Reaction Steps
To a solution of 4-(((3aR,5aR,5bR,7aR,9S,11aR,11bR,13aS)-3a-((S)-1-acetoxy-2-((4-chlorobenzyl)amino)ethyl)-1-isopropyl-5a,5b,8,8,11a-pentamethyl-2-oxo-3,3a,4,5,5a,5b,6,7,7a,8,9,10,11,11a,11b,12,13,13a-octadecahydro-2H-cyclopenta[a]chrysen-9-yl)oxy)-2,2-dimethyl-4-oxobutanoic acid (50) (150 mg, 0.192 mmol) in MeOH (6 mL) stirred in air at rt was added formaldehyde (31.2 mg, 0.384 mmol). The reaction mixture was stirred at rt for 2 h. Then it was added NaCNBH3 (85 mg, 1.345 mmol) portionwise and stirred for overnight. The solvent was evaporated and the residue diluted with DCM and washed with water and brine. The dried organics were filtered and concentrated to give 4-(((3aR,5aR,5bR,7aR,9S,11aR,11bR,13aS)-3a-((S)-1-acetoxy-2-((4-chlorobenzyl)(methyl)amino)ethyl)-1-isopropyl-5a,5b,8,8,11a-pentamethyl-2-oxo-3,3a,4,5,5a,5b,6,7,7a,8,9,10,11,11a,11b,12,13,13a-octadecahydro-2H-cyclopenta[a]chrysen-9-yl)oxy)-2,2-dimethyl-4-oxobutanoic acid (68) (120 mg, 65% yield) as a white solid. This material was used without further purification. 1H NMR (400 MHz, CHLOROFORM-d) δ=7.31-7.26 (m, 2H), 7.24-7.16 (m, 2H), 5.65 (d, J=8.5 Hz, 1H), 4.50 (br. s., 1H), 3.66-3.51 (m, 1H), 3.49-3.36 (m, 1H), 3.27-3.06 (m, 2H), 2.73-2.32 (m, 5H), 2.26 (s, 3H), 2.08-0.73 (m, 49H); LC/MS: m/z calculated 793.5. found 794.3 (M+1)+.
