Triethylamine in chloroform triggers the rearrangement of methoxymethyl 6β-p-nitrobenzylideniminopenicillanate to methoxymethyl 2,3,4,7-tetrahydro-2,2-dimethyl-6-p-nitrobenzylidenimino-7-oxo-1,4-thiazepine 3(S)-carboxylate, which rearranges further to the triethylamine salt of 3-(1-carboxy-2-methylprop-1-enyl)-2,3-dihydro-5-p-nitrobenzylidenimino-6-oxo-1,3-thiazine.