2-((Tert-butyldimethylsilyl)oxy)acetaldehyde → methyl 4-((1R,3aS,5aR,5bR,7aR,11aS,11bR,13aR,13bR)-3a-(2-(tert-butyldimethylsilyloxy)ethylamino)-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)benzoate

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To a solution of methyl 4-((1R,3aS,5aR,5bR,7aR,11aS,11bR,13aR,13bR)-3a-amino-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)benzoate (300 mg, 0.552 mmol) in DCE (4 mL) was added (tert-butyldimethylsilyloxy)acetaldehyde (131 mL, 0.690 mmol) and titanium(IV) isopropoxide (0.202 mL, 0.690 mmol). The mixture was stirred at rt for 45 minutes and sodium triacetoxyborohydride (234 mg, 1.103 mmol) was added. The mixture was stirred at rt for 23 h and an additional 0.1 mL of (tert-butyldimethylsilyloxy)acetaldehyde and an additional 150 mg of sodium triacetoxyborohydride were added. The mixture was stirred at rt for an additional 22 h, then was diluted with 15 mL of sat. NaHCO3 and was extracted with dichloromethane (3×15 mL). The combined organic layers were dried with Na2SO4, the drying agent was removed by filtration and the filtrate was concentrated under reduced pressure. The residue was adsorbed to silica gel and was purified by flash chromatography using a 0-10% EtOAc in hexanes gradient. The fractions containing the expected product were combined and were concentrated under reduced pressure to give the title compound (0.211 g, 0.301 mmol, 54.5% yield) as a clear yellow film. The product was used in the next step with no additional purification. LCMS: m/e 702.7 (M+H)+, 2.35 min (method 2).

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