4. Highly enantioselective synthesis of vinylepoxides via chiral sulfonium ylide
Optically active vinylepoxides are very useful intermediates in organic synthesis. Due to the multifunctional groups in the vinylepoxides, they are usually synthesized by indirect methods. The epoxidation reaction of allylide with aldehyde is one of the direct methods. However, the enantioselective synthesis of vinylepoxides by allylides is still rare in the literatures.
It was found that the allylide of sulfonium salt could react with aryl aldehydes to afford corresponding trans-epoxides in good yield with good trans/cis selectivity and excellent enantioselectivity.
Based on the results with several reaction conditions and the absolute configuration of epoxidation products, it was proposed that the excellent enantioselectivity was owed to the hydrogen bond between hydroxyl group and aldehyde.
In conclusion, three strategies (additive agent, chiral auxiliary or chiral reagent) were developed to improve the trans/cis selectivity, diastereoselectivity or enantioselectivity of the ylide cyclopropanation reactions. The success of these strategies provided facile and effective methodologies to the highly stereoselective synthesis of multisubstituted vinylcyclopropanes. The highly enantioselective synthesis of vinyl epoxide via chiral sulfonium ylide was also developed.
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