Lewis Acid-Catalyzed Cascade Ring Expansion and Intramolecular Friedel-Crafts Type Cyclization of α-Dithioacetyl Propargyl Alcohols: Access to S,S-Heterocycle-Fused Benzofulvenes and 3-Benzylidene-1-indanones.

The Lewis acid-catalyzed reaction of α-dithioacetyl propargyl alcohols has been reported. This cascade reaction involves the expansion of the dithioacetal ring, followed by a Friedel-Crafts cyclization of the resulting vinylic carbocation intermediates, efficiently producing six-, seven-, and eight-membered S,S-heterocycle-fused benzofulvenes in good yields (up to 98%) within short reaction times (5-40 min). When performed in the presence of water, the same reaction sequence afforded 3-benzylidene-1-indanone derivatives as single E isomers, albeit in lower yields (19-57%).