Trifluoroacetic acid → Dipyrromethane

In a foil-covered 250-mL three-necked flask, equipped with a magnetic stirrer and N2 inlet, was placed 5-thiazolecarboxaldehyde (1. 0.88 g, 7.81 mmol) (Dondoni, A.; Fantin, G.; Fogagnolo, M., Medici, A.; Pedrini, P. Synthesis 1987, 998–1001), CH2Cl2 (30 mL), and pyrrole (6 mL, 87 mmol). The reaction mixture was stirred for 10 min. then TFA (0.25 mL, 3.2 mmol) was added. After a stirring period of 2 h at room temperature, the reaction mixture was transferred to a separatory funnel and washed with saturated aqueous NaHCO3 (50 mL), H2O (50 mL) and brine (50 mL). The organic layer was dried (Na2SO4), filtered, and concentrated in vacuo. The residue was dissolved in CH2Cl2 (50 mL) and adsorbed onto silica gel (3 g). Purification by column chromatography (gradient elution 33–67% EtOAc hexanes) provided dipyrromethane 2 (0.95 g, 52%) as a gray solid: 1H NMR (300 MHz. CDCl3) δ 5.74 (s, 1H), 6.02 (m, 2H), 6.17 (m, 2H), 6.70 (m, 2H), 7.58 (s, 1 H). 8.19 (br s, 2H), 8.68 (s, 1H).