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.