Preorganized ligand arrays based on spirotetrahydrofuranyl motifs. Synthesis of the steyeoisomeric 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 directions as a consequence of a crossover in reaction selectivity when proceeding from nucleophilic attack on the cis dispiro ketone to oxygenation of the α,β-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 shows no measurable tendency to complex with alkali metal ions, 3 binds strongly 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. ¹³C NMR studies have defined slow exchange limits as the 2:1 sandwich complex with lithium ion is initially formed and transformed progressively into a 1:1 species upon the addition of more LiClO₄. Only the 2:1 complex with sodium ion is formed during comparable titration with NaClO₄. Association constants, molecular mechanics calculations, and X-ray crystallographic studies provide insight into the binding capacity of this belted tridentate ionophore.