[Si:1]([O:8][CH2:9][CH:10]1[CH2:14][S:13][CH:12](OCC)[S:11]1)([C:4]([CH3:7])([CH3:6])[CH3:5])([CH3:3])[CH3:2].[CH2:18]([O:20][P:21]([O:25]CC)[O:22][CH2:23][CH3:24])[CH3:19].P(Cl)(Cl)Cl>ClCCl.[Cl-].[Zn+2].[Cl-]>[Si:1]([O:8][CH2:9][C@@H:10]1[CH2:14][S:13][C@@H:12]([P:21]([O:22][CH2:23][CH3:24])([O:20][CH2:18][CH3:19])=[O:25])[S:11]1)([C:4]([CH3:5])([CH3:6])[CH3:7])([CH3:2])[CH3:3]
To a solution of 4-(t-butyldimethylsilyloxymethyl)-2-ethoxy-1,3-dithiolane (example 48) (37.1 g, 126 mmol) and triethylphosphite (14.3 mL, 83.0 mmol, 0.66 eq) in dichloromethane (100 mL) were added phosphorus trichloride (3.6 mL, 42.0 mmol, 0.33 eq) and zinc chloride (catalytic) at 0° C. The solution was stirred at 0° C. for 2 hr and at room temperature for 18 hr. The solvent was evaporated and the crude material was purified by flash chromatography with a mixture of hexane and ethyl acetate (80:20 to 60:40) to give 3.64 g of the cis isomer, 8.62 g of the trans isomer and 9.95 g of the mixture (2:1 trans:cis) for a total yield of 46%.
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