[CH2:1]([C@H:8]1[CH2:12][O:11][C:10](=[O:13])[N:9]1[C:14](=[O:20])[CH2:15][CH2:16][CH:17]([CH3:19])[CH3:18])[C:2]1[CH:7]=[CH:6][CH:5]=[CH:4][CH:3]=1.CCN(C(C)C)C(C)C.[O:30]1COCO[CH2:31]1>C(Cl)Cl.[Ti](Cl)(Cl)(Cl)Cl>[CH2:1]([C@H:8]1[CH2:12][O:11][C:10](=[O:13])[N:9]1[C:14](=[O:20])[C@@H:15]([CH2:31][OH:30])[CH2:16][CH:17]([CH3:18])[CH3:19])[C:2]1[CH:3]=[CH:4][CH:5]=[CH:6][CH:7]=1
Intermediate (2a) (14.5 g, 46.3 mmol) was dissolved in DCM (151 mL, 2.4 mol) and stirred at 0° C. under nitrogen. 1 M titanium tetrachloride in DCM (48.6 mL) was added and stirred for 15 minutes. DIPEA (8.9 mL, 51.0 mmol) was added dropwise at 0° C. and the mixture was stirred for 75 minutes. 1,3,5-Trioxane (4.6 g, 51.0 mmol) in DCM (30 mL) was then added. After 10 minutes a second equivalent of 1 M titanium tetrachloride in DCM (48.6 mL) was added and the mixture stirred at 0° C. for 5 hours. The reaction was then quenched with 250 mL of saturated ammonium chloride. Water and DCM were added, and the aqueous phase was extracted twice more with DCM. The organic layers were combined, dried over MgSO4, filtered and concentrated. The resulting material was purified by silica gel chromatography (0-60% EtOAc:hexanes) to provide intermediate (2b) (13.9 g).
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