Scheme 2. Synthesis of [¹¹C]L2-b Precursor (6) and Alternate Methylation Reference Standard (8).

Reagents and conditions: (i) Boc-anhydride, 4-DMAP, DCM, 12 h, 40 °C; Fe, NH₄Cl, MeOH, THF, 3 h, 70 °C, 74%; (ii) 4-pyridinecarboxyaldehyde, NaSO₄, MeOH, r.t. 12 h, 60%; (iii) TMSCl, MeOH, 2 4h, 0 °C, 90%; (iv) formaldehyde, MeOH; NaBH₄, MeOH, 0 °C, 2 h, 4.3%; (v) TMSCl, MeOH, 24 h, 0 °C, 60%.