Molecular conformations. Part X. A skew boat cyclohexanone: X-ray analysis of lunarine hydrobromide hydrate

Abstract

The constitution and absolute stereochemistry of lunarine have been elucidated by X-ray analysis of the crystal structure of lunarine hydrobromide hydrate, C₂₅H₃₂N₃O₄Br,H₂O. The alkaloid contains a spermidine chain, in which the terminal nitrogen atoms are present as amides of two p-hydroxycinnamic acid residues constituting a 3-oxohexahydrodibenzofuran system. The cyclohexanone ring adopts a twist-boat conformation, and since the same feature occurs in the crystal structure of lunarine hydriodide it appears to be a genuine property of the isolated lunarine molecule. The non-chair conformation appears to be stabilized by the cis-fusion of the cyclohexanone and dihydrofuran rings, and extrapolation of the results to cis-hydrindan-5-one suggests that the energetically favoured conformation of the cyclohexanone ring in that molecule may also be the skew boat form. The crystals of the hydrobromide are monoclinic, space group P2₁, with Z= 2 in a cell of dimensions a= 11·37, b= 10·96, c= 12·41 Å, β= 120·3°. The atomic co-ordinates were determined by Fourier and least-squares calculations, the final value of R over 2055 independent reflections being 13·1%.