Cylindrospermopsin is a potent hepatotoxin produced by freshwater cyanobacteria. Blooms of cyanobacteria have recently increased dramatically, likely due to the rise in global temperatures and eutrophication. Release of the toxin into water that is subsequently ingested can lead to liver damage, potentially fatal. Monitoring the levels of cylindrospermopsin (CYN) in freshwater has become a human health issue, and is a subject of recent reports by the US Environmental Protection Agency and USDA. The problem is that accurate quantitative measurements of CYN concentration requires pure, isotopically labeled analytical standards, which can be used in isotope dilution mass-spectrometric assays. Producing this standard from cultures of cyanobacteria using feeding with labeled additives proved to be tedious, unreliable, and lengthy (over 2 years before results are known) process. This project proposes to access the analytical standard by total chemical synthesis, which holds a promise as a reliable source of the much-needed material. Our lab has a strong expertise in producing isotopically labeled complex molecules, including natural products, by total synthesis.