Improved Synthesis of 5-Nitrohomovanillic Acid and 6-Nitrohomovanillic Acid as Probes for Metabolism Studies of Endothelium-Derived Dopamines: Identification in Human Amniotic Fluid

6-Nitrodopamine is an endogenous catecholamine responsible for numerous biological activities. Here, an improved method for the synthesis of both 6-nitrohomovanillic acid (6-NHVA) and its regioisomer 5-nitrohomovanillic acid (5-NHVA) is reported. The developed one-step synthetic procedures ensured the efficient preparation of the target compounds in good yields. Comprehensive structural characterization was achieved through one- and two-dimensional NMR studies and by high-resolution mass spectrometry (HR-MS/MS). The presence of both substances was identified in human amniotic fluid by LC-MS/MS.