Synthesis of Spirolactones and Functionalized Benzofurans via Addition of 3‑Sulfonylphthalides to 2‑Formylaryl Triflates and Conversion to Benzofuroisocoumarins

A convenient protocol for the synthesis of spirobenzofuran-isobenzofurans and substituted benzofurans via a modified Hauser–Kraus reaction of 3-sulfonylphthalide with 2-formylaryl triflates is reported here. The initial reaction involved 1,2-addition of phthalide to the formyl group and intramolecular cyclization via substitution of triflate followed by a cascade of rearrangements leading to spirolactone or benzofuran derivatives. The electronic nature of substituents on aryl triflates affected the course and outcome of the reaction. The mechanism was supported by successful characterization of one of the intermediates by mass spectrometry. A medicinally relevant influenza virus type B inhibitor, benzofuroisocoumarin, was synthesized in a single step from the spiro compound, thus demonstrating the synthetic utility of our methodology.