Corey-Winter Olefination
The Corey-Winter olefination allows the stereospecific synthesis of alkenes from 1,2-diols through the formation of a cyclic thiocarbonate.
General features: 1. The reaction is completely stereospecific and high-yielding. 2. Highly substituted and hindered olefins can be obtained. 3. The stereochemistry of the product olefin is determined by the stereochemistry of the starting material. Besides, a cis alkene may be converted to the corresponding trans-1,2-diol and subjected to the Corey-Winter procedure to afford the corresponding trans olefin and vice versa.
Corey-Winter Olefination
Reaction Mechanism
1. The 1,2-diol is converted to the cyclic thiocarbonate derivative using thiocarbonyldiimidazole, TCDI. 2. Cis-elimination reaction upon treatment with excess trimethylphosphite. 2.1. The phosphite attacks the sulfur atom leaving a carbene. 2.2. The carbene reacts with a second equivalent of phosphite, generating an ylide. 2.3. The ylide delivers the product along with an acyl phosphite that decarboxylates.
Experimental Procedure
To a solution of the 1,2-diol (0.21 mmol, 1.0 eq) in toluene (20 mL), was added thiocarbonyl diimidazole (10.0 eq) at rt. The reaction mixture was refluxed and stirred for 48 hours and then cooled to room temperature. 1M HCl solution and ethyl acetate were added to the reaction mixture, and the organic phase was extracted with ethyl acetate, washed with brine, dried over anhydrous Na2SO4 and concentrated under reduced pressure. The residue was purified by column chromatography to give the corresponding cyclic thionocarbonate (80% yield).
A solution of the cyclic thionocarbonate (0.17 mmol, 1.0 eq) and P(OCH3)3 (10 mL) was refluxed and stirred for 48 hours. The reaction mixture was concentrated under reduced pressure, and the residue was purified by column chromatography to give the desired alkene (93% yield).
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