The hitherto longest molecular ribbons (4-61, composed of several [3.3lmetacyclophane units, are synthesized using a new iterative synthetic strategy, up to a length of seven fourfold-bridged benzene rings, and it is proved by X-ray crystal structural analyses and 1H NMR spectroscopy that these molecules form meander-type folded syn conformations leading to multiple n-staples.Molecular ribbon-structures were proposed years ago, yet only Stoddart and coworkers have found structure-directed syntheses.* Subsequent Diels-Alder reactions led to open-chain and cyclic ribbon structures, which consist of condensed sixmembered ring assemblies.3For some years, we have tried to obtain flexible ribbon structures which could be macrocyclized later on account of their four terminal functional group^.^ So far, we have synthesized only [3.3]metacyclophane structures la-3a with a maximum of four benzene ring units.5

Abstract:The longest molecular ribbons known to date (3-lo), composed of a series of [3.3]metacyclophane units, have been synthesised by means of a repetitive three steps: ester reduction to a tetrakis(hydroxymethy1) compound, derivatisation to the corresponding tetrakis(br0-synthetic strategy. These multiple ring systems with up to nine bridged benzene rings in a row are the longest structurally perfect cyclophane sequences known to date. The synthetic strategy comprises

The first synthesis of the ribbon-like compounds 6-10 is a face-to-face manner, and the tosyl units of 8-10 pointing described. The new building block diethyl 2,6-bis(bromo-outwards. The "intramolecular self-assembly" (folding) of methyl)pyridine-3,5-dicarboxylate 1 was prepared and cha-these molecular ribbons is primarily attributable to the enerracterized. Single-crystal X-ray structure analyses indicate getically more favourable syn conformation of both the that these ribbon-like molecules arrange in a folded confor-[3.3]metacyclophanes and the pyridinophanes. mation, with the pyridine and benzene units being folded inIn work published earlier we were able to show that ribbon-shaped molecules of type I, built up from The ribbon-shaped pyridine assemblies I are of interest because the integrated, complex-forming donor centres theyThe observed structures of such compounds may be due to a number of factors: the common tendency of many [3.3]metacyclophanes to arrange in a syn conformation, maximum packing in the crystal, and, as in the case of the self-assembly of folded proteins, the influence of the sidechains (tosyl in this case)L21.contain allow the complexation of cations and protons, which may solubilize compounds. We were also interested in ascertaining whether the ribbon-shaped molecules built from pyridine rings also undergo "self-assembly" leading to the folded structure described for their benzenoid counterparts. Ultimately, the open-chain tetrafunctionalized compounds I are intended to be converted by macrocyclization to tube-shaped molecules 11, which could serve as potential concave host compounds for guest ions and molecules [5].This work has shown that the methodologiesL61 developed for the syntheses of the ribbon-and belt-shaped (n)benzenophanes I, I1 may also be employed in the synthesis of analogous multiple pyridinophanes.We used diethyl 2,6-bis(bromomethyl)pyridine-3,5-dicarboxylate (1) as the basic building block, which can be considered as the pyridine analogue of diethyl 2,4-bis(bromomethy1)benzene-1,5-dicarboxylate['1. The mono sodium salt of 4-toluenesulfonic acid amide 2 and the amine 3 were chosen as the cyclization partners of 1 for the preparation of two-layered pyridino-or benzenopyridinophanes. Threelayered tetraaza-or tetrathiapyridinophanes were prepared by the reaction of 1 with the tetrakis(tosylaminomethy1) compound 4 or the tetrakis(mercaptomethy1) compound 5.