asymmetric synthesis of the bis-guanidinium poison, (+)-saxitoxin (STX), is described. Commencing from an N,O-acetal starting material made readily available through sulfamate ester C-H amination, the completed route to STX showcases the utility of oxathiazinane dioxide heterocycles for the assembly of polyfunctionalized amine derivatives. In the final preparative stages, an unusual nine-membered ring guanidine