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

Abstract

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 derivative of 2, provide calix[2]uracil[2]arenes 4. Their conformations, investigated through X-ray, variable temperature ¹H NMR, and molecular modeling MM2 calculations, are found to depend on the nature of substituent(s) on the position 2 of 1,3-phenylene rings. 4a exists in equilibrium between various conformations, and 4b, 4d-4i adopt an inward flattened partial cone conformation. In 4j, the equilibrium is restricted to two conformations at -60 °C, but the 4j.ethanol complex (2:1), in the solid state, has a cone conformation with ethanol being a H-bond donor through H of OH and CH₂ and an acceptor through O of OH. 4d, 4j, and 4k exhibit significant preferences toward alkaline earth metal ions over alkali metal ions though overall bindings are poor.