Preorganized ligand arrays based on spirotetrahydrofuranyl motifs. Synthesis of the stereoisomeric 1,8,14-trioxatrispiro[4.1.4.1.4.1]octadecanes and the contrasting conformational features and ionic binding capacities of these belted ionophores

The cis,trans trispiro ether 4 is accessible from several synthetic directi ons as a consequence of a crossover in reaction selectivity when proceeding from nucleophilic attack on the cis dispiro ketone to oxygenation: of the alpha,beta -unsaturated: ester 17. Its cis,cis isomer 3 was obtained in 17 steps and 14.6% overall yield from 3,5-dimethoxybenzoic acid by making use of the alicyclic side chain in N as a "conformational lock". Although 4 sho ws no measurable tendency to complex with alkali metal ions, 3 binds strong ly to Li+ and Na+ ions,as well as to CH3NH3+. Whereas the 3eq conformation is populated in the solid state and in solution, complex formation; occurs readily. C-13 NMR studies have defined slow exchange limits as the 2:1 sand wich Complex with lithium ion is; initially formed and transformed progress ively into a 1:1 species upon the addition of more LiClO4. Only the 2:1 com plex with sodium ion is formed during comparable titration with NaClO4. Ass ociation constants, molecular mechanics calculations, and X-ray crystallogr aphic studies provide insight into the binding capacity of this belted trid entate ionophore.