from the results it can be seen that not the slightest change of the two amide protons is tolerated without an important loss of activity : replacement of one or two amide protons with alkyl groups , extension of the amide to the hydrazide and its alkyl analogues , and exchange of the amide with an ester or a carboxylic acid all reduce the relative activity/affinity at least by 2-fold .
C0014898
ester
C0007066
carboxylic acid