In a 25 mL dry Schlenk flask, oleanolic acid (2.0 mmol, 1.00 eq.) was dissolved in 10 mL of anhydrous pyridine, followed by the addition of ethanoic anhydride (2.50 eq.). The reaction mixture was slowly warmed to room temperature (rt) and stirred overnight. Upon completion of the reaction, the mixture was washed several times with water and the organic phase was extracted with ethyl acetate. The organic phases were combined and dried with anhydrous magnesium sulfate. After filtration, the filtrate was concentrated under reduced pressure to give pure (4aS,6aS,6bR,8aR,10S,12aR,12bR,14bS)-10-acetoxy-2,2,6a,6b,9,9,12a-heptamethyl-1,3,4,5,6,6,6a,6b,7,8,8a,9,10,11,12,12a,12b,13. 14b-octadecahydroperylene-4a(2H)-carboxylic acid in >99% yield, which was directly used in the next step of the reaction.
