Reaction Steps
To a solution of (1R,3aS,5aR,5bR,7aR,11aR,11bR,13aR,13bR)-5a,5b,8,8,11a-pentamethyl-9-oxo-1-(prop-1-en-2-yl)icosahydro-1H-cyclopenta[a]chrysene-3a-carbaldehyde (17.6 g, 36.1 mmol) in t-BuOH (100 mL) was added 2-methyl-2-butene (40 mL, 476 mmol). A solution of sodium chlorite (15 g, 133 mmol) and sodium phosphate monobasic monohydrate (25 g, 181 mmol) in water (200 mL) was added drop wise over 1.25 h and the mixture was stirred at rt for an additional 45 minutes. The mixture was diluted with saturated aqueous ammonium chloride (100 mL) and extracted with ethyl acetate (3×125 mL). The combined organic layers were washed with brine and dried over sodium sulfate. The drying agent was removed by filtration and the filtrate was concentrated under reduced pressure. The residue was purified by using a 300 g Thomson silica gel column and a 10-50% ethyl acetate in hexanes gradient. The fractions containing the expected product were combined and concentrated under reduced pressure to give (1R,3aS,5aR,5bR,7aR,11aR,11bR,13aR,13bR)-5a,5b,8,8,11a-pentamethyl-9-oxo-1-(prop-1-en-2-yl)icosahydro-1H-cyclopenta[a]chrysene-3a-carboxylic acid as a white foam (16.4 g, 36.1 mmol, 100%). LCMS: m/e 453.2 (M−H)−, 2.61 min (method 3). 1H NMR (400 MHz, chloroform-d) δ=10.02 (br. s., 1H), 4.75 (d, J=1.8 Hz, 1H), 4.64-4.61 (m, 1H), 3.02 (td, J=10.8, 4.8 Hz, 1H), 2.55-2.36 (m, 3H), 2.33-2.19 (m, 2H), 2.08-1.86 (m, 4H), 1.70 (s, 3H), 1.08 (s, 3H), 1.02 (s, 3H), 1.00 (s, 3H), 0.98 (s, 3H), 0.93 (s, 3H), 1.82-0.90 (m, 15H).
