Encountered Isosteric and Bioisosteric Modifications

In 1970, Burger classified and snbdivided bioisosteres into two broad categories classic and non-classic. Grimm s Hydride Displacement Law and Erlenmeyer s definition of 

Some non-classical isosteres are reported in Table 15.5 and from a brief glance it can be noticed that they do not obey the steric and electronic definition of classical isosteres. A second notable characteristic of non-classical bioisosteres is that they do not have the same number of atoms as the substituent or moiety for which they are used as a replacement. 

As the distinction between isosteres and bioisosteres is rather of academic interest, it is preferred, in this chapter to treat both categories together. Conseqnently, for example, divalent series such as 0=, HN=, and H2C= can be discussed together with S=. However, the correct nomenclature will be used as much as possible, keeping in mind nevertheless that isosteric replacanent anbraces both true isosteres and bioisosteres. 

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