Synthesis and antibiotic activity of the tricyclic furo[3,2-c] isochromen-2-trione unit of the pyranonaphthoquinones

The elaboration and biological activity of 15, containing the proposed pharmacophore for the antibiotic activity of the pyranonaphthoquinones, are reported. The synthetic strategy involved acid-catalyzed lactonization of mandelate 17 for isochroman ring formation, in combination with a Wittig–oxa-Michael functionalization of isochroman-3-ol derivative 20, a lactonization involving configurational inversion of a benzylic alcohol and a final AgO oxidation. Compound 15 showed activity against Staphylococcus aureus and Bacillus subtilis with MIC of 64 and 32 μg/mL, respectively.