Boron-Containing Pseudocryptates

The structural organization of these boron-containing natural products has inspired the synthesis of pseudocryptand 110 [157], which combines the 

18-membered [2.2]macrocyclic core of the [2.2.2]cryptand with two bidentate catechol units that can be transformed with boric acid and a metal hydroxide MOH (M = Li, Na, K, Rb, Cs, NH4) to the corresponding charge neutral pseudocryptates 111-116 (Fig. 29) [157-159]. 

Based on these observations a selective receptor for lithium 117 has been synthesized from a smaller diazadioxa macrocycle [160]. Ligands 110 and 117 may be regarded also as binucleating ligands capable of binding two cations at a time. 

Another related host-guest complex has been constructed from citric acid, boric acid, and a strontium salt in a 2 1 1 stoichiometry. In this complex the strontium cation is surrounded by four water molecules, two monodentate carboxyl groups and one oxygen atom of the BO4 unit [161]. 

---