The carbon skeleton of the belt region of fullerene C84 (D2)

The synthesis and structural characterization of the double-stranded carbon skeleton in the belt region of a C-84 fullerene has been achieved. The synthetic methodology used is based on cyclic dimerization of diastereomeric AB-type monomers 18 by a non-diastereospecific Diels-Alder reaction with isobenzofuran and acenaphthylene groups as reactive termini. Four diasteromeric monomer precursors 17 were prepared for the first time by the use of dihydropyracylene (12) in a multistep synthesis. The synthesis of dihydropyracylene itself has been optimized to the degree that it is now available on the 10g scale. The belt-shaped macrocycle 19, obtained from dimerization of the monomers 17, could be partly aromatized by an acid-catalyzed dehydration reaction to give 23, which differs from the fully unsaturated belt by two water molecules. Semiempirical AMI calculations of the electronic and thermodynamic properties of cyclic fluoranthenes revealed strain energy as the essential reason for the incomplete aromatization of 19. The structures of the macrocycles 19 and 23, one of the monomer precursors, and two diastereomeric epoxybenzo[k]fluoranthenes were elucidated by single-crystal X-ray crystallography.