[Synthesis]
1. O-Anisaldehyde (10.0 g, 73.4 mmol, 1.0 eq.) was dissolved in ethanol (150 mL) and stirred at 25 °C. N,N-dimethylethylenediamine (8.70 mL, 80.8 mmol, 1.1 eq.) was added and stirring was continued at 25 °C for 24 hours. Upon completion of the reaction, it was filtered through a MgSO4 pad and concentrated to give white solid imidazolidine (15.0 g, 99% yield).
2. The above imidazolidine (15.0 g, 72.8 mmol, 1.0 eq.) was dissolved in ether (250 mL) and cooled to -40 °C. The reaction was carried out over a period of time. Tert-butyl lithium (1.7 M in pentane, 100 mL, 170 mmol, 2.34 eq.) was added dropwise over 1 hr. After dropwise addition, the temperature was slowly increased to -20 °C and stirring was continued for 7 hours. Subsequently, it was transferred to a solution of ether (250 mL) containing dibromodichloromethane (55.3 g, 170 mmol, 2.34 eq.) via cannula over 5 min. The reaction mixture was gradually warmed to 25 °C over 12 h, after which it was cooled to 0 °C and 1 M HCl (500 mL) was slowly added. After stirring at 0 °C for 1 h, the reaction was rapidly warmed to 25 °C and quenched by the addition of water (500 mL). Extracted with ethyl acetate (3 x 250 mL), the organic phases were combined, washed with water (500 mL) and brine (250 mL), dried (MgSO4) and concentrated to give the crude product. Purification by fast column chromatography (silica gel, hexane/ethyl acetate, 9/1) gave a white solid brominated product (8.12 g, 52% yield).
3. The brominated product (8.12 g, 37.8 mmol, 1.0 eq.) was suspended in methanol (100 mL) and cooled to 0 °C. The brominated product (8.12 g, 37.8 mmol, 1.0 eq.) was added to the mixture. Sodium borohydride (2.88 g, 75.6 mmol, 2.0 eq.) was added in batches and stirred at 0 °C for 1 hour. When the reaction was complete, it was quenched by the addition of water (100 mL) and concentrated. The mixture was redissolved in ethyl acetate (100 mL), poured into water (100 mL) and extracted with ethyl acetate (3 x 50 mL). The organic phases were combined, washed with water (150 mL) and brine (50 mL), dried (MgSO4) and concentrated to give a white solid alcohol (7.83 g, 96% yield).
4. The alcohol (7.83 g, 36.1 mmol, 1.0 eq.) was dissolved in ether (180 mL) and pyridine (0.437 mL, 5.41 mmol, 0.15 eq.) and phosphorus tribromide (3.41 mL, 36.1 mmol, 1.0 eq.) were added sequentially. After stirring at 25 °C for 4 h, the reaction was quenched by the addition of water (100 mL), poured into water (100 mL) and extracted with ethyl acetate (3 x 50 mL). The organic phases were combined, washed with water (200 mL) and brine (100 mL), dried (MgSO4), and concentrated to give white solid bromide (10.0 g, 99% yield). Note: The product is prone to decomposition after purification and should be used as soon as possible.
5. KHMDS (0.5 M in toluene, 129 mL, 64.5 mmol, 1.8 eq.) was added dropwise to a solution of diethyl phosphite (9.19 mL, 71.4 mmol, 2.0 eq.) in THF (100 mL) at 0 °C and stirred for 15 min. Subsequently, a solution of freshly prepared bromide (10.0 g, 35.7 mmol, 1.0 eq.) in THF (100 mL) was added dropwise, and the reaction mixture was gradually warmed up to 25 °C over 12 hours. Upon completion of the reaction, it was quenched with saturated NH4Cl (150 mL), poured into water (150 mL) and extracted with ethyl acetate (3 x 150 mL). The organic phases were combined, washed with water (100 mL) and brine (100 mL), dried (MgSO4) and concentrated to give a colorless oily phosphonate (10.79 g, 90% yield). |