Pyridine sulfur trioxide complex ring opening

In view of the development of the synthesis of various aromatic and heterocyclic compounds from pyrylium salts (80T679), the preparation of the unsubstituted pyrylium salt from pyridine is noteworthy (Scheme 269) (53CB1327). The pyridine-sulfur trioxide complex undergoes ring opening to the sodium salt of pent-2-ene-1,5-dial. This salt cyclizes in perchloric acid via the red oxonium salt. 

There are many examples of pyridinium salts, particularly, but not exclusively, those with powerful electron-withdrawing A-substituents, adding a nucleophile at C-2 and then undergoing a ring opening. The classic example is addition of hydroxide to the pyridine sulfur trioxide complex, which produces the sodium salt of glutaconaldehyde, as shown below. 

Generally, sulfur trioxide reacts with hydrocarbon olefins quite vigorously and has to be either diluted or used in complexes with pyridine or dioxane to moderate its reactivity. The corresponding cycloadducts, p-sultones, easily undergo ring opening and have not yet been isolated. In sharp contrast, fluorinated p-sultones are relatively stable materials and over 50 compounds of this type have been prepared and isolated. Synthesis and chemistry of this class of heterocycles is well documented and has been previously reviewed. ... 

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