[CH2:1]([O:8][C:9]([NH:11][C@H:12]([C:17]([OH:19])=O)[CH2:13][CH:14]([CH3:16])[CH3:15])=[O:10])[C:2]1[CH:7]=[CH:6][CH:5]=[CH:4][CH:3]=1.Cl.CN.C[CH2:24][N:25](C(C)C)C(C)C.CN(C(ON1N=NC2C=CC=CC1=2)=[N+](C)C)C.[B-](F)(F)(F)F>CN(C=O)C>[CH3:24][NH:25][C:17](=[O:19])[C@H:12]([CH2:13][CH:14]([CH3:16])[CH3:15])[NH:11][C:9]([O:8][CH2:1][C:2]1[CH:7]=[CH:6][CH:5]=[CH:4][CH:3]=1)=[O:10]
To a solution of N-Benzyloxycarbonyl-L-leucine (1.18 g, 4.45 mmol), methylamine hydrochloride (0.60 g, 8.90 mmol) and DIEA (3.0 mL, 17.2 mmol) in DMF (40 mL) was added TBTU (1.43 g, 5.6 mmol) at 0° C. in two batches over 10 minutes. After stirring at room temperature overnight, the reaction was quenched with water (5 mL) and evaporated under vacuum. The residue was partitioned between brine and EtOAc. The organic phase was dried over anhydrous Na2SO4, filtered and evaporated to dryness. The residue was filtered through a short pad of silica gel with EtOAc to give crude product 20A (1.20 g) which was used in the next step without purification. LCMS (+ESI) m/z 301.1 [M+Na]+.
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