Improved Synthesis of 6‐(p‐(Bromoacetamido)benzyl)‐1,4,8,11‐ tetraazacyclotetradecane‐N,N′,N′′,N′′′‐tetraacetic Acid (I) and Development of a Thin‐Layer Assay for Thiol‐Reactive Bifunctional Chelating Agents.