The general procedure for the synthesis of methyl 2,5-dibromovalerate from tetrahydro-2H-pyran-2-one and methanol was as follows: 518 g of 6-valerol and 5 ml of phosphorus tribromide were added to an 11-necked, three-necked flask. The mixture was heated to 95 °C to 105 °C under stirring conditions, followed by the slow addition of 550 g of bromine while controlling the reaction temperature between 100 °C and 120 °C. After the reaction system was stabilized, another 5 ml of phosphorus tribromide and 236 g of bromine were added at 110 °C. After the addition was completed, the reaction mixture was allowed to stand for 30 minutes and subsequently cooled to between 0°C and 10°C. Next, 11 ml of methanol and 1 g of p-toluenesulfonic acid were added to the reaction system and the reaction temperature was maintained at 25 °C. After refluxing the reaction mixture for 5 hours, the excess reagent was removed by distillation and the lower organic phase was isolated. The organic phase was washed sequentially with 500 ml of 10% sodium hydroxide solution and 500 ml of water. After the washing was completed, the organic layer was separated and purified by fractional distillation (139 °C to 142 °C/28 hPa) to give 612.7 g of methyl 2,5-dibromopentanoate in 45% yield and >96% GC purity.
