## 2 Results and Discussion
### 11C-Methylation of an Acid-Labile Protected Desmethyl Precursor  
Owing to its low stability, we next replaced the base-labile Pom-protecting group with a base-stable but acid-labile _tert_-butyloxycarbonyl (Boc) residue. To this end, \(O\)-benzyl urea **7** was reacted with di-_tert_-butyldicarbonate and 4-dimethylaminopyridine (DMAP)/triethylamine in THF, which afforded the \(O\)-Bn-N-Boc diprotected intermediate **11** in a yield of 89%. Subsequent \(O\)-debenzylation of **11** by CTH under the same conditions as described for **7** furnished the desired \(N\)-Boc protected radiomethylation precursor **12** as bench-stable crystals in near-quantitative yield (Figure 7). Figure 5: Preparation of \(N\)-Pom-protected precursor **10**. Depending on reaction conditions, \(O\)-debenzylation of **9** by catalytic transfer hydrogenation furnished either **10** or (due to base-induced rearrangement) **10a**. Conditions: (a) Pom-CI, K\({}_{2}\)CO\({}_{3}\), DMF, 60 \({}^{\circ}\)C; (b) Pd/C, ammonium formate, DMF, 55 \({}^{\circ}\)C; (c) Pd/C, ammonium formate, MeOH-AcOH, 55 \({}^{\circ}\)C.  
Non-radioactive screening experiments similar to those performed with **10** (i.e., treatment of precursor in DMF with 1 N aqueous NaOH and heating to 60 \({}^{\circ}\)C) indicated a sufficient base stability of **12**. Accordingly, radiomethylation of **12** was performed with [11C]MeI in DMF and 1 N aqueous NaOH (2 equivalents) for 5 min at 60 \({}^{\circ}\)C (Figure 2, Route d). Subsequent Boc-deprotection with concentrated aqueous HCl at 100 \({}^{\circ}\)C for 5 min furnished the desired tracer [11C]tozadenant, albeit in highly variable yields. Likewise, small-scale manual radiolabeling experiments performed in close succession produced distinctly different results, with a moderate radiochemical conversion (RCC) of 43% and large quantities of polar side products in one case and an excellent RCC of 82% and few side products in the other (Figure 8A). These discrepancies could potentially be explained by hydroxide-assisted thermal decomposition of DMF with formation of dimethylamine as a competing nucleophile  
Figure 6: ESI HPLC-MS analysis of the products formed upon treatment of precursor **10** in DMF with two equivalents of NaOH and heating to 60 \({}^{\circ}\)C to simulate radiolabeling conditions. HPLC conditions: Column: Phenomenex Kinetex®F5 \(\upmu\)m, 250 \(\times\) 4.6 mm; eluent (solvent A = 0.2% HCOOH in H\({}_{2}\)O, solvent B = 0.2% HCOOH in McN): linear gradient: 0 min 20% B, 15 min: 80% B; flow rate: 1.5 mL/min. Figure 7: Preparation of [11C]tozadenant from the \(N\)-Boc protected precursor **12**. Conditions: (a) (Boc)\({}_{2}\)O, TEA, DMAP, THF, rt; (b) ammonium formate, Pd/C, DMF, 55 \({}^{\circ}\)C; (c) n.c.a. [11C]methyl iodide, NaOH, DMF, 60 \({}^{\circ}\)C; (d) conc. HCl, 100 \({}^{\circ}\)C.  
in amounts that depended on the exact timing [31]. With this in mind, additional small-scale experiments were performed with DMSO or DMA as alternative reaction solvents. While the use of DMSO proved to be unsuitable and resulted in RCCs of only 5-10% (Figure 8B), \({}^{11}\)C-methylation of **12** in DMA proceeded smoothly and afforded the desired tracer in reproducible RCCs of 78-81% with only a few minor side products (Figure 8C). Figure 8: HPLC chromatograms of crude reaction mixtures obtained by \({}^{11}\)C-methylation of precursor **12** with [\({}^{11}\)C]MeI in DMF (**A**), DMSO (**B**) or DMA (**C**) followed by de-protection of the radiolabeled intermediate. Two reactions were performed in close succession with each solvent (indicated in red and black) and all signals were normalized to the highest peak in the corresponding chromatogram.