To a solution of 4-[(2′-oxo-2,3-dihydrospiro[furo[2,3-g][1,4]benzodioxine-8,3′-indol]-1′(2′H)-yl)methyl]benzoic acid (0.75 g, 1.7 mmol) in dichloromethane (10 mL) was added oxalyl chloride (0.50 mL, 5.77 mmol) and N,N-dimethylformamide (2 drops, catalytic amount). The mixture was stirred for 16 h at ambient temperature and concentrated in vacuo. The residue was dissolved in pyridine (1 mL) and transferred into a 10 mL microwave reaction vessel with pyridine rinses (2×1 mL). To this solution was added N-hydroxyacetamidine (0.25 g, 3.4 mmol) and the reaction mixture was irradiated at 170° C. for 30 min in a microwave reactor. The reaction mixture was allowed to cool to ambient temperature, concentrated in vacuo and the residue was purified by column chromatography and eluted with a 15% to 50% gradient of ethyl acetate in hexanes, followed by recrystallization from dichloromethane/diethyl ether to afford 1′-[4-(3-methyl-1,2,4-oxadiazol-5-yl)benzyl]-2,3-dihydrospiro[furo[2,3-g][1,4]benzodioxine-8,3′-indol]-2′(1′H)-one (0.50 g, 63%) as a colorless solid: mp 177-178° C. (dichloromethane); 1H NMR (300 MHz, CDCl3) δ 8.08 (d, J=8.1 Hz, 2H), 7.46 (d, J=8.1 Hz, 2H), 7.26-7.13 (m, 2H), 7.06-6.98 (m, 1H), 6.74 (d, J=7.6 Hz, 1H), 6.49 (s, 1H), 6.21 (s, 1H), 5.14 (d, J=16.0 Hz, 1H), 4.94 (d, J=9.0 Hz, 1H), 4.87 (d, J=16.0 Hz, 1H), 4.66 (d, J=8.9 Hz, 1H), 4.21-4.07 (m, 4H), 2.44 (s, 3H); 13C NMR (75 MHz, CDCl3) δ 177.6, 174.9, 167.8, 155.3, 144.7, 141.7, 138.4, 132.2, 128.9, 128.6, 128.0, 124.1, 123.8, 123.8, 120.8, 111.5, 109.1, 99.5, 80.1, 64.5, 63.9, 58.1, 43.9, 11.7; MS (ES+) m/z 467.9 (M+1).
