Macrocyclic Hexaureas: Synthesis, Conformation, and Anion Binding

Varied flexibility : Cyclic oligoureas are formed by using anions as templates. Linking of six xanthene and/or diphenyl ether fragments by urea groups leads to the formation of five macrocyclic compounds with a 48‐membered ring with variable flexibility (see picture). Their interaction with anions shows a strong influence of acetate and chloride ions on the cyclization from four precursor molecules.Five macrocylic compounds , XXDXXD, XDXDXD, XDDXDD, and with 48‐membered rings, in which six xanthene and/or diphenyl ether fragments are linked through six urea (‐NH‐C(O)‐NH‐) groups, have been synthesized. In the cyclization step, a linear diamine was allowed to react with the appropriate diisocyanate by using a [5+1] (i.e., “XDXDX+D” for XDXDXD), [4+2] (), or [3+3] (XDDXDD) procedure. Compounds and XXDXXD were prepared from two molecules of the dimeric amine XX and two molecules of the respective monomeric diisocyanate (X or D) in a [2+1+2+1] (or 2×[2+1]) reaction. The (nonoptimized) yields in the cyclization step ranged from 45 to 80 %. The linear precursor diamines or diisocyanates were obtained by analogous condensation reactions by using partial protection with a tert ‐butoxycarbonyl group. All the macrocyclic compounds and synthetic intermediates were characterized by 1 H NMR and mass spectra. Three different crystal structures were obtained for XDDXDD, which show the molecule in a more or less strongly folded conformation determined by intramolecular hydrogen bonding. The interaction of the hexaureas with selected anions was studied by 1 H NMR spectroscopy and UV absorption spectrophotometry.