Reaction Steps
The following procedure was modified from J. Am. Chem. Soc. 2002, 124, 8001-8006. To a rbf containing diisopropyl 6-(((trifluoromethyl)sulfonyl)oxy)spiro[3.3]hept-5-ene-2,2-dicarboxylate (0.095 g, 0.229 mmol) was added bis(pinacolato)dibororon (0.064 g, 0.252 mmol), phenolate, K+(0.045 g, 0.344 mmol), triphenylphospine (3.61 mg, 0.014 mmol), and bis(triphenylphosphine)palladium(II) chloride (4.83 mg, 6.88 mmol). The mixture was diluted with toluene (2 mL), flushed with nitrogen, then was heated to 50° C. for 3 h. The mixture was cooled to rt and TLC showed all starting material had been consumed. To the mixture was added (1R,3aS,5aR,5bR,7aR,11aR,11bR,13aR,13bR)-benzyl 5a,5b,8,8,11a-pentamethyl-1-(prop-1-en-2-yl)-9-(((trifluoromethyl)sulfonyl)oxy)-2,3,3a,4,5,5a,5b,6,7,7a,8,11,11a,11b,12,13,13a,13b-octadecahydro-1H-cyclopenta[a]chrysene-3a-carboxylate (0.149 g, 0.220 mmol), phosphoric acid, potassium salt (0.146 g, 0.688 mmol) and PdCl2(dppf)-CH2Cl2 adduct (5.62 mg, 6.88 μmol). The mixture was diluted with DMF (2.000 mL), flushed with nitrogen, then was heated to 80° C. After heating the mixture for an additional 90 h, it was cooled to rt and was concentrated under reduced pressure, then was dissolved in DCM and MeOH and was filtered through a plug of silica gel and celite. The filtrate was concentrated under reduced pressure, and was purified by flash chromatography using a 0-25% ethyl acetate in hexanes gradient and a 12 g silica gel column. The major isolate from the purification contained a mixture of products, so it was further purified by flash chromatography using a slower, 0-5, 5-5, 5-10% ethyl acetate/hexanes gradient and a 12 g silica gel column. The fractions containing the expected product were combined and concentrated under reduced pressure to give diisopropyl 6-((1R,3aS,5aR,5bR,7aR,11aS,11bR,13aR,13bR)-3a-((benzyloxy)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 (0.053 g, 0.047 mmol (based on 70% purity), 20.4% yield) as a colorless foam. 1H NMR (500 MHz, chloroform-d) δ=7.40-7.30 (m, 5H), 5.86 (s, 1H), 5.51 (dd, J=6.4, 1.8 Hz, 1H), 5.19-5.02 (m, 4H), 4.73 (d, J=1.5 Hz, 1H), 4.60 (s, 1H), 3.08-2.98 (m, 1H), 2.73-2.66 (m, 4H), 2.62-2.54 (m, 2H), 1.69 (s, 3H), 2.53-0.74 (m, 49H). The material was used in the next step with no additional purification.
