Scheme 13. Synthesis of the compounds 26, 27, and 28. Reagents and conditions: (a) (i) (CH₂)₄Br₂, CH₃CN, K₂CO₃, 50 °C; (ii) piperidine, Et₃N, 50 °C; (b) cyanomethylenetributylphosphorane (CMBP), toluene, 100 °C; (c) Ph₃P, DCM, di-tertbutyl azodicarboxylate (DBAD), rt.