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0 °C

To a mixture of 250a (4.0 g, 20.0 mmol) in chloroform (100 mL) cooled at 0° C. was added the solution of POBr3 (22.9 g, 80 mmol) in chloroform (20 mL) over 30 minutes while maintaining the internal temperature below 5° C. The reaction mixture was warmed to 50° C. and stirred at this temperature for 3 h. It was then cooled to 0° C. and quenched with water. The organic layer was separated and evaporated under reduced pressure. The residue was purified by silica-gel column chromatography eluting with 30:1 dichloromethane/methanol to afford 250b (3.3 g, 62%) as yellow solid. MS-ESI: [M+H]+ 264.1

1. temperaturewhile maintaining the internal temperature below 5° C
2. temperatureThe reaction mixture was warmed to 50° C.
3. temperatureIt was then cooled to 0° C.
4. customquenched with water
5. customThe organic layer was separated
6. customevaporated under reduced pressure
7. customThe residue was purified by silica-gel column chromatography
8. washeluting with 30:1 dichloromethane/methanol