The dipolar cycloaddition of methyl acrylate to 5,6‐diethyl‐1‐methyl‐3‐oxidopyrazinium

5,6‐Diethylpyrazin‐2‐one reacts with iodomethane to give a quaternary salt, deprotonation of which, in situ , liberates a 3‐oxidopyrazinium which undergoes a 1,3‐dipolar cycloaddition with methyl acrylate to form methyl ( Z )‐5‐ethyl‐4‐ethylidene‐8‐methyl‐2‐oxo‐3,8‐diazabicyclo[3.2.1]octane‐6‐ endo ‐6‐carboxylate. The crystal structure revealed (i) the existence of the imine product as its enamine tautomer, (ii) the Z geometry of the exocyclic double bond, and (iii) the endo orientation of the ester group. Pairwise hydrogen bonding between the NH H atom and the amide carbonyl group links the molecules into centrosymmetric dimers.