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

1. by replacement of the 1-methyl group by an acyl, alkyl whether or not unsaturated, benzyl or phenethyl group, whether or not further substituted;
2. by substitution in the piperidine ring with alkyl or alkenyl groups or with a propano bridge, whether or not further substituted;
3. by substitution in the 4-phenyl ring with alkyl, alkoxy, aryloxy, halogeno or haloalkyl groups;
4. by replacement of the 4-ethoxycarbonyl by any other alkoxycarbonyl or alkoxyalkyl or acyloxy group;
5. by formation of an N-oxide or of a quaternary base.