In a 4-ml scintillating vial, a solution of 2-amino-5-aminomethyl-4,7-dihydro-5H-thieno[2,3-c]pyran-3-carboxylic acid tert-butyl ester (58 mg, 0.2 mmol) in tetrahydrofuran (2.0 ml) was treated with 4-nitrophthalic acid (63 mg, 0.3 mmol), diisopropylethylamine (190 μl, 1.1 mmol), and 1,3-diisopropylcarbodiimide (120 μl, 0.77 mmol). The reaction mixture was shaken vigorously for 10 seconds before being stirred at 50° C. for 43 hours and at room temperature for 20 hours. The solution was diluted with ethyl acetate (25 ml), washed with 0.5N aqueous hydrochloric acid (25 ml), saturated aqueous sodium bicarbonate (25 ml), and the brine (25 ml). The organic layer was dried (MgSO4), filtered and the solvent evaporated in vacuo. Crude 2-amino-5-(5-nitro-1,3-dioxo-1,3-dihydro-isoindol-2-ylmethyl)-4,7-dihydro-5H-thieno[2,3-c]pyran-3-carboxylic acid tert-butyl ester was obtained as a solid and used immediately in the next step.
