From Furan to Molecular Stairs: Syntheses, Structural Properties, and Theoretical Investigations of Oligocyclic Oligoacetals

The synthesis of oligocyclic oligoacetals using five-membered rings as repetitive unit is described. Furan was used as the starting material, which is converted by a three-step procedure consisting of twofold cyclopropanation, reduction, and oxidative ring enlargement into a tricyclic bis(enol ether). A repetition of this synthetic procedure leads to the formation of extended oligoacetal systems. Insights into the structures were gained by X-ray crystallographic investigations and revealed helical arrangements of the subunits in the solid-state. DFT (B3LYP) calculations have been carried out to elucidate the transition state of the ring enlargement and the flexibility of the annelated oligocyclic systems. Strain energies and topologies of potential cyclically condensed oligoacetals are predicted.