Scheme 2. Synthesis of AB-fragments with modified A or B building block. Conditions: a19, 18a–b, SOCl₂, 95 °C, then (20c or 20d to 19) or (20 to 18a or 18b) and NEt₃ or Na₂CO₃ (for 20d), THF, 0 °C → 25 °C; bi product of 19 and 20c or 20d, KOH, THF, 25 °C, then Boc₂O, DMAP, THF, 25 °C, 75% (21c, 3 steps), 59% (21d, 3 steps); bii product of 20 and 19a or 19b, TFA/CH₂Cl₂ (1 : 1), 0 °C, 57% (21a), 68% (21b), c [(21a or 21b), SOCl₂, 95 °C] or [(21c or 21d), DIPEA, DMAP, EDC, THF], then pentachlorophenol (PCP), THF, 25 °C, 51% (22a), 51% (22b), 12% (22c), 18% (22d).