A dipicolylamine (DPA) attached to an aldehyde moietycyclizes to form a pyridinium-fused heterocycle. This intramolecular reaction between the pyridine and aldehydemoieties is reversible, and the position of the equilibrium iscontrolled by pH. The fused-ring heterocycle is stabilizedunder acidic conditions, whereas the ring-opened form is themajor species under basic conditions. The ring-closed formcan be utilized as a masked aldehyde precursor to access avariety of functional metal chelators containing the DPA unit.