Reaction of 4-(Alkynyloxy)cyclohexa-2,5-dienones with Dimethyl Sulfoxide: A Catalyst-Free Formation of 6/5/3-Fused Tricyclic Enones

6/5/3-Fused tricyclic enones were obtained when 4-(alkynyloxy)cyclohexa-2,5-dienones were treated with DMSO at 150 °C. The reaction proceeded via a four-membered oxathietene intermediate. The protocol developed is atom economical, has a broad substrate scope, and is amenable to gram-scale synthesis. The products obtained are susceptible to further synthetic transformations.