Spiroketals diaxial configuration

The chemistry of spiroketals, especially l,7-dioxaspiro[5,5]undecanes, is well-studied and reviewed in the literature (15,16). Generally, the ratio of isomers of spiroketals may be controlled by several stabilizing factors such as stereoelectronic effects and 1,3-diaxial interaction of the substituents. Utilizing well-designed spiroketals, one can selectively prepare the most stable isomer by thermodynamic equilibration. Based on this idea, we planned to prepare a spiroketal represented by II which possesses several substituents with established (Cy and C5O and unestablished (Ca) configurations. The spiroketal center and its a-position in II can isomerize to the more stable forms by steric effects caused by the established stereocenters on the ring. The prepared most stable isomer... 

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