SYNTHESIS OF THE FIRST [N]PARACYCLOPHANE WITH AN ANNULENE AS THE AROMATIC, A NECKLACE-LIKE MOLECULE - NMR, X-RAY, AND MMX STRUCTURAL COMPARISONS

The [18]paracyclo[14]annulene, 6, a trans-10b,10c-dimethyldihydropyren e bridged by an 18 atom chain at the 2,7-positions, was prepared in 33 % yield from 1,14-tetradecanedioyl dichloride and 7-bis(hydroxymethyl) -10b,10c-dimethyldihydropyrene (7). Molecular mechanics calculations w ere carried out to correlate the lowest energy conformers of 6 with th e experimental X-ray structure. High field NMR spectra of 6 were fully assigned and correlated with the lower energy conformers, which in so lution equilibrate such that the annulene ring can freely spin. The mo lecule, which is necklace shaped, has the protons H(9/6), and not thos e at the centre of the chain of the necklace [H(8/7)], closest to the centre of the annulene ring, and most shielded by the ring current. Sh ielding of various protons of the chain approximately correlates with the distances of their appropriate carbons, averaged over the 10 lowes t energy conformers, from the centre of the ring current.