A reversible pH-dependent intramolecular pyridine-aldehyde cyclization.

A dipicolylamine (DPA) attached to an aldehyde moiety cyclizes to form a pyridinium-fused heterocycle. This intramolecular reaction between the pyridine and aldehyde moieties is reversible, and the position of the equilibrium is controlled by pH. The fused-ring heterocycle is stabilized under acidic conditions, whereas the ring-opened form is the major species under basic conditions. The ring-closed form can be utilized as a masked aldehyde precursor to access a variety of functional metal chelators containing the DPA unit.