Bronsted acid-catalyzed rapid enol-ether formation of 2-hydroxyindole-3-carboxaldehydes.

A one-step Bronsted acid-catalyzed synthetic methodology leading to 3-(alkoxymethylene)indolin-2-ones was developed starting from easily accessible 2-hydroxyindole-3-carboxaldehydes. The procedure simply involves a treatment of differently substituted 2-hydroxyindole-3-carboxaldehydes with various alcohols (primary/secondary/tertiary/allyl/propargyl/benzyl) in the presence of a catalytic amount of Bronsted acids such as [Formula: see text]-toluenesulfonic acid and trifluroacetic acid. A series of 19 indolin-2-one-based enol-ethers were synthesized in excellent yields, which implies the general character of our methodology. The enol-ethers produced could be used as a useful building block for the synthesis of indole-based heterocycles.