Lithiation five-membered heterocycles, positional

The inductive effect of the heteroatom, which withdraws electrons to a greater extent from an adjacent carbon atom (a-positions), allows direct a-lithiation of practically all five-membered heterocycles. The relative acidities of a-hydrogens in some different classes are illustrated in the table below. 

In many ways, the electron-rich five-membered aromatic heterocycles behave very much like carbocyclic aromatic compounds when it comes to lithiation. Lithiation a to O or S of furan and thiophene is straightforward (Scheme 130) . The usual selection of orf/io-directing groups allows lithiation at other positions and some examples... 

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