The compound from step 8b (294 mg, 1.2 mmol) was dissolved in toluene (10 mL), and the solution flushed with N2. N-benzyl-N(methoxymethyl)trimethylsilylmethylamine (284 mg, 0.227 mmol) was added, the mixture was stirred for 20 minutes and trifluoroacetic acid (1M, 0.1 mL, 0.10 mmol) was added dropwise. The mixture was stirred for 1.5 hour at room temperature, then diluted with 50 mL of ethyl acetate, washed with 5% aqueous NaHCO3 and dried over Na2SO4. The solvent was removed and the residue was dried under vacuum (542 mg). The isomers were separated by chromatography on silica gel, and the major isomer (3R,4R) was taken to the next step. MS m/z 427 (M+H)+.
