Reaction Steps
To a solution of diisopropyl 6-((1R,3aS,5aR,5bR,7aR,11aS,11bR,13aR,13bR)-3a-(((tert-butyldimethylsilyl)oxy)carbonyl)-5a,5b,8,8,11a-pentamethyl-1-(prop-1-en-2-yl)-2,3,3a,4,5,5a,5b,6,7,7a,8,11,11a,11b,12,13,13a,13b-octadecahydro-1H-cyclopenta[a]chrysen-9-yl)spiro[3.3]hept-5-ene-2,2-dicarboxylate (0.026 g, 0.021 mmol (based on 65% purity)) in THF (2 mL) was added TBAF (75% in water) (10.81 mg, 0.031 mmol). The mixture was stirred at rt for 30 minutes, then was diluted with 15 mL of 1N HCl and was extracted with ethyl acetate (3×20 mL). The combined organic layers were washed with brine, dried over magnesium sulfate, filtered, and concentrated under reduced pressure to give (1R,3aS,5aR,5bR,7aR,11aS,11bR,13aR,13bR)-9-(6,6-bis(isopropoxycarbonyl)spiro[3.3]hept-1-en-2-yl)-5a,5b,8,8,11a-pentamethyl-1-(prop-1-en-2-yl)-2,3,3a,4,5,5a,5b,6,7,7a,8,11,11a,11b,12,13,13a,13b-octadecahydro-1H-cyclopenta[a]chrysene-3a-carboxylic acid (0.024 g, 0.021 mmol (based on 60% purity), 100% yield). The material was used in the next step with no additional purification. LCMS: m/e 701.5 (M−H)−, 3.35 min (method 3). 1H NMR (400 MHz, chloroform-d) δ=10.58 (br. s., 1H), 5.87 (s, 1H), 5.52 (d, J=4.8 Hz, 1H), 5.13-5.02 (m, 2H), 4.75 (s, 1H), 4.62 (br. s., 1H), 3.07-2.97 (m, 1H), 2.70 (s, 4H), 2.63-2.55 (m, 2H), 1.70 (s, 3H), 2.54-0.77 (m, 49H). The material was used in the next step with no additional purification.
