Since the discovery 11 of the activating effect of the N- (p-Nitro-phenyl)-group on the methine hydrogen of halogenacetamino esters, it has been found that the p-Acetyl-phenyl and m-Nitro-phenyl groups exert a similar effect. It has been shown that the delocalisation of the N-non-bonded electrons across the orbitals of the N-Aryl substituent is the governing factor in the cyclisation of the above systems, and the effect of the bulk of the N-Aryl substituent is relegated to a minor position.