Directing effects of fused benzene rings

A [Z ]-fused benzene ring would be expected to favor (3-substitution in the heterocyclic ring (53) over a-substitution (54) based on the expected cr-complex stability. 

In fact, benzo[ ]furan undergoes mainly a-substitution, benzo[ ]thiophene undergoes mainly 13-substitution and indole undergoes almost exclusive (3-substitution. This again illustrates the very strong directing effect of the oxygen atom to the a-position. 

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