Concise Synthesis of (+)‐[13C4]‐Anatoxin‐a by Dynamic Kinetic Resolution of a Cyclic Iminium Ion

An asymmetric total synthesis of [¹³C₄]‐anatoxin‐a ([¹³C₄]‐ 1 ) has been developed from commercially available ethyl [¹³C₄]‐acetoacetate ([¹³C₄]‐ 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.