Annulative Synthesis of Benzofurans from General Alkenyl Sulfoxides and Phenols via Pummerer/Sigmatropic Cascade

<p>In addition to ketene dithioacetal monoxides that were specially designed and have been used so far, general alkenyl sulfoxides of moderate reactivity have now become applicable as substrates in the trifluoroacetic anhydride-mediated annulation with phenols for the shorter-step and transition-metal-free synthesis of benzofurans having diverse substituents. The improved method enabled concise formation of furan-fused complex polycyclic skeletons, which culminates in the two-step synthesis of dioxa[8]helicene. The modest reactivity of alkenyl sulfoxides has allowed isolation of dihydrobenzofurans as intermediates, which unveiled two interesting phenomena: (i) the initial cyclization reflects the stereochemistry and steric environment of alkenyl sulfoxides and (ii) the aromatization by the departure of alkanethiol is the rate-determining step.</p>