1,3-calix[4]arene crown ether conformers with a 3-thienyl pendant functionality at the lower rim

Synthetic protocols to novel thienyl-calix[4]crown building blocks are reported. The selective distal introduction of a 2-(3-thienyl)ethoxy functionality into mono-O-alkylated calix[4]arenes 1 (R = Me, Bn), followed by cyclization of the mixed di-O-alkylated intermediates 2 with tetra- and pentaethylene glycol ditosylate and base, produced mixtures of 1,3-alternate and cone 1,3-calix[4]arene crown ether conformers 3 and 4, respectively. On the other hand, the selective debenzylation (Me3SiCl) of 1-benzyloxy-3-propoxycalix[4]arene crown-5 (cone conformer 4c) led to cone monohydroxy-derivative 5, which upon alkylation with 2-(3-thienyl)ethanol tosylate and Cs2CO3 afforded the rigid partial cone calix[4]arene crown-5 6, having the heterocyclic pendant functionality anti to the polyether ring. The H-1 NMR resonances of Bu-t substituents at the upper rim of conformationally rigid mixed 1,3-dialkoxycalix[4]arene crown ethers provide a diagnostic tool for establishing the mutual inclinations of the opposing pairs of aromatic rings. The structures of 5,11,17,23-tetrakis-(1,1-dimethylethyl)-25-methoxy-27-{[2-(3-thienyl)ethoxy]}(beta)-26,28-(crown-6)(alpha)-calix[4]arene (anti-3aa) and 5,11,17,23-tetrakis(1,1-dimethylethyl)-25-methoxy-27-{[2-(3-thienyl)ethoxy]}(alpha)-26,28-(crown-6)(alpha)-calix[4]arene calix[4]arene (syn-4aa) conformers were determined by single-crystal X-ray analyses. Methoxy-containing calix[4]arene crown ethers possess fluxional properties, and the conformational equilibria in solution are strongly affected by alkali-metal complexation.