2-(Di-tert-butylphosphino)biphenyl → 6'-Isobutyltrispiro[cyclobutane-1,2'-chromene-4',4''-[1,3]oxazole-3',3'''-oxetan]-2''-amine

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To a stirred mixture of palladium (II) acetate (7.9 mg, 0.035 mmol) and biphenyl-2-yl(di-tert-butyl)phosphine (5.3 mg, 0.018 mmol) in THF (5 mL) under argon atmosphere was added di-tert-butyl (6′-bromotrispiro[cyclobutane-1,2′-chromene-4′,4″-[1,3]oxazole-3′,3′″-oxetan]-2″-yl)imidodicarbonate (100 mg, 0.177 mmol) followed by isobutylzinc bromide (0.5 M THF solution, 1.1 mL). The reaction mixture was stirred at room temperature for 17 hours, and then the reaction was quenched with H2O and brine. The resulting mixture was extracted with chloroform 3 times. The combined organic layer was dried over magnesium sulfate and concentrated under reduced pressure. The crude product was dissolved in toluene (6 mL). To the solution was added silica gel (neutral; 600 mg), and the resulting suspension was stirred at 100° C. for 1 hour. After cooling and concentration, the residue was purified by silica gel column chromatography (EtOH/CHCl3=0:100-20:80) followed by column chromatography (NH-silica gel, EtOAc/hexane=20:80-100:0) to afford 6′-isobutyltrispiro[cyclobutane-1,2′-chromene-4′,4″-[1,3]oxazole-3′,3′″-oxetan]-2″-amine (20 mg).

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