STRUCTURES OF 3 SUBSTITUTED PENTACYCLO[5.4.0.0(2,6).0(3,10).0(5,9)]UNDECANES AND A COMPUTATIONAL ANALYSIS OF BOND LENGTHS

o[5.4.0.0(2,6).0(3,10).0(5,9)]undecane-8,11-dione, [5.4.0.0(2,6).0(3,1 0).0(5.9)]undecane-8-one-11-ol, and 5-methyl-2-phenylpentacyclo[5.4.0. 0(2,6) .0(3,10).0(5,9)]undecane-8,11-diol are three,cage compounds whi ch differ only in the oxidation state at C(8) and C(11). The three com pounds contain a four-membered, a six-membered, and four five-membered rings fused into a cagelike structure. An X-ray structure analysis sh ows the C(1)-C(7) and C(9)-C(10) bonds in the diketo cage to be 1.606( 2) and 1.586(2) angstrom, which are significantly longer than in the o ther two molecules. In order to assess the effects of strain, steric, and electronic factors in these compounds, we investigated the unsubst ituted parent cage compounds and a series of derivatives by molecular mechanics (MM3), AMPAC (AM1), MOPAC (PM3), and GAUSSIAN 90 calculation s. These data suggest that dipolar through space interactions are resp onsible for the bond elongation and not a pi- --> sigma interaction, which has been postulated in parallel pi-systems originating from a co mmon bond. A small degree of pi- --> sigma ''through-bond'' interacti on may contribute to the lengthening in the dimethylene cage analogues .