Concise Synthesis of (+)-[ C ]-Anatoxin-a by Dynamic Kinetic Resolution of a Cyclic Iminium Ion.

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