4.3.2. Synthesis of 2,4-Dinitrobenzaldehyde Schiff Base 9

An aliquot of 5 (20 mg, 0.14 mmol) dissolved in anhydrous MeOH (5 mL) was treated with 2,4-dinitrobenzaldehyde (20 mg, 0.10 mmol) at room temperature for 4 h, after which the reaction mixture was quenched with water (5 mL), extracted with EtOAc (2 × 5 mL), concentrated in vacuo, and purified by preparative reversed phase HPLC (Phenomenex Luna-C₁₈, 21.2 mm × 25 cm, 10 μm, 20 mL/min, gradient elution 50% H₂O/MeCN to MeCN over 15 min) to yield 9 (15 mg, 46.6% yield): [α]²²_D –170.6 (c 0.01, MeOH); 1D and 2D NMR (600 MHz, methanol-d₄) see Table S1 and Figures S3–S5 in Supporting Information; HRESI(+)MS m/z 345.0819 [M + Na]⁺ (calcd for C₁₃H₁₄N₄NaO₆ 345.0806).