The para-chloro analogue was synthesized, which showed little effect in the biological activity. Since, the positive π (pie) (hydrophobicity) is found to increase the biological activity; it is possible that the substituents cannot be good for the activity.
Chloro group has positive π and positive σ (sigma) value than hydrogen. Therefore, one requires trying para-methyl substituent, which has a positive π but negative σ value. 4-methyl analogue was synthesized from the lead compound, which showed increase in the biological activity.
One can therefore, assume that + π and –σ (electron donating group) are good for the biological activities. The next step in topliss scheme is to synthesize the analogue having the tertiary butyl group at the para position, which has positive π and increased – σ value. It is due to stronger electron withdrawing effect.
It is a scheme in which, there is special series of various substituents in a sequence that are suitably substituted compounds and are aromatic. Those compounds had the biological properties that are superior to their parent compounds.