Metathesis reaction of formaldehyde acetals: An easy entry into the dynamic covalent chemistry of cyclophane formation

The acid-catalyzed transacetalation of formaldehyde acetals (formal metathesis) is a suitable reaction for the generation of well-behaved Dynamic Libraries of cyclophane formals. The composition of the equilibrated mixtures solely depends on concentration, and is totally independent of whether the feedstock is any of the pure cyclic oligomers, or a mixture of oligomers/polymers. Effective Molarities related to the formation of the lower cyclic oligomers were directly measured as their equilibrium molar concentrations above the critical monomer concentration. The finding that silver cation acts as a selective binder toward the cyclic dimer C-2, coupled with the "proof reading and editing" capability of our quickly equilibrating system, translated into significant amplifications of C-2 when the equilibrated mixtures were exposed to the action of the silver template. These results highlight the potential of Dynamic Combinatorial Chemistry as a powerful tool for the preparation in synthetically useful amounts of an otherwise elusive macrocyclic compound. The possibility of using a mixture of high molecular weight byproducts as feedstock adds considerably to the practical value of the procedure.