Process for preparing isopimaric acid

A process for preparing Compound I, the process comprising:

• dissolving thermally isomerized rosin in acetone having a total mass of 1-10 times the mass of the rosin,
• adding dropwise to the dissolved rosin an amount of an acetone solution of isobutanolamine (acetone/isobutanolamine=(1:1 mL/g)) to form a precipitate, wherein the amount of the acetone solution is 10-40% by weight of thermally isomerized rosin,
• standing, filtering, washing with 50% (v/v) ethanol, and drying the precipitate to obtain a crude ammonium salt of Compound I;
• recrystallizing the crude ammonium salt at least five times, and drying to obtain purified crystal of the ammonium salt of Compound I;
• dissolving the purified crystal in ether of 1-20 times the mass of the purified crystal,
• adding a solution of 1-20% (wt/wt) hydrochloric acid to the dissolved crystal until the crystal disappears,
• removing the water layer and washing the ether layer with water to neutral,
• evaporating the ether at one atmosphere and 30-50° C. to obtain a residue,
• dissolving the residue in acetone of 0.5-5 times the mass of the residue,
• adding water slowly into the solution in drops until precipitated crystals cease to grow, and
• filtering and drying the precipitated crystals to obtain purified Compound I.