Some investigations on [6]metacyclophanes

[G]Metacyclophanes 2 with chlorine or fluorine substituents have been synth esized by a novel, convenient version of the base-catalyzed double eliminat ion of hydrogen chloride from the corresponding halopropellanes 7. Conforma tional analysis by H-1 and C-13 NMR spectroscopy revealed remarkable analog ies of certain partial structures of the hexamethylene bridge of 2 with the known exo and endo conformers of [5]metacyclophanes 1. The Diels-Alder rea ctivity of 2 towards tetracyanoethene was investigated in order to clarify seemingly contradictory reports in the literature. As anticipated, it was o bserved that the reactivity of 2 is strongly reduced compared to that of th e more strained lower homologues 1, but there is a subtle fine tuning: Whil e the unsubstituted parent compound 2a still forms adducts with strong dien ophiles, chlorine substitution at the aromatic ring completely blocks the D iels-Alder reactions.