The reaction of 3‐aminopropionamide oxime (7) with two equiv. of benzaldehyde or of a substituted benzaldehyde gives cis‐3,5‐diaryl‐5,6,7,8‐tetrahydro‐3H[1,2,4]oxadiazolo[4,3‐c]‐pyrimidines (cis‐11a‐c,e). According to 1H‐, 13C‐, and 15N‐NMR studies these compounds form in solution a novel triple tautomeric equilibrium comprising cis‐ and trans‐11a‐c, e and 3‐[2‐(benzylidineamino)ethyl]‐5‐aryl‐4,5‐dihydro‐1,2,4‐oxaliazoles (10a‐c, e). However, the solid obtained from 7 and two equiv. of cinnamaldehyde proved to be an imine (10g), which again formed in solution a similar triple tautomeric system analogously to the mutarotation of sugars. The structure of 11a and 10g was confirmed by X‐ray crystallography.
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