Five-Membered Heterocycles with One Hetero Atom

Five-Membered Heterocycles with One Hetero Atom 

There are three heterocyclic ring systems that can be included imder this heading. These rings include oxygen, nitrogen, or sulfur hetero atoms while the most studied ones are those with oxygen. 

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In the reactions of electrophilic substitution, isoxazole is far less active than the five-membered heterocycles with one hetero atom and pyrazole. It is closer to pyridine, but more reactive. 

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