Concise Synthesis of (+)‐[ 13 C 4 ]‐Anatoxin‐a by Dynamic Kinetic Resolution of a Cyclic Iminium Ion

An asymmetric total synthesis of [ 13 C 4 ]‐anatoxin‐a ([ 13 C 4 ]‐ 1 ) has been developed from commercially available ethyl [ 13 C 4 ]‐acetoacetate ([ 13 C 4 ]‐ 15 ). The unique requirements associated with isotope incorporation inspired a new, robust, and highly scalable route, providing access to 0.110 g of this internal standard for use in the detection and precise quantification of anatoxin‐a in freshwater. A highlight of the synthesis is a method that leverages a cyclic iminium ion racemization to achieve dynamic kinetic resolution in an enantioselective Morita–Baylis–Hillman (MBH) cyclization.