Acylation, of benzene derivatives

In contrast with the acylation of benzene derivatives, which generally requires stoichiometric amounts of the Lewis acid,137 the acylation of five-membered heterocycles requires only catalytic amounts of the catalyst (generally 0.01 mole of catalyst for each mole of acid anhydride is sufficient). This is probably due either to the greater reactivity of the substrates or to the lower ability of the ketones formed to coordinate the catalyst. 

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