Obstructive halogenation

The isosterism between H and F will therefore often serve to give analogs that are more resistant to metabolic degradation (obstructive halogenation flunarizine and in flufenisal Figme 15.63). Similarly the CF3 group is biostable whereas CH3 is easily oxidized. ... 

The obstruction of a molecule by means of halogen substitution can impose certain conformations or mask certain functions. In the case of cloiudine the bulky halogen atoms prevent the free rotation and maintain the planes of the aromatic rings in a perpendicular position to each other (Figure 20.29a). 5... 

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