Optimization data for this reaction is shown in Table . Elimination of the succinyl fragment occurs readily at room temperature upon the addition of DBU (optimally, as a solution in DCM) to the N-succinyl species in up to 99% yield (Figure ). Using these optimized conditions, we proceeded to evaluate the scope of this reaction on a small library of pyrimidine containing compounds. These results are summarized in Figure . Some substrates exhibited major and minor sites of N incorporation as observed by N NMR; the positions exhibiting the higher N enrichment are depicted. The positional selectivity of N labeling is influenced by sterics; greater labeling occurs at the less hindered pyrimidine nitrogen, corresponding to attack of the aspartate nucleophile at the least hindered carbon. Simple 4-arylpyrimidines 18-24 were found to undergo N labeling in high chemical and isotopic yields (Figure ). Other arenes were also tolerated at C4 (see 23 and 24). 5-Arylpyrimidines 25-27 also underwent labeling (Figure ), a Determined by 1 H NMR using 1,3,5-trimethoxybenzene as an internal standard. b 1.2 equiv of triflic anhydride and 1.5 equiv of dimethyl aspartate were used. albeit with lower efficiency. This discrepancy is likely due to the lower electron density (and thus nucleophilicity) of the participating pyrimidine nitrogen in the 5-aryl substrates relative to the 4aryl substrates. Pyrimidines bearing electron poor arenes do not undergo appreciable amounts of labeling, supporting this hypothesis. The successful participation of 4,5-diaryl pyrimidine 33 implies that electronics at nitrogen dictate the success of the reaction to a greater degree than steric encumbrance about the heterocyclic carbon atoms. We posit that 4-aryl pyrimidines undergo triflylation to a greater degree and feature a stronger N-S bond, rendering the activated species more resistant to detriflylation to give unlabeled starting material. The low chemical yields for the 5-arylpyrimidines likely result from decomposition under the reaction conditions, possibly due to bis-addition of the aspartate nucleophile to the pyrimidine heterocycle (as determined by mass spectrometry) and subsequent side reactions of the bis-adduct. These hypotheses are supported by our computational studies (Figure -G, vide infra).