SYNTHETIC STRATEGIES TOWARDS C-70 - MOLECULAR MECHANICS AND MNDO CALCULATIONS ON PINAKENE, C28H14 AND RELATED MOLECULES

Molecular mechanics (MM2):calculations have been performed to estimate the strain energies of hydrocarbons derived from the fragments of C,, which retain C, or C,symmetry. There are two ways of dissecting C-70 retaining C-2 symmetry, C-2(a) and C-2(h). The key fragment along the C-2(a) route is pinakene. C28H14 (12). It exists in a bowl shape. Its strain energy is close to that of the corresponding C-28 fragment of C-70. The bowl depth was calculated to be 2.03 Angstrom at the MNDO level, and the 'bowl-to-bowl' inversion barrier calculated to be about 48.6 kcal mol(-1), implying that this is a very rigid bowl. The strain energy build-up from the sequential placement of methylene bridges in going from dibenzo[fg,op]naphthacene (26) to pinakene was estimated. Methyl derivatives of dibenzo[fg,op]naphthacene were taken as the key precursors in this study. Strain energies of various positional isomers were also evaluated. Synthetic strategies towards pinakene are delineated.