Ring‐Chain Tautomerism (part II): Synthetic Applications of Biologically Active Five‐Membered Heterocycles With Two Heteroatoms

ABSTRACT Recent examples provide clear evidence that ring‐chain tautomerism can be exploited successfully in the synthesis of wide varieties of five‐membered heterocyclic systems. Owing to the versatile chemotherapeutical activities of azoles, the current review mainly focused on employing the ring‐chain tautomeric phenomenon in synthesizing azoles, the five‐membered heterocyclic systems having two hetero‐atoms such as pyrazoles, isoxazoles, oxazoles, imidazoles, thiazoles. Moreover, we discuss the simultaneous existence of ring‐chain tautomerism in two similar or different five‐membered heterocycles, which is known as ring‐ring tautomerism. Also, the three‐, four‐ and five‐component equilibria are reported. In addition, the biological activities of the target azoles are reported.