Imines 1,3-prototropic rearrangement

This ready nucleophilic substitution at the 6-position is surprising since this position is electron-rich in both dihydrodiazepines and dihydrodiaze-pinium salts and is the site at which electrophilic substitution occurs. The likely explanation is that in the presence of base some prototropic rearrangement of the normal dihydrodiazepine base into a bis-imino form takes place. Although the equilibrium concentration of the bis-imine is likely to be very small (it has not been observed spectroscopically) it would be strongly electrophilic at the 6-position owing to the combined effects of the bromine atom and the two azomethine groups, and could well be the reactive species in the nucleophilic substitution of the bromine atom ... 

Heating imines derived from 3-amino-l,4-benzodiazepin-2-ones with iV-methylmaleimide in boiling toluene provided adducts derived from the stereospecific cycloaddition of the resonance-stabilized azo-methine ylide 45, formed by a 1,2-prototropic rearrangement, in 82-89% yield (Scheme 16) <1996T13455>. The relative stereochemistry was established by analysis of H NMR NOE data and comparison with the single crystal X-ray structure of an analogous compound. 

The reaction proceeds by way of the imine (109) which is converted into the enamine (110) by prototropic rearrangement before being cyclised with concentrated sulphuric acid. 

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