The compound from step 7b (490 mg, 2 mmol) was dissolved in methylene chloride (9 mL), and the solution was cooled to 0° C. and flushed with N2. N-benzyl-N-(methoxymethyl)trimethylsilylmethylamine (498 mg, 2.1 mmol) was added, the mixture was stirred for 20 minutes and trifluoroacetic acid (1M, 0.2 mL, 0.20 mmol) was added dropwise over 20 minutes with stirring at 0° C. The mixture was stirred at 0° C. for 1 hour and at room temperature for 24 hours, then diluted with 50 mL of methylene chloride, washed with 10% aqueous K2CO3 and dried over Na2SO4. The solvent was removed and the residue was dried under vacuum (767 mg). The isomers were separated by medium pressure liquid chromatography on silica gel, eluting with 0.5% methanol/methylene chloride. The major isomer (3R,4R) (290 mg) was taken to the next step. MS m/z 379 (M+H)+.
