Direct kinetic formation of nonanomeric [6.5]-spiroketals in aqueous media.

The direct kinetic formation of spiroketals from mixed ketal-alcohol precursors under acid catalysis was studied using four differently substituted systems. In all cases, the exclusive formation of the anomeric isomer was observed under equilibrating conditions. However, the formation of the nonanomeric spiroketal isomer was observed if the reaction was performed under kinetic conditions using an appropriately tuned acid. Water had a dramatic accelerating effect on the spiroketalization reactions that were performed in THF, and the highest yields of the nonanomeric products were obtained in aqueous THF. The nonanomeric/anomeric product ratio was also strongly affected by the substituents and the stereochemistry of the starting alcohol.