Scheme 1

Synthesis of compounds 4a–4c and 7a–7c. Reagents and conditions: (a) EtOH, H₂SO₄, reflux, overnight; (b) DIBAL-H, DCM, N₂, −70 °C to rt, 12 h, 90–95 %; (c) NaNO₂, HOAc, H₂O, 0 °C to rt, 25–32 %; (d) MnO₂, DCM, water–ice to rt, 4 h, 92 %; (e) NH₄OH-HCl, pyridine, EtOH, 90 °C, 3 h, 49 %; (f) SOCl₂, DMF, N₂, 30 °C, 12 h, 70–75 %