[CH2:1]([C:5]1[NH:6][C:7]([C:14]([O:16][CH3:17])=[O:15])=[C:8]([C:10]([O:12][CH3:13])=[O:11])[N:9]=1)[CH2:2][CH2:3][CH3:4].CC(C)([O-])C.[K+].[CH3:24][Si:25]([CH3:32])([CH3:31])[CH2:26][CH2:27][O:28][CH2:29]Cl>CN(C=O)C.C1COCC1>[CH3:24][Si:25]([CH3:32])([CH3:31])[CH2:26][CH2:27][O:28][CH2:29][N:9]1[C:8]([C:10]([O:12][CH3:13])=[O:11])=[C:7]([C:14]([O:16][CH3:17])=[O:15])[N:6]=[C:5]1[CH2:1][CH2:2][CH2:3][CH3:4]The diester 1 from Step (a) (23 g, MW 240, 0.096 mol) was dissolved in 300 ml DMF. Potassium t-butoxide (100 ml of a 1M solution in dry THF) was added through a syringe. The solution was stirred for 15 min at 25° C. [2-(trimethylsilyl)ethoxy]methyl chloride (SEM-chloride, 18.6 ml, 1.1 eq, MW 166, d 0.94) was added and a precipitate instantly formed. The reaction was stirred for 90 min at 25° C. The solvent was removed and the residue partitioned between ethyl acetate and water. The organic extract was dried over K2CO3 and concentrated. By TLC (SiO2, MeOH-CHCl3 /1-10) a single major spot was observed. This crude product (38 g, 106% yield) was used in subsequent steps without further purification.
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