Metalloids-toxic isosters

In this section we describe some exotic applications of the bioisostery concept implying the utilization of unusual elements such as silicon, boron, selenium, arsenic, and antimony. The use of those elements as bioisosteres of carbon in existing drugs is a different approach enabling the introduction of a new drug-hke chemical space into the drug discovery and development process. 

Sihcon is directly below carbon in the periodic table so, according to the Erlenmeyer s expansion of the isoster-ism concept, carbon and silicon can be considered as true isosters. Sila-substitution (C/Si exchange) of biologically active substances is an approach to search for new druglike candidates with improved pharmacological properties and stronger IP position. The application of this isosterism remains however limited. For reviews on the subject, see Fessenden and Fessenden, Tacke and Zilch, - Ricci et and Showell and Mills.  

Silicon is more electropositive than carbon (and even more if compared to oxygen and nitrogen) and the covalent silicon-carbon bonds in the sp hybridization state, are 

FIGURE 15.66 Friedman s ether oxygen-methylene group paradox.  

Boron is essential for plant growth and development. In medicinal chemistry its main use is related to that of coupling reagent. The most important employment of boron as drug relies upon the treatment of certain tumors by Boron Neutron 

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