Scheme 2.

Synthetic route for final compounds 5 a–c and 6 a–c. Reagents and conditions: a) boc‐proline, DCC, DMAP, CH₂Cl₂, 2 h to o/n, 41–71 %; b) 4 n HCl, dioxane, RT, 4 h, 98 %–quant.; c) methyl (2‐chloroacetyl)prolinate, Et₃N, RT, CH₃CN, 2.5 d or reflux, 2‐ethoxyethanol, 2–3 d, then 2 n HCl, CH₃CN, RT, 4 h, 6–61 % (2 steps).