Scheme 3.

Synthesis of compounds 6, 8, 10 and 12^a

^aReagents and conditions: (a) R₁-X, Cs₂CO₃, MeCN, reflux, 12 h, 32-99 %; (b) R₂-SO₂Cl, Et₃N, DMAP, THF, reflux, 5 h or 100 °C, 3 h, MW, 35-78 %; (c) R₃-NCO, Et₃N, DMAP, THF, 100 °C, MW, 10 h, 51-81 %; (d) LiOH, MeOH, THF, H₂O, 12 h, 21-85%; (e) 6M HCl, THF, rt, 5 days, 80 %; (f) hexyl chloroformate, Et₃N, DMAP, THF, 100 °C, 3 h, MW, 85 %; (g) 6M HCl, THF, rt, 3 days, 55 %.