5-Bromoisatin → 5'-Bromospiro[[1,3]dioxane-2,3'-indolin]-2'-one

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A mixture of 5-bromoisatin (5.00 g, 22.1 mmol), 1,3 propanediol (4.86 mL, 66.4 mmol, 3 eq) and p-toluenesulfonic acid monohydrate (0.84 g, 4.42 mmol, 0.2 mole %) in benzene (430 mL) was refluxed with a Dean Stark Trap for 5 hr. The mixture was concentrated and washed with sat. aq. NaHCO3 (2×) and brine (1×) and flash chromatographed (Biotage KP silica gel, step gradient 70/30-60/40 Petroleum ether/EtOAc) to give the title compound as a bright yellow solid (2.86 g, 46% ). Anal. Calc'd for C11H10BrNO3: C, 46.50; H, 3.55; N, 4.93. Found: C, 46.82; H, 3.54; N, 4.82. IR: consistent. NMR (300 Mz, DMSO-d6): consistent. MS: (ESI−) m/z 281/283 [M−H], 1 Br pattern observed. m.p.: 202-203° C.

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