Fig 3. Synthetic scheme of compounds 23, 24, 33, 34, 35, 38.

Reagents and conditions: i) from 1 for 23, from 5 for 24, from 32 for 33: H₂SO₄, MeOH, reflux, 1 h; ii) from 32: Py · SO₃, DMF dry, 120°C, 25 min; iii) CH₃I, anhydrous K₂CO₃, dry DMF, reflux, 1.5 h; iv) 1N NaOH, EtOH.