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Efficient intermolecular [3+2] trapping of the Nazarov intermediate with vinyl sulfides.

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Tandem or "domino" processes have emerged as a powerful tool for organic synthesis. In these reactions, multiple new bonds or rings are formed in a single stereoselective operation generating complex polycyclic systems. Trapping the Nazarov intermediate with suitable nucleophiles is one of these processes. It is called the interrupted Nazarov reactions. This project is an example of the interrupted Nazarov reactions. Several simple 1,4-pentadien-3-ones were prepared and then treated with BF3·OEt2 to undergo Nazarov cyclization to produce oxyallyl cations. Generating this cationic intermediate in the present of electron-rich vinyl sulfides yielding functionalized bicyclo[2.2.1]heptanone products. Although this trapping occurred via stepwise cationic [3+2] cycloaddition, the diastereoselectivity of the products was good. This methodology resulted in the formation of three carbon-carbon bonds and up to six new stereocenters.;The ability to transfer a sulfur group in the bicyclo[2.2.1]heptanone products to another functionality was also examined. The polycyclic products contain a sulfur functionality was successfully desulfurized by treating it with Raney nickel. In the process, the carbonyl group was also reduced to an alcohol. In another modification of the sulfide moiety, the functionalized bicyclo[2.2.1]heptanone product was oxidized by m-CPBA to its corresponding sulfoxide. Stirring the sulfoxide product in refluxing toluene successfully eliminated the sulfoxide group and furnished the alkene.
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