Abstract
Usnic acid is a well-known secondary lichen metabolite exhibiting a broad spectrum of biological activity. Previously it was shown that the reaction of usnic acid with various amines resulted in enamine-bond formation instead of the C(11)=O carbonyl group. Enamines obtained have a pronounced biological activity. In this work, we have shown that the reaction of usnic acid with ammonia can be regioselective if the solvent is replaced by water. The regioselectivity of that reaction depends on temperature and ammonia quantity. The C-1 enamine as only product formation has been obtained by the usnic acid reaction with an excess of ammonia (20 eq.) in water with cooling (+9 °C).
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