Design and Synthesis of Urea-Linked Aromatic ligomers-A Route Towards Convoluted Foldamers

Herein we report the design and synthesis of crescent-shaped and helical urea-based foldamers, the vature of which is controlled by varying the constituent building a blocks and their connectivity These oligomers are comprised of two, three or five alternating aromatic heterocycles (pyridalzine. pyrimidine or pyrazine) and methyl-substituted aromatic carbocycles (tolvl, o-xylvl of In-xylyl) connected together through urea linkages A crescent-Shaped conformational preference is encoded within these pi-conjugated Urea-linked oligomers based oil intrarmolecular hydrogen bonding and steric Interactions, the degree of curvature IS toiled by the Urea connectivity to the heterocycles and the aryl groups. NMR characterization of these foldamers confirms the intramolecular hydrogen-bonded conformation expected (Z,E configuration of the urea bond) in both the pyridazyl and pyrimidyl foldamers in solution An X-ray crystal structure of the N3,N6-diisobutylpyridazine-4,6-diamine-o-tolyl urea-linked foldamer (4) confirms the presence of N-H N hydrogen bonds between the heterocyclic nitrogen atom and the free hydrogen of the urea linkage Additionally. the tolyl methyl group interacts unfavourably with the urea Carbonyl oxygen. thus destabilising the alternate planar conformation.