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Wender's group at Stanford has been developing a homolog of the Diels-Alder reaction: a [5 + 2] cycloaddition process for synthesizing seven-membered rings. A five-carbon vinylcyclopropane unit is substituted for the four-carbon diene used in the classic Diels-Alder reaction. But [5 + 2] cycloadditions do not proceed thermally, as Diels-Alder reactions do. Transition-metal catalysis proved the key to the researchers' first success, in 1995, with intramolecular [5 + 2] cycloadditions. Subsequently, Wender and coworkers found they could carry out the metal-catalyzed reaction intermolecularly using alkoxy-activated vinylcyclopropanes. Now the researchers have found that the [5 + 2] cycloaddition can be done with alkyl-substituted vinylcyclopropanes. "We've broken through and opened up the field," Wender told the Pacifichem symposium [ J. Am. Chem. Soc., 123, 179 (2001) ]. "Not having to have oxygen activation allows you to use a much wider range of five-carbon components in the cycloaddition than had been possible previously," he said. "It arguably establishes a tool for seven-membered ring synthesis that is as versatile and practical as the Diels-Alder cycloaddition is for six-membered ring synthesis." In addition, the Stanford researchers have been able to extend the strategy to [6 + 2] cycloaddition reactions, using vinylcyclobutanones. And they have found a water-soluble rhodium catalyst that allows the cycloadditions to be carried out in water, without the need for organic solvents. The organic materials can be stripped off, Wender reported, and the aqueous solution of catalyst reused. |
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