Carmofur and its analogs have been previously synthesized from 5-fluorouracil with phosgene and hexylamine in pyridine, or through direct nucleophilic addition of 5-fluorouracil to n-hexyl isocyanate in pyridine (Fig. ). However, previously recorded synthetic procedures have suffered from low yields and inefficient reaction times . Additionally, due to the limited solubility of 5-fluorouracil in most organic solvents, this reaction must be performed in exceptionally polar, non-volatile solvents such as pyridine, making it inconvenient to monitor by thin-layer chromatography 14 . Serendipitously, carmofur's fluorine atom serves as a spectroscopic handle for reaction monitoring by F nuclear magnetic resonance (NMR) spectroscopy. Here, we show that this can be performed on a benchtop NMR spectrometer directly in a synthetic chemistry laboratory, and that this workflow can be used for real-time quantitative reaction monitoring in the synthesis of carmofur in a manner that is less invasive and time consuming than other conventional methods for analytical quantitation such as high performance liquid chromatography (HPLC) or mass spectrometry .