In situ formed acetal changes the course of Brønsted acid-catalyzed reaction of ortho-alkynylbenzaldehydes with arylalkynes altogether. By utilizing this, an efficient domino approach for the regioselective synthesis of substituted benzo[a]fluorenes has been developed under mild reaction conditions. In situ formed acetal facilitates the intermolecular heteroalkyne metathesis and subsequent trans to cis isomerization of a double bond to effect the intramolecular annulation.