the cyclohexane ring. The configurations at the four asymmetric carbon atoms confirm the now accepted stereochemistry of α-santonin, including the recently revised configuration at the methyl-substituted carbon atom of the γ-lactone ring. The packing of the molecules is completely determined by van der Waals interactions.