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.