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. magnified image Five macrocylic compounds XXXXXX, XXDXXD, XDXDXD, XDDXDD, and DDDDDD 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] (DDDDDD), or [3+3] (XDDXDD) procedure. Compounds XXXXXX 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 1H 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 1H NMR spectroscopy and UV absorption spectrophotometry.