Heterocalixarenes. 1. Calix[2]uracil[2] arene: Synthesis, X-ray structure,conformational analysis, and binding character

1,3-Bis[(1-uracilyl)methyl)]benzene derivatives 3, formed by selective N-1 alkylation of 1,3-bis(trimethylsilyloxy)pyrimidine with 1,3-bis(bromomethyl )benzene derivatives 2, on cyclization with the same or different derivativ e of 2, provide calix[2]uracil[2]arenes 4. Their conformations, investigate d through X-ray, variable temperature H-1 NMR, and molecular modeling MM2 c alculations, are found to depend on the nature of substituent(s) on the pos ition 2 of 1,3-phenylene rings. 4a exists in equilibrium between various co nformations, and 4b, 4d-4i adopt an inward flattened partial cone conformat ion. In 4j, the equilibrium is restricted to two conformations at -60 degre es C, but the 4j ethanol complex (2:1), in the solid state, has a cone conf ormation with ethanol being a H-bond donor through H of OH and CH2 and an a cceptor through O of OH. 4d, 4j, and 4k exhibit significant preferences tow ard alkaline earth metal ions over alkali metal ions though overall binding s are poor.