5‐a]pyrimidines (5‐carbethoxy‐7‐methyl‐and 7‐carbethoxy‐5‐methyl‐), the structures of which were determined by 1H and 13C NMRspectroscopy. The 6‐carbethoxy‐7‐methyl‐regioisomer is shown to be the only product in the reaction of the same aminopyrazoles with 2‐ethoxymethylidene‐3‐oxobutyrate; the regiochemical assignment was independently achieved by multimuclear (13C and 15N) NMRspectroscopy.