The Crystal and Molecular Structure of 7,8-Epoxyhalochamigrene

The molecular structure of 7,8-epoxyhalochamigrene, C15H23OBr2Cl, has been confirmed by means of X-ray crystallographic analysis. The crystals are orthorhombic, with sixteen molecules in a unit cell with dimensions of a=10.303, b=71.31, and c=9.061 Å; the space group is P212121. 4241 independent intensity data were collected on a four-circle diffractometer with graphite-monochromated Cu Kα radiation. The structure was solved by the Monte Carlo direct method, using 20 reflections as a starting set, and was refined by the block-diagonal least-squares method. The final R value was 0.072. The molecular structure thus obtained corresponds to that proposed by Howard and Fenical on the basis of chemical and spectroscopic evidence. The four crystallographically-independent molecules have almost the same geometries. Of the two spiro-linked six-membered rings, the one having the epoxy group takes a half-chair conformation, while the other adopts a chair conformation. The crystal consists of alternating single and triple molecular layers parallel to the (010) plane; the latter layer has a molecular arrangement approximately of P212121 symmetry.