Triuracils – 1,3-Bis[ω-(N-methyluracil-1-yl)alkyl]thymines and Their 5,5′-Cyclic Counterparts

A series of acyclic compounds with two 3,6-dimethyluracil or 3-methyluracil units attached to a a thymine or isocyanurate fragment by polymethylene or xylylene spacers are investigated. Ring-closure reactions with paraformaldehyde in aqueous HCl in the presence of copper(I) chloride afforded pyrimidinophanes which can be considered macrocyclic counterparts of the acyclic triuracils. By X-ray diffraction data the structures of the macrocycles in the solid state are characterised by a close arrangement of the uracil units, but there are no π-π contacts between them. On the contrary, significant hypochromism of the macrocyclic triuracils compared with their acyclic counterparts indicates stacking between the thymine and 3,6-dimethyluracil units in chloroform and H2O solutions. The conformational behaviour of the acyclic and macrocyclic compounds in solution has been determined by NMR, UV and computational methods. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)