Cl.[F:2][C@@:3]12[C@:16]3([CH3:17])[C:11](=[CH:12][C:13](=[O:18])[CH:14]=[CH:15]3)[C@@H:10]([F:19])[CH2:9][C@H:8]1[C@@H:7]1[CH2:20][C@@H:21]3[C@:25]([C:26](=[O:32])[CH2:27][O:28][C:29](=[O:31])[CH3:30])([C@@:6]1([CH3:33])[CH2:5][C@@H:4]2[OH:34])[CH2:24][NH:23][CH2:22]3.[CH3:35][C:36]([CH3:41])([CH3:40])[CH2:37][CH:38]=O.C(O)=O>C(#N)C>[CH3:35][C:36]([CH3:41])([CH3:40])[CH2:37][CH2:38][N:23]1[CH2:24][C@:25]2([C:26](=[O:32])[CH2:27][O:28][C:29](=[O:31])[CH3:30])[C@@H:21]([CH2:20][C@H:7]3[C@H:8]4[C@@:3]([F:2])([C@:16]5([CH3:17])[C:11]([C@@H:10]([F:19])[CH2:9]4)=[CH:12][C:13](=[O:18])[CH:14]=[CH:15]5)[C@@H:4]([OH:34])[CH2:5][C@@:6]32[CH3:33])[CH2:22]1A solution of compound 20 (900 mg, 1.800 mmol), 3,3-dimethylbutyraldehyde (904 μl, 7.20 mmol), formic acid (552 μl, 14.40 mmol) and TEA (251 μl, 1.800 mmol) in acetonitrile (45 ml) is irradiated with microwaves (140° C., 15 minutes). The reaction is partitioned between AcOEt and NaHCO3 solution. The organic layers are then washed with brine, dried over Na2SO4 and filtered. The solvent is evaporated to give a oil which is purified by silica gel chromatography (from DCM:MeOH 100:0 to 98.5:1.5) to give the title compound (721 mg, 1.316 mmol, 73.1% yield).
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