A solution of tert-butyl (5S,12S)-12-[(R)-[(3R,4R,5R)-3,4-bis{[tert-butyl(dimethyl)silyl]oxy}-5-(2,4-dioxo-3,4-dihydro-1(2H)-pyrimidinyl)tetrahydro-2-furanyl](hydroxy)methyl]-5-isobutyl-3,6-dioxo-1-phenyl-2-oxa-4,7,11-triazatridecan-13-oate (62 mg, 0.086 mmol, obtained from Reference Example 6) and [(1R)-3-methyl-1-(3-oxo-propylcarbamoyl)butyl]-carbamic acid benzyl ester (33 mg, 0.103 mmol, obtained from N-CBZ-D-leucine in an similar manner described in Reference Example 15 and 16) in anhydrous tetrahydrofuran was stirred at room temperature under a nitrogen atmosphere for 15 minutes. Acetic acid (2 drops) and sodium triacetoxyborohydride (36 mg, 0.17 mmol) were added and the resulting solution was stirred under nitrogen for an additional 2.5 hours. The reaction mixture was basified to pH 9 with aqueous sodium carbonate solution and the aqueous layer was extracted with ethyl acetate (3×50 ml). The combined extracts were washed with saturated sodium chloride solution, dried over sodium sulfate, filtered and concentrated in vacuo. The residue was chromatographed (flash column, silica gel, 2.5% methanol in methylene chloride) to provide tert-butyl (5R,12S)-12-[(R)-[(3R,4R,5R)-3,4-bis{[tert-butyl(dimethyl)silyl]oxy}-5-(3-(4-methoxybenzyl)-2,4-dioxo-3,4-dihydro-1(2H)-pyrimidinyl)tetrahydro-2-furanyl](hydroxy)methyl]-5-isobutyl-3,6-dioxo-1-phenyl-2-oxa-4,7,11-triazatridecan-13-oate (37 mg, 42%) as a white amorphous solid.
