Impurity in H-NMR spectrum of Hantzsch ester after synthetic procedure

Question

In the $\ce{^1H}$-NMR spectrum of synthesized Hantzsch ester (a dihydropyridine derivative), there are some impurity peaks I cannot explain: a singlet at $\pu{8.81 ppm}$, a quartet at $\pu{4.4 ppm}$, a singlet at $\pu{2.99 ppm}$, and a triplet at $\pu{1.5 ppm}$. I think the quartet and the triplet are indicative of an acetate functionality. However, I cannot match the other two singlets to form a structure that aligns with a possible intermediate in the Hanztsch ester synthesis (see image below).

Would anyone have an idea what this impurity might be? Or is it more likely that the impurity is a result of contamination of starting material?

Answer 1

It is hard to assume what would be your byproduct without knowing your synthetic procedure and proton NMR spectrum of the crude product. However, it is a general fact that unidentified impurities arising from incomplete reactions including small amounts of the corresponding oxidized pyridine derivative, which can form if the reaction conditions are not carefully controlled (if the reaction conditions aren't optimized to prevent oxidation) and oxidation might occur during the synthesis:

The corresponding oxidized pyridine derivative is shown in the above scheme. It is a symmetric compound with one aromatic proton, which could be most probably resonate as a singlet at around $\delta$ $\pu{8.8 ppm}$ in your $\ce{^1H}$-MNR spectrum as noted in above diagram. The other singlet, $\delta$ $\pu{2.99 ppm}$ is very well be due to aromatic methyl groups next to electron withdrawing $\ce{N}$ atom in the pyridine ring.