Molecular conformations. Part IX. Bicyclo[3,2,1]octane system: X-ray analysis of an enmein derivative
Abstract
The dimensions of the bicyclo[3,2,1]octane skeleton in an enmein derivative (III) have been determined by X-ray analysis. The six-membered ring C adopts a distorted chair conformation with the bridging valency angle 8,14,13 (102°) substantially smaller than tetrahedral. The five-membered ring D of the bicyclo-octane has a half-chair conformation with atom 13 displaced by –0·35 Å and atom 14 by 0·43 Å from the plane through atoms 8, 15, and 16. Mean-plane calculations indicate that through the envelope conformation is encountered in methyl gibberellate di-p-bromobenzoate, the conformation of the five-membered ring in a number of other natural-product bicyclo-[3,2,1]octane derivatives is closer to a half-chair than to an envelope form. The crystals are orthorhombic, space group P212121, with a= 8·40, b= 12·29, and c= 22·02 Å, and Z= 4. The atomic co-ordinates were obtained by Fourier and least-squares calculations. The final value of R is 11·2% over 2114 reflections. The absolute configuration of the molecule was established by anomalous-dispersion methods.