Results. Leveraging our prior work on a formal C-atom deletion to convert pyrimidines to pyrazoles (Figure ), we hypothesized that N-triflylated pyrimidine heterocycles would undergo an addition of nucleophile, ring opening, and ring closure sequence to achieve a formal nitrogen deletion to yield 3 when exposed to an aminomalonate nucleophile (Figure ). Instead, exposing the N-triflylated heterocycle to the aminomalonate led to N-malonyl ylide 4, which was confirmed by X-ray crystallographic analysis. Because this conversion accomplished a nitrogento-nitrogen atom swap, we recognized the potential utility of this transformation if N-isotopically labeled nucleophiles were employed. With this application in mind, we sought to optimize for the formation of the N-malonyl pyrimidinium ylide, where a nitrogen in the skeleton of the heteroatom had been exchanged (see the Supplementary Materials for details). However, the variable protonation state of the malonyl ylide (methine C-H pKa ≈ 5.0) and competing side reactions such as bis-addition of the amine nucleophile to the activated heterocycle (giving 15, Figure ) led to variable yields. Additionally, removal of the malonyl group from the ylide intermediate proved challenging and similarly irreproducible. We hypothesized that these issues arose from the high C-H acidity of the malonyl fragment; therefore, we evaluated an aspartate diester in lieu of an aminomalonate (Figure ), which led to higher, reproducible reaction yields of up to 99%. This nucleophile also set the stage for dealkylation of the N-alkyl intermediate -which is precedented for the analogous pyridine system --to give a pyrimidine ring in which one of the nitrogen atoms had been exchanged.