Axial spiroketal oxygen

The fact that the substituents on the side chains can control the conformation of the spiroketal centre means that it is not necessary to worry about that centre in a synthesis, provided you are trying to make the spiroketal that has the double anomeric stabilization (both oxygens axial) and that has any substituents equatorial on the rings. A recent (1997) synthesis of a single enantiomer of some fruit-fly pheromones from an aspartic acid-derived bromodiol is shown overleaf. It involves three different-sized oxygen heterocycles. 

By contrast, Deslongchamps et al. observed selective equatorial attack of the Z-silyl ketene acetal in a spiroketal (Scheme 4.41). Axial attack would be presumably inhibited by 1,3-diaxial interactions between the silyl ketene acetal and the axial oxygen substituent [43]. 

---