Synthesis and guest binding properties of cyclophanes containing two benzo[b]furan rings

Abstract

The cyclophanes 1a-d containing two benzo[b]furan rings connected by various bridges have been prepared and their binding behaviors with N-benzylphenethylammonium cation 2 were examined by NMR titration method. The successive alkylation reactions of 4-hydroxyl groups and then 2-hydroxyl groups of 2,4-dihydroxybenzophenonse gave macrocycles 5a-c. Photoirradiation of the macrocycles 5a-c with 350 nm mercury lamp followed by dehydration afforded the cyclophanes 1a-c. Hydrolysis of two ester groups pendant on a bridge of 1b produced the carboxyl group-containing cyclophane 1d. The cyclophane 1a having a p-xylene bridge showed 1:1 binding with 2 with the binding constant of 36 ± 6 M-1 in 3:1 CDCl3/methanol-d4 solvent, while 1b and 1c which have neutral flexible bridges exhibited no appreciable binding with 2. The disodium salt of 1d showed much higher binding affinity for 2 forming 1:1 and 1:2 complexes.