Sulfuric Acid → Tert-butyl 2-bromo-4-methylpentanoate

The product from Step 1 above (63.3 g, 0.32 mol) was dissolved in dichloromethane (250 mL), concentrated sulfuric acid (2 mL) was added, and cooled to −40° C. in a pressure bottle. Isobutene was condensed into the solution to roughly double the volume, and the mixture was then allowed to warm to room temperature overnight. Following evaporation to half of the volume under reduced pressure, the solution was washed with 10% aqueous sodium bicarbonate, dried over Na2SO4, and thoroughly evaporated under reduced pressure at room temperature to leave the title compound as a yellow oil (79.9 g, 0.314 mol; 98%).