Synthesis of [2]- and [3]rotaxanes by an end-capping approach utilizing urethane formation

Several axle-shaped sec-ammonium salts having hydroxy groups at the terminal ends were allowed to react with 3,5-dimethylphenyl isocyanate in the presence of dibenzo-24-crown-8 ether (DB24C8) in CHCl3 by the catalysis of dibutyltin dilaurate to afford the corresponding [2]rotaxanes in good yields. Several Lewis acids other than dibutyltin dilaurate were effective for the end-capping. Although N,N-dimethylacetamide was not a suitable solvent, the use of chlorobenzene and nitromethane as solvents gave the [2]rotaxane in high yields. When a C-2-chiral 26-membered crown ether, prepared from biphenyl-2,2',6,6'-tetrol and L-tartaric acid, was employed instead of DB24C8, the corresponding optically active [2]rotaxane was isolated in 47% yield. The present method was successfully applied to the synthesis of [3]rotaxane by employing hexamethylene diisocyanate. Thus, end-capping by acid-catalyzed urethane formation from alcohol and isocyanate was demonstrated to be an effective protocol for the synthesis a variety of rotaxanes, based on the sec-ammonium salt/crown ether recognition motif.