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.