Diisopropylethylamine → (S)-22-(tert-Butoxycarbonyl)-45,45-dimethyl-10,19,24,43-tetraoxo-3,6,12,15,44-pentaoxa-9,18,23-triazahexatetracontanoic acid

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To a solution of 2-(19-tert-Butoxycarbonylnonadecanoylamino)pentanedioic acid 1-tert-butyl ester 5-(2,5-dioxopyrrolidin-1-yl) ester (2.50 g) and [2-(2-{2-[2-(2-aminoethoxy)ethoxy]acetyl-amino}ethoxy)ethoxy]acetic acid (alternative name: H-OEG-OEG-OH)(1.47 g) in ethanol (40 ml) was added DIPEA (1.26 ml). The mixture was stirred at room temperature overnight and then concentrated in vacuo. To the residue was added aqueous 0.1 N HCl (150 ml) and ethyl acetate (200 ml). The layers were separated and the aqueous layer was extracted with ethyl acetate (100 ml). The combined organic layers were washed with water and brine, dried (magnesium sulphate) and concentrated in vacuo to give an oil, which crystallised on standing. Yield 96% (3.1 g). LCMS: Theoretical mass: 874.2. Found: 874.49.

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