Synthesis and structure analysis of cyclic furfural acetals

Quantum chemical calculations and the PMR method are used to show that the preferable conformation of cyclic furfural acetals is a chair with an axial orientation of the furyl substituent. In 2-(furyl-2')-5-ethyl-5-oxymethyl-1,3-dioxane, the conformation equilibrium is shifted toward the trans-isomer with diaxial positions of the furyl and oxymethyl groups. The results of calculations suggest that the synthesis can lead to a cis-isomer with an axial orientation of the furyl and equatorial oxymethyl groups. It was shown experimentally that the synthesis leads to a mixture of trans- and cis-isomers. Mild conditions (room temperature, aqueous medium) lead to formation of the trans-isomer and small amounts of the cis-isomer (less than 2%). In rigid conditions (boiling in aromatic hydrocarbons), up to 20% of the cis-isomer is formed.