A facile access to substituted benzo[a]fluorenes from o-alkynylbenzaldehydes via in situ formed acetals

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