Scheme 1.

Synthesis of pyranoxanthone 2. Reagents and conditions: (i) H₂SO₄, methanol, reflux, 3: 95%; (ii) K₃PO₄, CuI, picolinic acid, dimethyl sulfoxide (DMSO), 90 °C, 4: 40%; (iii) LiOH, tetrahydrofuran/methanol, room temperature (RT), 5: 88%; (iv) CH₃COCl, H₂SO₄, RT, 6: 85%; (v) AlCl₃, toluene, reflux, 7: 79%; (vi) 3-methylpent-1-yn-3-ol, trifluoroacetic anhydride (TFAA), 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), CuCl₂, acetonitrile, −5 °C, 8: 67%; (vii) N,N-dimethylformamide (DMF), microwave (MW), 150 °C, 2: 86%.