Sulfur trioxide pyridine complex

Sulfur trioxide pyridine complex is the compound with the formula C₅H₅NSO₃. It is a colourless solid that dissolves in polar organic solvents. It is the adduct formed from the Lewis base pyridine and the Lewis acid sulfur trioxide. The compound is mainly used as a source of sulfur trioxide, for example in the synthesis of sulfate esters from alcohols:

This is useful for sulfonylation reactions, especially in the sulfonylation of furans. It is also an activating electrophile in a Parikh-Doering oxidation.