Unexpected Opening of the Benzodiazepine Ring During Acetylation

Reaction of 2,4-diaryl-2,3-dihydro-1H-1,5-benzodiazepines 1 either with acetyl chloride in dry CH2Cl2 in the presence of pyridine, or with acetic anhydride in the presence or absence of base afforded, after unexpected opening of the benzodiazepine ring, N-butenylidene-amino-phenylacetamides 3 in excellent yields instead of the expected ring tautomers 2.