Synthesis of avobenzone

Avobenzone is one of the most widely used UV-A filters in sunscreens. It is an aromatic 1,3-diketone that exists almost entirely as a conjugated, chelated enol; that system is what absorbs UV-A radiation.

It is made by a crossed Claisen condensation between methyl 4-tert-butylbenzoate, which is not enolizable and can act only as the electrophile, and 4-methoxyacetophenone, which provides the enolate. As in every Claisen condensation, deprotonation of the product, much more acidic than the reactants, drives the equilibrium.