SYNTHESIS AND STRUCTURE-ANALYSIS OF A LINEAR BENZANNULATED CIS-CISOID-CIS TETRAQUINANE

Contrary to expectations, the photoreaction of the benzannulated bicyc lo[3.3.0] octene 9 with 2-cyclohexene-1,4-dione (12) leads to a [2 + 2 ] cycloaddition preferentially on the endo- rather than on the exo-sid e. Upon treatment with trimethylsilyl iodide, the major compound 14 re arranges to 16 with a cis-cisoid-cis triquinane structure. The X-ray s tructure analysis of 16 revealed a ''closed'' U-type geometry. The fac t that 16 shows different conformations in solution and the solid stat e is interpreted in terms of a partial pseudorotation of the central c yclopentane ring. According to MM2 calculations, the barrier between t he U-shaped and the ''opened'' conformation is about 10-12 kJ/ mol.