Pyridinium para-toluenesulfonate

The most common ketals are dioxolane and dioxane these form more readily and have fewer problems with enol ether contamination. Sometimes, however, they can be difficult to hydrolyze, particularly when the molecule contains sensitive functional groups. The reaction is acid catalyzed, with some of the more common acids being para-toluenesulfonic, pyridinium para-toluenesulfonic, and eamphorsulfonie. The eluninated water can be removed either with an azeotroping solvent (such as toluene or benzene) or by the addition of a dehydrating agent such as trimethylorthoformate. 

The enedione intermediate was generated under mild oxidative conditions using A-bromosuccinimide (NBS) and pyridinium para-toluenesulfonic acid (PPTS) and, once formed, the intramolecular reactions occurred spontaneously. The diol substrates underwent the expected NBS-mediated ring expansion followed by bicycloketalization to give [3.2.1]bicycle 13, the single hydroxyl substrates underwent the same ring expansion... 

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