Non-planar sulfur- or phosphorus-derived

For the carboxylic function of active compounds, three different classes of bioisosters can be distinguished (a) the direct derivatives (Table 15.11) (b) the planar acidic heterocycles (Table 15.12) and (c) the non-planar sulfur- or phosphorus-derived acidic functions (Table 15.5). 

The carboxylic function of active compounds has been changed to direct derivatives such as hydroxamic acids R— CO—NH—OH, acyl-cyanamides R—CO—NH—CN and acyl-sulfonamides R—CO—NH—SO2—R to planar acidic heterocycles such as tetrazoles, hydroxy-isoxazoles, etc., or even to non-planar sulfur- or phosphorus-derived acidic functions (Table 13.8). 

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