4-Methylmorpholine → tert-butyl (5S,8S,11S,12R)-11-((S)-sec-butyl)-1-(9H-fluoren-9-yl)-5,8-diisopropyl-12-methoxy-4,10-dimethyl-3,6,9-trioxo-2-oxa-4,7,10-triazatetradecan-14-oate

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To a mixture of N-[(9H-fluoren-9-ylmethoxy)carbonyl]-N-methyl-L-valine (19.54 g, 55.29 mmol, 1.3 eq.) and #6 (15.25 g, 42.54 mmol, 1 eq.) in 2-methyltetrahydrofuran (152 mL, 0.28 M) was added 2-chloro-4,6-dimethoxy-1,3,5-triazine (CDMT) (9.71 g, 55.3 mmol, 1.3 eq.). After 10 minutes, N-methylmorpholine (6.6 mL, 60 mmol, 1.4 eq.) was slowly added, while keeping the internal temp below 25° C. The reaction was stirred for 4 hours and was then quenched by the addition of water (50 mL). After stirring for 15 minutes, the aqueous layer was separated and back-extracted with 2-methyltetrahydrofuran (50 mL). The combined organic layers were washed with saturated aqueous sodium bicarbonate solution (100 mL), then were dried over magnesium sulfate, filtered, and concentrated in vacuo. The resulting yellow foam was purified by silica gel chromatography (Gradient: 5% to 35% ethyl acetate in heptane) to give #7 (32 g, 97%). 1H NMR (400 MHz, DMSO-d6), presumed to be a mixture of rotamers, characteristic signals: δ 7.89 (d, J=7.4 Hz, 2H), 7.62 (d, J=7.4 Hz, 2H), 7.41 (br dd, J=7.4, 7.4 Hz, 2H), 7.29-7.34 (m, 2H), 4.52-4.69 (br m, 1H), 3.70-3.82 (br m, 1H), 3.22 and 3.25 (2 br s, total 3H), 2.94 and 2.96 (2 br s, total 3H), 2.78 and 2.81 (2 br s, total 3H), 2.11-2.23 (m, 1H), 1.90-2.10 (m, 2H), 1.68-1.83 (br m, 1H), 1.40 (s, 9H), 1.21-1.33 (br m, 1H).

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