Any compound (not being a compound for the time being specified in sub-paragraph (a) above structurally derived from Fentanyl by modification in any of the following ways, that is to say:

1. by replacement of the phenyl portion of the phenethyl group by any heteromonocycle whether or not further substituted in the heterocycle;
2. by substitution in the phenethyl group with alkyl, alkenyl, alkoxy, hydroxy, halogeno, haloalkyl, amino or nitro groups;
3. by substitution in the piperidine ring with alkyl or alkenyl groups;
4. by substitution in the aniline ring with alkyl, alkoxy, alkylenedioxy, halogeno or haloalkyl groups;
5. by substitution at the 4-position of the piperidine ring with any alkoxycarbonyl or alkoxyalkyl or acyloxy group;
6. by replacement of the N-propionyl group by another acyl group.