Replacement of univalent atoms or groups

Halogens (particularly chlorine) can be replaced by other electron-attracting functions such as frifluoromethyl or cyano groups. In the antibiotic chloramphenicol, both the chlorine atoms of the dichloroacetic moiety and of the para-nitro-phenyl group yielded productive isosteric replacements (Table 15.6). Many other examples of univalent atoms or groups replacements are found in the chapter dealing with substituent effects (Chapter 20) and with quantitative structure-activity relationships (Chapter 23). 

A first series of frequently interchanged divalent atoms or groups is represented by O, S, NH, and CH2 and many 

Similar changes can be applied to cyclic series, for example, to a series such as piperidine-morphoUne- 

CHAPTER 15 Molecular Variations Based on Isosteric Replacements 

The carbaisosteres are as potent as or even more potent than the corresponding physostigmines. In addition, the (-)-enantiomers which possess the same absolute configuration at Cga and Cga as that of physostigmine are generally 6 to 12 times more potent in inhibiting acetylcholinesterase than the corresponding (-l-)-enantiomers. 

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