New insights into the synthesis and characterization of 2‐methoxy‐3‐alkylpyrazines and their deuterated isotopologues

A previously described synthetic route for preparation of 2‐methoxy‐3‐alkylprazines (MPs) based on condensation of glyoxal with an α‐amino acid amide, followed by methylation with iodomethane yields 3‐alkyl‐1‐methyl‐1H‐pyrazin‐2‐ones (N‐methyl derivatives), rather than the designated 2‐methoxy‐3‐alkylpyrazines (O‐methyl derivatives). Despite similar nuclear magnetic resonance and mass spectral properties, gas chromatographic (GC) retention indices differ significantly, indicating chemical difference. With the example of 3‐sec‐butyl‐1‐methyl‐1H‐pyrazin‐2‐one and its 3‐sec‐butyl‐1‐[2H3]methyl‐1H‐pyrazin‐2‐one isotopologue, the position of the methyl group introduced could be assigned unambiguously, using heteronuclear multiple bond correlation (HMBC) NMR experiments. For future characterization, the spectroscopic (NMR, EI+MS) as well as GC retention index data on two stationary phases of the most aroma relevant MPs and their deuterated isotopologues are summarized.